Mercury anomalies across the Cryogenian-Ediacaran boundary in South China

Abstract

The Cryogenian-Ediacaran transition represents a pivotal geological period in the evolution of global climate, ocean chemistry, and early organisms. The transition is concurrent with the change from Marinoan glacial deposits to overlying cap carbonate/dolomite, which is followed by the appearance of novel animal and algae fossils. Unusual carbon cycling during the deposition of cap carbonate/dolomite is recorded by prominent negative carbonate carbon isotope (δ13Ccarb) anomalies. The mechanisms which drove melting of the Marinoan icesheets remain uncertain. To explore the cause of this dramatic climate warming and its effect on oceanic biogeochemical cycles, we measured Hg concentrations and isotopes, along with major and trace elements, of the sedimentary succession across the Cryogenian-Ediacaran boundary at the Jiulongwan and Huajipo sections, South China. Hg concentrations show spikes with a ∼ 2 times increase at the top of the Marinoan Nantuo Formation at both sections, which are likely associated with organic matter drawdown rather than enhanced volcanism as indicated by increased TOC contents and similar Hg isotopic signature as those of background Hg deposition. A conspicuous negative shift in δ202Hg along with a positive shift in Δ199Hg are observed in the cap dolomite of the Doushantuo Formation at both sections, which are ascribed to contribution of Hg from anoxic deep water and Hg associated with dissolved organic carbon (Hg-DOC), due to upwelling and oceanic oxygenation after deglaciation. Our Hg data argue against a sudden large igneous province (LIP) event causing Marinoan deglaciation. Results also indicate enhanced upwelling and oceanic oxygenation event during the Cryogenian to Ediacaran transition.