Calibrating lower-middle Miocene mammal faunas and unravelling climate change during the Miocene Climate Optimum; the Bardenas Reales de Navarra record (Ebro basin, NE Iberian Peninsula)

Calibrating lower-middle Miocene mammal faunas and unravelling climate change during the Miocene Climate Optimum; the Bardenas Reales de Navarra record (Ebro basin, NE Iberian Peninsula) | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Calibrating lower-middle Miocene mammal faunas and unravelling climate change during the Miocene Climate Optimum; the Bardenas Reales de Navarra record (Ebro basin, NE Iberian Peninsula) Juan Cruz Larrasoaña Gorosquieta, Oier Suarez-Hernando, Elisabet Beamud, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4447195/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 22 Oct, 2024 Read the published version in Journal of Iberian Geology → Version 1 posted 5 You are reading this latest preprint version Abstract The chronology of lower Miocene Iberian small mammal faunas is still poorly constrained given the scarcity of well dated sedimentary successions including small mammal fossil localities. Such scarcity has prevented also an accurate understanding of the response of European terrestrial ecosystem to global changes across the Miocene Climate Optimum (MCO), one of the best analogues of present-day global warming. Here we present an updated fossil small mammal record of the Bardenas Reales de Navarra (western Ebro basin, Spain), where an expanded lower to middle Miocene continental succession is superbly exposed. Previous and new magnetostratigraphic results from this succession have enabled us to propose, along with additional magnetostratigraphically-dated Iberian faunas, a new chronology for local zones Y to D (Mammal Neogene zones MN2 to MN5). In addition to that, the studied small mammal faunas point to a gradual increase in temperature and humidity conditions in SW Europe between 20 and 15.5 Ma, which appears to be coupled with the progressive shift towards warmer regional (Atlantic) and global conditions across the MCO, thereby pointing to gradual changes in oceanic circulation as the main driver of this period of global warmth. The evolution of sedimentary facies appears to indicate a threshold response of the Ebro basin hydrological balance to the MCO, whereas pedogenic formation of magnetic minerals seems to be linked to periods of enhanced climate variability. These results highlight the need of combining different paleoenvironmental indicators in order to obtain a reliable view of the response of continental ecosystems to global warming. Iberian Peninsula Ebro basin magnetochronology small mammals environmental magnetism Miocene Climate Optimum Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1. Introduction Establishing accurate chronologies for continental sedimentary successions is of major importance in paleoclimatology because it enables placing environmental changes occurred in continental areas within the same chronostratigraphic framework as changes reported from the neighbouring marine realm. This is a step that needs to be fulfilled in order to better understand the response of regional environmental changes to global climate variations (van der Meulen & Daams, 1992 ; Alcalá et al., 2000 , van Dam et al., 2006 , 2023 ). Amongst those periods of climate change occurred during the Cenozoic, the Miocene Climate Optimum (MCO) stands out because it represents the most pronounced period of global warming and enhanced CO 2 atmospheric concentrations of the last 33 Myr of the Earth´s history (Beerling and Royer, 2011 ; Westerhold et el., 2020) and, hence, is considered as one of the best analogues of present-day global change. Most marine MCO records point to global temperatures 6–8°C warmer than today between broadly 17 and 14 Ma (Mudelsee et al., 2014 ; Westerhold et al., 2020 ), which were accompanied by atmospheric CO 2 concentrations often exceeding 500 ppm (Beerling and Royer, 2011 ; Greenop et al., 2014 ). The MCO was followed by a sharp end (the Middle Miocene Climate Transition, ca. 14 − 13 Ma) that, given its amplitude, is recognized in most paleclimatological records (Mudelsee et al., 2014 ; Westerhold et al., 2020 ). As opposed to this rapid termination, the onset of the MCO is marked by a longer warming trend that started at about 20 Ma and is of lower magnitude in terms of global temperature changes (Mudelsee et al., 2014 ). Discussion on the causes of the MCO is open, with changes in ocean circulation, volcanic activity, atmospheric CO 2 levels, orbital forcing, and feedbacks between plant expansion and CO 2 levels being considered as possible driving factors (Pagani et al., 1999 ; Hamon et al., 2012 ; Henrot et al., 2017 ; Super et al. 2018 ; Goto et al., 2023 ). MCO marine data also indicate slight (0.5 myr) differences in its onset and termination as well as in amplitude of the response in different areas (Mudelsee et al., 2014 ), adding uncertainty on its underlying causes. As opposed to the oceanic realm, data from continental areas are rather scarce. Mean annual temperatures established for north America from low-resolution (250 kyr) paleosoil weathering indexes reveal a steady onset of the MCO, peak warm temperatures between 17 and 13 Ma, and a sharp cooling at around 12.5 Ma (Gallagher & Sheldon, 2013 ). In East Asia, high-resolution (20 kyr) rock magnetic data from different red clay sequences in the Chinese Loess Plateau point to a gradual onset of the MCO at 17 − 16 Ma and a more abrupt termination of peak warm conditions at 13.8–14.5 Ma (Zan et al., 2015 ; Zhao et al., 2017 ). In Europe, data on the MCO come from different sectors. Paleoclimatic reconstructions from central Europe based on ectothermic vertebrates (Bohme, 2003; Bohme et al., 2011 ), plant macrofossil assemblages (Mossbruger et al., 2005), pollen data (Donders et al., 2009 ) and carbonate clumped isotopes from paleosoils (Methner et al., 2020 ) point to overall warmer and wetter conditions between 17 − 15 Ma, but show significant regional discrepancies that are attributed to paleogeographic factors such as land-sea distribution or orographic factors (Mossbruger et al., 2005). It should be kept in mind, however, that the temporal resolution of these records is often too coarse (0.2-2 Myr) and their chronology is not fully resolved in some cases (Donders et al., 2009 , Bohme et al., 2011 ), which further limits the assessment of European climate changes across the MCO. This is especially relevant for its onset, since most records either do not extend beyond 17 Ma (see Donders et al., 2009 ; Bohme et al., 2011 ) or have a much lower (> 1 Myr) resolution before that time (Bohme, 2003; Mosbrugger et al., 2005 ; Methner et al., 2020 ), hampering identification of the underlying forcing mechanism(s). High-resolution (80 kyr) paleoclimatic reconstructions for southern Europe across the MCO derive form the magnetostratigraphically-dated fossil record of the Calatayud-Daroca basin (Daams et al., 1999a , 1999b ; van Dam et al., 2006 ). Using herpetological assemblages from this basin, overall wetter conditions have been reported for the peak of the MCO, with and abrupt drying at 15.5 Ma (Bohme et al., 2011 ). The fossil record of the Calatayud-Daroca basin extends back in time beyond 17 Ma but, unfortunately, the sedimentary succession is faulted and no reliable magnetostratigraphic calibration of fossil faunas can be produced (Daams et al., 1999a , b ), precluding precise delineation of the onset of the MCO. The Bardenas Reales de Navarra (BRN) area at the western Ebro basin host a 690 m-thick, lower-middle Miocene sedimentary succession of continental origin that includes several small mammal fossil faunas initially attributed to Iberian zones Z to A (equivalent to Mammal Neogene zone 3, MN3) (Murelaga, 2000 ; Murelaga et al., 2004 ). The outstanding outcrop conditions and the good paleomagnetic signal associated to the dominant fine-grained sediments within the succession has resulted in an independent and straightforward correlation of the well-resolved local magnetozones with the Geomagnetic Polarity Timescale (GPTS). The resulting magnetostratigraphic age model indicates that the succession spans from chron C6An.1n to chron C5Br (Larrasoaña et al., 2006 ), covering from ca. 20.5 to 15.2 Ma in the updated GPTS of Ogg ( 2020 ). Although later efforts have enabled the study of additional fossil faunas (Ruiz-Sánchez et al., 2012a , 2012b , 2012c , 2012d , 2013 ), most of the new fossil localities have been located in successions with no magnetostratigraphic constraint, so that their chronology has relied on the lithostratigraphic correlation to previously published magnetostratigraphic successions. In any case, these new findings have largely expanded the small mammal record of the BRN so that most of the lower Miocene Iberian zones prior to 17 Ma (Y1 to C) are encompassed, for the first time, in an expanded and well exposed succession in direct stratigraphic continuity with basal middle Miocene faunas (zone D) for which a reliable magnetostratigraphic calibration exists (Daams et al., 1999a , 1999b ; van Dam et al., 2006 ). Hence, the BRN area offers the possibility of extending the calibration of the middle Miocene Iberian zones performed in the Calatayud-Daroca basin to the lower Miocene, filling an important gap in the chronology of small mammal Iberian and European biochronology (Dams et al., 1999a, 1999b; Agustí et al., 2001 , van Dam et al., 2006 ). In addition to small mammal remains of biochronological significance, fossil localities at the BRN area host a diverse vertebrate fauna that includes amphibians, reptiles and large mammals (Murelaga, 2000 ; Murelaga et al., 1999 , 2002 , 2004 ). The fossil record of the succession is completed by ostracod (Martínez-García et al., 2014 ), charophyte (González-Pardos, 2012 ) and avian and reptilian eggshell remains (Grellet-Tinnet et al., 2012), avian and artiodactyl fossil tracks (Díaz-Martínez et al., 2016 , 2020 ), and by an aquatic avian fossilized nest (Grellet-Tinnet et al., 2012). Collectively, this varied fossil record portraits a warm, subtropical ecosystem where rainfall was likely submitted to a high seasonality, in line with previous inferences based on reptilian and amphibian faunas (Murelaga et al., 2002 ). In this study we present an update of the fossil small mammal record of the lower-middle Miocene sedimentary succession at the BRN area, which includes a review of previously published fossil localities and new, unpublished (Suarez-Hernando, 2017 ) fossil localities aimed at filling the gaps between different local zones. These data are presented along with three new magnetostratigraphic sections that have been studied in order to provide robust chronological constrains for the largest possible number of the newly studied fossil localities. Integration of these data with previously published results from the Calatayud-Daroca basin (Daams et al., 1999a , 1999b , van Dam et al., 2006 ) and other sectors of the Ebro basin (Agustí et al., 2011 ; Pérez-Rivarés et al., 2004 , 2018 ) enables a new calibration of Iberian and European mammal zones for the lower Miocene to be proposed. We have also used the updated small mammal record of the BRN to produce, following established methodologies (Daams et al., 1988 ; van Dam and Weltje, 1999), a low-resolution (ca. 0.3 Ma) reconstruction of relative variations in temperature and humidity conditions in southwestern Europe between 20.5 and 15.3 Ma, encompassing the onset and most of the MCO peak. This paleoenvironmental record has been complemented by a high-resolution (30 kyr) rock magnetic study of the fine-grained sediments of the BRN area deposited under subaerial conditions, which are subjected to incipient pedogenic processes during sedimentation and can, therefore, provide additional constraints on the underlying climatic conditions (Liu et al., 2012 ). The recent link established between enhanced pedogenic formation of magnetic minerals in response to wetter conditions in the Pliocene Teruel basin (Gao et al., 2022 ) suggests that this rock-magnetic approach might be successfully applied to other continental basins in the Iberian Peninsula, such as the Ebro basin considered here. Paleoenvironmental results derived from small mammal faunas and rock magnetic properties are used, in combination with additional sedimentological (Larena et al., 2020 ) and micropaleontological (Martínez-García et al., 2014 ) information derived from the BRN sedimentary succession to provide what might be considered the first comprehensive record of climate variations suffered by southwestern European ecosystems during the onset and peak warming conditions of the MCO. 2. Geological setting and methods 2.1. Geological setting The Ebro basin is an asymmetric, triangularly-shaped foreland basin that developed mainly during the Paleogene in response to tectonic loading in the Pyrenees fold-and-thrust belt (Muñoz et al., 2002 ; Pardo et al., 2004 ) (Fig. 1 ). Tectonic thrusting and uplift in the western Pyrenees cut the connection of the Ebro basin with the Atlantic Ocean in the late Eocene (ca. 36 Ma) (Costa et al., 2010 ), which subsequently developed as an endorheic, low-gradient depression enclosed also by the Iberian and the Catalan Coastal Ranges. This configuration persisted until the Late Miocene (12 − 7 Ma), when the Mediterranean drainage network captured the basin (García-Castellanos & Larrasoaña, 2015 ). As a result of this protracted evolution as an endorheic depression, the Ebro basin filled up with a sequence of Late Eocene to Miocene sediments that accumulated in a set of broadly concentrical facies belt that included: 1) a system of alluvial fans attached to the active margins of the enclosing mountain belts, where conglomerates accumulated; 2) a lacustrine system that occupied the central parts of the depression, when either carbonates or evaporites accumulated in response to varying climatic conditions; and 3) an intermediate zone where distal alluvial flood plains met the lacustrine areas, and which witnessed accumulation of mudstones interbedded with palustrine carbonates and fluvial sandstones (Arenas & Pardo, 1999 ; Muñoz et al., 2002 ; Pardo et al., 2004 ). The overall thickness of the Ebro basin sedimentary sequence reaches up to 6000 in its north-western sector, at the foothills of the Pyrenean thrust-and-fold belt, and decreases progressively to the south and east to reach less than 2000 m in the vicinity of the Iberian and Catalan margins. This difference attests to the flexural subsidence of the basin in response mainly to tectonic load of the Pyrenean allochthonous units (Muñoz et al., 2002 , Pardo et al., 2004 ). The BRN area host a sequence of ca. 694 meters accumulated in the western part of the central Ebro basin during the lower and middle Miocene (Fig. 1 ) (Larrasoaña et al. 2006 ). The sequence includes the uppermost 50 m of the Lerín Gypsum Formation (Salvany et al., 1994 ) and the overlying Tudela Formation, which has been divided into 5 units (Larrasoaña et al., 2006 ) (Fig. 2 ). The Lerín Gypsum Formation is represented in the area by brown and yellow mudstones that include some sandstone beds and two thick (< 15 m) packages constituted by gypsum levels and grey mudstones that attest to sedimentation in a saline mudflat. The Tudela Formation is mainly constituted by red, brown, yellow and grey mudstones that include frequent sandstones and limestone beds associated to grey marls. These sediments were deposited in a distal alluvial mud flat fed by fluvial courses that coexisted either with small ponds (units 1 and 4) or larger lacustrine areas (units 2, 3 and 5). The only exception to this pattern is the 10 m-thick Fustiñana Gypsum Member (middle of Unit 3), which attest to short-lived sedimentation in a saline lake formed under arid conditions (Salvany et al., 1994 ). The Lerín Gypsum Formation is part of the fourth tecto-sedimentary unit (TSU-4) in which the sedimentary infill of the Ebro basin has been divided (Muñoz et al., 2002 ; Pardo et al., 2004 ; Pérez-Rivarés et al., 2018 ). Units 1 to 4 of the Tudela Formation belong to TSU-5, whereas its Unit 5 is included within TSU-6 (Muñoz et al., 2002 ; Pardo et al., 2004 ; Pérez-Rivarés et al., 2018 ). The distribution of these sediments and their lateral equivalents can be envisaged in the context of a basin with an extremely low topographic gradient thanks to the sedimentary model developed by Arenas & Pardo ( 1999 ). According to this model, periods with a positive hydrological balance witnessed the development of large lakes in the central part of the Ebro basin, which resulted in accumulation of grey marls in the deepest (< 6 m) part of the lacustrine system and of carbonates in a shallower (< 2 m) fringe that separated the lake system (units 1–4 of the Tudela Formation and their lateral counterpart to the east, the lower part of the Alcubierre Formation) from the surrounding mudflats (Ujué and Sariñena formations towards the north). Periods with a negative hydrological balance led to accumulation of evaporites in the central part of a shrinking lacustrine system (Lerín and Zaragoza formations) and of distal alluvial sediments in mudflats that prograded up to 30 km basinward over areas previously occupied by lacustrine sedimentation. This configuration changed dramatically at the base of Unit 5 of the Tudela Formation (and its lateral equivalent, the upper part of Alcubierre Formation), when lacustrine sedimentation replaced previously accumulated sediments regardless of their origin and position. Magnetostratigraphic results across the basin have demonstrated that such widespread lacustrine expansion occurred at ca. 16.1 Ma (uppermost part of chron C5Cn, Larrasoaña et al., 2006 ; Pérez-Rivarés et al., 2004 , 2018 ), in what can be considered as the response of the hydrological balance of the Ebro basin to enhanced wetter conditions driven by the MCO (Arenas & Pardo, 1999 ; Pérez-Rivarés et el., 2018; Larena et al., 2020 ). 2.2. Stratigraphy and sedimentology We have logged and described the lithology, colour, grain size, thickness and sedimentary structures and textures of sediments along five new sections at the BRN, namely the Cuesta Agujeros, Cabezo Carbonera, Loma Negra, Punta del Olmo, and Punta de Riantón sections (Fig. 1 ). These sections have been studied because they cover stratigraphic gaps with a lack of fossil sites, mainly in the lowermost and upper parts of the BRN sedimentary succession. We have also revised and described the sedimentary facies of previously published sections in the BRN (Murelaga, 2020; Larrasoaña et al., 2006 ) in order to have a complete view of the sedimentological significance of the studied succession. Especial attention has been paid to sedimentary features and textures indicative of incipient edaphic features (e.g., mottling linked to root traces) in mudstones deposited under subaerial conditions (as signalled by yellow, brown and red colours) (Arenas & Pardo, 1999 ; Larena et al., 2020 ), for their bearing on environmental magnetic results (see section 2.4 ). The overall horizontal bedding and mild deformation conditions (Soto et al., 2009 ), together with excellent exposures and the presence of distinctive lithological marker beds, enable lithostratigraphic correlations between the new sections and those studied previously to be done (Fig. 2 ). This has enabled revision of the previous composite section described in Larrasoaña et al. ( 2006 ), for which only a minor correction (< 10 m at the base of the Barranco de Tudela section) has been introduced. 2.3. Small mammal chronology and palaeoenvironmental reconstruction We screened twenty-seven stratigraphic levels in the new sections logged, targeting mainly grey mudstones, where most fossil remains had been previously recovered (Murelaga, 2000 ; Murelaga et al., 2004 ; Ruiz-Sánchez et al., 2012a , 2012b , 2012c , 2012d , 2013 ). At each interval, about 25 kg of sediment were sampled, washed and sieved in order to produce concentrates with particles ranging in size between 3 and 0.5 mm. Such concentrates were treated with acetic acid (10% concentration) in order to facilitate the removal of the carbonate fraction and separation of small mammal fossil teeth (see Suarez-Hernando, 2017 ). Eleven intervals where at least one small-mammal tooth was found, plus seven previously published localities (Ruiz-Sánchez et al., 2012b , 2012c , 2013 ), were resampled by collecting between 80 and 1700 kg of sediment at each interval depending on initial richness estimates and prioritizing biozone boundaries. Overall, a total of 19.970 kg of sediment have been collected and studied over the last 25 years in order to find 1171 small mammal molars suitable for species identification, rendering an average richness of 0.059 molars/kg (for reference, rich localities are those yielding ~ 1 molar/kg). For details on the nomenclature of small mammal teeth identification, the reader is referred to Murelaga ( 2000 ), Murelaga et al. ( 2004 ), Suarez-Hernando ( 2017 ) and references therein. Paleoenvironmental inferences from fossil small mammal teeth were done following the schemes of Daams et al. ( 1988 ) and van Dam and Weltje (1999), who assign different species or genera of fossil small mammals to relative changes in humidity and temperature according to their ecological preferences. Thus, reconstructions of past humidity and temperature conditions have been calculated based on the percentage of faunas associated to wet, neutral and dry environments (humidity) and to cold, neutral and warm conditions (temperature). Quantitative estimates of past temperatures (Legendre et al., 2005 ) and precipitation (van Dam et al., 2023 ) have not been considered because the richness of the fauna do not reach the minimum required (100 molars) but only in a few localities. 2.4. Magnetostratigraphy and environmental magnetism We have sampled three of the five new sections considered in this study in order to provide a chronology for most of the previously published (Ruiz-Sánchez et al., 2012b , 2012c , 2013 ) and the new fossil localities presented here (Figs. 1 c and 2 ). The first section, named Cabezo Carbonera, includes 96 m of the lowermost unit of the Tudela Formation (Fig. 3 ). The second section (117 m-thick) corresponds with the downward continuation of the Cuesta Agujeros section studied previously by Larrasoaña et al. ( 2006 ) and includes Unit 1 and the lowermost part of Unit 2 of the Tudela Formation (Fig. 4 ). The third section, labelled as Punta del Olmo, includes 105 m of the uppermost half of Unit 3 of the Tudela Formation (Fig. 5 ). A total of 31, 45 and 43 stratigraphic intervals, mainly mudstones but also some limestone beds, were sampled throughout the Cabezo Carbonera, Cuesta Agujeros and Punta del Olmo sections, resulting in an average resolution of 2.8 meters. Samples were collected using a standard, water-refrigerated electrical drill powered by a generator. Samples were oriented in the field using a magnetic compass mounted on a core-orienting fixture. The succession shows a very gentle dipping of ~ 14º to the north at the Cabezo Carbonera section, whereas is horizonal at the Cuesta Agujeros and Punta del Olmo sections. The two remaining stratigraphic sections, Loma Negra and Punta de Riantón, include the Unit 4 and the lowermost 30 m of Unit 5 of the Tudela Formation. They were not sampled for magnetostratigraphic analyses because their lithostratigraphic correlation to the neighbouring Sancho Abarca and Pico del Fraile sections, for which magnetostratigraphic data are available (Larrasoaña et al., 2006 ), is straightforward and unequivocal given the presence of extensive, distinctive limestone beds. Paleomagnetic measurements were conducted at Paleomagnetic Laboratory of the Geo3BCN Institute (CCiTUB-CSIC) in Barcelona, Spain. The Natural Remanent Magnetization (NRM) of the studied samples was measured using a 2G superconducting rock magnetometer, which has a noise level of ~ 10 –6 A/m. Previous studies on Miocene sediments from the central and eastern part of the Ebro basin have demonstrated that thermal demagnetization is the most effective method for isolating the different paleomagnetic components of the NRM. Therefore, thermal demagnetization of the samples was conducted using a MMTD–80 furnace at intervals of 100°C, 50°C, 40°C and 30°C to a maximum temperature of 680°C. Stable Characteristic Remanent Magnetization (ChRM) directions were calculated by means of Principal Component Analysis (Kirschvink 1980 ) after they were identified through visual inspection of orthogonal demagnetization plots using the VPD software (Ramón et al., 2017 ). Environmental magnetic properties were measured using standard paleomagnetic specimens of mudstones deposited under subaerial conditions (as signalled by yellow, brown and red colours). A total of 198 samples distributed throughout the composite section yield an average resolution of 3.5 m, which corresponds to about 26 kyr taking into consideration previous magnetostratigraphic results (Larrasoaña et al., 2006 ). The low-field magnetic susceptibility of these samples has been measured at two frequencies of 470 (c LF ) and 4700 (c HF ) Hz using a Bartington MS3 susceptibility meter equipped with a MS2B sensor at the CN IGME, CSIC. These measurements enable calculation of the frequency-dependent (c fd =c LF -c HF ) and the percent frequency-dependent (c fd% =(c fd /c LF ) x 100) susceptibilities as proxies for the absolute and relative contents of superparamagnetic particles (SP, < 25 nm), respectively (Dearing et al., 1996 ). SP particles form as a result of pedogenic activity, so determining their abundances in continental sequences provide insights into climatic conditions under the assumption that pedogenic activity is enhanced under warmer and wetter conditions, at least within a certain low-middle humidity range that excludes water logging of soils (Liu et al., 2012 ; Jiang et al., 2018 ). These data have been combined with a description of the sedimentary facies for the same samples taking advantage of the excellent observation conditions provided by the clean surfaces of the paleomagnetic specimens. 3. Results 3.1. Stratigraphy and sedimentology Results from the five studied sections, coupled with results from earlier studies, enable description of the sedimentologic and stratigraphic significance of the composite BRN section. The sedimentary facies are similar to those described by other authors in the central part of the Ebro basin (Arenas & Pardo, 1999 , Larena et al., 2020 ). The most abundant clastic facies correspond to red, brown and yellow mudstones that are typically massive (facies Fm) and constitute tabular packages of up to 15 m-thick. These mudstones occasionally show horizontal lamination (facies Fh) and often display a faint aggregated texture that signals root bioturbation (facies Fmbt). Sandstones appear either as thin (< 40 cm), tabular strata of massive appearance (facies Sm) with occasional horizontal lamination (facies Sh) or as thick (< 3 m) irregular beds with channel morphology and cross-bedding stratification (facies St). Carbonate facies are dominated by limestones that constitute tabular beds between 10 cm and 1.5 m in thickness and display a massive texture that includes variable amounts of sand grains dispersed within the matrix (facies Lm). The massive appearance is modified at some horizons by the accumulation of intraclasts and gastropods, ostracods and charophytes remains and fragments. The uppermost 10–20 cm of these massive limestones are typically affected by vertical root traces and, less frequently, by desiccation cracks (facies Lb). Far less abundant are laminated limestones that constitute thin (< 20 cm) tabular beds that include frequent accumulation of intraclasts and gastropods, ostracods and charophytes fragments (Ll). No stromatolitic limestones have been found in the BRN area. Evaporite facies mostly correspond to nodular gypsum beds, a few cm to 1 m in thickness, that show an alabastrine texture and are associated with grey marls (facies Gn). More infrequent are gypsum beds that display a mm to cm-scale horizontal lamination embedded within grey marls (facies Gl). Finally, mixed facies are mostly represented by grey marls and mudstones that constitute tabular packages of up to 3 m thick, include variable amounts of sand grains dispersed within the matrix, and usually have a massive appearance (facies Mm). In fewer occasions, these marls and mudstones display mm-scale laminations and contain some ostracods and charophytes fragments (facies Ml). A distinctive mottling indicating subaerial exposure is sometimes observed affecting these marls and mudstones (Mmbt). These facies are arranged in distinctive facies associations (FA) that can be broadly grouped in the three major types described by Arenas & Pardo ( 1999 ) and Larena et al. ( 2020 ). FA1 represents sedimentation in a distal alluvial plain, and is mainly composed by sandstone facies St that signal the incision and later filling of low-sinuosity, high-energy fluvial channels. Avulsion of these channels during flooding events cause rapid deposition of mudstone facies Fm and Fh as well as accumulation of sandstone facies Sm and Sh in unconfined, low-energy flows at lateral and terminal splays of the main fluvial channels. In this context, facies Fmtb indicate periods of plant colonization and soil formation occurred between flooding events. FA1 is, by far, the most common facies association in the BRN area, so that it dominates units 1 and 4 of the Tudela formation as well as most of the Lerín Gypsum Formation and units 2 and 3 of the Tudela Formation. FA2 represents sedimentation in a lacustrine setting with a positive hydrological balance, and begins with the implantation of a stable lake as indicated by deposition of mudstone facies Mm and Ml. Stabilization of the lake system is marked by deposition of limestone facies Lm, being its occasional deepening signaled by accumulation of limestone facies Ll. Under this scenario, accumulation of sand grains, intraclasts and bioclasts attest for the influence of episodic fluvial inputs that deliver detrital and biogenic material from the surrounding plains and shallower areas of the lake system, where mudstones facies Mm also accumulate. Continued carbonate production and detrital supply eventually turn the lake into a shallow palustrine area that can be eventually subjected to subaerial exposure (limestone facies Lb and mudstone facies Mmbt). FA2 dominates Unit 5 of the Tudela Formation as well as thinner (< 8 m) intervals of its Unit 3 and the lower and uppermost parts of Unit 2. Finally, FA3 represents sedimentation in a lacustrine setting with a negative hydrological balance that begins with accumulation of mudstone facies Mm and Ml under strong evaporation conditions that, eventually, lead to formation of gypsum facies Gn by evaporitic pumping and even direct precipitation from water (facies Gl). FA3 is restricted to parts of the Lerín Gypsum Formation and to a thin (< 10 m) interval within Unit 3 of the Tudela Formation (Fustiñana member) that can be considered as the most widespread extension of the Zaragoza Gypsum Formation to the northwest. The wide lateral extension (from 5 to > 20 km) of most limestone beds and mudstone packages of FA1, FA2 and FA3 within the BRN area enables a straightforward correlation between the different studied sections, and provides important clues on the paleogeographic significance of the studied sediments (Fig. 2 ). First, Unit 1 of the Tudela Formation (dominated by distal alluvial FA1 sediments) appears to thicken towards the north between the Cabezo Marijuán and Cabezo Carbonera sections. Second, thick (up to 8 m) intervals of freshwater lacustrine FA2 sediments within units 2 and 3 of the Tudela Formation (e.g., Cabezo Marijuán and Barranco de Tudela sections) grade laterally to the north (Cuesta Agujeros and other sections studied by Murelaga, 2000 ) to distal alluvial (FA1) sediments. Third, the Fustiñana Gypsum member (FA3 in the middle of Unit 3) also thins out to the north, where it is replaced by distal alluvial (FA1) sediments. And fourth, thick (> 8 m) intervals of FA2 sediments are restricted to Unit 5 of the Tudela Formation, with facies indicative of deepest lacustrine conditions (Ll) being found only in the southernmost studied sections (Sancho Abarca and Pico del Fraile). All these circumstances indicate, in agreement with previous sedimentologic observations by Larena et al. ( 2020 ), that the BRN area was located in the transition zone between the lakes that occupied the central part of the Ebro basin (whether freshwater or saline) and the distal alluvial plains that drained the Pyrenean orogen to the north. They also indicate a significant shift in the entity of lakes, which became strikingly well developed in the transition from units 4 and 5 (TSU-5 and TSU-6) of the Tudela Formation. 3.2. Small mammal biochronology The attribution of the fossil faunas of the BRN area to different local zones can be done after considering the updated faunal list of all the studied fossil localities (Fig. 6 , Supplementary Table 1). The basal faunas found at the BRN area (localities CH1 and CA1) include Eucricetodon sp., E. gerandianus and Armantomys cf. bijmai , which enables their attribution to the lower part of local Y of the Agenian, labelled as Y1 (López-Martínez, 1989 ; Daams, 1990). Moving upwards in the succession, fossil localities CA2 and CC1 appear characterized by the associated occurrence of E. aquitanicus and A. daamsi , which are indicative of the upper part of Agenian biozone Y (Y2, Álvarez-Sierra et al., 1987 ). Identification of local zone Z of the Ramblian in fossil localities CM1 and CJ1 is possible based on the absence of Rhodanomys-Ritteneria and the combined presence of Cricetidae ( Eucricetodon spp. and E. infralactorensis ) and Gliridae ( Pseudodryomys ibericus and Ps. simplicidens , Peridyromys murinus ) (Daams et al., 1987 ). Faunas at fossil locality CA3 are consistent with local zone Z, but this attribution is mainly based on its stratigraphic position within localities CM1 and CJ1. Fossil locality CA3b has yielded only remains of Prodryomys cf. brailloni , which allows its tentative attribution also to local zone Z. The next local zone of the Ramblian, zone A, is characterized by the absence of Cricetidae, the predominance of Ligerimys , and the presence of P. murinus and Ps. ibericus (Daams et al., 1987 ; Daams et al., 1999a ). Faunas clearly attributable to zone A are found at several localities between m 235 and 495 of the composite section (Fig. 2 ). Of especial relevance are localities CA4 and CSA3. The cooccurrence of A. cf. jasperi , Altomiramys sp., L. aff. magnus and Pseudotheridomys sp. in CA4 makes this fossil locality the oldest one securely assigned to zone A. CSA3 is the youngest locality associated to the same zone A on the basis of its fossil remains, which lack Cricetidae and include P. cf. ibericus , Gliridinus sp., Vasseuromys rambliensis , Eomydae indet. Together with localities PO38 and PO73, CSA3 confirms the extension of local zone A up to the uppermost part of Unit 3 of the Tudela Formation, as was initially suggested by the isolated locality PF1 (Larrasoaña et al., 2006 ; Ruiz-Sánchez et al., 2012d ). The next fossil locality moving upwards in the stratigraphic section is BVG. This locality hosts remains of Megacricetodon cf. primitivus , A. jasperi , Simplomys sp., Vasseuromys cf. cristinae and Ochotonidae indet., and is hence attributed to local zone C of the lower Aragonian. Locality PR110 might be tentatively assigned also to local zone C, but its remains are too scarce for a secure attribution. Attempts at identifying zone B faunas between sites CSA3 and BVG have failed due to the barren nature of FA1-dominated sediments that constitute the lower part of Unit 4 of the Tudela Formation. The last zone identified at the BRN composite section corresponds to local zone D of the middle Aragonian, which is characterized by a decrease in the diversity of Gliridae, an increase in diversity of Cricetidae, and the absence of Eomydae (Daams et al., 1999a ; van der Meulen et al., 2012 ). The first fossil locality unequivocally assigned to zone D is LN64, where Democricetodon gracilis , Microdyromys cf. koenigswaldi , V. cristinae , Spermophilinus sp., Ochotonidae indet. and Soricidae indet. have been recovered. PF2 also host a rich assemblage (Fig. 6 ) that includes also Microdyromys cf. remmerti and enables its secure assignation to subzone Dc (van der Meulen et al., 2012 ). The rest of fossil localities found within Unit 5 of the Tudela Formation have faunas that are compatible with local zone D (Fig. 6 ), but their attribution is not unequivocal given that their content is either scarce (PR118) or characterized by elements with a wide biostratigraphic distribution (PR113, PR125; SA5 and SA6). 3.2. Magnetostratigraphy Thermal demagnetization of the studied samples reveals the presence of two stable paleomagnetic components after removal of a viscous magnetization below 150ºC that is often subparallel to the drilling direction (Fig. 7 ). Between 150ºC and 300ºC-420ºC in most cases, and up to 500ºC in some red mudstones, a low temperature component is identified with northerly directions and positive, step directions that are interpreted as a present-day field overprint that lacks of any geological significance (Fig. 7 a-e). After demagnetization of this component, an additional component (regarded as the characteristic remanent magnetization, ChRM) is identified showing a behaviour that is linked to the lithology. Thus, this component is completely removed by 420ºC in most limestones and some grey mudstones and marls, and reaches maximum unblocking temperatures of < 580ºC in yellow, brown and some red mudstones. Unblocking temperatures of up to 660ºC are observed for most of the red mudstones studied. This paleomagnetic behaviour is essentially identical to that reported previously in the BRN area (Larrasoaña et al., 2006 ) and other sectors of the central part of the Ebro basin (Pérez-Rivarés et al., 2004 , 2018 ), and indicates that both magnetite and hematite are the main magnetic carriers of the ChRM. As opposed to the paleomagnetic behaviour, the directional properties of the ChRM show no link with lithology. Thus, the ChRM shows either northerly directions with positive inclinations of around 55º or broadly antipodal southerly directions with negative inclinations of around − 40º regardless of lithology. Following previous authors (Larrasoaña et al., 2006 ; Pérez-Rivarés et al., 2004 , 2018 ), calculated ChRM directions have been assigned to three categories according to their quality. Type 1 directions show rectilinear trends that yield low (< 5º) mean angular deviations (MAD) (Fig. 7 a, b), whereas type 2 directions display less-developed linear trends (MAD of 5–15º) or uncomplete demagnetizations due to the growth of new magnetic minerals upon thermal treatment (Fig. 7 c, d). Type 3 directions have highly scattered directions with large MAD (> 15º) (Fig. 7 e). Only types 1 and 2 ChRM directions provide straightforward and highly reliable polarity determination and are, therefore, consider hereafter for constructing the sequence of polarity zones identified on the studied sections. The Cabezo Carbonera section is divided into a lower interval of normal polarity (labelled N2 following Larrasoaña et al., 2006 ) and an upper interval of reverse polarity (R2) (Figs. 2 and 3 ). The Cuesta Agujeros section is characterized by a large reversal that spans the middle part of the succession (R2) and two normal polarity intervals in the lowermost (N2) and uppermost (N3) parts of the section (Figs. 2 and 4 ). Finally, the Punta del Olmo section includes a reverse zone that spans the lower and middle part of the section (R4) and two overlying normal (N5) and reverse (R5) intervals located in its uppermost parts (Figs. 2 and 5 ). Since only the Cabezo Carboneras section shows a subtle bedding dip of 14ºC, no significant fold test can be performed. Nevertheless, comparison of ChRM directions in this section before and after tilt correction suggest that the ChRM represents a pre-folding direction. Although ChRM directions are seemingly antipodal, they do not pass the reversal test due to the partial overlap that exists between the ChRM and the present-day field overprint (Fig. 7 f), a circumstance often observed in continental sediments of the Ebro basin that tends to preferentially flatten reverse directions (Larrasoaña et al., 2006 ; Pérez-Rivarés et al., 2004 , 2018 ). Nevertheless, the means of both normal and reverse directions overlap with that of the Miocene reference direction, which along with the consistent pattern of polarity intervals indicates that the ChRM is a primary component acquired during or shortly after deposition of the studied rocks. Correlation of the studied successions to the GPTS 2020 is straightforward in light of the previous magnetostratigraphic dating performed by Larrasoaña et al. ( 2006 ). The resulting composite section spans from chron C6An.1n to chron C5Br, covering from ca. 20.5 to 15.2 Ma (Fig. 1 , Supplementary Table 1). 3.3. Palaeoenvironmental reconstruction based on small mammals Before addressing the paleoecological results derived from small mammal remains, it is important to recall that all the fossil localities at the BRN are found within grey mudstones and marls (facies Mm, Ml, Mmbt) indicative of shallow (< 2 m) palustrine environments receiving low energy fluvial currents (Murelaga, 2020; Murelaga et al., 2004 ; Larrasoaña et al., 2006 ; Ruiz-Sánchez et al., 2012a , 2012b , 2012c , 2012d , 2013 ). The only exception is the CJ1 locality, where the grey mudstones include a microconglomeratic component indicating a fluvial current with higher energy. In any case, all fossil localities include both demic and ademic elements that ensure a good representativity of the faunas inhabiting the region at the time of sediment accumulation. With this caveat in mind, and considering the ecological preferences of the different small mammals and their relative importance in the studied localities, the variations of relative humidity and temperature conditions can be established for the studied sedimentary sequence (Supplementary Table 1). The fraction of wet-adapted fauna is negligible at the lower part of the sequence and increases rapidly to 20–60% around 230–350 m (Fig. 8 b). After a transient return to low values at around 440 m, the proportion of wet-adapted fauna increase again at 470 m to values of > 60% that remain high throughout the rest of the succession. Although the record is conditioned by the irregular distribution of fossil localities, a clear trend towards wetter conditions can be established for the succession, which shows sustained wet conditions between 470 m and the top of the sequence. The fraction of faunas indicative of warm conditions shows a similar trend through the section (Fig. 8 c), so that a progressive shift to warmer conditions is found in the lower 450 m of the succession (punctuated by a colder period at 430 m) before showing sustained values of 60–100% throughout the rest of the sequence. 3.4. Palaeoenvironmental reconstruction based on magnetic properties Rock magnetic data have been produced only for distal alluvial mudstone facies that denote accumulation of fine-grained (e.g., clay and silt) detrital material under subaerial conditions (Fm, Fl and Fmbt) (Supplementary Table 2). Of these facies, those displaying evidence for root bioturbation (Fmbt) attest for pedogenic process within paleo-alfisols similar to those reported by Hamer et al. ( 2007 ) in other lower Miocene sediments from the central Ebro basin. Other pedotypes identified at the BRN area, such as inceptisols (developed on palustrine mudstones, facies Mmbt) and entisols (developed on lacustrine limestones, facies Lb) (see Hamer et al., 2007 ), have not been considered: 1) to avoid interference of subaerial pedogenic processes with earlier diagenetic process occurred under aqueous conditions; and 2) to account for a different parent material that involves a larger carbonate fraction. cfd% values for the studied mudstones range between 0 and 8 (Fig. 9 a-c). Overall, about 61% of the mudstones show cfd%> 2 regardless of colour and sedimentary facies, which indicates that they include detectable amounts of SP particles (Dearing et al., 1996 ). It is worth noting that the fraction of samples with cfd%> 2 is of about 75, 67 and 40% for red, brown and yellow mudstone samples, respectively, which indicates that the contribution of SP particles to their magnetic assemblage is relatively more important in red mudstones and less relevant in the case of yellow mudstones. In any case, the fact that both massive (Fm) and bioturbated (Fmbt) mudstones display a significant contribution of SP particles regardless of colour indicate that pedogenic processes leading to formation SP material begun before development of macroscopic evidence for paleosoil formation (e.g., root bioturbation). A comparison between cfd% and c LF values indicates an overall weak correlation between both variables (correlation coefficients between 0.35 and 0.78) (Fig. 9 , upper panel). The correlations increase notably (coefficients between 0.47 to 0.93) when c LF and cfd are compared (Fig. 9 , lower panel). This suggests that cfd values are more influenced by the initial susceptibility than cfd% values, and indicates that this later parameter provides a better proxy to signal the presence of SP particles in the studied sediments. Depth variations in cfd% indicate that the lowest contributions from SP particles are found between 50–330 and 520–690 m of the composite section, where most cfd% values range between 0 and 2 (Fig. 8 e). The lowermost 50 m and the interval between 330–520 m of the composite section display cfd% values that typically range between 3 and 7 and point to a higher contribution from SP pedogenic particles. 4. Discussion 4.1. Calibration of lower to middle Miocene Iberian mammal faunas A combination of the new results presented and summarized for the BRN area in this study, along with those obtained in other sector of the Ebro basin (Odin et al., 1997 ; Agustí et al., 2011 ; Pérez-Rivarés et al., 2014, 2018 ) and other Iberian (e.g., Calatayud-Daroca) basins (Daams et al., 1999a , 1999b ; van Dam et al., 2006 ; van der Meulen et al., 2012 ; García-Paredes et al., 2016 ), enables the proposition of a new magnetostratigraphic calibration for the continental record of the Iberian Peninsula spanning from the Agenian to the middle Aragonian (Fig. 10 ). Localities CH1 and CA1 from the BRN represent the first magnetostratigraphically-dated Y1 faunas from the Iberian Peninsula. Their chronology, ranging from the upper part of chron C6An to the lower part of chron C6r, is consistent with that of Y1 localities MO and BU4B from the Calatayud-Daroca basin, whose age was established, in the absence of magnetostratigraphic data, by a temporal interpolation of evolutionary features of the genus Eucricetodon between the late Oligocene and the lower Miocene (van Dam et al., 2006 ). Only 5 meters above CA1, assigned to zone Y1, locality CA2 assigned to Y2 is found. Considering mean accumulation rates, and placing the Y1/Y2 boundary halfway between both localities, an age of 19.82 Ma for this boundary can be established. Hence, a new age ranging between ca. 21.6 Ma (van Dam et al., 2006 ) and 19.82 Ma (this paper) can be proposed for Y1. With regards to zone Y2, represented at the BRN area by localities CA2 and CC1, it spans a very narrow stratigraphic interval dated to the middle part of chron C6r at around 19.8 Ma. This is very close to the age of the Tardienta locality in the Ebro basin, which is considered to mark the Y2/Z boundary and is also placed in the middle part of C6r (Pérez-Rivarés et al., 2018 ) at an age of 19.7 ± 0.3 Ma (van Dam et al. 2006 ), as indicated by the revised 40 Ar/ 39 Ar age of the volcanic ash layer with which the locality is associated (Odin et al., 1997 ). This age is also consistent with that established for Y2 localities CR, Al3B and Ata from the Daroca-Teruel basin on the basis of the evolutionary changes described for Eucricetodon (van Dam et al., 2006 ). Given that these later localities are not magnetostratigraphically dated, we consider that the chronology of localities CA2, CC1 and Tardienta yield a robust age for zone Y2, which is constrained to the interval between 19.82 Ma and 19.7 Ma. This very short duration, of hardly 0.2 Myr, calls into question the biostratigraphic usefulness of subzone Y2, and leads us to discourage its use in favor of a broader Y zone that spans, according to the results presented here, from 21.6 to 19.7 Ma. Since the boundary between zones Y and Z marks the end of the Agenian (Daams et al., 1987 ), an age of 19.7 Ma can be assigned to its boundary with the Ramblian. Zone Z is represented by three localities at the BRN area, CM1, CA3 and CJ1. They also span a short stratigraphic interval within the lower half of chron C6n. Given that no other Z locality has been dated by magnetostratigraphy in the Iberian Peninsula, the chronology of this zone has to rely on our results from the BRN area. The boundary between zones Z and A can be placed halfway between CJ1 and CA4, which represent the youngest and oldest localities belonging to zones Z and A, respectively. Considering mean accumulation rates, this results in an age of 19.2 Ma and in an overall duration for zone Z of about 0.5 Myr (with its base dated at 19.7 Ma). Zone A is represented by a large number (14) of localities at the BRN area, whose age spans from the middle part of chron C6n to chron C5Dn. This long duration is confirmed by magnetostratigraphically calibrated A faunas from the Alcubierre area in the Ebro basin, where the uppermost boundary of zone A was found between localities LN142 and LN145 at an age of 17.15 Ma (Agustí et al., 2011 ; Pérez-Rivarés et al., 2018 ). These robust and mutually consistent results from the Ebro basin contrast strikingly with the ages assigned for the fossil record of the Calatayud-Daroca basin, where A fauna begin at ca. 18 Ma (van Dam et al., 2006 ) in what appeared to be a much shorter Z zone and a significantly longer A zone. Keeping in mind that the chronology of these zones was based on the interpolation of Eucricetodon evolutionary changes, we interpret that a yet unrecognized sedimentary event must have affected the record of the Calatayud-Daroca basin. Since the chronology assigned to Y fossil localities in that basin (established following the same method) is consistent with the results from the Ebro basin, we interpret that the most likely explanation for the discrepancy observed on the timing and duration of zones Z and A is an abrupt shift from very high accumulation rates during de former to a condensed sedimentation during the later. In any case, a duration from 19.2 to 17.15 Ma can be established for zone A in what represents its first comprehensive magnetostratigraphic calibration in the Iberian Peninsula. Keeping in mind this new calibration, is seems that the inferences made on small fossil turnovers in association to cyclic climatic changes (van Dam et al., 2006 ) need to be reconsidered and reassessed. An additional implication of our results is that the age of the Ramblian/Aragonian boundary, which is marked by the limit between zones A and B (Daams et al., 1987 , 1999a ), can be dated to 17.15 Ma. In the absence of faunas assigned to zone B in the Ebro basin, the duration of this zone and the age of its upper boundary with zone C has to rely on the distribution of B faunas from the Calatayud-Daroca basin, for which magnetostratigraphic data are available (Dams et al., 1999b; van Dam et al., 2006 ), and of C faunas from this basin and from the BRN area (e.g. fossil locality VBG). Overall, they provide a consistent age of 16.64 Ma for the B/C boundary and indicate a duration of ca. 0.5 Myr for zone B. Finally, a robust chronology can be also provided for local zone D, since magnetostratigraphic data available from Iberian basins yield a mutually consistent record that spans the entire chron C5Br. The age of the C/D boundary (and hence of the lower/upper Aragonian), dated to 15.93 Ma at the Calatayud-Daroca basin (van Dam et al., 2006 ; van der Meulen et al. 2012 ; García-Paredes et al. 2016 ), is fully supported by localities LN64, PR121 and PF2, which have a just younger age. A last issue deserving attention is the fact that most of the taxa that mark different subzones within zone D (van der Meulen et al., 2012 ) have not been found in the BRN or Alcubierre areas. This illustrates the point that using subzones does not necessarily provide a better tool for biostratigraphic correlations, specially if different basins are under consideration. This, coupled with the shorter duration of subzones (as we showed to be the case for Y1), forces us to discourage their use in the Iberian Peninsula. Our biostratigraphic results also provide new insights into the chronology of Early Miocene mammal Neogene (MN) units, especially for those whose boundaries are subjected to large (e.g., 0.8–2.1 Ma) uncertainties (e.g., MN2/MN3 and MN3/MN4, see Agustí et al., 2001 ). Concerning the MN2/MN3 boundary, which corresponds to the boundary between local zones Y and Z and marks the transit between the Agenian and the Ramblian (Daams et al., 1987 ), an age of 19.7 Ma can be established from the magnetostratigraphic calibration of the Tardienta locality and the chronology of MN2 (CH1 to CC1) and MN3 (CM1 to CSA3) localities from the BRN area (Fig. 10 ). With respect to the MN3/MN4 boundary, its age is placed at 17.15 Ma keeping in mind the magnetostratigraphic calibration of MN3 faunal assemblages from the BRN (CM1 to CSA3) and Alcubierre (LN142) areas and of MN4 localities from the Ebro (LN145) and Daroca-Teruel (SR4A to VL2A) basins (Fig. 10 ). The chronology of MN5 localities at the BRN area (LN64 to PF2) gives also full support to the 15.93 Ma age established for the MN4/MN5 boundary in the Daroca-Teruel basin (Fig. 10 ). 4.2. Climate change in SW Europe during the MCO Small mammal fauna from the BRN area point to a gradual increase in temperature and humidity between the bottom (ca. 20 Ma) and top (15.3 Ma) of the record, which appears to be punctuated by relatively warmer and wetter conditions at three intervals between 19.2–18.6, 17.6–17.1 and 15.7–15.3 Ma (Fig. 11 d, e). This trend seems to contrast with the sedimentary evolution of the basin, which witnessed the alternation of alluvial and lacustrine (both fresh-water and evaporitic) sedimentation before experimenting a basin-wide expansion of lacustrine conditions at ca. 16.1 Ma (Arenas & Pardo, 1999 ; Pérez-Rivarés et el., 2018; Larena et al., 2020 ) (Fig. 11 a). The most likely interpretation is that the hydrological balance of the basin responded in a non-linear fashion to humidity conditions, so that an overall lacustrine expansion occurred after certain humidity threshold was achieved. This seems to be supported by the important, yet gradual increase in the fraction of the charophyte Harrisichara tuberculata and the ostracod Pseudocandona parallela observed in the uppermost part of the BRM record, which indicates a shift from shallow and warm stagnant ponds to stable lake conditions characterized by deeper, cooler and fresh to mesohaline waters at around 16.1 Ma (González-Pardos, 2012 ; Martínez-García et al., 2014 ; Larena et al., 2020 ) (Fig. 11 c). In this regard, it is important to note that the temperature and humidity curves from the BRN bear a stronger resemblance to the evolution of sea surface temperatures reported for the Atlantic Ocean off the Iberian Peninsula (Deep Sea Drilling Program Site 608, Super et al., 2018 ) (Fig. 11 g) than to global temperatures as reported from the global benthic compilation of d 18 O records (Westerhold et al., 2020 ) (Fig. 11 h). This is important because the tropical-subtropical North Atlantic corridor that extends from the Gulf of Mexico till nearly the position of Site 608 represents the main source of moisture for rainfall in the Iberian Peninsula (Gimeno et al., 2010 ) (Fig. 1 a), and its surface temperature conditions in the past emerge as a plausible modulator for rainfall during the MCO, when subtropical climates expanded northward well into southern Europe (Hamon et al., 2012 ; Henrot et al., 2017 ). Thus, warmer (and wetter) than background conditions reported at the BRN between 19.2–18.6, 17.6–17.1 and 15.7–15.3 Ma are also observed, albeit with some offset, in the Site 608 sea surface temperature record regardless of variations in global temperatures (Fig. 11 d, e, g). In any case, it seems that the rather gradual increase in temperature and humidity conditions in SW Europe indicated by our data between 20 and 17 Ma seems to rule out volcanism as the main driver of the MCO, since it experienced a rather abrupt activity peak around the optimum between 17.2 and 15.3 Ma (Goto et al., 2023 ). Instead, the apparent coupling observed between climate shifts in SW Europe and oceanic (e.g., Atlantic) and global records preceding the MCO point to gradual restriction of oceanic circulation, likely in the circum-Antarctic domain (Pagani et al., 1999 ), as the main driver of the MCO. The magnetic properties (e.g., c fd %) of the studied mudstones suggest an evolution of climatic conditions that differs markedly from those signalled by the fossil record of the BRN. Thus, instead of the warming and wetting trend broadly suggested by fossil fauna through the onset and peak of the MCO, enhanced pedogenic processes presumably signalling warmer and wetter conditions are found between 20.5–20.3 and 18.5–17.2 Ma, and mark a decline from 17.2 Ma onwards precisely during the peak of the MCO (Fig. 11 f). This suggest that pedogenic formation of SP particles was not linked to warmer global temperatures or increased sea surface temperatures off the Iberian Peninsula. Noticeably, the two periods of enhanced SP formation are centred around the two intervals of evaporitic sedimentation marked by the top of the Lerín Formation and the Fustiñana Gypsum Member (Fig. 11 ). Moreover, these two periods appear to coincide with maxima in the long-term, 2.4 Myr eccentricity cycle of the Earth´s orbit (Fig. 11 i), a pacemaker widely known to amplify seasonal climate variability due to the modulation it exerts on the amplitude of precession and, therefore, on boreal summer insolation (Laskar et al., 2004 ). From this perspective, it seems that pedogenic formation of SP particles appears to be linked more to the alternance of wet and dry conditions, enabling the initial formation of lakes and its later evolution to evaporitic conditions, than to wet conditions alone. This interpretation, although tantalizing, faces two main problems. The first one is that the 2.4 kyr eccentricity maximum centred around 15.5 Ma is not associated with high SP contents, but rather by low concentrations of these type of particles. In this regard, it might be recalled that the correlation between moisture conditions and pedogenic formation of SP particles breaks down for exceedingly wet conditions, that lead to the transformation of SP particles into hematite and, under even wetter conditions, goethite (Liu et al., 2012 ; Jiang et al., 2018 ). Under this perspective, it might be argued that the low SP content detected around the eccentricity maximum at 15.5 Ma responds to specially wet conditions during the peak of the MCO and the completion of pedogenic processes well beyond the transformation of SP particles. Peak wet conditions in the BRN as imaged by sedimentological and paleontological, coupled with the conspicuous yellow colour of mudstones after 16 Ma (Fig. 11 b), that suggests the dominance of goethite (Jiang et al., 2018 ), make this interpretation plausible and testable by future studies aimed at quantifying the amount of hematite and goethite in the studied mudstones. The second problem is that the correlation of evaporitic sedimentation and 2.4 Myr eccentricity maxima inferred for the BRN area is at odds with the link established between these maxima and periods of fresh-water lake expansion deduced in the central part of the Ebro basin by other authors (e.g., Valero et al., 2014 ). In this regard, it can be argued that other processes aside from climate conditions, such as tectonic pulses and the amount of sediment supply, are also known to affect the architecture of endorheic foreland basin sequences (Valero et al., 2014 ). The western part of the Ebro basin is considerably narrower than its central part (Fig. 1 b), which means that all the material eroded from the neighbouring margins of the Pyrenees and the Iberian Range accumulated over a smaller area. Moreover, younger tectonic deformation in the western Pyrenees led to enhanced supply of detrital sediment to the western part of the Ebro basin during the Miocene (Curry et al., 2021 ). Finally, the western Ebro basin is affected by a set of folds detached along Oligocene and Miocene evaporitic formations (Larrasoaña et al., 2003 ) that uplifted the central fringe of the basin during the lower Miocene (Inglés et al., 1997 ). All these factors have resulted in significantly higher Miocene accumulation rates in the western part of the Ebro basin (Larrasoaña et al., 2006 ), which could easily explain why shallow (filled evaporitic) and deep (underfilled fresh-water) lakes coexisted in the western and central parts of the basin, respectively. If so, it follows that conditions wet enough to disrupt this situation occurred only during the 2.4 Myr eccentricity maximum coeval with the peak of the MCO, when fresh-water lacustrine sedimentation spread over the entire basin regardless of previous paleoenvironmental cotexts. 5. Conclusions The updated fossil small mammal record of the BRN area presented in this study, along with previous and new accompanying magnetostratigraphic results, enable the first comprehensive dating of lower Miocene Iberian fossil localities found in direct stratigraphic continuity with middle Miocene faunas. These data, coupled with results from other magnetostratigraphically dated fossil localities from the Ebro and Daroca-Teruel basins, yield the first robust calibration of Iberian local biozone boundaries Y/Z (19.7 Ma) Z/A (19.2 Ma), A/B (17.15 Ma), B/C (16.64 Ma) and C/D (15.93 Ma). These results also fill an important gap in lower Miocene European biochronology, since they robustly place the MN2/3 and MN3/4 boundaries at around 19.7 and 17.15 Ma, respectively. They further indicate that some local subzones are either too short (e.g., Y2) or difficult to identify unless very rich localities are unearthed (subzones within zone D), and lead us to discourage their use in view of their limited utility for biostratigraphic purposes. In addition to this, we have used the updated small mammal content of the BRN area to provide relative records of temperature and humidity variations across the onset and peak of the MCO, that have been combined with paleoclimatic inferences based on the sedimentary evolution of the Ebro basin and with a record of pedogenic formation of magnetic particles. The small mammal faunas from the BRN area point to a gradual increase in temperature and humidity conditions in SW Europe between ca. 20 and 15.2 Ma, which appears to be punctuated by relatively warmer and wetter conditions at three intervals between 19.2–18.6, 17.6–17.1 and 15.7–15.3 Ma. This evolution seems to bear more resemblance with the record of eastern north Atlantic surface temperatures than to global temperatures, which is consistent with the main role of the eastern north Atlantic as the source for moisture in SW Europe. In any case, the apparent coupling observed between climate shifts in SW Europe and Atlantic and global records between 20 and 17 Ma points to gradual changes in oceanic (likely peri-Antarctic) circulation as the main driver of the MCO, as opposed to other mechanism (e.g., volcanism) whose activity underwent a more abrupt intensification restricted to the peak of the MCO after 17 Ma. In comparison with the gradual shift to wetter conditions envisaged by the small mammal faunas across the MCO, the sedimentary evolution of the BRN in particular, and of the Ebro basin in general, appears to mark a more abrupt response. The most likely interpretation is that the hydrological balance of the basin responded in a non-linear fashion to humidity conditions, so that an overall lacustrine expansion during the peak of the MCO occurred after certain humidity threshold was achieved. As opposed to these shifts, paleoenvironmental variations recorded by the relative concentration of SP particles point to enhanced pedogenic conditions between 20.5–20.3 and 18.5–17.2 Ma, coinciding with maxima in the long-term cycle of the eccentricity of the Earth´s orbit but with no clear link with development of the MCO. This suggest that pedogenic formation of SP particles was not linked to regional or global warmer and wetter conditions, but with periods of enhanced seasonal climate variability. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4447195","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":304980910,"identity":"234cdd4d-fad0-4f24-9e69-f2730d462cb1","order_by":0,"name":"Juan Cruz Larrasoaña Gorosquieta","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzElEQVRIiWNgGAWjYBACxgYGxgMgBj8zCVoYwFokm0mxCazF4ACxypnbDzAc+LnnnrzxcfarGz4w2NgTdlhPAsPBnmfFhtsO85TdnMGQlthAUMsMoMN4DiQwArWk3eZhOJxA2BagloN/DiTYb24GavnD8J8IhwG1HAbakriBmf3YbaCNjIQd1pPYcFjmQELyjMM8bDd7DJIJ+8Ww/fDBh28OJNj29x9/duNHhR1hhxk2wF3CYwCMHYIaGBjkEUz2B0SoHwWjYBSMgpEIAA6WQSikVHr0AAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0003-4568-631X","institution":"IGME: Instituto Geologico y Minero de Espana","correspondingAuthor":true,"prefix":"","firstName":"Juan","middleName":"Cruz Larrasoaña","lastName":"Gorosquieta","suffix":""},{"id":304980911,"identity":"ba54b631-a39e-4dba-a36f-8e6c78f244d5","order_by":1,"name":"Oier Suarez-Hernando","email":"","orcid":"","institution":"Universidad del País Vasco - Campus Bizkaia: Universidad del Pais Vasco - Campus Bizkaia","correspondingAuthor":false,"prefix":"","firstName":"Oier","middleName":"","lastName":"Suarez-Hernando","suffix":""},{"id":304980912,"identity":"f4c1e2f4-58cd-41c1-a149-223a6196641f","order_by":2,"name":"Elisabet Beamud","email":"","orcid":"","institution":"Universitat de Barcelona","correspondingAuthor":false,"prefix":"","firstName":"Elisabet","middleName":"","lastName":"Beamud","suffix":""},{"id":304980913,"identity":"9148a2ba-f276-4554-aa1c-d369f6d83890","order_by":3,"name":"Miguel Garcés","email":"","orcid":"","institution":"Universitat de Barcelona","correspondingAuthor":false,"prefix":"","firstName":"Miguel","middleName":"","lastName":"Garcés","suffix":""},{"id":304980914,"identity":"c392ccac-a1ba-4d5f-96e4-1049fa1d43e2","order_by":4,"name":"José Ignacio Pérez-Landazábal","email":"","orcid":"","institution":"UPNA: Universidad Publica de Navarra","correspondingAuthor":false,"prefix":"","firstName":"José","middleName":"Ignacio","lastName":"Pérez-Landazábal","suffix":""},{"id":304980915,"identity":"91505050-cc0b-4901-a48a-94cc698d74de","order_by":5,"name":"Cristina Gómez-Polo","email":"","orcid":"","institution":"UPNA: Universidad Publica de Navarra","correspondingAuthor":false,"prefix":"","firstName":"Cristina","middleName":"","lastName":"Gómez-Polo","suffix":""},{"id":304980916,"identity":"fa62545a-40a6-4e37-9732-bc5f9477ad89","order_by":6,"name":"Francisco Javier Ruiz-Sánchez","email":"","orcid":"","institution":"Universitat de València: Universitat de Valencia","correspondingAuthor":false,"prefix":"","firstName":"Francisco","middleName":"Javier","lastName":"Ruiz-Sánchez","suffix":""},{"id":304980917,"identity":"c6595bb5-a3fe-4fb3-bef1-1275fdfd64ba","order_by":7,"name":"Maria Pilar Mata","email":"","orcid":"","institution":"IGME: Instituto Geologico y Minero de Espana","correspondingAuthor":false,"prefix":"","firstName":"Maria","middleName":"Pilar","lastName":"Mata","suffix":""},{"id":304980918,"identity":"2a3586ab-15f5-4d10-80ce-81abea82b071","order_by":8,"name":"Xabier Murelaga","email":"","orcid":"","institution":"Universidad del Pais Vasco - Campus Bizkaia","correspondingAuthor":false,"prefix":"","firstName":"Xabier","middleName":"","lastName":"Murelaga","suffix":""}],"badges":[],"createdAt":"2024-05-20 07:01:15","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4447195/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4447195/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s41513-024-00265-7","type":"published","date":"2024-10-22T15:57:41+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":57640384,"identity":"3d988d1e-92c5-4c75-91ef-d73cdd91a65b","added_by":"auto","created_at":"2024-06-03 17:10:29","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":408269,"visible":true,"origin":"","legend":"\u003cp\u003ea) Geological map of the Bardenas Reales de Navarra, with location of the new sections and fossil sites studied in this work and in previous publications, shown along with its context in a) Iberian Peninsula and b) the Ebro Basin. TSNA in a) stands for the tropical-subtropical North Atlantic, the main source for moisture in the Iberian Peninsula. Red (grey) stars denote the location of sections for which new magnetostratigraphic (only stratigraphic) data have been produced. White starts indicate section studied in previous papers (Murelaga, 2000; Larrasoaña et al., 2006). CC: Cabezo Carbonera; CA: Cuesta Agujeros; PO: Punta del Olmo; LN: Loma Negra; PR: Punta de Riantón; CM: Cabezo Marijuán; RB: Rincón del Bu; NA: Nasa; BT: Barranco Tudela; CV: Cabejo Vaquero; CN: Cabezo Miñón; BF: Barranco del Fraile; SI: Sisares; MB: Muga Blanca; PF: Pico del Fraile; SA: Sancho Abarca.\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/c5f4ff2383f750bcf9f2d5ed.png"},{"id":57640831,"identity":"6d0d3ed7-744a-4859-9e8c-18d99d8fbe67","added_by":"auto","created_at":"2024-06-03 17:18:29","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":426656,"visible":true,"origin":"","legend":"\u003cp\u003eLithostratigraphy of the BRN composite section, with location of fossil sites, lithostratigraphic correlation between different sections, and correlation of the polarity sequence to the GPTS 2020. Green bars indicate intervals used to build up the composite section. Red labels indicate the new sections presented in this study.\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/146aaeaf1434f7c2284db0a4.png"},{"id":57640393,"identity":"403ec920-6a31-46b3-b831-0afa0808ebc1","added_by":"auto","created_at":"2024-06-03 17:10:29","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":135579,"visible":true,"origin":"","legend":"\u003cp\u003eMagnetostratigraphy of the Cabezo Carbonera section.\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/80839b7c4ed04eecd57195c6.png"},{"id":57640386,"identity":"f8b30c15-086c-4ab8-b73e-f09ec9a85317","added_by":"auto","created_at":"2024-06-03 17:10:29","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":139833,"visible":true,"origin":"","legend":"\u003cp\u003eMagnetostratigraphy of the Cuesta Agujeros section. For the stratigraphic log and paleomagnetic data, see the key in Figure 3.\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/d5dcd29f8149542ee938aa6f.png"},{"id":57640830,"identity":"6f8eacd7-d3e3-4aa4-8ead-f202e3176aff","added_by":"auto","created_at":"2024-06-03 17:18:29","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":119910,"visible":true,"origin":"","legend":"\u003cp\u003eMagnetostratigraphy of the Punta del Olmo section. For the stratigraphic log and paleomagnetic data, see the key in Figure 3.\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/6dc0cf443beb34c3fbf5915c.png"},{"id":57640394,"identity":"ef080d4f-2d20-48ba-b58a-74d237674335","added_by":"auto","created_at":"2024-06-03 17:10:29","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":335070,"visible":true,"origin":"","legend":"\u003cp\u003eSmall mammal assemblages of the fossil localities described in this work and in previous studies of the Tudela Formation, with indication of their biostratigraphic assignation to the local biozones. Black squares denote the presence of a taxon, whereas grey (light grey) squares denote its widely-accepted (less commonly reported) biostratigraphic distribution. aff.: affinis (affinity, but no identical to). cf. confer (provisionally assigned to). * mark data from Murelaga (2000), Murelaga et al. (2004) and Larrasoaña et al. (2006). [] mark data from Ruiz-Sánchez et al. (2012b, c, d).\u003c/p\u003e","description":"","filename":"Figure6.png","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/8b368883a6157897f5704183.png"},{"id":57640389,"identity":"1c2a2c4d-9b8d-493d-abd6-312ecfe857ac","added_by":"auto","created_at":"2024-06-03 17:10:29","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":86173,"visible":true,"origin":"","legend":"\u003cp\u003eOrthogonal demagnetization diagrams of samples representative for the different types of paleomagnetic behaviour (a-e). f) Reversal test performed with the ChRM directions used for constructing the magnetostratigraphy (quality types 1 and 2) of the three studied sections.\u003c/p\u003e","description":"","filename":"Figure7.png","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/f2b538add5b6d3b277293e45.png"},{"id":57640392,"identity":"4bd8f811-4b57-4500-8ce7-89ca210e83ff","added_by":"auto","created_at":"2024-06-03 17:10:29","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":182816,"visible":true,"origin":"","legend":"\u003cp\u003eStratigraphic variations of paleoclimatic proxies derived from the BRN area. a) Composite BRN section; b) Relative humidity; c) Relative temperature; d) Magnetic susceptibility at high (c\u003csub\u003eHF\u003c/sub\u003e) and low (c\u003csub\u003eLF\u003c/sub\u003e) frequencies; e) c\u003csub\u003efd\u003c/sub\u003e% as a proxy for pedogenesis.\u003c/p\u003e","description":"","filename":"Figure8.png","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/97be520c62b6e417582e6858.png"},{"id":57640832,"identity":"345c039e-4176-4aa5-9d9d-c224529fa9cf","added_by":"auto","created_at":"2024-06-03 17:18:29","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":109793,"visible":true,"origin":"","legend":"\u003cp\u003eBi-plots between the low-frequency magnetic susceptibility (c\u003csub\u003eLF\u003c/sub\u003e) and the frequency dependence of magnetic susceptibility, expressed both as bulk value (c\u003csub\u003efd\u003c/sub\u003e) and as percentage (c\u003csub\u003efd\u003c/sub\u003e%) as a function of sedimentary textures for: a) red, b) brown, and c) yellow mudstones.\u003c/p\u003e","description":"","filename":"Figure9.png","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/0ea4263685a231ebd143d39b.png"},{"id":57640390,"identity":"6fcfe152-f355-4cf0-8fdb-cf1ade57bcdc","added_by":"auto","created_at":"2024-06-03 17:10:29","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":129720,"visible":true,"origin":"","legend":"\u003cp\u003eA new chronology for the Early and Middle Miocene faunas from the Iberian Peninsula, based on our new data from the BRN area and other sections from the Ebro (Agustí et al., 2011; Pérez-Rivarés et al., 2004, 2018) and the Daroca-Teruel (Daams et al., 1999a, 1999b, van Dam et al., 2006) basins.\u003c/p\u003e","description":"","filename":"Figure10.png","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/1179b79789a56870f65d61ad.png"},{"id":57640397,"identity":"90f92a00-8580-4e4e-8d92-7e2b8a845745","added_by":"auto","created_at":"2024-06-03 17:10:30","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":357838,"visible":true,"origin":"","legend":"\u003cp\u003eChronology of paleoclimatic proxies retrieved from the BRN area and their comparison with regional and global records of climate variability. a) Sedimentary evolution of the Ebro Basin; b) colour of the studied BRN mudstones; c) relative lake level variations inferred from ostracod and charophyte remains (González-Pardos, 2012; Martínez-García et al., 2014); d) relative temperature curve based on small mammal remains; e) relative humidity curve based on small mammal remains; f) degree of pedogenesis based on magnetic data; g) sea surface temperature record in the North Atlntic Ocean off the Iberian Peninsula (Super et al., 2018); h) global temperatures deduced from d\u003csup\u003e18\u003c/sup\u003eO benthic foraminiferal records (Westerhold et al., 2020); i) 2.4 Myr and 0.4 Myr band-pass filters of the Earth´s eccentricity (Laskar et al., 2004).\u003c/p\u003e","description":"","filename":"Figure11.png","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/5cbf13490747ce6e0fb7a6bf.png"},{"id":67681904,"identity":"e8cce19b-8dcb-4012-baf4-ab1a3313c5a2","added_by":"auto","created_at":"2024-10-28 16:11:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2552809,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/a44ee1f5-bf3a-4ee4-8db9-4ba75723308c.pdf"},{"id":57640396,"identity":"9b468647-3a97-4122-9785-334e529430e1","added_by":"auto","created_at":"2024-06-03 17:10:30","extension":"xlsx","order_by":16,"title":"","display":"","copyAsset":false,"role":"supplement","size":14019,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryTable1.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/4c988632e8b4c852a2df23f7.xlsx"},{"id":57640395,"identity":"2c3c3c72-314d-4430-ac1e-b1f7ed872241","added_by":"auto","created_at":"2024-06-03 17:10:29","extension":"xlsx","order_by":17,"title":"","display":"","copyAsset":false,"role":"supplement","size":41246,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryTable2.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-4447195/v1/423845f810a87cff781a793e.xlsx"}],"financialInterests":"","formattedTitle":"Calibrating lower-middle Miocene mammal faunas and unravelling climate change during the Miocene Climate Optimum; the Bardenas Reales de Navarra record (Ebro basin, NE Iberian Peninsula)","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eEstablishing accurate chronologies for continental sedimentary successions is of major importance in paleoclimatology because it enables placing environmental changes occurred in continental areas within the same chronostratigraphic framework as changes reported from the neighbouring marine realm. This is a step that needs to be fulfilled in order to better understand the response of regional environmental changes to global climate variations (van der Meulen \u0026amp; Daams, \u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Alcal\u0026aacute; et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2000\u003c/span\u003e, van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e, \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Amongst those periods of climate change occurred during the Cenozoic, the Miocene Climate Optimum (MCO) stands out because it represents the most pronounced period of global warming and enhanced CO\u003csub\u003e2\u003c/sub\u003e atmospheric concentrations of the last 33 Myr of the Earth\u0026acute;s history (Beerling and Royer, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Westerhold et el., 2020) and, hence, is considered as one of the best analogues of present-day global change.\u003c/p\u003e \u003cp\u003eMost marine MCO records point to global temperatures 6\u0026ndash;8\u0026deg;C warmer than today between broadly 17 and 14 Ma (Mudelsee et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Westerhold et al., \u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), which were accompanied by atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentrations often exceeding 500 ppm (Beerling and Royer, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Greenop et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The MCO was followed by a sharp end (the Middle Miocene Climate Transition, ca. 14\u0026thinsp;\u0026minus;\u0026thinsp;13 Ma) that, given its amplitude, is recognized in most paleclimatological records (Mudelsee et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Westerhold et al., \u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). As opposed to this rapid termination, the onset of the MCO is marked by a longer warming trend that started at about 20 Ma and is of lower magnitude in terms of global temperature changes (Mudelsee et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Discussion on the causes of the MCO is open, with changes in ocean circulation, volcanic activity, atmospheric CO\u003csub\u003e2\u003c/sub\u003e levels, orbital forcing, and feedbacks between plant expansion and CO\u003csub\u003e2\u003c/sub\u003e levels being considered as possible driving factors (Pagani et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Hamon et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Henrot et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Super et al. \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Goto et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). MCO marine data also indicate slight (0.5 myr) differences in its onset and termination as well as in amplitude of the response in different areas (Mudelsee et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), adding uncertainty on its underlying causes.\u003c/p\u003e \u003cp\u003eAs opposed to the oceanic realm, data from continental areas are rather scarce. Mean annual temperatures established for north America from low-resolution (250 kyr) paleosoil weathering indexes reveal a steady onset of the MCO, peak warm temperatures between 17 and 13 Ma, and a sharp cooling at around 12.5 Ma (Gallagher \u0026amp; Sheldon, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). In East Asia, high-resolution (20 kyr) rock magnetic data from different red clay sequences in the Chinese Loess Plateau point to a gradual onset of the MCO at 17\u0026thinsp;\u0026minus;\u0026thinsp;16 Ma and a more abrupt termination of peak warm conditions at 13.8\u0026ndash;14.5 Ma (Zan et al., \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Zhao et al., \u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). In Europe, data on the MCO come from different sectors. Paleoclimatic reconstructions from central Europe based on ectothermic vertebrates (Bohme, 2003; Bohme et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), plant macrofossil assemblages (Mossbruger et al., 2005), pollen data (Donders et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2009\u003c/span\u003e) and carbonate clumped isotopes from paleosoils (Methner et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) point to overall warmer and wetter conditions between 17\u0026thinsp;\u0026minus;\u0026thinsp;15 Ma, but show significant regional discrepancies that are attributed to paleogeographic factors such as land-sea distribution or orographic factors (Mossbruger et al., 2005). It should be kept in mind, however, that the temporal resolution of these records is often too coarse (0.2-2 Myr) and their chronology is not fully resolved in some cases (Donders et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2009\u003c/span\u003e, Bohme et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), which further limits the assessment of European climate changes across the MCO. This is especially relevant for its onset, since most records either do not extend beyond 17 Ma (see Donders et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Bohme et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) or have a much lower (\u0026gt;\u0026thinsp;1 Myr) resolution before that time (Bohme, 2003; Mosbrugger et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Methner et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), hampering identification of the underlying forcing mechanism(s). High-resolution (80 kyr) paleoclimatic reconstructions for southern Europe across the MCO derive form the magnetostratigraphically-dated fossil record of the Calatayud-Daroca basin (Daams et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1999a\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1999b\u003c/span\u003e; van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Using herpetological assemblages from this basin, overall wetter conditions have been reported for the peak of the MCO, with and abrupt drying at 15.5 Ma (Bohme et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). The fossil record of the Calatayud-Daroca basin extends back in time beyond 17 Ma but, unfortunately, the sedimentary succession is faulted and no reliable magnetostratigraphic calibration of fossil faunas can be produced (Daams et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1999a\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003eb\u003c/span\u003e), precluding precise delineation of the onset of the MCO.\u003c/p\u003e \u003cp\u003eThe Bardenas Reales de Navarra (BRN) area at the western Ebro basin host a 690 m-thick, lower-middle Miocene sedimentary succession of continental origin that includes several small mammal fossil faunas initially attributed to Iberian zones Z to A (equivalent to Mammal Neogene zone 3, MN3) (Murelaga, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Murelaga et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). The outstanding outcrop conditions and the good paleomagnetic signal associated to the dominant fine-grained sediments within the succession has resulted in an independent and straightforward correlation of the well-resolved local magnetozones with the Geomagnetic Polarity Timescale (GPTS). The resulting magnetostratigraphic age model indicates that the succession spans from chron C6An.1n to chron C5Br (Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), covering from ca. 20.5 to 15.2 Ma in the updated GPTS of Ogg (\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Although later efforts have enabled the study of additional fossil faunas (Ruiz-S\u0026aacute;nchez et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2012a\u003c/span\u003e, \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2012b\u003c/span\u003e, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2012c\u003c/span\u003e, \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2012d\u003c/span\u003e, \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), most of the new fossil localities have been located in successions with no magnetostratigraphic constraint, so that their chronology has relied on the lithostratigraphic correlation to previously published magnetostratigraphic successions. In any case, these new findings have largely expanded the small mammal record of the BRN so that most of the lower Miocene Iberian zones prior to 17 Ma (Y1 to C) are encompassed, for the first time, in an expanded and well exposed succession in direct stratigraphic continuity with basal middle Miocene faunas (zone D) for which a reliable magnetostratigraphic calibration exists (Daams et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1999a\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1999b\u003c/span\u003e; van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Hence, the BRN area offers the possibility of extending the calibration of the middle Miocene Iberian zones performed in the Calatayud-Daroca basin to the lower Miocene, filling an important gap in the chronology of small mammal Iberian and European biochronology (Dams et al., 1999a, 1999b; Agust\u0026iacute; et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2001\u003c/span\u003e, van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn addition to small mammal remains of biochronological significance, fossil localities at the BRN area host a diverse vertebrate fauna that includes amphibians, reptiles and large mammals (Murelaga, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Murelaga et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e1999\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2002\u003c/span\u003e, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). The fossil record of the succession is completed by ostracod (Mart\u0026iacute;nez-Garc\u0026iacute;a et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), charophyte (Gonz\u0026aacute;lez-Pardos, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) and avian and reptilian eggshell remains (Grellet-Tinnet et al., 2012), avian and artiodactyl fossil tracks (D\u0026iacute;az-Mart\u0026iacute;nez et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2016\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), and by an aquatic avian fossilized nest (Grellet-Tinnet et al., 2012). Collectively, this varied fossil record portraits a warm, subtropical ecosystem where rainfall was likely submitted to a high seasonality, in line with previous inferences based on reptilian and amphibian faunas (Murelaga et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2002\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn this study we present an update of the fossil small mammal record of the lower-middle Miocene sedimentary succession at the BRN area, which includes a review of previously published fossil localities and new, unpublished (Suarez-Hernando, \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) fossil localities aimed at filling the gaps between different local zones. These data are presented along with three new magnetostratigraphic sections that have been studied in order to provide robust chronological constrains for the largest possible number of the newly studied fossil localities. Integration of these data with previously published results from the Calatayud-Daroca basin (Daams et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1999a\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1999b\u003c/span\u003e, van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and other sectors of the Ebro basin (Agust\u0026iacute; et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; P\u0026eacute;rez-Rivar\u0026eacute;s et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2004\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) enables a new calibration of Iberian and European mammal zones for the lower Miocene to be proposed.\u003c/p\u003e \u003cp\u003eWe have also used the updated small mammal record of the BRN to produce, following established methodologies (Daams et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1988\u003c/span\u003e; van Dam and Weltje, 1999), a low-resolution (ca. 0.3 Ma) reconstruction of relative variations in temperature and humidity conditions in southwestern Europe between 20.5 and 15.3 Ma, encompassing the onset and most of the MCO peak. This paleoenvironmental record has been complemented by a high-resolution (30 kyr) rock magnetic study of the fine-grained sediments of the BRN area deposited under subaerial conditions, which are subjected to incipient pedogenic processes during sedimentation and can, therefore, provide additional constraints on the underlying climatic conditions (Liu et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). The recent link established between enhanced pedogenic formation of magnetic minerals in response to wetter conditions in the Pliocene Teruel basin (Gao et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) suggests that this rock-magnetic approach might be successfully applied to other continental basins in the Iberian Peninsula, such as the Ebro basin considered here. Paleoenvironmental results derived from small mammal faunas and rock magnetic properties are used, in combination with additional sedimentological (Larena et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) and micropaleontological (Mart\u0026iacute;nez-Garc\u0026iacute;a et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) information derived from the BRN sedimentary succession to provide what might be considered the first comprehensive record of climate variations suffered by southwestern European ecosystems during the onset and peak warming conditions of the MCO.\u003c/p\u003e"},{"header":"2. Geological setting and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Geological setting\u003c/h2\u003e \u003cp\u003eThe Ebro basin is an asymmetric, triangularly-shaped foreland basin that developed mainly during the Paleogene in response to tectonic loading in the Pyrenees fold-and-thrust belt (Mu\u0026ntilde;oz et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Pardo et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Tectonic thrusting and uplift in the western Pyrenees cut the connection of the Ebro basin with the Atlantic Ocean in the late Eocene (ca. 36 Ma) (Costa et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), which subsequently developed as an endorheic, low-gradient depression enclosed also by the Iberian and the Catalan Coastal Ranges. This configuration persisted until the Late Miocene (12\u0026thinsp;\u0026minus;\u0026thinsp;7 Ma), when the Mediterranean drainage network captured the basin (Garc\u0026iacute;a-Castellanos \u0026amp; Larrasoa\u0026ntilde;a, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). As a result of this protracted evolution as an endorheic depression, the Ebro basin filled up with a sequence of Late Eocene to Miocene sediments that accumulated in a set of broadly concentrical facies belt that included: 1) a system of alluvial fans attached to the active margins of the enclosing mountain belts, where conglomerates accumulated; 2) a lacustrine system that occupied the central parts of the depression, when either carbonates or evaporites accumulated in response to varying climatic conditions; and 3) an intermediate zone where distal alluvial flood plains met the lacustrine areas, and which witnessed accumulation of mudstones interbedded with palustrine carbonates and fluvial sandstones (Arenas \u0026amp; Pardo, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Mu\u0026ntilde;oz et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Pardo et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). The overall thickness of the Ebro basin sedimentary sequence reaches up to 6000 in its north-western sector, at the foothills of the Pyrenean thrust-and-fold belt, and decreases progressively to the south and east to reach less than 2000 m in the vicinity of the Iberian and Catalan margins. This difference attests to the flexural subsidence of the basin in response mainly to tectonic load of the Pyrenean allochthonous units (Mu\u0026ntilde;oz et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2002\u003c/span\u003e, Pardo et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2004\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe BRN area host a sequence of ca. 694 meters accumulated in the western part of the central Ebro basin during the lower and middle Miocene (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) (Larrasoa\u0026ntilde;a et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The sequence includes the uppermost 50 m of the Ler\u0026iacute;n Gypsum Formation (Salvany et al., \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e1994\u003c/span\u003e) and the overlying Tudela Formation, which has been divided into 5 units (Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The Ler\u0026iacute;n Gypsum Formation is represented in the area by brown and yellow mudstones that include some sandstone beds and two thick (\u0026lt;\u0026thinsp;15 m) packages constituted by gypsum levels and grey mudstones that attest to sedimentation in a saline mudflat. The Tudela Formation is mainly constituted by red, brown, yellow and grey mudstones that include frequent sandstones and limestone beds associated to grey marls. These sediments were deposited in a distal alluvial mud flat fed by fluvial courses that coexisted either with small ponds (units 1 and 4) or larger lacustrine areas (units 2, 3 and 5). The only exception to this pattern is the 10 m-thick Fusti\u0026ntilde;ana Gypsum Member (middle of Unit 3), which attest to short-lived sedimentation in a saline lake formed under arid conditions (Salvany et al., \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e1994\u003c/span\u003e). The Ler\u0026iacute;n Gypsum Formation is part of the fourth tecto-sedimentary unit (TSU-4) in which the sedimentary infill of the Ebro basin has been divided (Mu\u0026ntilde;oz et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Pardo et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; P\u0026eacute;rez-Rivar\u0026eacute;s et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Units 1 to 4 of the Tudela Formation belong to TSU-5, whereas its Unit 5 is included within TSU-6 (Mu\u0026ntilde;oz et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Pardo et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; P\u0026eacute;rez-Rivar\u0026eacute;s et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe distribution of these sediments and their lateral equivalents can be envisaged in the context of a basin with an extremely low topographic gradient thanks to the sedimentary model developed by Arenas \u0026amp; Pardo (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). According to this model, periods with a positive hydrological balance witnessed the development of large lakes in the central part of the Ebro basin, which resulted in accumulation of grey marls in the deepest (\u0026lt;\u0026thinsp;6 m) part of the lacustrine system and of carbonates in a shallower (\u0026lt;\u0026thinsp;2 m) fringe that separated the lake system (units 1\u0026ndash;4 of the Tudela Formation and their lateral counterpart to the east, the lower part of the Alcubierre Formation) from the surrounding mudflats (Uju\u0026eacute; and Sari\u0026ntilde;ena formations towards the north). Periods with a negative hydrological balance led to accumulation of evaporites in the central part of a shrinking lacustrine system (Ler\u0026iacute;n and Zaragoza formations) and of distal alluvial sediments in mudflats that prograded up to 30 km basinward over areas previously occupied by lacustrine sedimentation. This configuration changed dramatically at the base of Unit 5 of the Tudela Formation (and its lateral equivalent, the upper part of Alcubierre Formation), when lacustrine sedimentation replaced previously accumulated sediments regardless of their origin and position. Magnetostratigraphic results across the basin have demonstrated that such widespread lacustrine expansion occurred at ca. 16.1 Ma (uppermost part of chron C5Cn, Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; P\u0026eacute;rez-Rivar\u0026eacute;s et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2004\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), in what can be considered as the response of the hydrological balance of the Ebro basin to enhanced wetter conditions driven by the MCO (Arenas \u0026amp; Pardo, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; P\u0026eacute;rez-Rivar\u0026eacute;s et el., 2018; Larena et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Stratigraphy and sedimentology\u003c/h2\u003e \u003cp\u003eWe have logged and described the lithology, colour, grain size, thickness and sedimentary structures and textures of sediments along five new sections at the BRN, namely the Cuesta Agujeros, Cabezo Carbonera, Loma Negra, Punta del Olmo, and Punta de Riant\u0026oacute;n sections (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These sections have been studied because they cover stratigraphic gaps with a lack of fossil sites, mainly in the lowermost and upper parts of the BRN sedimentary succession. We have also revised and described the sedimentary facies of previously published sections in the BRN (Murelaga, 2020; Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) in order to have a complete view of the sedimentological significance of the studied succession. Especial attention has been paid to sedimentary features and textures indicative of incipient edaphic features (e.g., mottling linked to root traces) in mudstones deposited under subaerial conditions (as signalled by yellow, brown and red colours) (Arenas \u0026amp; Pardo, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Larena et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), for their bearing on environmental magnetic results (see section \u003cspan refid=\"Sec6\" class=\"InternalRef\"\u003e2.4\u003c/span\u003e). The overall horizontal bedding and mild deformation conditions (Soto et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2009\u003c/span\u003e), together with excellent exposures and the presence of distinctive lithological marker beds, enable lithostratigraphic correlations between the new sections and those studied previously to be done (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). This has enabled revision of the previous composite section described in Larrasoa\u0026ntilde;a et al. (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), for which only a minor correction (\u0026lt;\u0026thinsp;10 m at the base of the Barranco de Tudela section) has been introduced.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Small mammal chronology and palaeoenvironmental reconstruction\u003c/h2\u003e \u003cp\u003eWe screened twenty-seven stratigraphic levels in the new sections logged, targeting mainly grey mudstones, where most fossil remains had been previously recovered (Murelaga, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Murelaga et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Ruiz-S\u0026aacute;nchez et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2012a\u003c/span\u003e, \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2012b\u003c/span\u003e, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2012c\u003c/span\u003e, \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2012d\u003c/span\u003e, \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). At each interval, about 25 kg of sediment were sampled, washed and sieved in order to produce concentrates with particles ranging in size between 3 and 0.5 mm. Such concentrates were treated with acetic acid (10% concentration) in order to facilitate the removal of the carbonate fraction and separation of small mammal fossil teeth (see Suarez-Hernando, \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Eleven intervals where at least one small-mammal tooth was found, plus seven previously published localities (Ruiz-S\u0026aacute;nchez et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2012b\u003c/span\u003e, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2012c\u003c/span\u003e, \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), were resampled by collecting between 80 and 1700 kg of sediment at each interval depending on initial richness estimates and prioritizing biozone boundaries. Overall, a total of 19.970 kg of sediment have been collected and studied over the last 25 years in order to find 1171 small mammal molars suitable for species identification, rendering an average richness of 0.059 molars/kg (for reference, rich localities are those yielding\u0026thinsp;~\u0026thinsp;1 molar/kg). For details on the nomenclature of small mammal teeth identification, the reader is referred to Murelaga (\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2000\u003c/span\u003e), Murelaga et al. (\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2004\u003c/span\u003e), Suarez-Hernando (\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) and references therein.\u003c/p\u003e \u003cp\u003ePaleoenvironmental inferences from fossil small mammal teeth were done following the schemes of Daams et al. (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1988\u003c/span\u003e) and van Dam and Weltje (1999), who assign different species or genera of fossil small mammals to relative changes in humidity and temperature according to their ecological preferences. Thus, reconstructions of past humidity and temperature conditions have been calculated based on the percentage of faunas associated to wet, neutral and dry environments (humidity) and to cold, neutral and warm conditions (temperature). Quantitative estimates of past temperatures (Legendre et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) and precipitation (van Dam et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) have not been considered because the richness of the fauna do not reach the minimum required (100 molars) but only in a few localities.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Magnetostratigraphy and environmental magnetism\u003c/h2\u003e \u003cp\u003eWe have sampled three of the five new sections considered in this study in order to provide a chronology for most of the previously published (Ruiz-S\u0026aacute;nchez et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2012b\u003c/span\u003e, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2012c\u003c/span\u003e, \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) and the new fossil localities presented here (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The first section, named Cabezo Carbonera, includes 96 m of the lowermost unit of the Tudela Formation (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The second section (117 m-thick) corresponds with the downward continuation of the Cuesta Agujeros section studied previously by Larrasoa\u0026ntilde;a et al. (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and includes Unit 1 and the lowermost part of Unit 2 of the Tudela Formation (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The third section, labelled as Punta del Olmo, includes 105 m of the uppermost half of Unit 3 of the Tudela Formation (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). A total of 31, 45 and 43 stratigraphic intervals, mainly mudstones but also some limestone beds, were sampled throughout the Cabezo Carbonera, Cuesta Agujeros and Punta del Olmo sections, resulting in an average resolution of 2.8 meters. Samples were collected using a standard, water-refrigerated electrical drill powered by a generator. Samples were oriented in the field using a magnetic compass mounted on a core-orienting fixture. The succession shows a very gentle dipping of ~\u0026thinsp;14\u0026ordm; to the north at the Cabezo Carbonera section, whereas is horizonal at the Cuesta Agujeros and Punta del Olmo sections. The two remaining stratigraphic sections, Loma Negra and Punta de Riant\u0026oacute;n, include the Unit 4 and the lowermost 30 m of Unit 5 of the Tudela Formation. They were not sampled for magnetostratigraphic analyses because their lithostratigraphic correlation to the neighbouring Sancho Abarca and Pico del Fraile sections, for which magnetostratigraphic data are available (Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), is straightforward and unequivocal given the presence of extensive, distinctive limestone beds.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePaleomagnetic measurements were conducted at Paleomagnetic Laboratory of the Geo3BCN Institute (CCiTUB-CSIC) in Barcelona, Spain. The Natural Remanent Magnetization (NRM) of the studied samples was measured using a 2G superconducting rock magnetometer, which has a noise level of ~\u0026thinsp;10\u003csup\u003e\u0026ndash;6\u003c/sup\u003e A/m. Previous studies on Miocene sediments from the central and eastern part of the Ebro basin have demonstrated that thermal demagnetization is the most effective method for isolating the different paleomagnetic components of the NRM. Therefore, thermal demagnetization of the samples was conducted using a MMTD\u0026ndash;80 furnace at intervals of 100\u0026deg;C, 50\u0026deg;C, 40\u0026deg;C and 30\u0026deg;C to a maximum temperature of 680\u0026deg;C. Stable Characteristic Remanent Magnetization (ChRM) directions were calculated by means of Principal Component Analysis (Kirschvink \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e1980\u003c/span\u003e) after they were identified through visual inspection of orthogonal demagnetization plots using the VPD software (Ram\u0026oacute;n et al., \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eEnvironmental magnetic properties were measured using standard paleomagnetic specimens of mudstones deposited under subaerial conditions (as signalled by yellow, brown and red colours). A total of 198 samples distributed throughout the composite section yield an average resolution of 3.5 m, which corresponds to about 26 kyr taking into consideration previous magnetostratigraphic results (Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The low-field magnetic susceptibility of these samples has been measured at two frequencies of 470 (c\u003csub\u003eLF\u003c/sub\u003e) and 4700 (c\u003csub\u003eHF\u003c/sub\u003e) Hz using a Bartington MS3 susceptibility meter equipped with a MS2B sensor at the CN IGME, CSIC. These measurements enable calculation of the frequency-dependent (c\u003csub\u003efd\u003c/sub\u003e=c\u003csub\u003eLF\u003c/sub\u003e-c\u003csub\u003eHF\u003c/sub\u003e) and the percent frequency-dependent (c\u003csub\u003efd%\u003c/sub\u003e=(c\u003csub\u003efd\u003c/sub\u003e/c\u003csub\u003eLF\u003c/sub\u003e) x 100) susceptibilities as proxies for the absolute and relative contents of superparamagnetic particles (SP, \u0026lt;\u0026thinsp;25 nm), respectively (Dearing et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). SP particles form as a result of pedogenic activity, so determining their abundances in continental sequences provide insights into climatic conditions under the assumption that pedogenic activity is enhanced under warmer and wetter conditions, at least within a certain low-middle humidity range that excludes water logging of soils (Liu et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Jiang et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). These data have been combined with a description of the sedimentary facies for the same samples taking advantage of the excellent observation conditions provided by the clean surfaces of the paleomagnetic specimens.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Stratigraphy and sedimentology\u003c/h2\u003e \u003cp\u003eResults from the five studied sections, coupled with results from earlier studies, enable description of the sedimentologic and stratigraphic significance of the composite BRN section. The sedimentary facies are similar to those described by other authors in the central part of the Ebro basin (Arenas \u0026amp; Pardo, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1999\u003c/span\u003e, Larena et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The most abundant clastic facies correspond to red, brown and yellow mudstones that are typically massive (facies Fm) and constitute tabular packages of up to 15 m-thick. These mudstones occasionally show horizontal lamination (facies Fh) and often display a faint aggregated texture that signals root bioturbation (facies Fmbt). Sandstones appear either as thin (\u0026lt;\u0026thinsp;40 cm), tabular strata of massive appearance (facies Sm) with occasional horizontal lamination (facies Sh) or as thick (\u0026lt;\u0026thinsp;3 m) irregular beds with channel morphology and cross-bedding stratification (facies St).\u003c/p\u003e \u003cp\u003eCarbonate facies are dominated by limestones that constitute tabular beds between 10 cm and 1.5 m in thickness and display a massive texture that includes variable amounts of sand grains dispersed within the matrix (facies Lm). The massive appearance is modified at some horizons by the accumulation of intraclasts and gastropods, ostracods and charophytes remains and fragments. The uppermost 10\u0026ndash;20 cm of these massive limestones are typically affected by vertical root traces and, less frequently, by desiccation cracks (facies Lb). Far less abundant are laminated limestones that constitute thin (\u0026lt;\u0026thinsp;20 cm) tabular beds that include frequent accumulation of intraclasts and gastropods, ostracods and charophytes fragments (Ll). No stromatolitic limestones have been found in the BRN area. Evaporite facies mostly correspond to nodular gypsum beds, a few cm to 1 m in thickness, that show an alabastrine texture and are associated with grey marls (facies Gn). More infrequent are gypsum beds that display a mm to cm-scale horizontal lamination embedded within grey marls (facies Gl).\u003c/p\u003e \u003cp\u003eFinally, mixed facies are mostly represented by grey marls and mudstones that constitute tabular packages of up to 3 m thick, include variable amounts of sand grains dispersed within the matrix, and usually have a massive appearance (facies Mm). In fewer occasions, these marls and mudstones display mm-scale laminations and contain some ostracods and charophytes fragments (facies Ml). A distinctive mottling indicating subaerial exposure is sometimes observed affecting these marls and mudstones (Mmbt).\u003c/p\u003e \u003cp\u003eThese facies are arranged in distinctive facies associations (FA) that can be broadly grouped in the three major types described by Arenas \u0026amp; Pardo (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1999\u003c/span\u003e) and Larena et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). FA1 represents sedimentation in a distal alluvial plain, and is mainly composed by sandstone facies St that signal the incision and later filling of low-sinuosity, high-energy fluvial channels. Avulsion of these channels during flooding events cause rapid deposition of mudstone facies Fm and Fh as well as accumulation of sandstone facies Sm and Sh in unconfined, low-energy flows at lateral and terminal splays of the main fluvial channels. In this context, facies Fmtb indicate periods of plant colonization and soil formation occurred between flooding events. FA1 is, by far, the most common facies association in the BRN area, so that it dominates units 1 and 4 of the Tudela formation as well as most of the Ler\u0026iacute;n Gypsum Formation and units 2 and 3 of the Tudela Formation.\u003c/p\u003e \u003cp\u003eFA2 represents sedimentation in a lacustrine setting with a positive hydrological balance, and begins with the implantation of a stable lake as indicated by deposition of mudstone facies Mm and Ml. Stabilization of the lake system is marked by deposition of limestone facies Lm, being its occasional deepening signaled by accumulation of limestone facies Ll. Under this scenario, accumulation of sand grains, intraclasts and bioclasts attest for the influence of episodic fluvial inputs that deliver detrital and biogenic material from the surrounding plains and shallower areas of the lake system, where mudstones facies Mm also accumulate. Continued carbonate production and detrital supply eventually turn the lake into a shallow palustrine area that can be eventually subjected to subaerial exposure (limestone facies Lb and mudstone facies Mmbt). FA2 dominates Unit 5 of the Tudela Formation as well as thinner (\u0026lt;\u0026thinsp;8 m) intervals of its Unit 3 and the lower and uppermost parts of Unit 2.\u003c/p\u003e \u003cp\u003eFinally, FA3 represents sedimentation in a lacustrine setting with a negative hydrological balance that begins with accumulation of mudstone facies Mm and Ml under strong evaporation conditions that, eventually, lead to formation of gypsum facies Gn by evaporitic pumping and even direct precipitation from water (facies Gl). FA3 is restricted to parts of the Ler\u0026iacute;n Gypsum Formation and to a thin (\u0026lt;\u0026thinsp;10 m) interval within Unit 3 of the Tudela Formation (Fusti\u0026ntilde;ana member) that can be considered as the most widespread extension of the Zaragoza Gypsum Formation to the northwest.\u003c/p\u003e \u003cp\u003eThe wide lateral extension (from 5 to \u0026gt;\u0026thinsp;20 km) of most limestone beds and mudstone packages of FA1, FA2 and FA3 within the BRN area enables a straightforward correlation between the different studied sections, and provides important clues on the paleogeographic significance of the studied sediments (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). First, Unit 1 of the Tudela Formation (dominated by distal alluvial FA1 sediments) appears to thicken towards the north between the Cabezo Mariju\u0026aacute;n and Cabezo Carbonera sections. Second, thick (up to 8 m) intervals of freshwater lacustrine FA2 sediments within units 2 and 3 of the Tudela Formation (e.g., Cabezo Mariju\u0026aacute;n and Barranco de Tudela sections) grade laterally to the north (Cuesta Agujeros and other sections studied by Murelaga, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) to distal alluvial (FA1) sediments. Third, the Fusti\u0026ntilde;ana Gypsum member (FA3 in the middle of Unit 3) also thins out to the north, where it is replaced by distal alluvial (FA1) sediments. And fourth, thick (\u0026gt;\u0026thinsp;8 m) intervals of FA2 sediments are restricted to Unit 5 of the Tudela Formation, with facies indicative of deepest lacustrine conditions (Ll) being found only in the southernmost studied sections (Sancho Abarca and Pico del Fraile). All these circumstances indicate, in agreement with previous sedimentologic observations by Larena et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), that the BRN area was located in the transition zone between the lakes that occupied the central part of the Ebro basin (whether freshwater or saline) and the distal alluvial plains that drained the Pyrenean orogen to the north. They also indicate a significant shift in the entity of lakes, which became strikingly well developed in the transition from units 4 and 5 (TSU-5 and TSU-6) of the Tudela Formation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Small mammal biochronology\u003c/h2\u003e \u003cp\u003eThe attribution of the fossil faunas of the BRN area to different local zones can be done after considering the updated faunal list of all the studied fossil localities (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, Supplementary Table\u0026nbsp;1). The basal faunas found at the BRN area (localities CH1 and CA1) include \u003cem\u003eEucricetodon\u003c/em\u003e sp., \u003cem\u003eE. gerandianus\u003c/em\u003e and \u003cem\u003eArmantomys\u003c/em\u003e cf. \u003cem\u003ebijmai\u003c/em\u003e, which enables their attribution to the lower part of local Y of the Agenian, labelled as Y1 (L\u0026oacute;pez-Mart\u0026iacute;nez, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e1989\u003c/span\u003e; Daams, 1990). Moving upwards in the succession, fossil localities CA2 and CC1 appear characterized by the associated occurrence of \u003cem\u003eE. aquitanicus\u003c/em\u003e and \u003cem\u003eA. daamsi\u003c/em\u003e, which are indicative of the upper part of Agenian biozone Y (Y2, \u0026Aacute;lvarez-Sierra et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). Identification of local zone Z of the Ramblian in fossil localities CM1 and CJ1 is possible based on the absence of \u003cem\u003eRhodanomys-Ritteneria\u003c/em\u003e and the combined presence of \u003cem\u003eCricetidae\u003c/em\u003e (\u003cem\u003eEucricetodon\u003c/em\u003e spp. and \u003cem\u003eE. infralactorensis\u003c/em\u003e) and Gliridae (\u003cem\u003ePseudodryomys ibericus\u003c/em\u003e and \u003cem\u003ePs. simplicidens\u003c/em\u003e, \u003cem\u003ePeridyromys murinus\u003c/em\u003e) (Daams et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). Faunas at fossil locality CA3 are consistent with local zone Z, but this attribution is mainly based on its stratigraphic position within localities CM1 and CJ1. Fossil locality CA3b has yielded only remains of \u003cem\u003eProdryomys\u003c/em\u003e cf. \u003cem\u003ebrailloni\u003c/em\u003e, which allows its tentative attribution also to local zone Z. The next local zone of the Ramblian, zone A, is characterized by the absence of Cricetidae, the predominance of \u003cem\u003eLigerimys\u003c/em\u003e, and the presence of \u003cem\u003eP. murinus and Ps. ibericus\u003c/em\u003e (Daams et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Daams et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1999a\u003c/span\u003e). Faunas clearly attributable to zone A are found at several localities between m 235 and 495 of the composite section (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Of especial relevance are localities CA4 and CSA3. The cooccurrence of \u003cem\u003eA.\u003c/em\u003e cf. \u003cem\u003ejasperi\u003c/em\u003e, \u003cem\u003eAltomiramys\u003c/em\u003e sp., \u003cem\u003eL.\u003c/em\u003e aff. \u003cem\u003emagnus\u003c/em\u003e and \u003cem\u003ePseudotheridomys\u003c/em\u003e sp. in CA4 makes this fossil locality the oldest one securely assigned to zone A. CSA3 is the youngest locality associated to the same zone A on the basis of its fossil remains, which lack Cricetidae and include \u003cem\u003eP.\u003c/em\u003e cf. \u003cem\u003eibericus\u003c/em\u003e, \u003cem\u003eGliridinus\u003c/em\u003e sp., \u003cem\u003eVasseuromys rambliensis\u003c/em\u003e, \u003cem\u003eEomydae\u003c/em\u003e indet. Together with localities PO38 and PO73, CSA3 confirms the extension of local zone A up to the uppermost part of Unit 3 of the Tudela Formation, as was initially suggested by the isolated locality PF1 (Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Ruiz-S\u0026aacute;nchez et al., \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2012d\u003c/span\u003e). The next fossil locality moving upwards in the stratigraphic section is BVG. This locality hosts remains of \u003cem\u003eMegacricetodon\u003c/em\u003e cf. \u003cem\u003eprimitivus\u003c/em\u003e, \u003cem\u003eA. jasperi\u003c/em\u003e, \u003cem\u003eSimplomys\u003c/em\u003e sp., \u003cem\u003eVasseuromys\u003c/em\u003e cf. \u003cem\u003ecristinae\u003c/em\u003e and Ochotonidae indet., and is hence attributed to local zone C of the lower Aragonian. Locality PR110 might be tentatively assigned also to local zone C, but its remains are too scarce for a secure attribution. Attempts at identifying zone B faunas between sites CSA3 and BVG have failed due to the barren nature of FA1-dominated sediments that constitute the lower part of Unit 4 of the Tudela Formation. The last zone identified at the BRN composite section corresponds to local zone D of the middle Aragonian, which is characterized by a decrease in the diversity of Gliridae, an increase in diversity of Cricetidae, and the absence of Eomydae (Daams et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1999a\u003c/span\u003e; van der Meulen et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). The first fossil locality unequivocally assigned to zone D is LN64, where \u003cem\u003eDemocricetodon gracilis\u003c/em\u003e, \u003cem\u003eMicrodyromys\u003c/em\u003e cf. \u003cem\u003ekoenigswaldi\u003c/em\u003e, \u003cem\u003eV. cristinae\u003c/em\u003e, \u003cem\u003eSpermophilinus\u003c/em\u003e sp., Ochotonidae indet. and Soricidae indet. have been recovered. PF2 also host a rich assemblage (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e) that includes also \u003cem\u003eMicrodyromys\u003c/em\u003e cf. \u003cem\u003eremmerti\u003c/em\u003e and enables its secure assignation to subzone Dc (van der Meulen et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). The rest of fossil localities found within Unit 5 of the Tudela Formation have faunas that are compatible with local zone D (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e), but their attribution is not unequivocal given that their content is either scarce (PR118) or characterized by elements with a wide biostratigraphic distribution (PR113, PR125; SA5 and SA6).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Magnetostratigraphy\u003c/h2\u003e \u003cp\u003eThermal demagnetization of the studied samples reveals the presence of two stable paleomagnetic components after removal of a viscous magnetization below 150\u0026ordm;C that is often subparallel to the drilling direction (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). Between 150\u0026ordm;C and 300\u0026ordm;C-420\u0026ordm;C in most cases, and up to 500\u0026ordm;C in some red mudstones, a low temperature component is identified with northerly directions and positive, step directions that are interpreted as a present-day field overprint that lacks of any geological significance (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea-e). After demagnetization of this component, an additional component (regarded as the characteristic remanent magnetization, ChRM) is identified showing a behaviour that is linked to the lithology. Thus, this component is completely removed by 420\u0026ordm;C in most limestones and some grey mudstones and marls, and reaches maximum unblocking temperatures of \u0026lt;\u0026thinsp;580\u0026ordm;C in yellow, brown and some red mudstones. Unblocking temperatures of up to 660\u0026ordm;C are observed for most of the red mudstones studied. This paleomagnetic behaviour is essentially identical to that reported previously in the BRN area (Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and other sectors of the central part of the Ebro basin (P\u0026eacute;rez-Rivar\u0026eacute;s et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2004\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), and indicates that both magnetite and hematite are the main magnetic carriers of the ChRM. As opposed to the paleomagnetic behaviour, the directional properties of the ChRM show no link with lithology. Thus, the ChRM shows either northerly directions with positive inclinations of around 55\u0026ordm; or broadly antipodal southerly directions with negative inclinations of around \u0026minus;\u0026thinsp;40\u0026ordm; regardless of lithology. Following previous authors (Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; P\u0026eacute;rez-Rivar\u0026eacute;s et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2004\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), calculated ChRM directions have been assigned to three categories according to their quality. Type 1 directions show rectilinear trends that yield low (\u0026lt;\u0026thinsp;5\u0026ordm;) mean angular deviations (MAD) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea, b), whereas type 2 directions display less-developed linear trends (MAD of 5\u0026ndash;15\u0026ordm;) or uncomplete demagnetizations due to the growth of new magnetic minerals upon thermal treatment (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec, d). Type 3 directions have highly scattered directions with large MAD (\u0026gt;\u0026thinsp;15\u0026ordm;) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ee). Only types 1 and 2 ChRM directions provide straightforward and highly reliable polarity determination and are, therefore, consider hereafter for constructing the sequence of polarity zones identified on the studied sections. The Cabezo Carbonera section is divided into a lower interval of normal polarity (labelled N2 following Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and an upper interval of reverse polarity (R2) (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The Cuesta Agujeros section is characterized by a large reversal that spans the middle part of the succession (R2) and two normal polarity intervals in the lowermost (N2) and uppermost (N3) parts of the section (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Finally, the Punta del Olmo section includes a reverse zone that spans the lower and middle part of the section (R4) and two overlying normal (N5) and reverse (R5) intervals located in its uppermost parts (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSince only the Cabezo Carboneras section shows a subtle bedding dip of 14\u0026ordm;C, no significant fold test can be performed. Nevertheless, comparison of ChRM directions in this section before and after tilt correction suggest that the ChRM represents a pre-folding direction. Although ChRM directions are seemingly antipodal, they do not pass the reversal test due to the partial overlap that exists between the ChRM and the present-day field overprint (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ef), a circumstance often observed in continental sediments of the Ebro basin that tends to preferentially flatten reverse directions (Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; P\u0026eacute;rez-Rivar\u0026eacute;s et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2004\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Nevertheless, the means of both normal and reverse directions overlap with that of the Miocene reference direction, which along with the consistent pattern of polarity intervals indicates that the ChRM is a primary component acquired during or shortly after deposition of the studied rocks. Correlation of the studied successions to the GPTS 2020 is straightforward in light of the previous magnetostratigraphic dating performed by Larrasoa\u0026ntilde;a et al. (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The resulting composite section spans from chron C6An.1n to chron C5Br, covering from ca. 20.5 to 15.2 Ma (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Supplementary Table\u0026nbsp;1).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Palaeoenvironmental reconstruction based on small mammals\u003c/h2\u003e \u003cp\u003eBefore addressing the paleoecological results derived from small mammal remains, it is important to recall that all the fossil localities at the BRN are found within grey mudstones and marls (facies Mm, Ml, Mmbt) indicative of shallow (\u0026lt;\u0026thinsp;2 m) palustrine environments receiving low energy fluvial currents (Murelaga, 2020; Murelaga et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Ruiz-S\u0026aacute;nchez et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2012a\u003c/span\u003e, \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2012b\u003c/span\u003e, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2012c\u003c/span\u003e, \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2012d\u003c/span\u003e, \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). The only exception is the CJ1 locality, where the grey mudstones include a microconglomeratic component indicating a fluvial current with higher energy. In any case, all fossil localities include both demic and ademic elements that ensure a good representativity of the faunas inhabiting the region at the time of sediment accumulation. With this caveat in mind, and considering the ecological preferences of the different small mammals and their relative importance in the studied localities, the variations of relative humidity and temperature conditions can be established for the studied sedimentary sequence (Supplementary Table\u0026nbsp;1). The fraction of wet-adapted fauna is negligible at the lower part of the sequence and increases rapidly to 20\u0026ndash;60% around 230\u0026ndash;350 m (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb). After a transient return to low values at around 440 m, the proportion of wet-adapted fauna increase again at 470 m to values of \u0026gt;\u0026thinsp;60% that remain high throughout the rest of the succession. Although the record is conditioned by the irregular distribution of fossil localities, a clear trend towards wetter conditions can be established for the succession, which shows sustained wet conditions between 470 m and the top of the sequence. The fraction of faunas indicative of warm conditions shows a similar trend through the section (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec), so that a progressive shift to warmer conditions is found in the lower 450 m of the succession (punctuated by a colder period at 430 m) before showing sustained values of 60\u0026ndash;100% throughout the rest of the sequence.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Palaeoenvironmental reconstruction based on magnetic properties\u003c/h2\u003e \u003cp\u003eRock magnetic data have been produced only for distal alluvial mudstone facies that denote accumulation of fine-grained (e.g., clay and silt) detrital material under subaerial conditions (Fm, Fl and Fmbt) (Supplementary Table\u0026nbsp;2). Of these facies, those displaying evidence for root bioturbation (Fmbt) attest for pedogenic process within paleo-alfisols similar to those reported by Hamer et al. (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) in other lower Miocene sediments from the central Ebro basin. Other pedotypes identified at the BRN area, such as inceptisols (developed on palustrine mudstones, facies Mmbt) and entisols (developed on lacustrine limestones, facies Lb) (see Hamer et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), have not been considered: 1) to avoid interference of subaerial pedogenic processes with earlier diagenetic process occurred under aqueous conditions; and 2) to account for a different parent material that involves a larger carbonate fraction.\u003c/p\u003e \u003cp\u003ecfd% values for the studied mudstones range between 0 and 8 (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea-c). Overall, about 61% of the mudstones show cfd%\u0026gt; 2 regardless of colour and sedimentary facies, which indicates that they include detectable amounts of SP particles (Dearing et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). It is worth noting that the fraction of samples with cfd%\u0026gt; 2 is of about 75, 67 and 40% for red, brown and yellow mudstone samples, respectively, which indicates that the contribution of SP particles to their magnetic assemblage is relatively more important in red mudstones and less relevant in the case of yellow mudstones. In any case, the fact that both massive (Fm) and bioturbated (Fmbt) mudstones display a significant contribution of SP particles regardless of colour indicate that pedogenic processes leading to formation SP material begun before development of macroscopic evidence for paleosoil formation (e.g., root bioturbation).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA comparison between cfd% and c\u003csub\u003eLF\u003c/sub\u003e values indicates an overall weak correlation between both variables (correlation coefficients between 0.35 and 0.78) (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, upper panel). The correlations increase notably (coefficients between 0.47 to 0.93) when c\u003csub\u003eLF\u003c/sub\u003e and cfd are compared (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, lower panel). This suggests that cfd values are more influenced by the initial susceptibility than cfd% values, and indicates that this later parameter provides a better proxy to signal the presence of SP particles in the studied sediments. Depth variations in cfd% indicate that the lowest contributions from SP particles are found between 50\u0026ndash;330 and 520\u0026ndash;690 m of the composite section, where most cfd% values range between 0 and 2 (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ee). The lowermost 50 m and the interval between 330\u0026ndash;520 m of the composite section display cfd% values that typically range between 3 and 7 and point to a higher contribution from SP pedogenic particles.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e4.1. Calibration of lower to middle Miocene Iberian mammal faunas\u003c/h2\u003e \u003cp\u003eA combination of the new results presented and summarized for the BRN area in this study, along with those obtained in other sector of the Ebro basin (Odin et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Agust\u0026iacute; et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; P\u0026eacute;rez-Rivar\u0026eacute;s et al., 2014, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) and other Iberian (e.g., Calatayud-Daroca) basins (Daams et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1999a\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1999b\u003c/span\u003e; van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; van der Meulen et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Garc\u0026iacute;a-Paredes et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), enables the proposition of a new magnetostratigraphic calibration for the continental record of the Iberian Peninsula spanning from the Agenian to the middle Aragonian (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eLocalities CH1 and CA1 from the BRN represent the first magnetostratigraphically-dated Y1 faunas from the Iberian Peninsula. Their chronology, ranging from the upper part of chron C6An to the lower part of chron C6r, is consistent with that of Y1 localities MO and BU4B from the Calatayud-Daroca basin, whose age was established, in the absence of magnetostratigraphic data, by a temporal interpolation of evolutionary features of the genus \u003cem\u003eEucricetodon\u003c/em\u003e between the late Oligocene and the lower Miocene (van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Only 5 meters above CA1, assigned to zone Y1, locality CA2 assigned to Y2 is found. Considering mean accumulation rates, and placing the Y1/Y2 boundary halfway between both localities, an age of 19.82 Ma for this boundary can be established. Hence, a new age ranging between ca. 21.6 Ma (van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and 19.82 Ma (this paper) can be proposed for Y1.\u003c/p\u003e \u003cp\u003eWith regards to zone Y2, represented at the BRN area by localities CA2 and CC1, it spans a very narrow stratigraphic interval dated to the middle part of chron C6r at around 19.8 Ma. This is very close to the age of the Tardienta locality in the Ebro basin, which is considered to mark the Y2/Z boundary and is also placed in the middle part of C6r (P\u0026eacute;rez-Rivar\u0026eacute;s et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) at an age of 19.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 Ma (van Dam et al. \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), as indicated by the revised \u003csup\u003e40\u003c/sup\u003eAr/\u003csup\u003e39\u003c/sup\u003eAr age of the volcanic ash layer with which the locality is associated (Odin et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). This age is also consistent with that established for Y2 localities CR, Al3B and Ata from the Daroca-Teruel basin on the basis of the evolutionary changes described for \u003cem\u003eEucricetodon\u003c/em\u003e (van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Given that these later localities are not magnetostratigraphically dated, we consider that the chronology of localities CA2, CC1 and Tardienta yield a robust age for zone Y2, which is constrained to the interval between 19.82 Ma and 19.7 Ma. This very short duration, of hardly 0.2 Myr, calls into question the biostratigraphic usefulness of subzone Y2, and leads us to discourage its use in favor of a broader Y zone that spans, according to the results presented here, from 21.6 to 19.7 Ma. Since the boundary between zones Y and Z marks the end of the Agenian (Daams et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1987\u003c/span\u003e), an age of 19.7 Ma can be assigned to its boundary with the Ramblian.\u003c/p\u003e \u003cp\u003eZone Z is represented by three localities at the BRN area, CM1, CA3 and CJ1. They also span a short stratigraphic interval within the lower half of chron C6n. Given that no other Z locality has been dated by magnetostratigraphy in the Iberian Peninsula, the chronology of this zone has to rely on our results from the BRN area. The boundary between zones Z and A can be placed halfway between CJ1 and CA4, which represent the youngest and oldest localities belonging to zones Z and A, respectively. Considering mean accumulation rates, this results in an age of 19.2 Ma and in an overall duration for zone Z of about 0.5 Myr (with its base dated at 19.7 Ma).\u003c/p\u003e \u003cp\u003eZone A is represented by a large number (14) of localities at the BRN area, whose age spans from the middle part of chron C6n to chron C5Dn. This long duration is confirmed by magnetostratigraphically calibrated A faunas from the Alcubierre area in the Ebro basin, where the uppermost boundary of zone A was found between localities LN142 and LN145 at an age of 17.15 Ma (Agust\u0026iacute; et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; P\u0026eacute;rez-Rivar\u0026eacute;s et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). These robust and mutually consistent results from the Ebro basin contrast strikingly with the ages assigned for the fossil record of the Calatayud-Daroca basin, where A fauna begin at ca. 18 Ma (van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) in what appeared to be a much shorter Z zone and a significantly longer A zone. Keeping in mind that the chronology of these zones was based on the interpolation of \u003cem\u003eEucricetodon\u003c/em\u003e evolutionary changes, we interpret that a yet unrecognized sedimentary event must have affected the record of the Calatayud-Daroca basin. Since the chronology assigned to Y fossil localities in that basin (established following the same method) is consistent with the results from the Ebro basin, we interpret that the most likely explanation for the discrepancy observed on the timing and duration of zones Z and A is an abrupt shift from very high accumulation rates during de former to a condensed sedimentation during the later. In any case, a duration from 19.2 to 17.15 Ma can be established for zone A in what represents its first comprehensive magnetostratigraphic calibration in the Iberian Peninsula. Keeping in mind this new calibration, is seems that the inferences made on small fossil turnovers in association to cyclic climatic changes (van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) need to be reconsidered and reassessed. An additional implication of our results is that the age of the Ramblian/Aragonian boundary, which is marked by the limit between zones A and B (Daams et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1987\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1999a\u003c/span\u003e), can be dated to 17.15 Ma.\u003c/p\u003e \u003cp\u003eIn the absence of faunas assigned to zone B in the Ebro basin, the duration of this zone and the age of its upper boundary with zone C has to rely on the distribution of B faunas from the Calatayud-Daroca basin, for which magnetostratigraphic data are available (Dams et al., 1999b; van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), and of C faunas from this basin and from the BRN area (e.g. fossil locality VBG). Overall, they provide a consistent age of 16.64 Ma for the B/C boundary and indicate a duration of ca. 0.5 Myr for zone B.\u003c/p\u003e \u003cp\u003eFinally, a robust chronology can be also provided for local zone D, since magnetostratigraphic data available from Iberian basins yield a mutually consistent record that spans the entire chron C5Br. The age of the C/D boundary (and hence of the lower/upper Aragonian), dated to 15.93 Ma at the Calatayud-Daroca basin (van Dam et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; van der Meulen et al. \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Garc\u0026iacute;a-Paredes et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), is fully supported by localities LN64, PR121 and PF2, which have a just younger age. A last issue deserving attention is the fact that most of the taxa that mark different subzones within zone D (van der Meulen et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) have not been found in the BRN or Alcubierre areas. This illustrates the point that using subzones does not necessarily provide a better tool for biostratigraphic correlations, specially if different basins are under consideration. This, coupled with the shorter duration of subzones (as we showed to be the case for Y1), forces us to discourage their use in the Iberian Peninsula.\u003c/p\u003e \u003cp\u003eOur biostratigraphic results also provide new insights into the chronology of Early Miocene mammal Neogene (MN) units, especially for those whose boundaries are subjected to large (e.g., 0.8\u0026ndash;2.1 Ma) uncertainties (e.g., MN2/MN3 and MN3/MN4, see Agust\u0026iacute; et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). Concerning the MN2/MN3 boundary, which corresponds to the boundary between local zones Y and Z and marks the transit between the Agenian and the Ramblian (Daams et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1987\u003c/span\u003e), an age of 19.7 Ma can be established from the magnetostratigraphic calibration of the Tardienta locality and the chronology of MN2 (CH1 to CC1) and MN3 (CM1 to CSA3) localities from the BRN area (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e). With respect to the MN3/MN4 boundary, its age is placed at 17.15 Ma keeping in mind the magnetostratigraphic calibration of MN3 faunal assemblages from the BRN (CM1 to CSA3) and Alcubierre (LN142) areas and of MN4 localities from the Ebro (LN145) and Daroca-Teruel (SR4A to VL2A) basins (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e). The chronology of MN5 localities at the BRN area (LN64 to PF2) gives also full support to the 15.93 Ma age established for the MN4/MN5 boundary in the Daroca-Teruel basin (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e4.2. Climate change in SW Europe during the MCO\u003c/h2\u003e \u003cp\u003eSmall mammal fauna from the BRN area point to a gradual increase in temperature and humidity between the bottom (ca. 20 Ma) and top (15.3 Ma) of the record, which appears to be punctuated by relatively warmer and wetter conditions at three intervals between 19.2\u0026ndash;18.6, 17.6\u0026ndash;17.1 and 15.7\u0026ndash;15.3 Ma (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ed, e). This trend seems to contrast with the sedimentary evolution of the basin, which witnessed the alternation of alluvial and lacustrine (both fresh-water and evaporitic) sedimentation before experimenting a basin-wide expansion of lacustrine conditions at ca. 16.1 Ma (Arenas \u0026amp; Pardo, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; P\u0026eacute;rez-Rivar\u0026eacute;s et el., 2018; Larena et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea). The most likely interpretation is that the hydrological balance of the basin responded in a non-linear fashion to humidity conditions, so that an overall lacustrine expansion occurred after certain humidity threshold was achieved. This seems to be supported by the important, yet gradual increase in the fraction of the charophyte \u003cem\u003eHarrisichara tuberculata\u003c/em\u003e and the ostracod \u003cem\u003ePseudocandona parallela\u003c/em\u003e observed in the uppermost part of the BRM record, which indicates a shift from shallow and warm stagnant ponds to stable lake conditions characterized by deeper, cooler and fresh to mesohaline waters at around 16.1 Ma (Gonz\u0026aacute;lez-Pardos, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Mart\u0026iacute;nez-Garc\u0026iacute;a et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Larena et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ec). In this regard, it is important to note that the temperature and humidity curves from the BRN bear a stronger resemblance to the evolution of sea surface temperatures reported for the Atlantic Ocean off the Iberian Peninsula (Deep Sea Drilling Program Site 608, Super et al., \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eg) than to global temperatures as reported from the global benthic compilation of d\u003csup\u003e18\u003c/sup\u003eO records (Westerhold et al., \u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eh). This is important because the tropical-subtropical North Atlantic corridor that extends from the Gulf of Mexico till nearly the position of Site 608 represents the main source of moisture for rainfall in the Iberian Peninsula (Gimeno et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea), and its surface temperature conditions in the past emerge as a plausible modulator for rainfall during the MCO, when subtropical climates expanded northward well into southern Europe (Hamon et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Henrot et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Thus, warmer (and wetter) than background conditions reported at the BRN between 19.2\u0026ndash;18.6, 17.6\u0026ndash;17.1 and 15.7\u0026ndash;15.3 Ma are also observed, albeit with some offset, in the Site 608 sea surface temperature record regardless of variations in global temperatures (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ed, e, g). In any case, it seems that the rather gradual increase in temperature and humidity conditions in SW Europe indicated by our data between 20 and 17 Ma seems to rule out volcanism as the main driver of the MCO, since it experienced a rather abrupt activity peak around the optimum between 17.2 and 15.3 Ma (Goto et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Instead, the apparent coupling observed between climate shifts in SW Europe and oceanic (e.g., Atlantic) and global records preceding the MCO point to gradual restriction of oceanic circulation, likely in the circum-Antarctic domain (Pagani et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1999\u003c/span\u003e), as the main driver of the MCO.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe magnetic properties (e.g., c\u003csub\u003efd\u003c/sub\u003e%) of the studied mudstones suggest an evolution of climatic conditions that differs markedly from those signalled by the fossil record of the BRN. Thus, instead of the warming and wetting trend broadly suggested by fossil fauna through the onset and peak of the MCO, enhanced pedogenic processes presumably signalling warmer and wetter conditions are found between 20.5\u0026ndash;20.3 and 18.5\u0026ndash;17.2 Ma, and mark a decline from 17.2 Ma onwards precisely during the peak of the MCO (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ef). This suggest that pedogenic formation of SP particles was not linked to warmer global temperatures or increased sea surface temperatures off the Iberian Peninsula. Noticeably, the two periods of enhanced SP formation are centred around the two intervals of evaporitic sedimentation marked by the top of the Ler\u0026iacute;n Formation and the Fusti\u0026ntilde;ana Gypsum Member (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e). Moreover, these two periods appear to coincide with maxima in the long-term, 2.4 Myr eccentricity cycle of the Earth\u0026acute;s orbit (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ei), a pacemaker widely known to amplify seasonal climate variability due to the modulation it exerts on the amplitude of precession and, therefore, on boreal summer insolation (Laskar et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). From this perspective, it seems that pedogenic formation of SP particles appears to be linked more to the alternance of wet and dry conditions, enabling the initial formation of lakes and its later evolution to evaporitic conditions, than to wet conditions alone. This interpretation, although tantalizing, faces two main problems. The first one is that the 2.4 kyr eccentricity maximum centred around 15.5 Ma is not associated with high SP contents, but rather by low concentrations of these type of particles. In this regard, it might be recalled that the correlation between moisture conditions and pedogenic formation of SP particles breaks down for exceedingly wet conditions, that lead to the transformation of SP particles into hematite and, under even wetter conditions, goethite (Liu et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Jiang et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Under this perspective, it might be argued that the low SP content detected around the eccentricity maximum at 15.5 Ma responds to specially wet conditions during the peak of the MCO and the completion of pedogenic processes well beyond the transformation of SP particles. Peak wet conditions in the BRN as imaged by sedimentological and paleontological, coupled with the conspicuous yellow colour of mudstones after 16 Ma (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb), that suggests the dominance of goethite (Jiang et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), make this interpretation plausible and testable by future studies aimed at quantifying the amount of hematite and goethite in the studied mudstones. The second problem is that the correlation of evaporitic sedimentation and 2.4 Myr eccentricity maxima inferred for the BRN area is at odds with the link established between these maxima and periods of fresh-water lake expansion deduced in the central part of the Ebro basin by other authors (e.g., Valero et al., \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). In this regard, it can be argued that other processes aside from climate conditions, such as tectonic pulses and the amount of sediment supply, are also known to affect the architecture of endorheic foreland basin sequences (Valero et al., \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The western part of the Ebro basin is considerably narrower than its central part (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), which means that all the material eroded from the neighbouring margins of the Pyrenees and the Iberian Range accumulated over a smaller area. Moreover, younger tectonic deformation in the western Pyrenees led to enhanced supply of detrital sediment to the western part of the Ebro basin during the Miocene (Curry et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Finally, the western Ebro basin is affected by a set of folds detached along Oligocene and Miocene evaporitic formations (Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) that uplifted the central fringe of the basin during the lower Miocene (Ingl\u0026eacute;s et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). All these factors have resulted in significantly higher Miocene accumulation rates in the western part of the Ebro basin (Larrasoa\u0026ntilde;a et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), which could easily explain why shallow (filled evaporitic) and deep (underfilled fresh-water) lakes coexisted in the western and central parts of the basin, respectively. If so, it follows that conditions wet enough to disrupt this situation occurred only during the 2.4 Myr eccentricity maximum coeval with the peak of the MCO, when fresh-water lacustrine sedimentation spread over the entire basin regardless of previous paleoenvironmental cotexts.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThe updated fossil small mammal record of the BRN area presented in this study, along with previous and new accompanying magnetostratigraphic results, enable the first comprehensive dating of lower Miocene Iberian fossil localities found in direct stratigraphic continuity with middle Miocene faunas. These data, coupled with results from other magnetostratigraphically dated fossil localities from the Ebro and Daroca-Teruel basins, yield the first robust calibration of Iberian local biozone boundaries Y/Z (19.7 Ma) Z/A (19.2 Ma), A/B (17.15 Ma), B/C (16.64 Ma) and C/D (15.93 Ma). These results also fill an important gap in lower Miocene European biochronology, since they robustly place the MN2/3 and MN3/4 boundaries at around 19.7 and 17.15 Ma, respectively. They further indicate that some local subzones are either too short (e.g., Y2) or difficult to identify unless very rich localities are unearthed (subzones within zone D), and lead us to discourage their use in view of their limited utility for biostratigraphic purposes.\u003c/p\u003e \u003cp\u003eIn addition to this, we have used the updated small mammal content of the BRN area to provide relative records of temperature and humidity variations across the onset and peak of the MCO, that have been combined with paleoclimatic inferences based on the sedimentary evolution of the Ebro basin and with a record of pedogenic formation of magnetic particles. The small mammal faunas from the BRN area point to a gradual increase in temperature and humidity conditions in SW Europe between ca. 20 and 15.2 Ma, which appears to be punctuated by relatively warmer and wetter conditions at three intervals between 19.2\u0026ndash;18.6, 17.6\u0026ndash;17.1 and 15.7\u0026ndash;15.3 Ma. This evolution seems to bear more resemblance with the record of eastern north Atlantic surface temperatures than to global temperatures, which is consistent with the main role of the eastern north Atlantic as the source for moisture in SW Europe. In any case, the apparent coupling observed between climate shifts in SW Europe and Atlantic and global records between 20 and 17 Ma points to gradual changes in oceanic (likely peri-Antarctic) circulation as the main driver of the MCO, as opposed to other mechanism (e.g., volcanism) whose activity underwent a more abrupt intensification restricted to the peak of the MCO after 17 Ma.\u003c/p\u003e \u003cp\u003eIn comparison with the gradual shift to wetter conditions envisaged by the small mammal faunas across the MCO, the sedimentary evolution of the BRN in particular, and of the Ebro basin in general, appears to mark a more abrupt response. The most likely interpretation is that the hydrological balance of the basin responded in a non-linear fashion to humidity conditions, so that an overall lacustrine expansion during the peak of the MCO occurred after certain humidity threshold was achieved. As opposed to these shifts, paleoenvironmental variations recorded by the relative concentration of SP particles point to enhanced pedogenic conditions between 20.5\u0026ndash;20.3 and 18.5\u0026ndash;17.2 Ma, coinciding with maxima in the long-term cycle of the eccentricity of the Earth\u0026acute;s orbit but with no clear link with development of the MCO. This suggest that pedogenic formation of SP particles was not linked to regional or global warmer and wetter conditions, but with periods of enhanced seasonal climate variability. These results highlight the importance of combining different paleoenvironmental indicators, because each of them can provide information on different paleoclimatic variables and it is only their combination what can produce a comprehensive understanding of past paleoenvironmental shifts undergone by continental areas in response to climate change.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of Interest\u003c/h2\u003e \u003cp\u003eOn behalf of all authors, the corresponding author states that there is no conflict of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis study was supported by project BARCLIM (BR23/5), granted by the Junta de Bardenas Reales de Navarra, and by the Consolidated Research Group IT-1602-22 of the Basque Government Research System. This work is dedicated to our dear friend and colleague Pilar Mata, who passed away much too soon this 2024.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAgust\u0026iacute;, J., Cabrera, L., Garc\u0026eacute;s, M., Krijgsman, W., Oms, O., \u0026amp; Par\u0026eacute;s, J. M. (2001). A calibrated mammal scale for the Neogene of Western Europe. State of the art. \u003cem\u003eEarth-Science Reviews\u003c/em\u003e, \u003cem\u003e52\u003c/em\u003e, 247\u0026ndash;260.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAgust\u0026iacute;, J., P\u0026eacute;rez-Rivar\u0026eacute;s, F. J., Cabrera, L., Garc\u0026eacute;s, M., Pardo, G., \u0026amp; Arenas, C. (2011). The Ramblian-Aragonian boundary and its significance for the European Neogene continental chronology. 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Iron oxide characteristics of mid-Miocene Red Clay deposits on the western Chinese Loess Plateau and their paleoclimatic implications. \u003cem\u003ePalaeogeography Palaeoclimatology Palaeoecology\u003c/em\u003e, \u003cem\u003e468\u003c/em\u003e, 162\u0026ndash;172.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-iberian-geology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jibg","sideBox":"Learn more about [Journal of Iberian Geology](http://link.springer.com/journal/41513)","snPcode":"41513","submissionUrl":"https://www.editorialmanager.com/jibg/default2.aspx","title":"Journal of Iberian Geology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Iberian Peninsula, Ebro basin, magnetochronology, small mammals, environmental magnetism, Miocene Climate Optimum","lastPublishedDoi":"10.21203/rs.3.rs-4447195/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4447195/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe chronology of lower Miocene Iberian small mammal faunas is still poorly constrained given the scarcity of well dated sedimentary successions including small mammal fossil localities. Such scarcity has prevented also an accurate understanding of the response of European terrestrial ecosystem to global changes across the Miocene Climate Optimum (MCO), one of the best analogues of present-day global warming. Here we present an updated fossil small mammal record of the Bardenas Reales de Navarra (western Ebro basin, Spain), where an expanded lower to middle Miocene continental succession is superbly exposed. Previous and new magnetostratigraphic results from this succession have enabled us to propose, along with additional magnetostratigraphically-dated Iberian faunas, a new chronology for local zones Y to D (Mammal Neogene zones MN2 to MN5). In addition to that, the studied small mammal faunas point to a gradual increase in temperature and humidity conditions in SW Europe between 20 and 15.5 Ma, which appears to be coupled with the progressive shift towards warmer regional (Atlantic) and global conditions across the MCO, thereby pointing to gradual changes in oceanic circulation as the main driver of this period of global warmth. The evolution of sedimentary facies appears to indicate a threshold response of the Ebro basin hydrological balance to the MCO, whereas pedogenic formation of magnetic minerals seems to be linked to periods of enhanced climate variability. These results highlight the need of combining different paleoenvironmental indicators in order to obtain a reliable view of the response of continental ecosystems to global warming.\u003c/p\u003e","manuscriptTitle":"Calibrating lower-middle Miocene mammal faunas and unravelling climate change during the Miocene Climate Optimum; the Bardenas Reales de Navarra record (Ebro basin, NE Iberian Peninsula)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-03 17:10:24","doi":"10.21203/rs.3.rs-4447195/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2024-05-22T11:39:20+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-05-21T10:58:49+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Journal of Iberian Geology","date":"2024-05-21T09:24:50+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-05-20T11:37:27+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Iberian Geology","date":"2024-05-20T03:00:15+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-iberian-geology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jibg","sideBox":"Learn more about [Journal of Iberian Geology](http://link.springer.com/journal/41513)","snPcode":"41513","submissionUrl":"https://www.editorialmanager.com/jibg/default2.aspx","title":"Journal of Iberian Geology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"69c4d42b-509d-474b-9d34-4dba42fc0ef5","owner":[],"postedDate":"June 3rd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-10-28T16:03:19+00:00","versionOfRecord":{"articleIdentity":"rs-4447195","link":"https://doi.org/10.1007/s41513-024-00265-7","journal":{"identity":"journal-of-iberian-geology","isVorOnly":false,"title":"Journal of Iberian Geology"},"publishedOn":"2024-10-22 15:57:41","publishedOnDateReadable":"October 22nd, 2024"},"versionCreatedAt":"2024-06-03 17:10:24","video":"","vorDoi":"10.1007/s41513-024-00265-7","vorDoiUrl":"https://doi.org/10.1007/s41513-024-00265-7","workflowStages":[]},"version":"v1","identity":"rs-4447195","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4447195","identity":"rs-4447195","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}