Impact of Orbital Changes on Hillslope Geomorphology

Background: Many hillslope processes - physical, chemical, and biological - depend on subsurface temperature and water availability. As the subsurface temperature field varies both in space and through climate cycles, the dominant processes of mobile regolith production and transport and the rate at which they act will vary. These processes include the chemical weathering of minerals, cracking of rocks through frost action and tree roots, presence and impact of vegetation on soil cohesion, location and activity of burrowing and trampling animals, frost creep, and solifluction. Methods: We calculate the top-of-atmosphere insolation at any time in the Quaternary, honoring the variations in orbit over Milankovitch timescales. We then incorporate spatial and temporal variations in incoming short-wave radiation on sub-daily timescales due to elevation, latitude, aspect, and shading. Outgoing long-wave radiation is taken to depend on the surface temperature and may be modified by allowing back-radiation from the atmosphere. We solve for the subsurface temperature field using a numerical model that acknowledges depth-varying material properties, water content, and phase change. Armed with an ability to predict the thermal field in time and in space over timescales relevant to landscape evolution, we target variations in regolith production and motion over the long timescales on which periglacial hillslopes evolve. We implement a basic parameterization of temperature-dependent chemical and physical weathering linked to mobile regolith generation. We incorporate multiple regolith transport processes including frost heave and creep. Our intention is not to parameterize all operative processes, but to include sufficient detail to identify how the different processes interact. Results: We find that at many latitudes, slope and aspect are just as important as geologic time in controlling the mean annual temperature and the geomorphic processes and their efficiencies that temperature governs. For example, both the magnitude and the pattern of frost heave activity through time are dependent on slope and aspect and show larger variations across aspects than through time. In addition, the mean annual temperature exerts control on both the magnitude and timing of process efficiency. Efficiency of temperature-dependent processes, including frost cracking, frost heave, and chemical reactions differ from one another in space and time, implying that they do not always co-vary. Conclusions: Coupling a model for the surface energy balance based in orbital variations with a subsurface temperature model allows us to start making predictions about how geomorphic processes efficiencies have changed in space and time. These predictions have implications for long-term landscape evolution.