Reconstruction of paleo-redox conditions and early sulfur cycling during deposition of the Cryogenian Datangpo Formation in South China

• Published in: Gondwana research 2010-11, Vol.18 (4), p.632-637
• Main Authors: Feng, Lian-Jun, Chu, Xue-Lei, Huang, Jing, Zhang, Qi-Rui, Chang, Hua-Jin
• Format: Article
• Language: English
• Subjects: Cryogenian; Iron species; Molybdenum; Redox conditions; Sulfur isotope
• ISSN: 1342-937X; 1878-0571
• DOI: 10.1016/j.gr.2010.02.011

In South China, the Datangpo black shales (663 Ma–654.5 Ma) were deposited during the Cryognian interglacial time between the Sturtian and Marinoan glaciations. Multi-geochemical proxies, including different iron speciation and relevant ratios (Fe HR/Fe T, Fe P/Fe HR and Fe T/Al ratios) and molybdenum concentrations, were used to reconstruct the paleo-depositional environment of this black shale horizon. The ratios of different iron species (Fe HR/Fe T > 0.38 and Fe P/Fe HR < 0.80) suggest an overall anoxic conditions (ferruginous) over the deposition of the black shales, although intermittent euxinic (Fe HR/Fe T > 0.38 and Fe P/Fe HR ≈ 0.80) and oxic (Fe HR/Fe T < 0.38) intervals could have occurred. Furthermore, Fe T/Al ratios (Fe T/Al ≤ 0.51) confirm that water column may not be persistent euxinia during the deposition of the Datangpo black shales. Meanwhile, molybdenum concentrations show a decreasing trend towards the top of the black shales, reconciling the gradual oxygenating trend during this period as stated above. Compared to δ 34S Py values in the Mesoproterozoic deep ocean, more positive δ 34S Py values of this study may result from a small size of sulfate reservoir. The small-size sulfate reservoir and concurrent enrichment of molybdenum indicate that the ocean chemistry in the Cryogenian Period is similar to that in the Archean Eon.