Suboxic-dominated conditions in the Ediacaran shallow ocean: Geochemistry of pelitic siltstones in the Doushantuo Formation, South China

Abstract

The Ediacaran witnessed a significant oxygenation of the Earth's surface in line with the emergence and diversification of early animals, but the redox conditions of the shallow waters during this period are still uncertain. Here, we present geochemical profiles from neritic pelitic siltstones of the Doushantuo Formation at the village of Xiaoyang on the northwestern margin of the Yangtze Platform in South China. Most sediments show moderate to low FeHR/FeT ratios (≤0.38), low FePy/FeHR ratios (<0.7), and low enrichment factors (EF ≤ 1) for most redox-sensitive elements, suggesting predominately oxygenated shallow waters. However, we also find moderately enriched Re (EF ~10–25) combined with depleted U and Mo (EF ≤ 1), indicating suboxic conditions. Furthermore, a few samples show FeHR/FeT > 0.38, suggesting episodic anoxia in the shallow ocean. Overall, the Xiaoyang section represents a suboxic shallow ocean with occasional episodes of anoxia during the Doushantuo stage, implying that the oxygenation level in the early-mid Ediacaran ocean was still limited, even under some shallow conditions. It also supports the high redox heterogeneity in the early-mid Ediacaran ocean, which could have restricted the evolution and distribution of early metazoans.