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Authigenic carbonates and pyrite associated with sulfate-driven anaerobic oxidation of methane (AOM) at methane seeps provide archives to explore the biogeochemical processes involved and seepage dynamics over time. The wide range and extremely high δ34S values of pyrite (δ34Spy) have been used to trace the AOM-related processes. However, the detailed mechanism for this phenomenon is not well understood. We propose that the characteristics of δ34Spy were mainly controlled by the competition between sulfate reduction and sulfate supply, as well as the redox condition. To test this hypothesis, we investigated Sr/Ca and Mg/Ca ratios, trace element compositions, pyrite contents and sulfur isotopic compositions in seep carbonates from Site F and Haima in the northern South China Sea. Calcite and aragonite contents were distinguished through the Sr/Ca and Mg/Ca ratios. The data show that aragonites are always associated with relatively low δ34Spy values compared to calcites. The Mo contents show a good correlation with pyrite contents in calcites and aragonites, and the slope in aragonites is larger than that in calcites. This relationship indicates that the aragonite precipitated in a relatively open system with higher Mo availability. Thus, we conclude that sulfides with low δ34S values formed at high supply of sulfate under the relatively open system with respect to diffusive replenishment of sulfate, where the carbonate precipitation occurred close to the seafloor due to a strong methane flux. Under vigorous methane flux simultaneously, the high potential of less anoxic conditions, which could limit the additional pyrite accumulation and/or favor the microbial disproportionation, could also be the cause of the low δ34Spy, as supported by samples from the Haima sites. Evidence for this assumption is based on the occurrence of bivalve shells and less enrichment in As and Sb. Conversely, the positive δ34Spy values result from near to complete exhaustion of dissolve sulfate via AOM within a deeper sulfate-methane transition zone, where Mo is less available. The combination of a detailed elemental study of authigenic carbonates with sulfur isotopic composition of sulfide minerals in carbonates is a promising tool for reconstructing the dynamics of seep intensities at modern and, potentially, geological seep sites.

Hydrocarbon-seep carbonates were discovered to be hosted within Miocene marine mudstone in the central Western Foothills near the Kuohsing area, Central Taiwan. Here, we present field observations, petrography, mineralogy, stable carbon isotope data, as well as rare earth element compositions. We evaluate the role of diagenetic alteration on the Kuohsing seep carbonates and constrain their fluid sources and formation conditions. Four main morphologies in the field were observed, including cylindrical columns, massive forms, lenticular nodules, and stratiform bodies. Petrographically, the carbonates are predominantly composed of partially recrystallized calcite and dolomite, with abundant amounts of biogenic detritus, pyrite aggregations and unidentified filaments. Isotopic analyses show that carbon isotope values inversely vary with oxygen isotope values, indicating substantial modification of the carbon isotope compositions of the carbonates by diagenetic processes. Extremely negative δ13C values as low as −47.6‰ recognized as a near-primary isotopic signature recorded in the carbonates, an end member of the trend, reveals that methane was incorporated during the carbonate precipitation. On the other hand, the carbonates have well preserved primary REE compositions, exhibiting MREE enrichment, no La and Ce anomalies, and supra-chondritic Y/Ho ratios. These REE patterns suggest that the Kuohsing authigenic carbonates were precipitated from reducing seep fluids with limited admixture of seawater.

Three decades after the discovery of cold seep systems, various sites of hydrocarbon seepage have been found in the South China Sea (SCS). Over the past decade, these sites have become model systems for understanding the variability of hydrocarbon seepage and associated biogeochemical processes. In this review, we describe the cold seep systems of the SCS with an emphasis on seafloor manifestations, fluid sources, biogeochemical processes, and macroecology. Seafloor features associated with seeps include mud volcanoes, pockmarks, and carbonate deposits. A common characteristic of cold seeps is the occurrence of authigenic (i.e., in situ precipitated) carbonate minerals. These carbonates commonly exhibit low δ13C and high δ18O values, suggesting the incorporation of methane-derived carbon and oxygen derived from gas hydrate water. Biogeochemical processes such as sulfate-driven anaerobic oxidation of methane (SD-AOM), the key process at seeps, have been studied in detail with the aim of establishing geochemical proxies to trace these processes into the geological past. We also detail the features characterizing seep ecosystems. Understanding the impact of decomposing methane hydrate on the marine carbon budget remains challenging and requires additional seafloor observations as well as models predicting how gas hydrate responds to changing conditions such as temperature increase, sea level rise, and episodic mass wasting.

Anomalously low δ13C values of foraminifera in the stratigraphy are usually interpreted to reflect activities of paleo-methane seepage. The application of this proxy, however, requires to understand the cause of anomalously low δ13C signals in foraminifera. Here we present stable carbon and oxygen isotopes (δ13C and δ18O), Mg/Ca ratios and accelerator mass spectrometry (AMS) radiocarbon data from Cores GMGS2-08B (∼0–5.5 m below the seafloor [mbsf]) and 08C (2–23 mbsf) at ∼798 m water depth, recovered from the gas hydrate-bearing sediments in the northeastern South China Sea (SCS). The main objectives are to investigate the primary versus secondary signals of extremely negative δ13C values of foraminifera and further recognize the methane emission events (MEEs). The δ13C values of benthic (Uvigerina peregrina and Cibicidoides spp.) and planktonic (Globigerinoides ruber and Pulleniatina obliquiloculata) foraminifera in these two cores show distinct intervals with negative values (as low as −15.85‰) that indicate the shift of the sulfate methane transition zone (SMTZ) and the change in methane release flux in the past. The δ13C records of U. peregrina by a more exhaustive cleaning procedure designed to remove authigenic carbonate coatings show that most δ13C values are still lower relative to those in the normal marine environment. The Mg/Ca ratios of Uvigerina spp. by this exhaustive cleaning procedure display higher values and more variable for anomalous tests (as high as 13.40 mmol/mol) than normal ones. These observations indicate the 13C-depleted signals of foraminifera are mainly originated from diagenetic alteration of authigenic carbonate precipitates with extremely negative δ13C values. Furthermore, the mass-balance approach is adopted to assess the relative contribution of methane-derived authigenic carbonate in the low-δ13C signals in foraminifera and reveals the maximum contribution can be as high as ∼30% among the δ13C anomalies. Additionally, several major MEEs with distinct characteristics can be recognized according to remarkable δ13C excursions of foraminifera in these two cores.

Over the past decade more than 30 cold seep sites have been reported along the continental slope of the South China Sea (SCS). These seep sites have all been located in the northern SCS, and no methane seepage has been discovered in the southern SCS. Here, we report on the analysis results of the geochemical and geophysical of the marine core sediments (58S) collected from the southern continental slope of the SCS. We suggest that the sedimentary total sulfur (TS), total inorganic carbon (TIC), δ34SCRS values of chromium reducible sulfur (CRS) and carbonate δ13CTIC values of the bulk sediments, coupled with the ratio of (NRMAF60mT-NRMAF80mT)/NRM can be used to trace the methane release events in a methane-rich environment. At least two methane release events were identified at depths of 5.5–6.2 m and 6.8–7.2 m. Moreover, according to the characteristic of the TS/TOC and (NRMAF60mT-NRMAF80mT)/NRM ratio of sediments, we suggest the methane flux has changed episodically. This inference is supported by the occurrence of the strongly negative carbonate δ13CTIC values of bulk sediments and the positive δ34SCRS values, which indicate the occurrence of anaerobic oxidation of methane (AOM) and the location of a past or present sulfate methane transition zone (SMTZ), respectively. Ultimately, these new data suggest that the combined geochemical and geophysical study of marine sediments provides a valuable tool with which to infer the temporal variations in methane seepage in a marine environment.

Several tens of methane (CH4) seepage sites have been discovered in the northern South China Sea (SCS) during the investigation and exploration of gas hydrates and cold seeps over the past decade. However, our knowledge about methane seepage in the southern SCS remains scarce, especially in the Beikang Basin on the southern slope of the SCS. Four piston cores (BH-CL18, BH-CL23, BH-CL29 and BH-CL35) were collected in the Beikang Basin to understand CH4 cycling in shallow sediments. The extrapolated depths of the sulfate–methane transition zones (SMTZ) of the four cores are between 5.3 and 8.8 mbsf. By simulating the porewater profiles of SO42−, CH4, DIC and PO43− using the reaction-transport model, we estimated that the dissolved SO42− was predominantly consumed by the anaerobic oxidation of methane (AOM) at rates of 27.5 mmol m−2 yr−1 in BH-CL18, 28.1 mmol m−2 yr−1 in BH-CL23, 43.1 mmol m−2 yr−1 in BH-CL29 and 39.4 mmol m−2 yr−1 in BH-CL35, respectively. Due to intense AOM, almost all the methane sourced from subsurface sediments was depleted within the SMTZ, resulting in negligible benthic methane fluxes. The kink-type depth SO42− profile of BH-CL23 was apparently attributable to an enhanced methane flux, which was not captured by the steady-state modelling. Furthermore, highly depleted δ13C values of the dissolved inorganic carbon (DIC) in BH-CL18 (−43.6‰), BH-CL23 (−44‰) and BH-CL29 (−51.2‰) also indicated a methane-derived DIC. In contrast, relatively higher δ13C values in BH-CL35 suggested a mixture of 13C-enriched DIC produced by methanogenesis, as shown by the reversal toward more 13C-enriched DIC below 4.2 mbsf in BH-CL29. To our knowledge, this is the first quantitative study dealing with methane cycling in the shallow sediments of the southern SCS and may have considerable implications for understanding the poorly investigated cold seep and gas hydrate systems on the continental slope of the southern SCS.

AbstractNumerous marine hydrocarbon seeps have been discovered in the past three decades, the majority of which are dominated by methane-rich fluids. However, an increasing number of modern oil seeps and a few ancient oil-seep deposits have been recognized in recent years. Oil seepage exerts significant control on the composition of the seep-dwelling fauna and may have impacted the marine carbon cycle through geological time to a greater extent than previously recognized. Yet, distinguishing oil-seep from methane-seep deposits is difficult in cases where δ13Ccarb values are higher than approximately -30‰ due to mixing of different carbon sources. Here, we present a comparative study of authigenic carbonates from oil-dominated (site GC232) and methane-dominated (site GC852) seep environments of the northern Gulf of Mexico, aiming to determine the geochemical characteristics of the two types of seep carbonates. We analyzed (1) Major and trace element compositions of carbonates, (2) total organic carbon (TOC), total nitrogen (TN) and carbon isotope (δ13CTOC) of residue after decalcification, (3) sulfur isotope signatures of chromium reducible sulfur (CRS, δ34SCRS) and residue after CRS extraction (δ34STOS ), as well as (4) sulfur contents (TOS) of residue after CRS extraction. Carbonates from the studied oil seep are dominated by aragonite and exhibit lower δ34SCRS values, suggesting carbonate precipitation close to the sediment surface. In addition, oil-seep carbonates are characterized by higher TOC and TOS contents and higher TOC/TN ratios, as well as less negative δ13CTOC values compared to methane-seep carbonates, probably reflecting a contribution of residual crude oil enclosed in oil-seep carbonates. Very low δ13CTOC values (as low as −68.7‰, VPDB) and low TOC/TN ratios of methane-seep carbonates indicate that the enclosed organic matter is derived mainly from the biomass of methanotrophic biota. This study presents new geochemical data that will allow the discrimination of oil-seep from methane-seep deposits. Although some of the geochemical patterns are likely to be affected by late diagenesis, if applied with caution, such patterns can be used to discern the two end-member types of seepage – oil seeps and methane seeps – in the geological record.

Widespread cold seeps along continental margins are significant sources of dissolved carbon to the ocean water. However, little is known about the methane turnovers and possible impact of seepage on the bottom seawater at the cold seeps in the South China Sea (SCS). We present seafloor observation and porewater data of six push cores, one piston core and three boreholes as well as fifteen bottom-water samples collected from four cold seep areas in the northwestern SCS. The depths of the sulfate&ndash, methane transition zone (SMTZ) are generally shallow, ranging from ~7 to &lt, 0.5 mbsf (meters below seafloor). Reaction-transport modelling results show that methane dynamics were highly variable due to the transport and dissolution of ascending gas. Dissolved methane is predominantly consumed by anaerobic oxidation of methane (AOM) at the SMTZ and trapped by gas hydrate formation below it, with depth-integrated AOM rates ranging from 59.0 and 591 mmol m&minus, 2 yr&minus, 1. The &delta, 13C and &Delta, 14C values of bottom-water dissolved inorganic carbon (DIC) suggest discharge of 13C- and 14C-depleted fossil carbon to the bottom water at the cold seep areas. Based on a two-endmember estimate, cold seeps fluids likely contribute 16&ndash, 26% of the bottom seawater DIC and may have an impact on the long-term deep-sea carbon cycle. Our results reveal the methane-related carbon inventories are highly heterogeneous in the cold seep systems, which are probably dependent on the distances of the sampling sites to the seepage center. To our knowledge, this is the first quantitative study on the contribution of cold seep fluids to the bottom-water carbon reservoir of the SCS, and might help to understand the dynamics and the environmental impact of hydrocarbon seep in the SCS.

International audience; Marine authigenic carbonates form shallow-water microbialites, mud mounds, and hydrocarbon-seep deposits and contain appreciable amounts of trace elements that yield information on paleoenvironments. Element patterns of some of these carbonates archive metabolic processes through geologic time since many trace elements are redox-sensitive and participate in biological cycling. Trace element distributions in microbial carbonates not only yield information on the redox state of ancient oceans, but also on the chemical evolution of Earth's hydrosphere and atmosphere. Trace element patterns can be used to distinguish marine from freshwater sources, to estimate water depth, and can help identify microbial metabolisms through time. A major issue concerning the use of marine carbonates as a paleoenvironmental archive is assessing the degree of early and late diagenetic alteration, which can modify or even reset the original content and distribution of trace elements. The degree and effect of early and late diagenetic alteration can be evaluated by comparing element contents to organic and other inorganic geochemical proxies. Authigenic hydrocarbon-seep carbonates forming in seafloor sediments are the product of microbial oxidation of methane and other hydrocarbon compounds. Seep carbonates are excellent archives, whose trace element contents yield information on sedimentary redox processes, as well as information on seepage intensity and fluid composition. Trace elements serve as proxies in these highly dynamic environments shaped by fluid seepage and chemosynthesis, and can help to reconstruct the evolution of chemosynthesis-based life at seeps through the Phanerozoic.

The modern southern Mariana Trench is characterized by oligotrophic surface waters, resulting in low primary productivity and well-oxygenated bottom waters. This study investigates changes in the redox conditions of bottom waters in the southern Mariana Trench during the Last Glacial Maximum (LGM) and their potential causes. We measured major, trace, and rare earth elements (REE) in three gravity cores (GC03, GC04, and GC05) and one box core (BC11) retrieved from the southern Challenger Deep at water depths from 5289 to 7118 m. The upper sediment layers of both GC05 and BC11 are dominated by valve fragments of the giant diatom Ethmodiscus rex, forming laminated diatom mats (LDMs). 14C-AMS dates of bulk organic matter show that the LDMs accumulated between 18.4 and 21.8 kyr B.P., corresponding to the LGM. Modest enrichments of U and Mo along with weak or absent Ce anomalies in the LDM point to suboxic conditions during the LGM. In contrast, non-LDM samples exhibit little to no enrichment of redox-sensitive elements as well as negative Ce anomalies, indicating deposition under oxic bottom-water conditions. The Ce anomalies are considered valid proxies for bottom-water redox conditions because REE signatures were acquired in the early diagenetic environment, as indicated by strong P-REE correlations and middle-REE enrichment associated with early diagenetic cycling of Fe-Mn oxyhydroxides in the sediment column followed by capture of the REE signal by biogenic and/or authigenic apatite. We postulate that the more reducing bottom-water conditions during the LGM were linked to increased primary productivity induced by enhanced Asian dust input. As shown in earlier studies, the increased primary productivity associated with Ethmodiscus rex blooms in the eastern Philippine Sea played a significant role in capturing atmospheric CO2 during the LGM. Consequently, the magnitude of atmospheric CO2 sequestration by giant diatom blooms during the LGM may have been greater than previously envisaged.

Chemosymbiotic bivalves harboring endosymbiotic, chemotrophic bacteria have been investigated from a variety of hydrocarbon seeps worldwide. It has been shown that carbon, nitrogen, and sulfur isotopic compositions of the animal soft body parts are excellent indicators for evaluating energy transfer and food sources for the respective deep-sea habitats. However, recognition of chemosymbiosis has proven to be difficult for bivalves that dwelled at ancient seeps due to the lack of soft tissue. Here, we investigated δ13C, δ15N, and δ34S signatures of the tissue (mantle) and the shell organic matrix (SOM) of the same specimens of three bathymodiolin mussel species with different chemotrophic symbionts (methanotrophs in Bathymodiolus platifrons and B. childressi and thiotrophs in B. aduloides) and one vesicomyid clam (Calyptogena sp.) from a variety of hydrocarbon seeps from the South China Sea and the Gulf of Mexico. The data obtained demonstrate that all seep bivalves regardless of species or locations reveal overall small differences in δ13C (≤+4‰), δ15N (≤+1‰), and δ34S (≤+5‰) values between SOM and mantle (∆SOM-mantle) of the specimens. Relatively larger ∆SOM-mantle for δ13C values (as high as +10‰) in B. platifrons and larger ∆SOM-mantle for δ34S values (up to 16‰) in B. aduloides and Calyptogena sp. might be due to different symbionts in their gills. Since SOM can be extracted from fossil bivalve shells, the proxy can be used as a fingerprint of chemosynthesis-based food chains, although its utility will depend on the quality of preservation of the shell organic matter. Despite this uncertainty, the new proxy has great potential to reconstruct energy flow through different types of chemosynthesis-based ecosystems.

Sulfate-driven anaerobic oxidation of methane (AOM) limits the release of methane from marine sediments and promotes the formation of carbonates close to the seafloor in seepage areas along continental margins. It has been established that hydrocarbon seeps are a source of methane, dissolved inorganic carbon, and dissolved organic carbon to marine environments. However, questions remain about the contribution of deep-sourced carbon from hydrocarbon seeps to the sedimentary organic carbon pool. In this study, we analyzed carbon quantity, radiocarbon content (as percent modern carbon, pMC), stable carbon isotopic compositions (as δ13C) of organic matter enclosed within seep carbonates from the Gulf of Mexico and the South China Sea to assess if sediment organic matter may be used as a proxy for methane seepage intensity. The δ13C values of organic matter (δ13Corg) exhibited a large range from −81.4‰ to −23.9‰. Radiocarbon contents of the carbonate-bound organic matter in seep carbonates ranged from 6% to 28% pMC, suggesting organic matter of the carbonates is a mixture of marine particulate organic matter (δ13C = −22‰ VPDB and 90% modern carbon) and biomass resulting from methane oxidation (assumed to have 0% modern carbon). Assuming constant productivity in the marine photic zone, it is proposed that seepage intensity and duration are the most important factors controlling the contribution of methane-derived carbon to the sedimentary column. This study reinforces the potential for using δ13C values of organic carbon to discern methane-rich environments in ancient sedimentary environments where authigenic carbonate is not present and to constrain the record of AOM through Earth history.

Seven short cores collected along two transects from the shelf to trench bottom in the New Britain Trench (NBT) area, Papua New Guinea (PNG), were studied to illuminate the provenance and burial of sediments across the shelf-trench continuum. Here, we present geochemical data, including major, minor and rare earth elements, bulk and clay mineralogy, grain size, and Sr–Nd isotopes. The results indicate that sediments in the NBT area were mainly derived from intermediate volcanogenic materials of the surrounding islands with the contribution of volcanogenic materials up to 90%. Volcanogenic materials from Rabaul Volcano had a significant influence on the western NBT (Transect 1), whereas the eastern NBT (Transect 2) was more strongly affected by the Tabar-Lihir-Tanga-Feni (TLTF) volcanic chain. In addition, pollen and spores analyses showed a decreasing trend in their concentrations in the order of the landward site, the trench axis site, and the abyssal site on the Transect 1 and the dominance of Cyperaceae in the herb fraction at the trench axis site NBT01. This observation, accompanied by the significant differences in the major and minor elements as well as Sr–Nd isotopes between upper 4 cm sediments and those below at NBT01, suggests that the upper 4 cm sediments were probably sourced from river sediments. Together with low chemical index of alteration (CIA) values, we suggest that terrigenous riverine materials are rapidly transported and buried at the trench axis. Our findings highlight the important role of hadal trenches in trapping terrigenous materials and have implication for understanding the fate of terrigenous materials in the deep ocean.

Hypersaline seafloor environments formed by brine seepage represent role model examples of extreme marine ecosystems; they may for example help to identify potential habitats for life on other planets. Trace and rare earth elements (REEs) are crucial components enabling microbial life at brine and hydrocarbon seeps. However, the behavior of trace elements at brine seeps is currently poorly understood. Because authigenic carbonates forming at brine seeps archive information on the composition of seepage fluids, we have analyzed carbonates collected from a brine seep site (Mississippi Canyon 709, MC709 for short) of the northern Gulf of Mexico (GoM). The dominance of low-magnesium (Mg) calcite among the carbonate minerals, moderately low δ13Ccarb values (average: −26.1‰, VPDB), as well as the presence of oil in carbonates suggest concomitant seepage of brine fluids and crude oil at this site. High δ18Ocarb values (up to +4.5‰, VPDB) suggest the involvement of 18O-rich fluids during carbonate precipitation, likely resulting from mineral-water reactions in deeper sediments. Significant enrichments of barium (Ba), strontium (Sr), and lithium (Li) in the samples suggest that deep ascending brine fluids contributed to carbonate formation. Enrichments in molybdenum (Mo), arsenic (As), and antimony (Sb) and a correlation between Mo, As, and Sb enrichment factors (EF) and the authigenic iron fraction (Fe/Al ratios) suggest that iron particulate shuttle processes were involved in scavenging of dissolved Mo, As, and Sb from seawater. This notion is strengthened by the co-variation of MoEF and UEF and the shale-normalized REE patterns of the brine-seep carbonates. This study allows to constrain the enrichment mechanisms of trace and REE elements at brine seeps, providing the foundation for an improved understanding of the distribution and behavior of trace metals, as well as their role in microbial metabolisms in the local hypersaline environment.

Sulfate-driven anaerobic oxidation of methane (SD-AOM) and organoclastic sulfate reduction (OSR) are the two processes that consume sulfate in methane-rich marine sediments. Determining the relative contribution of each process to overall sulfate reduction is critical to assess the global carbon and sulfur cycle in marine settings. The analysis of sulfur and oxygen isotope compositions of porewater sulfate (δ18OSO4 and δ34SSO4) provides crucial insight into sulfate reduction pathways and the dynamics of the sedimentary sulfur cycle. The δ18OSO4/δ34SSO4 ratio is governed by net sulfate reduction rates and has been widely used to distinguish SD-AOM from OSR. Furthermore, δ18OSO4/δ34SSO4 slopes are believed to vary with changing methane flux. To enhance the applicability of δ18OSO4 vs. δ34SSO4 plots for the tracing of SD-AOM activity and methane flux, we investigate sulfate sulfur and oxygen isotopes of porewaters from three piston cores collected from the Haima seeps of the South China Sea. Reaction-transport modeling previously indicated that sulfate consumption was dominated by SD-AOM under changing methane fluxes (1126 mmol m−2 yr−1 to 26 mmol m−2 yr−1). Our new data demonstrate an increase of the initial δ18OSO4/δ34SSO4 slope when methane flux decreases. An initial slope of 0.43 corresponds to a methane flux of 194 mmol m−2 yr−1, the lowest currently known methane flux producing a small δ18OSO4/δ34SSO4 slope diagnostic for SD-AOM in sediments close to the seafloor. When methane flux decreases to 26 mmol m−2 yr−1, a small slope (0.48) is only observed in deeper sediment layers. Our results also reveal an increase of δ18OSO4/δ34SSO4 slopes with decreasing SD-AOM rates. We estimate that net sulfate reduction rates can be as low as 10−9 mol cm−3 yr−1 to produce slopes smaller than 0.5 in SD-AOM-dominated settings. In OSR-dominated settings, δ18OSO4/δ34SSO4 slopes tend to be significantly higher than 0.5, while net sulfate reduction rates tend to be lower than 10−5 mol cm−3 yr−1. This study reveals that quantification of methane flux is required to utilize the full potential of δ18OSO4 vs. δ34SSO4 plots in the endeavor of tracing SD-AOM activity and the dynamics of the sulfur cycle in methane-rich environments.

The stable isotopic composition of benthic foraminifera tests is widely used to trace past methane release events. However, gaps in our knowledge on the ecology of benthic foraminifera at methane seeps impede the utility of test composition as proxy of methane emission histories and the influence of redox zonation on the local fauna. Here, we use sediment geochemistry and foraminiferal assemblages of a seep area of the South China Sea to reveal that seepage duration and intensity affect benthic foraminifera. Taxa tolerating eutrophic conditions like Uvigerina thrive at times of reduced seepage when aerobic methanotrophy is favored. In contrast, intense methane seepage leads to high sulfide levels, and a positioning of the sulfate-methane transition zone close to the seafloor. At these times, bottom waters may become oxygen depleted and benthic foraminifera will decline. This study provides (1) the means to understand how seepage dynamics control the distribution of benthic foraminifera and (2) new constraints on the use of foraminifera as paleoceanographic proxies, including possible mechanisms of mass mortality in the geological past.

Microbial sulfate reduction occurs in diverse depositional sedimentary environments, especially in fine-grained sedimentary turbidites. However, the extent to which turbidites impact the mechanism of sulfidization and the carbon–sulfur–iron cycle is poorly understood. Here, a sediment gravity core (10GC) with 50-cm thick turbidites was collected from the Nansha Trough of the southwestern South China Sea (SSCS) to unravel the biogeochemistry and sulfidization processes. One accelerator mass spectrometric 14C date in the planktonic foraminifera indicates that the turbidite layer was deposited at or immediately before 16 ka. Using a suite of elemental (Fe, K, Si Al, and Zr) and sediment grain size proxies suggests that the sediments within the turbidites were derived from the Sunda Shelf and the Malay Peninsula. The TOC/TN values (9–20) of the turbidites indicate deposition of terrestrial organic matter with a mixture of C3 and C4 types of vegetation. The relatively negative 13CTIC values (−7.76‰ to 2.09‰) are robust evidence for the accumulation of authigenic carbonates, which was attributed to organoclastic sulfate reduction. The rather positive δ34SCRS (CRS: chromium reducible sulfur) (−37.5‰ to −10.5‰) and high CRS contents of the turbidites compared to the hemipelagic sediments suggest that the enhanced sulfidization occurred due to a high rate of sulfate reduction in the closed system. In contrast to the turbidites, the more negative δ34SCRS values (−50.9‰ to −47.4‰) in the hemipelagic sediments reflect early diagenetic pyritization at the sediment–water interface. This study highlights the importance of turbidite sedimentation on the biogeochemical cycling of sulfur, carbon, and iron. However, further research is needed to better understand and quantify the contribution of marine turbidite sediments to the global ocean C–S–Fe cycle.

The magnesium (Mg) isotopic composition of marine carbonate has been used as a proxy to constrain the geochemical composition of seawater and to trace the global Mg cycle. Sulfate-driven anaerobic oxidation of methane (SD-AOM) is a prominent process that facilitates the formation of marine authigenic carbonate. Yet the understanding of the impact of SD-AOM on the Mg isotope composition of authigenic carbonate under natural conditions is limited. Here we present the analysis of two tubular methane-seep carbonates from the South China Sea. The tubular carbonates reveal a trend of decreasing δ13C values and increasing carbonate contents from the periphery (i.e., the outer surface of the carbonate) to the inner portion (i.e., close to the inner surface of the carbonate pipes). The tubular carbonates are interpreted as former fluid conduits, indicating intense and lasting seepage that apparently increased with time. The observed variability in composition probably results from changes in the intensity of SD-AOM and correlating carbonate precipitation rates. Trends of enrichment of middle rare earth elements (MREEs), molybdenum (Mo), and uranium (U) from the periphery inward indicate more reducing formation conditions toward the inner surface of the former conduit. Along the same transects, δ26Mg values decrease from periphery to inner portion in one tubular carbonate (−2.63‰ to −3.24‰), while δ26Mg values are close to constant for the second tubular carbonate (−3.42‰ to −3.37‰); such patterns highlight the complexity of Mg isotope fractionation during seep carbonate formation. A positive correlation between δ26Mg and δ13C values and a negative correlation between δ26Mg and Mg/Ca molar ratios suggests that enhanced SD-AOM facilitates the incorporation of light Mg isotopes into the carbonate lattice. However, such assumption contradicts with the fact that higher carbonate precipitation rates reduce Mg isotope fractionation due to incomplete dehydration of Mg ions. It is hypothesized that hydrogen sulfide produced by SD-AOM may enlarge the extent of fractionation by weakening the hydration of Mg. If this interpretation is correct, the effect of sulfide on the fractionation of Mg isotopes can even offset or exceed the effect of precipitation rate. Our study indicates that SD-AOM has an effect on the Mg isotope fractionation during carbonate precipitation. Similar effects may have occurred in geological periods with more widespread carbonate authigenesis. A better understanding of the factors affecting the Mg isotopic fractionation during marine carbonate authigenesis is needed to efficiently use this archive of paleoenvironments and element cycling.

The lacustrine black shales in the Chang7 Member from the Upper Triassic Yanchang Formation of the Ordos Basin in Central China are considered one of the most important hydrocarbon source rocks. However, the mechanism of organic accumulation in the black shales remains controversial. To resolve the controversy, with the former paleontological data of Yanchang Formation and sedimentation rate data of the Chang7 black shales, we investigated the typical intervals of the Chang7 black shales (TICBS) which were obtained by drilling in Yaowan at the southern margin of the Ordos Basin and performed various sedimentary, isotopic and geochemical analysis, including the sedimentary petrography, pyrite morphology, total organic carbon (TOC) and total sulfur (TS), the ratio of pyritic Fe to total Fe (DOPT), major and trace elements, together with pyritic sulfur isotopes (δ34Spy). The high sulfur content, enrichment of redox-sensitive trace metals, and the lower sedimentation rate of the TICBS in addition to the presence of marine spined acritarchs and coelacanth fossils indicate that the TICBS were deposited in a lacustrine environment possibly influenced by seawater. The petrographic observations show a thick layer of black shale with interlayers of thin layered siltstone (silty mudstone) and laminated tuff, which were related to the turbidity currents and volcanism, respectively. The U/Th, C-S, and Mo-U covariations, pyrite morphology, DOPT, combined with the δ34Spy, suggest that the deposition occurred beneath the anoxic-sulfidic bottom waters, which was intermittently influenced by the oxygen-containing turbidity. The Ni/Al and Cu/Al possibly show extremely high to high primary productivity in the water column, which might be connected with the substantial nutrients input from seawater or frequently erupted volcanic ash entering the lake. In addition, the coincidence of an increased abundance of TOC with increased P/Al, Ni/Al, Cu/Al and U/Th, as well as relatively consistent Ti/Al suggest that the accumulation of the organic matter might be irrelevant to the clastic influx, and was mainly controlled by the high primary productivity and anoxic-sulfidic conditions. Further, the covariations of TOC vs. P/Al and TOC vs. Ba/Al indicate that the high primary productivity led to the elevated accumulation and burial of organic matter, while the anoxic to sulfidic conditions were likely resulted from an intense degradation of the organic matter during the early diagenesis. In summary, the organic matter accumulation is ultimately attributed to the high primary productivity possibly resulted from seawater or volcanic ash entering the lake.

Authigenic carbonates recovered from two newly discovered active cold seeps on the northwestern slope of the South China Sea have been studied using petrography, mineralogy, stable carbon and oxygen isotopic, as well as trace element compositions, together with AMS 14C ages of shells of seep-dwelling bivalves to unravel fluid sources, formation conditions, and seepage dynamics. The two seeps (ROV1 and ROV2), referred to as ‘Haima seeps’ herein, are approximately 7 kilometers apart, and are typified by abundant carbonate rocks represented bycrusts and nodules. Aragonite and high-Mg calcite are the main carbonate minerals. Based on low δ13Ccarbonate values ranging from −43.0‰ to −27.5‰ (V-PDB) methane is apparently the predominant carbon source of seep carbonates. The corresponding δ18O values, varying from 2.5‰ to 5.8‰ (V-PDB), mostly are higher than calculated values representing precipitation in equilibrium with seawater (2.5‰ to 3.8‰), which probably reflects past destabilization of locally abundant gas hydrates. In addition, we found that carbonates with bivalve shells are generally aragonite-dominated, and bear no barium enrichment but uranium enrichments, reflecting shallow formation depths close to the seafloor. In contrast, carbonate crusts without bivalve shells and nodules contain more calcite, and are characterized by major molybdenum enrichment and different degrees of barium enrichment, agreeing with precipitation at greater depth under strictly anoxic conditions. AMS 14C ages suggest that a major episode of carbonate precipitation occurred between 6.1 ka and 5.1 ka BP at the Haima seeps, followed by a possibly subordinate episode from approximately 3.9 ka to 2.9 ka BP. The common occurrence of dead bivalves at both sites indicates that chemosynthesis-based communities flourished to a greater extent in the past, probably reflecting a decline of seepage activity in recent times. Overall, these results confirm that authigenic carbonates from gas hydrate-bearing areas can provide insight into long-term seepage dynamics and the genesis and fate of marine gas hydrate reservoirs.

Natural hydrocarbon seeps in the marine environment are important contributors to greenhouse gases in the atmosphere. Such gases include methane, which plays a significant role in global carbon cycling and climate change. To accurately quantify the methane flux from hydrocarbon seeps on the seafloor, a specialized in situ and online gas flux measuring (GFM) device was designed to obtain high-resolution time course gas fluxes using the process of equal volume exchange. The device consists of a 1.0-m diameter, 0.9-m tall, inverted conical tent and a GFM instrument that contains a solenoid valve, level transducer, and gas collection chamber. Rising gas bubbles from seeps were measured by laboratory-calibrated GFM instruments attached to the top of the tent. According to the experimental data, the optimal anti-shake time interval was 5 s. The measurement range of the device was 0–15 L min−1, and the relative error was ± 1.0%. The device was initially deployed at an active seep site in the Lingtou Promontory seep field in South China Sea. The amount of gas released from a single gas vent was 30.5 m3 during the measurement period, and the gas flow rate ranged from 22 to 72 L h−1, depending on tidal period, and was strongly negatively correlated with water depth. The measurement results strongly suggest that oceanic tides and swells had a significant forcing effect on gas flux. Low flow rates were associated with high tides and vice versa. The changes in gas volume escaping from the seafloor seeps could be attributed to the hydrostatic pressure induced by water depth. Our findings suggest that in the marine environment, especially in the shallow shelf area, sea level variation may play an important role in controlling methane release into the ocean. Such releases probably also affect atmospheric methane levels.

The sediments in gravity core DH-1, collected from Shenhu area on the northern slope of the South China Sea, are composed of two dark grey clay silt layers (A1 and A3) and two foraminifera-rich layers (A2 and A4) (A1 is the top layer and A4 is the bottom layer). Major and trace metal contents, along with grain size and mineralogic data suggest that these silt layers represent background hemipelagic sedimentation. Unlike silt layers, the intervening foraminifera-rich layerA4 exhibits a fining-upward trend, is distal turbidite. Grain size CM diagram and the sharp contacts between the foraminifera-rich layer A2 and the upper and lower layers suggest layer A2 is the result of depositional interaction between turbidite and bottom currents. AMS 14C dating of planktonic foraminifera indicate the age of the turbidite in A2 is between15.3 ka and 22.6 ka. We hypothesize that turbidite formation in this period was related to the increase supply of terrigenous matter since the Last Glacial Maximum. The rapid deposition of each turbidite led to reduced rates of oxygen diffusion into the sediment, allowing anoxic conditions develop in sediments and fostering U enrichment (>3.1 μg/g) in foraminifera-rich layer A4b. In contrast, decrease concentration of U and U/Mo ratios in foraminifera-rich layerA2, which is possibly due to U remobilization as a consequence of downward penetration of the oxidation front from the oxygen rich bottom currents. These new data suggest that authigenic U was quite sensitive to the inflow of bottom currents, and U/Mo ratios may indicate the depositional environmental change in relation to the bottom current action on the turbidite.

Sediments at marine methane seep sites provide potential archives of past fluid flow that serve to explore seepage activities over time. Three gravity cores (D-8, D-F, and D-7) were collected from seep sites on the northern slope of the South China Sea where gas hydrates were drilled in the subsurface. Various carbon and sulfur contents, δ13C values of total inorganic carbon (δ13CTIC), δ34S values of chromium reducible sulfur (δ34SCRS), trace element contents, grain size, and AMS 14C dating of planktonic Foraminifera in the sediments were determined to explore the availability of related proxies at seeps and to trace past methane seepage activities. Evidence for the presence of methane seepage and consequently anaerobic oxidation of methane comes from the occurrence of 13C-depleted authigenic carbonate nodules (δ13C values as low as −49‰) discovered at an interval of 150–200 cm in core D-7. This finding is supported by high S/C ratios and molybdenum enrichment in the same interval. However, low contents of CRS and negative δ34SCRS values are present. It is suggested to reflect a transient methane seepage event, which continued for about 1 ka based on the 14C ages. Cores D-8 and D-F have δ13CTIC values close to zero, low S/C ratios and CRS contents, negative δ34SCRS values, and no trace element enrichment, suggesting a negligible impact of methane-seepage on the sediments. The negative δ34SCRS values of the studied seep-impacted and background sediments suggest that the application of δ34SCRS alone as a proxy to identify AOM-related process may be insufficient. Sediment carbon-sulfur-trace element systematics and 14C ages used here have the potential to be a promising tool to recognize transient methane seepages and constrain their timescales.

It is generally accepted that organic matter content and accumulation rate at the seafloor attenuate with increasing water depth. However, hadal trenches, which represent the deepest portion of the hydrosphere on Earth, do not abide by this universal rule. It has been speculated that hadal trenches would serve as organic matter depocenters, where bacteria-mediated organic matter degradation intensifies. Here we examine the contents of total organic carbon (TOC) and total nitrogen (TN), as well as the δ13C values of TOC in three gravity cores, two box cores, and three grab samples with water depth from 4900 to 7118 m, in order to reveal the provenances of organic matter, spatial distribution and accumulation rates of particulate organic carbon (POC) in the southern Mariana Trench rim and slope. Although the deepest area was not sampled, trench rim and slope is also of importance in terms of transportation and accumulation processes of organic matter. A vast majority of the sediment samples have bulk TOC/TN molar ratios and δ13C values of TOC ranging from 4.2 to 11 and from − 21.8 to − 18.9‰, respectively, implying that the organic matter was primarily sourced from marine algae. Two exceptions have been found at 101 cm and 201 cm depths of core GC05 with a possible input of terrestrial material suggested by an abrupt increase in TOC contents and TOC/TN molar ratios accompanied by marked decreases in δ13C values of TOC. Moreover, TOC contents in surface sediments basically increased with water depth. Based on the published excess 210Pb data and by assuming a simplified one-dimensional sediment loading despite of frequent occurrences of mass wasting transports in hadal trenches, the sedimentation rate in the southern Challenger Deep (6037 m) was estimated to be 0.02 cm yr− 1. This much higher sedimentation rate compared to the globally averaged values in deep ocean sediments coupled with the trend of increasing in TOC contents with water depth may serve as evidence for the lateral transport of sediment particles induced by the funnel-like topography and localized current dynamics within the trench. The average POC accumulation rate within the southern Challenger Deep roughly amounts to 1.5 × 10− 5 g cm− 2 yr− 1, equivalent to about a seventh of the globally averaged POC accumulation rate in deep-ocean seafloor. To our knowledge, POC accumulation in hadal trenches has been quantified for the first time, and we highlight that organic matter degradation in deep hadal trenches should not be neglected and may represent a significant component of the global carbon cycle.

The timing of the establishment of the Yangtze River, whether prior to the early Miocene (~24 Ma) or more recently (~2 Ma), has been a point of much debate. Here we applied detrital zircon U-Pb dating to Miocene sedimentary rocks from Taiwan and to estuary sands from modern rivers in SE China to trace sediment provenance and to further constrain the evolution of the Yangtze River. Detrital zircon U-Pb ages from Miocene sandstones of the Western Foothills show similar age spectra to Miocene and modern sediments in the Yangtze River drainage and some similarity to the Minjiang River sediments. However, they differ significantly from ages in some sandstones from the Hengchun Peninsula accretionary prism and from the estuary sands of the Jiulongjiang River. This information, together with petrographic and sedimentary facies analysis, argues that the Jiulongjiang and Minjiang Rivers were major sources to some Hengchun Peninsula turbidites (~12 Ma), while synchronous sedimentation in the Western Foothills was supplied from the Yangtze, Minjiang (or similar river), and possibly even the Yellow River. These sediments were transported southward/eastward via rivers or channels to the marginal sedimentary basins now inverted in the Western Foothills in Taiwan. The Yangtze River must have been established prior to the middle Miocene.

Seep carbonates provide excellent records of past seepage activities, and have been commonly considered to preserve primary, unaltered stable carbon isotope signatures. However, late diagenetic reactions may overprint original isotopic compositions, but the mode and effect of such alterations are poorly understood. In particular, there are significant uncertainties regarding how carbon and strontium isotopic compositions of seep carbonates respond to diagenesis. This study reports recently discovered Cretaceous hydrocarbon-seep deposits from the Yarlung-Zangbo Suture Zone, Tibet, China that have experienced substantial diagenetic alteration that is shown by recrystallization and secondary veins. Unitary linear recursive analysis was applied to δ 13 C values and 87 Sr/ 86 Sr ratios of the seep carbonates to evaluate the degree of secondary modification and to quantitatively constrain the compositions of primary carbonates and late diagenetic fluids. The δ 18 O values range from − 11.8‰ to − 2.2‰, δ 13 C values from − 34.1‰ to − 12.9‰ and 87 Sr/ 86 Sr ratios from 0.706221 to 0.706808. The heterogeneity in isotopic compositions and the observation that the most negative δ 18 O values occur in samples with the most extensive recrystallization indicate significant and spatially heterogeneous modification of isotope compositions during late diagenesis. The linear correlations between δ 13 C values and δ 18 O values for matrix micrites (R 2 = 0.54), and between bulk carbonate 87 Sr/ 86 Sr ratios and δ 18 O values (R 2 = 0.85) are best explained by burial diagenetic overprinting of oxygen, strontium, and even carbon isotopic compositions rather than by meteoric water hypergenesis. Extrapolated values of δ 13 C and ratios of 87 Sr/ 86 Sr against a δ 18 O value of − 2‰ (average value of calcite precipitated in isotopic equilibrium with coeval Cretaceous seawater) that would characterize the primary carbonate, give an end member δ 13 C value of − 34‰ and an end member 87 Sr/ 86 Sr ratio of 0.7072. The end member isotopic values obtained by this extrapolation suggest that the primary seep carbonates with low δ 13 C values and high 87 Sr/ 86 Sr ratios were formed by anaerobic oxidation of methane near the seafloor. In contrast, the measured δ 18 O values and 87 Sr/ 86 Sr ratios reflect late diagenetic fluids represented by burial pore water characterized by a low 87 Sr/ 86 Sr ratio and high temperature. Our findings reveal that δ 13 C values can only be moderately and 87 Sr/ 86 Sr ratios can be significantly altered during late diagenesis, and show that it is possible to quantitatively assess the primary composition of diagenetically altered seep carbonates.

Takahe seep, located on the Opouawe Bank, Hikurangi Margin, is characterized by a well-defined subsurface seismic chimney structure ca. 80,500 m2 in area. Sub-seafloor geophysical data based on acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimney structure. Reaction-transport modeling was applied to porewater data from 11 gravity cores to constrain methane turnover rates and benthic methane fluxes in the upper 10 m. Model results show that methane dynamics were highly variable due to transport and dissolution of ascending gas. The dissolution of gas (up to 3761 mmol m−2 yr−1) dwarfed the rate of methanogenesis within the simulated sediment column (2.6 mmol m−2 yr−1). Dissolved methane is mainly consumed by anaerobic oxidation of methane (AOM) at the base of the sulfate reduction zone and trapped by methane hydrate formation below it, with maximum rates in the central part of the chimney (946 and 2420 mmol m−2 yr−1, respectively). A seep-wide methane budget was constrained by combining the biogeochemical model results with geophysical data and led to estimates of AOM rates, gas hydrate formation and benthic dissolved methane fluxes of 3.68 × 104 mol yr−1, 73.85 × 104 mol yr−1and 1.19 × 104 mol yr−1, respectively. A much larger flux of methane probably escapes in gaseous form through focused bubble vents. The approach of linking geochemical model results with spatial geophysical data put forward here can be applied elsewhere to improve benthic methane turnover rates from limited single spot measurements to larger spatial scales.

Natural hydrocarbon seeps in a marine environment are one of the important contributors to greenhouse gases in the atmosphere, including methane, which is significant to the global carbon cycling and climate change. Four hydrocarbon seep areas, the Lingtou Promontory, the Yinggehai Rivulet mouth, the Yazhou Bay and the Nanshan Promontory, occurring in the Yinggehai Basin delineate a near-shore gas bubble zone. The gas composition and geochemistry of venting bubbles and the spatial distribution of hydrocarbon seeps are surveyed on the near-shore Lingtou Promontory. The gas composition of the venting bubbles is mainly composed of CO2, CH4, N2 and O2, with minor amounts of non-methane hydrocarbons. The difference in the bubbles’ composition is a possible consequence of gas exchange during bubble ascent. The seepage gases from the seafloor are characterized by a high CO2 content (67.35%) and relatively positive δ 13CV-PDB values (–0.49×10–3–0.86×10–3), indicating that the CO2 is of inorganic origin. The relatively low CH4 content (23%) and their negative δ 13CV-PDB values (–34.43×10–3—–37.53×10–3) and high ratios of C1 content to C1–5 one (0.98–0.99) as well point to thermogenic gases. The hydrocarbon seeps on the 3.5 Hz sub-bottom profile display a linear arrangement and are sub-parallel to the No. 1 fault, suggesting that the hydrocarbon seeps may be associated with fracture activity or weak zones and that the seepage gases migrate laterally from the central depression of the Yinggehai Basin.

Release of methane from the seafloor throughout the world's oceans and the biogeochemical processes involved may have significant effects on the marine sedimentary environment. Identification of such methane release events in marine sediment records can hence provide a window into the magnitude of ancient seeps. Here, we report on analysis of the geochemical composition of samples in a 12.3 m long sediment core (DH-5) collected from a seep site in the South China Sea (SCS). Our aim has been to investigate whether the evidence for the presence of methane release event within sediments is discernible from solid-phase sediment geochemistry. We show that sedimentary total sulfur (TS), δ 34 S values of chromium reducible sulfur (δ 34 S CRS ) along with total organic carbon (TOC) and total inorganic carbon (TIC) content can be used to infer the presence of methane release events in cold seep settings. At least three methane release events were identified in the studied core (Unit I at 400–550 cm, Unit II at 740–820 cm, and Unit III at 1000–1150 cm). According to the characteristic of redox-sensitive elements (eg., Mo, U and Mn), we suggest that methane flux has been changed from relatively high (Unit I) to low (Unit II and III) rates. This inference is supported by the coupled occurrence of 34 S-enriched sulfides in Unit II and III. AMS 14 C dates from planktonic foraminifera in Unit I suggest that high methane flux event occurred at ∼15.4–24.8 kyr BP, which probably resulted in locally-focused aerobic methane oxidation. Overall, our results suggest that TS, TOC, TIC and δ 34 S CRS have potential for identifying present and fossil methane release events in marine sediments.

Sulfate-driven anaerobic oxidation of methane (SD-AOM) supports chemosynthesis-based communities and limits the release of methane from marine sediments. Formation of authigenic carbonates at active methane seeps is promoted by SD-AOM stoichiometry. While distinctively small δ 18 O/δ 34 S slopes of pore fluid sulfate have been shown to typify modern methane-rich environments, identification of such environments has been difficult for the geological past due to the lack of sedimentary pore fluids. However, if the isotopic composition of sulfate were archived in authigenic carbonate during early diagenesis, carbonate-associated sulfate (CAS) should display the characteristic δ 18 O-δ 34 S pattern. To test this hypothesis, we investigated the δ 18 O CAS , δ 34 S CAS , and 87 Sr/ 86 Sr signatures of authigenic carbonate minerals from three modern and two ancient methane-seep provinces. The data obtained demonstrate that all deposits regardless of age or location display consistently small δ 18 O CAS /δ 34 S CAS slopes (∼0.3) and CAS does not represent ambient seawater but pore-water sulfate. This finding confirms the utility of CAS as a recorder of SD-AOM in methane-rich environments. In addition, we report that aragonites bear higher CAS contents, 87 Sr/ 86 Sr ratios closer to that of contemporary seawater, and a larger δ 18 O CAS /δ 34 S CAS slope than calcites, reflecting the shallower formation depth of aragonite where pore-water has a composition close to that of seawater with high concentrations of sulfate. The new proxy can be used to constrain the record of SD-AOM through most of Earth history by measuring the δ 18 O and δ 34 S values of CAS of methane-derived diagenetic carbonates including but not limited to seep carbonates.

Trace elements are actively cycled in seawater, are essential components for marine life, and are transported to the seafloor where they can become enriched in marine sediments. Trace element enrichment in marine sediments depends on local hydrography, redox conditions, and biological activity. Most redox-sensitive trace elements partake in biological cycling in seawater and sedimentary pore waters, and their distribution in the rock record can help to constrain the dynamics and changes in environmental conditions on geologic timescales. Carbonate minerals including calcite, aragonite, and dolomite readily adsorb and/or incorporate trace elements during precipitation. Although experimental laboratory studies concerning incorporation of trace elements into carbonate minerals have not always yielded unambiguous results, they have generally confirmed a strong tendency of carbonate minerals to accumulate and retain certain trace elements. Because natural carbonate minerals can archive trace elements, they have attracted attention as sources of information on biogeochemical processes. Nonetheless, drawbacks of using trace element contents of marine carbonates as paleoproxies exist due to mineral alteration during early diagenesis, subsequent burial diagenesis, and meteoric diagenesis. These processes can affect trace element contents in marine carbonates to different degrees, possibly leading to alteration or even a complete loss of their original trace element composition. This review summarizes current knowledge on the behavior of trace elements in seawater and interstitial pore water by integrating trace element cycling in the water column, their transport to the seafloor, and their subsequent incorporation into carbonate minerals. This work provides a comprehensive overview for the carbonate sedimentologist, detailing the water column and early diagenetic processes that affect trace element patterns of unaltered or poorly altered carbonate rocks used as archives of paleoenvironmental change.

Authigenic low-magnesium calcite (LMC)&mdash, a mineral phase that should precipitate in calcite seas rather than today&rsquo, s aragonite sea&mdash, was recently discovered at the seafloor of the Gulf of Mexico (GoM) at water depths of 65 m (site SS296) and 189 m (site GC53). This study investigates the mineralogical, petrographic, and geochemical characteristics of LMC from both sites to reveal its formation process. The &delta, 18O values of LMC from site SS296 cluster in two groups (&minus, 0.6&permil, to 1.7&permil, 6.3&permil, to 7.5&permil, ) and the presence of cone-in-cone texture in the samples with lower &delta, 18O values suggest precipitation at higher temperatures and greater depth. Low &delta, 18O values of LMC from site GC53 ranging from &minus, 9.4&permil, to &minus, 2.5&permil, indicate an influence of meteoric waters during formation. LMC at both sites reveals a wide range of &delta, 13C values (&minus, 17.4&permil, to 2.6&permil, ), indicating various carbon sources including seawater and/or organic matter. This interpretation is further supported by the &delta, 13C values of organic carbon extracted from the LMC lithologies (&delta, 13Corg: from &minus, 26.8&permil, 18.9&permil, ). Relatively low Sr concentrations of LMC samples regardless of variable 87Sr/86Sr ratios, ranging from 0.707900 to 0.708498 for site GC53 and from 0.709537 to 0.710537 for site SS396, suggest the exchange of Sr between pore fluids and ambient sediments/rocks. The observed wide range of 87Sr/86Sr ratios and the enrichment of Fe and Mn in LMC is in accordance with pore fluids deriving from the dissolution of Louann salt. Overall, this study reveals that the formation of LMC at sites SS296 and GC53 was favored by the presence of low Mg/Ca ratio pore fluids resulting from salt dissolution in subsurface environments when sufficient dissolved inorganic carbon was available. These results are essential for understanding the formation of marine LMC at times of an aragonite sea, highlighting the role of formation environments&mdash, open environments close to or at the seafloor vs. confined subseafloor environments typified by pore waters with a composition largely different from that of seawater.

Cold seeps frequently occur at the seafloor along continental margins. The dominant biogeochemical processes at cold seeps are the combined anaerobic oxidation of methane and sulfate reduction, which can significantly impact the global carbon and sulfur cycles. The circulation of methane-rich fluids at margins is highly variable in time and space, and assessing past seepage activity requires the use of specific geochemical markers. In this study, we report multiple sedimentary proxy records for three piston gravity cores (QDN-14A, QDN-14B, and QDN-31) from the Haima seep of the South China Sea (SCS). By combining total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), total sulfur (TS), acid insoluble carbon and sulfur isotope (δ13Corganic carbon and δ34Sacid-insoluble), and δ34S values of chromium reducibility sulfur (δ34SCRS), as well as carbon isotopes of TIC (δ13CTIC) in sediments, our aim was to provide constraints on methane seepage dynamics in this area. We identified three sediment layers at about 260–300 cm, 380–420 cm and 480–520 cm sediment depth, characterized by particular anomalies of low δ13CTIC values and high TS content, high TS and CRS contents, and high δ34Sacid-insoluble and δ34SCRS values, respectively. On this basis, we propose that these sediment horizons correspond to distinct methane release events preserved in the sediment record. While the exact mechanisms accounting for the presence (or absence) of these particular geochemical signals in the sediment are not known, we propose that they correspond to variations in methane flux and their duration through time. Overall, our results suggest that sedimentary carbon and sulfur and their isotopes are useful tracers for better understanding of methane seepage dynamics over time.

Methane of various origins is widespread in the sediments of continental margins, representing a key component of the carbon cycle in the ocean. Its occurrence in shallow marine sediments can be linked to the gas hydrate reservoir in the deep sedimentary column. However, this type of methane is difficult to track due to complex reactions occurred during its long distance migration. Porewater geochemical analyses of two ∼7 m piston cores (D17-15 and W01-16) combined with a reaction-transport model were applied to quantify methane-related biogeochemical processes and to explore the linkage of shallow methane to gas hydrate reservoir in the hydrate drilling areas of Shenhu and Dongsha in the South China Sea (SCS). The model results revealed that anaerobic oxidation of methane (AOM) is the predominant biogeochemical process in both cores, consuming approximately 85% and 74% of sulfate in the modeled domain, respectively. The crossplot of dissolve inorganic carbon (DIC) accumulation versus sulfate depletion of the system suggests that methane consumed during the AOM is mainly originated from external methane sources rather than local methanogenesis. Using a δ13CDIC mass balance calculation together with model-derived fractions of different DIC sources, we estimated that the δ13C value of the external methane in core D17-15 is −66.6‰, which is in accordance with the previous reported δ13C values of hydrate-bound methane in the area. These results suggest that methane consumed in shallow sediments in the hydrate drilling areas of the SCS is mainly originated from local gas hydrate reservoir. The proposed approach has the potential to be used to differentiate the sources of methane, which will provide constraints on dynamics of methane in gas hydrate-bearing marine sediments.

Seepage of methane and oil is widespread at active and passive continental margins worldwide. Large-scale methane release may have caused bottom water oxygen depletion and acidification in the geological past, but direct evidence of low-oxygen conditions caused by methane leakage in natural systems is scarce. Here we report on the geochemical composition of sediments of two deepwater seep sites (>1700 m) from the South China Sea. The δ34S values of chromium reducible sulfur (δ34SCRS), δ13C values of total inorganic carbon (δ13CTIC), total sulfur (TS) and total organic carbon (TOC) contents, as well as redox-sensitive elements (Fe, U, and Mn) are used to constrain the intensity of fluid seepage and its impact on bottom water redox conditions. Sediment horizons affected by methane seepage are characterized by relatively high TS/TOC ratios (>1.4), low δ13CTIC values ( 22‰). The identified horizons show elevated authigenic U contents, high U/Th, as well as low Mn/Al ratios, suggesting oxygen depletion during their deposition. It is shown that local oxygen concentration of bottom waters was related to methane flux, with periods of larger methane release initiating during the Last Glacial Maximum. Much oxygen was apparently consumed during times of intense methane seepage, resulting in low oxygen conditions of bottom waters – conditions that persisted for ten thousand years and more. Our observations suggest that the interplay of flow intensity and microbial processes at seeps can locally affect the oxygen concentration in the deep ocean. The impact of microbial methane consumption on the oxygen concentration in the deep ocean should consequently be considered when looking into potential effects of future warming of the ocean, as more gas hydrate is susceptible to destabilization during ocean warming.

Marine sediments represent the largest reservoir of methane on Earth. Anaerobic oxidation of methane (AOM) mediated by anaerobic methanotrophic archaea (ANME) and associated sulfate-reducing bacteria is the main process controlling methane emission from marine sediments. Recent laboratory studies have revealed that ANME or AOM consortia can mediate nitrogen (N) fixation and ammonium assimilation. These processes are known to preferentially utilize 14N rather than the 15N isotope. However, geochemical fingerprints of microbial nitrogen uptake in marine sediments are lacking, limiting our understanding of its role in modern and ancient sedimentary environments. Here we report prominent negative excursions of bulk sediment δ15N values (up to −5‰) at sulfate-methane transitions (SMTs) where ANME show peak abundances in sediments at four sites of the Haima seeps of the South China Sea. Such δ15N excursions are restricted to SMTs, suggesting that 15N depletion results from ANME-mediated nitrogen uptake. We posit that this process is ubiquitous at the SMT – a reaction front in coastal and continental margin sediments worldwide – resulting in similar isotope patterns. Negative δ15N excursions are consequently a promising proxy to track AOM in the geological record and to better constrain carbon and nitrogen cycling in ancient sedimentary environments.

Sulfate-driven anaerobic oxidation of methane (SD-AOM) controls methane release from marine sediments to the ocean. A variety of authigenic precipitates within sediments has been widely used to identify past occurrences of SD-AOM. However, the lack of a systematic evaluation of the formation conditions of these precipitates sometimes impedes the recognition of SD-AOM in past environments characterized by diffusive transport of methane. Carbon, sulfur, and trace element geochemistry of pore-water and sediments was investigated at a site affected by upward methane diffusion in the Shenhu area of the South China Sea. Here, the sulfate-methane transition zone (SMTZ) is located ~7.6 m below the seafloor based on sulfate and methane concentrations. The slope of δ34S vs. δ18O values of sulfate is consistent with diffusive transport of methane. Concentration and isotope profiles of pore-water species point to diffusive rather than advective transport of solutes. Enhanced sulfate reduction inferred from δ34S values of sulfate agrees with the local occurrence of relatively abundant, 34S-enriched iron sulfide minerals. The time required to produce the observed authigenic iron sulfides around the SMTZ is estimated to be ~1.1 ka based on the amount of iron sulfide minerals and the present diffusive flux of sulfate. No enrichment of authigenic carbonate, barium (Ba), and molybdenum (Mo) was detected in the studied sediments. This observation is consistent with the calculation that the current fluxes of pore-water calcium, magnesium, Ba, and Mo are too low to allow for authigenic enrichments. The low fluxes are largely controlled by diffusion, which facilitates the formation of 34S-enriched iron sulfide minerals. The observed enrichment patterns – unlike those of sediments affected by advective seepage – are expected to be prevalent in modern and ancient continental-margin sediments, and may contribute to the identification of past methane-rich zones and overlying SD-AOM zones as sinks for methane in the geological record.