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Many chemosynthesis-based communities prospering in deep-sea environments rely on the metabolic activity of sulfur-oxidizing bacteria. This is also the case for vestimentiferan siboglinid tubeworms, whose demand for nutrition is entirely satisfied by their endosymbiotic bacteria harbored in the trophosome. Such chemosymbiosis leads to a significantly lower nitrogen isotope composition of the trophosome than in other types of soft tissue. However, the specific process of nitrogen utilization by siboglinids remains unclear. As a key element in the relevant enzymes (nitrogenase, nitrate reductase), molybdenum (Mo) is indispensable in the biogeochemical cycling of nitrogen. The Mo isotope composition (δ98Mo) of siboglinids is thus a potential proxy to decode the mode of nitrogen utilization. In this study, we found that δ98Mo along the chitinous tube of the vestimentiferan siboglinid Paraescarpia echinospica from the Haima seep of the South China Sea yields values as negative as -4.59‰ (-1.13 ± 1.75‰, n = 19) – the lowest δ98Mo signature ever reported for any kind of natural material. It is suggested that this extremely negative Mo isotope composition is caused by preferential utilization of isotopically light Mo by the tubeworm symbionts during nitrate reduction. Such Mo isotope signature could provide a means to identify siboglinid tubeworms in the rock record, a group of annelids that has previously escaped unambiguous identification due to the lack of mineralized skeleton.

Authigenic minerals forming at marine seeps constitute an excellent archive of past methane seepage and biogeochemical processes. Over the past two decades, authigenic carbonate and sulfur-bearing minerals from methane seeps of the South China Sea (SCS) have been widely investigated, providing insight into fluid sources and seepage dynamics and facilitating the establishment of geochemical proxies to trace sulfate-driven anaerobic oxidation of methane (SD-AOM). Authigenic carbonates from all seep sites in the SCS commonly exhibit low δ13C and high δ18O values, confirming the incorporation of methane-derived carbon and oxygen from a pore water pool probably affected by gas hydrate dissociation. Pyrite is a common authigenic mineral at methane seeps, also forming at low methane flux where authigenic carbonate tends to be absent. The identification of methane seepage and SD-AOM activity consequently benefited from the advancement of sulfur isotope geochemistry, particularly from in situ measurements of δ34Spyrite values using nanoSIMS and multiple sulfur isotopes. Quantification of carbon and sulfur fluxes in the course of SD-AOM in modern and ancient marine sedimentary environments remains challenging, highlighting the need for more field-based research and modeling work. Furthermore, other elemental cycles and biogeochemical processes at methane seeps archived in authigenic minerals, such as nitrogen-based metabolisms, remain largely unknown. We highlight that SCS seeps are fascinating natural laboratories to better understand methane-driven biogeochemical processes and their signatures in authigenic minerals, representing a rewarding but also challenging object of research in the field of geomicrobiology.

The natural dynamics of fluid flow at methane seeps and increasingly human activities influence the biogeochemistry of the microenvironment and further determine the activity of the chemosynthetic communities within these ecosystems. However, ways to reconstruct short-term fluid flow dynamics and to decipher the influence of scientific exploration at seeps are limited. In this study, we present high-resolution trace elements/Ca ratios (Li/Ca, Mg/Ca, Ti/Ca, Mn/Ca, Co/Ca, Cu/Ca, Zn/Ca, Sr/Ca, Zr/Ca, Mo/Ca, Ba/Ca, Th/Ca and U/Ca ratios) from the shells of two species of chemosymbiotic bivalves (the thiotrophic vesicomyid clam Archivesica marissinica and the methanotrophic mussel Gigantidas haimaensis) from the Haima cold seeps of the South China Sea. We found that the complex distribution patterns of some trace elements (Mg/Ca, Sr/Ca, Mo/Ca and U/Ca ratios) in G. haimaensis are largely controlled by mineral composition or age. The observation of Co/Ca and Ba/Ca ratios in both species indicate strong physiological and environmental control on the incorporation of trace elements during the biomineralization process. Besides, the distribution patterns of other trace elements provide information that can be used to discuss open issues such as the loss of trace elements after death of the bivalves, and the possible influence of human activities such as sediment disturbance. Overall, this study emphasizes the potential for using high-resolution element geochemistry of seep bivalve shells to reveal the physiological and environmental factors that control the growth of bivalves, and to elucidate the potential history of fluid discharge at cold seeps.

Modern cold seeps are of fluctuant flux, which could result in variabilities of geochemical archives through intensively influencing the redox condition in pore fluids. However, the geochemical archives are not fully understood when the redox condition changes. Here, tubular carbonates from the Shenhu Sea Area were used to reconstruct the formation environment and redox conditions. The moderately negative δ13C values of the carbonates (−40.1‰ to −30.8‰, VPDB) indicate a mixed carbon source of thermogenic and biogenic methane. The low δ18O values (−2.7‰ to 1.0‰, VPDB) suggest a type of 18O-depleted pore fluid possibly influenced by gas hydrate formation. Co-variation of MoEF, WEF, CoEF, and CrEF suggests that high Fe contents in the rims of samples R1 and R2 are induced by Fe (oxyhydr)oxidation enrichment, while the positive correlation between MoEF and Mn/Al ratio indicates that high Mn contents in the rims of samples R3 and R4 are induced by Mn (oxyhydr)oxidation enrichment. The occurrence of Fe or Mn enrichment in the rims and the absence of Fe/Mn enrichment in the cores suggest Fe/Mn (oxyhydr)oxides forming in pore fluid rather than in bottom seawater. The carbonate phases of the rims enriched in Fe (oxyhydr)oxides are dominated by high magnesium calcite, while those of the rims enriched in Mn (oxyhydr)oxides are dominated by aragonite. The occurrence of Fe or Mn (oxyhydr)oxides corresponds to the previously proposed formation depth for the carbonate phase. The occurrence of dissolution textures in these rims indicates episodic oxic conditions, which would facilitate Fe2+/Mn2+ oxidation. We suppose that the Fe2+ and Mn2+ could be supplied through fluid seepage or diffusion from underlying sediments when the flux decreased. Similar archives may be applied to qualitatively reflect the changes of redox conditions in seep systems. Similar scenarios may help us understand the geochemical records in seeps of fluctuant flux.

The magnesium (Mg) isotopic composition of marine authigenic carbonates is considered as promising archive of ancient seawater geochemistry and paleoenvironments. Previous experimental and theoretical work has shown that Mg isotope fractionation during carbonate mineral formation is a function of mineralogy and precipitation rate. However, information on Mg isotope fractionation is limited for well-defined precipitation rates in natural settings. Here, we investigate pore waters from sediments of an area of active methane seepage in the South China Sea. Low δ13C values (< −48.3‰ VPDB) of dissolved inorganic carbon (DIC) near the sulfate-methane transition zone (SMTZ) indicate that sulfate-driven anaerobic oxidation of methane (SD-AOM) is the predominant biogeochemical process. Pore water composition of dissolved Mg, calcium (Ca), and strontium (Sr) agrees with aragonite as the dominant carbonate mineral at the site ROV1, and high Mg-calcite at sites ROV2 and ROV4. Calculated carbonate precipitation rates are 0.92 μmol cm−2 yr−1 for site ROV2 and 1.24 μmol cm−2 yr−1 for site ROV4; these estimates are similar to previous calculations for seeps from other areas. The pore water δ26Mg values (−0.88‰ to −0.71‰) obtained for the three study sites are similar to those of seawater, in accord with a minor effect of Rayleigh fractionation due to abundant supply of Mg from seawater and insignificant consumption of Mg during carbonate precipitation. The modeled Mg isotope fractionation (ϵ = −2.0‰ to −1.0‰ for core ROV2; ϵ = −1.3‰ to −0.3‰ for core ROV4) can be explained by kinetic isotope fractionation during carbonate precipitation. The calculated carbonate precipitation rates and the degree of fractionation of Mg isotopes support the notion that fractionation is small at high precipitation rates. However, the carbonate precipitation rates calculated for the studied seep environments are much smaller than those in laboratory experiments, documenting a discrepancy of isotopic fractionation between carbonate authigenesis in laboratory experiments and natural environments. These results, including the modeled precipitation rates, provide new constraints for Mg isotope fractionation in natural settings.

Gas hydrates represent a huge reservoir of methane in marine sediments, prone to dissociation in response to environmental changes. There is consensus that past events of gas hydrate dissociation in the marine environment mainly occurred during periods of low sea‐level. Here, we report geochemical data for two meter‐thick layers of seep carbonate collected from a hydrate‐bearing drill core from ~800 m water depth in the northern South China Sea. The aragonite‐rich carbonates reveal positive δ18O values, confirming a genetic link with gas hydrate dissociation. Uranium‐thorium dating of seep carbonates indicates that gas hydrates at the study site dissociated between 133,300 and 112,700 yr BP, hence coinciding with the Last Interglacial (MIS 5e) sea level highstand. We put forward the concept that a climate‐driven increase in temperature was responsible for a period of pronounced gas hydrate dissociation. Plain Language Summary The gas hydrate reservoir is a dynamically changing system extremely susceptible to variations of seafloor temperature and pressure. Therefore, gas hydrate dissociation and subsequent methane seepage frequently occur during times of global climate change, especially during sea level lowstands with reduced seabed pressure. However, this conclusion was mainly based on dating of seep carbonates sampled from the seabed. As a consequence, one cannot exclude that previous results have been compromised by a sampling bias since seafloor samples are easier to collect. Authigenic seep carbonates from drill cores represent a continuous record of gas hydrate dynamics. Our uranium‐thorium dating of seep carbonate from drill cores provides a unique example of the effects of temperature and pressure on the stability of the hydrate system in the Dongsha area, northern South China Sea (SCS), during the last interglacial stage (MIS 5e, about 130,000 years BP). Representing the most similar and most contemporary analogue to the current interglacial, the study of a methane release event in the SCS during MIS 5e will shed light on the expected trend of methane release events in the future, while providing insight into the response of low latitude oceans to climate change.

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.

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.

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.

A remarkable exposure of tubular authigenic carbonates was found on the seafloor in the Dongsha area of the South China Sea (SCS). The tubular carbonates, around 2&ndash, 3 cm in diameter and usually less than 10 cm in length, represent broken fragments of once-larger pipes that now protrude from muddy sediments. The morphology, carbon and oxygen stable isotope compositions, and trace and rare earth element contents of the carbonates were analyzed to decipher the mode of carbonate formation. The tubular carbonates exhibit a dark brown coating of iron and manganese hydrous oxides, indicating prolonged exposure to oxic bottom waters. The carbonate content of the micritic pipes falls between 12.5 and 67.3 wt.% with an average of 42.0 wt.%, suggesting formation within the sediment. This inference is supported by trace and rare earth element patterns including a moderate enrichment of middle rare earth elements. Low &delta, 13C values (as low as &minus, 50.3&permil, Vienna Pee Dee Belemnite (VPDB)) suggest that carbonate precipitation was induced by the anaerobic oxidation of methane. The unusually positive &delta, 18O values of the carbonates (as high as +5.3&permil, VPDB) are believed to reflect the destabilization of locally abundant gas hydrate. Taken together, it is suggested that pipe formation was initiated by sediment-dwelling organisms, such as crustaceans or bivalves. The burrows subsequently acted as conduits for upward fluid migration. The lithification of the sediment directly surrounding the conduits and the partial filling of the conduits with carbonate cement resulted in the formation of tubular carbonates. Turbidity currents, sediment slumps, or the vigorous emission of fluids probably induced the fragmentation of tubular carbonates within the sediment. The carbonate fragments had been further subjected to winnowing by bottom currents. This study provides insight into the interaction of megafauna burrowing with fluid migration and carbonate formation at hydrocarbon seeps, highlighting the role of bottom currents and mass wasting on the formation of fragmented tubular carbonates.

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.