The Importance of Hillslope Scale in Responses of Chemical Erosion Rate to Changes in Tectonics and Climate

Abstract

Chemical erosion is of wide interest due to its influence on topography, nutrient supply to streams and soils, sediment composition, and Earth's climate. While controls on chemical erosion rate have been studied extensively in steady-state models, few studies have explored the controls on chemical erosion rate during transient responses to external perturbations. Here we develop a numerical model for the coevolution of soil-mantled topography, soil thickness, and soil mineralogy, and we use it to simulate responses to step changes in rates of rock uplift, soil production, soil transport, and mineral dissolution. These simulations suggest that tectonic and climatic perturbations can generate responses in soil chemical erosion rate that differ in speed, magnitude, and spatial pattern and that climatic and tectonic perturbations may impart distinct signatures on hillslope mass fluxes, soil chemistry, and sediment composition. The response time of chemical erosion rate is dominantly controlled by hillslope length and is secondarily modulated by rates of rock uplift, soil production, transport, and mineral dissolution. This strong dependence on drainage density implies that a landscape's chemical erosion response should depend on the relative efficiencies of river incision and soil transport and thus may be mediated by climatic and biological factors. The simulations further suggest that the timescale of the hillslope response may be long relative to that of river channel profiles, implying that chemical erosion response times may be limited more by the sluggishness of the hillslopes than by the rate of signal propagation through river channel profiles. Plain Language Summary The flux of dissolved solutes out of mountainous regions is of wide interest due to its influence on topography, ecosystems, sediment, and climate. Here we develop a new model to calculate the responses of chemical erosion rate to changes in rates of rock uplift, soil production, soil transport, and mineral dissolution. These simulations suggest that tectonic and climatic perturbations can generate distinct responses in soil chemical erosion rate. They also show that the time it takes chemical erosion rates to respond to changes in climate or tectonics is primarily controlled by the length of the hillslope and secondarily by rates of rock uplift, soil production, soil transport, and mineral dissolution. This strong dependence on hillslope length implies that chemical erosion responses may be affected by climate and life to the extent that these factors affect how easily rivers cut through rock and how easily soil moves downhill. This further implies that chemical erosion response times may be limited mainly by how slowly hillslopes respond to changes in climate and tectonics, rather than by how slowly river networks do.

Modeling Cosmogenic Nuclides in Transiently Evolving Topography and Chemically Weathering Soils

“…Response times in the climatic experiment were longer than those in the tectonic experiment for all variables (Figure 7g–7l). This differs from a similar experiment in Ferrier and Perron (2020) and is likely due to the introduction of more complex topography and different parameter values. The mean (± s.d.)…”

“…Numerical modeling and experimental work have shown that L h is set by the competition between advective fluvial processes and diffusive hillslope processes (Perron et al., 2009; Sweeney et al., 2015). In numerical simulations in Ferrier and Perron (2020), chemical erosion rates took longer to respond to perturbations in landscapes with longer hillslopes. This matches previous modeling of topographic response times of a single hillslope for both linear and nonlinear hillslope sediment transport to changes in bedrock uplift (Fernandes & Dietrich, 1997; Roering et al., 2001).…”

“…In all modeling scenarios, we used a 150 × 150 regular grid with a resolution of 10 m. The boundaries of the model domain were pinned to zero elevation, allowing the boundaries to act as a regional base‐level. Mass transfers to the channel network do not change the channel elevation (Ferrier & Perron, 2020). We used a timestep of 10 years to maintain an acceptable agreement between D act and D inf at steady state.…”

“…The model does not treat bare-bedrock hillslopes, so application of this model is restricted to soil-mantled landscapes. We incorporated nonlinear hillslope soil transport and TCN conservation into the coupled landscape evolution-soil mineralogy model of Ferrier and Perron (2020). The model tracks and conserves TCN concentrations in the soil, at the soil-bedrock interface, and in deeper bedrock (Figure 1).…”

Comment on soil-2021-78

“…The soil catena can be understood from the retention and movement of water and chemical elements linked to topography and vegetation (Reuter and Bell, 2001). Recently, Ferrier and Perron (2020) constructed a numerical model for the coevolution of topography, soils and soil mineralogy that allowed them to conclude that the hillslope scale has a critical importance in the response of chemical weathering rates to changes in tectonics and climate. However, most existing soil formation models do not account for hydrology, nor for chemical weathering reactions.…”