Mid-Miocene palaeohydrology archived in paleosol of La Tatacoa, Colombia

Mid-Miocene palaeohydrology archived in paleosol of La Tatacoa, Colombia | 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 Mid-Miocene palaeohydrology archived in paleosol of La Tatacoa, Colombia Susana Salazar-Jaramillo, Juan Carlos Loaiza-Usuga, José Luis Sotelo Buitrago This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7594667/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract The mid-Miocene paleosols of the Baraya Member (Villavieja Formation, La Tatacoa, Colombia) preserve a detailed palaeohydrological record during the Middle Miocene Climatic Transition. Developed in distal flood basins of meandering fluvial system, the Ferruginous Red Bed (FRB) paleosols and La Venta Red Bed (LVRB) paleosols represent cumulative, polygenetic soils strongly shaped by alternating wet–dry cycles. Geochemical and micromorphological evidence reveal distinct pedogenic pathways. The FRB exhibits extreme weathering (Al₂O₃ up to 26.7%; CIA > 89), depletion of base cations, and abundant clay coatings, gley pedofeatures, and Fe–Mn nodules, reflecting illuviation and reduction–segregation processes under wetter udic conditions. In contrast, the LVRB shows vertic features, homogenized CIA values (~ 80–88%), and clay infillings within shrinkage cracks, recording pedoturbation and stronger seasonal drying (udic–ustic). Secondary carbonates in both profiles indicate periodic decalcification–reprecipitation, though aridic conditions were not reached. These paleosols classify as Alfisol (Udalf) and Vertisol (Udert/Ustert) respectively, both reflecting subhumid climates. Integration with the fossil record indicates a transitional riparian mosaic rather than a continuous rainforest habitat associated with the La Venta Fauna. The transition zone is determined by two intertwined factors rainfall (~ 1000–2000 mm/year) and at least one moderate dry season (~ 3 months). These findings underscore the role of soil geomorphological position and rainfall seasonality as primary drivers of redox processes and soil moisture balance, while situating Neotropical palaeosols within the broader framework of mid-Miocene climate dynamics and the northward migration of the Intertropical Convergence Zone. paleoclimate paleosols soil moisture regimes tropical soils soil micromorphology Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction The mid-Miocene was a turning point in Earth’s climate system, marked by the transition from the Mid-Miocene Climatic Optimum (MMCO) to subsequent global cooling associated with the expansion of the Antarctic ice sheet (Holbourn et al., 2010). Around ~ 13.1 Ma, the establishment of permanent Antarctic glaciation coincided with a northward migration of the Intertropical Convergence Zone (ITCZ), which produced stronger rainfall seasonality and intensified chemical weathering across the tropical belt (John et al., 2003 ; Holbourn et al., 2010). These global dynamics provide an essential framework for evaluating how Neotropical ecosystems and landscapes responded to shifts in hydrology and precipitation regimes. Within this context, La Tatacoa (Huila, Colombia) stands out as one of the most important Neotropical fossil localities. The La Venta Fauna, preserved in the Villavieja Formation, represents one of the richest Neogene vertebrate assemblages of South America (Kay and Madden, 1997 ; Catena et al., 2017; Cadena et al., 2020 ). Yet its palaeoclimate remains contested: fossil flora and fauna indicate humid tropical environments (Wellman, 1970 ; Kay and Madden, 1997 ; Guerrero, 1997 ), while the pedogenic properties of the associated paleosols point toward drier and highly seasonal conditions (Flórez-Molina et al., 2013; Flórez-Molina et al., 2018 ). This apparent contradiction has fueled debate over the environmental backdrop of the Tatacoa region during the Serravallian. Clarifying whether the soils formed under wetter sub-humid or drier semi-arid regimes is critical for reconciling these records and for understanding habitat heterogeneity within the La Venta ecosystem. Paleosols provide unrivaled terrestrial archives of palaeoclimate and palaeohydrology, preserving signatures of weathering, drainage, and vegetation cover (Alekseev et al., 2019 ; Loaiza-Usuga et al., 2022 ; Orr and Roberts, 2024 ). In La Tatacoa, the lowermost red-bed paleosols of the Baraya Member (Fig. 2 A; Guerrero, 1997 ) offer a unique opportunity to examine mid-Miocene soil hydrology. These profiles—the Ferruginous Red Bed (FRB) and the La Venta Red Bed (LVRB; equivalent to the Lower Red Beds of Fields, 1959)—formed in overbank deposits of meandering rivers and are chronologically constrained between Chron C5AAn (13.183–13.032 Ma; Flynn et al., 1997 ; Ogg, 2012) and Chron C5Ar.3r (13.032–12.887 Ma), spanning ~ 120 kyr (Salazar-Jaramillo et al., 2021). This study aims to characterize the pedogenetic processes of the FRB and LVRB paleosols through micromorphology, major oxide geochemistry, and clay mineralogy, to reconstruct soil moisture regimes, and to evaluate whether they record a transition from wetter sub-humid to drier sub-humid conditions. By integrating soil-based evidence with the broader context of mid-Miocene climate dynamics, we hypothesize that the La Tatacoa paleosols reflect hydrological variability driven by ITCZ migration, and that their contrasting moisture regimes help explain the mosaic of habitats that sustained the La Venta Fauna. 2. Geological and stratigraphic setting The Tatacoa area, located north of Villavieja (Huila, Fig. 1 ), belongs to the Neiva Sub-basin of the Upper Magdalena Valley (UMV; Villarroel et al., 1996). The UMV constitutes a major basin formed during the northern Andean orogeny, where repeated deformation episodes controlled both the structural framework and the style of sedimentation (Cediel et al., 2003 ). Bounded by reverse faults extending from the bifurcation of the Central and Eastern Cordilleras (Mojica and Franco, 1990 ), the basin accumulated thick successions of continental deposits whose evolution was closely tied to Andean uplift. Before the rise of the Eastern Cordillera, the region was part of a vast “pan-Amazonian” fluvial system connected to the Pebas mega-wetland, draining toward the present Amazon, Orinoco, and Magdalena fluvial basins (Hoorn et al., 2010 ; Bicudo et al., 2019 ; Kirschner and Hoorn, 2020 ). The sedimentary fill includes Jurassic volcano-sedimentary rocks of the Saldaña Formation, sourced mainly from the Central Cordillera (Villarroel et al., 1996). These are unconformably overlain by the Honda Group, which is subdivided into the La Victoria and Villavieja formations (Guerrero, 1997 ; Fig. 2 A). The Honda Group records a transition from gray mudstones and sandstones of the La Victoria Formation to the reddish mudstones and paleosol-rich successions of the Villavieja Formation, the latter being the focus of this study. Recent chronostratigraphic reevaluations using Bayesian modeling of U-Pb zircon data have reinterpreted the age of the Honda Group, extending the sedimentary record of the Honda Gorup from ~ 16.0 Ma at the base to ~ 10.5 Ma at the top. (Mora et al., 2023). This updated model suggests the fossiliferous sequence of La Venta is slightly older than previous estimates. Above the Honda Group lies the Gigante Formation, of Tortonian age (~ 8.5 Ma; van Houten, 1976 ; Guerrero, 1997 ), characterized by gravels interbedded with volcanic deposits of ash, pumice, and sand (Takemura and Danhara, 1983). A major tectonic event at ~ 12.9 Ma initiated the uplift of the Eastern Cordillera (Guerrero, 1997 ), which by ~ 6–3 Ma had reached elevations sufficient to act as an orographic barrier (Mora et al., 2008). This uplift reorganized drainage networks, enhanced sediment supply, and promoted the deposition of braided river systems such as the La Cerbatana Conglomerate, which marks the boundary between the La Victoria and Villavieja formations (Guerrero, 1997 ). Within the Villavieja Formation, the Baraya Member is particularly significant for its fossiliferous horizons and paleosols. These soils directly overlie the Monkey and Fish beds of the La Venta Fauna, one of the most important Neotropical vertebrate assemblages of the Miocene (Kay and Madden, 1997 ; Catena et al., 2017; Cadena et al., 2020 ). A recent chronostratigraphic model by Mora et al. (2023) proposes and revised age range for the Villavieja Formation; however, for the purpose of this study, the Baraya Member is considered within its established chronostratigraphic framework (Guerrero, 1997 ), where it directly overlies the Cerbatana Conglomerate and include the Monkey and Fish beds of the La Venta Fauna (Kay and Madden, 1997 ; Catena et al., 2017; Cadena et al., 2020 ). Sedimentological evidence indicates that the Baraya Member was deposited in a low-energy meandering fluvial system, with small channels (2–3 m deep), point-bar sequences, and extensive overbank deposits (Guerrero, 1997 ). Fine-grained floodplain sediments favored the development of cumulative soils under low sedimentation rates, generating polygenetic paleosols with distinct pedogenic features (Kraus, 1999 ). Their position in distal flood basins strongly conditioned soil hydrology: poorly drained topography promoted water stagnation and redox reactions, while alternations of wet and dry periods enhanced clay illuviation, vertic features, and carbonate precipitation. These palaeotopographic and hydrological controls are well expressed in the Ferruginous Red Bed (FRB; Fig. 2 B) and La Venta Red Bed (LVRB; Fig. 2 C) profiles, situated ~ 50 m and ~ 100 m above the Cerbatana Conglomerate sampled at 3°13′ 38.03′′ N, 75° 8′53.19′′W and at 3°14′6.88′′ N, 75° 9′ 16.22′′W, respectively (Fig. 2 ). These profiles preserve micromorphological and mineralogical evidence of mid-Miocene soil moisture regimes, offering critical insights into the palaeohydrology of La Tatacoa. 3. Methods Two paleosol profiles were selected for study: the Ferruginous Red Bed (FRB) and the La Venta Red Bed (LVRB). Their stratigraphic positions within the Baraya Member are well established (Guerrero, 1997 ; Fig. 2 ). Profile selection was based on their stratigraphic continuity, preservation of pedogenic features, and proximity to key fossiliferous horizons. Field descriptions followed the guidelines of the Soil Survey Manual (Soil Science Division Staff, 2017), and classification employed both Soil Taxonomy (Soil Survey Staff, 2022 ) and the World Reference Base for Soil Resources (IUSS Working Group WRB, 2022 ). Detailed morphological descriptions were undertaken, and samples were collected systematically for multi-proxy analysis, including physical, chemical, mineralogical, and micromorphological studies (Loaiza-Usuga et al., 2015 ). Undisturbed blocks for thin-section preparation were processed at the IGAC National Soil Laboratory (Bogotá, Colombia). Samples were first air-dried for two months at room temperature, then impregnated with polyester resin (ref. 744) and hardener (ref. 2744) in a 5:1 ratio, with 0.75 g acetone as a diluent, following the protocol of Murphy ( 1986 ). After six weeks of curing, samples were cut with a diamond saw, mounted with resin, and further thinned using the Petrothin Buehler system to a standard thickness of ~ 20 µm. Micromorphological observations were conducted with an Olympus CX31 microscope, and descriptions followed the criteria of Stoops ( 2021 ) and Loaiza-Usuga et al. ( 2015 ). X-ray diffraction (XRD) analyses were performed on both bulk powders and oriented clay fractions at the Lithogeochemical Characterization Laboratory, National University of Colombia (Bogotá). Clay separates were prepared using the pipette method to obtain oriented mounts on glass slides. Each sample was analyzed under four conditions: bulk, untreated oriented, glycolated, and calcined (515–550°C, 4 h). Diffraction patterns were obtained using a Bruker D2 Phaser diffractometer with Ni-filtered Cu-Kα radiation (30 kV, 10 mA). Scans were run from 2.5° to 40° 2θ, with a step size of 0.014°, a dwell time of 0.1 s/step, and a PSD detector angle of 5.33°. Mineral phases were identified and semi-quantified with DIFFRAC.EVA V4.2.2 software, and clay minerals were verified following Thorez ( 1976 ) and Chen ( 1977 ). For X-ray fluorescence (XRF) analysis, a 1.0-gram paleosol subsample was diluted with a lithium borate flux (10:1) and fused into a 37 mm glass disc. Major and minor element oxide concentrations (Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, K₂O, CaO, TiO₂, MnO, Fe₂O₃) were determined using wavelength-dispersive X-ray fluorescence (WD-XRF) on a Philips MagixPro PW-2440 spectrometer. The analysis, performed at the National University of Colombia, employed a semi-quantitative method calibrated against a suite of certified reference materials (NIM-G, NIM-S, NIM-N, NIM-P). Based on replicate measurements and deviations from certified values, the reported oxide data have an estimated accuracy error of approximately 5% relative. To interpret the intensity and processes of weathering during pedogenesis, a suite of geochemical proxies was employed. The Chemical Index of Alteration (CIA), calculated as [Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O)] × 100, serves as a primary index to quantify the total degree of chemical weathering by measuring the proportion of stable alumina relative to mobile cations (Nesbitt & Young, 1982 ). The extent of clay mineral formation, or argillization, was assessed using the Al/Si ratio (Ashley & Driese, 2000 ; Retallack, 2001 ). The loss of base cations through leaching and hydrolysis was evaluated using the ratio Al/(Na + Ca + K + Mg) (Ashley & Driese, 2000 ; Hamer et al., 2007 ). Finally, specific pedogenic pathways were identified using the Na/K ratio to indicate salinization processes and the Ca/Mg ratio to indicate calcification ( e.g ., Flaig et al., 2011 ). To quantitatively reconstruct mean annual precipitation (MAP) during the period of pedogenesis, climofunctions based on bulk geochemistry were applied. The CALMAG proxy ([Al₂O₃/(Al₂O₃+CaO + MgO)]*100) provided a precise estimate via the equation MAP = 22.69(CALMAG) − 435.80 (R² = 0.90; SE = ± 108 mm; Nordt & Driese, 2010 ). The CIA offered a broader assessment using MAP = 221e^(0.0197*CIA) (R² = 0.72; SE = ± 182 mm; Nesbitt & Young, 1982 ), allowing for interpretation through two proxies. Field and laboratory data were integrated to reconstruct soil moisture regimes and pedogenic environments. Taxonomic classification (Soil Survey Staff, 2022 ; IUSS Working Group WRB, 2022 ) provided the basis for assigning moisture regimes (udic–ustic), while micromorphological features guided the identification of specific pedogenic processes such as redoximorphism, argilluviation, calcification, and vertization. Clay mineralogy and major oxides geochemistry served as a complementary proxy for weathering intensity and soil hydrological conditions. Interpretations were framed in relation to established paleoclimate proxies, enabling reconstruction of wet–dry cycles and their broader climatic significance during the mid-Miocene. 4. Results 4.1. Paleosol Morphology and Classification The two studied paleosol profiles from the Baraya Member exhibit distinct macromorphological characteristics that reflect their different pedogenic histories and environmental conditions (Fig. 2). Their features are summarized in Table 1. Ferruginous Red-bed (FRB) Paleosol Features The FRB paleosol profile is thick (~9 m) and is dominated by well-developed Bt argillic horizons. These horizons exhibit strong, primarily subangular blocky (sbk) and columnar (p) structures, indicating a high degree of pedogenic development. The consistence is predominantly hard (h) to very firm (vfir), plastic (p), and sticky (s). A key macromorphological feature is the pervasive gleying, indicated by the dominant grey to greyish-green (Gley) Munsell colors across most horizons ( e.g ., Gley1 7/10Y, Gley2 8/10G), often occurring alongside reddish ( e.g ., 2.5YR 5/8) and yellowish mottles. This suggests fluctuating redox conditions due to periodic saturation. Evidence of clay illuviation (argilluviation) is common, with many horizons containing clay coatings (cc) on ped faces and within channels. The upper part of the profile contains two C horizons (FRB 16, 17) with a sandy texture and a coarse/fine (c/f) related distribution estimated between 2/1 and 3/1. Carbonate nodules are present in the uppermost horizon (FRB 18), though this feature is often masked by the more dominant clay coatings and gley colors. The presence of root channels (rch) throughout the profile indicates significant biological activity. La Venta Red-bed (LVRB) Paleosol Features The LVRB paleosol is also well-developed (~9 m thick) and is characterized by horizons with vertic properties. These are evidenced by the common presence of slickensides (ss) and wedge-shaped structures, derived from the expansion and contraction of clays. The structure is a mix of subangular blocky (sbk), columnar (p), and prismatic, with a consistence that is very firm (vfir), plastic (p), and sticky (s). In contrast to the FRB paleosol, the matrix colors are dominantly reddish ( e.g ., 5YR 4/4, 5YR 5/6), indicating better drainage and more oxidizing conditions, though gley mottles (g) are present in some horizons, suggesting periodic wetness. Impregnations of iron oxides and clay coatings (cc) filling channels and fissures are common. The textural distribution in the Bt horizons is dominantly porphyric (c/f ~1/8), confirming the clay-rich nature of the parent material. The base of the profile is a C horizon (LVRB 1) with prominent gley colors (Gley1 7/5GY) and a sandy texture, which contains bone remains (br) and subrounded quartz grains (Qs), linking it to the rich paleontological heritage of the area. Taxonomic Classification as Alfisols and Vertisols Based on the diagnostic morphological features we classified the FRB paleosol as an Alfisol. This classification is supported by the presence of argillic (Bt) horizons with significant clay illuviation (clay coatings) and a base saturation >35% based on the mineral association and previous studies (Flórez-Molina et al. , 2018). The pervasive gleying and redoximorphic features indicate a soil moisture regime that was seasonally saturated, aligning with an Udalf suborder (udic moisture regime). By contrast, we classified the LVRB paleosol as a Vertisol. This is determined by the vertic properties, including slickensides and wedge-shaped structures, formed by the shrink-swell behavior of smectitic clays. The combination of redoximorphic features (mottles) with a generally reddish, well-drained matrix suggests a seasonally contrasting moisture regime, supporting a Udert suborder (udic moisture regime). 4.2. Micromorphological Features Micromorphological analysis revealed a suite of pedogenic features that elucidate the paleoenvironmental conditions during the formation of the FRB (Alfisol) and LVRB (Vertisol) paleosols. The key characteristics are categorized and described below and summarized in Table 2 and shown in Figure 3. Redoximorphic Characteristics and Stagnic Properties The FRB paleosol is dominated by redoximorphic features indicating periodic saturation. Key evidence includes gleyed, brown dotted micromasses with stipple speckled b-fabric (Fig. 3a, b), and dense incomplete Fe-Mn oxide infillings within planes and channels set in a moderately to strongly impregnated yellowish and reddish micromass (Fig. 3f). Abundant typic and anorthic Fe and Mn nodules are common throughout the profile. These features collectively point to fluctuating redox conditions driven by a seasonally high-water table. Clay Coatings, Slickensides, and Vertic Features Both profiles show evidence of clay illuviation, but with different implications. The FRB paleosol exhibits well-developed, continuous clay coatings (argillans) on ped faces and in channels (Fig. 3f, XPL), confirming the argillic horizon designation. By contrast, the LVRB paleosol is defined by its vertic features. The most remarkable characteristics are the prominent shrinkage cracks (Fig. 3h, i) and the presence of slickensides. The process of clay illuviation is ongoing, evidenced by impure clay coatings in the groundmass and infilling planes and channels (Fig. 3f). Pedoturbation from biological activity is also indicated by relict features (Fig. 3d). Iron and Manganese Oxide Distributions The distribution of Fe oxides differs between the two paleosols, reflecting their drainage regimes. In the FRB paleosol, Fe oxides are primarily present as impregnations in the micromass and as hypocoatings and quasicoatings resulting from reduction-segregation processes. In the LVRB paleosol, the distribution indicates a more oxidizing environment. Well-developed hematite is a key feature (Fig. 3k), and Fe oxides are also found within cracks (Fig. 3j), as coatings, and as illuviated material within pores. Carbonate Pedofeatures Carbonate features are present in specific contexts. In the FRB paleosol, carbonates occur as micritic crystallitic b-fabric (Fig. 3b), typic nodules, and calcitic infillings inside root channels (Fig. 3c). In the LVRB paleosol, the basal C horizon contains carbonate hypocoatings and micritic crystallitic b-fabric (Fig. 3l), associated with abundant lithic fragments, quartz, and bone remains, indicating a less pedogenically altered parent material. 4.3. Geochemistry and Pedogenic Processes Clay mineralogy Figure 4 shows the XRD results for bulk and oriented (glycolated) samples, which reveal a clay mineralogy dominated by smectite, illite, illite/smectite mixed-layer clays, and kaolinite. The 001 smectite peak expands to ~17 Å upon ethylene glycol solvation. Illite is present as a discrete phase, exhibiting stable 10.1 Å and 5.0 Å reflections in natural, glycolated, and calcinated preparations. A broad 8.58 Å peak in glycolated samples signifies illite/smectite mixed-layering. The identification of kaolinite is validated by the disappearance of its peaks post-calcination. The presence of hematite is confirmed by its 2.69 Å (002) reflection. Weathering intensity indicators and paleoprecipitation (MAP) The XRF data (Table 3) highlights a pedogenic contrast between the FRB and LVRB paleosols, reflecting differences in weathering intensity and associated paleoclimatic conditions (Fig. 5). Chemical Index of Alteration (CIA), argillization (Al/Si), leaching–hydrolysis [Al/(Na+Ca+K+Mg)], and mean annual precipitation (MAP) show strong positive correlations, whereas salinization (Na/K) and calcification (Ca/Mg) indices display negative correlations with CIA and the other weathering-related parameters. CIA Values: The FRB paleosol shows a wide range of CIA values (66.58 - 90.23), indicating a strong weathering gradient within the profile. The lower (older) horizons (FRB7, FRB8) exhibit intense chemical weathering (CIA > 89), characteristic of advanced pedogenesis. By contrast, the upper (younger) horizons (FRB16-18) show significantly lower CIA values (66 - 74), suggesting weaker weathering or a closer affinity to the unaltered parent material. The LVRB paleosol displays consistently high but slightly less intense weathering, with CIA values tightly clustered between 80.01 and 87.64. In both profiles, an increase in CIA reflects intensified leaching of base cations and enhanced hydrolysis, while salinization and calcification decrease accordingly, confirming the weathering trend (Fig. 5). The CALMAG and CIA-K derived MAP estimates are broadly consistent, though CALMAG typically yields higher values. This discrepancy is expected due to the different chemical bases of each proxy. Consistently with CIA values, FRB MAP values at the bottom (older (FRB7, FRB8) indicate a very high precipitation (~1680-1300 mm/yr) in the most weathered horizons, while the upper horizons (FRB16-18) indicate a much drier climate (~1250-820 mm/yr). This suggests the FRB profile represents a cumulative paleosol where the lower part formed under a significantly wetter climate than the upper part. The LVRB paleosol formed under consistently high precipitation, with MAP estimates ranging from ~1630 to 1100 mm/yr. The climate was humid but potentially less so than the peak wet conditions recorded at the top of the FRB sequence. Pedogenic processes Argillization (Al/Si): The FRB paleosol shows higher Al/Si ratios (up to 0.42) horizons compared to the LVRB (max 0.33), indicating a greater abundance of clay minerals. However, a more intense hydrolysis is reflected in the lower horizons (FRB7, FRB8), where CIA > 89, and also along the LVRB profile with CIA between 80.01 and 87.64. This suggests in situ clay formation and supports the interpretation of a wetter climate during its formation. The upper horizons (FRB16-18) may be more detrital in origin ( e.g . Salazar-Jaramillo et al., 2022). Salinization vs. carbonation (Na/K and Ca/Mg): The high Na/K and Ca/Mg values in the less-weathered FRB horizons (FRB16-18) suggest salinization and carbonation were active processes under drier climate conditions. Since the geochemistry of the LVRB is more homogeneous (due to pedoturbation), the salinization index is also more uniform, showing consistent values throughout the profile. By contrast, carbonation shows localized enrichments, as expected, due to the presence of calcium carbonate (fossil remains of bones) in LVRB. 5. Discussion Developed in the overbank of meandering rivers (2–3 m depth, 20–35 m width and a meander belt amplitude of 190–350 m; Guerrero, 1997 ), the FRB and LVRB polygenetic paleosols are of cumulative nature ( e.g . Kraus and Hasiotis 2006). During the time interval that represents both profiles (~ 120 kyr), low sedimentation rates (~ 0.413 mm/yr; Guerrero, 1997 ) favored pedogenesis, yielding well-developed paleosol in the distal side of the fluvial channels ( i.e . the floodbasin; Kraus, 1999 ). These floodplain profiles, known as cumulative soils, developed when the erosion process became insignificant and sedimentation was constant (Kraus, 1999 ). Thus, pedogenetic processes were favored ( i.e . increased pedogenesis), giving rise to polygenesis. In the floodbasin, soils became progressively more poorly drained because the topographic position was lower and the sediment was finer-grained and, thus, less permeable (Kraus, 1999 , Lizzoli et al., 2025 ). Soil drainage in the topographic lows is clearly subject to seasonality, which, in turn, responds to the balance between water input by precipitation/runoff and water loss to evaporation and transpiration (Alekseev et al., 2019 , Loaiza-Usuga et al., 2022 , Lizzoli et al., 2025 ). For this reason, the pedogenetic features evidenced by the micromorphology suggest that FRB and LVRB paleosol development and maturity were controlled to a greater extent by changes in drainage conditions. As a consequence, processes such as fersialitization, ferrugination, illuviation of clays, redox processes, hydromorphism (stagnic characteristics), leaching of base cations, reddening (rubefaction) and, to a lesser extent, carbonation, took place. Clay mineralogy (Fig. 4 ), on the other hand, allowed us to identify additional processes of vermiculitization and smectitization. 5.1. Pedogenic processes in detail Pedogenesis in the FRB and LVRB paleosols reflects the interplay of multiple processes—fersiallitization, rubefaction, clay illuviation, vertic processes, carbonate cycling, and redoximorphism—superimposed within cumulative flood basin soils. The combined micromorphological and geochemical evidence indicates in situ weathering and soil development, rather than simple inheritance from alluvial parent material. While the micromorphology reveals the processes and fabrics—the how of pedogenesis—, the bulk geochemistry quantifies the intensity and composition—the how much of weathering. Together, these approaches allowed us to reconstruct the soil development driven by climate and landscape change in the Baraya Member. Weathering intensity and clay formation and illuviation (argillization) Lower horizons of the FRB profile show intense in situ weathering. They are highly enriched in Al₂O₃ ( e.g ., FRB7: 26.68%; FRB8: 26.11%; Table 3 ), the most immobile major element, reflecting the leaching away of other elements, concentrating alumina. Correspondingly, CIA values exceeding 89 provide quantitative confirmation of extreme weathering and hydrolysis. This signal is reinforced by the depletion of base cations such as CaO, Na₂O, MgO, and K₂O ( e.g ., FRB7: CaO = 0.90%, Na₂O = 0.53%). Micromorphological observations complement these results: thin sections reveal abundant clay coatings and illuviation features ( e.g ., FRB14, FRB12, FRB10; Table 2 ), the physical imprint of translocated Al-rich clays. In the LVRB profile micromorphological observations provide compelling evidence that clay translocation was also an active process. Features such as “impure clay coatings,” “clay-dense incomplete infillings” (LVRB4), and “clay with Fe–Mn oxide discontinuous infillings” (LVRB6) demonstrate the mobilization and deposition of fine materials along structural voids (Table 2 ). Unlike the FRB, where clay coatings develop on ped surfaces and within pores in a relatively stable Alfisol environment, the LVRB illuviation occurs through shrinkage cracks characteristic of Vertisols. The association of clays with Fe and Mn oxides further indicates periodic redox activity linked to wet–dry cycles. Bulk geochemistry supports this interpretation. Elevated Al₂O₃ concentrations ( e.g ., LVRB4: 21.35%; Table 3 ) in the B horizons reflect the enrichment of aluminous clays, quantitatively documenting the same process observed microscopically. Together, these datasets reveal that while both FRB and LVRB paleosols record clay illuviation, the operative mechanisms differ significantly. In the FRB, illuviation manifests as clay coatings deposited in pores by illuviation. In the LVRB, clays are flushed into cracks during wet phases, only to be redistributed and homogenized during subsequent swelling events. Based on the observed pedogenic features, the reddening of the paleosols is interpreted as the product of in-situ fersiallitization and rubefaction, rather than an inherited characteristic from sedimentary provenance ("brown alluvium" sensu van Houten, 1972 ). Fersiallitization, the in-situ weathering of primary silicates to form 2:1 clays (Targulian and Krasilnikov, 2007 ), is evidenced by neoformed smectite and vermiculite (Fig. 4 ), stipple speckled b-fabric, and subangular blocky microstructure (Figs. 3 a-c). Concurrent rubefaction occurred as iron, released during weathering, oxidized and precipitated as authigenic hematite (Fig. 3 k), binding to clays (Figs. 3 h-j) and forming features such as iron nodules, impregnations, and hypocoatings (Table 2 ; Figs. 3 f-g). A nucleic Fe oxide nodule forming around a muscovite grain (Fig. 3 g) provides definitive evidence for this in-situ pedogenic origin, which requires specific conditions including Fe-bearing parent material, warm temperatures, and poor drainage to immobilize silica via 2:1 clay formation (Duchaufour, 1982; Loaiza-Usuga et al., 2022 ). The clay mineral assemblage (Fig. 4 ) captures an intermediate stage of pedogenic evolution, revealing a weathering sequence that began with vermiculitization and smectitization but did not proceed to terminal ferrallitization. Vermiculitization was triggered by the oxidation and leaching of Fe²⁺ and Mg²⁺ from mineral structures (Velde and Meunier, 2008 ). The released cations, particularly Mg²⁺ and Fe²⁺, subsequently became available for the neoformation of Fe-rich smectitic clays (Velde and Meunier, 2008 ; Lizzoli et al., 2025 ; Salazar-Jaramillo et al., 2022 ). Concurrently, the presence of well-crystallized kaolinite, likely authigenic given the plagioclase-rich parent material (van Houten and Travis, 1968; Wellman, 1970 ; Anderson et al., 2016 ), signals the onset of the next stage: desilication and ferrugination, involving the neoformation of simpler 1:1 clays (Duchaufour, 1982; Lizzoli et al., 2025 ). The co-occurrence of these 2:1 and 1:1 clay minerals indicate a slowdown of weathering. Thus, the FRB and LVRB paleosols exemplify Duchaufour's (1982) first stage of ferruginous soil development, retaining strong fersiallitic characteristics rather than evolving into highly advanced, kaolinite-dominated ferrallitic profiles. Redoximorphic processes The FRB and LVRB paleosols display clear evidence of redoximorphic processes, though with contrasting intensities and expressions. In the FRB, elevated Fe₂O₃ values ( e.g ., FRB11: 10.47%; FRB5: 8.26%) correspond to micromorphological features such as gleyed micromass, Fe–Mn nodules, oxide infillings, and hypocoatings (FRB14, FRB12; Table 2 ). Grey-blue hues in FRB17, FRB16, and FRB14 record anoxic conditions and prolonged water saturation, while reddish to dark red matrices (Fig. 3 f) indicate that much of the pore system remained oxygenated for most of the year (Loaiza-Usuga et al., 2017 , 2022 ). These alternating colors reflect fluctuating water tables and the alternation between oxic and anoxic regimes typical of reduction–segregation pedogenesis. Redoximorphic features such as coatings and hypocoatings in the basal mass, together with iron nodules (Fig. 3 f–g), confirm the mobilization of Fe/Mn under reducing conditions and their subsequent reprecipitation during oxidation (Vepraskas et al., 2018 ; Kovda and Mermut, 2018 ). A nucleic Fe oxide nodule formed around muscovite (Fig. 3 g) illustrates localized precipitation of hematite during these cycles. Importantly, these pedofeatures suggest short-lived saturation or rapid fluctuations of the water table, rather than long-term waterlogging. While gley processes are typically associated with perennial groundwater influence, the FRB paleosols better match stagnic conditions (pseudogley; Pipujol and Buurman, 1994 ; IUSS Working Group WRB, 2022 ), where excess water is seasonal and restricted to wet periods. The LVRB paleosols, in contrast, developed under more oxidizing conditions. Their reddish and yellowish matrices (LVRB5, LVRB4, LVRB6; Table 1 ) reflect well-drained soil environments, with redoximorphic features largely limited to hypocoatings, quasicoatings, and Fe–Mn nodules (Table 2 ). These formed through temporary saturation within peds or following vertic shrink–swell events, when water was trapped internally before draining. Moderate Fe₂O₃ values ( e.g ., LVRB2: 6.37%; LVRB3: 6.35%; Table 3 ) correspond well with these localized features. Only the deeper LVRB1 horizon (7Cg4) shows gleyed micromass, confirming that reducing conditions were confined to less-weathered parent material beneath the active vertic zone. The geomorphic and depositional context of both paleosols helps explain these patterns. Situated in flood basin positions, far from active channels, these soils developed in fine-grained, slowly permeable sediments (Kraus, 1999 ). Under such conditions, intermittent fluvial input and low relief favor stagnic hydromorphism: soils are not saturated year-round, but seasonal rainfall and poor drainage cause temporary saturation in parts of the profile (Pipujol and Buurman, 1994 ; Lizzoli et al., 2025 ). Organic matter also plays a role in Fe–Mn cycling, as soluble organo-complexes of reduced Fe and Mn migrate short distances and subsequently reprecipitate as concretions, infillings, and coatings (Lizzoli et al., 2019; Loaiza-Usuga et al., 2022 ). The presence of hematite (Figs. 3 k, 5 ), particularly in stagnic matrices, indicates strong desiccation following seasonal anoxia, as ferrihydrite dehydrates under high temperatures, low water activity, and slightly basic pH (Pipujol and Buurman, 1994 ; Flórez-Molina et al., 2013, 2018 ; Salazar-Jaramillo et al., 2022 ). Taken together, the FRB paleosols reflect more pervasive reduction and Fe–Mn redistribution, consistent with wetter, low-relief flood basins subject to fluctuating saturation. The LVRB paleosols, though still influenced by transient stagnic conditions, were generally better drained, with redoximorphism largely confined to localized ped interiors and lower horizons. Both profiles thus capture the hydropedological consequences of seasonal waterlogging in distal flood basins, but with differing intensity and pedogenic outcomes. Parent material and pedogenic overprinting Geochemical and micromorphological evidence together clarify the nature of the parent material and its alteration by pedogenesis. High SiO₂ contents ( e.g ., LVRB7: 76.49%; FRB18: 61.12%) and the persistence of Zr and TiO₂ indicate a siliciclastic source. Micromorphology corroborates this, with porphyric groundmass textures and abundant quartz, plagioclase, and lithic grains observed in thin section (FRB4, FRB2). Fossil bone fragments in the LVRB1 C horizon explain localized P₂O₅ enrichment and link the soil directly to the La Venta fossiliferous beds. Importantly, the parent material signal is overprinted by strong pedogenesis: upper horizons are enriched in Al₂O₃, clay coatings, and Fe–Mn nodules, reflecting a progressive transformation from siliciclastic sediments into mature paleosols. Vertic properties and pedoturbation Micromorphological evidence strongly supports the role of shrink–swell dynamics as the defining pedogenic process in the LVRB paleosols (Table 3 ). In LVRB3 (5Bb5t5w5ss4g2), the presence of shrinkage cracks and a variegated pattern represents the microscopic signature of repeated expansion and contraction. Across multiple horizons, descriptions such as “subangular blocky microstructure with unaccommodated peds” (LVRB6, LVRB4) and “accommodated planes” further indicate that the soil fabric was continually reworked and sheared by swelling and desiccation. The horizon designations with “ss” ( e.g ., Bb1t1w1ss1), coupled with evidence of sheared fabrics and accommodated planes, are the micromorphological correlates of slickensides, a diagnostic feature of Vertisols. An important complement to these vertic features is the presence of clay coatings. In both profiles, illuviated materials occur in cavities, pores, and fissures, but they are notably more frequent in the LVRB. This is unusual because vertic processes typically destroy cutans (Fig. 3 h–i). Their preservation suggests that clay translocation was active even within vertic horizons, perhaps during intervals of reduced shrink–swell activity. Such features may represent an environmental shift, with diminished vertic intensity allowing illuviation to operate. In this sense, cutans in Vertisols can be interpreted as evidence of a “genetic pathway to another soil order” (Buol et al., 2011 ), signaling incipient development toward Alfisol conditions under better-drained settings. This interpretation is reinforced by hematite and iron oxide coatings (Fig. 3 f–g), which indicate more oxygenated soil environments (Kovda and Mermut, 2018 ). Geochemical data reinforce this interpretation (Table 3 ; Fig. 5 ). The LVRB paleosols exhibit consistently high CIA values (~ 80–88), but unlike the FRB, there is no strong vertical gradient. This homogeneity reflects pedoturbation: repeated shrink–swell activity mixes material from different depths, incorporating less-weathered substrates into the upper horizons and redistributing more weathered material downward. As a result, the chemical profile is homogenized, showing uniformly high but not extreme weathering intensity throughout the solum. Moderate depletion of base cations, with MgO ranging from ~ 0.64–1.27% and CaO from ~ 0.66–1.04%, indicates leaching, but at levels less intense than the more leached FRB topsoils. The constant renewal of mineral surfaces through pedoturbation retards the complete depletion of bases while sustaining advanced weathering overall. Carbonate Dynamics Carbonation and decarbonation processes complement the weathering, redox, and vertic dynamics described above, further illustrating the strong influence of seasonal wet–dry cycles on these paleosols. Macroscopically, calcic nodules and calcic horizons are well developed (Guerrero, 1997 ; Flórez-Molina et al., 2018 ), while thin sections reveal secondary carbonates within the micromass (Fig. 3 l). These appear as infillings of root channels and planes (Fig. 3 c) with stipple- and speckled-distributed micritic crystallitic b-fabrics (Fig. 3 l), confirming their secondary pedogenic origin. The seasonal mechanism of carbonate cycling is well established: during the rainy season, carbonates are solubilized and bases are leached; during the subsequent dry season, dissolved bicarbonate migrates within the profile and reprecipitates at depth (Duchaufour, 1982; Buol et al., 2011 ). This alternation not only controls carbonate redistribution but also coincides with the release of Fe and enhanced leaching of bases during wet phases. Clay illuviation, likewise linked to wet–dry alternations, occurred simultaneously within these horizons (Buol et al., 2011 ). The association of fersiallitic pedogenesis, illuviation, and vertic properties is consistent with previous descriptions of Alfisol- and Vertisol-like paleosols (Flórez-Molina et al., 2013, 2018 ), developed over Fe-rich parent materials (van Houten and Travis, 1968; Wellman, 1970 ; Anderson et al., 2016 ). Moreover, the coexistence of secondary carbonates with Fe/Mn oxides (Figs. 3 f–g, 3 l, 5 ) supports their interpretation as Alfisol– and Vertisol-like systems, in agreement with Mermut and Dasog ( 1986 ), Kovda and Mermut ( 2018 ), and Soil Survey Staff ( 2022 ). Soil Moisture Regimes, Paleosol Classification, and Paleoenvironmental Implications The FRB and LVRB paleosols of the Baraya Member display mature pedogenetic development, with Bt horizons, rubefaction overprinting hydromorphism, and evidence of alternating wet–dry cycles. Processes such as fersiallitization, rubefaction, and carbonation–decarbonation point to a strongly seasonal, subhumid climate (Driese and Ober, 2005; Alekseev et al., 2019 ; Hasiotis et al., 2007 ). Micromorphological features such as illuvial coatings, clay-filled channels, and crack infillings (Fig. 3 f) confirm that fine material was mobilized during wet phases and redistributed along fissures and pores during dry phases (Kraus and Hasiotis, 2006; Catena et al., 2016; Loaiza-Usuga et al., 2017 , 2022 ; Vepraskas et al., 2018 ). Carbonates and hematite provide additional evidence of climatic oscillations. Rapid crystallization of both occurs during intense desiccation phases (Duchaufour, 1982), whereas strong decalcification requires the higher rainfall thresholds typical of subhumid settings (Yaalon, 1997 ). Stagnic micromass (Fig. 3 f–g) demonstrates temporary waterlogging, while bases, silica, and other weathering products were retained in the dry season by capillary forces and biogeochemical cycling (Duchaufour, 1982). Together, these processes indicate a tropical wet–dry subhumid climate, marked by a pronounced dry season (max ~ 3 months but more humid than other equatorial zones where Oxisols form (Cecil, 2003; Feddema, 2005 ; Hasiotis et al., 2007 ). Despite these commonalities, the two paleosols differ in their hydropedological regimes. The FRB profile is characterized by grayish matrices, strong structure, and pervasive redoximorphism, suggesting higher soil moisture availability throughout much of the year (Vepraskas et al., 2018 ; Soil Survey Staff, 2022 ). Such conditions are consistent with a subhumid–humid environment, where soils remain moist for 6–9 consecutive months (Cecil, 2003), equivalent to a udic moisture regime and classification as an Alfisol (Udalf suborder) (Soil Survey Staff, 2022 ). By contrast, the LVRB paleosols exhibit thick argillic (Bt) horizons combined with vertic properties—wedge structures, slickensides, and clay coatings formed along shrinkage cracks. These features reflect repeated expansion–contraction cycles under pronounced seasonal drying (Flórez-Molina et al., 2013). Moisture seasonality was greater than in FRB, with soils remaining moist only 3–5 consecutive months (Cecil, 2003), corresponding to a udic regime near the ustic limit, and classification as a Vertisol (Udert to Ustert suborders) (Soil Survey Staff, 2022 ). Secondary carbonates, present in both profiles, formed as calcium carbonate precipitated from percolating solutions in cracks and pores, aided by root respiration and localized CO₂ variations (Durand et al., 2018 ). Micromorphological evidence (Figs. 3 c, 3 l) indicates weak impregnation and secondary origins (Sehgal and Stoops, 1972 ; Durand et al., 2018 ). Their presence points to prolonged dry periods, but the absence of gypsum suggests that aridic conditions were never reached (Wanas and Abu El-Hassan, 2006). These findings differ from Flórez-Molina et al. (2013, 2018 ), who interpreted a regime shift from torric (arid) in FRB equivalents to ustic (semi-arid) in LVRB equivalents; however, the present evidence supports a contrast between udic and ustic limits within subhumid settings. The paleoenvironmental significance of these profiles lies in their geomorphic context. Developed in distal flood basins, their fine-grained textures and low topographic positions promoted seasonal stagnic processes (Kraus, 1999 ). Soils were not waterlogged year-round, but experienced episodic saturation during wet seasons, followed by desiccation and hematite formation under high temperatures, water scarcity, and slightly basic conditions (Pipujol and Buurman, 1994 ; Flórez-Molina et al., 2013, 2018 ; Salazar-Jaramillo et al., 2022 ). This alternation produced the mosaic of gley features, hematite coatings, and carbonate accumulations observed in both paleosols. The paleontological record of La Venta adds a crucial ecological dimension to the paleoenvironmental reconstruction. Macrofauna (mammals, reptiles, birds, and fish) and macroflora (Kay and Madden, 1997 ) consistently indicate humid conditions (Stille, 1907 , 1938; Stirton, 1953 ; van Houten and Travis, 1968; Wellman, 1970 ; Takemura and Danhara, 1986 ; Setoguchi and Rosenberger, 1987 ; Guerrero, 1997 ; Villarroel et al., 1996; Kay et al., 1997; Spradley et al., 2019), with mammalian proxies suggesting mean annual rainfall between 1500–2000 mm/yr for the Los Monos bed (Kay and Madden, 1997 ). Importantly, Kay and Madden ( 1997 ) found no strong evidence for pronounced seasonal drought, a conclusion that complements rather than contradicts the paleosol evidence. Micromorphological features show that rainfall seasonality—not total precipitation—governed pedogenesis and soil moisture regimes. In subhumid climates, precipitation and soil water are decoupled by evapotranspiration, with soil moisture availability lagging behind peak rainfall (Cecil, 2003; Driese and Ober, 2005; Hasiotis et al., 2007 ). The alternating wet–dry signals recorded in the paleosols are consistent with a transitional subhumid regime, but not with the prolonged droughts characteristic of arid or savanna systems. Thus, faunal and pedological evidence converge on La Venta as a transitional environment between humid forests and more open habitats, best described as a riparian mosaic rather than a continuous evergreen rainforest (Kay and Madden, 1997 ). This hydrological seasonality resolves the apparent contradiction between paleontological signals of humid forests and pedological evidence of seasonality. The FRB paleosols (Alfisol-like, udic) suggest wetter conditions that supported denser, more forested vegetation, while the LVRB paleosols (Vertisol-like, ustic) reflect stronger seasonal contrasts, favoring more open shrub- and grass-dominated cover. Yet this shift does not necessarily imply a complete vegetation replacement. Instead, the change from udic Alfisols to ustic Vertisols reflects differences in topographic setting and hydrological regime within the floodbasin, where geomorphology drove soil development. Both paleosol types consistently record a transitional subhumid climate, indicating that the environment was not “halfway between a desert and a jungle,” but a dynamic, patchy mosaic of forests and open habitats—a heterogeneous landscape shaped by intermediate rainfall and seasonal drought. In the global climatic framework, the FRB and LVRB paleosols represent a terrestrial expression of the major reorganization triggered by Antarctic ice expansion, which increased the interhemispheric temperature gradient and drove a northward migration of the ITCZ (Holbourn et al., 2010). During the Middle Miocene, successive glacial episodes at ~ 14.6, 14.2, 13.9, and 13.1 Ma culminated in the last of these shifts (Holbourn et al., 2010), to which the Tatacoa paleosols are chronologically tied. This latitudinal displacement of the tropical rain belt intensified rainfall seasonality across equatorial South America, imposing alternating wet and dry conditions on floodplain landscapes. In this way, the Tatacoa paleosols capture on land the hydrological consequences modulated by ITCZ dynamics during the Mid-Miocene Climate Transition. 6. Conclusions The FRB and LVRB paleosols of the Baraya Member record cumulative soil development in flood basin settings under low sedimentation rates that favored polygenesis. Micromorphology and geochemistry demonstrate that pedogenesis was dominated by fersiallitization, rubefaction, clay illuviation, vertic dynamics, carbonate cycling, and redoximorphism, all modulated by seasonal wet–dry cycles. While both paleosols reflect a subhumid climate, their differences—udic Alfisol-like features in the FRB versus ustic Vertisol-like properties in the LVRB—are best explained by drainage dynamics (water budget). The paleontological record, indicating humid environments with mean annual rainfall of 1500–2000 mm and no evidence for prolonged drought, aligns with pedological evidence that seasonality governed soil moisture regimes in a transition zone. In other words, an intermediate rainfall (~ 1000–2000 mm/year) and a moderate dry season (3 months). Together, faunal, floral, and pedological records converge on the interpretation of La Venta as a transitional subhumid landscape, a heterogeneous riparian mosaic rather than a continuous rainforest. At the global scale, these paleosols provide terrestrial evidence for the hydrological impacts of Antarctic ice growth, which intensified interhemispheric temperature gradients and forced northward migration of the ITCZ during the Middle Miocene (Holbourn et al., 2010). Thus, the Tatacoa paleosols capture how regional soil formation and habitat heterogeneity were directly modulated by global climate dynamics during the Mid-Miocene Climate Transition. Declarations 7. Acknowledgments We gratefully acknowledge the financial support provided by Universidad Nacional de Colombia (Convocatoria nacional de proyectos para el fortalecimiento de la investigación, creación e innovación de la Universidad Nacional de Colombia 2016–2018; Código: 37506). 8. Conflicts of interest The authors declare that they have no conflicts of interest. 9. Author contributions Conceptualization: Susana Salazar, Jose Luis Sotelo; writing – original draft: Susana Salazar; writing – review & editing: Susana Salazar, José Luis Sotelo, Juan Carlos; visualization (figures/graphs): Susana Salazar; data curation: Susana Salazar; Juan Carlos Loaiza; supervision: Susana Salazar, Juan Carlos Loaiza; soil description: José Luis Sotelo; sample preparation (micromorphology and geochemistry): José Luis Sotelo; micromorphology – description & guidance: Juan Carlos Loaiza, José Luis Sotelo; photography: Juan Carlos Loaiza. References Alekseev, A.O., Kalinin, P.I. & Alekseeva, T.V. 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H., Soils in the Mediterranean region: What makes them different?, Catena , 1997, vol. 28, no. 3–4, pp. 157–169. https://doi.org/10.1016/S0341-8162(96)00035-5 Tables Tables 1 to 3 are available in the Supplementary Files section Supplementary Files TABLES.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major Revisions Needed 24 Nov, 2025 Reviewers agreed at journal 29 Sep, 2025 Reviewers invited by journal 25 Sep, 2025 Editor assigned by journal 16 Sep, 2025 First submitted to journal 12 Sep, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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1","display":"","copyAsset":false,"role":"figure","size":317002,"visible":true,"origin":"","legend":"\u003cp\u003eRegional context showing the location of the Tatacoa Desert, the geological setting and the Neiva sub-basin in the Upper Magdalena Valley (modified from ANH, 2007). The right square is the several stratigraphic levels at the Baraya Member of the Villavieja Formation, Honda Group (after Guerrero, 1997), and the two paleosol profiles described. Ferruginous Red Bed (FRB) and La Venta Red Bed (LVRB).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7594667/v1/5ecdb11bb39559af65d62a2c.png"},{"id":93053783,"identity":"b005e13e-f58a-4bdf-a30b-f5f11213dc7f","added_by":"auto","created_at":"2025-10-08 14:34:30","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":184804,"visible":true,"origin":"","legend":"\u003cp\u003eStratigraphy of study stratigraphic profiles in La Tatacoa, Colombia.\u003c/p\u003e\n\u003cp\u003eA. Stratigraphy of the Baraya Member at La Tatacoa, Colombia, with lithology, position and identifiers of marker beds and paleosols; B. FRB sampled paleosol horizons, Munsell color and main soil morphology; C. LVRB sampled paleosol horizons, Munsell color and main soil morphology; D. Legend of pedological features in paleosols.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7594667/v1/031376e36f9219118fc32cec.png"},{"id":93053789,"identity":"d5d1a8f3-4644-4af6-b445-238fb4849d24","added_by":"auto","created_at":"2025-10-08 14:34:30","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":901713,"visible":true,"origin":"","legend":"\u003cp\u003eMain micromorphological characteristics in the study paleosols. (a, b) shrinkage cracks brown dotted micromass (PLL), and stipple speckled and micritic crystallitic b-fabric (PLX), c) calcitic bioclast, root channel, subangular blocky microstructure, planes and vughs (PLL), d) biological activity inside chamber (PLL), e) vughy microstructure enaulic c/f related distribution (PLL), f) vughs and planes reduction conditions, dense incomplete Fe and Mn oxides infilling inside planes and channels, moderately impregned Fe quasicoatings, yellowish and reddish micromass(PLL), g) nucleic Fe oxides nodule formed around muscovite, stipple speckled b-fabric, planes with Fe and Mn hypocoatings strong impregned (PLL), h, i) clay coatings and infilling in planes and vughs, shrinkage cracks (vertic features).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7594667/v1/e57e226fbef088e4c45be88f.png"},{"id":93053788,"identity":"cd51d447-3cd2-4efe-bf09-fff37e90ca7a","added_by":"auto","created_at":"2025-10-08 14:34:30","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":204315,"visible":true,"origin":"","legend":"\u003cp\u003eFerruginous Red Bed (FRB) profile, samples FRB 9 (bottom), FRB 18 (top), La Venta Red Bed (LVRB) profile, samples LVRB 4 (bottom) and LVRB 7 (top). X-ray diffraction patterns acquired from oriented clay mineral mounts of the total clay fraction pre (black diffractogram) and post ethylene glycolation (EG, blue diffractogram) and heat treatments at 515–550 °C (red diffractogram).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7594667/v1/b658f3fcde27e31bea121568.png"},{"id":93054742,"identity":"acdd8d00-1d06-419f-a943-fbb9d64e53ac","added_by":"auto","created_at":"2025-10-08 14:42:30","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":141843,"visible":true,"origin":"","legend":"\u003cp\u003eFerruginous Red Bed (FRB) profile (bottom), La Venta Red Bed (LVRB) profile, samples LVRB 4 (top) showing weathering indexes and MAP.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7594667/v1/83c036aa3ebac888ae695433.png"},{"id":93056772,"identity":"e8472964-0f38-4627-ba89-7afb087ec15c","added_by":"auto","created_at":"2025-10-08 15:06:31","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2420442,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7594667/v1/d03eecc3-9e89-4ef3-b5ec-12a697db9241.pdf"},{"id":93053786,"identity":"66037b15-f1d0-4102-8189-2aec87b06e65","added_by":"auto","created_at":"2025-10-08 14:34:30","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":39483,"visible":true,"origin":"","legend":"","description":"","filename":"TABLES.docx","url":"https://assets-eu.researchsquare.com/files/rs-7594667/v1/3945572b0b9121baa0c19f8d.docx"}],"financialInterests":"","formattedTitle":"Mid-Miocene palaeohydrology archived in paleosol of La Tatacoa, Colombia","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe mid-Miocene was a turning point in Earth\u0026rsquo;s climate system, marked by the transition from the Mid-Miocene Climatic Optimum (MMCO) to subsequent global cooling associated with the expansion of the Antarctic ice sheet (Holbourn et al., 2010). Around ~\u0026thinsp;13.1 Ma, the establishment of permanent Antarctic glaciation coincided with a northward migration of the Intertropical Convergence Zone (ITCZ), which produced stronger rainfall seasonality and intensified chemical weathering across the tropical belt (John et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Holbourn et al., 2010). These global dynamics provide an essential framework for evaluating how Neotropical ecosystems and landscapes responded to shifts in hydrology and precipitation regimes.\u003c/p\u003e\u003cp\u003eWithin this context, La Tatacoa (Huila, Colombia) stands out as one of the most important Neotropical fossil localities. The La Venta Fauna, preserved in the Villavieja Formation, represents one of the richest Neogene vertebrate assemblages of South America (Kay and Madden, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Catena et al., 2017; Cadena et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Yet its palaeoclimate remains contested: fossil flora and fauna indicate humid tropical environments (Wellman, \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e1970\u003c/span\u003e; Kay and Madden, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e), while the pedogenic properties of the associated paleosols point toward drier and highly seasonal conditions (Fl\u0026oacute;rez-Molina et al., 2013; Fl\u0026oacute;rez-Molina et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). This apparent contradiction has fueled debate over the environmental backdrop of the Tatacoa region during the Serravallian. Clarifying whether the soils formed under wetter sub-humid or drier semi-arid regimes is critical for reconciling these records and for understanding habitat heterogeneity within the La Venta ecosystem.\u003c/p\u003e\u003cp\u003ePaleosols provide unrivaled terrestrial archives of palaeoclimate and palaeohydrology, preserving signatures of weathering, drainage, and vegetation cover (Alekseev et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Loaiza-Usuga et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Orr and Roberts, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). In La Tatacoa, the lowermost red-bed paleosols of the Baraya Member (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA; Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e) offer a unique opportunity to examine mid-Miocene soil hydrology. These profiles\u0026mdash;the Ferruginous Red Bed (FRB) and the La Venta Red Bed (LVRB; equivalent to the Lower Red Beds of Fields, 1959)\u0026mdash;formed in overbank deposits of meandering rivers and are chronologically constrained between Chron C5AAn (13.183\u0026ndash;13.032 Ma; Flynn et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Ogg, 2012) and Chron C5Ar.3r (13.032\u0026ndash;12.887 Ma), spanning\u0026thinsp;~\u0026thinsp;120 kyr (Salazar-Jaramillo et al., 2021).\u003c/p\u003e\u003cp\u003eThis study aims to characterize the pedogenetic processes of the FRB and LVRB paleosols through micromorphology, major oxide geochemistry, and clay mineralogy, to reconstruct soil moisture regimes, and to evaluate whether they record a transition from wetter sub-humid to drier sub-humid conditions. By integrating soil-based evidence with the broader context of mid-Miocene climate dynamics, we hypothesize that the La Tatacoa paleosols reflect hydrological variability driven by ITCZ migration, and that their contrasting moisture regimes help explain the mosaic of habitats that sustained the La Venta Fauna.\u003c/p\u003e"},{"header":"2. Geological and stratigraphic setting","content":"\u003cp\u003eThe Tatacoa area, located north of Villavieja (Huila, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), belongs to the Neiva Sub-basin of the Upper Magdalena Valley (UMV; Villarroel et al., 1996). The UMV constitutes a major basin formed during the northern Andean orogeny, where repeated deformation episodes controlled both the structural framework and the style of sedimentation (Cediel et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Bounded by reverse faults extending from the bifurcation of the Central and Eastern Cordilleras (Mojica and Franco, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e1990\u003c/span\u003e), the basin accumulated thick successions of continental deposits whose evolution was closely tied to Andean uplift. Before the rise of the Eastern Cordillera, the region was part of a vast \u0026ldquo;pan-Amazonian\u0026rdquo; fluvial system connected to the Pebas mega-wetland, draining toward the present Amazon, Orinoco, and Magdalena fluvial basins (Hoorn et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Bicudo et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Kirschner and Hoorn, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe sedimentary fill includes Jurassic volcano-sedimentary rocks of the Salda\u0026ntilde;a Formation, sourced mainly from the Central Cordillera (Villarroel et al., 1996). These are unconformably overlain by the Honda Group, which is subdivided into the La Victoria and Villavieja formations (Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA). The Honda Group records a transition from gray mudstones and sandstones of the La Victoria Formation to the reddish mudstones and paleosol-rich successions of the Villavieja Formation, the latter being the focus of this study. Recent chronostratigraphic reevaluations using Bayesian modeling of U-Pb zircon data have reinterpreted the age of the Honda Group, extending the sedimentary record of the Honda Gorup from ~\u0026thinsp;16.0 Ma at the base to ~\u0026thinsp;10.5 Ma at the top. (Mora et al., 2023). This updated model suggests the fossiliferous sequence of La Venta is slightly older than previous estimates.\u003c/p\u003e\u003cp\u003eAbove the Honda Group lies the Gigante Formation, of Tortonian age (~\u0026thinsp;8.5 Ma; van Houten, \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e), characterized by gravels interbedded with volcanic deposits of ash, pumice, and sand (Takemura and Danhara, 1983). A major tectonic event at ~\u0026thinsp;12.9 Ma initiated the uplift of the Eastern Cordillera (Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e), which by ~\u0026thinsp;6\u0026ndash;3 Ma had reached elevations sufficient to act as an orographic barrier (Mora et al., 2008). This uplift reorganized drainage networks, enhanced sediment supply, and promoted the deposition of braided river systems such as the La Cerbatana Conglomerate, which marks the boundary between the La Victoria and Villavieja formations (Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eWithin the Villavieja Formation, the Baraya Member is particularly significant for its fossiliferous horizons and paleosols. These soils directly overlie the Monkey and Fish beds of the La Venta Fauna, one of the most important Neotropical vertebrate assemblages of the Miocene (Kay and Madden, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Catena et al., 2017; Cadena et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). A recent chronostratigraphic model by Mora et al. (2023) proposes and revised age range for the Villavieja Formation; however, for the purpose of this study, the Baraya Member is considered within its established chronostratigraphic framework (Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e), where it directly overlies the Cerbatana Conglomerate and include the Monkey and Fish beds of the La Venta Fauna (Kay and Madden, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Catena et al., 2017; Cadena et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eSedimentological evidence indicates that the Baraya Member was deposited in a low-energy meandering fluvial system, with small channels (2\u0026ndash;3 m deep), point-bar sequences, and extensive overbank deposits (Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). Fine-grained floodplain sediments favored the development of cumulative soils under low sedimentation rates, generating polygenetic paleosols with distinct pedogenic features (Kraus, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Their position in distal flood basins strongly conditioned soil hydrology: poorly drained topography promoted water stagnation and redox reactions, while alternations of wet and dry periods enhanced clay illuviation, vertic features, and carbonate precipitation. These palaeotopographic and hydrological controls are well expressed in the Ferruginous Red Bed (FRB; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB) and La Venta Red Bed (LVRB; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eC) profiles, situated\u0026thinsp;~\u0026thinsp;50 m and ~\u0026thinsp;100 m above the Cerbatana Conglomerate sampled at 3\u0026deg;13\u0026prime; 38.03\u0026prime;\u0026prime; N, 75\u0026deg; 8\u0026prime;53.19\u0026prime;\u0026prime;W and at 3\u0026deg;14\u0026prime;6.88\u0026prime;\u0026prime; N, 75\u0026deg; 9\u0026prime; 16.22\u0026prime;\u0026prime;W, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). These profiles preserve micromorphological and mineralogical evidence of mid-Miocene soil moisture regimes, offering critical insights into the palaeohydrology of La Tatacoa.\u003c/p\u003e"},{"header":"3. Methods","content":"\u003cp\u003eTwo paleosol profiles were selected for study: the Ferruginous Red Bed (FRB) and the La Venta Red Bed (LVRB). Their stratigraphic positions within the Baraya Member are well established (Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Profile selection was based on their stratigraphic continuity, preservation of pedogenic features, and proximity to key fossiliferous horizons. Field descriptions followed the guidelines of the Soil Survey Manual (Soil Science Division Staff, 2017), and classification employed both Soil Taxonomy (Soil Survey Staff, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) and the World Reference Base for Soil Resources (IUSS Working Group WRB, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Detailed morphological descriptions were undertaken, and samples were collected systematically for multi-proxy analysis, including physical, chemical, mineralogical, and micromorphological studies (Loaiza-Usuga et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eUndisturbed blocks for thin-section preparation were processed at the IGAC National Soil Laboratory (Bogot\u0026aacute;, Colombia). Samples were first air-dried for two months at room temperature, then impregnated with polyester resin (ref. 744) and hardener (ref. 2744) in a 5:1 ratio, with 0.75 g acetone as a diluent, following the protocol of Murphy (\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e1986\u003c/span\u003e). After six weeks of curing, samples were cut with a diamond saw, mounted with resin, and further thinned using the Petrothin Buehler system to a standard thickness of ~\u0026thinsp;20 \u0026micro;m. Micromorphological observations were conducted with an Olympus CX31 microscope, and descriptions followed the criteria of Stoops (\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) and Loaiza-Usuga et al. (\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eX-ray diffraction (XRD) analyses were performed on both bulk powders and oriented clay fractions at the Lithogeochemical Characterization Laboratory, National University of Colombia (Bogot\u0026aacute;). Clay separates were prepared using the pipette method to obtain oriented mounts on glass slides. Each sample was analyzed under four conditions: bulk, untreated oriented, glycolated, and calcined (515\u0026ndash;550\u0026deg;C, 4 h). Diffraction patterns were obtained using a Bruker D2 Phaser diffractometer with Ni-filtered Cu-Kα radiation (30 kV, 10 mA). Scans were run from 2.5\u0026deg; to 40\u0026deg; 2θ, with a step size of 0.014\u0026deg;, a dwell time of 0.1 s/step, and a PSD detector angle of 5.33\u0026deg;. Mineral phases were identified and semi-quantified with DIFFRAC.EVA V4.2.2 software, and clay minerals were verified following Thorez (\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e1976\u003c/span\u003e) and Chen (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1977\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eFor X-ray fluorescence (XRF) analysis, a 1.0-gram paleosol subsample was diluted with a lithium borate flux (10:1) and fused into a 37 mm glass disc. Major and minor element oxide concentrations (Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, K₂O, CaO, TiO₂, MnO, Fe₂O₃) were determined using wavelength-dispersive X-ray fluorescence (WD-XRF) on a Philips MagixPro PW-2440 spectrometer. The analysis, performed at the National University of Colombia, employed a semi-quantitative method calibrated against a suite of certified reference materials (NIM-G, NIM-S, NIM-N, NIM-P). Based on replicate measurements and deviations from certified values, the reported oxide data have an estimated accuracy error of approximately 5% relative.\u003c/p\u003e\u003cp\u003eTo interpret the intensity and processes of weathering during pedogenesis, a suite of geochemical proxies was employed. The Chemical Index of Alteration (CIA), calculated as [Al₂O₃/(Al₂O₃ + CaO* + Na₂O\u0026thinsp;+\u0026thinsp;K₂O)] \u0026times; 100, serves as a primary index to quantify the total degree of chemical weathering by measuring the proportion of stable alumina relative to mobile cations (Nesbitt \u0026amp; Young, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e1982\u003c/span\u003e). The extent of clay mineral formation, or argillization, was assessed using the Al/Si ratio (Ashley \u0026amp; Driese, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Retallack, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). The loss of base cations through leaching and hydrolysis was evaluated using the ratio Al/(Na\u0026thinsp;+\u0026thinsp;Ca\u0026thinsp;+\u0026thinsp;K\u0026thinsp;+\u0026thinsp;Mg) (Ashley \u0026amp; Driese, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Hamer et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). Finally, specific pedogenic pathways were identified using the Na/K ratio to indicate salinization processes and the Ca/Mg ratio to indicate calcification (\u003cem\u003ee.g\u003c/em\u003e., Flaig et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eTo quantitatively reconstruct mean annual precipitation (MAP) during the period of pedogenesis, climofunctions based on bulk geochemistry were applied. The CALMAG proxy ([Al₂O₃/(Al₂O₃+CaO\u0026thinsp;+\u0026thinsp;MgO)]*100) provided a precise estimate via the equation MAP\u0026thinsp;=\u0026thinsp;22.69(CALMAG) \u0026minus;\u0026thinsp;435.80 (R\u0026sup2; = 0.90; SE\u0026thinsp;=\u0026thinsp;\u0026plusmn;\u0026thinsp;108 mm; Nordt \u0026amp; Driese, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). The CIA offered a broader assessment using MAP\u0026thinsp;=\u0026thinsp;221e^(0.0197*CIA) (R\u0026sup2; = 0.72; SE\u0026thinsp;=\u0026thinsp;\u0026plusmn;\u0026thinsp;182 mm; Nesbitt \u0026amp; Young, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e1982\u003c/span\u003e), allowing for interpretation through two proxies.\u003c/p\u003e\u003cp\u003eField and laboratory data were integrated to reconstruct soil moisture regimes and pedogenic environments. Taxonomic classification (Soil Survey Staff, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; IUSS Working Group WRB, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) provided the basis for assigning moisture regimes (udic\u0026ndash;ustic), while micromorphological features guided the identification of specific pedogenic processes such as redoximorphism, argilluviation, calcification, and vertization. Clay mineralogy and major oxides geochemistry served as a complementary proxy for weathering intensity and soil hydrological conditions. Interpretations were framed in relation to established paleoclimate proxies, enabling reconstruction of wet\u0026ndash;dry cycles and their broader climatic significance during the mid-Miocene.\u003c/p\u003e"},{"header":"4. Results","content":"\u003ch3\u003e4.1.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; Paleosol Morphology and Classification\u003c/h3\u003e\n\u003cp\u003eThe two studied paleosol profiles from the Baraya Member exhibit distinct macromorphological characteristics that reflect their different pedogenic histories and environmental conditions (Fig. 2). Their features are summarized in Table 1.\u003c/p\u003e\n\u003cp\u003eFerruginous Red-bed (FRB) Paleosol Features\u003c/p\u003e\n\u003cp\u003eThe FRB paleosol profile is thick (~9 m) and is dominated by well-developed Bt argillic horizons. These horizons exhibit strong, primarily subangular blocky (sbk) and columnar (p) structures, indicating a high degree of pedogenic development. The consistence is predominantly hard (h) to very firm (vfir), plastic (p), and sticky (s). A key macromorphological feature is the pervasive gleying, indicated by the dominant grey to greyish-green (Gley) Munsell colors across most horizons (\u003cem\u003ee.g\u003c/em\u003e., Gley1 7/10Y, Gley2 8/10G), often occurring alongside reddish (\u003cem\u003ee.g\u003c/em\u003e., 2.5YR 5/8) and yellowish mottles. This suggests fluctuating redox conditions due to periodic saturation. Evidence of clay illuviation (argilluviation) is common, with many horizons containing clay coatings (cc) on ped faces and within channels. The upper part of the profile contains two C horizons (FRB 16, 17) with a sandy texture and a coarse/fine (c/f) related distribution estimated between 2/1 and 3/1. Carbonate nodules are present in the uppermost horizon (FRB 18), though this feature is often masked by the more dominant clay coatings and gley colors. The presence of root channels (rch) throughout the profile indicates significant biological activity.\u003c/p\u003e\n\u003cp\u003eLa Venta Red-bed (LVRB) Paleosol Features\u003c/p\u003e\n\u003cp\u003eThe LVRB paleosol is also well-developed (~9 m thick) and is characterized by horizons with vertic properties. These are evidenced by the common presence of slickensides (ss) and wedge-shaped structures, derived from the expansion and contraction of clays. The structure is a mix of subangular blocky (sbk), columnar (p), and prismatic, with a consistence that is very firm (vfir), plastic (p), and sticky (s). In contrast to the FRB paleosol, the matrix colors are dominantly reddish (\u003cem\u003ee.g\u003c/em\u003e., 5YR 4/4, 5YR 5/6), indicating better drainage and more oxidizing conditions, though gley mottles (g) are present in some horizons, suggesting periodic wetness. Impregnations of iron oxides and clay coatings (cc) filling channels and fissures are common. The textural distribution in the Bt horizons is dominantly porphyric (c/f ~1/8), confirming the clay-rich nature of the parent material. The base of the profile is a C horizon (LVRB 1) with prominent gley colors (Gley1 7/5GY) and a sandy texture, which contains bone remains (br) and subrounded quartz grains (Qs), linking it to the rich paleontological heritage of the area.\u003c/p\u003e\n\u003cp\u003eTaxonomic Classification as Alfisols and Vertisols\u003c/p\u003e\n\u003cp\u003eBased on the diagnostic morphological features we classified the FRB paleosol as an Alfisol. This classification is supported by the presence of argillic (Bt) horizons with significant clay illuviation (clay coatings) and a base saturation \u0026gt;35% based on the mineral association and previous studies (Flórez-Molina \u003cem\u003eet al.\u003c/em\u003e, 2018). The pervasive gleying and redoximorphic features indicate a soil moisture regime that was seasonally saturated, aligning with an Udalf suborder (udic moisture regime).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eBy contrast, we classified the LVRB paleosol as a Vertisol. This is determined by the vertic properties, including slickensides and wedge-shaped structures, formed by the shrink-swell behavior of smectitic clays. The combination of redoximorphic features (mottles) with a generally reddish, well-drained matrix suggests a seasonally contrasting moisture regime, supporting a Udert suborder (udic moisture regime).\u003c/p\u003e\n\u003ch3\u003e4.2.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; Micromorphological Features\u003c/h3\u003e\n\u003cp\u003eMicromorphological analysis revealed a suite of pedogenic features that elucidate the paleoenvironmental conditions during the formation of the FRB (Alfisol) and LVRB (Vertisol) paleosols. The key characteristics are categorized and described below and summarized in Table 2 and shown in Figure 3.\u003c/p\u003e\n\u003cp\u003eRedoximorphic Characteristics and Stagnic Properties\u003c/p\u003e\n\u003cp\u003eThe FRB paleosol is dominated by redoximorphic features indicating periodic saturation. Key evidence includes gleyed, brown dotted micromasses with stipple speckled b-fabric (Fig. 3a, b), and dense incomplete Fe-Mn oxide infillings within planes and channels set in a moderately to strongly impregnated yellowish and reddish micromass (Fig. 3f). Abundant typic and anorthic Fe and Mn nodules are common throughout the profile. These features collectively point to fluctuating redox conditions driven by a seasonally high-water table.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eClay Coatings, Slickensides, and Vertic Features\u003c/p\u003e\n\u003cp\u003eBoth profiles show evidence of clay illuviation, but with different implications. The FRB paleosol exhibits well-developed, continuous clay coatings (argillans) on ped faces and in channels (Fig. 3f, XPL), confirming the argillic horizon designation. By contrast, the LVRB paleosol is defined by its vertic features. The most remarkable characteristics are the prominent shrinkage cracks (Fig. 3h, i) and the presence of slickensides. The process of clay illuviation is ongoing, evidenced by impure clay coatings in the groundmass and infilling planes and channels (Fig. 3f). Pedoturbation from biological activity is also indicated by relict features (Fig. 3d).\u003c/p\u003e\n\u003cp\u003eIron and Manganese Oxide Distributions\u003c/p\u003e\n\u003cp\u003eThe distribution of Fe oxides differs between the two paleosols, reflecting their drainage regimes. In the FRB paleosol, Fe oxides are primarily present as impregnations in the micromass and as hypocoatings and quasicoatings resulting from reduction-segregation processes. In the LVRB paleosol, the distribution indicates a more oxidizing environment. Well-developed hematite is a key feature (Fig. 3k), and Fe oxides are also found within cracks (Fig. 3j), as coatings, and as illuviated material within pores.\u003c/p\u003e\n\u003cp\u003eCarbonate Pedofeatures\u003c/p\u003e\n\u003cp\u003eCarbonate features are present in specific contexts. In the FRB paleosol, carbonates occur as micritic crystallitic b-fabric (Fig. 3b), typic nodules, and calcitic infillings inside root channels (Fig. 3c). In the LVRB paleosol, the basal C horizon contains carbonate hypocoatings and micritic crystallitic b-fabric (Fig. 3l), associated with abundant lithic fragments, quartz, and bone remains, indicating a less pedogenically altered parent material.\u003c/p\u003e\n\u003ch3\u003e4.3.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; Geochemistry and Pedogenic Processes\u003c/h3\u003e\n\u003cp\u003eClay mineralogy\u003c/p\u003e\n\u003cp\u003eFigure 4 shows the XRD results for bulk and oriented (glycolated) samples, which reveal a clay mineralogy dominated by smectite, illite, illite/smectite mixed-layer clays, and kaolinite. The 001 smectite peak expands to ~17 Å upon ethylene glycol solvation. Illite is present as a discrete phase, exhibiting stable 10.1 Å and 5.0 Å reflections in natural, glycolated, and calcinated preparations. A broad 8.58 Å peak in glycolated samples signifies illite/smectite mixed-layering. The identification of kaolinite is validated by the disappearance of its peaks post-calcination. The presence of hematite is confirmed by its 2.69 Å (002) reflection.\u003c/p\u003e\n\u003cp\u003eWeathering intensity indicators and paleoprecipitation (MAP)\u003c/p\u003e\n\u003cp\u003eThe XRF data (Table 3) highlights a pedogenic contrast between the FRB and LVRB paleosols, reflecting differences in weathering intensity and associated paleoclimatic conditions (Fig. 5). Chemical Index of Alteration (CIA), argillization (Al/Si), leaching–hydrolysis [Al/(Na+Ca+K+Mg)], and mean annual precipitation (MAP) show strong positive correlations, whereas salinization (Na/K) and calcification (Ca/Mg) indices display negative correlations with CIA and the other weathering-related parameters.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eCIA Values: The FRB paleosol shows a wide range of CIA values (66.58 - 90.23), indicating a strong weathering gradient within the profile. The lower (older) horizons (FRB7, FRB8) exhibit intense chemical weathering (CIA \u0026gt; 89), characteristic of advanced pedogenesis. By contrast, the upper (younger) horizons (FRB16-18) show significantly lower CIA values (66 - 74), suggesting weaker weathering or a closer affinity to the unaltered parent material. The LVRB paleosol displays consistently high but slightly less intense weathering, with CIA values tightly clustered between 80.01 and 87.64. In both profiles, an increase in CIA reflects intensified leaching of base cations and enhanced hydrolysis, while salinization and calcification decrease accordingly, confirming the weathering trend (Fig. 5).\u003c/p\u003e\n\u003cp\u003eThe CALMAG and CIA-K derived MAP estimates are broadly consistent, though CALMAG typically yields higher values. This discrepancy is expected due to the different chemical bases of each proxy. Consistently with CIA values, FRB MAP values at the bottom (older (FRB7, FRB8) indicate a very high precipitation (~1680-1300 mm/yr) in the most weathered horizons, while the upper horizons (FRB16-18) indicate a much drier climate (~1250-820 mm/yr). This suggests the FRB profile represents a cumulative paleosol where the lower part formed under a significantly wetter climate than the upper part. The LVRB paleosol formed under consistently high precipitation, with MAP estimates ranging from ~1630 to 1100 mm/yr. The climate was humid but potentially less so than the peak wet conditions recorded at the top of the FRB sequence.\u003c/p\u003e\n\u003cp\u003ePedogenic processes\u003c/p\u003e\n\u003cp\u003eArgillization (Al/Si): The FRB paleosol shows higher Al/Si ratios (up to 0.42) horizons compared to the LVRB (max 0.33), indicating a greater abundance of clay minerals. However, a more intense hydrolysis is reflected in the lower horizons (FRB7, FRB8), where CIA \u0026gt; 89, and also along the LVRB profile with CIA between 80.01 and 87.64. This suggests in situ clay formation and supports the interpretation of a wetter climate during its formation. The upper horizons (FRB16-18) may be more detrital in origin (\u003cem\u003ee.g\u003c/em\u003e. Salazar-Jaramillo et al., 2022).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSalinization vs. carbonation (Na/K and Ca/Mg): The high Na/K and Ca/Mg values in the less-weathered FRB horizons (FRB16-18) suggest salinization and carbonation were active processes under drier climate conditions. Since the geochemistry of the LVRB is more homogeneous (due to pedoturbation), the salinization index is also more uniform, showing consistent values throughout the profile. By contrast, carbonation shows localized enrichments, as expected, due to the presence of calcium carbonate (fossil remains of bones) in LVRB.\u003c/p\u003e"},{"header":"5. Discussion","content":"\u003cp\u003eDeveloped in the overbank of meandering rivers (2\u0026ndash;3 m depth, 20\u0026ndash;35 m width and a meander belt amplitude of 190\u0026ndash;350 m; Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e), the FRB and LVRB polygenetic paleosols are of cumulative nature (\u003cem\u003ee.g\u003c/em\u003e. Kraus and Hasiotis 2006). During the time interval that represents both profiles (~\u0026thinsp;120 kyr), low sedimentation rates (~\u0026thinsp;0.413 mm/yr; Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e) favored pedogenesis, yielding well-developed paleosol in the distal side of the fluvial channels (\u003cem\u003ei.e\u003c/em\u003e. the floodbasin; Kraus, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). These floodplain profiles, known as cumulative soils, developed when the erosion process became insignificant and sedimentation was constant (Kraus, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Thus, pedogenetic processes were favored (\u003cem\u003ei.e\u003c/em\u003e. increased pedogenesis), giving rise to polygenesis.\u003c/p\u003e\u003cp\u003eIn the floodbasin, soils became progressively more poorly drained because the topographic position was lower and the sediment was finer-grained and, thus, less permeable (Kraus, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1999\u003c/span\u003e, Lizzoli et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Soil drainage in the topographic lows is clearly subject to seasonality, which, in turn, responds to the balance between water input by precipitation/runoff and water loss to evaporation and transpiration (Alekseev et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2019\u003c/span\u003e, Loaiza-Usuga et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2022\u003c/span\u003e, Lizzoli et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). For this reason, the pedogenetic features evidenced by the micromorphology suggest that FRB and LVRB paleosol development and maturity were controlled to a greater extent by changes in drainage conditions. As a consequence, processes such as fersialitization, ferrugination, illuviation of clays, redox processes, hydromorphism (stagnic characteristics), leaching of base cations, reddening (rubefaction) and, to a lesser extent, carbonation, took place. Clay mineralogy (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), on the other hand, allowed us to identify additional processes of vermiculitization and smectitization.\u003c/p\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e5.1. Pedogenic processes in detail\u003c/h2\u003e\u003cp\u003ePedogenesis in the FRB and LVRB paleosols reflects the interplay of multiple processes\u0026mdash;fersiallitization, rubefaction, clay illuviation, vertic processes, carbonate cycling, and redoximorphism\u0026mdash;superimposed within cumulative flood basin soils. The combined micromorphological and geochemical evidence indicates in situ weathering and soil development, rather than simple inheritance from alluvial parent material. While the micromorphology reveals the processes and fabrics\u0026mdash;the \u003cem\u003ehow\u003c/em\u003e of pedogenesis\u0026mdash;, the bulk geochemistry quantifies the intensity and composition\u0026mdash;the \u003cem\u003ehow much\u003c/em\u003e of weathering. Together, these approaches allowed us to reconstruct the soil development driven by climate and landscape change in the Baraya Member.\u003c/p\u003e\u003cp\u003e\u003cb\u003eWeathering intensity and clay formation and illuviation (argillization)\u003c/b\u003e\u003c/p\u003e\u003cp\u003eLower horizons of the FRB profile show intense in situ weathering. They are highly enriched in Al₂O₃ (\u003cem\u003ee.g\u003c/em\u003e., FRB7: 26.68%; FRB8: 26.11%; Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), the most immobile major element, reflecting the leaching away of other elements, concentrating alumina. Correspondingly, CIA values exceeding 89 provide quantitative confirmation of extreme weathering and hydrolysis. This signal is reinforced by the depletion of base cations such as CaO, Na₂O, MgO, and K₂O (\u003cem\u003ee.g\u003c/em\u003e., FRB7: CaO\u0026thinsp;=\u0026thinsp;0.90%, Na₂O\u0026thinsp;=\u0026thinsp;0.53%). Micromorphological observations complement these results: thin sections reveal abundant clay coatings and illuviation features (\u003cem\u003ee.g\u003c/em\u003e., FRB14, FRB12, FRB10; Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), the physical imprint of translocated Al-rich clays.\u003c/p\u003e\u003cp\u003eIn the LVRB profile micromorphological observations provide compelling evidence that clay translocation was also an active process. Features such as \u0026ldquo;impure clay coatings,\u0026rdquo; \u0026ldquo;clay-dense incomplete infillings\u0026rdquo; (LVRB4), and \u0026ldquo;clay with Fe\u0026ndash;Mn oxide discontinuous infillings\u0026rdquo; (LVRB6) demonstrate the mobilization and deposition of fine materials along structural voids (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Unlike the FRB, where clay coatings develop on ped surfaces and within pores in a relatively stable Alfisol environment, the LVRB illuviation occurs through shrinkage cracks characteristic of Vertisols. The association of clays with Fe and Mn oxides further indicates periodic redox activity linked to wet\u0026ndash;dry cycles.\u003c/p\u003e\u003cp\u003eBulk geochemistry supports this interpretation. Elevated Al₂O₃ concentrations (\u003cem\u003ee.g\u003c/em\u003e., LVRB4: 21.35%; Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) in the B horizons reflect the enrichment of aluminous clays, quantitatively documenting the same process observed microscopically. Together, these datasets reveal that while both FRB and LVRB paleosols record clay illuviation, the operative mechanisms differ significantly. In the FRB, illuviation manifests as clay coatings deposited in pores by illuviation. In the LVRB, clays are flushed into cracks during wet phases, only to be redistributed and homogenized during subsequent swelling events.\u003c/p\u003e\u003cp\u003eBased on the observed pedogenic features, the reddening of the paleosols is interpreted as the product of in-situ fersiallitization and rubefaction, rather than an inherited characteristic from sedimentary provenance (\"brown alluvium\" \u003cem\u003esensu\u003c/em\u003e van Houten, \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e1972\u003c/span\u003e). Fersiallitization, the in-situ weathering of primary silicates to form 2:1 clays (Targulian and Krasilnikov, \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), is evidenced by neoformed smectite and vermiculite (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), stipple speckled b-fabric, and subangular blocky microstructure (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea-c). Concurrent rubefaction occurred as iron, released during weathering, oxidized and precipitated as authigenic hematite (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ek), binding to clays (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh-j) and forming features such as iron nodules, impregnations, and hypocoatings (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef-g). A nucleic Fe oxide nodule forming around a muscovite grain (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg) provides definitive evidence for this in-situ pedogenic origin, which requires specific conditions including Fe-bearing parent material, warm temperatures, and poor drainage to immobilize silica via 2:1 clay formation (Duchaufour, 1982; Loaiza-Usuga et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe clay mineral assemblage (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) captures an intermediate stage of pedogenic evolution, revealing a weathering sequence that began with vermiculitization and smectitization but did not proceed to terminal ferrallitization. Vermiculitization was triggered by the oxidation and leaching of Fe\u0026sup2;⁺ and Mg\u0026sup2;⁺ from mineral structures (Velde and Meunier, \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). The released cations, particularly Mg\u0026sup2;⁺ and Fe\u0026sup2;⁺, subsequently became available for the neoformation of Fe-rich smectitic clays (Velde and Meunier, \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Lizzoli et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Salazar-Jaramillo et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Concurrently, the presence of well-crystallized kaolinite, likely authigenic given the plagioclase-rich parent material (van Houten and Travis, 1968; Wellman, \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e1970\u003c/span\u003e; Anderson et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), signals the onset of the next stage: desilication and ferrugination, involving the neoformation of simpler 1:1 clays (Duchaufour, 1982; Lizzoli et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). The co-occurrence of these 2:1 and 1:1 clay minerals indicate a slowdown of weathering. Thus, the FRB and LVRB paleosols exemplify Duchaufour's (1982) first stage of ferruginous soil development, retaining strong fersiallitic characteristics rather than evolving into highly advanced, kaolinite-dominated ferrallitic profiles.\u003c/p\u003e\u003cp\u003e\u003cb\u003eRedoximorphic processes\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe FRB and LVRB paleosols display clear evidence of redoximorphic processes, though with contrasting intensities and expressions. In the FRB, elevated Fe₂O₃ values (\u003cem\u003ee.g\u003c/em\u003e., FRB11: 10.47%; FRB5: 8.26%) correspond to micromorphological features such as gleyed micromass, Fe\u0026ndash;Mn nodules, oxide infillings, and hypocoatings (FRB14, FRB12; Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Grey-blue hues in FRB17, FRB16, and FRB14 record anoxic conditions and prolonged water saturation, while reddish to dark red matrices (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef) indicate that much of the pore system remained oxygenated for most of the year (Loaiza-Usuga et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2017\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). These alternating colors reflect fluctuating water tables and the alternation between oxic and anoxic regimes typical of reduction\u0026ndash;segregation pedogenesis.\u003c/p\u003e\u003cp\u003eRedoximorphic features such as coatings and hypocoatings in the basal mass, together with iron nodules (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef\u0026ndash;g), confirm the mobilization of Fe/Mn under reducing conditions and their subsequent reprecipitation during oxidation (Vepraskas et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Kovda and Mermut, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). A nucleic Fe oxide nodule formed around muscovite (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg) illustrates localized precipitation of hematite during these cycles. Importantly, these pedofeatures suggest short-lived saturation or rapid fluctuations of the water table, rather than long-term waterlogging. While gley processes are typically associated with perennial groundwater influence, the FRB paleosols better match stagnic conditions (pseudogley; Pipujol and Buurman, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; IUSS Working Group WRB, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), where excess water is seasonal and restricted to wet periods.\u003c/p\u003e\u003cp\u003eThe LVRB paleosols, in contrast, developed under more oxidizing conditions. Their reddish and yellowish matrices (LVRB5, LVRB4, LVRB6; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) reflect well-drained soil environments, with redoximorphic features largely limited to hypocoatings, quasicoatings, and Fe\u0026ndash;Mn nodules (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). These formed through temporary saturation within peds or following vertic shrink\u0026ndash;swell events, when water was trapped internally before draining. Moderate Fe₂O₃ values (\u003cem\u003ee.g\u003c/em\u003e., LVRB2: 6.37%; LVRB3: 6.35%; Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) correspond well with these localized features. Only the deeper LVRB1 horizon (7Cg4) shows gleyed micromass, confirming that reducing conditions were confined to less-weathered parent material beneath the active vertic zone.\u003c/p\u003e\u003cp\u003eThe geomorphic and depositional context of both paleosols helps explain these patterns. Situated in flood basin positions, far from active channels, these soils developed in fine-grained, slowly permeable sediments (Kraus, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Under such conditions, intermittent fluvial input and low relief favor stagnic hydromorphism: soils are not saturated year-round, but seasonal rainfall and poor drainage cause temporary saturation in parts of the profile (Pipujol and Buurman, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Lizzoli et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Organic matter also plays a role in Fe\u0026ndash;Mn cycling, as soluble organo-complexes of reduced Fe and Mn migrate short distances and subsequently reprecipitate as concretions, infillings, and coatings (Lizzoli et al., 2019; Loaiza-Usuga et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The presence of hematite (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ek, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), particularly in stagnic matrices, indicates strong desiccation following seasonal anoxia, as ferrihydrite dehydrates under high temperatures, low water activity, and slightly basic pH (Pipujol and Buurman, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Fl\u0026oacute;rez-Molina et al., 2013, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Salazar-Jaramillo et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eTaken together, the FRB paleosols reflect more pervasive reduction and Fe\u0026ndash;Mn redistribution, consistent with wetter, low-relief flood basins subject to fluctuating saturation. The LVRB paleosols, though still influenced by transient stagnic conditions, were generally better drained, with redoximorphism largely confined to localized ped interiors and lower horizons. Both profiles thus capture the hydropedological consequences of seasonal waterlogging in distal flood basins, but with differing intensity and pedogenic outcomes.\u003c/p\u003e\u003cp\u003e\u003cb\u003eParent material and pedogenic overprinting\u003c/b\u003e\u003c/p\u003e\u003cp\u003eGeochemical and micromorphological evidence together clarify the nature of the parent material and its alteration by pedogenesis. High SiO₂ contents (\u003cem\u003ee.g\u003c/em\u003e., LVRB7: 76.49%; FRB18: 61.12%) and the persistence of Zr and TiO₂ indicate a siliciclastic source. Micromorphology corroborates this, with porphyric groundmass textures and abundant quartz, plagioclase, and lithic grains observed in thin section (FRB4, FRB2). Fossil bone fragments in the LVRB1 C horizon explain localized P₂O₅ enrichment and link the soil directly to the La Venta fossiliferous beds. Importantly, the parent material signal is overprinted by strong pedogenesis: upper horizons are enriched in Al₂O₃, clay coatings, and Fe\u0026ndash;Mn nodules, reflecting a progressive transformation from siliciclastic sediments into mature paleosols.\u003c/p\u003e\u003cp\u003e\u003cb\u003eVertic properties and pedoturbation\u003c/b\u003e\u003c/p\u003e\u003cp\u003eMicromorphological evidence strongly supports the role of shrink\u0026ndash;swell dynamics as the defining pedogenic process in the LVRB paleosols (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). In LVRB3 (5Bb5t5w5ss4g2), the presence of shrinkage cracks and a variegated pattern represents the microscopic signature of repeated expansion and contraction. Across multiple horizons, descriptions such as \u0026ldquo;subangular blocky microstructure with unaccommodated peds\u0026rdquo; (LVRB6, LVRB4) and \u0026ldquo;accommodated planes\u0026rdquo; further indicate that the soil fabric was continually reworked and sheared by swelling and desiccation. The horizon designations with \u0026ldquo;ss\u0026rdquo; (\u003cem\u003ee.g\u003c/em\u003e., Bb1t1w1ss1), coupled with evidence of sheared fabrics and accommodated planes, are the micromorphological correlates of slickensides, a diagnostic feature of Vertisols.\u003c/p\u003e\u003cp\u003eAn important complement to these vertic features is the presence of clay coatings. In both profiles, illuviated materials occur in cavities, pores, and fissures, but they are notably more frequent in the LVRB. This is unusual because vertic processes typically destroy cutans (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh\u0026ndash;i). Their preservation suggests that clay translocation was active even within vertic horizons, perhaps during intervals of reduced shrink\u0026ndash;swell activity. Such features may represent an environmental shift, with diminished vertic intensity allowing illuviation to operate. In this sense, cutans in Vertisols can be interpreted as evidence of a \u0026ldquo;genetic pathway to another soil order\u0026rdquo; (Buol et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), signaling incipient development toward Alfisol conditions under better-drained settings. This interpretation is reinforced by hematite and iron oxide coatings (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef\u0026ndash;g), which indicate more oxygenated soil environments (Kovda and Mermut, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eGeochemical data reinforce this interpretation (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The LVRB paleosols exhibit consistently high CIA values (~\u0026thinsp;80\u0026ndash;88), but unlike the FRB, there is no strong vertical gradient. This homogeneity reflects pedoturbation: repeated shrink\u0026ndash;swell activity mixes material from different depths, incorporating less-weathered substrates into the upper horizons and redistributing more weathered material downward. As a result, the chemical profile is homogenized, showing uniformly high but not extreme weathering intensity throughout the solum. Moderate depletion of base cations, with MgO ranging from ~\u0026thinsp;0.64\u0026ndash;1.27% and CaO from ~\u0026thinsp;0.66\u0026ndash;1.04%, indicates leaching, but at levels less intense than the more leached FRB topsoils. The constant renewal of mineral surfaces through pedoturbation retards the complete depletion of bases while sustaining advanced weathering overall.\u003c/p\u003e\u003cp\u003e\u003cb\u003eCarbonate Dynamics\u003c/b\u003e\u003c/p\u003e\u003cp\u003eCarbonation and decarbonation processes complement the weathering, redox, and vertic dynamics described above, further illustrating the strong influence of seasonal wet\u0026ndash;dry cycles on these paleosols. Macroscopically, calcic nodules and calcic horizons are well developed (Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Fl\u0026oacute;rez-Molina et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), while thin sections reveal secondary carbonates within the micromass (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003el). These appear as infillings of root channels and planes (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec) with stipple- and speckled-distributed micritic crystallitic b-fabrics (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003el), confirming their secondary pedogenic origin.\u003c/p\u003e\u003cp\u003eThe seasonal mechanism of carbonate cycling is well established: during the rainy season, carbonates are solubilized and bases are leached; during the subsequent dry season, dissolved bicarbonate migrates within the profile and reprecipitates at depth (Duchaufour, 1982; Buol et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). This alternation not only controls carbonate redistribution but also coincides with the release of Fe and enhanced leaching of bases during wet phases. Clay illuviation, likewise linked to wet\u0026ndash;dry alternations, occurred simultaneously within these horizons (Buol et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe association of fersiallitic pedogenesis, illuviation, and vertic properties is consistent with previous descriptions of Alfisol- and Vertisol-like paleosols (Fl\u0026oacute;rez-Molina et al., 2013, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), developed over Fe-rich parent materials (van Houten and Travis, 1968; Wellman, \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e1970\u003c/span\u003e; Anderson et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Moreover, the coexistence of secondary carbonates with Fe/Mn oxides (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef\u0026ndash;g, \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003el, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) supports their interpretation as Alfisol\u0026ndash; and Vertisol-like systems, in agreement with Mermut and Dasog (\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e1986\u003c/span\u003e), Kovda and Mermut (\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), and Soil Survey Staff (\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003cb\u003eSoil Moisture Regimes, Paleosol Classification, and Paleoenvironmental Implications\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe FRB and LVRB paleosols of the Baraya Member display mature pedogenetic development, with Bt horizons, rubefaction overprinting hydromorphism, and evidence of alternating wet\u0026ndash;dry cycles. Processes such as fersiallitization, rubefaction, and carbonation\u0026ndash;decarbonation point to a strongly seasonal, subhumid climate (Driese and Ober, 2005; Alekseev et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Hasiotis et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). Micromorphological features such as illuvial coatings, clay-filled channels, and crack infillings (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef) confirm that fine material was mobilized during wet phases and redistributed along fissures and pores during dry phases (Kraus and Hasiotis, 2006; Catena et al., 2016; Loaiza-Usuga et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2017\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Vepraskas et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eCarbonates and hematite provide additional evidence of climatic oscillations. Rapid crystallization of both occurs during intense desiccation phases (Duchaufour, 1982), whereas strong decalcification requires the higher rainfall thresholds typical of subhumid settings (Yaalon, \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). Stagnic micromass (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef\u0026ndash;g) demonstrates temporary waterlogging, while bases, silica, and other weathering products were retained in the dry season by capillary forces and biogeochemical cycling (Duchaufour, 1982). Together, these processes indicate a tropical wet\u0026ndash;dry subhumid climate, marked by a pronounced dry season (max\u0026thinsp;~\u0026thinsp;3 months but more humid than other equatorial zones where Oxisols form (Cecil, 2003; Feddema, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Hasiotis et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2007\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eDespite these commonalities, the two paleosols differ in their hydropedological regimes. The FRB profile is characterized by grayish matrices, strong structure, and pervasive redoximorphism, suggesting higher soil moisture availability throughout much of the year (Vepraskas et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Soil Survey Staff, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Such conditions are consistent with a subhumid\u0026ndash;humid environment, where soils remain moist for 6\u0026ndash;9 consecutive months (Cecil, 2003), equivalent to a udic moisture regime and classification as an Alfisol (Udalf suborder) (Soil Survey Staff, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eBy contrast, the LVRB paleosols exhibit thick argillic (Bt) horizons combined with vertic properties\u0026mdash;wedge structures, slickensides, and clay coatings formed along shrinkage cracks. These features reflect repeated expansion\u0026ndash;contraction cycles under pronounced seasonal drying (Fl\u0026oacute;rez-Molina et al., 2013). Moisture seasonality was greater than in FRB, with soils remaining moist only 3\u0026ndash;5 consecutive months (Cecil, 2003), corresponding to a udic regime near the ustic limit, and classification as a Vertisol (Udert to Ustert suborders) (Soil Survey Staff, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eSecondary carbonates, present in both profiles, formed as calcium carbonate precipitated from percolating solutions in cracks and pores, aided by root respiration and localized CO₂ variations (Durand et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Micromorphological evidence (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003el) indicates weak impregnation and secondary origins (Sehgal and Stoops, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e1972\u003c/span\u003e; Durand et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Their presence points to prolonged dry periods, but the absence of gypsum suggests that aridic conditions were never reached (Wanas and Abu El-Hassan, 2006). These findings differ from Fl\u0026oacute;rez-Molina et al. (2013, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), who interpreted a regime shift from torric (arid) in FRB equivalents to ustic (semi-arid) in LVRB equivalents; however, the present evidence supports a contrast between udic and ustic limits within subhumid settings.\u003c/p\u003e\u003cp\u003eThe paleoenvironmental significance of these profiles lies in their geomorphic context. Developed in distal flood basins, their fine-grained textures and low topographic positions promoted seasonal stagnic processes (Kraus, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Soils were not waterlogged year-round, but experienced episodic saturation during wet seasons, followed by desiccation and hematite formation under high temperatures, water scarcity, and slightly basic conditions (Pipujol and Buurman, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Fl\u0026oacute;rez-Molina et al., 2013, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Salazar-Jaramillo et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). This alternation produced the mosaic of gley features, hematite coatings, and carbonate accumulations observed in both paleosols.\u003c/p\u003e\u003cp\u003eThe paleontological record of La Venta adds a crucial ecological dimension to the paleoenvironmental reconstruction. Macrofauna (mammals, reptiles, birds, and fish) and macroflora (Kay and Madden, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1997\u003c/span\u003e) consistently indicate humid conditions (Stille, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e1907\u003c/span\u003e, 1938; Stirton, \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e1953\u003c/span\u003e; van Houten and Travis, 1968; Wellman, \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e1970\u003c/span\u003e; Takemura and Danhara, \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Setoguchi and Rosenberger, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Guerrero, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Villarroel et al., 1996; Kay et al., 1997; Spradley et al., 2019), with mammalian proxies suggesting mean annual rainfall between 1500\u0026ndash;2000 mm/yr for the Los Monos bed (Kay and Madden, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). Importantly, Kay and Madden (\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1997\u003c/span\u003e) found no strong evidence for pronounced seasonal drought, a conclusion that complements rather than contradicts the paleosol evidence. Micromorphological features show that rainfall seasonality\u0026mdash;not total precipitation\u0026mdash;governed pedogenesis and soil moisture regimes. In subhumid climates, precipitation and soil water are decoupled by evapotranspiration, with soil moisture availability lagging behind peak rainfall (Cecil, 2003; Driese and Ober, 2005; Hasiotis et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). The alternating wet\u0026ndash;dry signals recorded in the paleosols are consistent with a transitional subhumid regime, but not with the prolonged droughts characteristic of arid or savanna systems. Thus, faunal and pedological evidence converge on La Venta as a transitional environment between humid forests and more open habitats, best described as a riparian mosaic rather than a continuous evergreen rainforest (Kay and Madden, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1997\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThis hydrological seasonality resolves the apparent contradiction between paleontological signals of humid forests and pedological evidence of seasonality. The FRB paleosols (Alfisol-like, udic) suggest wetter conditions that supported denser, more forested vegetation, while the LVRB paleosols (Vertisol-like, ustic) reflect stronger seasonal contrasts, favoring more open shrub- and grass-dominated cover. Yet this shift does not necessarily imply a complete vegetation replacement. Instead, the change from udic Alfisols to ustic Vertisols reflects differences in topographic setting and hydrological regime within the floodbasin, where geomorphology drove soil development. Both paleosol types consistently record a transitional subhumid climate, indicating that the environment was not \u0026ldquo;halfway between a desert and a jungle,\u0026rdquo; but a dynamic, patchy mosaic of forests and open habitats\u0026mdash;a heterogeneous landscape shaped by intermediate rainfall and seasonal drought.\u003c/p\u003e\u003cp\u003eIn the global climatic framework, the FRB and LVRB paleosols represent a terrestrial expression of the major reorganization triggered by Antarctic ice expansion, which increased the interhemispheric temperature gradient and drove a northward migration of the ITCZ (Holbourn et al., 2010). During the Middle Miocene, successive glacial episodes at ~\u0026thinsp;14.6, 14.2, 13.9, and 13.1 Ma culminated in the last of these shifts (Holbourn et al., 2010), to which the Tatacoa paleosols are chronologically tied. This latitudinal displacement of the tropical rain belt intensified rainfall seasonality across equatorial South America, imposing alternating wet and dry conditions on floodplain landscapes. In this way, the Tatacoa paleosols capture on land the hydrological consequences modulated by ITCZ dynamics during the Mid-Miocene Climate Transition.\u003c/p\u003e\u003c/div\u003e"},{"header":"6. Conclusions","content":"\u003cp\u003eThe FRB and LVRB paleosols of the Baraya Member record cumulative soil development in flood basin settings under low sedimentation rates that favored polygenesis. Micromorphology and geochemistry demonstrate that pedogenesis was dominated by fersiallitization, rubefaction, clay illuviation, vertic dynamics, carbonate cycling, and redoximorphism, all modulated by seasonal wet\u0026ndash;dry cycles. While both paleosols reflect a subhumid climate, their differences\u0026mdash;udic Alfisol-like features in the FRB versus ustic Vertisol-like properties in the LVRB\u0026mdash;are best explained by drainage dynamics (water budget). The paleontological record, indicating humid environments with mean annual rainfall of 1500\u0026ndash;2000 mm and no evidence for prolonged drought, aligns with pedological evidence that seasonality governed soil moisture regimes in a transition zone. In other words, an intermediate rainfall (~\u0026thinsp;1000\u0026ndash;2000 mm/year) and a moderate dry season (3 months). Together, faunal, floral, and pedological records converge on the interpretation of La Venta as a transitional subhumid landscape, a heterogeneous riparian mosaic rather than a continuous rainforest. At the global scale, these paleosols provide terrestrial evidence for the hydrological impacts of Antarctic ice growth, which intensified interhemispheric temperature gradients and forced northward migration of the ITCZ during the Middle Miocene (Holbourn et al., 2010). Thus, the Tatacoa paleosols capture how regional soil formation and habitat heterogeneity were directly modulated by global climate dynamics during the Mid-Miocene Climate Transition.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e7. \u0026nbsp; Acknowledgments\u003c/p\u003e\n\u003cp\u003eWe gratefully acknowledge the financial support provided by Universidad Nacional de Colombia (Convocatoria nacional de proyectos para el fortalecimiento de la investigaci\u0026oacute;n, creaci\u0026oacute;n e innovaci\u0026oacute;n de la Universidad Nacional de Colombia 2016\u0026ndash;2018; C\u0026oacute;digo: 37506).\u003c/p\u003e\n\u003cp\u003e8.\u0026nbsp; \u0026nbsp;Conflicts of interest\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflicts of interest.\u003c/p\u003e\n\u003cp\u003e9.\u0026nbsp; \u0026nbsp;Author contributions\u003c/p\u003e\n\u003cp\u003eConceptualization: Susana Salazar, Jose Luis Sotelo; writing \u0026ndash; original draft: Susana Salazar; writing \u0026ndash; review \u0026amp; editing: Susana Salazar, Jos\u0026eacute; Luis Sotelo, Juan Carlos; visualization (figures/graphs): Susana Salazar; data curation: Susana Salazar; Juan Carlos Loaiza; supervision: Susana Salazar, Juan Carlos Loaiza; soil description: Jos\u0026eacute; Luis Sotelo; sample preparation (micromorphology and geochemistry): Jos\u0026eacute; Luis Sotelo; micromorphology \u0026ndash; description \u0026amp; guidance: Juan Carlos Loaiza, Jos\u0026eacute; Luis Sotelo; photography: Juan Carlos Loaiza. \u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAlekseev, A.O., Kalinin, P.I. \u0026amp; Alekseeva, T.V. 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(ANH and B\u0026amp;M Exploration Ltda, Bogot\u0026aacute;, Colombia, 2019). https://www.anh.gov.co/documents/12/colombian_sedimentary_basins.pdf\u003c/li\u003e\n \u003cli\u003eAshley, G. M., \u0026amp; Driese, S. G. (2000). 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Monger, \u003cem\u003eUSDA Handbook 18\u003c/em\u003e (Government Printing Office, Washington, D.C, 2017).\u003c/li\u003e\n \u003cli\u003eSoil Survey Staff., Keys to Soil Taxonomy, 13\u003csup\u003eth\u003c/sup\u003e ed, (USDA-Natural Resources Conservation Service, Government Printing Office, Washington, D.C, 2022)\u003c/li\u003e\n \u003cli\u003eSpradley, J.P., Glazer, B.J., Kay, R.F., Mammalian faunas, ecological indices, and machine-learning regression for the purpose of paleoenvironment reconstruction in the Miocene of South America, \u003cem\u003ePalaeogeography, Palaeoclimatology, Palaeoecology\u003c/em\u003e, 2019, vol. 518, pp. 155\u0026ndash;171. https://doi.org/10.1016/j.palaeo.2019.01.014\u003c/li\u003e\n \u003cli\u003eStille, H, \u0026ldquo;Geologische Studien im Gebiete des Rio Magdalena\u0026rdquo;. Festchr. Adolf V.Koenen, 1907, pp. 277\u0026ndash;358. http://catalog.hathitrust.org/Record/011929316\u003c/li\u003e\n \u003cli\u003eStirton, R., Vertebrate paleontology and continental stratigraphy in Colombia. \u003cem\u003eGeological Society of America Bulletin\u003c/em\u003e, 1953, vol. 64, pp. 603\u0026ndash;622. https://doi.org/10.1130/0016-7606(1953)64[603:VPACSI]2.0.CO;2\u003c/li\u003e\n \u003cli\u003eStoops, G., Guidelines for analysis and description of soil and regolith thin sections, 2\u003csup\u003end\u003c/sup\u003e edition (John Wiley \\\u0026amp; Sons, 2021). https://doi.org/10.1002/9780891189763\u003c/li\u003e\n \u003cli\u003eTakemura, K., Danhara, T., Fission-track dating the upper part of Miocene Honda Group in La Venta Badlands, Colombia. \u003cem\u003eKyoto University\u003c/em\u003e \u003cem\u003eOverseas Research Reports of New World Monkeys\u003c/em\u003e, 1986, pp. 31\u0026ndash;38. http://hdl.handle.net/2433/199624\u003c/li\u003e\n \u003cli\u003eTargulian, V. O., Krasilnikov, P. V., Soil system and pedogenic processes: Self-organization, time scales, and environmental significance, \u003cem\u003eCatena\u003c/em\u003e, 2007, vol. 71, pp. 373\u0026ndash;381. https://doi.org/10.1016/j.catena.2007.03.007\u003c/li\u003e\n \u003cli\u003eThorez, J, Practical Identification of Clay Minerals, 1976, Ed. G. Lelotte, Dison, 90 p.\u003c/li\u003e\n \u003cli\u003evan der Wiel, A. M., van den Bergh, G. D., Hebeda, E. H., Uplift, subsidence, and volcanism in the southern Neiva Basin, Colombia, Part 2: Influence on fluvial deposition in the Miocene Gigante Formation, \u003cem\u003eJournal of South American Earth Sciences\u003c/em\u003e, 1992, vol. 5, no. 2, pp. 175\u0026ndash;196. https://doi.org/10.1016/0895-9811(92)90037-Y\u003c/li\u003e\n \u003cli\u003evan Houten, F., Iron and clay in tropical savanna alluvium, Northern Colombia: a contribution to the origin of red beds, \u003cem\u003eGeological Society of America Bulletin\u003c/em\u003e, 1972, vol. 83, pp. 2761\u0026ndash;2772. https://doi.org/10.1130/0016-7606(1972)83[2761:IACITS]2.0.CO;2\u003c/li\u003e\n \u003cli\u003evan Houten, F., Late Cenozoic volcaniclastic deposits, Andean foredeep, Colombia, \u003cem\u003eGeological Society of America Bulletin\u003c/em\u003e, 1976, vol. 87, pp. 481\u0026ndash;495. https://doi.org/10.1130/0016-7606(1976)87\u0026lt;481\u003c/li\u003e\n \u003cli\u003evan Houten, F., Travis, R., Cenozoic deposits, Upper Magdalen Valley, Colombia. \u003cem\u003eThe American Association of Petroleum Geologists Bulletin\u003c/em\u003e, 1968, vol. 52, no. 4, pp. 695\u0026ndash;702. https://doi.org/10.1306/5D25C455-16C1-11D7-8645000102C1865D\u003c/li\u003e\n \u003cli\u003eVelde, B., Meunier, A, \u0026ldquo;The development of soils and weathering profile\u0026rdquo;, in \u003cem\u003eThe origin of clay minerals in soils and weathered rocks\u003c/em\u003e (Springer, Berlin, Heidelberg, 2008), pp. 113-142. https://doi.org/10.1007/978-3-540-75634-7\u003c/li\u003e\n \u003cli\u003eVepraskas, M. J., Lindbo, D. L., Stolt, M. H, \u0026ldquo;Redoximorphic Features\u0026rdquo;. in \u003cem\u003eInterpretation of Micromorphological Features of Soils and Regoliths\u003c/em\u003e (Elsevier, 2018), pp. 425\u0026ndash;445. https://doi.org/10.1016/B978-0-444-63522-8.00015-2\u003c/li\u003e\n \u003cli\u003eVillarroael, C., Setoguchi, T., Brieva, J., Macia, C., Geology of the La Tatacoa \u0026ldquo;Desert\u0026rdquo; (Huila, Colombia): Precisions on the Stratigraphy of the Honda Group, the Evolution of the \u0026ldquo;Pata High\u0026rdquo; and the Presence of the La Venta Fauna, In: Memoirs of the Faculty of Science, Kyoto University, \u003cem\u003eSeries of Geology and Mineralogy,\u003c/em\u003e 1996, vol. 58, no. 1-2, pp. 41\u0026ndash;66. http://hdl.handle.net/2433/186679\u003c/li\u003e\n \u003cli\u003eWellman, S.S., Stratigraphy and Petrology of the Nonmarine Honda Group (Miocene), Upper Magdalena Valley, Colombia. \u003cem\u003eGSA Bulletin\u003c/em\u003e, 1970, vol. 81, pp. 2353\u0026ndash;2374. https://doi.org/10.1130/0016-7606(1970)81[2353:SAPOTN]2.0.CO;2\u003c/li\u003e\n \u003cli\u003eWesterhold, T., Bickert, T., R\u0026ouml;hl, U., Middle to late Miocene oxygen isotope stratigraphy of ODP site 1085 (SE Atlantic): New constrains on Miocene climate variability and sea-level fluctuations, \u003cem\u003ePalaeogeography, Palaeoclimatology, Palaeoecology\u003c/em\u003e, 2005, vol. 217, no. 3\u0026ndash;4, pp. 205\u0026ndash;222. https://doi.org/10.1016/j.palaeo.2004.12.001\u003c/li\u003e\n \u003cli\u003eYaalon, D. H., Soils in the Mediterranean region: What makes them different?, \u003cem\u003eCatena\u003c/em\u003e, 1997, vol. 28, no. 3\u0026ndash;4, pp. 157\u0026ndash;169. https://doi.org/10.1016/S0341-8162(96)00035-5\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTables 1 to 3 are available in the Supplementary Files section\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"palaeobiodiversity-and-palaeoenvironments","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"pbpe","sideBox":"Learn more about [Palaeobiodiversity and Palaeoenvironments](https://www.springer.com/journal/12549)","snPcode":"12549","submissionUrl":"https://www.editorialmanager.com/pbpe/default2.aspx","title":"Palaeobiodiversity and Palaeoenvironments","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"paleoclimate, paleosols, soil moisture regimes, tropical soils, soil micromorphology","lastPublishedDoi":"10.21203/rs.3.rs-7594667/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7594667/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe mid-Miocene paleosols of the Baraya Member (Villavieja Formation, La Tatacoa, Colombia) preserve a detailed palaeohydrological record during the Middle Miocene Climatic Transition. Developed in distal flood basins of meandering fluvial system, the Ferruginous Red Bed (FRB) paleosols and La Venta Red Bed (LVRB) paleosols represent cumulative, polygenetic soils strongly shaped by alternating wet\u0026ndash;dry cycles. Geochemical and micromorphological evidence reveal distinct pedogenic pathways. The FRB exhibits extreme weathering (Al₂O₃ up to 26.7%; CIA\u0026thinsp;\u0026gt;\u0026thinsp;89), depletion of base cations, and abundant clay coatings, gley pedofeatures, and Fe\u0026ndash;Mn nodules, reflecting illuviation and reduction\u0026ndash;segregation processes under wetter udic conditions. In contrast, the LVRB shows vertic features, homogenized CIA values (~\u0026thinsp;80\u0026ndash;88%), and clay infillings within shrinkage cracks, recording pedoturbation and stronger seasonal drying (udic\u0026ndash;ustic). Secondary carbonates in both profiles indicate periodic decalcification\u0026ndash;reprecipitation, though aridic conditions were not reached. These paleosols classify as Alfisol (Udalf) and Vertisol (Udert/Ustert) respectively, both reflecting subhumid climates. Integration with the fossil record indicates a transitional riparian mosaic rather than a continuous rainforest habitat associated with the La Venta Fauna. The transition zone is determined by two intertwined factors rainfall (~\u0026thinsp;1000\u0026ndash;2000 mm/year) and at least one moderate dry season (~\u0026thinsp;3 months). These findings underscore the role of soil geomorphological position and rainfall seasonality as primary drivers of redox processes and soil moisture balance, while situating Neotropical palaeosols within the broader framework of mid-Miocene climate dynamics and the northward migration of the Intertropical Convergence Zone.\u003c/p\u003e","manuscriptTitle":"Mid-Miocene palaeohydrology archived in paleosol of La Tatacoa, Colombia","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-08 14:34:25","doi":"10.21203/rs.3.rs-7594667/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revisions Needed","date":"2025-11-24T07:22:47+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-09-29T17:05:50+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-25T13:43:00+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-09-16T05:57:53+00:00","index":"","fulltext":""},{"type":"submitted","content":"Palaeobiodiversity and Palaeoenvironments","date":"2025-09-12T16:05:53+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"palaeobiodiversity-and-palaeoenvironments","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"pbpe","sideBox":"Learn more about [Palaeobiodiversity and Palaeoenvironments](https://www.springer.com/journal/12549)","snPcode":"12549","submissionUrl":"https://www.editorialmanager.com/pbpe/default2.aspx","title":"Palaeobiodiversity and Palaeoenvironments","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"1de38398-eb28-4cc7-aefc-87f5bcaa1dd3","owner":[],"postedDate":"October 8th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-02-12T14:34:22+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-08 14:34:25","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7594667","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7594667","identity":"rs-7594667","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}