Distinguishing microbial from thermochemical sulfate reduction from the upper Ediacaran in South China

Abstract

It is hard to distinguish pyrites generated by microbial sulfate reduction (MSR) from thermochemical sulfate reduction (TSR). The upper Ediacaran Dengying Formation (ca 551–541 Ma) of Sichuan basin is chosen to study for this purpose by means of petrological, bulk and micrometer-scale analyses on barite, pyrite, calcite, and dolomite for carbon and sulfur isotope and rare earth elements and yttrium (REY) compositions. We find that MSR-derived pyrites precipitated from near-surface to shallow burial settings are characterized by seawater-like REY patterns and superchondritic Y/Ho ratios (74.3 ± 9.5), and have δ34Spyrite values in a wide range from −36.8‰ to 34.8‰, positively correlated with arsenic contents, likely through cryptic sulfur cycles driven by arsenic. In contrast, TSR-derived pyrites generated during deep burial settings have REY characterized by non-seawater-like patterns and chondritic Y/Ho ratios (33.6 ± 5.1), and show small sulfur isotopic fractionations (~10.5‰). TSR-pyrites yield both U-shaped and bell-shaped δ34S transects compared with U-shaped distributions of MSR-pyrites. The calcites associated with the TSR-pyrites show LREE enrichment, elevated Y/Ho ratios (85.8 ± 14.2) and negative δ13C values, likely resulting from decomposition of organic matter. This study highlights a combination method from REY patterns, δ13C and δ34S values from analyses on micro-sized pyrite and the associated calcites can be used to effectively distinguish MSR from TSR.