The barium (Ba) isotopic composition of carbonate minerals (δ138Bacarb) is a promising new geochemical tracer for past Ba cycling. However, further study of the potential influence of early diagenetic processes on δ138Bacarb records remains an important avenue for this new tracer’s development. Marine methane seep environments are sites of intense Ba cycling during early diagenesis. Seeps may be responsible for much of the flux of Ba from the sedimentary subsurface into seawater, while the authigenic carbonate minerals precipitating at seeps may represent an archive of this potential Ba source. Here we present δ138Bacarb data from authigenic carbonates collected at eight methane seeps from the South China Sea, the Gulf of Mexico, the Congo Fan, and the Black Sea. The obtained δ138Bacarb values display a broad range from −0.15‰ to 0.37‰, indicating authigenic seep carbonates archive highly variable Ba isotope signals of local pore waters. Aragonite-dominated carbonates tend to show higher δ138Bacarb values than high-Mg calcite-dominated carbonates. This pattern suggests a greater influence of seawater during aragonite precipitation caused by its formation at shallower depths compared to high-Mg calcite. Accordingly, low δ138Bacarb values and relatively high Ba/Ca molar ratios are interpreted to reflect the release of Ba from sediments through barite dissolution. However, under conditions of increased upward flux of Ba reflected by high Ba/Ca ratios, seep carbonates display highly variable δ138Bacarb values independent of Ba/Ca molar ratios, suggesting that additional processes (i.e. local reprecipitation of 134Ba-enriched barite) control pore water barium isotope compositions. Taken together, these observations showcase the complexity of Ba isotope systematics at marine seeps, governed by the mixing of Ba from different sources, mineral dissolution in the subsurface, and local authigenesis (carbonate minerals and barite). These new findings highlight the dynamic nature of early diagenetic modifications on Ba isotopic fingerprints, while proposing a process-based conceptual framework for the combined use of Ba/Ca ratios and mineralogical analysis to assess diagenetic signals in carbonate-hosted Ba isotope archives.

Distinguishing methane and oil seeps using marine authigenic carbonates is crucial for reconstructing past seep activity and assessing their environmental impact. Current methodologies for such a discrimination lack direct analysis on the organic matter preserved within seep carbonates. Here we investigated the optical properties of iron-bound organic carbon (OC-Fe-R) in authigenic carbonates from methane and oil seeps from the Gulf of Mexico and the South China Sea. Methane-seep carbonates display a mean OC-Fe-R fraction (f(OC-FeR)) relative to total organic carbon (TOC) of 14.8 +/- 10.3%, which is significantly higher than the 3.1 +/- 2.2% observed for oil-seep carbonates. This difference is attributed to the predominance of low molecular weight carboxylic acids at methane seeps and the high aromaticity of the larger molecular weight compounds at oil seeps. Furthermore, a significant negative correlation was observed between the biological index (BIX) and TOC in oil-seep carbonates (R-2 = 0.40; p < 0.05), a relationship that was not evident for methane seeps. This result suggests that the TOC at oil seeps consisting of a recalcitrant component is resistant to biodegradation. Additionally, a strong negative correlation (R-2 = 0.60; p < 0.01) was identified between marine, microbial humic-like components and the delta C-13 compositions of oil-seep carbonates. We suggest that low f(OC-FeR) ratios, negative correlations between BIX and TOC and between marine, microbial humic-like components and delta C-13 values in authigenic carbonates serve as criteria for discriminating oil seeps from methane seeps. This optical fingerprinting enables rapid and straightforward discrimination of hydrocarbon sources in authigenic carbonates and advances our understanding of the contributions of seeps to the marine carbon cycle.

The presence of fossilised fungi within deep crustal rock formations has been established based on fossil evidence from 400 Ma continental crust and 81 Ma oceanic basaltic crust. Moreover, the Palaeoproterozoic Ongeluk Formation contains putative fungal remains reaching 2.4 Ga. The resulting gap of 2 billion years raises questions regarding the history of fungi in marine subsurface environments, in particular the lack of bona fide fossils in ophiolites, sections of layered basalts from mid-ocean ridges. Devonian examples of cryptoendolithic microorganisms preserved in marine pillow basalt stem from the Arnstein locality, Rheinisches Schiefergebirge, and the Kahlleite locality, Thüringer Wald, Germany, and have previously been described as filaments of microorganisms with uncertain biological affinity. The filamentous fossils were investigated using environmental scanning electron microscopy, Raman spectroscopy, confocal microscopy, widefield microscopy, and optical light microscopy. Energy dispersive spectroscopy analyses of several of the inferred microfossils revealed a presence of clay minerals, pointing to a mode of mineralisation in association with organic matter and agreeing with a biological origin. Raman spectroscopy showed carbon localised within the studied filaments and revealed that particularly iron oxide minerals are associated with carbon. Element compositions similar to younger mineralised fungal remains and morphologies resembling sporophores and hyphae agree with the interpretation of the Arnstein and Kahlleite fossils as marine fungi, shedding new light on many of the previously undetermined fossils and plausibly narrowing the fossil gap of oceanic deep subsurface fungi by at least 300 million years.

Convergent margins are key engines of mass transfer between Earth’s surface and interior, governing long-term fluxes of volatiles, redox-sensitive elements, and carbon. Subduction transports large quantities of water and carbon into the mantle, while fluid release and metasomatic reactions move these components into the mantle wedge and, in places, toward the surface. Central to this exchange is mantle wedge serpentinization, the slab-derived hydration of mantle wedge peridotite that alters redox conditions and generates reduced compounds such as H2 and CH4, directly linking deep Earth processes to carbon cycling and energy availability for subseafloor biosphere.In the modern Mariana forearc, serpentinite mud volcanoes provide a rare natural laboratory to directly interrogate the products and consequences of mantle wedge serpentinization. Our recent geochemical, isotopic, and lipid biomarker findings demonstrate that the availability of abiotic geofuels produced during serpentinization exerts a first-order control on the subsurface chemosynthetic microbial communities on a temporal scale (Kumawat et al., 2025). We present in situ stable oxygen isotope measurements of serpentine from several Mariana mud volcanoes, combined with published pore fluid δ18O compositions. They define systematic spatial trends in serpentinization temperature, from cold, trench-proximal settings to progressively hotter conditions deeper in the mantle wedge. Using our newly developed serpentine–water calibration, our data imply that these thermal gradients regulate redox evolution and the production of reduced volatiles and organic components, establishing dynamic energy landscapes that sustain life under high pH, nutrient limitation, and episodic substrate delivery.While mantle wedge serpentinization, serpentinite mud volcanism, and associated biospheres are increasingly well-constrained in the modern Mariana forearc, their occurrence and significance in the geological record remains largely unconstrained. We also present geochemical evidence for Early Cretaceous serpentinite mud volcanism preserved within the paleo-forearc basin within the Coast Ranges of California. Elevated fluid-mobile element inventories, systematic oxygen isotope record of serpentine, and textural evidence for mud-supported serpentinite transport are complemented by an extensive methane-seep fossil record and lipid biomarker signatures indicative of chemosynthesis-based ecosystems. Together, these observations suggest that mantle wedge serpentinization and focused fluid discharge have been major volatile and energy providers in Mesozoic convergent margins.By integrating modern forearc observations with ancient geological archives, this work highlights serpentinization as a persistent and efficient mechanism for mass transfer, redox modulation, and volatile cycling at convergent margins. These processes not only shape mantle metasomatism and arc evolution but also link deep Earth volatile pathways to the limits of habitability in the deep biosphere through Earth history. Kumawat, P., Albers, E., Bach, W. et al. Biomarker evidence of a serpentinite chemosynthetic biosphere at the Mariana forearc. Commun Earth Environ 6, 659 (2025). https://doi.org/10.1038/s43247-025-02667-

Abstract Black carbon (BC), a refractory component of organic carbon, exhibits poorly constrained environmental behavior. Recent culture experiments suggested anaerobic methanotrophic archaea (ANME) produce BC via anaerobic oxidation of methane, a pathway supported by studies of seep carbonates — mineral deposits restricted to settings characterized by high methane flux. However, it remains unconfirmed in sediments with low methane flux, settings that are far more widespread in the global ocean. Here, we examine BC distribution and δ 13 C BC values in two seepage‐influenced sediment cores (PC‐01, PC‐02) and a reference core (C‐S05) from the Haima seeps in the South China Sea. Core PC‐01, influenced by higher methane flux, shows significantly 13 C‐depleted BC (−28.9‰ to −25.4‰) at the sulfate‐methane transition zone compared to global non‐seep sediments (−26.4‰ to −10.9‰). Furthermore, not all seepage‐affected sediments exhibit this signal, as demonstrated by core PC‐02. This discrepancy is likely governed by site‐specific variations in methane flux. Consequently, when methane flux is low, the limited production of ANME‐derived BC is insufficient to measurably shift the isotopic signature of the bulk BC pool. A Bayesian mixing model estimates that ANME‐derived BC accounts for ∼6.3% of the total BC at core PC‐01. Although quantitatively minor, ANME‐derived BC may represent a previously overlooked source of potentially aged BC in marine environments, consistent with microbial BC production in methane‐rich sediments.

In marine sulfidic surface sediments, the reduction of iron (Fe) oxides releases substantial Fe-bound phosphorus (P). However, the balance between its retention in solid phases versus its escape to the water column remains poorly constrained near the sediment-water interface (SWI). To investigate this, we analyzed authigenic carbonates formed close to the SWI at cold seeps in the South China Sea. Our results show that high-magnesium calcite forming under restricted sulfidic conditions with limited fluid mixing, as evidenced by its low S13C values and high S34S values of associated iron sulfides (S34SCRS), acts as a major sink of P, exhibiting higher contents of reactive P than nearby sediments unaffected by seepage. In contrast, aragonite, precipitating in more open, sulfate-reducing settings with prominent fluid mixing near the SWI, is depleted in reactive P relative to sediment unaffected by seepage. Critically, contents of reactive P, including Fe-bound P and authigenic P, correlate positively with Mg/Ca ratios and S34SCRS values but negatively with Sr/Ca ratios and S13C values, indicating that carbonate mineralogy and fluid-seawater exchange are the primary controls on P sequestration. Our findings suggest that the openness of the seabed interface, not just redox conditions, ultimately governs P efflux from sulfidic sediments. This insight from modern seeps offers a new framework for assessing P recycling and burial in ancient anoxic oceans.

Molybdenum isotope compositions (delta 98Mo) of sedimentary rocks have been widely used as a paleo-redox proxy in marine systems. However, incomplete constraints on Mo behavior during deposition and early diagenesis of marine sediments limit the application of this isotope proxy to ancient settings. Here, we report new delta 98Mo data from modern cold seep sediments from the South China Sea. Our results reveal consistent authigenic delta 98Mo signatures (delta 98Moauth = +1.58 +/- 0.17%o, 1a) of seep sediments hosting chemosymbiotic communities that thrive below oxic bottom waters. With the delta 98Moauth value of seep sediments approximately 0.7%o lower than the value of modern seawater, the isotopic offset is similar to isotope patterns characteristic of modern anoxic continental margin sediments, where dissolved oxygen and sulfide concentrations are close to zero in the overlying water column. Combing the new Mo isotope data with those from other seep sites, we suggest that the Mo isotope offsets between seep sediments and bottom seawater are variable but seep sediment values are substantially higher than values of typical oxic sediments, likely regulated by diagenetic scavenging of Mo during sulfide mineral formation through hydrogen sulfide production during anaerobic oxidation of methane (AOM) in pore water directly below the seafloor. The fractionation of Mo isotopes observed in seep sediments challenges the paradigm that a sedimentary delta 98Mo value approximately 0.7%o lower than global seawater value invariably indicates regional anoxic conditions of bottom waters, and highlight the role of regional methane seepage intensity in shaping global seawater delta 98Mo and reconstructing global marine oxygenation levels. Our new data underscore the necessity to establish multi-proxy constraints on local redox conditions and early diagenesis before the delta 98Mo signatures of marine sediments can be interpreted with confidence.

The isoprenoid lycopane occurs in the membranes of cultured archaea, but in environmental samples it has traditionally been interpreted to derive from phytoplankton. In the deposits of the Muschelkalk Sea, a Triassic marginal sea, lycopane contents increased during a freshening trend at the Anisian-Ladinian boundary. Based on peak abundances of lycopane at pycnoclines, we hypothesize an episodic occurrence of haloclines in the Muschelkalk Sea, with freshwater runoff mostly affecting surface waters. Freshwater dissolved inorganic carbon is typically 13C-depleted. An up-section trend of decreasing delta 13C values of the biomarkers of phytoplankton (phytane) and heterotrophic consumers (2,6,10,14,18-pentamethylicosane; regular PMI) from -33%o to -41%o is therefore in accord with freshening. Even lower delta 13C values of lycopane and the co-occurring archaeal lipid biphytane (average: -48%o and -47%o, respectively) agree with archaea as source organisms of lycopane in the Muschelkalk Sea. We put forward a novel application of a common isoprenoid biomarker, linking the occurrence of 13C-depleted lycopane to marine freshening events.

Marine carbonates are widely regarded as excellent archives for recording the geochemical evolution of seawater. However, the long-standing “dolomite problem” has hindered the reconstruction of formation mechanisms and depositional settings of the widespread occurrences of dolomite throughout Earth’s history. Cold seeps, where sulfate-driven anaerobic oxidation of methane (SD-AOM) promotes the formation of dolomite, are key to deciphering low-temperature dolomitization processes. Here, we investigate late Miocene to early Pleistocene seep dolomite from Chiahsien, Taiwan, together with modern seep dolomite from the Gulf of Mexico and South China Sea, using a coupled molybdenum (Mo) and magnesium (Mg) isotope approach to constrain the dolomitization environment and processes within seep systems. The wide range of authigenic Mo isotope compositions (δ98Moauth: 0.29 to 2.94‰), correlating with Fe/Al ratios, tracks changes of methane flux spanning from diffusion-dominated (enrichment of heavy Mo) to oxide-shuttle-dominated (enrichment of light Mo) regimes. This contrasts with the homogeneous δ26Mg values of seep dolomite (−2.72 ± 0.21‰), reflecting equilibration with seawater during dolomitization in a shallow, porewater environment with high replenishment of seawater Mg. The Chiahsien seep dolomite further displays higher crystallographic ordering expressed as I(015)/I(110), generally lower Sr/(Mg + Ca) ratios, and more euhedral crystals than modern seep dolomite. Taken together, our new observations suggest that precursor carbonate minerals (aragonite, calcite) were dolomitized during early diagenesis at shallow depth. The lack of Rayleigh fractionation effects of Mg isotope distinguishes seep dolomite from dolomite formed in more restricted, deeper settings, such as environments dominated by organic sulfate reduction and methanogenesis. This study demonstrates that combined Mo–Mg isotope systematics can effectively decipher the redox history and the process of dolomitization. Furthermore, it suggests that seep dolomite, which tends to form in open, seawater-buffered porewater environments, is a promising archive for reconstructing the Mg isotope composition of ancient seawater.

Active nitrogen cycling, including microbially mediated nitrogen fixation and nitrogen loss, occurs at marine cold seeps and may influence nitrogen budgets in the deep ocean. However, the record of these processes in seep environments remains poorly constrained. Here, we investigate the nitrogen isotopic composition of organic nitrogen (delta 15NON) along with carbon isotopes and elemental composition of authigenic carbonates from four methane-seep sites and three brine-seep sites in the Gulf of Mexico and the South China Sea. Methane-seep carbonates exhibit significantly lower delta 13C values of total organic carbon (delta 13CTOC; mean: -34.7 +/- 8.0 parts per thousand, n = 32) than authigenic carbonates from brine seeps (mean: -27.9 +/- 8.4 parts per thousand, n = 24). In contrast, delta 15NON values are generally low for both types of seeps, with mean values of 0.1 +/- 2.2 parts per thousand (n = 32) for methane seeps and -1.2 +/- 0.8 parts per thousand (n = 24) for brine seeps. delta 15NON values below zero are common and delta 15NON values are largely decoupled from delta 13CTOC values at most sites, which is interpreted to reflect dynamic nitrogen cycling near the sediment-water interface at seeps, involving processes such as dissimilatory nitrate reduction to ammonium, denitrification, and anaerobic ammonium oxidation. The nitrogen involved in these processes is subsequently taken up by methanotrophic consortia and associated microorganisms, preserving its isotopic signature in the organic matter incorporated into seep carbonates. Our results demonstrate that delta 15NON values of seep carbonates represent a novel archive of a hotspot of benthic nitrogen cycling, providing new insight into nitrogen cycling in deep-sea seep environments.

Lake Faiyum provides a comprehensive Holocene sediment record, offering more insight into Nile flood deposition than fluvial sediments in the Nile Valley. Geochemical analysis of lake sediments reveals changes in climate, productivity, organic matter source and composition, salinity, and redox conditions. We integrated lipid biomarker and elemental data from core F1-08 taken at the southern margin of the lake. Our findings indicate that the Early Holocene (ca. >10-8.2 cal. ka BP) was characterized by a humid climate with high lake-levels. This is supported by low Ti/Al and Zr/Al ratios, low terrestrial/aquatic n-alkane ratios (TARs), and a significant contribution of freshwater algal homologues (low TOC/TN ratio). Salinity was low, indicated by low Sr/Ba and S/TOC ratios. Nutrient-rich Nile discharge fostered productivity and oxygen depletion, promoting organic carbon preservation. During the period from ca. 8.2 to 6.2 cal. ka BP (within the Middle Holocene), the lake experienced a slight increase in salinity, which caused a change from freshwater to brackish water conditions accompanied by a slight lake-level drop and delta-like wet land, indicated by increased Sr/Ba and S/TOC ratios. Brackish conditions in the lake coincide with the appearance and sharp increase of various botryococcenes, caused by elevated algal productivity including Botryococcus braunii and typified by low TOC/TN ratios. The Middle to Late Holocene (ca. 6.2 cal. ka BP to present) witnessed gradual aridification and declining lake-levels, organic productivity, and preservation. Evidence includes high Ti/Al and Zr/Al ratios, low TOC contents, and reduced contribution of freshwater algal homologues (higher TOC/TN) and a sharp decrease of botryococcenes. Salinity gradually increased, reflected by higher Sr/Ba and S/TOC ratios. An increase in Mo enrichment since ca. 4.2 cal. ka BP, without a corresponding rise in TOC contents, may be due to human-induced hydrological changes or transport to the lake through wind and limited rainfall. Elevated Ti levels at that time may be linked to increased aridity in the Ethiopian Highlands, potentially impacting Nile floods ca. 4.2 cal. ka BP and contributing to the decline of the Old Kingdom in Egypt.

The Sturtian and Marinoan glaciations shaped Neoproterozoic palaeoenvironmental evolution. While methane emission likely intensified the Marinoan greenhouse effect, its role during the Sturtian glaciation-coinciding with widespread iron formations (IFs)-remains poorly understood. Here, we analysed bio-essential metals (Ni, Co, Zn), rare earth elements and yttrium (REY), Fe (δ56Fe) and Ni (δ60Ni) isotopes in hematite and magnetite, alongside bulk-rock and in-situ C isotopes of Mn-rich carbonates from five well-preserved Sturtian-aged IFs in South China. Our findings provide geochemical evidence for a methane-related biogeochemical pathway driving Fe-bearing mineral transformation via methanogenesis and metal-driven anaerobic methane oxidation (AOM), mediated by methanogens and anaerobic methane-oxidizing archaea (ANME) in ferruginous settings. Additionally, the Sturtian deglaciation facilitated atmospheric-oceanic O2 exchange, increased nutrient influx from weathering, and methane release under slow AOM oxidation kinetics, potentially aiding ice sheet melting or prolonging glacial waning.

The association of organic carbon (OC) to reactive iron oxides (Fe R ), forming OC‐Fe R complexes, represents a significant OC sink in marine sediments. However, the impact of diagenetic processes, such as sulfate reduction and iron sulfide formation, on the stability of OC‐Fe R in marine sediments remains poorly understood. Here, we compare sulfidic sediments from three cores taken at methane seeps with a non‐sulfidic sediment record from a nearby site. Our results show that an overall 6.3% decrease in OC‐Fe R is associated with a 42% reduction in Fe R during the transformation from iron oxides to iron sulfides, suggesting that OC‐Fe R is resistant to sulfidization. We observed highly 13 C‐depleted OC‐Fe R in the sulfidic sediments, likely due to the interaction between OC and Fe R during anaerobic oxidation of methane. Our findings highlight the stability of OC‐Fe R in natural sulfidic sediments, offering new insights into the role of OC‐Fe R in continental margin sediments.

Carbon is removed from Earth’s surface and may be stored within carbonate minerals over long periods of time. The formation of authigenic carbonate in marine sediments accounts for much of this sequestered carbon, whereby the rate of sequestration depends on mineral precipitation rates. Among the catalyzing agents of carbonate precipitation are biofilms and microbial mats, which are ubiquitous in Earth surface environments. Microbial carbonates are abundant at methane seeps where they form by the sulfate-driven anaerobic oxidation of methane (SD-AOM), mediated by anaerobic archaea and sulfate-reducing bacteria. We investigated a 5 m long core composed almost entirely of two microbially-derived carbonate cements from an active methane seep in the South China Sea, offshore Taiwan. Phase-specific U/Th dating, lipid biomarker analyses, and calcium isotope data suggest that one of these phases is a direct product of biofilm mineralization, typified by high precipitation rates. This study is the first to estimate the accretion rate of individual carbonate phases in microbial limestones, and provides first-order constraints on the catalytic effect of microbial activity on carbonate precipitation. This has implications on the rate of global carbon burial, which may be significantly increased by the influence of biofilms and microbial mats on carbonate precipitation.

Sedimentary pyrite is becoming one of the most promising and reliable archives for biogeochemical processes and environmental evolution of the Earth’s surface today. It represents a major reservoir of sulfur within the global sulfur cycle, with most of its formation taking place in organic-rich sediments along continental margins. Authigenic pyrite typically forms through microbial sulfate reduction coupled to organic matter remineralization or anaerobic oxidation of methane in sediments. Pyrite formation in marine sediments influences global seawater sulfate concentrations and sulfur isotope patterns, reflecting local microbial activities or environmental change, and tracking past seawater chemistry. Applications as a paleoenvironmental proxy rely on characteristic geochemical signatures archived in pyrite, including its sulfur isotopic and trace element compositions. Therefore, a comprehensive understanding of the controls on pyrite geochemistry is critical for the effective application of this proxy in studying the Earth system.Marine methane-rich sediments alone continental margins, such as seeps, are excellent natural laboratories to study mineral authigenesis, while also being global hotspots of sulfate consumption and authigenic pyrite formation. We present various geochemical datasets including multiple sulfur (32S, 33S, 34S, 36S), iron (54Fe, 56Fe), and molybdenum (95Mo, 98Mo) isotopic compositions, along with trace element patterns of authigenic pyrite from modern and ancient methane-rich sediments deposited along continental margins. Our results highlight the potential of pyrite geochemistry as a tool to distinguish and characterize different modes and intensities of microbial sulfate reduction during early diagenesis. Furthermore, this study reveals that the trace element inventory of pyrite formed during early diagenesis is affected by sediment composition rather than by seawater. A comprehensive understanding of early diagenetic processes improves our understanding of pyrite formation and its geological implications.

Some of the carbon removed from Earth’s surface is stored within authigenic carbonate in marine sediments. Methane seeps are crucial sites of global marine carbon cycling sustaining microbial activity, enabling carbonate formation and the transfer of methane-derived carbon to the geosphere. Carbon sequestration rates depend on carbonate precipitation rates, which can be accelerated by mat-forming microorganisms that are ubiquitous at methane seeps and other Earth surface environments today. We investigate a 5-m-long drill core from an active methane seep at 1350 m water depth in the South China Sea with an exceptional abundance of pink and clear aragonite cement derived from the sulfate-driven anaerobic oxidation of methane, yet both cements precipitated under different conditions. Phase-specific 230Th/U-based ages, lipid biomarker compositions, and calcium isotope data suggest that pink aragonite is a product of in situ biofilm mineralization. First estimated precipitation rates of these individual cements in the seep carbonates range from 0.04 cm/ka for clear aragonite to 1.0 cm/ka for pink aragonite, suggesting an up to 25-fold increase in precipitation rates associated with biofilm mineralization. These results provide first kinetic constraints for future quantitative carbon cycle models, emphasizing the role of biofilms in accelerating carbon sequestration in marine authigenic carbonates. Carbonate precipitation by microbial mats in active hydrocarbon seeps can accelerate carbon sequestration in these settings by up to 25 times, according to analysis of a drill core from the South China Sea.

At marine methane seeps, siboglinid tube worms influence the exchange of elements between sediments and seawater and potentially represent a recorder of seep activity. The organ of siboglinids produces the chitin of the tube wall, facilitating the formation of tubes that protect the siboglinid's soft tissue. However, the mechanisms underlying element transfer and isotope fractionation between soft tissue and the chitinous tube are poorly constrained. This study analyzes the carbon, nitrogen, and sulfur isotopes as well as trace elements, including rare earth elements (REEs) and copper (Cu), compositions of tissues and tubes of the siboglinid tube worm Paraescarpia echinospica from the Haima seeps of the South China Sea. The stable isotope values of both tissue and the chitinous tube primarily reflect sulfur oxidation processes and carbon fixation by endosymbiotic sulfur-oxidizing bacteria. The trace element composition of the soft tissue suggests the utilization of light rare earth elements (LREEs) and Cu during aerobic oxidation of methane supposedly performed by epibiotic aerobic methanotrophic bacteria. In contrast, the trace element composition (e.g., REEs, Cu) of the chitinous tube primarily records the influence of diagenetic processes and source effects. The chitinous tube is therefore apparently not a straightforward archive of the local environment and metabolic characteristics of the tube worms. Still, the discrepancies between the information stored in tissue and the chitinous tube may offer some insight for the identification of fossil seep-dwelling tube worms if parts of the chitinous tube are preserved. Integrating element and isotope geochemistry within paleontological studies may consequently enhance our understanding of siboglinid evolution.

Marine authigenic carbonates are considered a major long-term carbon sink, yet the role of silicate weathering in their formation remains unclear. Here we use lithium isotope compositions of authigenic carbonates from the Gulf of Mexico to trace the coupling between silicate weathering and carbonate authigenesis in marine sediment. In contrast to the positive delta 7Licarb values in seep carbonates formed near the seafloor, carbonates originating from organoclastic sulphate reduction in deeper burial settings exhibit negative delta 7Licarb values (-6.6 parts per thousand to -1.2 parts per thousand). Calculated delta 7Lifluid values (-4 parts per thousand to +1.4 parts per thousand) in pore fluids closely match the delta 7Lisilicate values of the silicate component in the rocks, suggesting congruent silicate weathering is approached. A positive correlation between delta 7Lifluid and delta 13Ccarb values indicates organoclastic sulphate reduction enhances silicate weathering, which together promote carbonate authigenesis in anoxic sediments. Our findings demonstrate that lithium isotopes are a valuable tool for tracing carbon-silicate interactions and reconstructing carbon cycling in anoxic sediments.

Carbonate concretions accompanied by elemental sulfur are found in many upper Miocene marine successions across the Mediterranean area (e.g. SE-Spain, Sicily, Apennine, Cyprus). Most of these rocks are characterized by molds of evaporitic minerals (mostly gypsum) suggesting an early (syngenetic) or late (epigenetic) diagenetic origin. In contrast to these findings, a case study from the Ripa dello Zolfo area in northern Italy lacks evidence of carbonate and sulfur replacement of preexisting sulfate minerals. An integrated approach including sedimentological, petrographical, stable isotope (carbon, oxygen, and multiple sulfur isotopes), and lipid biomarker analyses was used for the study of three main lithofacies: a) laminated lithofacies representing aphotic carbonate stromatolites enclosing fossils of filamentous sulfide-oxidizing bacteria; b) brecciated lithofacies deriving from the brecciation of carbonate stromatolites by mud injections; c) sulfur-bearing lithofacies deriving from the precipitation of thin laminae of elemental sulfur at or close to the sediment-water interface. The δ13C and δ18O values of authigenic carbonate minerals and δ13C of lipid biomarkers indicate that the initial formation of the laminated lithofacies was favored by organoclastic sulfate reduction in the shallow subsurface close to the sediment-water interface, producing sulfide that sustained dense microbial mats of sulfide-oxidizing bacteria at the seafloor. Calcification of the mats and consequent formation of stromatolites were possibly favored by nitrate-driven sulfide oxidation at the seafloor. The subsequent brecciation of the stromatolites was apparently the consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM) in an underlying sulfate-methane transition zone (SMTZ). Focused fluid flow from a deeper zone was not only causing the brecciation of the stromatolites, but also delivered bicarbonate ions for the subsequent precipitation of additional, 13C-depleted calcite (δ13C values as low as -52‰). Along with bicarbonate, also hydrogen sulfide was produced by SD-AOM within an SMTZ in a zone below the stromatolites and was transported upwards. The oxidation of hydrogen sulfide at or close to the seafloor promoted the formation of elemental sulfur characterized by δ34S and Δ33S values close to coeval seawater sulfate. This study highlights that a multi-proxy approach has great potential for the reconstruction of spatially and temporarily separated biogeochemical processes in the shallow subsurface or at the seafloor (i.e., anaerobic oxidation of methane, sulfate reduction, sulfide oxidation) – processes that may induce the syngenetic formation of authigenic carbonate and sulfur deposits in marine sediments.

Quantifying the contribution of different carbon sources to dissolved inorganic carbon (DIC) flux in cold seep environments is critical for understanding the global carbon cycle. Pore water geochemical compositions provide insights into the biogeochemical processes of different DIC sources at cold seeps. Here, delta 13 C DIC values as well as SO42-,DIC, Ca2+, Mg2+, and PO4 3- concentrations of three push cores were analyzed with a reactive transport model to distinguish the DIC sources and calculate the DIC budget in the shallow sediments of the Haima cold seeps. The shallow depths of the sulfate methane transition zone (SMTZ) indicate significant methane flux and anaerobic oxidation of methane (AOM). The model results confirm that AOM is the primary biogeochemical process consuming sulfate at three sites, accounting for 99.5%, 91.5%, and 52.1%, respectively. The Sr/Ca vs. Mg/Ca ratio shows that high-Mg calcite precipitation occurred at ROV4 and ROV5 sites, while the carbonated phase precipitating at ROV01 site was 73.8% aragonite accompanied by 26.2% of high-Mg calcite. Moreover, extremely low delta 13 C DIC values indicate the presence of deep-sourced biogenic methane at the three sites. Based on the delta 13 C mass balance, the contribution of methane to DIC by AOM and methanogenesis is 99.6%, 95.4%, and 62.1%, respectively. Thus, methanogenesis is another primary source of DIC at the Haima cold seeps. Our study documents the influence of deep-sourced methane and methanogenesis on DIC flux at seeps and demonstrates that the DIC budget of seep sediments is a major contributor to the marine carbon pool and the marine carbon cycle.