Ediacaran sulfur cycle: Insights from sulfur isotope measurements (Δ33S and δ34S) on paired sulfate–pyrite in the Huqf Supergroup of Oman

Abstract

The Ediacaran Period (635–541Ma) was a time of dramatic environmental and ecological change. Sulfur isotopic records preserved in marine strata provide insights into these changes. Previous work using δ34S in pyrite and carbonate-associated sulfate in the Ediacaran-aged Huqf Supergroup (Sultanate of Oman) suggested increases in marine sulfate concentrations, changing microbial pathways and redox conditions, and a long-term increase in δ34S of sulfate delivery to the ocean. However, δ34S-based analyses on their own were insufficient to uniquely reconstruct these environmental and ecological changes. Here we present new paired δ34S and Δ33S data from carbonate-associated sulfate and pyrite in these strata. The results allow us to provide three new constraints on Ediacaran sulfur cycling. First, a notable increase Δ34SCAS-PY, previously attributed to possible disproportionation, in upper Buah strata is associated with Δ33S-evidence for an increase in sulfide reoxidation, indicating a transition to more oxidizing conditions, possibly associated with the onset of bioturbation. Secondly, Δ33S data indicate that a large positive shift in both sulfate and pyrite δ34S in Ara Group strata is mostly likely due to an increase in delivery of 34S-enriched sulfate to the basin. Finally paired δ34S–Δ33S data constrain possible changes in seawater sulfate concentrations, suggesting an increase in sulfate supply related to enhanced evaporite weathering, consistent with geological and geochemical observations that imply high [SO4] (∼17mM) during deposition of the Ara Group.