Large-scale methane venting and clathrite-like structures recorded in the Late Miocene foredeep deposits of the northern Apennines (Marnoso arenacea Fm., Italy)

Large-scale methane venting and clathrite-like structures recorded in the Late Miocene foredeep deposits of the northern Apennines (Marnoso arenacea Fm., Italy) | 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 Large-scale methane venting and clathrite-like structures recorded in the Late Miocene foredeep deposits of the northern Apennines (Marnoso arenacea Fm., Italy) Simone Muzzioli, Stefano Conti, Claudio Argentino, Daniela Fontana This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6980851/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 10 Nov, 2025 Read the published version in Geo-Marine Letters → Version 1 posted 9 You are reading this latest preprint version Abstract Ancient methane-seep deposits exposed in the Apennine mountains of northern Italy are key to reconstruct the geodynamic evolution of the Miocene foredeep basin prior to the Messinian Salinity Crisis. Here, we report on a new Miocene outcrop of seep-carbonates in the Romagna region which showed indications of paleo-gas hydrate destabilization. We combined field work facies analyses, petrography of thin sections, scanning-electron microscopy and stable isotope analyses to interpret the origin of these authigenic carbonates and biogeochemical conditions at the time of their formation. These deposits show vuggy, brecciated and massive-with-lucinids lithofacies and are characterized by light carbon isotope signatures, as negative as − 47.8‰. The oxygen isotope compositions of these carbonates are out of equilibrium with coeval bottom seawater, exhibiting anomalously-heavy values. We calculated the stability of paleo-gas hydrates, which revealed that gas hydrates type I (pure methane) were stable at depths > 485 m, in agreement with the paleo-bathymetric domain of this outcrop. This area was geologically active in the Late Miocene and affected by continuous bathymetric changes caused by the eastwardly-migrating accretionary wedge and foredeep basin. We propose that gas hydrate destabilization and prolonged anaerobic oxidation of methane in the shallow sediment produced the studied carbonate deposits. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction In marine environments, subsurface hydrocarbon generation, migration and seafloor emission lead to the formation of cold seeps, which are sites where hydrocarbon-charged fluids are released into the water column (Campbell 2006 ) and potentially reach the atmosphere (Weber et al. 2019 ). Distinct features of submarine hydrocarbon emissions are a lush chemosymbiotic macrofauna (Dubilier et al. 2008 ) and authigenic carbonates, formed by microbially-mediated anaerobic oxidation of methane (AOM; Boetius et al. 2000 ; Luff et al. 2004 ). These carbonates, hereafter named seep-carbonates, are typified by a distinct geochemical composition which is particularly depleted in 13 C, due to incorporation of methane-derived carbon (Luff and Wallmann 2003 ; Peckmann and Thiel 2004 ; Cochran et al. 2022 ). The dynamic nature of cold seeps contributes to the variability in the morphology, size and distribution of seep-carbonates (Jakubowicz et al. 2020 ) as well as of the chemosynthetic habitats at the seafloor (Levin 2005 ; Judd and Hovland 2009 ). Moderate methane fluxes favor the precipitation of carbonate (Aloisi et al 2000 ) and sustain widespread chemosynthetic communities including microbial mats, tubeworms and bivalves, disposed in a concentric zonation around the seep or forming chemoherm-type build-ups (Roberts 2001 ; Taludaker 2012 ; Ho et al. 2012 ). Conversely, high flow rates inhibit benthic macrofauna to a large extent due to toxicity of sulfide released by AOM, while favoring the development of seafloor depressions such as pockmarks and larger craters in low permeability sediments (Judd and Hovland 2009 ). Seep-carbonates associated with seafloor mounds have been reported from gas hydrate pingoes (Serov et al. 2017 ) and mud volcanoes (Loher et al. 2018 ; Panieri et al. 2025 ; Argentino et al. 2025 ) and often display brecciated facies and typical vuggy fabric, multiple cement generations and chaotic arrangements of fractures and veins. These structures are indicative of high-energy conditions during seep carbonate growth and, together with heavy oxygen isotope signatures (δ 18 O), can be proxies for gas-hydrate dissociation (Bohrmann et al. 2002 ; Teichert et al. 2005 ). Gas hydrate dissociation releases 18 O-enriched fresh water and methane into sediments, thus affecting the bulk δ 18 O composition of seep-carbonates (Naher et al. 2007; Tong et al. 2016 ; Han et al. 2014 ; Loyd et al. 2016 ). Gas hydrate-associated carbonates, also known as clathrites, generally form bodies of large dimension and show peculiar structures and lithologies called clathrites (Kennett and Fackler-Adams 2000 ), relatively well described in modern seep settings (Bohrmann and Torres 2006 ; Abegg et al. 2007 ): carbonate crusts growing within pure gas hydrate layers oriented parallel to the bedding surfaces, vacuolar structures that mimics the shape of gas hydrate bubbles, associations of pure aragonitic and gas hydrate layers (zebra-like structures), monomictic and polimictic breccias resulting from the rapid destabilization of gas hydrates formerly present within the sediment pore spaces. Following present-day analogues, the possible role of gas hydrates in the formation of fossil seep-carbonates can the deduced by textural and geochemical (δ 18 O) proxies applied in several studies in the Oligocene and Miocene of the Apennine and Carpathian chains (Dela Pierre et al. 2010 ; Conti et al. 2013; Bojanowski et al. 2021 ; Argentino et al. 2019 ). Fossil seep-carbonates are therefore crucial in reconstructing the style and magnitude of past fluid circulation through the upper crust with implications for geodynamic and past atmosphere composition (Hryniewicz 2022 ). In the northern Apennines of Italy, the late Miocene paleo-tectonic and paleo-environmental context is particularly debated due to the complex relationships among tectonics, climatic changes and sea level drops, with different and contrasting reconstructions which may be improved by the analyses of the numerous Tortonian to Messinian seep-carbonates outcrops. Here, Messinian seep-carbonates are often strongly brecciated with vuggy fabric and authigenic micrite depleted in 13 C and enriched in 18 O, as expected during hydrate destabilization. In this study we investigated for the first time the well exposed outcrops of seep-carbonates contained within the Late Miocene foredeep deposits of the Romagna Apennines in the Predappio area (Fig. 1 ). They are of special importance for highlighting the occurrence and evolution of paleo gas-hydrates in deep marine environments during the complex transition to the pre-evaporitic Messinian stage linked to the Mediterranean desiccation. Geological setting The Northern Apennine chain has been formed since Late Eocene-early Oligocene as a fold and thrust belt, related to the convergence between the European and African plates, with the interposition and the collision of two microplates (Corsica-Sardinia and Adria). During the continental collision, clastic wedges accumulated in foreland basins in front of the deforming thrust belt associated with the progressive roll-back of the subducting Adriatic slab (Le Breton et al. 2017 ). Deposition mainly occurred in wedge-top and foredeep basins, progressively migrating towards NE (Argnani and Ricci Lucchi 2001 ; Faccenna et al. 2001 ; Conti et al. 2016 ). In the Miocene, the foredeep basin in the area now corresponding to the Romagna-Tuscan-Umbrian region was filled by the thick turbidite succession of the Marnoso-arenacea Fm. During its migration (Langhian-Messinian), the Apennine thrust-front incorporated foredeep siliciclastic deposits in the accretionary wedge causing the closure of the foredeep. The closure stage is marked by silty-clayey hemipelagic slope mudstones, sporadic channelized arenaceous bodies and organic-rich mudstones informally named Euxinic shales. These deposits enclose numerous seep-carbonate bodies, lens-like to stratiform, late Tortonian to early Messinian in age (Terzi et al. 1994 ; Conti et al. 2024 ). During the late Tortonian to early Messinian, numerous seep-carbonates formed and accumulated in pelitic marls corresponding to the Tossignano and Ghioli di letto mudstones (Fig. 1 ). These carbonates formed in different settings and have various morphologies: - isolated blocks with a metric extension included in poorly cemented sandstones deposited in submarine canyons (Fontanelice Member and minor Basins); - large (up to 100 m) stratiform bodies included in fine-grained apron sediments over the slope (Borgo Tossignano marls); - lenticular to stratiform bodies (1-100 m long) in restricted basins hosted in euxinic shales draping thrust bounded faults and buried ridges, forming intrabasinal highs (Ghioli di letto mudstones) (Conti et al. 2024 ). The Messinian succession ends upward with evaporitic deposits of the Gessoso-solfifera Fm, consisting of up to 16 massive gypsum beds, about 30 m thick, made of both primary (selenitic) and clastic (gessarenite) gypsum (Lugli et al. 2007 ). The passage to the evaporites is marked by 2 m-thick laminated brown to white carbonates (Calcare di Base auctorum ) interpreted in many outcrops as microbial-mediated carbonates (Guido et al. 2007 ) and recently re-interpreted locally as being methane-derived carbonates (Conti et al. 2024 ). The carbonate outcrop investigated in our study pertains to this context and is a new key to determining the temporal and spatial scales of cold seepage at the onset of the Messinian Salinity Crisis. The Messinian succession ends with fluvio-deltaic and lacustrine deposits with brackish water fauna of the Lago-Mare biofacies (Bassetti et al. 2004 ) ascribed to various continental and transitional environments. Climatic oscillations and tectonics determined the evolution from brackish “lake-sea” to marginal fluvio-deltaic and marsh conditions. Methods A detailed field work of the Predappio area has allowed us to draft a high-resolution geological map including the distribution, facies and geometries of seep-carbonates. A stratigraphic log, 30 m thick, was measured in detail in the well-exposed Sasseto outcrop where seep-carbonates exhibit different facies and are covered with erosive contact by conglomerates of the Colombacci Fm. We distinguished lithology, fossil content, depositional and fluid related structures, focusing more on the brecciated and vuggy fabrics due to their significance in relation to paleo-seep dynamics. Eleven samples of carbonate rocks, subdivided in 29 sub-samples, were sampled from all lithofacies for petrographic and isotopic analyses in order to identify different microfacies and authigenic phases. Thin sections were studied using a Leitz optical microscope and investigated via Scanning Electron Microscopy hosted at the Department of Chemical and Geological Sciences, University of Modena and Reggio Emilia (Italy); semi-quantitative elemental information were acquired via energy-dispersive spectrometry (EDS) at accelerating voltage of 20 kV and spot size 60 mm. SEM-EDS analyses were focused on three samples (PR8, PR9, PR10) collected from a brecciated and vuggy carbonate facies. All results from spot analyses are reported in the Supplementary Information. Material for stable C and O isotope analysis was collected from the microcrystalline groundmass, and from different cement phases. 18 powder samples of carbonates, massive and laminated, were analyzed at the Department of Chemical, Life and Environmental Sustainability of the University of Parma. About 10 mg were reacted with 100% phosphoric acid at 25°C for 24 h. The purified CO2 gas was analyzed using a Finnigan MAT 252 gas isotope ratio mass spectrometer. Replicate analyses of reference materials throughout the session yielded a precision better than 0.1‰ (1s) for both δ 13 C and δ 18 O values. A second batch of 11 powders were prepared and analyzed at the University of Modena and Reggio Emilia. Powders were obtained using a handheld microdrill from thin section slabs or polished rock surfaces. Around 0.5 mg were reacted with 100% phosphoric acid at 25°C for 4 h. The purified gas was analyzed on a Isoprime precisION (Elementar) stable isotope ratio mass spectrometer at Centro Interdipartimentale Grandi Strumenti (C.I.G.S.) of the University of Modena e Reggio Emilia. Accuracy of the measurements was evaluated by running six replicates of both NBS18 and NBS19 reference materials, yielding results in agreement with recommended values. Repeatability precision (1s) of three duplicate samples measured in the same session was better than 0.13‰ and 0.084‰ for δ 13 C and δ 18 O, respectively. All isotope results are reported in standard δ notation relative to the Vienna-PeeDee Belemnite (V-PDB) standard. Results Outcrop geology The examined outcrop (Sasseto) is characterized by numerous bodies of methane-derived carbonates distributed over an extension of approximately 800 m in the WNW-ESE direction, in correspondence with two important tectonic structures (Figs. 1 , 2 ). The western part of the outcrop is parallel to one of the major buried external fronts of the northern Apennine fold-and-thrust belt over the Tortonian Marnoso-arenacea Fm. over the upper Messinian deposits. The eastern part has a N-S direction parallel to the still active Forlì fault line. The carbonate bodies have variable geometries, morphologies and distribution varying from lenticular, amygdaloid, pinnacles, stratiform and scattered irregular, with thickness of individual bodies from 2 m up to 30 m and lateral extension up to 100 m. Stratiform carbonate bodies are concordant with the encasing deposits, except for the basal pinnacular contact which is almost vertical. The carbonate bodies form reliefs that stand out on the encasing marly sediments, separated by valley incisions along which landslide phenomena develop (Fig. 2 ). The Sasseto outcrop is folded into an anticlinal structure with the axis dipping towards SE. Stratigraphy and seep carbonate facies In the Sasseto outcrop we measured a 30 m-thick stratigraphic section from the basal contact with the Marnoso arenacea Fm. (Borgo Tossignano Member) to the upper discordant contact with alluvial conglomerate deposits of the Colombacci Fm (Fig. 3 ). This section includes seep-carbonates exhibiting different facies from brecciated, massive and to microbial laminated carbonates, each characterized by distinct lithologies, textures, presence/absence of veins or fractures and fossil content (Fig. 4 a, b, c, d). Although these associations are partially intertwined, this study reconstructs their vertical succession which is recurrent in the whole examined area and has a transitional trend from the base to the top (Fig. 3 ). Facies 1 (P1) consists of brecciated limestones, with gray, calcilutite, angular to subangular monogenic clasts of 1–10 cm in size, made up of different micritic lithologies, referable to fragmentation of previously formed carbonate crusts (Fig. 4 c). Clasts are chaotically dispersed in the authigenic micritic matrix or in a fine to medium-grained sandy matrix. Larger clasts derive from the coalescence of heterometric smaller clasts, testifying various cycles of cementation and fragmentation. This facies commonly lacks macrofossils. Facies 1 characterizes the basal portion of the carbonate bodies where it can be up to 15 meters thick, but can be found also at different stratigraphic levels and in the upper portion of single bodies, less intensely brecciated and a minor occurrence of veins and druses. Brecciated portions are pervaded by veins and extensional fractures; vein dimensions range from 1–2 mm to 2 cm, they are not systematic and show a sinuous to irregular distribution; in most cases they can join forming veins of major dimensions (1–2 cm) containing abundant black iron sulfides. Frequently elongate fractures merge to form vuggy and spongy fabrics with cavities ranging from 1 to 3 cm (Fig. 4 e, f). The abundance of veins and fractures of centimetric dimension may give to the carbonate masses the appearance of megabreccias. The vein infillings may contain coquina debris (shell fragments), coarse calcarenites and carbonate micro-breccias. In this lithofacies the fossil content is scarce, represented by disarticulated clams and rare articulated lucinids. Micritic, mottled carbonates pervaded by irregular networks of veins and druses (from few mm up to 3 cm in diameter) and cut by millimetric-scale fractures are scattered. Facies 2 (P2) consists of massive marly limestones, pale brown in colour, with abundant fossil content. Fossils are densely packed articulated and disarticulated bivalves, mostly lucinids (Fig. 4 b), as irregularly distributed clusters, in several cases in life position. Fossils range from a few centimeters, up to fifteen cm in diameter. Facies 2 dominates in the intermediate portion of the carbonate bodies beneath the microbial laminated carbonates, but it can be present also in the basal portion, with heteropic contact with the brecciated facies P1. Typically, this facies has a maximum thickness of 5–7 m. Facies 3 (P3) is characterized by laminated microbialitic carbonates composed of millimetric-scale alternations of brown-dark gray to light grey-whitish laminae (Fig. 4 d). The laminate pattern is sub-horizontal to wavy; macrofossils are absent. Facies 3 characterizes the upper portion of the section, with a thickness of about 5 m. It is cut by an erosive contact with fluvial conglomerates of the Colombacci Fm. Facies 3 locally is heteropic with stratified calcarenites with decimetric beds and occasional weak lamination. Microfacies Most of the samples from the massive facies in the lower-middle part of the sections are typical seep-carbonates with abundant chemosymbiotic macrofauna, largely fragments of lucinid bivalves (Fig. 5 ). They are predominantly made of micrite with minor early-diagenetic microspar (Fig. 5 a). Micrite is composed of dolomite but calcite is also present. Microspar may be calcitic and dolomitic. Pyrite, a typical mineral in seep-carbonates, was observed and is usually developed as small, partly oxidized framboids, or forming patches of thin microbial laminae more frequently observed in the dolomite micrite (Figs. 5 c). Massive carbonates locally contain planktonic foraminifera as well as sand- to silt-sized detrital material mainly aligned along fractures and conduits (Fig. 5 a). Detrital fraction is largely composed of quartz, K feldspar, phillites and muscovite similar to the detritus of the underlying Marnoso arenacea turbidites. In the brecciated /vuggy microfacies, micritic intraclasts of various shape, commonly angular, and color (light micrite and brown micrite) with apparent chaotic arrangements are associated to a complex network of fractures and veins (Fig. 5 d, e, f). Micritic intraclasts derive from the fragmentation of previously formed carbonate concretions. Abundant microcavities (mm to cm size) giving a “spongy” appearance are present within the cementing dolomicrite and dolomicrosparite. Larger cavities, vacuolar structures are also present with an irregular, often elongated shape, oriented parallel to the bed/laminae surfaces. They are marked by a rim of dark dolomicrite and filled by equant calcite spars. The carbonate samples collected from the upper laminated lithofacies are made up of millimetre-scale alternations of laminae with different microfabrics: dark micrite alternated with light microspar levels, white to gray in color (Fig. 5 g, j). They are predominantly composed of calcite or dolomite and calcite (Table 1 ). Micritic laminae consist mainly of a dense arrangement of peloidal particles: peloids are rounded, tens of microns in size, with ovoidal to subcylindrical shape. Clotted dolomicrite locally shows a dendritic-like arrangement. Rounded and elongated dense aggregates of dolomicrite are present with a preferred orientation parallel to the surfaces of laminae, enclosed in a matrix of calcite microspar. Micritic laminae are alternated with gray laminae in which calcite microspar includes variable amounts of silica and clay particles. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) The SEM-EDS analyses of three samples (PR8, PR9, PR10) from outcrop facies P1, revealed a recurring chemistry of the carbonate phases (Supplementary Information). In all the samples, we recognized a low backscatter dolomitic phase (Fig. 6 ) containing 30.3–71.6% m/m CaO and 20.3–44.8% m/m MgO, in contact with a higher-backscatter low-Mg calcite phase (up to 2.2% m/m MgO) (Fig. 6 a, c, d). Calcite also fills fractures in the dolomite cements, indicating it formed later in the paragenetic succession. Pyrite is abundant and occurs with framboidal morphologies forming aggregates ranging in size from < 5 mm to ~ 30 mm. Pyrite is locally distributed in a laminated fashion (Fig. 6 b). Particles of barite have also been identified via SEM-EDS within the dolomite cements (Supplementary Information). Isotope geochemistry The stable carbon and oxygen isotopic composition of the investigated samples is highly heterogeneous, with common trends at microfacies scale. Starting from the stratigraphically lower samples (PR8, PR9 and PR10) from the brecciated facies P1, the δ13C values range between − 44.2‰ and − 31.2‰ (Table 1 ; Fig. 7 ). We distinguished mainly 5 microfacies in these isotope investigations (Fig. 7 ). Overall, micrite (vuggy microfacies, light and dark micrites) has heavier δ 18 O and slightly less negative δ 13 C compared to the sparry and rim cements. Two micritic samples from outcrop facies 2 rich in lucinid clams yielded δ 13 C values of − 32.7‰ and − 27.0‰, slightly heavier than in facies P1 (Fig. 7 ) and δ 18 O values of 5.8‰ and 5.6‰. Moving stratigraphically upward, samples from the laminated facies P3 show isotopic differences depending on the type of laminae, with the ones rich in peloids having more negative δ 13 C values than the others. The peloidal laminae show the most negative δ 13 C values of the entire dataset ranging from − 47.8‰ to − 14.5‰, and δ 18 O values between 0.8‰ to 2.7‰. Two micritic laminae show δ 13 C values of − 23.1‰ and − 8.7‰ and δ 18 O values of 3.8‰ and 4.8‰. A microsparitic laminae yielded δ 13 C = − 11.3‰ and δ 18 O = 1.8‰. Microsparitic cement in a sample collected from facies P4 has δ 13 C = − 34.8‰ and δ 18 O = 0.7‰. In general, micritic samples have heavier δ 18 O compared to the sparry and rim cements (Fig. 7 ). Table 1 Carbonate mineralogy, δ13C and δ18O values, for samples from Predappio area representing all three facies and different cements. Carbonate mineralogy was established by thin section petrography and SEM. Sample Facies Target phase Predominant Mineralogy δ 13 C (‰) δ 18 O (‰) SM10a P3 Cement laminae Calcite −11.33 1.81 SM10b P3 Peloidal laminae Calcite −14.47 1.81 PR11 P3 Microsparitic cement Calcite −34.76 −0.74 PR3a P3 Micritic laminae Dolomite > Calcite −8.68 4.81 PR3b P3 Peloidal laminae Dolomite > Calcite −28.1 2.7 PR2a P3 Micritic laminae Calcite −23.14 3.78 PR2b P3 Peloidal laminae Calcite −47.83 0.79 PR6 P2 Micrite Dolomite −26.99 5.58 PR5 P2 Micrite Dolomite −32.66 5.76 PR4 P1 Light micrite Dolomite −39.83 6.98 PR10a P1 Rim cement Dolomite −41.56 1.48 PR10b P1 Light micrite Dolomite −34.95 6.25 PR10c P1 Light micrite Dolomite −35.91 6.53 PR10d P1 Rim cement Dolomite −42.38 3.11 PR10e P1 Vuggy micrite Dolomite −41.6 6.1 PR9a P1 Light micrite Dolomite −39.51 7.2 PR9b P1 Light micrite Dolomite −39.75 7.91 PR9c P1 Light micrite Dolomite −39.72 7.92 PR9d P1 Rim cement Dolomite −39.72 2.72 PR9e P1 Sparitic cement Calcite −38.5 2.66 PR9f P1 Dark micrite Dolomite −39.64 7.04 PR8a P1 Light micrite Dolomite −36.74 6.31 PR8b P1 Sparitic Cement Calcite −44.22 0.02 PR8c P1 Sparitic Cement Calcite −39.62 −0.17 PR8d P1 Light micrite Dolomite −41.05 7.69 PR8e P1 Rim cement Dolomite −44.12 0.6 PR8f P1 Sparitic cement Calcite −43.03 1.18 PR8g P1 Dark micrite Dolomite −41.15 6.27 SM3 P1 Sparitic cement Calcite −31.18 0.07 Discussion The studied seep-carbonate outcrop included in marine euxinic sediments of the Apennine foredeep displays an evident change of lithofacies passing from brecciated carbonate facies dominating the lower-medium portion of the stratigraphic section to massive micritic facies rich in large bivalve mollusks and planktonic foraminifera (also found laterally to the breccias). The seep-impacted sequence ultimately evolves at the top into laminated microbialites without macrofauna. The microcrystalline groundmass is mostly composed of primary micrite, largely dolomitic and subordinately calcitic, and by early-diagenetic microspar cements. Clasts in brecciated facies are micrites, light and dark in color. Different generations of cements fill voids and fractures, rim dolomitic cements and later sparitic calcite filling voids. The laminated deposits are mostly composed of prevalent calcite (subordinate dolomite) microlaminae rich in peloidal particles alternated with microsparitic layers including scarce amounts of clay minerals. The samples from our study showed wide isotopic ranges (Fig. 7 ): strongly depleted δ 13 C values (− 44‰ to − 27‰) occur in the massive and brecciated facies, both micrite and cements; whereas in the upper laminated facies seep-carbonates show less depleted values in δ 13 C (up to − 8‰). The anaerobic oxidation of methane releases isotopically light dissolved inorganic carbon (DIC), which adds up to other DIC sources already present in the pore water, i.e. seawater-derived DIC (δ 13 C~0‰) and DIC from organic matter oxidation (>~ −25‰). AOM-derived DIC carries the composition of methane being oxidized, which in turn is generally much lighter than − 30‰ (Judd and Hovland, 2009 ). Since these authigenic carbonates form in the sediment interval of highest alkalinity production by AOM, they tend to record δ 13 C compositions < − 30‰. The values measured in our samples clearly point to variable amounts of incorporation of methane-derived carbon in a methane-charged sedimentary setting. The distribution and trend in isotope values of the different seep-carbonates are likely consequence of variations in fluid intensity and possibly composition, with the establishment of diverse environmental settings favoring the development of different facies. The massive carbonate facies with large bivalves (P1) indicates a seafloor environment with low-moderate seepage conditions and rates of methane oxidation. In fact, high AOM rates would generate high fluxes of HS − toward the seafloor which can cause temporary toxic conditions to macrobenthos (Bagarinao 1992 ; Riesch et al. 2015 ) relying on well-oxygenated bottom waters for their metabolic processes. We do not have clear indications to estimate paleodepth, but the regional geology indications (Di Giulio et al. 2013 ) and, for some extents, the abundance of planktonic foraminifera could confirm a moderately deep slope environment, at least several hundred meter deep (between approximatively 500 and 1000 m) .The siliciclastic detritus dispersed in micritic carbonates and concentrated in fractures within this facies (Fig. 4 a) belongs to the encasing sediments (MA Fm) confirming a fluid circulation originated from the substratum. A dominant portion of the studied seep bodies is strongly fragmented, to form monogenic breccias with clasts made up of light and brown micrites and abundant vuggy, vacuolar, bubble-like, drusy fabrics. Voids and fractures are filled by two main cements: an earlier dolomite rim cement and later calcite cement filling void and fractures. In this facies, the micrite and cements are remarkably enriched in 18 O, in particular, δ 18 O in micrite is invariably comprised from 6 and 8‰. The δ 18 O composition of authigenic carbonates forming in equilibrium with coeval seawater can be calculated by applying thermodynamic equations. Assuming an average δ 18 O = − 0.4‰ (vSMOW) (James et al., 1994 ; Lear et al. 2000 ) and bottom water temperature ~ 5°C (Barbieri et al. 2000 ) we obtain values of δ 18 O= 0.94‰ for low-Mg calcite (Tarutani 1969; Friedman and O'Neil 1977) and δ 18 O= 3.5‰ for dolomite (Vasconcelos et al. 2005 ). Therefore, the values measured in our samples are not compatible with precipitation in isotopic equilibrium with coeval seawater as they are heavier than calculated. We also rule out post-depositional diagenetic alteration, since the temperature gradients in the sedimentary column and/or meteoric weathering would alter the carbonate isotopic composition toward lighter values, which is the opposite to what we observed. We hypothesize that these carbonates have similar origin to other Apennine paleo-seeps in this region (Conti et al. 2021 ) and related to paleo-gas hydrate destabilization. Gas hydrate stability at the time of seep carbonate formation is difficult to extrapolate since there are many uncertainties on the exact paleo-environmental conditions. However, we estimated the minimum paleo-seafloor depth at which gas hydrates were stable by assuming: 1) pure methane-hydrate (structure I); 2) normal salinity 37 psu (Flecker et al. 2015 ); 3) normal geothermal gradient 31°C/km; 4) steady state conditions. The model was implemented on the graphical user interface CAGEHYD (Vadakkepuliyambatta 2019 ) based on the CSMHYD code (Sloan 2006 ). Our model indicates that gas hydrates were stable at water depths > 485 m, which is consistent with lithological constraints imposed by our field work datasets. The heavy values in the samples could also be explained by the occurrence of deep-sourced fluids rich in 18 O from clay dehydration at greater depth within the sediments (Kastner et al. 1991 ). However, the petrographic features resembling modern clathrites together with methane-related carbon isotopic signatures, support the hydrate hypothesis for the origin of the carbonates in the Predappio outcrop. The upper portion of the examined seep-carbonate bodies contain several structures typical of microbial sediments, such as laminations, peloids, elongated micritic filament-like particles and clotted micrite. As observed in other outcrops (Brisighella, Conti et al. 2024 ) these features could be interpreted as cyanobacterial (or some other bacterial) or algal origin. Yet, the δ 13 C values as negative as − 47.8‰ in a peloidal laminae in this facies still points to variable amounts of incorporation of methane-derived carbon thus suggesting a methane-rich sediment conditions. Similar to the examined deposits are also stromatolites from the Lower Messinian of the Calcare di Base Fm from Sicily (Oliveri et al. 2010 ); based on morphology, the filaments have been interpreted as Beggiatoa -like sulphur bacteria, The laminated pattern in the Brisighella section is also comparable to structures known both from modern and fossil benthic coccoidal cyanobacterial mats that underwent early mineralization (Kempe and Ka´zmierczak 1993 ) such as cyanobacterial mats from Sulej´ow Dam, Poland, and subfossil mats from a crater lake on Satonda Island, Indonesia. Petrographic structures and isotope compositions similar to the ones from this study have been reported from Messinian Monferrato ( Septarian Like-Beds in Natalicchio et al. 2012 ), Montepetra (Conti et al. 2010 ) and Brisighella outcrops (Conti et al. 2024 ) and the paleo occurrence of gas hydrates has been based on the strict similarity with and geochemistry and features reported for clathrites described by various authors (Greinert et al. 2001 ; Pierre and Rouchy 2004 ; Teichert et al. 2005 ; Bohrmann and Torres 2006 ; Suess 2020 ; Bojanowski et al. 2021 ). In fact, following these indications, Bohrmann et al. ( 2002 ) and Kennett and Fackler-Adams ( 2000 ) proposed that these methane derived authigenic carbonates ‘may be classified as one type of clathrites and can be used as markers of gas hydrates in the geological record. Nevertheless not all fractured seep-carbonates are related to gas hydrate system and some may be from the deeper gas reservoirs as for the origin of fluids from a secondary circuit in the Montepetra outcrop (Conti et al. 2010 ). The complex history of development of overpressure and episodic hydrofracturing would have enlarged the conduits and dismembered the previously precipitated crusts, forming a further reworking of the breccias (Fig. 4 c, e). As reported in modern analogue systems in foredeep basins worldwide, the hydrate destabilization and high methane fluxes to the paleo-seafloor would have been favored by the active geodynamic conditions during the late Miocene, in the eastwardly-migrating Apennine foredeep. Conclusions In this study we report on a new Miocene seep-carbonate outcrop in the northern Apennines (Italy) which shows strong evidence for paleo-gas hydrate destabilization. This outcrop is composed of carbonate bodies with stratiform to pinnacular geometries, and having thicknesses up to 30 m and lateral extension up to 100 m. The entire outcrop is 800 m in lateral extent and is hosted in the Tortonian interval of the Marnoso arenacea Fm. (turbiditic foredeep deposits). We interpreted three main carbonate facies in a stratigraphic section encompassing the seep-impacted interval. The lowermost carbonate bodies mainly consist of brecciated limestones in a micritic or sandy matrix, characterized by veins, fractures, and scarce fossil content. Moving upward, carbonates are made up of marly limestones with densely packed bivalve fossils (mostly lucinids) in life position. The uppermost carbonates show a laminated microbialites barren of macrofossils. This facies succession can be traced also within individual bodies. Stable isotope analyses on different facies revealed anomalously heavy isotope oxygen signatures (7.9‰) in the basal massive facies associated with vuggy appearance and d 13 C 485 m, in agreement with the expected paleo-bathymetry of this slope setting. Similarly to what observed in modern analogue systems in foredeep basins worldwide, the active geodynamic conditions in the eastwardly-migrating Apennine foredeep during the Miocene would have caused hydrate destabilization and high methane fluxes to the paleo-seafloor. This process eventually resulted in the formation of widespread seep-carbonate deposits now exposed onshore. This model, combined with previous studies on coeval outcrops in northern Italy, provides new insights into cold seepage dynamics in this sector of the Western Mediterranean during the stages prior to the onset of the Messinian Salinity Crisis. Declarations Funding This research has been founded by FAR Unimore 2024. CA position is funded by EMAN7 project (Research Council of Norway Grant No. 320100). Author Contribution S.M., S.C. and D.F. conceived the study, and together with C.A. they designed the experiments. S.M., S.C. and D.F. performed field work and stratigraphic logging; all the authors discussed and interpreted the microscopy and geochemistry data. S.M., S.C. and D.F. prepared Figures 1, 2, 3 and Table 1, and C.A. prepared Figures 4, 5, 6 and 7 and conducted thermodynamic modeling. All co-authors contributed to the manuscript. Acknowledgement We are grateful to Federico Lugli and Anna Cipriani (Unimore) for analytical assistance during isotope analyses and for the use of MEGIC Metallomics and Geochemistry laboratory (Unimore) tools for sample preparation. We are indebted to Paolo Serventi (Unimore) for assistance in the field and sample collection and to Simona Marchetti Dori (Unimore) for SEM analytical assistance. 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Supplementary Files SupplementaryInformation.xlsx Cite Share Download PDF Status: Published Journal Publication published 10 Nov, 2025 Read the published version in Geo-Marine Letters → Version 1 posted Editorial decision: Revision requested 16 Aug, 2025 Reviews received at journal 12 Aug, 2025 Reviews received at journal 11 Aug, 2025 Reviewers agreed at journal 27 Jul, 2025 Reviewers agreed at journal 22 Jul, 2025 Reviewers invited by journal 22 Jul, 2025 Editor assigned by journal 02 Jul, 2025 Submission checks completed at journal 28 Jun, 2025 First submitted to journal 26 Jun, 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. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6980851","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":489632734,"identity":"58b47114-9bfb-42cd-9255-e1d0f2243970","order_by":0,"name":"Simone Muzzioli","email":"","orcid":"","institution":"University of Modena and Reggio Emilia","correspondingAuthor":false,"prefix":"","firstName":"Simone","middleName":"","lastName":"Muzzioli","suffix":""},{"id":489632735,"identity":"017d4900-d148-4a09-94e3-486cf403d6cc","order_by":1,"name":"Stefano Conti","email":"","orcid":"","institution":"University of Modena and Reggio Emilia","correspondingAuthor":false,"prefix":"","firstName":"Stefano","middleName":"","lastName":"Conti","suffix":""},{"id":489632736,"identity":"0acafc18-5a9e-4a6b-99a3-c7cca9074a29","order_by":2,"name":"Claudio Argentino","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA/klEQVRIiWNgGAWjYDACdiDmMYBzJYAiDQwH8GphRtGSANTCc4DhAF49YC1wXgLIIiCBTwt/M/MziTcFNgz8s49ffFz4wyKPX/KN4eEPDHfkcGmROMxmJjnHII1B4lxOsfGMBIliydk5BkCHPTPGpcWAmcFMmsfgMAPDGZ40aZ4EicQNt8FaDic24NTC/g2sRf4MT/pvsJabZwhp4YHYYnCG/RgzWMsNHvxaJA7zFFsC/cJjeIaHWZonTSJxZk9awYEzBodx+oW/vX3jjTd/bOTkzrA//MxjU5fYz35484eKisM4QwwGeFDSANDBhDSAAfsDopSNglEwCkbByAMAd11SWaZn72kAAAAASUVORK5CYII=","orcid":"","institution":"UiT The Arctic University of Norway","correspondingAuthor":true,"prefix":"","firstName":"Claudio","middleName":"","lastName":"Argentino","suffix":""},{"id":489632737,"identity":"23658b94-7d42-40c6-ae4b-bd108d24e37c","order_by":3,"name":"Daniela Fontana","email":"","orcid":"","institution":"University of Modena and Reggio Emilia","correspondingAuthor":false,"prefix":"","firstName":"Daniela","middleName":"","lastName":"Fontana","suffix":""}],"badges":[],"createdAt":"2025-06-26 07:53:16","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6980851/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6980851/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00367-025-00830-8","type":"published","date":"2025-11-10T15:57:46+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":87499961,"identity":"3b8717f9-3640-4e2c-acc5-c14843aaafcc","added_by":"auto","created_at":"2025-07-24 13:40:46","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":183851,"visible":true,"origin":"","legend":"\u003cp\u003eSimplified structural map of the Romagna Apennines (Italy) showing the location of the studied outcrops (Predappio area).\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6980851/v1/5bc9c1fd0664b6e714cc7055.jpeg"},{"id":87499959,"identity":"5633609b-4c00-4f58-9b8b-be3cf47cd74a","added_by":"auto","created_at":"2025-07-24 13:40:46","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1292725,"visible":true,"origin":"","legend":"\u003cp\u003eDetailed geological map of the studied carbonate outcrops in the Predappio area showing seep facies distribution and enclosing rocks: slope mudstones and euxinic shales of the upper Miocene Marnoso arenacea Fm. and overlying conglomerates of the upper Messinian Colombacci Fm. The location of the measured log and collected samples is reported.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6980851/v1/18e1a574367fc8684b1fd0ac.jpeg"},{"id":87501552,"identity":"df084f02-4bc5-4297-b32c-8922545d8d50","added_by":"auto","created_at":"2025-07-24 13:56:46","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":323464,"visible":true,"origin":"","legend":"\u003cp\u003eStratigraphic log of the Sasseto seep-carbonate outcrop (Predappio area) showing facies distribution, structures and samples.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6980851/v1/a25e0fb7ef0a2063694dab20.jpeg"},{"id":87499960,"identity":"dd320d30-1ae8-49c5-8b11-fb9ccfbbcb2e","added_by":"auto","created_at":"2025-07-24 13:40:46","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1184332,"visible":true,"origin":"","legend":"\u003cp\u003eSeep-carbonate facies in the studied outcrops: (a) stratiform seep-carbonate body containing abundant macrofauna; (b) densely packed lucinid-like clams in the P2 facies (c) carbonate monogenic breccias in the basal portion of the seep-carbonate sequence (P1); (d) laminated carbonates upsection (P3 facies); (e) network of fractures filled by authigenic micrite in the brecciated facies; clasts derive from fragmentation of previously precipitated micrite; (f) vuggy fabric with abundant microcavities showing an irregular elongated shape, oriented parallel to the bed/laminae surfaces, marked by a rim of dark dolomicrite (P1 facies).\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6980851/v1/7f78df1ace1b0f41ef9272c6.jpeg"},{"id":87500514,"identity":"813e88e9-9d95-467e-b38d-15c0872e24ef","added_by":"auto","created_at":"2025-07-24 13:48:46","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1364605,"visible":true,"origin":"","legend":"\u003cp\u003ePhotomicrograph of the studied seep-carbonates: (a) micritic carbonates of the massive facies, made up of dolomite and subordinate calcite, locally containing planctonic foraminifera and silt-sized siliciclastic detrital material sourced from the underlying Marnoso-arenacea turbidites. Grains are mainly aligned along fractures; (b) Different generations of microsparitic to sparitic and isopachous cements bordering fractures in the massive facies; (c) Pyrite in dark dolomicrite of the massive facies forming patches of thin microbial laminae; light pyrite in the lower right; (d) Different types of micritic intraclasts (dark micrite and peloidal) in the brecciated microfacies; (e) Isopachous cement and microbial spherules among dolomicrite and dolomicrosparite in the brecciated/vuggy microfacies; (f) network of fractures and veins and microcavities within \u0026nbsp;dolomicrite in the vuggy microfacies. Vacuolar structures are marked by a rim of dark dolomicrite and filled by equant calcite spars; (g) millimetric alternations of dark micrite laminae alternated with light microspar levels composed of calcite or both dolomite and calcite Micritic laminae consist of a dense arrangement of peloidal particles with ovoidal to subcylindrical shape (upper P3 facies); (j) Micritic laminae alternated with laminae of calcite microsparite rich in peloids in the upper laminated P3 facies.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6980851/v1/9a095c4affc4f9256260e63d.jpeg"},{"id":87499964,"identity":"bcf8ddd7-b212-4929-81ba-6d8fb54fb08f","added_by":"auto","created_at":"2025-07-24 13:40:46","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1142691,"visible":true,"origin":"","legend":"\u003cp\u003eBackscattered SEM images of thin sections obtained from the massive and fossiliferous carbonates. Lighter colors correspond to heavier compounds. a) Contact between microcrystalline dolomite and calcite phases in sample PR8. b) Laminated pyrite (iron sulfide) accumulations in sample PR9. Inset shows a close-up view of pyrite framboids. c) Higher magnification on dolomite cements lining a cavity in sample PR9. d) Calcite and dolomite phases in sample PR10; calcite fills the fractures in the dolomite.\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6980851/v1/f85a646a3792e45ee802e299.jpeg"},{"id":87500508,"identity":"35105972-4792-4e9d-a30e-c0d6c45b5af0","added_by":"auto","created_at":"2025-07-24 13:48:46","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":84068,"visible":true,"origin":"","legend":"\u003cp\u003eIsotopic composition (δ\u003csup\u003e13\u003c/sup\u003eC, δ\u003csup\u003e18\u003c/sup\u003eO) of carbonate samples from the Predappio outcrop. Different symbols and colors refer to the major outcrop facies (figs 2 and 3) and microfacies identified on thin sections, respectively (Table 1).\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6980851/v1/765dce54024a8afff4742463.jpeg"},{"id":96105016,"identity":"5799afa2-085d-4a22-adf3-3f488ffc3a52","added_by":"auto","created_at":"2025-11-17 16:07:11","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6351279,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6980851/v1/11800feb-b9bd-4741-afef-4263155b2d21.pdf"},{"id":87500516,"identity":"3cb8b781-0a74-4232-ba38-2941f3a1bab7","added_by":"auto","created_at":"2025-07-24 13:48:46","extension":"xlsx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":3366587,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryInformation.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-6980851/v1/a0a00d100d5f69125772daf1.xlsx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Large-scale methane venting and clathrite-like structures recorded in the Late Miocene foredeep deposits of the northern Apennines (Marnoso arenacea Fm., Italy)","fulltext":[{"header":"Introduction","content":"\u003cp\u003eIn marine environments, subsurface hydrocarbon generation, migration and seafloor emission lead to the formation of cold seeps, which are sites where hydrocarbon-charged fluids are released into the water column (Campbell \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and potentially reach the atmosphere (Weber et al. \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Distinct features of submarine hydrocarbon emissions are a lush chemosymbiotic macrofauna (Dubilier et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) and authigenic carbonates, formed by microbially-mediated anaerobic oxidation of methane (AOM; Boetius et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Luff et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). These carbonates, hereafter named seep-carbonates, are typified by a distinct geochemical composition which is particularly depleted in \u003csup\u003e13\u003c/sup\u003eC, due to incorporation of methane-derived carbon (Luff and Wallmann \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Peckmann and Thiel \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Cochran et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The dynamic nature of cold seeps contributes to the variability in the morphology, size and distribution of seep-carbonates (Jakubowicz et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) as well as of the chemosynthetic habitats at the seafloor (Levin \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Judd and Hovland \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Moderate methane fluxes favor the precipitation of carbonate (Aloisi et al \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) and sustain widespread chemosynthetic communities including microbial mats, tubeworms and bivalves, disposed in a concentric zonation around the seep or forming chemoherm-type build-ups (Roberts \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Taludaker \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Ho et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Conversely, high flow rates inhibit benthic macrofauna to a large extent due to toxicity of sulfide released by AOM, while favoring the development of seafloor depressions such as pockmarks and larger craters in low permeability sediments (Judd and Hovland \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Seep-carbonates associated with seafloor mounds have been reported from gas hydrate pingoes (Serov et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) and mud volcanoes (Loher et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Panieri et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Argentino et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2025\u003c/span\u003e) and often display brecciated facies and typical vuggy fabric, multiple cement generations and chaotic arrangements of fractures and veins. These structures are indicative of high-energy conditions during seep carbonate growth and, together with heavy oxygen isotope signatures (δ\u003csup\u003e18\u003c/sup\u003eO), can be proxies for gas-hydrate dissociation (Bohrmann et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Teichert et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Gas hydrate dissociation releases \u003csup\u003e18\u003c/sup\u003eO-enriched fresh water and methane into sediments, thus affecting the bulk δ\u003csup\u003e18\u003c/sup\u003eO composition of seep-carbonates (Naher et al. 2007; Tong et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Han et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Loyd et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Gas hydrate-associated carbonates, also known as clathrites, generally form bodies of large dimension and show peculiar structures and lithologies called clathrites (Kennett and Fackler-Adams \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2000\u003c/span\u003e), relatively well described in modern seep settings (Bohrmann and Torres \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Abegg et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2007\u003c/span\u003e): carbonate crusts growing within pure gas hydrate layers oriented parallel to the bedding surfaces, vacuolar structures that mimics the shape of gas hydrate bubbles, associations of pure aragonitic and gas hydrate layers (zebra-like structures), monomictic and polimictic breccias resulting from the rapid destabilization of gas hydrates formerly present within the sediment pore spaces. Following present-day analogues, the possible role of gas hydrates in the formation of fossil seep-carbonates can the deduced by textural and geochemical (δ\u003csup\u003e18\u003c/sup\u003eO) proxies applied in several studies in the Oligocene and Miocene of the Apennine and Carpathian chains (Dela Pierre et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Conti et al. 2013; Bojanowski et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Argentino et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eFossil seep-carbonates are therefore crucial in reconstructing the style and magnitude of past fluid circulation through the upper crust with implications for geodynamic and past atmosphere composition (Hryniewicz \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In the northern Apennines of Italy, the late Miocene paleo-tectonic and paleo-environmental context is particularly debated due to the complex relationships among tectonics, climatic changes and sea level drops, with different and contrasting reconstructions which may be improved by the analyses of the numerous Tortonian to Messinian seep-carbonates outcrops. Here, Messinian seep-carbonates are often strongly brecciated with vuggy fabric and authigenic micrite depleted in \u003csup\u003e13\u003c/sup\u003eC and enriched in \u003csup\u003e18\u003c/sup\u003eO, as expected during hydrate destabilization. In this study we investigated for the first time the well exposed outcrops of seep-carbonates contained within the Late Miocene foredeep deposits of the Romagna Apennines in the Predappio area (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). They are of special importance for highlighting the occurrence and evolution of paleo gas-hydrates in deep marine environments during the complex transition to the pre-evaporitic Messinian stage linked to the Mediterranean desiccation.\u003c/p\u003e\n\u003ch3\u003eGeological setting\u003c/h3\u003e\n\u003cp\u003eThe Northern Apennine chain has been formed since Late Eocene-early Oligocene as a fold and thrust belt, related to the convergence between the European and African plates, with the interposition and the collision of two microplates (Corsica-Sardinia and Adria). During the continental collision, clastic wedges accumulated in foreland basins in front of the deforming thrust belt associated with the progressive roll-back of the subducting Adriatic slab (Le Breton et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Deposition mainly occurred in wedge-top and foredeep basins, progressively migrating towards NE (Argnani and Ricci Lucchi \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Faccenna et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Conti et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). In the Miocene, the foredeep basin in the area now corresponding to the Romagna-Tuscan-Umbrian region was filled by the thick turbidite succession of the Marnoso-arenacea Fm. During its migration (Langhian-Messinian), the Apennine thrust-front incorporated foredeep siliciclastic deposits in the accretionary wedge causing the closure of the foredeep. The closure stage is marked by silty-clayey hemipelagic slope mudstones, sporadic channelized arenaceous bodies and organic-rich mudstones informally named Euxinic shales. These deposits enclose numerous seep-carbonate bodies, lens-like to stratiform, late Tortonian to early Messinian in age (Terzi et al. \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Conti et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eDuring the late Tortonian to early Messinian, numerous seep-carbonates formed and accumulated in pelitic marls corresponding to the Tossignano and Ghioli di letto mudstones (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These carbonates formed in different settings and have various morphologies: - isolated blocks with a metric extension included in poorly cemented sandstones deposited in submarine canyons (Fontanelice Member and minor Basins); - large (up to 100 m) stratiform bodies included in fine-grained apron sediments over the slope (Borgo Tossignano marls); - lenticular to stratiform bodies (1-100 m long) in restricted basins hosted in euxinic shales draping thrust bounded faults and buried ridges, forming intrabasinal highs (Ghioli di letto mudstones) (Conti et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The Messinian succession ends upward with evaporitic deposits of the Gessoso-solfifera Fm, consisting of up to 16 massive gypsum beds, about 30 m thick, made of both primary (selenitic) and clastic (gessarenite) gypsum (Lugli et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). The passage to the evaporites is marked by 2 m-thick laminated brown to white carbonates (Calcare di Base \u003cem\u003eauctorum\u003c/em\u003e) interpreted in many outcrops as microbial-mediated carbonates (Guido et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) and recently re-interpreted locally as being methane-derived carbonates (Conti et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The carbonate outcrop investigated in our study pertains to this context and is a new key to determining the temporal and spatial scales of cold seepage at the onset of the Messinian Salinity Crisis. The Messinian succession ends with fluvio-deltaic and lacustrine deposits with brackish water fauna of the Lago-Mare biofacies (Bassetti et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) ascribed to various continental and transitional environments. Climatic oscillations and tectonics determined the evolution from brackish “lake-sea” to marginal fluvio-deltaic and marsh conditions.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eA detailed field work of the Predappio area has allowed us to draft a high-resolution geological map including the distribution, facies and geometries of seep-carbonates. A stratigraphic log, 30 m thick, was measured in detail in the well-exposed Sasseto outcrop where seep-carbonates exhibit different facies and are covered with erosive contact by conglomerates of the Colombacci Fm. We distinguished lithology, fossil content, depositional and fluid related structures, focusing more on the brecciated and vuggy fabrics due to their significance in relation to paleo-seep dynamics. Eleven samples of carbonate rocks, subdivided in 29 sub-samples, were sampled from all lithofacies for petrographic and isotopic analyses in order to identify different microfacies and authigenic phases. Thin sections were studied using a Leitz optical microscope and investigated via Scanning Electron Microscopy hosted at the Department of Chemical and Geological Sciences, University of Modena and Reggio Emilia (Italy); semi-quantitative elemental information were acquired via energy-dispersive spectrometry (EDS) at accelerating voltage of 20 kV and spot size 60 mm. SEM-EDS analyses were focused on three samples (PR8, PR9, PR10) collected from a brecciated and vuggy carbonate facies. All results from spot analyses are reported in the Supplementary Information. Material for stable C and O isotope analysis was collected from the microcrystalline groundmass, and from different cement phases. 18 powder samples of carbonates, massive and laminated, were analyzed at the Department of Chemical, Life and Environmental Sustainability of the University of Parma. About 10 mg were reacted with 100% phosphoric acid at 25°C for 24 h. The purified CO2 gas was analyzed using a Finnigan MAT 252 gas isotope ratio mass spectrometer. Replicate analyses of reference materials throughout the session yielded a precision better than 0.1‰ (1s) for both δ\u003csup\u003e13\u003c/sup\u003eC and δ\u003csup\u003e18\u003c/sup\u003eO values. A second batch of 11 powders were prepared and analyzed at the University of Modena and Reggio Emilia. Powders were obtained using a handheld microdrill from thin section slabs or polished rock surfaces. Around 0.5 mg were reacted with 100% phosphoric acid at 25°C for 4 h. The purified gas was analyzed on a Isoprime precisION (Elementar) stable isotope ratio mass spectrometer at Centro Interdipartimentale Grandi Strumenti (C.I.G.S.) of the University of Modena e Reggio Emilia. Accuracy of the measurements was evaluated by running six replicates of both NBS18 and NBS19 reference materials, yielding results in agreement with recommended values. Repeatability precision (1s) of three duplicate samples measured in the same session was better than 0.13‰ and 0.084‰ for δ\u003csup\u003e13\u003c/sup\u003eC and δ\u003csup\u003e18\u003c/sup\u003eO, respectively. All isotope results are reported in standard δ notation relative to the Vienna-PeeDee Belemnite (V-PDB) standard.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003eOutcrop geology\u003c/h2\u003e\u003cp\u003eThe examined outcrop (Sasseto) is characterized by numerous bodies of methane-derived carbonates distributed over an extension of approximately 800 m in the WNW-ESE direction, in correspondence with two important tectonic structures (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The western part of the outcrop is parallel to one of the major buried external fronts of the northern Apennine fold-and-thrust belt over the Tortonian Marnoso-arenacea Fm. over the upper Messinian deposits. The eastern part has a N-S direction parallel to the still active Forl\u0026igrave; fault line. The carbonate bodies have variable geometries, morphologies and distribution varying from lenticular, amygdaloid, pinnacles, stratiform and scattered irregular, with thickness of individual bodies from 2 m up to 30 m and lateral extension up to 100 m. Stratiform carbonate bodies are concordant with the encasing deposits, except for the basal pinnacular contact which is almost vertical. The carbonate bodies form reliefs that stand out on the encasing marly sediments, separated by valley incisions along which landslide phenomena develop (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The Sasseto outcrop is folded into an anticlinal structure with the axis dipping towards SE.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eStratigraphy and seep carbonate facies\u003c/h3\u003e\n\u003cp\u003eIn the Sasseto outcrop we measured a 30 m-thick stratigraphic section from the basal contact with the Marnoso arenacea Fm. (Borgo Tossignano Member) to the upper discordant contact with alluvial conglomerate deposits of the Colombacci Fm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This section includes seep-carbonates exhibiting different facies from brecciated, massive and to microbial laminated carbonates, each characterized by distinct lithologies, textures, presence/absence of veins or fractures and fossil content (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, b, c, d). Although these associations are partially intertwined, this study reconstructs their vertical succession which is recurrent in the whole examined area and has a transitional trend from the base to the top (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFacies 1 (P1) consists of brecciated limestones, with gray, calcilutite, angular to subangular monogenic clasts of 1\u0026ndash;10 cm in size, made up of different micritic lithologies, referable to fragmentation of previously formed carbonate crusts (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). Clasts are chaotically dispersed in the authigenic micritic matrix or in a fine to medium-grained sandy matrix. Larger clasts derive from the coalescence of heterometric smaller clasts, testifying various cycles of cementation and fragmentation. This facies commonly lacks macrofossils. Facies 1 characterizes the basal portion of the carbonate bodies where it can be up to 15 meters thick, but can be found also at different stratigraphic levels and in the upper portion of single bodies, less intensely brecciated and a minor occurrence of veins and druses. Brecciated portions are pervaded by veins and extensional fractures; vein dimensions range from 1\u0026ndash;2 mm to 2 cm, they are not systematic and show a sinuous to irregular distribution; in most cases they can join forming veins of major dimensions (1\u0026ndash;2 cm) containing abundant black iron sulfides. Frequently elongate fractures merge to form vuggy and spongy fabrics with cavities ranging from 1 to 3 cm (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, f). The abundance of veins and fractures of centimetric dimension may give to the carbonate masses the appearance of megabreccias. The vein infillings may contain \u003cem\u003ecoquina debris\u003c/em\u003e (shell fragments), coarse calcarenites and carbonate micro-breccias. In this lithofacies the fossil content is scarce, represented by disarticulated clams and rare articulated lucinids. Micritic, mottled carbonates pervaded by irregular networks of veins and druses (from few mm up to 3 cm in diameter) and cut by millimetric-scale fractures are scattered.\u003c/p\u003e\u003cp\u003eFacies 2 (P2) consists of massive marly limestones, pale brown in colour, with abundant fossil content. Fossils are densely packed articulated and disarticulated bivalves, mostly lucinids (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), as irregularly distributed clusters, in several cases in life position. Fossils range from a few centimeters, up to fifteen cm in diameter. Facies 2 dominates in the intermediate portion of the carbonate bodies beneath the microbial laminated carbonates, but it can be present also in the basal portion, with heteropic contact with the brecciated facies P1. Typically, this facies has a maximum thickness of 5\u0026ndash;7 m.\u003c/p\u003e\u003cp\u003eFacies 3 (P3) is characterized by laminated microbialitic carbonates composed of millimetric-scale alternations of brown-dark gray to light grey-whitish laminae (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). The laminate pattern is sub-horizontal to wavy; macrofossils are absent. Facies 3 characterizes the upper portion of the section, with a thickness of about 5 m. It is cut by an erosive contact with fluvial conglomerates of the Colombacci Fm. Facies 3 locally is heteropic with stratified calcarenites with decimetric beds and occasional weak lamination.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eMicrofacies\u003c/h3\u003e\n\u003cp\u003eMost of the samples from the massive facies in the lower-middle part of the sections are typical seep-carbonates with abundant chemosymbiotic macrofauna, largely fragments of lucinid bivalves (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). They are predominantly made of micrite with minor early-diagenetic microspar (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). Micrite is composed of dolomite but calcite is also present. Microspar may be calcitic and dolomitic. Pyrite, a typical mineral in seep-carbonates, was observed and is usually developed as small, partly oxidized framboids, or forming patches of thin microbial laminae more frequently observed in the dolomite micrite (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). Massive carbonates locally contain planktonic foraminifera as well as sand- to silt-sized detrital material mainly aligned along fractures and conduits (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). Detrital fraction is largely composed of quartz, K feldspar, phillites and muscovite similar to the detritus of the underlying Marnoso arenacea turbidites.\u003c/p\u003e\u003cp\u003eIn the brecciated /vuggy microfacies, micritic intraclasts of various shape, commonly angular, and color (light micrite and brown micrite) with apparent chaotic arrangements are associated to a complex network of fractures and veins (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed, e, f). Micritic intraclasts derive from the fragmentation of previously formed carbonate concretions. Abundant microcavities (mm to cm size) giving a \u0026ldquo;spongy\u0026rdquo; appearance are present within the cementing dolomicrite and dolomicrosparite. Larger cavities, vacuolar structures are also present with an irregular, often elongated shape, oriented parallel to the bed/laminae surfaces. They are marked by a rim of dark dolomicrite and filled by equant calcite spars.\u003c/p\u003e\u003cp\u003eThe carbonate samples collected from the upper laminated lithofacies are made up of millimetre-scale alternations of laminae with different microfabrics: dark micrite alternated with light microspar levels, white to gray in color (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg, j). They are predominantly composed of calcite or dolomite and calcite (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Micritic laminae consist mainly of a dense arrangement of peloidal particles: peloids are rounded, tens of microns in size, with ovoidal to subcylindrical shape. Clotted dolomicrite locally shows a dendritic-like arrangement. Rounded and elongated dense aggregates of dolomicrite are present with a preferred orientation parallel to the surfaces of laminae, enclosed in a matrix of calcite microspar. Micritic laminae are alternated with gray laminae in which calcite microspar includes variable amounts of silica and clay particles.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eScanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)\u003c/h2\u003e\u003cp\u003eThe SEM-EDS analyses of three samples (PR8, PR9, PR10) from outcrop facies P1, revealed a recurring chemistry of the carbonate phases (Supplementary Information). In all the samples, we recognized a low backscatter dolomitic phase (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e) containing 30.3\u0026ndash;71.6% m/m CaO and 20.3\u0026ndash;44.8% m/m MgO, in contact with a higher-backscatter low-Mg calcite phase (up to 2.2% m/m MgO) (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, c, d). Calcite also fills fractures in the dolomite cements, indicating it formed later in the paragenetic succession. Pyrite is abundant and occurs with framboidal morphologies forming aggregates ranging in size from \u0026lt;\u0026thinsp;5 mm to ~\u0026thinsp;30 mm. Pyrite is locally distributed in a laminated fashion (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). Particles of barite have also been identified via SEM-EDS within the dolomite cements (Supplementary Information).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eIsotope geochemistry\u003c/h3\u003e\n\u003cp\u003eThe stable carbon and oxygen isotopic composition of the investigated samples is highly heterogeneous, with common trends at microfacies scale. Starting from the stratigraphically lower samples (PR8, PR9 and PR10) from the brecciated facies P1, the δ13C values range between \u0026minus;\u0026thinsp;44.2\u0026permil; and \u0026minus;\u0026thinsp;31.2\u0026permil; (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). We distinguished mainly 5 microfacies in these isotope investigations (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). Overall, micrite (vuggy microfacies, light and dark micrites) has heavier δ\u003csup\u003e18\u003c/sup\u003eO and slightly less negative δ\u003csup\u003e13\u003c/sup\u003eC compared to the sparry and rim cements. Two micritic samples from outcrop facies 2 rich in lucinid clams yielded δ\u003csup\u003e13\u003c/sup\u003eC values of \u0026minus;\u0026thinsp;32.7\u0026permil; and \u0026minus;\u0026thinsp;27.0\u0026permil;, slightly heavier than in facies P1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) and δ\u003csup\u003e18\u003c/sup\u003eO values of 5.8\u0026permil; and 5.6\u0026permil;. Moving stratigraphically upward, samples from the laminated facies P3 show isotopic differences depending on the type of laminae, with the ones rich in peloids having more negative δ\u003csup\u003e13\u003c/sup\u003eC values than the others. The peloidal laminae show the most negative δ\u003csup\u003e13\u003c/sup\u003eC values of the entire dataset ranging from \u0026minus;\u0026thinsp;47.8\u0026permil; to \u0026minus;\u0026thinsp;14.5\u0026permil;, and δ\u003csup\u003e18\u003c/sup\u003eO values between 0.8\u0026permil; to 2.7\u0026permil;. Two micritic laminae show δ\u003csup\u003e13\u003c/sup\u003eC values of \u0026minus;\u0026thinsp;23.1\u0026permil; and \u0026minus;\u0026thinsp;8.7\u0026permil; and δ\u003csup\u003e18\u003c/sup\u003eO values of 3.8\u0026permil; and 4.8\u0026permil;. A microsparitic laminae yielded δ\u003csup\u003e13\u003c/sup\u003eC\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;11.3\u0026permil; and δ\u003csup\u003e18\u003c/sup\u003eO = 1.8\u0026permil;. Microsparitic cement in a sample collected from facies P4 has δ\u003csup\u003e13\u003c/sup\u003eC\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;34.8\u0026permil; and δ\u003csup\u003e18\u003c/sup\u003eO = 0.7\u0026permil;. In general, micritic samples have heavier δ\u003csup\u003e18\u003c/sup\u003eO compared to the sparry and rim cements (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eCarbonate mineralogy, δ13C and δ18O values, for samples from Predappio area representing all three facies and different cements. Carbonate mineralogy was established by thin section petrography and SEM.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSample\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFacies\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eTarget phase\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003ePredominant Mineralogy\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eδ\u003csup\u003e13\u003c/sup\u003eC\u003c/p\u003e\u003cp\u003e(\u0026permil;)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eδ\u003csup\u003e18\u003c/sup\u003eO\u003c/p\u003e\u003cp\u003e(\u0026permil;)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSM10a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCement laminae\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCalcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;11.33\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e1.81\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSM10b\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePeloidal laminae\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCalcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;14.47\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e1.81\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMicrosparitic cement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCalcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;34.76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u0026minus;0.74\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR3a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMicritic laminae\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u0026thinsp;\u0026gt;\u0026thinsp;Calcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;8.68\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e4.81\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR3b\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePeloidal laminae\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u0026thinsp;\u0026gt;\u0026thinsp;Calcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;28.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2.7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR2a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMicritic laminae\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCalcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;23.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e3.78\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR2b\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePeloidal laminae\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCalcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;47.83\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.79\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMicrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;26.99\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e5.58\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMicrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;32.66\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e5.76\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLight micrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;39.83\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e6.98\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR10a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eRim cement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;41.56\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e1.48\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR10b\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLight micrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;34.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e6.25\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR10c\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLight micrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;35.91\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e6.53\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR10d\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eRim cement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;42.38\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e3.11\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR10e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eVuggy micrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;41.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e6.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR9a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLight micrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;39.51\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e7.2\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR9b\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLight micrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;39.75\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e7.91\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR9c\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLight micrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;39.72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e7.92\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR9d\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eRim cement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;39.72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2.72\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR9e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSparitic cement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCalcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;38.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2.66\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR9f\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eDark micrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;39.64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e7.04\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR8a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLight micrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;36.74\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e6.31\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR8b\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSparitic Cement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCalcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;44.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.02\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR8c\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSparitic Cement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCalcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;39.62\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u0026minus;0.17\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR8d\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLight micrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;41.05\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e7.69\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR8e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eRim cement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;44.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR8f\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSparitic cement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCalcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;43.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e1.18\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePR8g\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eDark micrite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDolomite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;41.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e6.27\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSM3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSparitic cement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCalcite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u0026minus;31.18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.07\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe studied seep-carbonate outcrop included in marine euxinic sediments of the Apennine foredeep displays an evident change of lithofacies passing from brecciated carbonate facies dominating the lower-medium portion of the stratigraphic section to massive micritic facies rich in large bivalve mollusks and planktonic foraminifera (also found laterally to the breccias). The seep-impacted sequence ultimately evolves at the top into laminated microbialites without macrofauna.\u003c/p\u003e\u003cp\u003eThe microcrystalline groundmass is mostly composed of primary micrite, largely dolomitic and subordinately calcitic, and by early-diagenetic microspar cements. Clasts in brecciated facies are micrites, light and dark in color. Different generations of cements fill voids and fractures, rim dolomitic cements and later sparitic calcite filling voids. The laminated deposits are mostly composed of prevalent calcite (subordinate dolomite) microlaminae rich in peloidal particles alternated with microsparitic layers including scarce amounts of clay minerals.\u003c/p\u003e\u003cp\u003eThe samples from our study showed wide isotopic ranges (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e): strongly depleted δ\u003csup\u003e13\u003c/sup\u003eC values (\u0026minus;\u0026thinsp;44\u0026permil; to \u0026minus;\u0026thinsp;27\u0026permil;) occur in the massive and brecciated facies, both micrite and cements; whereas in the upper laminated facies seep-carbonates show less depleted values in δ\u003csup\u003e13\u003c/sup\u003eC (up to \u0026minus;\u0026thinsp;8\u0026permil;). The anaerobic oxidation of methane releases isotopically light dissolved inorganic carbon (DIC), which adds up to other DIC sources already present in the pore water, i.e. seawater-derived DIC (δ\u003csup\u003e13\u003c/sup\u003eC~0\u0026permil;) and DIC from organic matter oxidation (\u0026gt;~ \u0026minus;25\u0026permil;). AOM-derived DIC carries the composition of methane being oxidized, which in turn is generally much lighter than \u0026minus;\u0026thinsp;30\u0026permil; (Judd and Hovland, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Since these authigenic carbonates form in the sediment interval of highest alkalinity production by AOM, they tend to record δ\u003csup\u003e13\u003c/sup\u003eC compositions\u0026thinsp;\u0026lt;\u0026thinsp;\u0026minus;\u0026thinsp;30\u0026permil;. The values measured in our samples clearly point to variable amounts of incorporation of methane-derived carbon in a methane-charged sedimentary setting.\u003c/p\u003e\u003cp\u003eThe distribution and trend in isotope values of the different seep-carbonates are likely consequence of variations in fluid intensity and possibly composition, with the establishment of diverse environmental settings favoring the development of different facies. The massive carbonate facies with large bivalves (P1) indicates a seafloor environment with low-moderate seepage conditions and rates of methane oxidation. In fact, high AOM rates would generate high fluxes of HS\u003csup\u003e\u0026minus;\u003c/sup\u003e toward the seafloor which can cause temporary toxic conditions to macrobenthos (Bagarinao \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Riesch et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) relying on well-oxygenated bottom waters for their metabolic processes. We do not have clear indications to estimate paleodepth, but the regional geology indications (Di Giulio et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) and, for some extents, the abundance of planktonic foraminifera could confirm a moderately deep slope environment, at least several hundred meter deep (between approximatively 500 and 1000 m) .The siliciclastic detritus dispersed in micritic carbonates and concentrated in fractures within this facies (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea) belongs to the encasing sediments (MA Fm) confirming a fluid circulation originated from the substratum.\u003c/p\u003e\u003cp\u003eA dominant portion of the studied seep bodies is strongly fragmented, to form monogenic breccias with clasts made up of light and brown micrites and abundant vuggy, vacuolar, bubble-like, drusy fabrics. Voids and fractures are filled by two main cements: an earlier dolomite rim cement and later calcite cement filling void and fractures. In this facies, the micrite and cements are remarkably enriched in \u003csup\u003e18\u003c/sup\u003eO, in particular, δ\u003csup\u003e18\u003c/sup\u003eO in micrite is invariably comprised from 6 and 8\u0026permil;. The δ\u003csup\u003e18\u003c/sup\u003eO composition of authigenic carbonates forming in equilibrium with coeval seawater can be calculated by applying thermodynamic equations. Assuming an average δ\u003csup\u003e18\u003c/sup\u003eO\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;0.4\u0026permil; (vSMOW) (James et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Lear et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) and bottom water temperature\u0026thinsp;~\u0026thinsp;5\u0026deg;C (Barbieri et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) we obtain values of δ\u003csup\u003e18\u003c/sup\u003eO= 0.94\u0026permil; for low-Mg calcite (Tarutani 1969; Friedman and O'Neil 1977) and δ\u003csup\u003e18\u003c/sup\u003eO= 3.5\u0026permil; for dolomite (Vasconcelos et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Therefore, the values measured in our samples are not compatible with precipitation in isotopic equilibrium with coeval seawater as they are heavier than calculated. We also rule out post-depositional diagenetic alteration, since the temperature gradients in the sedimentary column and/or meteoric weathering would alter the carbonate isotopic composition toward lighter values, which is the opposite to what we observed.\u003c/p\u003e\u003cp\u003eWe hypothesize that these carbonates have similar origin to other Apennine paleo-seeps in this region (Conti et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) and related to paleo-gas hydrate destabilization. Gas hydrate stability at the time of seep carbonate formation is difficult to extrapolate since there are many uncertainties on the exact paleo-environmental conditions. However, we estimated the minimum paleo-seafloor depth at which gas hydrates were stable by assuming: 1) pure methane-hydrate (structure I); 2) normal salinity 37 psu (Flecker et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2015\u003c/span\u003e); 3) normal geothermal gradient 31\u0026deg;C/km; 4) steady state conditions. The model was implemented on the graphical user interface CAGEHYD (Vadakkepuliyambatta \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) based on the CSMHYD code (Sloan \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Our model indicates that gas hydrates were stable at water depths\u0026thinsp;\u0026gt;\u0026thinsp;485 m, which is consistent with lithological constraints imposed by our field work datasets. The heavy values in the samples could also be explained by the occurrence of deep-sourced fluids rich in \u003csup\u003e18\u003c/sup\u003eO from clay dehydration at greater depth within the sediments (Kastner et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1991\u003c/span\u003e). However, the petrographic features resembling modern clathrites together with methane-related carbon isotopic signatures, support the hydrate hypothesis for the origin of the carbonates in the Predappio outcrop.\u003c/p\u003e\u003cp\u003eThe upper portion of the examined seep-carbonate bodies contain several structures typical of microbial sediments, such as laminations, peloids, elongated micritic filament-like particles and clotted micrite. As observed in other outcrops (Brisighella, Conti et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) these features could be interpreted as cyanobacterial (or some other bacterial) or algal origin. Yet, the δ\u003csup\u003e13\u003c/sup\u003eC values as negative as \u0026minus;\u0026thinsp;47.8\u0026permil; in a peloidal laminae in this facies still points to variable amounts of incorporation of methane-derived carbon thus suggesting a methane-rich sediment conditions. Similar to the examined deposits are also stromatolites from the Lower Messinian of the Calcare di Base Fm from Sicily (Oliveri et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2010\u003c/span\u003e); based on morphology, the filaments have been interpreted as \u003cem\u003eBeggiatoa\u003c/em\u003e-like sulphur bacteria, The laminated pattern in the Brisighella section is also comparable to structures known both from modern and fossil benthic coccoidal cyanobacterial mats that underwent early mineralization (Kempe and Ka\u0026acute;zmierczak \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1993\u003c/span\u003e) such as cyanobacterial mats from Sulej\u0026acute;ow Dam, Poland, and subfossil mats from a crater lake on Satonda Island, Indonesia.\u003c/p\u003e\u003cp\u003ePetrographic structures and isotope compositions similar to the ones from this study have been reported from Messinian Monferrato (\u003cem\u003eSeptarian Like-Beds\u003c/em\u003e in Natalicchio et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), Montepetra (Conti et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) and Brisighella outcrops (Conti et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) and the paleo occurrence of gas hydrates has been based on the strict similarity with and geochemistry and features reported for clathrites described by various authors (Greinert et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Pierre and Rouchy \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Teichert et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Bohrmann and Torres \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Suess \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Bojanowski et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). In fact, following these indications, Bohrmann et al. (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) and Kennett and Fackler-Adams (\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) proposed that these methane derived authigenic carbonates \u0026lsquo;may be classified as one type of clathrites and can be used as markers of gas hydrates in the geological record. Nevertheless not all fractured seep-carbonates are related to gas hydrate system and some may be from the deeper gas reservoirs as for the origin of fluids from a secondary circuit in the Montepetra outcrop (Conti et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). The complex history of development of overpressure and episodic hydrofracturing would have enlarged the conduits and dismembered the previously precipitated crusts, forming a further reworking of the breccias (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, e). As reported in modern analogue systems in foredeep basins worldwide, the hydrate destabilization and high methane fluxes to the paleo-seafloor would have been favored by the active geodynamic conditions during the late Miocene, in the eastwardly-migrating Apennine foredeep.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn this study we report on a new Miocene seep-carbonate outcrop in the northern Apennines (Italy) which shows strong evidence for paleo-gas hydrate destabilization. This outcrop is composed of carbonate bodies with stratiform to pinnacular geometries, and having thicknesses up to 30 m and lateral extension up to 100 m. The entire outcrop is 800 m in lateral extent and is hosted in the Tortonian interval of the Marnoso arenacea Fm. (turbiditic foredeep deposits). We interpreted three main carbonate facies in a stratigraphic section encompassing the seep-impacted interval. The lowermost carbonate bodies mainly consist of brecciated limestones in a micritic or sandy matrix, characterized by veins, fractures, and scarce fossil content. Moving upward, carbonates are made up of marly limestones with densely packed bivalve fossils (mostly lucinids) in life position. The uppermost carbonates show a laminated microbialites barren of macrofossils. This facies succession can be traced also within individual bodies. Stable isotope analyses on different facies revealed anomalously heavy isotope oxygen signatures (7.9\u0026permil;) in the basal massive facies associated with vuggy appearance and d\u003csup\u003e13\u003c/sup\u003eC\u0026thinsp;\u0026lt;\u0026thinsp;\u0026minus;\u0026thinsp;40\u0026permil;. Our thermodynamic modelling of the paleo-gas hydrate stability zone indicates that hydrates were potentially occurring at depths\u0026thinsp;\u0026gt;\u0026thinsp;485 m, in agreement with the expected paleo-bathymetry of this slope setting. Similarly to what observed in modern analogue systems in foredeep basins worldwide, the active geodynamic conditions in the eastwardly-migrating Apennine foredeep during the Miocene would have caused hydrate destabilization and high methane fluxes to the paleo-seafloor. This process eventually resulted in the formation of widespread seep-carbonate deposits now exposed onshore. This model, combined with previous studies on coeval outcrops in northern Italy, provides new insights into cold seepage dynamics in this sector of the Western Mediterranean during the stages prior to the onset of the Messinian Salinity Crisis.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eThis research has been founded by FAR Unimore 2024. CA position is funded by EMAN7 project (Research Council of Norway Grant No. 320100).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eS.M., S.C. and D.F. conceived the study, and together with C.A. they designed the experiments. S.M., S.C. and D.F. performed field work and stratigraphic logging; all the authors discussed and interpreted the microscopy and geochemistry data. S.M., S.C. and D.F. prepared Figures 1, 2, 3 and Table 1, and C.A. prepared Figures 4, 5, 6 and 7 and conducted thermodynamic modeling. All co-authors contributed to the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe are grateful to Federico Lugli and Anna Cipriani (Unimore) for analytical assistance during isotope analyses and for the use of MEGIC Metallomics and Geochemistry laboratory (Unimore) tools for sample preparation. We are indebted to Paolo Serventi (Unimore) for assistance in the field and sample collection and to Simona Marchetti Dori (Unimore) for SEM analytical assistance.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAbegg F, Bohrmann G, Freitag J, Kuhs W (2007) Fabric of gas hydrate in sediments from the Hydrate Ridge\u0026mdash; results from ODP 204 samples. 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Sedimentology 57:142\u0026ndash;161.\u003c/li\u003e\n\u003cli\u003ePanieri G, Argentino C, Savini A, Ferr\u0026eacute; B, Hemmateenejad F, Eilertsen HM, Mattingsdal R, Ramalho SP, Eidvin T, Youngs S, Colson BC, Michel APM, Kapit JA, Swanborn D, Rogers AD, Angeles IB, Polteau S, Kalenitchenko D, Buenz S, Mazzini A (2025) Sanctuary for vulnerable Arctic species at the Borealis Mud Volcano. Nature Communications16, Article number: 504.\u003c/li\u003e\n\u003cli\u003ePeckmann J, Thiel V (2004) Carbon cycling at ancient methane-seeps. Chem Geol, 205, 443\u0026ndash;467.\u003c/li\u003e\n\u003cli\u003ePierre C, Rouchy JM (2004) Isotopic compositions of diagenetic dolomites in the Tortonian marls of the western Mediterranean margins: evidence of past gas hydrate formation and dissociation. Chemical Geology, 205(3-4):469-484. \u003c/li\u003e\n\u003cli\u003eRiesch, R., Tobler, M. and Plath, M., 2015. Hydrogen sulfide-toxic habitats. 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Nature communications 10(1):4584.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"geo-marine-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"gmle","sideBox":"Learn more about [Geo-Marine Letters](http://link.springer.com/journal/367)","snPcode":"367","submissionUrl":"https://submission.nature.com/new-submission/367/3","title":"Geo-Marine Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6980851/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6980851/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAncient methane-seep deposits exposed in the Apennine mountains of northern Italy are key to reconstruct the geodynamic evolution of the Miocene foredeep basin prior to the Messinian Salinity Crisis. Here, we report on a new Miocene outcrop of seep-carbonates in the Romagna region which showed indications of paleo-gas hydrate destabilization. We combined field work facies analyses, petrography of thin sections, scanning-electron microscopy and stable isotope analyses to interpret the origin of these authigenic carbonates and biogeochemical conditions at the time of their formation. These deposits show vuggy, brecciated and massive-with-lucinids lithofacies and are characterized by light carbon isotope signatures, as negative as \u0026minus;\u0026thinsp;47.8\u0026permil;. The oxygen isotope compositions of these carbonates are out of equilibrium with coeval bottom seawater, exhibiting anomalously-heavy values. We calculated the stability of paleo-gas hydrates, which revealed that gas hydrates type I (pure methane) were stable at depths\u0026thinsp;\u0026gt;\u0026thinsp;485 m, in agreement with the paleo-bathymetric domain of this outcrop. This area was geologically active in the Late Miocene and affected by continuous bathymetric changes caused by the eastwardly-migrating accretionary wedge and foredeep basin. We propose that gas hydrate destabilization and prolonged anaerobic oxidation of methane in the shallow sediment produced the studied carbonate deposits.\u003c/p\u003e","manuscriptTitle":"Large-scale methane venting and clathrite-like structures recorded in the Late Miocene foredeep deposits of the northern Apennines (Marnoso arenacea Fm., Italy)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-24 13:40:41","doi":"10.21203/rs.3.rs-6980851/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-08-16T05:56:17+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-12T08:44:09+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-12T00:12:57+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"131874026504718406152549200997607245179","date":"2025-07-27T06:38:11+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"225760775082288466448309889034414355634","date":"2025-07-22T23:58:28+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-22T14:49:10+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-02T10:57:28+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-28T04:36:01+00:00","index":"","fulltext":""},{"type":"submitted","content":"Geo-Marine Letters","date":"2025-06-26T07:41:25+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"geo-marine-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"gmle","sideBox":"Learn more about [Geo-Marine Letters](http://link.springer.com/journal/367)","snPcode":"367","submissionUrl":"https://submission.nature.com/new-submission/367/3","title":"Geo-Marine Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a1234925-e33a-4e7a-b98f-0bed207e6471","owner":[],"postedDate":"July 24th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-11-17T16:01:21+00:00","versionOfRecord":{"articleIdentity":"rs-6980851","link":"https://doi.org/10.1007/s00367-025-00830-8","journal":{"identity":"geo-marine-letters","isVorOnly":false,"title":"Geo-Marine Letters"},"publishedOn":"2025-11-10 15:57:46","publishedOnDateReadable":"November 10th, 2025"},"versionCreatedAt":"2025-07-24 13:40:41","video":"","vorDoi":"10.1007/s00367-025-00830-8","vorDoiUrl":"https://doi.org/10.1007/s00367-025-00830-8","workflowStages":[]},"version":"v1","identity":"rs-6980851","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6980851","identity":"rs-6980851","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}