Characterizing deposition and diagenesis history of post-Marinoan Snowball Earth carbonates from sedimentary pyrites

Abstract

This study presents a comprehensive investigation of petrography, clumped isotope thermometry (T-∆₄₇), and NanoSIMS sulfur isotopic analysis of pyrites for the basal Doushantuo cap carbonates from the Jiulongwan (JLW) section. The majority of the analyzed pyrites originate from reduction of marine sulfates, primarily through thermochemical sulfate reduction (TSR), with minor contributions from microbial sulfate reduction (MSR) during dolomitization. Overall, the occurrence of pyrites and their δ³⁴S populations are controlled by H₂S generation, transportation, and storage. MSR- and TSR-derived H₂S are recorded by different pyrite generations, each exhibiting distinctive δ³⁴S_py values within a cm-sized specimen. In the case of TSR-derived pyrites, fluid pulses stored and mixed in cavities produce pyrites with variable occurrences but limited isotopic variation. In contrast, prolonged fluid alteration involves gradual pyrite precipitation during fluid transportation along the grain boundaries. This results in a broad δ³⁴S variation, even under high temperature conditions. The ∆³⁴S_CAS-py values of MSR-derived pyrites collected from two depths indicate a slight increase in seawater sulfate concentration during two transgression stages in the aftermath of the Marinoan Snowball glaciation. This study highlights the significance of employing multiple approaches at microscopic scales to elucidate sedimentation processes during post-glacial periods.