Ecological and environmental change recorded through stratigraphy and sedimentology at selected Ediacaran-Cambrian boundary sections in East and Central Asia

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We suggest investigating three aspects of sedimentary-stratigraphic change at the Precambrian-Cambrian Boundary (PCB; ca. 543 Ma) that complement geochemical and paleobiological subprojects: (1) the effects of the agronomic revolution, recorded by paleoichnology; (2) the significance of karsting at the PCB on the Yangtze Platform; and (3) the comparative stratigraphic architecture of Ediacaran-Cambrian sections from the Yangtze Platform, the Tarim/Yili microcontinents, and southern Kazakhstan.(1) The fossil record of Ediacaran to Lower Cambrian shelf deposits demonstrates how significant volumes of fine-grained sediment became incorporated in the global chemicaloceanographic interchanges through the demise of microbially sealed surfaces during the agronomic revolution. To date, the timescale, facies dependency and degree of this enlargement of the Earth system cannot be incorporated in geochemical modeling calculations nor can it be convincingly linked to the expansion of the metazoa. In this subproject, we will link the bioturbation index from key sections across the PCB to geochemical proxies and relate those in turn to the chemostratigraphic record from profiles in West China (Tarim and Yili microcontinents) and Kazakhstan (Karatau Range).(2) Because the contact between Ediacaran and Cambrian deposits in the fossiliferous shallow-water facies the Yangtze Platform is virtually everywhere unconformable and karsted, resulting from one or several periods of relative sea level fall and subaerial exposure of Dengying Fm. carbonates, approximately 6-10 Ma of critical geologic time at the PCB is apparently missing. This record can be studied in numerous well-exposed stratigraphic sections of Sichuan, Guizhou and Hubei Provinces. We suggest to assess this event to infer duration and intensity of chemical weathering and compare our results to other well-exposed PCB reference sections worldwide. Our results, complemented by a concurrent biostratigraphic study (subproject Steiner), will fill in a neglected but potentially critical gap in our understanding of the metazoan base-Cambrian radiation in South China.