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

Abstract

The drawdown of atmospheric CO2 by the chemical weathering of silicate rocks represents one of
the 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 CO2
concentrations, 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 in
palaeo 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 weathering
rates were examined at the profile-scale using geochemical data from granitic field sites in the
literature, as well as new analyses from a temperate granitic critical zone. Secondly, Reactive
Transport Modelling was used to simulate the response of simple granitic weathering profiles
to differing climate regimes and subsurface architecture conditions. Finally, insights from field
site investigations and Reactive Transport Modelling were applied to the global scale using
a coupled geochemical and climate model, GEOCLIM, to investigate the impact of changing
palaeogeography on potential weathering fluxes through the Cretaceous-Eocene period.
This thesis identifies a strong hydrological control on chemical weathering rates, supporting the
findings of previous studies (Maher, 2010; Godd´eris et al., 2014; Ibarra et al., 2016) and further
challenging the concept of global temperatures as a strong control on chemical weathering rates.
Furthermore, climate conditions determine the morphology of a weathering profile. Under humid
conditions, weathering fronts become sharper and the impact of temperature is amplified. Finally,
palaeogeographical configurations have a substantial impact on long-term CO2 concentrations by
affecting the distribution of precipitation/runoff and thus chemical weathering rates.

Date of Award	1 Oct 2019
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Heather L Buss (Supervisor) & Dan J Lunt (Supervisor)