Weathering pathways and limitations in biogeochemical models: Application to Earth system evolution
Thesis or dissertation
University of East Anglia
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Current biogeochemical box models for Phanerozoic climate are reviewed and reduced to a robust, modular system, allowing application to the Precambrian. It is shown that stabilisation of climate following a Neoproterozoic snowball Earth should take more than 10(7) years, due to long-term geological limitation of global weathering rates. The timescale matches the observed gaps between extreme glaciations at this time, suggesting that the late Neoproterozoic system was oscillating around a steady state temperature below the snowball threshold. In the model, the period of disequilibrium following snowball glaciations is characterised by elevated ocean nutrient and organic burial rates, providing fair correlation with available geochemical proxies. Extending the analysis to consider carbon removed from the ocean via seafloor carbonatization does not result in a signifi�cant reduction in stabilisation time. Model timeframe is extended over the last 2Ga. Predicted oxygen concentration is shown to depend on the balance between terrestrial and sea floor weathering, which alters the global nutrient delivery rate and therefore global productivity. Under reasonable assumptions, broad predictions for Proterozoic climate fall within, or close to the bounds imposed by geological proxies. A mechanism for atmospheric oxygenation over Earth history is proposed: the combination of declining mantle heat flux and increasing continental area, aided by colonising land biota, results in a steadily increasing ocean nutrient supply, driving increasing rates of organic carbon burial. Methods currently used for assessing Phanerozoic O2 assume only terrestrial weathering fluxes, and are found to give unreasonable results when applied to the Precambrian. Phanerozoic predictions from the model developed here show a significant�cant reduction in the large oxygen peak at 300Ma found in previous studies. This is due to consideration of terrestrial and sea floor weathering balance, and to the longer model timeframe - which allows prediction of crustal abundances in the Cambrian, rather than assuming present day conditions.