Multiple controls on the paleoenvironment of the early Cambrian black shale-chert in the northwest Tarim Basin, NW China: Trace element, iron speciation and Mo isotopic evidence

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Research on early Cambrian sedimentary successions of different continental margins are important in better understanding the redox heterogeneity of the early Cambrian ocean, and further the evolutionary relationship between the early animals and their environment. In the present study, we carried out trace element, iron speciation, and Mo isotope analysis of the basal Cambrian Yurtus black shales and cherts in the inner ramp Shiairike section of the northwest Tarim Basin. Our integrated data, combined with previously published data of correlative units in this region, indicate strong basin restriction during deposition in the inner ramp area, with more severe restriction for the upper part of the studied unit. Our investigation supports strong upwelling of nutrient-rich deep water and high primary productivity, with predominately anoxic bottom waters during deposition of the rocks. The anoxia in bottom waters and preservation of organic carbon were likely maintained by both high primary productivity and bottom water stagnancy. Despite the overall anoxic bottom water condition, intermittent oxic intervals may have existed; and the variabilities in redox conditions and related Mn and/or Fe cycling in combination with low and variable [H2S]aq in bottom waters may have attributed to the highly variable δ98Mo values in the studied unit. Furthermore, a high δ98Mo value of 2.14‰ documented in the lower part of the unit is indicative of high δ98Mo of contemporary seawater, pointing to a relatively oxygenated ocean with limited seafloor anoxia.