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Sulfate-driven anaerobic oxidation of methane (SD-AOM) plays a critical role in regulating the global methane budget. Determination of the diagnostic triple isotope exponent θ 33 ( ≡ ln 33 ⁡ α / ln 34 ⁡ α ) for SD-AOM can help to identify and quantify microbial sulfate reduction via SD-AOM in the environment. The history of Earth's surface redox conditions can also be examined through the measurement of triple sulfur isotope compositions in sedimentary rocks. Due to difficulties in both culturing anaerobic methanotrophs and sampling pore-water sulfate in SD-AOM-dominated environments, however, the θ 33 values for the processes of SD-AOM have not been constrained. We propose that a set of modern cold-seep associated barite samples with low Δ δ 18 O / Δ δ 34 S values bear a record of residual pore-water sulfate during SD-AOM, and therefore the triple sulfur isotope composition of these barites can be used to deduce θ 33 values. We applied a 1-D diagenetic reaction–transport model to fit Δ 33 S and δ ′ 34 S results from modern cold seep barites collected from five sites in the Gulf of Mexico. Based on revealed negative correlations ( R 2 = 0.77 ) between Δ 33 S and δ ′ 34 S values we calculated an upper-limit θ 33 value of 0.5100 to 0.5112 (±0.0005) given a 1000 ln 34 ⁡ α value of − 30 ‰ to − 10 ‰ . This θ 33 value is distinctively lower than that of organoclastic sulfate reduction (OSR) in marine environments where the diagnostic isotope fractionation ( 1000 ln 34 ⁡ α ) is typically more negative than that of SD-AOM. In addition, cold seep barite data display a negative Δ 33 S – δ ′ 34 S correlation whereas pore-water sulfates of all OSR-dominated settings show a positive one. Therefore, the diagnostic triple-sulfur isotope exponent and associated negative Δ 33 S – δ ′ 34 S correlation may allow for the identification of SD-AOM in sedimentary records.

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.

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.

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.

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.

Numerous 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.

Hydrocarbon-rich brine seeps – representing extreme environments typified by halophilic and chemotrophic organisms – may provide insight into the ecology of microorganisms on an early Earth and possibly other planetary bodies. Authigenic low magnesium calcite (LMC) is common at brine seeps and represents an archive of fluid compositions, fluid origin, and the conditions under which this carbonate mineral formed. Here we present the analysis of authigenic LMC from three hydrocarbon-rich brine seeps (GB260, ~460 m; GB697, ~1280 m; AC601, ~2340 m) from the Gulf of Mexico. The formation of LMC was favored by brine fluids with extremely low Mg/Ca mole ratios (e.g.