The sulfur isotopic composition of Neoproterozoic seawater sulfate: implications for a snowball Earth?

The present study employs a method for analysis of the sulfur isotopic composition of trace sulfate extracted from carbonates collected in Namibia 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 profound climatic events that have been described as the snowball Earth hypothesis. The carbonates in the Otavi Group of Northwest Namibia contain 3-295 ppm sulfate. Positive excursions, to a high of 40parts per thousand (CDT), occur above the lower (Chuos Formation) and upper (Ghaub Formation) glacial intervals in the Rasthof and Maieberg cap carbonates, respectively. Positive excursions at the top of the Rasthof Formation (reaching 51parts per thousand) and within the overlying Gruis Formation (34parts per thousand) do not appear to correspond to glaciation. The delta(34)S(sulfate) values within the Ombaatjie Formation exhibit shifts over relatively short stratigraphic intervals (tens of meters), varying between similar to15 and 25parts per thousand. Cap carbonates from Australia exhibit positive delta(34)S(pyrite) trends with amplitudes similar to those of Namibian delta(34)S(sulfate), although, more data are necessary to firmly establish these delta(34)S trends as global in nature. delta(34)S(sulfate) excursions found in Namibian cap carbonates are consistent with the snowball Earth hypothesis in that they appear to reflect nearly complete reduction of sulfate in an isolated, anoxic global ocean, although, there are other mechanisms that may have facilitated these large shifts in delta(34)S(sulfate). Regardless, the low sulfate concentrations in Otavi carbonates, the high amplitude variability of the delta(34)S(sulfate) curve, and the apparently full reduction of sulfate (as implied from delta(34)S(pyrite) data), even in strata low in C-org, suggest that Neoproterozoic oceanic sulfate concentrations were much lower than modern values. Additionally, the buildup of ferrous iron and banded-iron formations during the Sturtian glacial event would indicate that Fe supply exceeded sulfide availability during the glacials and/or that all sulfide was fixed and buried. This could be construed as further evidence in support of low oceanic sulfate (and sulfide) at this time. (C) 2002 Elsevier Science B.V. All rights reserved.