The Effect of Past Climate Change on Regolith Erosion Rates for the Past 100 ka in the Eastern Sierra Nevada

Abstract:

Global climate change over the last 100 ka is known to have resulted in glacial fluctuations in the eastern Sierra Nevada evidenced today by moraines. The effect of past climate on regolith erosion rates is quantified by modeling hillslope diffusion and evolution of a profile cross-section of Mono Basin moraine in the eastern Sierra Nevada. The degradation in the model is described by the linear transport law, q = -κ(dz/dx), where the diffusivity coefficient, κ, is a parameter to account for factors affecting regolith transport rate (q) unrelated to slope (dz/dx), such as climate and substrate. Three scenarios were modeled with respective κ values. In the first, κ is held constant through the age of the landform and optimized to reproduce the current moraine cross-section. In the second, κ varies with time based on the documented variation in the paleoclimate and related degradation rate. In the third, κ is held constant and defined by present-day degradation values measured in the region. In all the scenarios the moraine initial slopes are at the angle of repose. Comparisons of the first (constant κ) and second (variable κ) scenarios show that the former can generate erosion rates that overestimate crest elevation by 30% during the first quarter of the landform age and underestimate it by 20% during the latter three-quarters. However, the third (current κ) scenario underestimates the degradation up to 58% of the current profile. This indicates that the past erosion rates were higher than what is observed today. Increased regolith erosion rates in the past were driven by climates colder and wetter than the present.