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

Abstract

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 40‰ (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 51‰) and within the overlying Gruis Formation (34‰) do not appear to correspond to glaciation. The δ34Ssulfate values within the Ombaatjie Formation exhibit shifts over relatively short stratigraphic intervals (tens of meters), varying between ∼15 and 25‰. Cap carbonates from Australia exhibit positive δ34Spyrite trends with amplitudes similar to those of Namibian δ34Ssulfate, although, more data are necessary to firmly establish these δ34S trends as global in nature. δ34Ssulfate 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 δ34Ssulfate. Regardless, the low sulfate concentrations in Otavi carbonates, the high amplitude variability of the δ34Ssulfate curve, and the apparently full reduction of sulfate (as implied from δ34Spyrite data), even in strata low in Corg, 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.