Robinson, S; (2017) Marine carbon isotopes, carbonate mineralogy and indices of chemical weathering during the Tonian and Cryogenian periods: examples from North and South China. Doctoral thesis , UCL (University College London).

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Abstract

In the mid to late Neoproterozoic, Earth is thought to have experienced climate change more extreme than at any other time in history, swinging from warm to globally freezing conditions and back again twice, possibly as many as four times. Yet life in the late Neoproterozoic flourished as never before. Not much later, phylogenetically disparate and morphologically complex animals appeared as if from nowhere, their descendants having survived millions of years of temperatures below zero. The thesis examines this scenario. Shallow-marine carbonate chemistry in the Neoproterozoic is characterised by a predisposition toward dolomite and huge variability in carbon-isotope composition. Carbonate rocks terminating the putative ‘snowball’ glaciations manifest both characteristics, but why 13C-depleted dolomites immediately followed is not well understood. The first part of the thesis considers the carbonate system in the light of two early-Neoproterozoic (Tonian) successions from North China: a mixed-clastic-carbonate succession that abruptly switched from calcite to dolomite, and another, not previously described, in which carbonate δ13C went from +4‰ to -6‰ and back to 0‰. The second part focuses on weathering. The carbonate strontium isotope record from the mid Tonian onward suggests that silicate weathering – a function of temperature – was intense, even during the Cryogenian. Siliciclastic sediments reflect weathering more directly. The continuous record of such sediments from South China analysed here, spanning the late Tonian to the start of the Ediacaran, enable silicate weathering to be tracked through the critical transitions when temperatures supposedly plunged. They show no abatement in weathering. However, even in the period before the formations analysed, most of the chemical alteration took place after deposition. Conclusions include the following. (1) Carbonate and siliciclastic chemistry were interrelated via the common factor of high atmospheric CO2. Seas were alkaline, and silicate dissolution took place under water as well as on land. Dolomite, inhibited today by kinetic barriers and low activities of Mg2+ and CO32–, was favoured by high temperatures and high concentrations of these ions, in many cases precipitating direct from seawater. (2) Calcium carbonate was mostly aragonite or vaterite, and primarily authigenic. (3) Carbon isotope ratios were controlled by water depth, varying from around -6‰ in deep water to high positive values in evaporitic settings. (4) While the ocean subsurface appears to have been anoxic through the Precambrian, modern levels of carbon isotope fractionation and organic carbon burial do not support the view that atmospheric oxygen was low. (5) Faulting of the continental crust during the Neoproterozoic produced hydrothermal enrichments of 87Sr, Fe, Mn and Mg (among other elements), and cap carbonates exemplify such enrichment. They were depleted in 13C, initially, because sea-level in their respective rift settings was high. Temperatures did not fall in the approach to the Sturtian and Marinoan intervals, and epicontinental water bodies in the Cryogenian were never frozen.

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