A coupled geomorphic and geochemical model for testing the dominant controls on chemical weathering rates in eroding landscapes

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A vigorous debate has emerged over the primary driver of chemical weathering rates. One hypothesis states that because weathering reactions are driven by pore water chemistry, climate, specifically rainfall, controls chemical weathering rates. In contrast, another hypothesis states that chemical weathering is driven by the supply of 'fresh' minerals to the weathering zone, and the dominant driver of chemical weathering is physical erosion. Studies evaluating the relative importance of these two hypotheses have had limited success. On the one hand, detailed geochemical studies that focus on pore water chemistry make no provision for geomorphic processes such as physical denudation and lateral sediment transport. Such studies cannot yield insights into the mechanisms that drive increased chemical weathering rates from eroding landscapes. This is because chemical weathering rates are spatially heterogeneous (as a century of soil science can attest) and eroding materials continuously move laterally through parts of the landscape with varying chemical weathering rates. On the other hand, studies focusing on weathering rates driven by erosion are largely based on empirical studies of basin wide weathering rates and make no provision for weathering reactions. To truly examine the relative importance of climate and physical erosion on chemical weathering rates, one must account for both weathering reactions and the generation and transport of sediment. In this study the PI proposes, for the first time, to combine a state of the art geochemical model with a detailed geomorphic model. The proposed model will be capable of predicting the coupled geochemical and geomorphic evolution of hillslope soils using both end member chemical weathering hypotheses. To test the model, and the relative importance of the two drivers of chemical weathering, a field site has been identified where the two end member hypothesis predict contrasting spatial distributions of chemical weathering. This field site has a uniquely comprehensive series of both geomorphic and geochemical measurements: at the site measurements exist to independently calibrate the model and compare model results with long term chemical weathering rates and solid state chemistry. Thus, by using a combination of state of the art numerical modelling and an exhaustive geochemical and geomorphic dataset, this project will test if climate (via rainfall and pore water chemistry) or physical erosion rates are dominant in controlling chemical weathering rates in an eroding landscape.