Critically Evaluating The Role of The Deep Subsurface In The Chemical Weathering Thermostat

The drawdown of atmospheric CO2 by the chemical weathering of silicate rocks represents one ofthe primary controls on the long-term climate of the Earth (Walker et al., 1981; Berner et al., 1983).Chemical weathering rates are themselves modulated by climate conditions and atmospheric CO2concentrations, with this feedback cycle termed the ”ChemicalWeathering Thermostat”. However,the depth at which weathering occurs, and to what extent different parameters affect weathering,is poorly constrained and hinders our ability to investigate the impact of weathering on climate inpalaeo settings.This thesis investigated weathering-climate interactions at a range of spatial and temporal scales.Initially, the impact of climate on weathering profile morphology and chemical weatheringrates were examined at the profile-scale using geochemical data from granitic field sites in theliterature, as well as new analyses from a temperate granitic critical zone. Secondly, ReactiveTransport Modelling was used to simulate the response of simple granitic weathering profilesto differing climate regimes and subsurface architecture conditions. Finally, insights from fieldsite investigations and Reactive Transport Modelling were applied to the global scale usinga coupled geochemical and climate model, GEOCLIM, to investigate the impact of changingpalaeogeography on potential weathering fluxes through the Cretaceous-Eocene period.This thesis identifies a strong hydrological control on chemical weathering rates, supporting thefindings of previous studies (Maher, 2010; Godd´eris et al., 2014; Ibarra et al., 2016) and furtherchallenging the concept of global temperatures as a strong control on chemical weathering rates.Furthermore, climate conditions determine the morphology of a weathering profile. Under humidconditions, weathering fronts become sharper and the impact of temperature is amplified. Finally,palaeogeographical configurations have a substantial impact on long-term CO2 concentrations byaffecting the distribution of precipitation/runoff and thus chemical weathering rates