Determination of 88Sr/86Sr mass-dependent isotopic and radiogenic isotope variation of 87Sr/86Sr in the Neoproterozoic Doushantuo Formation

We measured both mass-dependent isotope fractionation of delta Sr-88 (Sr-88/Sr-86) and radiogenic isotopic variation of Sr (Sr-87/Sr-86) for the Neoproterozoic Doushantuo Formation that deposited as a cap carbonate immediately above the Marinoan-related Nantuo Tillite. The delta Sr-88 and Sr-87/Sr-86 compositions showed three remarkable characteristics: (1) high radiogenic 87Sr/86Sr values and gradual decrease in the Sr-87/Sr-86 ratios, (2) anomalously low delta Sr-88 values at the lower part cap carbonate, and (3) a clear correlation between Sr-87/Sr-86 and delta Sr-88 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 Sr-87/Sr-86 and lighter delta Sr-88 ratios, was formed during the extreme global warming after the glacial event. High atmospheric CO2 content caused sudden precipitation of cap carbonate from the surface seawater with high Sr-87/Sr-86 and lighter delta Sr-88 ratios. Subsequently, the mixing of the underlying seawater, with unradiogenic Sr isotope compositions and normal delta Sr-88 ratios, probably caused gradual decrease of the Sr-87/Sr-86 ratios of the seawater and deposition of carbonate with normal delta Sr-88 ratios. The combination of Sr-87/Sr-86 and delta Sr-88 isotope systematics gives us new insights on the surface evolution after the Snowball Earth. (C) 2007 International Association for Gondwana Research. Published by Elsevier B.V All rights reserved.