AGE-EROSION CONSTRAINTS ON AN EARLY PLEISTOCENE PALEOSOL IN YUKON, CANADA, WITH PROFILES OF 10BE AND 26AL: EVIDENCE FOR A SIGNIFICANT LOESS COVER EFFECT ON COSMOGENIC NUCLIDE PRODUCTION RATES

Wounded Moose paleosols developed on remnant deposits of Late Pliocene to Early Pleistocene [pre-Reid] Cordilleran Ice Sheet [CIS] glaciations in central Yukon, Canada. They are an important regional soil-geomorphic marker at the boundary between early CIS advances and the non-glaciated regions of Yukon and Alaska. Yet, at present, their age is poorly constrained between the Reid [0.2 Ma] and earliest [2.84 Ma] CIS advances. Here, we apply depth profiles of in situ-produced cosmogenic ²⁶Al and ¹⁰Be to obtain a minimum exposure age [1.12 ^+0.44/_-0.36 Ma, 2σ] and maximum surface erosion rate [1.1 ^+0.9/_-0.5 m Myr^-1] for this soil marker from a high-level glaciofluvial terrace near Dawson, Yukon. Our results show that this soil formed under exceptionally stable conditions [max erosion rate similar to polar bedrock erosion rates] and that it pre-dates the emergence of the 100 ka [eccentricity] climate cycle. Contrasting results from single- and joint-nuclide depth profile models reveals a significant discrepancy between calculated and effective ¹⁰Be and ²⁶Al production rates [40-65% lower than expected values]. We interpret this difference as the result of intermittent loess cover [time-averaged depth between 60 and 110 cm] that significantly reduced apparent exposure ages obtained from the single–nuclide model. The observation of such a significant loess-cover effect on inferred cosmogenic nuclide production rates has implications for exposure dating in glacial and periglacial environments. A multi-nuclide sampling strategy is required to quantify this effect.