He, Tianchen; (2017) Isotopic constraints on the palaeoenvironmental conditions during the Cambrian radiation of animals. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Debates around the triggers for the Cambrian radiation have long focused on oxygenation events, weathering history and marine chemistry during the Ediacaran–Cambrian transition. However, high-resolution palaeoenvironmental constraints remain unsatisfactory regarding both long-term evolution and short-term shifts that directly tie into the early Cambrian fossil records and geological events. In the following thesis, a multi-proxy approach combining stratigraphic and geochemical analyses is presented that investigates the marine successions in Siberia and China, and draws on the environmental backdrop and triggers during the animal radiation events and Botomian–Toyonian extinction. This study miniaturises and validates the existing carbonate-associated sulphate (CAS) extraction methods, and suggests that a thorough leaching in NaCl solution reduces the risk of contamination from non-CAS sulphur phases. Applying high-fidelity geochemical methods and rigid screening techniques for diagenesis evaluation, this thesis presents paired, continuous and high-resolution carbonate carbon, CAS sulphur and carbonate strontium isotope trends for the Cambrian stages 2-4. These new data suggest that cyclical redox fluctuations exerted a direct control on the episodic pattern of early animal radiations, while low oceanic sulphate concentrations below modern levels likely persisted through the early Cambrian. High continental weathering flux may have dominated through Cambrian stages 2 and 3, implying high marine ion (e.g. Ca2+) and nutrient availability in the coeval oceans that could have facilitated animal diversification and calcium- favouring biomineralisation. This thesis also reports the first global seawater LOWESS- fitted 87Sr/86Sr curve from the latest Ediacaran to Cambrian Stage 4. New stratigraphic interpretations and detrital zircon ages provide the first robust constraints on the timing and position of the ‘Great Unconformity’ on the North China Craton (NCC). Provenance analysis further demonstrates the affinity between NCC and eastern Gondwana during the Cambrian Period.

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