Paper No. 304-1
Presentation Time: 1:45 PM
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.