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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 14 C 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 δ 13 C carbonate values ranging from −43.0‰ to −27.5‰ (V-PDB) methane is apparently the predominant carbon source of seep carbonates. The corresponding δ 18 O 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 14 C 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. [ABSTRACT FROM AUTHOR]

Integrated investigations have revealed abundant resources of gas hydrates on the northern slope of the South China Sea (SCS). Regarding the gas hydrate research of northern SCS, the gas hydrate related environment problem such as seabed landslides were also concentrated on in those areas. Based on 2D seismic data and sub-bottom profiles of the gas hydrate areas, submarine landslides in the areas of Qiongdongnan, Xisha, Shenhu, and Dongsha have been identified, characterized, and interpreted, and the geophysical characteristics of the northern SCS region investigated comprehensively. The results show 6 major landslides in the gas hydrate zone of the northern SCS and 24 landslides in the Shenhu and Dongsha slope areas of the northern SCS. The landslide zones are located mainly at water depths of 200–3000 m, and they occur on the sides of valleys on the slope, on the flanks of volcanoes, and on the uplifted steep slopes above magmatic intrusions. All landslides extend laterally towards the NE or NEE and show a close relationship to the ancient coastline and the steep terrain of the seabed. We speculate that the distribution and development of submarine landslides in this area has a close relationship with the tectonic setting and sedimentary filling characteristics of the slopes where they are located. Seismic activity is the important factor controlling the submarine landslide in Dongsha area, but the important factor controlling the submarine landslides in Shenhu area is the decomposition of natural gas hydrates. [ABSTRACT FROM AUTHOR]

Fluid flow patterns at cold seeps provide insights into the mechanism and influence of methane emission into the ocean, which is critical in its environmental impact assessment. Here, we report pore fluid compositions of three ∼8 m long piston cores (QDN-14A, QDN-14B and R1) collected from the newly-discovered active Haima cold seeps on the northwestern slope of the South China Sea. Reaction-transport models were further applied to quantify related biogeochemical processes and to reveal the patterns of fluid flow. Extremely low δ13C values (<−52‰) of dissolved inorganic carbon (DIC) near the sulfate-methane transition in the three cores suggest that anaerobic oxidation of methane is the predominant biogeochemical process. The presence of small pieces of gas hydrates along with negative anomalies of porewater chloride and sodium concentrations reflects gas hydrate dissociation. Nearly invariable concentrations of sulfate, DIC, and calcium on a meter-scale were observed in the uppermost part of the sediment cores QDN-14A and QDN-14B. This irrigation-like feature is inferred to result from enhanced methane flux in QDN-14A and QDN-14B. We infer that lateral migration of methane-rich fluids from R1 site to QDN-14A and QDN-14B sites together with upward migrated methane is responsible for the enhanced methane flux. This speculation is supported by the occurrence of gas hydrates which might have clogged the fluid channel in the seepage center (R1) and driven the transportation of methane-bearing fluid along a coarser sediment layer in surrounding sediments (QDN-14A and QDN-14B). The proposed scenario is further demonstrated using a non-steady-state modeling that reconstructed the porewater irrigation-like feature assuming an increased methane flux. The modeling result predicts that gas-hydrate formation in core R1 started at least 150 yr B.P. The proposed fluid flow pattern within a localized seep site may have a great implication for understanding the heterogeneity of sedimentary records. • Pore fluid compositions at newly-discovered active cold seeps were reported in the SCS. • Numerical simulations were used to reveal fluid flow patterns at Haima cold seeps. • The lateral migration of fluid might prevail at the seeping area of Haima cold seeps. • The lateral migration of fluid is caused by the sealing of gas hydrates in the seeping center. [ABSTRACT FROM AUTHOR]

The sources and sinks of dissolved CH4 in offshore waters are becoming diversified with the rapid increase in human activities. The concentration and air–sea exchange flux of dissolved CH4 present new characteristics of more intense spatiotemporal evolution, and the contribution to atmospheric CH4 continues to increase. Herein, a new model based on navigable air–sea exchange flux observations was proposed, which replaced the traditional station-based sampling analysis and testing method, realizing the synchronous measurement of methane in the atmosphere and surface seawater carried by ships. Based on the Marine Geological Survey project of the China Geological Survey, comprehensive environmental surveys were conducted in April 2018, September 2018, and June 2019 in the Qiongdongnan area in the northern part of the South China Sea, and the dissolved methane content in the sea surface atmosphere and surface seawaters in 2019 were simultaneously obtained. The methane exchange flux ranges of the southeastern sea area were calculated as −0.001~−0.0023 μmol·m−2·d−1 and −0.00164~−0.00395 μmol·m−2·d−1 by using the Liss and Merlivat formula (LM86), the Wanninkhof formula (W92), and the field-measured wind speed. The feasibility of the navigational air–sea methane exchange flux observation system was proven in a sea trial, and the measurement accuracy and observation efficiency of air-sea flux were improved with the designed system, providing a new technical means for further research on multiscale air–sea interactions and global climate change. [ABSTRACT FROM AUTHOR]

Understanding the controls on the sulfur isotope compositions of syndepositional pyrite (δ34S py) during early diagenesis is critical for environmental reconstructions, especially for the reconstruction of the marine sulfur cycle. Commonly, δ34S py values increase with depth in marine sediments; this phenomenon has been extensively studied, fostering the reconstruction of both global and regional depositional environments. However, more recently a decrease of δ34S py values with depth has been increasingly observed, but the corresponding controls on sulfur stable isotopes remain poorly constrained. This study investigated a gravity core collected from the northern continental slope of the South China Sea. Morphology, content, and δ34S values of pyrite, concentration and sulfur isotopic composition of porewater sulfate (δ34S SO4), carbon, nitrogen, and sulfur contents, and grain sizes of terrigenous sediments were analyzed and AMS14C dating of planktonic foraminifera was conducted. The δ34S py values were found to decrease with depth from −16.8‰ to −42.8‰, indicating low sulfur isotope fractionation (<38‰) during microbial sulfate reduction in the uppermost sediments and a larger fractionation (>64‰) at greater depth. Significant contributions of relatively 34S-enriched pyrite formed at shallow depth leads to overall δ34S py variations >26‰, whereby low δ34S py values coincide with higher sedimentation rates and higher organic matter supply. This study highlights the potential significance for relatively 34S-enriched, shallow-depth pyrite for δ34S py records, which may affect the reconstruction of depositional conditions and the global sulfur cycle. • Relatively 34S-enriched pyrite formed at shallow depth results from low sulfur isotope fractionation during MSR (<38‰). • Higher sedimentation rate and high organic matter supply led to lower δ34S py values. • Relatively 34S-enriched pyrite formed in uppermost sediments causes overall δ34S py variation >26‰. • δ34S py values provide new insight into the global sulfur cycle and past depositional processes. [ABSTRACT FROM AUTHOR]

Upward diffusion of dissolved methane and associated anaerobic oxidation of methane (AOM) coupled with microbial sulfate reduction (MSR) are widespread in normal hemipelagic sediments of continental margins. The occurrence of authigenic sulfur enrichment in such sediments is usually regarded as an indicator of AOM; however, extensive MSR and authigenic iron sulfide generation also occur in turbidites. Therefore, research attention should be directed toward determining the geochemical features of authigenic sulfur mineralization in methane-rich background sediments and turbidites. Here we report the sedimentology and geochemical composition of sediments from four piston cores (CL30, CL44, CL47, and CL3A) containing turbidite layers from methane-rich areas of the South China Sea. The turbidite layers were deposited at 17–15 kyr BP or earlier during the last glacial period and were located above the present sulfate–methane transition zone (SMTZ). The median grain size, proportion of sand-sized grains, and Si, Ti, K, and Zr contents show marked elevation in the turbidite layers compared with the background sediments, indicating the input of large amounts of terrigenous detrital matter with coarser grains. High total sulfur (TS) contents and low δ34S AIS values (AIS: acid-insoluble sulfur) (−35.2‰ to −26.1‰) are observed within turbidite layers relative to the background hemipelagic sediments in core CL44. These results suggest that intensive sulfidization occurred as a result of enhanced organoclastic sulfate reduction rates in turbidite horizons, which were probably caused by the resultant decrease in downward oxygen flux and waning bioturbation after rapid deposition of turbidites. In comparison, TS contents are higher and δ34S AIS values are positive (0.2‰–46.8‰) within the SMTZ on account of the rapid consumption of sulfate and build-up of isotopically heavier hydrogen sulfide pools via intensive AOM. Moreover, authigenic barium (Ba) enrichments commonly occur in the depth interval immediately above the SMTZ rather than within turbidite layers; therefore, they serve as a useful indicator of the position of the SMTZ. This study presents distinct differences in the geochemical compositions of turbidity deposits and methane-rich background sediments and highlights that a multi-proxy approach including sedimentological and geochemical analyses should be used to constrain the types of MSR in deep-sea sediments, given the common occurrence of turbidites and subsurface methane release in marginal deep-sea areas. It is also suggested that if applied with caution, S–C–Ba geochemical patterns can be used to identify palaeo-SMTZs and palaeo methane-rich zones in turbidite-containing strata. • Both authigenic sulfur enrichment occurred in turbidite and seep sediments in the SCS. • Sulfidization in turbidite layer may be characterized by OSR with high SRR. • Combined S, C, and Ba geochemical patterns can be used to identify turbidite and seeps. [ABSTRACT FROM AUTHOR]

The formation of authigenic phosphorus (P) minerals near the sulfate-methane transition (SMT) represents an important pathway of P burial in marine sediments. However, our current understanding of the coupling between the formation of authigenic phosphate and carbon and iron (Fe) cycling is insufficient. In this study, a piston core recovered from the Haima cold seeps of the Qiongdongnan basin, northern South China Sea, was used to analyze pore-water parameters and solid-phase Fe and P speciation. Results from a previous study indicate that the current SMT is located at approximately 1 m below the seafloor (mbsf) and a fossil SMT located at approximately 6.5 mbsf. Dissolved phosphate concentrations and solid-phase iron oxide (Fe Ox) contents are found to decrease with depth, while Fe-bound P (P Fe) and authigenic phosphate (P Auth) contents increase with depth in the zone between the current and fossil SMTs where gas hydrate is present, suggesting that vivianite and authigenic apatite formed in this zone. Gas hydrate below the current SMT will dissociate once methane is undersaturated, which could persistently provide dissolved methane to prevent the downward shift of the current SMT where methane is continually consumed. Such a methane-rich environment maintained by gas hydrate may favor the persistent reduction of Fe Ox and the formation of vivianite. Dissolved phosphate concentrations are high, but decrease rapidly with depth at the fossil SMT. Combined with higher Fe Ox and P Auth contents and lower P Fe contents and lower P Org /total organic carbon (TOC) ratios, these results suggest the formation of authigenic apatite at the fossil SMT. The high dissolved phosphate concentration and low P Org /TOC ratios at the fossil SMT are interpreted to reflect preferential release of P during the degradation of organic matter generated by methanotrophic microorganisms. Overall, our study shows that P speciation in methane-rich sediments provides insight into the formation processes of authigenic P minerals near the SMT and associated carbon and Fe cycling processes. • P and Fe cycling was investigated between the current and fossil sulfate-methane transition (SMT). • An apparent relationship was observed between the occurrence of gas hydrate and the formation of vivianite. • Iron-driven anaerobic oxidation of methane may be the key for a co-occurrence of gas hydrate and vivianite. • Preferential release of P during degradation of organic matter can occur at fossil SMTs. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Sediments from Haima cold seeps have variable compositions. • AOM caused high δ34S CRS and low δ13C TIC values, and high CRS contents. • AOM indicators of sediments are variable under variable methane flux and duration. • Three methane release events were identified in the seep site. Abstract 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 (δ13C organic carbon and δ34S acid-insoluble), and δ34S values of chromium reducibility sulfur (δ34S CRS), as well as carbon isotopes of TIC (δ13C TIC) 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 δ13C TIC values and high TS content, high TS and CRS contents, and high δ34S acid-insoluble and δ34S CRS 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. [ABSTRACT FROM AUTHOR]

Highlights • δ34S of pyrite in seep carbonates from Site F and Haima were investigated. • Mo, As, Sb contents and Sr/Ca, Mg/Ca ratios reveal variable redox conditions. • Pyrites in calcite have more positive δ34S values than aragonite at seeps. • The variable δ34S values are useful to assess the intensities of past seepages. Abstract 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 (δ34S py) 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 δ34S py 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 δ34S py 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 δ34S py , 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 δ34S py 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. [ABSTRACT FROM AUTHOR]

Highlights • Lipid biomarker patterns of active Haima seep carbonates were studied. • UCM in seep carbonate reflects the presence of crude oil at Haima seeps. • The predominant consortia in carbonates from both sites were ANME-1/DSS. • Methane seepage was at least intermittently more intense at site ROV2 than at ROV1. Abstract Authigenic carbonates retrieved from sites ROV1 and ROV2 of the Haima hydrocarbon seeps of the South China Sea at approximately 1390 m water depth were studied using lipid biomarker analyses. Abundant molecular fossils of anaerobic methane oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB) with strong 13C-depletions (δ13C values as low as −126‰), in combination with low δ13C carbonate values (−42.7‰ to −36.8‰), provide evidence that anaerobic oxidation of methane (AOM) was the major process driving the precipitation of the studied seep carbonates. The extremely low δ13C values of archaeal biomarkers confirm that biogenic methane was the main carbon source, but the seepage of accessory crude oil is also suggested by the presence of unresolved complex mixtures in the hydrocarbon fractions. A suite of 13C-depleted biomarkers indicate the predominance of ANME-1/ DSS consortia at both sites, which indicates that the studied carbonates formed during low to medium methane flux. Somewhat higher contents of archaeal biomarkers and their stronger 13C-depletion at site ROV2 probably reflect at least temporarily higher seepage intensities than at site ROV1. Abundant bacterial dialkyl glycerol diethers (DAGEs), revealing a large offset of their δ13C values compared to SRB-derived terminally branched fatty acids, were possibly produced by distinct SRB species other than members of the DSS cluster. The encountered hopanoids are attributed to aerobic methanotrophic bacteria based on their moderate 13C-depletion. The application of molecular fossils in combination with their compound-specific isotope signatures is an efficient tool to reconstruct the composition of seepage fluids and seepage intensities. [ABSTRACT FROM AUTHOR]

Highlights • Porewater geochemistry of two piston cores in hydrate drilling areas was investigated. • A numerical model was applied to quantify involved biogeochemical processes. • Methane consumed mainly originated from external sources rather than local methanogenesis. • The external methane is most likely to originate from local gas hydrate systems. Abstract 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 δ13C DIC 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. [ABSTRACT FROM AUTHOR]

Methane seepage activity caused by gas hydrate dissociation plays an essential role in the global carbon cycle and climate change in Earth's history. However, the triggering mechanisms of gas hydrate dissociation remain elusive because of the lack of a relatively complete history of methane seeps on long-term timescales. Here, we present major and trace elements, stable C–O isotopes, and U–Th dating analyses for cold seep carbonates from drill cores in the South China Sea. Our results reveal a 209,00-year-old history of methane seepage activity, which involves multi-stage changes in seepage intensity and methane source (microbial or thermogenic). Chronological data suggest that hydrostatic pressure reduction and dynamic depositional environment mainly controlled the gas hydrate dynamics in the northwestern South China Sea, which is different from the gas hydrate dissociation in the northeastern South China Sea controlled by bottom water warming. This discrepancy may be caused by the differences in physicochemical conditions (e.g., water depth) in different sea areas. Therefore, the findings of this study help to further clarify the relationship between oceanic hydrate reservoirs and environmental change in other world regions. • Seep carbonates from drill cores in the Qiongdongnan Basin were investigated. • Two methane seepage events were identified in the northwestern South China Sea. • Multi-stage changes in seepage intensity and methane source. • Seepage activity controlled by hydrostatic pressure and depositional environment. [ABSTRACT FROM AUTHOR]

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, δ 13 C values of total inorganic carbon (δ 13 C TIC ), δ 34 S values of chromium reducible sulfur (δ 34 S CRS ), trace element contents, grain size, and AMS 14 C 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 13 C-depleted authigenic carbonate nodules (δ 13 C 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 δ 34 S CRS values are present. It is suggested to reflect a transient methane seepage event, which continued for about 1 ka based on the 14 C ages. Cores D-8 and D-F have δ 13 C TIC values close to zero, low S/C ratios and CRS contents, negative δ 34 S CRS values, and no trace element enrichment, suggesting a negligible impact of methane-seepage on the sediments. The negative δ 34 S CRS values of the studied seep-impacted and background sediments suggest that the application of δ 34 S CRS alone as a proxy to identify AOM-related process may be insufficient. Sediment carbon-sulfur-trace element systematics and 14 C ages used here have the potential to be a promising tool to recognize transient methane seepages and constrain their timescales. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]