Stratification and mixing of a post-glacial Neoproterozoic ocean: Evidence from carbon and sulfur isotopes in a cap dolostone from northwest China |
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| Authors: | Bing Shen Shuhai Xiao Alan J Kaufman Huiming Bao Chuanming Zhou Haifeng Wang |
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| Institution: | 1. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan;2. Precambrian Ecosystem Laboratory, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan;3. Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan;4. Department of Geology and Key Laboratory for Continental Dynamics, Northwest University, Xi''an 710069, China;5. Department of Earth Science and Astronomy, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan;1. Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, United States;2. Department of Earth and Planetary Sciences/GEOTOP, McGill University, Montreal, QC H3A 2T5, Canada;3. Department of Geological Sciences and Geological Engineering, Queen''s University, Kingston, Ontario K7L 3N6, Canada;4. Princeton University, Guyot Hall, Princeton, NJ 08544, United States |
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| Abstract: | To improve our knowledge about the geochemical and environmental aftermath of Neoproterozoic global glaciations, we analyzed stable isotopes (δ13C, δ18O, δ34S) and elemental concentrations (Ca, Mg, S, Sr, Fe, and Mn) of the ～ 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‰ and ? 4.6‰, respectively. In contrast, sulfur isotopes show significant stratigraphic variations. Carbonate associated sulfate (CAS) abundance decreases rapidly in the basal cap dolostone and δ34SCAS composition varies between + 9‰ and + 15‰ in the lower 2.5 m. In the overlying interval, CAS abundance remains low while δ34SCAS rises ～ 5‰ and varies more widely between + 10‰ and + 21‰. The range of δ34Spy of the cap dolostone overlaps with that of δ34SCAS, but direct comparison shows that δ34Spy is typically greater than δ34SCAS 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 34S 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 34S 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 δ34SCAS, which together with strong temporal variation in δ34SCAS, points to generally low sulfate concentrations in post-Marinoan oceans. |
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