Continental weathering intensity during the termination of the Marinoan Snowball Earth: Mg isotope evidence from the basal Doushantuo cap carbonate in South China

Abstract

The Neoproterozoic global glaciations (720–635 Ma) represent the most severe icehouse climatic conditions in Earth's history, collectively known as the ‘Snowball Earth events’. The ‘Snowball Earth’ hypothesis (Hoffman et al. 1998) proposed that the termination of the global glaciation was driven by the extremely high atmospheric pCO2 level (300 times of present atmospheric level, PAL) that was accumulated during the global glaciation by volcanic degassing for tens of million years. Because of extremely high temperature, the deglaciation might associate with intense continental weathering that brought abundant bicarbonate and alkaline earth metals (Mg, Ca) to the ocean, resulting in the rapid precipitation of cap carbonate all over the world. Thus, cap carbonate precipitation marks the termination of global glaciation. The deglacial intense continental weathering has been recently supported by Mg isotopes. However, the Mg isotope data also indicate that the deglaciation might have significantly predated the cap carbonate precipitation, and limited data suggest that the weathering intensity was already low during the cap carbonate precipitation. In order to further test whether the cap carbonate precipitation was associated with low weathering intensity, here we analyzed Mg isotopic composition of siliciclastic component (δ26Mgsil) of the Doushantuo cap carbonate in the Yangtze Block, South China. Four cap carbonate sections (the Tongle (TL) drill core, the Fanyang (FY) section, the Wuluo (WL) drill core, and the Jiulongwan (JLW) section), spanning from shelf to basin environments, were analyzed. δ26Mgsil ranges from −0.43‰ to +0.39‰ (n = 10), from −0.31‰ to +0.48‰ (n = 9), from −0.06‰ to +0.10‰ (n = 7), and from −1.42‰ to −0.03‰ (n = 8) for the JLW, FY, WL, and TL samples, respectively. There are negative correlations between Mg/Al and δ26Mgsil in JLW, WL, and TL. The geochemical data indicate that the variation of δ26Mgsil might be attributed to authigenic clay formation during the cap carbonate precipitation. The binary mixing model indicates that the isotopic compositions of terrestrial clays range from 0‰ to +0.3‰, comparable to the values of modern soil clays but significantly lower than the values of the upper Nantuo Formation (up to ~ + 0.90‰), suggesting the moderate to low weathering intensity during the cap carbonate deposition. Our study suggests low atmospheric pCO2 level during cap carbonate precipitation probably due to the consumption by intense continental weathering