Evolution from an anoxic to oxic deep ocean during the Ediacaran–Cambrian transition and implications for bioradiation

Abstract

The Ediacaran–Cambrian transition, one of the most critical intervals in Earth's history, is marked by dramatic biological, oceanic and geochemical turnovers. Here high-resolution carbon and sulfur isotopic data respectively for organic carbon and pyrite, and iron speciation data are presented from the deep-water Liuchapo and Niutitang Formations on the Yangtze block, South China. The carbon isotopic data, together with biostratigraphic and radiometric dating, offer the compelling evidence for the placement of Ediacaran–Cambrian boundary within the Liuchapo Formation (chert succession), and for its correlation with shallow-water equivalents elsewhere. In this context, iron speciation and sulfur isotopic data further suggest a predominant anoxic and ferruginous deep ocean over the transitional time until the middle Early Cambrian (Atdabanian or Stage 3) when the deep ocean was rapidly oxygenated. Coincidently, during this interval, large-body metazoans (i.e., sponges) abruptly appeared in the deep ocean, which was temporally associated with the highly diversified large-body skeletonized animals (i.e., Chengjiang Biota) which colonized in shallow-water niches particularly in southwestern China. This scenario suggests a causal link between deep oceanic oxygenation and the explosive diversification of large-body skeletonized organisms in the Early Cambrian.