Natural hydrogen is increasingly recognized as a pivotal zero-carbon energy resource for the future. Recent subsurface exploration indicates that within deep aquifers, natural hydrogen is typically mixed with methane under high-pressure and low-temperature regimes, providing an ideal thermodynamic setting for gas hydrate crystallization. However, solubility data for H2CH4 mixtures under two-phase hydrate-liquid equilibrium remain completely absent. To address this data gap and predict phase behaviors under these unmeasured conditions, we established a comprehensive thermodynamic model. Theoretically, the hydrate phase is described using the van der Waals and Platteeuw theory with Langmuir constants derived from ab initio intermolecular potentials, whereas the non-idealities of the aqueous phase are computed by coupling the Holder approach with Pitzer's specific ion-interaction equations. Crucially, this theoretical model ensures robust predictive capability without relying on empirical parameter fitting to H-L data. Based on the developed model, systematic simulations were conducted using selected typical geological environmental parameters: pressures of 50–400 bar, temperatures of 274.15–282.15 K, dissolved hydrogen ratios (RH2) covering the entire valid H-L equilibrium region, and liquid phase compositions ranging from pure water to 2.0 m NaCl solutions. Simulation results reveal unique thermodynamic characteristics. First, an anomalous negative pressure dependence of gas solubility is identified, primarily driven by methane's competitive occupation of large cavities. Second, solubility exhibits a positive response to temperature, as higher solute concentrations are necessary to counterbalance the thermal destabilization of the crystal lattice. Finally, the introduction of electrolytes substantially reduces the dissolved gas capacity due to the salting-out phenomenon. These findings yield critical thermodynamic parameters required for evaluating native hydrogen reserves and optimizing hydrate-based storage solutions.

Hadal trenches have been taken as hotspots for organic carbon burial and microbial activity in the deep-sea settings. In this study, water-extractable (WEOM) and porewater dissolved organic matter (PWOM) were analyzed from two sites in the Mariana Trench-MT04 (clay-dominated) and MT05 (laminated diatom mat, LDM sediments)-to evaluate the influence of mineral association on dissolved organic matter composition, optical characteristics, and stable carbon isotopic signatures. Compared to the LDM core, WEOM in the clay-dominated core shows higher SUVA(254), humification index (HIX), and A(253)/A(203) values, but the fluorescence index (FI) is lower. These optical characteristics indicate higher aromaticity, a greater degree of humification and lower bioavailability of WEOM. Such patterns suggest that clay minerals selectively adsorb and occlude aromatic- and carboxyl-rich dissolved organic matter. This mineral association reduces microbial accessibility and enhances long-term preservation. Comparison between WEOM and PWOM further reveals distinct compositional and functional differences within the sedimentary organic carbon pool. WEOM is tightly associated with mineral surfaces, enriched in aromatic and humified components, and exhibits stronger stability, whereas PWOM contains more protein-like and labile constituents. The decreasing trend in delta C-13 (delta C-13 of sediment total organic carbon > delta C-13 of WEOM > delta C-13 of PWOM) indicates that selective microbial degradation and mineral protection jointly regulate dissolved organic matter transformation pathways. Therefore, WEOM represents a mineral-associated, relatively stable carbon reservoir that plays a crucial role in long-term organic carbon preservation in the deep-sea environment.

Accurate prediction of gas solubility in formation brines is critical for both natural hydrogen exploration and underground hydrogen storage, where hydrogen and methane frequently coexist as binary mixtures. However, existing theoretical studies are largely restricted to pure component systems, lacking a unified model to describe the complex competitive dissolution behavior of gas mixtures in realistic subsurface environments. To address this gap, this study presents a rigorous thermodynamic model by integrating the Peng-Robinson Equation of State (PR-EOS) with van der Waals mixing rules for the vapor phase and the Pitzer ion-interaction model for the liquid phase. By further incorporating an Approximation Principle, the model extends its applicability to complex multi-component brines with diverse ionic compositions without requiring extensive parameter regression. The model was validated against recent experimental data, demonstrating robust performance. Thermodynamic analysis reveals a critical pressure-induced solubility crossover: while methane exhibits higher solubility at low pressures, hydrogen solubility surpasses that of methane at elevated pressures, driven by the remarkably high fugacity coefficient of hydrogen. Furthermore, the two gases display distinct thermal sensitivity heterogeneity, where methane solubility decreases significantly with temperature while hydrogen remains relatively stable. Additionally, the solubility of each component shows a near-linear dependence on its mole fraction in the gas phase, and high-salinity environments impose a strong thermodynamic constraint on gas dissolution. Collectively, this work establishes a robust theoretical foundation for predicting phase behavior in subsurface systems, offering critical insights for both the efficient deployment of large-scale hydrogen storage and the assessment of natural hydrogen resources in the global energy transition.

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 Okinawa Trough (OT), situated in the southeastern expanse of the East China Sea shelf, is an active back-arc basin profoundly shaped by the subduction of the Philippine Sea Plate. Although magmatic processes across different segments of the OT have been extensively studied, key uncertainties remain regarding the composition of magma sources, the dominant mechanism governing magmatic evolution and the specific contribution of subduction-derived components-particularly the Iheya Ridge (IR) in the middle Okinawa Trough (MOT). To address these issues, this study aims to decipher the mantle source characteristics, magmatic evolution processes and role of subduction-related materials in the IR by analysing major and trace element compositions together with Sr-Nd isotopic characteristics of three volcanic rock samples from the IR. The samples span a compositional range from basalt to andesite (SiO2 = 51.55-60.07 wt%), reflecting the evolution of intermediate-mafic magmas. Consistent trace element patterns and restricted Sr-Nd isotopic variations (87Sr/86Sr = 0.705043-0.707050; 143Nd/144Nd = 0.512734-0.512862) indicate derivation from a common magma source, with compositional variations attributed to fractional crystallisation. Isotopic and geochemical modelling reveal that the mantle source is a ternary mixture of Depleted Mantle (DM), Enriched Mantle Type II (EMII) and subducted sediments, with subducted sediments contributing 1.5%-4.0% and mantle-derived melts accounting for 96.0%-98.5%. Fractional crystallisation involving plagioclase, clinopyroxene and apatite is identified as the dominant process driving magmatic differentiation, yielding intermediate magmas from basaltic parents. This study provides new constraints on mantle dynamics and magmatic evolution in young back-arc systems, highlighting the interplay between slab-derived inputs and extension-driven mantle upwelling.

The accelerating exploration of natural hydrogen and the development of hydrate-based gas storage technologies have ignited significant interest in the phase behavior of hydrogen-containing hydrates. However, accurately determining the phase equilibrium conditions of multicomponent hydrogen hydrates, particularly when thermodynamic promoters are introduced, remains a formidable challenge. From an experimental perspective, accurately measuring the phase equilibrium conditions of these complex high-pressure systems is notoriously difficult and hazardous. From a theoretical standpoint, traditional thermodynamic models struggle to describe the severe non-ideality introduced by multicomponent gas mixtures and liquid promoters, frequently suffering from parameter explosion and iterative non-convergence. To bridge these gaps, this study proposes a data-driven model based on a Bayesian-optimized interpretable XGBoost algorithm. A comprehensive dataset was established, starting from 921 raw experimental data points and yielding 724 high-quality samples after rigorous physical screening. To enhance practical applicability, three categories of key parameters-system temperature, initial feed gas composition, and promoter concentration-were utilized as input features to predict the phase equilibrium pressure. The optimized model exhibited exceptional predictive performance, achieving a high R2 of 0.9421 and MAE of 0.9323 MPa on the independent testing set. Furthermore, the SHAP method was integrated to unbox the model and reveal complex non-linear feature interactions, quantifying the compounding destabilizing penalties of high temperature and hydrogen, uncovering the distinct thermodynamic limitations of conventional co-guests (CH4 and CO2), and highlighting that the absolute pressure-reducing efficacy of liquid promoters (THF and TBAB) is markedly augmented under high-temperature and hydrogen-rich environments. This work not only provides a highly accurate, non-iterative prediction tool but also offers valuable physical insights into the phase behavior of complex hydrogen hydrates, providing reliable theoretical support for natural hydrogen exploration and the optimal design of solid-state hydrogen storage systems.

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.

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.

Although per- and polyfluoroalkyl substances (PFAS) are widespread in global ecosystems, their presence in the hadal zone, particularly that of novel compounds, remains unexplored. In this study, 15 PFAS were detected in amphipods from the Mariana, Mussau, and New Britain Trenches (ranging from 0.4-37.5 ng g-1 dry weight), whereas all seawater and sediment samples fell below the detection limit. We quantified organism-water partitioning, analyzed structural and concentration similarity via COSMO-RS, applied neural networks to predict bioaccumulation, and prioritized PFAS risks using a persistence-bioaccumulation-toxicity framework. Short-chain novel PFAS (e.g., PFBA and PFPeA) formed the largest share of total PFAS loads (up to 4.2 ng g-1 dw) but contributed minimally to risk. In contrast, long-chain PFAS (PFTrDA and PFUnDA), though less abundant, exhibited substantially higher risk potential. The novel compound F-53B was detected exclusively in Mariana amphipods. Overall, the accumulation patterns across PFAS classes reflect the combined influence of external exposure and internal partitioning constraints. These findings demonstrate that structural modification does not inherently reduce PFAS hazards and highlight the necessity of including hadal organisms in global chemical risk evaluation.

Natural cold seeps in the marine environment are significant contributors to atmospheric greenhouse gases. To accurately quantify the methane flux from cold seeps, a novel in situ acoustic measuring device was designed to determine the methane bubble flux using the attenuation in acoustic signal intensity caused by bubbles passing through a set pipe. The acoustic measuring device consists of an inverted conical tent and an acoustic measurement pipe with a rectangular cross-section. The acoustic measurement pipe comprises a bubble-fragmented homogenization unit and an acoustic cross-correlation sensor subsystem. The bubble-fragmented homogenization unit adjusts the rising gas bubbles to a diameter of less than 3 mm with a uniform spatial distribution. The acoustic cross-correlation sensor subsystem measures the bubble-rise rate and section void fraction to calculate bubble flux. The accuracy of the bubble rising rate is +/- 3%, and the relative error of bubble flux is 4.6%. The bubble flux of a single vent in the Lingtou promontory seep area is 2358.1 L, and the bubble rising rate ranges from 0.29 to 0.52 m/s, controlled by changes in ocean tides. Therefore, we believe this device can be a powerful tool for quantifying bubble flux in deep-sea environments.

Compared to the growing evidence of what drives methane seepage along upper continental slopes, our understanding of the factors controlling seepage in deep-marine environments is insufficient; this limitation is partly caused by the lack of constraints on the history of seepage in deep-sea settings. Here, we use uranium-thorium and radiocarbon dating of seep carbonates and bivalve shells sampled from cores taken in the Qiongdongnan Basin of the South China Sea (similar to 1700 m water depth). The carbonate and shell ages indicate two episodes of methane emission, one period during Marine Isotope Stage 5e (ca. 133-121 ka), and another period after the Last Glacial Maximum (ca. 18-3 ka). Modeling reveals that changes of bottom water temperature and sea level during periods of methane release had only limited influence on the extent of the hydrate stability zone, suggesting that-unlike for shallower water settings of the South China Sea-a change of bottom water temperature was not the main trigger of methane seepage. Considering that sedimentation rates during periods of seepage were significantly lower than during periods of dormancy, we put forward the new concept that a decline of sediment loading favored episodic seepage in the studied deep-sea setting. During periods of increased deposition, changes in gas pressure due to the gradual thickening of the gas hydrate stability zone resulted in the closure of fractures and, thus, reduced methane release. Vice versa, methane seepage increased when sedimentation rates declined, and hydraulic fracturing regained momentum. On Quaternary time scales, deepwater gas reservoirs may have been more sensitive to changes in sedimentation than they were to warming or sea level fall. The deep-sea reservoir of methane hydrate should, therefore, be considered separately when estimating the impact of hydrate stability on climate change.

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.

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.

Hadal trenches have recently been recognized as hotspots for organic carbon burial and degradation in the deep sea owing to its distinctive “funnel‐shaped” topography and frequent tectonic activity. In this study, we analyzed dissolved organic carbon (DOC) concentration, optical properties, and molecular compositions of water‐extractable organic matter (WEOM) in the sediment samples collected along two transects spanning diverse marine environments of the New Britain Trench (NBT) area. A significant positive correlation between DOC concentrations and total organic carbon (TOC) contents in sediments emphasizes the crucial role of organic matter (OM) supply for DOC production in the hadal environment. In addition, the optical parameters (e.g., fluorescence index, a 350 /DOC) suggest remarkable influences of terrestrial OM on the composition of dissolved organic matter (DOM) in the NBT sediments. Indeed, the correlations between the stable carbon isotopes of sedimentary TOC (δ 13 C) and related optical parameters (SUVA 254 , a 350 /DOC, S R and terrestrial humic‐like) indicate that microbial degradation of terrestrial OM has an important impact on the sedimentary DOM. Moreover, analysis of DOM molecular compositions showed increased aromaticity and double bond equivalents double bond equivalent in the western landward core and two axis cores, also suggesting enhanced microbial degradation of terrestrial OM. The input of terrestrial OM into the NBT has led to production of aromatic, refractory, and high molecular weight DOM. Our findings have implications for understanding the fate of terrestrial OM in the deep ocean.

Microbial sulfate reduction (MSR) and associated pyritic sulfide formation are important diagenetic processes in marine sediments. The sulfur isotopic composition of pyrite (delta(34)Spyr) is proven to be sensitive to changes in sedimentation rates and the content and reactivity of organic carbon, especially on the continental shelves and upper slopes (water depth < 350 m). However, the diagenetic responses of sulfur to variations in climatic and depositional conditions in the deep-sea sediments are still poorly understood. This study combines element contents and isotopes to characterize diagenetic interplays of sulfur, organic carbon, and methane in the continental slope sediments of the northern South China Sea since the mid-Pleistocene. Our data suggest that the total organic carbon (TOC) increased during glacial times, implying enhanced primary productivity due to increased nutrient supply by the East Asian Winter Monsoon, in addition to efficient transfer of organic carbon and better preservation of organic carbon due to reduced bottom water oxygen. Total sulfur and chromium reduction sulfur contents varied concomitantly with the TOC, suggesting an increased burial of organic carbon that enhanced the organoclastic sulfate reduction (OSR) and the formation of authigenic pyrite. The environmental changes did not induce a significant shift in delta(34)Spyr, due most likely to relatively low sedimentation rates and large fractionation in sulfur isotope through OSR during the glacial-interglacial cycles. Instead, it is hypothesized that the sulfate-driven anaerobic methane oxidation promoted the formation of a higher amount of authigenic pyrite. Consequently, it created a closed diagenetic system leading to positive excursions in delta(34)Spyr at the sulfate-methane transition zone. Our results suggest the vulnerability of pyrite formation and its sulfur isotopic composition to the changes in monsoon-driven primary productivity and the methane-rich fluid migrations in the continental margin sediments. This study complements the growing evidence for the local diagenetic controls on sedimentary sulfur geochemical records by highlighting the importance of early diagenesis in paleoenvironment reconstruction based on the content and sulfur isotopic composition of pyrite.

Cold seep carbonates,as the main product of submarine cold seep activities,can reflect past fluid environment information through their biogeochemical characteristics.A large number of Late Triassic cold seep carbonates are developed in southern Turkey.To reveal the fluid characteristics and depositional environment of their formation,this study conducts a detailed analysis of their carbon and oxygen isotopes,major and trace elements,and rare earth elements(REEs).The results show that theδ13CV-PDB values of the Turkish cold seep carbonates range from-15.5‰ to-2.8‰,indicating that their carbon sources likely resulted from mixing between methane and seawater.The REE distribution patterns show either no anomaly or slight depletion in cerium,accompanied by MREE enrichment.Additionally,the Ni/Co ratio(5.38 to 24.08)suggests the depositional environment of these carbonates was reducing.