Abstract
The mineral assemblages of lacustrine mixed sediments have vital significance for reconstructing paleo-lake environments, and typical lacustrine mixed sediments deposited during the Eocene are distributed throughout the Yingxi area in the western Qaidam Basin. Accordingly, the framboid size distributions and sulfur isotope (δ34Spy) characteristics of pyrite in addition to the carbon isotope (δ13Ccarb) characteristics and redox-sensitive element ratios (Th/U, V/(V + Ni) and V/Cr) of authigenic carbonates were investigated to reconstruct the high-frequency redox variations in cyclic lacustrine sediments.Corresponding to different redox conditions, three types of mixed sediments developed during transgression and regression stages within the saline basin in the Yingxi area during the Eocene: mixed terrigenous clasts formed in oxic-dysoxic water column conditions during lake transgression, mixed carbonates formed in anoxic water column conditions during early-stages lake regression, and mixed evaporites formed in anoxic-sulfidic water column conditions during middle-late-stage lake regression. The redox proxies correlate well with the water salinity, exhibit little or no correlation with the water temperature, and show an incongruity with the water depth, suggesting that the redox conditions were predominantly controlled by the water salinity rather than the water depth or temperature. Therefore, based on the water redox condition variations and the salinity, three dynamic formation models of the high-frequency sedimentary cycles in a saline basin are established: a deep-water, low-salinity, oxic-dysoxic model (Model I), a medium-water, medium-salinity, anoxic model (Model II) and a shallow-water, high-salinity, anoxic-sulfidic model (Model III). Because the climate gradually turned dry and hot from lake transgression to regression, the continued closure of the basin enhanced evaporation and salinization. In addition, mixed terrigenous clasts, mixed carbonates and mixed evaporites deposited successively. Correspondingly, salinity stratification gradually developed within the lake water, increasing the extent of bacterial sulfate reduction in the bottom water.
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