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 type paleosols developed on remnant deposits of Late Pliocene to Early Pleistocene [pre-Reid] Cordilleran Ice Sheet [CIS] glaciations in central Yukon, Canada. It is 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, its 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 both a minimum exposure age [1.12+0.44/−0.36 Ma, 2σ] and maximum erosion rate [1.1+0.9/−0.5 m Myr−1] for the Wounded Moose paleosol. 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 our results from single- and joint-nuclide depth profile models reveals a significant discrepancy between calculated and effective 10Be and 26Al production rates [40-65% of expected values]. We interpret this discrepancy as the result of intermittent loess cover-with a time-averaged depth between 60 and 110 cm-which significantly reduced apparent exposure ages obtained from the single-nuclide model. The observation of such a significant loess-cover effect on cosmogenic nuclide production has implications for exposure dating in glacial and periglacial environments; a multi-nuclide sampling strategy is required to quantify this effect.