Sulfur cycling in the aftermath of a 635-Ma snowball glaciation: Evidence for a syn-glacial sulfidic deep ocean

We have analyzed delta S-34 (sulfate and pyrite), delta O-18(carbonate) and delta C-13(carbonate), 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. delta S-34(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 delta S-34(sulfate) of similar to 20 parts per thousand above the cap dolostone accompanies a change in mineralogy from dolomite to limestone, a sharp decrease in delta O-18 and delta C-13, and a positive spike in Fe and Mn concentrations. In the latter, dolomite persists above the cap dolostone, delta S-34(sulfate) increases abruptly, delta O-18 and delta C-13 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 cold, euxinic deep water and a cap of warm, oxic, brackish water that flooded the continental shelf during the post-glacial transgression. The geochemical differences between open-ocean and restricted sections diminishes upsection, presumably recording the gradual mixing and homogenization of the waters along the Otavi platform. (c) 2006 Elsevier B.V. All rights reserved.