Seawater-Fluid Composition Records From Molybdenum Isotopes of Sequentially Extracted Phases of Seep Carbonate Rocks

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 (delta 98Moauth) distribution patterns remain poorly constrained. Here, we report Mo abundances and delta 98Mo 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 mu g/g and generally dominate the Mo budget of seep carbonate rocks. Our data indicate large ranges of delta 98Mobulk and delta 98Moauth values relative to NIST 3134 (0.25 parts per thousand), varying from 1.02 to 1.98 parts per thousand (n = 4) and from 0.15 to 3.07 parts per thousand (n = 34), respectively. The difference in delta 98Mo 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 delta 98Moauth and delta 98Mopy, delta 98Mocarb of seep carbonate rocks formed under sulfidic conditions shows a relatively narrow range with an average of 1.98 +/- 0.31 parts per thousand (1 SD; n = 10), providing constraints on the delta 98Mo composition of seawater in the course of Earth history. Overall, our findings show that the delta 98Mo 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. Sulfate reduction has created widespread euxinic conditions in the water column of ancient oceans and prevalent sulfidic conditions in subseafloor pore-water environments of the modern ocean. Ancient seawater delta 98Mo compositions have been considered to reflect the overall extent of oxic versus euxinic conditions of the global ocean, highlighting the importance of a robust reconstruction of the marine Mo isotope inventory largely unaffected by diagenesis. Under sulfidic conditions, the seawater delta 98Mo value can be archived by authigenic Mo in marine carbonate sediments formed during early diagenesis. However, the conditions of capturing the seawater delta 98Mo signature remain poorly constrained. Using a sequentially chemical leaching procedure, we investigated Mo contents and isotopic compositions of different fractions of modern seafloor seep carbonate rocks. Our results show that a delta 98Mo average value of 1.98 +/- 0.31 parts per thousand (1 SD; n = 10) of the carbonate mineral phase results from small or no Mo isotope fractionation during near-quantitative authigenic Mo sequestration under sulfidic conditions, potentially capturing seawater delta 98Mo. Our study shows that delta 98Mo of sequentially extracted phases of sedimentary carbonate rocks is a robust tool to constrain seawater and pore fluid properties. delta 98Mo of sequentially extracted phases of carbonates is useful to trace Mo isotope fractionation during early diagenesisDegree of Mo isotope fractionation during carbonate precipitation is controlled by hydrogen sulfide concentrationsAuthigenic Mo sequestration into the carbonate phase under sulfidic conditions generates small Mo isotope fractionation