Constraining the Ediacaran oceanic dissolved organic carbon reservoir: Insights from carbon isotopic records from a drill core from South China

The evolution of the atmospheric oxygen content through Earth’s history is a key issue in paleoclimatic and paleoenvironmental research. There were at least two oxygenation events in the Precambrian that involved fundamental changes in both biotic innovation and the surface environment. However, a large dissolved organic carbon (DOC) pool maintained in deep oceans during the Neoproterozoic may have extended the time interval between the two oxygenation events. To test the DOC hypothesis, we conducted detailed micro-drilled analyses of carbonate carbon isotopes (δ¹³C_carb) of a long Ediacaran drill core (the Wangji drill core), for which whole-rock δ¹³C_carb and organic carbon isotope (δ¹³C_org) records were available. The micro-drilled δ¹³C_carb values obtained in this study are consistent with whole-rock δ¹³C_carb results, precluding the influence of severe authigenic carbonate incorporation. Importantly, the multiple negative δ¹³C_carb excursions in the Wangji drill core were likely linked with upwelling events, during which DOC was supplied to the surface water and oxidized. Using box models, we estimate that ~3.6 × 10¹⁹ mol and ~2.0 × 10¹⁹ mol DOC were converted to bicarbonate during two negative δ¹³C_carb excursions spanning millions of years. The estimations are approximately 1000 times the modern marine DOC reservoir. Our results support a relatively high oxidation capacity (elevated atmospheric pO₂ and/or oceanic \rmSO_4^2 - ) of the Earth’s surface during the early Ediacaran Period.