Reexamination and reidentification of ocean oxygenation event in the wake of the Marinoan glaciation

Abstract

The earliest Ediacaran oceanic oxygenation event in the wake of the Marinoan glaciation defined as OOE-A (∼635–632 Ma) constitutes an integral component of the Neoproterozoic Oxygenation Event. It was largely identified based on high enrichments of redox-sensitive elements (RSEs) in the basal Ediacaran Doushantuo Formation in South China, but this proposal has been challenged on a global scale. Here, we reexamine the purported OOE-A through an integrative study of correlative successions across the shelf-to-basin transect in South China, along with records from other continents. Sedimentary and geochemical evidence, including iron speciation, RSE concentrations, organic carbon isotopes, and pyrite sulfur isotopes, collectively suggest an oxic shelf and an anoxic (intermittently euxinic) basin with a fluctuating chemocline around the upper slope environment in South China. Importantly, a diffusion-reaction model integrating sedimentation rates and authigenic RSE concentrations suggests that the discrepancies in RSE enrichment levels between the basal Ediacaran black shales of South China and northwestern Canada can be reconciled if seawater Mo and U concentrations were only 0.5–3 % and 1.5–6 % of their modern values, respectively. This finding contradicts the traditional viewpoint of the expansion of seawater RSE reservoirs in response to a widely oxygenated ocean, while emphasizing a non-negligible role of slow sedimentation rate in driving authigenic RSE enrichments in anoxic marine facies locally. From spatial and temporal perspectives, this study further refines the magnitude of the OOE-A in South China, characterizing it as a short-lived oxygenated shelf environment in a redox-stratified ocean. The reexamination of OOE-A offers new insights into understanding other documented oceanic oxygenation events, as well as the cause-and-effect relationship between marine oxygenation and biological evolution during the Neoproterozoic Oxygenation Event.