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

Abstract Authigenic molybdenum (Mo auth ) 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 (δ 98 Mo auth ) distribution patterns remain poorly constrained. Here, we report Mo abundances and δ 98 Mo 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 δ 98 Mo bulk and δ 98 Mo auth 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 δ 98 Mo values between carbonate and pyrite fractions of seep carbonate rocks formed under sulfidic conditions increases with higher Mo auth contents, suggesting a control of dissolved hydrogen sulfide concentrations on Mo isotope fractionation during carbonate precipitation. Compared with δ 98 Mo auth and δ 98 Mo py , δ 98 Mo carb 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 δ 98 Mo composition of seawater in the course of Earth history. Overall, our findings show that the δ 98 Mo 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.