SULFUR CYCLING IN THE AFTERMATH OF A NEOPROTEROZOIC SNOWBALL GLACIATION: EVIDENCE FOR A SYN-GLACIAL SULFIDIC DEEP OCEAN?

We have analyzed δ³⁴S(sulfate and pyrite), δ¹⁸O and δ¹³C compositions, and major and trace elemental concentrations, including extractable Fe and Mn phases, in four sections of the Maieberg Formation, the cap carbonate sequence to the Marinoan glaciation in northern Namibia. δ³⁴S_sulfate profiles and other geochemical characteristics in the basal, transgressive cap dolostone (Keilberg Member) are nearly identical in all sections and indicate deposition from a water mass with very low sulfate concentrations. In the overlying interval that consists of rhythmites deposited during the transgressive high-stand, large geochemical disparities occur between sections that we interpret to have been deposited in open-ocean versus restricted settings. In the former, a large negative shift in δ³⁴S_sulfate compositions of ca. 20‰ above the cap dolostone accompanies a change in mineralogy from dolomite to limestone, a sharp decrease in δ¹⁸O and δ¹³C compositions, and a positive spike in Fe and Mn concentrations. In the latter, dolomite persists above the cap dolostone, δ³⁴S_sulfate increases abruptly, δ¹⁸O and δ¹³C compositions are invariant, and a spike in Mn and Fe concentrations is present, but subtle. These contrasting geochemical signatures in coeval sections can be explained by strong lateral chemical gradients that developed as a stratified ocean with cold, euxinic deep water and a cap of warm, oxic, brackish water flooded the continental shelf during the post-glacial transgression. The geochemical disparities between open-ocean and restricted sections diminishes upsection, presumably recording the gradual mixing and homogenization of the ocean.