Chen, Xi; (2024) An improved Precambrian seawater Sr isotope curve, guiding into a Mesoproterozoic weathering study. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The continental weathering process plays a pivotal role in Earth's systems, governing global climate changes and carbon cycle, determining nutrient supply to ecosystems, and potentially impacting biological evolution. However, our understanding of the weathering processes during Precambrian times has been impeded, partly due to restricted investigations and utilisation of geochemical weathering proxies (e.g., Sr, Li isotopes) in Earth’s deep time. In this thesis, an advanced study of the Precambrian strontium isotope stratigraphy (SIS) is presented, involving data compilation, leaching method development, and its application in identifying the weathering event in conjunction with Li isotopes. The existing Precambrian strontium isotope curve, which is still widely used for reconstructing continental weathering and tectonic cyclicity over time, is now almost twenty years old and has inadequate temporal resolution. In this study, 2422 strontium isotope values were compiled for Precambrian marine carbonate rocks published since 2002. Each data point underwent a rigorous quality assessment, leading to an improved Precambrian seawater 87Sr/86Sr curve primarily derived from high-quality data. Using the updated database, the seawater Sr isotope curve was subsequently normalised against its primary sources to effectively illustrate the shifting proportions between continental weathering and mantle input. The normalised curve informs an early initiation of continent weathering, lithology-dominated Sr isotope fluctuation and a CO2 outgassing controlled integrated weathering rate. Moreover, to ensure the least altered carbonate fraction of ancient marine sediment samples can be extracted effectively and to facilitate the widespread application of SIS, a leaching technique was developed for the commonly encountered but less-favoured argillaceous and dolomitic limestones. The result shows that the first 10%–30% of the carbonate fraction following an NH4Ac prewash yielded the lowest and most distinctly unaltered seawater 87Sr/86Sr ratios. Applying the newly developed method, Sr isotope ratios through four carbonate sections of the c. 1.57 Ga Gaoyuzhuang Formation, North China Craton were measured, alongside paired Li isotopes. The new data demonstrate a positive seawater Sr and Li isotope excursion, which indicates a weathering event characterised by an enhanced weathering rate coupled with reduced weathering congruency. This weathering event coincides with a negative C isotope excursion and the appearance of decimeter-scale, multicellular eukaryotic fossils, highlighting interactions among tectonics, weathering, climate, carbon cycling, and biological innovation during the Mesoproterozoic Era.

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