Multi-proxy records of seawater chemistry and environmental change from Precambrian-Cambrian carbonate-phosphorite and chert-phosphorite assemblages: Implications for enhanced bioproductivity and phosphogenesis
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 skeleton-forming 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 sequences in basinal settings. The previous project has focused to create 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.
The main hypothesis tested in this project is that the availibility of nutrients, mainly phosphate, is essential to the development of metazoans. Hence it is key to understand whether phosphate was in surplus, and when it was a limiting nutrient and to compare the timing of nutrient availibility with that of the evolution of metazoans. Key is also to understand the sources of these nutrients.
Based on this hypothesis, 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, how weathering, hydrothermal activity and/or bioproductivity influenced the oceanic system and biomineralization. This will be achieved by studying well-exposed and well-documented Ediacaran to Early Cambrian sections in South China in close collaboration with the other geochemical subprojects 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, palaeontological data and geochemical proxies that are known to indicate with high sensitivity the effect of skeleton-forming organisms on element cycling in the oceans, may provide new insights which factors trigger the Precambrian-Cambrian bioradiation. We explore the possible processes that have led to the massiv chert deposits in these Early Cambrian deposits. Silicon stable isotopes are suitable proxies for silicification processes, and we will use these to explore variations in silicon sources along stratigraphic sequences, and also explore the silicon stable isotope fractionation associated with mineralisation at the micro scale. These stable isotope observations will be calibrated on siliceous sediment from the modern ocean that has been subjected varying degrees of chert formation in the younger geologic past.