Zhang, Kun; (2024) Proxy development for local-regional redox conditions with application to Cryogenian carbonate rocks. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

How life and oxygen on Earth coevolved remains a challenging question. One of the approaches to decipher the mutual relationship relies on the comprehensive understanding of marine redox evolution throughout Earth history. This requires reliable application and interpretation of geochemical redox proxies. In this thesis, I explore the secular change of Ce anomalies in marine sedimentary rocks and find that it responds sensitively to oceanic redox changes. Nevertheless, I illustrate that carbonate Ce anomalies are sensitive to early diagenesis and hence the interpretation of the proxy should consider the corresponding rare earth element patterns. I also develop the method for extracting carbonate-associated iodine and suggest that 3% EDTA in 3% NH4OH is a reliable stabiliser. I identify organic matter and phosphates as potential significant contaminants, the effects of which can be minimised through oxidative cleaning of 12% NaClO and 0.3 M acetic acid leaching. Applying the developed methods, I present a multiproxy dataset for carbonate rocks from the lowermost Taishir Formation in Mongolia that was deposited following the Sturtian ‘Snowball’ deglaciation. The geochemical data reveal an episode of unprecedented ocean deoxygenation in the immediate aftermath of Sturtian deglaciation, followed by a shift toward less reducing conditions in a postglacial ocean characterised by high phosphate and low sulfate concentrations. The marine redox changes are attributed to varying sulfate and nutrient input and gradual cooling of the atmosphere-ocean system. I propose that sustained primary production and limited oxidant availability facilitated the buildup of recalcitrant dissolved organic carbon pool in the deep ocean, which together with concomitantly enhanced organic matter burial, resulted in the elevated carbonate carbon isotope baseline. The late appearance of diversified eukaryotes after the interval of widespread marine anoxia suggests a possible link between ocean deoxygenation and delayed biological diversification.

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