Stratification and mixing of a post-glacial Neoproterozoic ocean: Evidence from carbon and sulfur isotopes in a cap dolostone from northwest China

To improve our knowledge about the geochemical and environmental aftermath of Neoproterozoic global glaciations, we analyzed stable isotopes (delta C-13, delta O-18,delta S-34) and elemental concentrations (Ca, Mg, S, Sr, Fe, and Mn) of the similar to 10-m-thick Zhamoketi cap dolostone atop the Tereeken diamictite in the Quruqtagh area, eastern Chinese Tianshan. Available chemostratigraphic data suggest that the Tereeken diamictite is probably equivalent to the Marinoan glaciation. Our new data indicate that organic and carbonate carbon isotopes of the Zhamoketi cap dolostone show little stratigraphic variations, averaging -28.2%omicron and -4.6%omicron, respectively. In contrast, sulfur isotopes show significant stratigraphic variations. Carbonate associated sulfate (CAS) abundance decreases rapidly in the basal cap dolostone and delta S-34(CAS) composition varies between +9%omicron and + 15%omicron in the lower 2.5 m. In the overlying interval, delta S-34(CAS) abundance remains low while delta S-34(CAS) rises similar to 5%omicron and varies more widely between + 10%omicron and +21%omicron. The range of delta S-34(py) of the cap dolostone overlaps with that of delta S-34(CAS) but direct comparison shows that delta S-34(py) is typically greater than delta S-34(CAS) measured from the same samples. Hypotheses to explain the observations must account for both the remarkable sulfur isotope enrichment of pyrites and the inverse fractionation. We propose that CAS and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin or a basin with a sulfate minimum zone. In this model, CAS was derived from shallow, oxic surface waters with moderate sulfate concentration and depleted in S-34 due to the post-glacial influx of sulfur from continental weathering. In contrast, pyrite was derived from anoxic bottom waters (or a sulfate minimum zone) with low sulfate concentration and S-34 enrichment due to long-term syn-glacial sulfate reduction. The rapid shift in CAS abundance and sulfur isotope composition within the cap dolostone is interpreted to reflect the mixing of the two reservoirs after initial deglaciation. Comparison with other post-Marinoan cap carbonates shows significant spatial heterogeneity in delta S-34(CAS), which together with strong temporal variation in delta S-34(CAS), points to generally low sulfate concentrations in post-Marinoan oceans. (c) 2007 Elsevier B.V. All rights reserved.