Multi-proxy records of seawater chemistry and environmental change from Precambrian-Cambrian carbonate-phosphorite and chert-phosphorite assemblages: Implications for enhanced bioproductivity and phosphorgenesis

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The Precambrian to early Cambrian transition comprises an episode of major environmental changes which are believed to be relevant for the most prominent bioradiation in Earth’s history: from an ocean in which algae and microbes were the dominant form of life to one in which significant skeletonforming higher life spread. Concomitant with these global changes is the onset of biomineralization and widespread calcium phosphate deposition, the latter which is often associated with the deposition of abundant bedded chert in basinal settings. The previous project has focused to reconstruct Total Sediment Phosphorus (TSP) curves, together with sedimentary and geochemical parameters. It documented three important phosphogentic events, with evidence for excess phosphorus in the Precambrian-Cambrian ocean covering the Yangtze Platform. However, so far, neither the source of phosphate for massive phosphorite nor the importance and function of micro-organisms in phosphogenesis are known with certainty. The same holds true for the origin and formation of associated cherts. Based on these results, our aims are twofold: (1) we aim to explore the relevance of primary depositional (anorganic and/or biomediated) versus diagenetic processes leading to the formation of massive phosphorites, and (2) investigate how the remarkable retrieval and concentration of dissolved P and Si can be achieved, and how weathering, hydrothermal activity and/or bioproductivity influenced the oceanic system and biomineralization. This will be achieved by studying well-exposed and welldocumented Ediacaran to Early Cambrian sections in South China in close collaboration with the other geochemical subprojects and within a multi-proxy approach. To identify regional versus global biogeochemical phenomena, additional samples using the same proxies from the Tarim block of NW China, from Kazachstan and India will be integrated. Particularly, the use of sedimentological and palaeontological data and geochemical proxies that are known to indicate the effect of skeletonforming organisms on element cycling in the oceans with high sensitivity may provide new insights which factors triggered the Precambrian-Cambrian bioradiation.