METEORIC 10BE, HILLSLOPE EROSION AND LANDSCAPE EVOLUTION ALONG THE COLORADO FRONT RANGE

Fallout radionuclides, such meteoric ¹⁰Be, are valuable as geochronometers and as tracers of hillslope sediment transport, vertical mixing, and residence time. Meteoric ¹⁰Be is particularly useful for long-term geomorphic assessments because it readily adsorbs to mineral matter in the near surface and its half-life is long (~1.36 Ma). Here, we discuss the application of meteoric ¹⁰Be to study hillslope erosion and landscape evolution along the Colorado Front Range. Local calibration of meteoric ¹⁰Be deposition is essential for geomorphic studies. Meteoric ¹⁰Be inventories for 6 dated landforms indicate: (1) long-term deposition varies spatially and temporally across this mountain landscape; (2) soil erosion and site-specific deposition from snowdrifts account for differences; (3) the region has 30-50% higher deposition over past 20ka than predicted by current models. Meteoric ¹⁰Be depth profiles for 10 hillslope pits indicate that concentrations consistently decrease with depth over ~40 cm. Shallow bulges and lower overall concentrations on south-facing hillslopes compared to exponential/declining profiles and higher overall concentrations on north-facing hillslopes imply that more rapid vertical mixing and lateral transport and significantly greater erosion have stripped fines and meteoric 10Be over the last 15-20 ka. Meteoric ¹⁰Be inventories for 40 hillslope locations within Gordon Gulch watershed indicate that average soil residence time for mobile regolith is ~19 ka, but that significant spatial variation exists. Meteoric ¹⁰Be inventories consistently increase downslope on north-facing hillslopes, but not on south-facing slopes. Regolith thickness patterns and meteoric ¹⁰Be age constraints indicate transport of hillslope material to toeslope areas prior to and during the colder climates associated with the end of the last glacial maximum, with latest Pleistocene and Holocene regolith currently stored at the bottom of hillslopes. Meteoric ¹⁰Be inventories can be compared with inventories of fallout radionulcides ¹³⁷Cs and ²¹⁰Pb, which measure sediment movement over the past century, to analyze behavior at short vs. long timescales. Recent fire activity in the region allows us to use meteoric ¹⁰Be to evaluate the role of wildfires in longer-term hillslope evolution.