Variations of redox conditions in the atmosphere and Yangtze Platform during the Ediacaran-Cambrian transition: Constraints from Cr isotopes and Ce anomalies

The Ediacaran-Cambrian transition was characterized by the evolution from Ediacaran soft-body to Cambrian skeletal organisms, and fluctuations of the marine carbon cycle. Marine redox changes should have played an essential role in both ecological and environmental evolutions; however, temporary changes and spatial distribution of redox conditions in the Ediacaran-Cambrian oceans remain elusive and highly debated. In this study, we analyzed chromium (Cr) isotope and cerium (Ce) anomaly profiles in carbonates from the Upper Ediacaran-Lower Cambrian Dengying and Zhujiaqing formations at the Xiaotan section, South China, in an attempt to provide new insights helping unravel Earth's surface redox changes during the Ediacaran-Cambrian transition. Our results show that these carbonates record generally positive delta Cr-53(auth) values (up to 1.15 parts per thousand) with fluctuations and prominent negative Ce anomalies (Ce/Ce-star values down to 0.35), implying a broad increase in oxygenation in the atmosphere and the inner shelf of the Yangtze Platform during the Ediacaran-Cambrian transition. The first appearance and subsequent diversification of the Cambrian animals may have benefited from this oxygenation event. The pronounced fluctuations of delta Cr-53(auth) and delta C-13(carb) values can be explained by episodic oxidation of a deep-water DOC pool in a stratified water column across the chemocline through upwelling, releasing light carbon isotopes and organic-matter-bound isotopically unfractionated Cr (III) into shallow waters.