STRONTIUM ISOTOPES OF BARITE IN NEOPROTEROZOIC CAP CARBONATES RECORD MIXING BETWEEN SNOWBALL DEEPWATER AND GLACIAL MELTWATER

The Snowball Earth hypothesis predicts that the hydrological cycle was effectively shut down for millions of years, inhibiting silicate weathering of the continental crust. In this scenario the deep ocean should have equilibrated with the geochemistry of the mantle through hydrothermal exchange at mid-ocean ridges. During deglaciation, barium from Snowball deepwater mixed with sulfate-rich meltwater to form barite (barium sulfate) fans within cap carbonates. Barite’s mineral structure results in significantly higher Sr concentrations compared to carbonates, which goes along with barite’s resistance to dissolution and diagenesis.

Here we report TIMS Sr isotope data from barite in Marinoan cap carbonates from Mongolia, Mauritania, NW Canada and a Sturtian cap carbonate in Oman. The ⁸⁷Sr/⁸⁶Sr ratio of the cap carbonates is used as a geochemical tracer of mixing between the radiogenic glacial meltwater and the unradiogenic Snowball deepwater. Marinoan cap carbonate deposits exhibit a spread of more radiogenic values due to the shorter Snowball duration and position of barite at the top of the cap. Basal Sturtian deposits exhibit mantle-like values, which is consistent with the longer duration of the Sturtian glaciation.