The Linkage of Chemical and Mechanical Processes in the Evolution of High Surfaces of the Front Range Crest, Colorado

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An understanding of the long-term evolution of hillslopes requires knowledge of how bedrock is converted to mobile regolith, and how regolith is transported downslope. In many cases, the rate of conversion of bedrock to regolith controls not only the rate of lowering of the landscape, but its shape as well. While conceptual models of regolith production are well established, the empirical observations needed to support these conceptual models are few. A mechanistic understanding of the process of regolith formation remains a matter of conjecture. Similarly, few empirical studies provide sufficient constraint on a process-based understanding of regolith transport. An overarching question, What is the dependence of regolith production rate on regolith thickness? reflects the reigning paradigm, in which regolith thickness is the controlling parameter. Clearly, thickness is a proxy for other variables, such as water contact time and chemical saturation state, and freeze-thaw cycle frequency. The interaction of mechanical and chemical processes that produce and transport regolith will be explored in a simple landscape: the alpine high surfaces in the Front Range of Colorado. These high surfaces are an ideal laboratory because their parabolic shape implies that they are steady state landforms, the regolith is thin and accessible and is generated from granitoid rock, and easily characterized frost processes are expected to dominate the mechanical processes. The approach used is to document the chemical development and mechanical properties in the rock and regolith, paying particular attention to the interface between these materials. Modern and several-thousand year process rates will be documented; all strongly constrain conceptual and numerical models of landscape evolution. Current chemical and physical processes will be monitored with soil water samplers, moisture probes, temperature strings, frost-heave sensors, and strain gauges. Sampling sites will be arrayed along a transect down a parabolic, steady state hillslope. Topography (using laser altimetry) and snow cover will be documented in detail, and variations in regolith thickness and chemical evolution surveyed. Long-term rates of regolith production and its motion down slope will be documented using the concentration of cosmogenic 10Be in bedrock and regolith profiles, respectively. An existing model of regolith production will be refined by incorporation of the specific processes documented in this study. The development of this model will integrate the field measurements outlined, and the information gleaned from modern and long-term process data. This will require attention to thermal, hydrologic, and chemical processes. The model will serve as a test of the process understanding developed from the field data.Broader Impacts. This work will be used in teaching at CU, outreach to the public, and training graduate and undergraduate students in research. One graduate student and 2 undergraduate students will be exposed to surface processes research, including analysis of laser altimetry, cosmogenic nuclide dating, field monitoring, and numerical modeling. As the site is close to CU, lectures, lab exercises and field trips will be possible. Data will be made available through the web sites of the PIs and through that of the Niwot Ridge LTER. The PIs are committed to taking their science to the broader public. Simulations of landscape evolution are being shared (see sample simulations of both glaciers and tor-dotted ridges at http://instaar.colorado.edu/rmnp). The model of alpine hillslopes produced will add understanding of the interactions of chemical and physical processes in eroding landscapes.