Redox-sensitive elements of Ediacaran black shales in South China with implications for a widespread anoxic ocean

The high enrichment of redox-sensitive elements (RSEs), Mo, U, V, and Cr, in Ediacaran shales was attributed to the Neoproterozoic Oxygenation Event (NOE). However, recent studies have shown that contemporaneous shales from NW Canada do not exhibit RSE enrichment, whereas those from South China exhibit varying degrees of RSE enrichment. Here we investigate RSE records in a broader spatial and temporal distribution of shales within the Ediacaran Doushantuo Formation from South China. In general, RSEs in these shales do not display high en-richments, suggesting an extensively anoxic Ediacaran ocean with low RSE contents. There are also some outliers with high RSE enrichments of various depositional facies, which are closely tied to anoxic conditions. The abnormally high RSE enrichments in shales of the Doushantuo Formation were primarily related to the ocean environment, not due to secondary processes. We suggest that local RSE-enriched events intermittently occurred in the Nanhua Basin, possibly as a result of pulses of RSEs being transported from the surface ocean to the deep ocean. This would enhance the local RSE reservoirs, yielding high RSE enrichments in sediments. Our results suggest a widespread anoxic and RSE-depleted ocean in the Nanhua Basin during the early-middle Ediacaran Period. Contemporarily, RSE and sulfate reservoirs in the surface ocean were likely enhanced, supporting at-mospheric and oceanic oxidation. Moreover, the exchange of materials between the surface and deep sea was significantly strengthened, facilitating oceanic oxidation via the downward transportation of dissolved O2 from the surface.