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Abstract Understanding the interaction between molybdenum (Mo) and organic matter during microbial sulfate reduction is critical for the use of Mo to reconstruct marine redox conditions throughout Earth's history. However, little is known about Mo isotope fractionation and how it relates to organic matter remineralization during microbial sulfate reduction. Here, we report Mo abundances and isotopic (δ98Mo) compositions for bulk‐rock, non‐lithogenic and sequentially extracted fractions, including carbonate (carb), pyrite, and organic matter (OM), of seep carbonate rocks. Our data indicate that the difference between δ98Mocarb and δ98MoOM (Δ98Mocarb‐OM) displays significant variability in the studied samples, ranging between 0.72 and 1.01‰. Remarkably, the obtained Δ98Mocarb‐OM values indicate correlative trends with stable carbon isotope ratios and bulk abundances of (a) total organic carbon, (b) Mo, and (c) pyrite in seep carbonates, which we interpret as reflecting sustained adsorption of isotopically light Mo onto organic matter during enhanced sulfate reduction. On this basis, we put forward the concept that Δ98Mocarb‐OM of authigenic carbonate rocks can be used as a measure of the intensity of sulfate reduction and for reconstructing past interactions between Mo and organic matter in marine sediments.

Abstract Authigenic molybdenum (Moauth) in marine sediments holds great potential to archive the Mo isotopic composition of seawater and biogeochemical processes. However, the factors that control authigenic Mo isotope (δ98Moauth) distribution patterns remain poorly constrained. Here, we report Mo abundances and δ98Mo compositions for bulk‐rock (bulk) and sequentially extracted fractions—including total authigenic (auth; i.e., non‐lithogenic fraction), carbonate (carb), iron and manganese oxyhydroxides, pyrite (py), and organic fractions (OM)—of authigenic carbonates recovered from various hydrocarbon seep sites in the Gulf of Mexico and the South China Sea. Extracted pyrite fractions exhibit Mo contents varying from 0.1 to 23.4 μg/g and generally dominate the Mo budget of seep carbonate rocks. Our data indicate large ranges of δ98Mobulk and δ98Moauth values relative to NIST 3134 (0.25‰), varying from 1.02 to 1.98‰ (n = 4) and from 0.15 to 3.07‰ (n = 34), respectively. The difference in δ98Mo values between carbonate and pyrite fractions of seep carbonate rocks formed under sulfidic conditions increases with higher Moauth contents, suggesting a control of dissolved hydrogen sulfide concentrations on Mo isotope fractionation during carbonate precipitation. Compared with δ98Moauth and δ98Mopy, δ98Mocarb of seep carbonate rocks formed under sulfidic conditions shows a relatively narrow range with an average of 1.98 ± 0.31‰ (1 SD; n = 10), providing constraints on the δ98Mo composition of seawater in the course of Earth history. Overall, our findings show that the δ98Mo composition of sequentially extracted phases of carbonate‐rich sedimentary rocks can provide insights into seawater‐sediment interactions and biogeochemical pathways of Mo during early diagenesis.

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 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.

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.

Cold seeps where methane-rich fluids escape from the seafloor generally support enormous biomass of chemosynthetic organisms and associated fauna. In addition to transporting a great amount of methane toward the seafloor, cold seeps also contribute to the aged, dissolved organic carbon (DOC) pool in the deep ocean. Here, two sediment cores from the “Haima cold seeps,” northern South China Sea and a nearby reference core were analyzed for pore-water sulfate and DOC concentrations, δ13C of DOC, and optical properties of dissolved organic matter (DOM). High DOC concentrations (0.9–3.7 mM) accompanied by extremely low δ13C values (−43.9 to −76.2‰) suggest the conversion of methane into sedimentary DOC pool in the seep sediments. Parallel factor analysis (PARAFAC) of the fluorescence excitation-emission matrices shows higher fluorescent intensities of labile protein-like components (C2 and C4) and lower fluorescent intensities of refractory humic-like components (C1 and C3) in the seep cores compared to the reference core. The intensity of C2 is positively correlated with DOC concentrations and δ13C-DOC in the seep sediments, suggesting that the labile protein-like DOM was produced by the anaerobic oxidation of methane (AOM). Moreover, low humification index (HIX) and high biological index (BIX) values also indicate intensified production of relatively labile DOM with lower degradation degree in the seep cores compared to the reference core. Hence, we highlight that methane-derived DOC may serve as important carbon and energy sources for heterotrophic microbial communities due to its relatively labile nature.

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 2–3 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 δ13C values (as low as −50.3‰, Vienna Pee Dee Belemnite (VPDB)) suggest that carbonate precipitation was induced by the anaerobic oxidation of methane. The unusually positive δ18O values of the carbonates (as high as +5.3‰, 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.

High-magnesium calcite (HMC) and aragonite are metastable minerals, which tend to convert into low-magnesium calcite (LMC) and dolomite. During this process, primary compositions are frequently altered, resulting in the loss of information regarding the formation environment and the nature of fluids from which the minerals precipitated. Petrological characteristics have been used to recognize primary LMC, however, neither the element distribution within primary LMC nor the effect of diagenetic alteration on element composition have been studied in detail. Here, two mostly authigenic carbonate lithologies from the northern Gulf of Mexico dominated by primary LMC were investigated to distinguish element compositions of primary LMC from LMC resulting from diagenetic alteration. Primary LMC reveals similar or lower Sr/Ca ratios than primary HMC. The lack of covariation between Sr/Ca ratios and Mg/Ca ratios in the studied primary LMCs are unlike compositions observed for LMC resulting from diagenetic alteration. The Sr/Mn ratios and Mn contents of the primary LMCs are negatively correlated, similar to secondary, diagenetic LMC. Element mapping for Sr and Mg in the primary LMC lithologies revealed no evidence of conversion from aragonite or HMC to LMC, and a homogenous distribution of Mn is in accordance with the absence of late diagenetic alteration. Our results confirm that Sr/Ca ratios, Mg/Ca ratios, and element systematics of primary LMC are indeed distinguishable from diagenetically altered carbonates, enabling the utilization of element geochemistry in recognizing primary signals in carbonate archives.

Authigenic low-magnesium calcite (LMC)—a mineral phase that should precipitate in calcite seas rather than today’s aragonite sea—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 δ18O values of LMC from site SS296 cluster in two groups (−0.6‰ to 1.7‰; 6.3‰ to 7.5‰) and the presence of cone-in-cone texture in the samples with lower δ18O values suggest precipitation at higher temperatures and greater depth. Low δ18O values of LMC from site GC53 ranging from −9.4‰ to −2.5‰ indicate an influence of meteoric waters during formation. LMC at both sites reveals a wide range of δ13C values (−17.4‰ to 2.6‰), indicating various carbon sources including seawater and/or organic matter. This interpretation is further supported by the δ13C values of organic carbon extracted from the LMC lithologies (δ13Corg: from −26.8‰ to −18.9‰). 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—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.

Many chemosynthesis-based communities prospering in deep-sea environments rely on the metabolic activity of sulfide-oxidizing bacteria. This is the case for vestimentiferan siboglinid tubeworms, whose demand for nutrition is satisfied predominantly by their en-dosymbiotic bacteria harbored in a specialized organ called the trophosome. Such chemo-symbiosis 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 sibo-glinids remains unclear. As a key element in the relevant enzymes (nitrogenase and nitrate reductase), molybdenum (Mo) is indispensable in the biogeochemical cycling of nitrogen. The Mo isotope composition (δ98Mo) of siboglinids is thus a potential proxy for decoding the processes involved in nitrogen metabolism. In this study, we found δ98Mo values along the chitinous tube of the vestimentiferan siboglinid Paraescarpia echinospica from the Haima seeps of the South China Sea as negative as -4.59%c (-1.13%c ± 1.75%o, 1SD, n = 19)-the lowest δ98Mo value 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's endosymbionts or epibionts during nitrate reduction. Such Mo isotope signature could provide a means for identifying siboglinid tubeworms, a group of annelids that has previously escaped unambiguous identification due to the lack of mineralized skeleton, in the rock record. [ABSTRACT FROM AUTHOR]