Abstract
We measured both mass-dependent isotope fractionation of δ 88Sr ( 88Sr/ 86Sr) and radiogenic isotopic variation of Sr ( 87Sr/ 86Sr) for the Neoproterozoic Doushantuo Formation that deposited as a cap carbonate immediately above the Marinoan-related Nantuo Tillite. The δ 88Sr and 87Sr/ 86Sr compositions showed three remarkable characteristics: (1) high radiogenic 87Sr/ 86Sr values and gradual decrease in the 87Sr/ 86Sr ratios, (2) anomalously low δ 88Sr values at the lower part cap carbonate, and (3) a clear correlation between 87Sr/ 86Sr and δ 88Sr values. These isotopic signatures can be explained by assuming an extreme greenhouse condition after the Marinoan glaciation. Surface seawater, mixed with a large amount of freshwater from continental crusts with high 87Sr/ 86Sr and lighter δ 88Sr ratios, was formed during the extreme global warming after the glacial event. High atmospheric CO 2 content caused sudden precipitation of cap carbonate from the surface seawater with high 87Sr/ 86Sr and lighter δ 88Sr ratios. Subsequently, the mixing of the underlying seawater, with unradiogenic Sr isotope compositions and normal δ 88Sr ratios, probably caused gradual decrease of the 87Sr/ 86Sr ratios of the seawater and deposition of carbonate with normal δ 88Sr ratios. The combination of 87Sr/ 86Sr and δ 88Sr isotope systematics gives us new insights on the surface evolution after the Snowball Earth.
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