THE SULFUR ISOTOPIC COMPOSITION OF TRACE SULFATE AND PYRITE ASSOCIATED WITH NEOPROTEROZOIC 'CAP CARBONATES' FROM NAMIBIA, SOUTH AUSTRALIA AND DEATH VALLEY, CA, USA: IMPLICATIONS FOR A SNOWBALL EARTH

The present study employs a method for analysis of the sulfur isotopic composition of trace sulfate extracted from carbonates in order to document secular variations in the sulfur isotopic composition of Neoproterozoic oceanic sulfate and to assess variations in the sulfur cycle that may have accompanied the "Snowball Earth". The sensitivity of d³⁴S_sulfate to biogeochemical change makes it a candidate to evaluate aspects of the Snowball Earth hypothesis, particularly whether or not the oceans were effectively isolated from riverine runoff as the result of long-term global sea-ice cover.

Trace sulfate from carbonate samples in Namibia (Rasthof, Gruis, Ombaatjie and Maieberg Fms), South Australia (Nuccaleena Fm) and Death Valley, CA (Noonday Dolomite) were analyzed for their sulfur isotopic composition. Dramatic positive excursions, reaching 40‰, appear stratigraphically above some, but not all, of the glacial intervals in what have been termed 'cap carbonates.'

We hypothesize that the large positive d³⁴S_sulfate excursions found in some Neoproterozoic cap carbonates and the relationship between d³⁴S_sulfate and d¹³C_carbonate generally are consistent with hypothesized "Snowball Earth" events, although, there are other mechanisms that could have caused these large shifts in d³⁴S_sulfate. If the hydrologic cycle was disabled because Earth's oceans were mostly covered with ice for millions of years, the source of relatively depleted d³⁴S from the continents via pyrite weathering would diminish or stop. If sulfate-reducing bacteria continued to function and preferentially dissimilate ³²S, the isolated oceanic sulfate pool would be driven to more enriched d³⁴S values and the concentration decreased. During deglaciation, ocean stratification probably broke down and deepwater overturn occurred. As a result, the trace sulfate concentrations associated with post-glacial carbonates was lower and enriched in ³⁴S. As the delivery of sulfur via riverine inputs was reestablished, the d³⁴S_sulfate gradually returned to more typical values of 20‰ perhaps over tens of millions of years. Neoproterozoic d³⁴S_pyrite records from western Canada and Australia (Amadeus Basin) parallel our d³⁴S_sulfate results from Namibia.