Exploring climate variability during the Marinoan glaciation: A study of black shale geochemistry

Abstract

The Marinoan glaciation is the younger of the two extraordinary glaciations during the Cryogenian Period. It has been debated whether the glacial world was featured by a complete ice cover or if parts of the oceans remained ice free. Here, we present petrographic and geochemical data of two black shale units found within diamictites of the Marinoan-aged Nantuo Formation in South China. The lower black shales exhibit low values of the chemical index of alteration (CIA) and muted enrichments of uranium, while the upper black shales show elevated CIA and uranium concentrations. These distinct geochemical signals are best explained by intensified chemical weathering and terrestrial runoff, which might be linked to global/local temperature changes. Our study is consistent with the notion that significant climate variations occurred during the Marinoan glaciation. The cyclic isolation and expansion of habitable areas might have acted as a biodiversity pump, contributing to the rapid radiation of eukaryotic life in the aftermath of the glaciation.