CONSTRAINING PERTUBATIONS TO THE SULFUR CYCLE IN THE AFTERMATH OF SNOWBALL EARTH: GEOCHEMISTRY OF THE NEOPROTEROZOIC MAIEBERG FORMATION, NAMIBIA

Significant variations in the sulfur isotopic composition of trace sulfate bound in post-glacial carbonates suggest that sulfur cycling in the late Neoproterozoic may have been strongly influenced by global glaciations. Here we examine major and trace elemental concentrations (Ca, Mg, Fe, Mn, Sr, Ba), the concentration of distinct Fe phases, and carbon and oxygen isotopic compositions in order to constrain the oceanographic conditions responsible for excursions in d³⁴S_sulfate of 16 to 20 per mil in the Maieberg cap carbonate in Namibia.

In the locality studied, the Maieberg Formation contains approximately 30 meters of cap dolostone (Keilberg Member) followed by 95 meters of limestone and then 80 meters of dolostone. The transition from cap dolostone to limestone attends a sharp increase in siliciclastic input and Fe³⁺ phases, negative shifts in both d¹³C and d¹⁸O, and a 16 per mil decrease in d³⁴S_sulfate from 31 to 15 per mil. d³⁴S_sulfate then increases gradually through the limestone before rising abruptly to >40 per mil in the overlying dolostone. The rise in d³⁴S_sulfate through the limestone correlates with a rise in Fe²⁺.

We speculate that the large shifts in d³⁴S_sulfate reflect a complex interplay between riverine inputs, oceanic upwelling and the post-glacial transgression of an ocean with low sulfate concentrations enriched in ³⁴S. The enriched d³⁴S_sulfate values in both the cap and upper dolostones are consistent with the snowball Earth hypothesis in that they appear to reflect nearly complete reduction of sulfate in an anoxic global ocean. We interpret the abrupt decrease in d³⁴S_sulfate, together with increases in siliciclastics and Fe³⁺ phases, as recording a dramatic increase in ³⁴S-depleted continental run-off to an ocean with low sulfate concentrations.