Molybdenum Isotope Fingerprinting of Microbial Sulfate Reduction in Seep Carbonate Rocks

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 (δ 98 Mo) 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 δ 98 Mo carb and δ 98 Mo OM (Δ 98 Mo carb‐OM ) displays significant variability in the studied samples, ranging between 0.72 and 1.01‰. Remarkably, the obtained Δ 98 Mo carb‐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 Δ 98 Mo carb‐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.