Sulfur Cycling During Progressive Burial in Sulfate‐Rich Marine Carbonates

Abstract

AbstractThe isotopic composition of sulfate in the rock record has been frequently used to track the changes in the Earth's surface environments. By considering isotopic fractionation imparted by microbial sulfate reduction (MSR) and thermochemical sulfate reduction (TSR), in this study, we aim to develop a holistic understanding of the mixed effects of MSR and TSR on δ34S signals in sulfate‐rich carbonate systems. We report the occurrence of various types of sulfur‐bearing components from the Cambrian‐Ordovician carbonate system in the Tarim Basin, NW China, coupled with a well‐established diagenesis framework for these rocks. Our results indicate that most of the sulfur‐bearing species possess δ34S values slightly lower than both the source sulfate and the sulfide generated by TSR, yet these sulfur‐bearing species have substantially higher δ34S values than sulfide that resulted from MSR. Hence, a combination of sulfides sourced from MSR and TSR can adequately explain the sulfur isotope data in the studied interval. Building upon this hypothesis, we developed a new sulfur diagenesis model in order to quantify the accumulated H2S from the combined effects of MSR and TSR. Our new model can be used to explain the origin of sulfur‐bearing species in many other deep burial carbonate systems, including the Sichuan Basin, China, and the Gulf of Mexico, USA. We propose that greater attention should be paid to isotopic modulation through mixed diagenetic processes in order to gain a better mechanistic understanding of the primary geochemistry signals (e.g., δ34S) in marine carbonates.