26AL/10BE BURIAL DATING OF KLONDIKE AND UPPER WHITE CHANNEL GRAVELS SUPPORTS A GAUSS AGE FOR THE EARLIEST ADVANCE OF THE CORDILLERAN ICE SHEET

The most extensive glaciation of the northern Cordillera of Canada is also thought to be the earliest advance of the Cordilleran Ice Sheet (CIS). Outwash gravel associated with this advance is magnetically normal, and predates the Mosquito Gulch tephra (ca. 1.4 Ma) in the Klondike area, suggesting either an Olduvai (1.77-1.95 Ma) or Gauss age (2.58-3.58 Ma). In the lower Klondike valley, this outwash (Klondike gravel) is interbedded with Upper White Channel gravel, which is elsewhere associated with tephra beds dating to ca. 3 Ma on the basis of glass fission-track ages.

Here, using in situ-produced cosmogenic ²⁶Al and ¹⁰Be, we test the age model for the earliest CIS advancement by burial dating sediment at the top of the Upper White Channel gravel and at the base of the Klondike gravel near Dawson City, YT. Using an inferred thickness (40 m) and density (2.0 g cm^-3) for the outwash, and assuming muon production of Heisinger et al. (2002a, 2002b), we calculate a mean burial age of 4.08 ± 0.33 Ma (1σ, standard error) for the Klondike gravel. Neglecting muon production, we calculate a minimum burial age of 2.27 ± 0.15 Ma (1σ, standard error). These results assume a depositional ²⁶Al/¹⁰Be ratio of 6.75 (spallogenic). This value can be significantly affected by i) temporary burial of sediment during transport (sediment storage), ii) deep-seeded mass wasting of material whose production ratio is controlled by muons, and iii) long-term (>1 Ma) stability followed by erosion of surfaces contributing sediment to the deposit. However, the similarity of ²⁶Al/¹⁰Be ratios measured in these two units—distinctly different in terms of sediment source and transport mechanism—suggests that the depositional ratio does not deviate significantly from the spallogenic production ratio. Our results support a Gauss age for the earliest advance of the CIS, and imply that large ice volumes existed in the northern Cordillera at the onset of extensive glaciation in the northern hemisphere.