Title: The global carbon cycle on geologic timescales
Description: Although we are currently limited to a sample size of one, the evolution of complex life appears to require a fairly narrow range of global surface temperatures (0 ± 50°C). How planetary temperatures are maintained on these long timescales remains one of the great outstanding questions in the study of the chemical evolution of the Earth. The stability of global climate over Earth history is even harder to understand given that models of solar evolution predict a large change in the amount of solar energy delivered to the Earth over geologic time. James Walker proposed a solution to this paradox in when he suggested that the increase in solar luminosity could be compensated for by a decline in the concentration of atmospheric CO 2 because the rate at which CO 2 is removed from the atmosphere depends on climate. In this thesis we explore questions of the global carbon cycle on a range of timescales using both analytical and numerical approaches to better understand how the chemistry of the ocean and atmosphere has evolved over Earth history and what this means for the CO 2 -thermostat. Using simple numerical models of the global carbon cycle, we examine the response of the global carbon cycle on short timescales (<100 kyr) to an episode of intense global warming thought to be associated with the massive release of carbon to the ocean-atmosphere system at the Paleocene-Eocene boundary (∼55.5 Ma).

We also explore the effects of the progressive oxidation of the ocean and atmosphere on the distribution of carbonate mineral saturation in the water column and sediments. Finally, we try to understand the global magnesium cycle using measurements of magnesium isotopes in deep-sea pore fluids and pelagic carbonates. Because the global magnesium cycle is intimately tied to the global carbon cycle, reconstructions of the global magnesium cycle in seawater provide unique insights into how processes responsible for observed changes in the global carbon cycle over the Cenozoic. Our results indicate a dynamic magnesium cycle in the Cenozoic driven by changes in the rates of continental weathering and/or the formation of dolomite (Ca,Mg)CO 3 .