Using cosmogenic nuclides from amalgamated talus cobbles to assess alpine erosion in the Teton Range

Abstract:

Measuring spatial patterns and rates of erosion in alpine landscapes is challenging because locations are steep and remote, and hillslope processes including rockfalls, avalanches and landslides are stochastic. This study evaluates how variability between quantitative erosion rates and surface ages and qualitative weathering features on talus deposits may influence interpretations of the spatial distribution of hillslope erosion in the Teton Range. I use cosmogenic nuclide concentrations to estimate erosion rates and surface ages of bedrock and talus deposits. I then compare these cosmogenic nuclide results to talus volumetric estimates of erosion rates. I also compare the quantitative estimates of surface ages and erosion to qualitative weathering observations on talus cobbles. The resulting erosion rates from cosmogenic nuclides are slower than rates estimated from talus volumes, however, both methods produced the same pattern of erosion. The fastest and slowest cosmogenic nuclide and volumetric erosion rates were associated with the same fans. Exposure ages of talus surfaces decrease as elevation of the fans increase. Qualitative observations of weathering on cobble surfaces were not reliable to predict the relative rates of erosion on the talus fans. Using amalgamated samples from hillslope deposits adjacent to steep sided canyons for cosmogenic nuclide analysis has high uncertainty when finding accurate erosion rates or exposure ages due to complicated shielding on steeply sloping walls. The similar patterns of rapid or slow erosion rates, however, suggest that cosmogenic nuclides or volume estimates are both reasonable methods to understand relative timing of hillslope events, even if the absolute age or erosion rate cannot be determined.