USING COSMOGENIC 10BE IN BOTH BEDROCK AND SEDIMENT TO TRACK GLACIAL AND PARAGLACIAL EROSION OF THE NORTHWEST SCOTTISH HIGHLANDS

To quantify the erosional effect of large ice-sheets and post-glacial processes along passive margins we measured cosmogenic ¹⁰Be in both sediment and bedrock across the Highlands of northwest Scotland specifically on the Isle of Skye, Glen Shiel, Glen Nevis, and near Inverness. Ridges and polished bedrock from across the study area have ¹⁰Be bedrock exposure ages ranging from 10–33 ka and dominantly predate both the last (Loch Lomond Advance [LLA]) and penultimate (Dimlington Stadial) glacial retreats indicating that high elevation bedrock was preserved during the last two glaciations. Understanding the erosional history of the lower elevation portions of the landscape is more difficult since using the standard method of calculating ¹⁰Be catchment averaged denudation rates is not valid in paraglacial environments. We investigated the sediment-evacuation pathways of glacially produced sediment and the relative magnitudes of glacial and paraglacial erosion in these lower elevation portions of the landscape by measuring ¹⁰Be concentrations from possible upstream sediment sources and comparing them to ¹⁰Be concentrations in catchment-wide sand. Low ¹⁰Be concentrations in deep, presently shielded till (0.12×10⁴ atoms g^-1 SiO₂) suggest that most ¹⁰Be was produced post-glacially, requiring efficient glacial stripping of low elevation bedrock to remove most of the ¹⁰Be inventory. Comparable ¹⁰Be concentrations from catchment-wide samples within the LLA limits (2–5×10⁴ atoms g^-1 SiO₂) to those from shallow sediment deposits (~3×10⁴ atoms g^-1 SiO₂) indicates that a process of remobilization and mixing of the top few meters of till yields the dominant source of catchment-wide sand. Most post-glacially produced ¹⁰Be remains stored in the catchments indicating slow rates of paraglacial sediment evacuation and insignificant paraglacial bedrock denudation. Higher ¹⁰Be concentrations are present in bedrock (7–34×10⁴ atoms g^-1 SiO₂) and from catchment-wide sand sampled outside of the LLA limits (6–11×10⁴ atoms g^-1 SiO₂). Overall, the data suggests that Quaternary landscape evolution in the highlands of northwest Scotland is characterized by polythermal glacial bedrock denudation and slow inter- and post- glacial remobilization and evacuation of glacially produced sediment.