Manganese Cycling Driven by Fluctuating Redox Chemocline in the Ediacaran Ocean

Abstract

The Ediacaran Period, immediately following the Neoproterozoic Cryogenian glaciations, marked a crucial phase in the Earth's evolutionary history. The paleo-oceanic environment helped shape the habitability, yet the links between oceanic redox state, hydrochemistry, biological activity, and elemental cycling during this time remain poorly understood. Here, we address this scientific issue based on high-resolution geochemical data from black shale-hosted, manganese (Mn)-rich carbonates within the upper Ediacaran Doushantuo Formation, located on the northern margin of the Yangtze Block, South China. Multiple redox proxies indicate that while the open ocean may have been intermittently oxygenated during the late Ediacaran glaciation, the continental margin slope was characterized by redox stratified conditions with a fluctuating redox interface. These fluctuations were driven by climatic shifts. During the deposition of black shales, a relatively warm and humid climate caused a notable increase in freshwater runoff and glacial meltwater, raising water levels and lowering salinity. The influx of bioessential elements carried by terrestrial runoff promoted primary productivity. By contrast, the deposition of the Mn(II) carbonates occurred under colder, more arid conditions, with limited terrigenous nutrient input, leading to lower organic carbon production. This resulted in the deepening of the redox chemocline over the continental shelf, promoting Mn(II) oxidation and subsequent mineralization. Paleoclimate-driven redox fluctuations were key in controlling elemental cycling during the late Ediacaran. This may offer insights into similar processes throughout Earth's history.