Controls on Bedrock Hillslope Erosion in Cold, High-Alpine Regions

Frost-cracking is believed to set rates of erosion in cold, bedrock hillslopes where temperatures are below freezing for a substantial portion of the year. Nevertheless, studies from the European Alps, North American Cordillera, and the Himalaya suggest that permafrost-thaw-induced rockfalls, a consequence of (warming) temperatures, may also contribute non-trivially to the long-term erosion rate.

 

Here, we present an assessment of bedrock, hillslope erosion rates and mechanisms in cold regions using a global dataset compiled from published literature. We additionally report 23 new bedrock hillslope erosion rates from across the European Alps estimated using in-situ cosmogenic ¹⁰Be. The hillslopes and glacial headwalls sampled span elevations from 2100 m to 4040 m, and include diverse lithologies, thermal regimes, and glaciation histories. Our new ¹⁰Be-based erosion rates range over several orders of magnitude, from 0.01 mm yr^-1 to 1.5 mm yr^-1, and integrate over timescales from 400 to 30000 years. To evaluate the relative rates of frost-cracking at both our field sites and those within the global compilation dataset, we calculate site-specific frost-cracking intensities using modern ground surface temperatures and modelled paleoclimatic conditions. Comparing these calculated frost-cracking intensities against the measured erosion rates suggests that frost-cracking alone may not be the rate-limiting erosional process in cold, alpine regions. We likewise observe no correlation between hillslope angle and erosion rate. Our compiled dataset does suggest, however, that erosion rates estimated using methodologies that integrate over long timescales are lower than those which integrate over short timescales, highlighting the potential for stochastic events, like rockfalls due to permafrost thaw, and/or paraglacial adjustment processes, to bias erosion rate estimates which integrate over short timescales.