Early Cambrian sedimentary rocks in South China: A link between oceanic oxygenation and biological explosion

Early Cambrian basin in South China has deposited successive sedimentary rocks, which not only recorded geochemical information of the variation of palaeo-ocean environment, but also preserved fossils of most phyla of modern animals, such as the large-body Chengjiang and Qingjiang biotas. Although the relation between gradually oxic Cambrian Ocean and biotic diversity is recognized, the temporal and spatial variations of redox condition, as well as the influence of the variations on the biotic evolution during the early Cambrian, remain unclear. Here, we focus on all of the early Cambrian sedimentary rocks deposited in inner-shelf, outer-shelf, slope, and deep basin in the South China Block. According to the geochemical datasets, such as iron speciation, major- and trace element, Hg contents and Hg isotopic data, we have summarized and discussed the changing of marine redox condition, hydrothermal activity and basin restriction during the Cambrian Terreneuvian to the early Cambrian Age 3. In the early Cambrian Terreneuvian, oxic environment appeared in the inner shelf facies, accompanied by the occurrence of small shelly fossils. Then, hydrothermal activities occurred in the center South China block of the outer-shelf and slope facies with a ferruginous environment in the late Terreneuvian and early Cambrian Age 3. However, the water column in the block margin of this two facies was euxinic during this period. In the early Cambrian Age 3, oxic water column had expanded to the slope and deep basin facies. Based on the compiled data, we propose that strong hydrothermal activities had provided abundant of Fe and other nutrients, and terrigenous inputs which also provided amounts of terrigenous sulfur that led to distinct environments in the slope facies. In the early Cambrian Age 3, the expanding oxic water column and nutrients after the hydrothermal activities stimulated the evolution of the biological diversity together, such as the Qingjiang and Chengjiang biotas. These variations highlight the effects of redox condition and hydrothermal activities on the biotic flourish of the early Cambrian.