SEQUENTIAL SULFUR EXTRACTION AND ISOTOPIC ANALYSIS OF EARLY EDIACARAN PHOSPHORITE FROM THE DOUSHANTUO FORMATION AT WENG'AN, SOUTH CHINA

The Ediacaran Doushantuo Formation hosts some of the largest phosphorus reserves in the world and preserves extraordinary fossils that can illuminate our understanding of biological evolution and environmental change. However, the origin of phosphorite and depositional environment of the Doushantuo Formation is not entirely resolved. Here, we conducted sequential extraction of different sulfur species present in both black and gray phosphorite of the Doushantuo Formation at Weng’an. Sulfur species were extracted using dichloromethane (for elemental sulfur, ES), sodium chloride solution (for water-soluble sulfates, WSS), acetic acid (presumably for carbonate-associated sulfate or CAS), hydrochloric acid (for phosphate-associated sulfate or PAS), diethylenetriamine pentaacetate and sodium hydroxide solution (for barite), chromium reduction (for pyrite), and combustion of final residual (for kerogen sulfur). The sequential extraction products were analyzed for sulfur isotope compositions. The results show that, in general, δ³⁴S_PAS and δ³⁴S_kerogen have the highest values of 30–35‰ VCDT, and these are interpreted to have recorded the sulfur isotope composition of sulfate in seawater and porewaters. In contrast, δ³⁴S_WSS, δ³⁴S_barite, and δ³⁴S_CAS are mostly 10–20‰ VCDT, similar to δ³⁴S_pyrite, likely compromised by pyrite oxidation on the outcrop or in the laboratory. δ³⁴S_ES is also mostly 10–20‰ VCDT, but with greater variability, in part due to the instability of elemental sulfur at geological time scale. The data indicate that kerogen and early diagenetic phosphorite can be valuable archives of seawater sulfate δ³⁴S, and that pyrite oxidation can be pervasive and poses a considerable challenge in sequential sulfur extraction and isotopic analysis. Thus, careful quantification of sulfur species and their isotopic compositions is important to understand sulfur cycles, redox conditions, and phosphogenesis in Ediacaran oceans.