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

摘要

In South China, the Datangpo black shales (663Ma-654.5Ma) 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~(34)S_(Py) values in the Mesoproterozoic deep ocean, more positive δ~(34)S_(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.