The Source of Oxidants in the Shuram Ocean Oxygenation Event

The Ediacaran Period (635-539 Ma) witnessed the longest-lasting (~574-567 Ma), largest-scale and globally distributed negative carbonate carbon isotope (δ13Ccarb) excursion in Earth’s history——the minimum negative δ13Ccarb value reaching -12‰. This event is referred to as the “Shuram Excursion” (SE; also termed to DOUNCE/EN3/Wonoka). It is widely believed to be resulted from the large-scale oxidation of dissolved organic carbon in the ocean, yet the source of oxidants remains debated. One hypothesis emphasizes enhanced terrestrial weathering inputs (such as gypsum), whereas the other attributes to dissolved oxygen from atmosphere. Resolving this controversy is critical for understanding the emergence of complex multicellular eukaryotes and metazoans during this interval. Here, we analyzed sulfur and oxygen-isotope (δ34S-δ18O-∆'17O) compositions of carbonate-associated sulfate (CAS) of Ediacaran sections from different water depths in South China. The results showed that during the SE, both δ34SCAS and δ18OCAS obviously decreased across all sections, indicating an increase in the oxidation flux of sulfide/sulfur or/and a decrease in the reduction flux of sulfate, which suggested increased sulfate concentrations in the oceans. However, the ∆'17OCAS exhibits distinct heterogeneity: the most negative 17O anomalies (with ∆'17OCAS value reaching to -0.55‰) were observed in the mid-depth sections (Zhengjiatang, Jiulongwan, Panmen), whereas no detectable ∆'17O anomalies were observed in both shallow-water (Lianghong, ZK6305) and deeper-water (Liulongshan) sections. Given that mass-independent fractionation 17O anomalies are a fingerprint signature of atmospheric O2, the observed Δ′17OCAS spatial heterogeneity implied that, compared to shallow and deep-water conditions, sulfate in med-depth settings recorded a greater proportion of oxidation of reduced sulfur (H2S/HS−/S0) by paleo-atmospheric O2, mainly occurring near the chemocline. These results revealed that during the SE, in addition to the potential enhanced continental weathering inputs, paleo-atmospheric O2 and its derived sulfate by the oxidation of marine internal reduced sulfur likely constituted an important source of oxidants for organic carbon oxidation. This study provides key constraints on the coupled dynamics of paleoatmospheric-oceanic oxygenation and had important implications for their driving role in biological evolution during the Ediacaran Period.