A Model for Interpreting 10Be Basin-Wide Erosion Rates in Post-Glacial Environments, Northwest Scotland

Abstract:

Meaningful interpretation of in-situ cosmogenic ¹⁰Be basin-wide erosion rates in slowly eroding postglacial catchments is complicated because ¹⁰Be is inherited through shifts between glacial and fluvial regimes and ice shielding prevents ¹⁰Be production. Such environments do not attain isotopic and landscape steady state, conditions necessary for the current method of calculating basin-wide erosion rates. We propose an alternate set of assumptions, specific to postglacial regions, which make it possible to calculate basin-wide erosion rates in the post-glacial Highlands of NW Scotland. From 20 Scottish basins basin-wide ¹⁰Be concentrations range from 2.129 x 10⁴ to 4.870 x 10⁴ atoms/g qtz. Average ¹⁰Be concentrations from shallow till and bank deposits within the basins are 2.856 x 10⁴ atoms/g qtz, similar to the basin-wide concentrations, whereas average bedrock concentrations in the basins are 1.747 x 10⁵ atoms/g qtz. This suggests that during the postglacial time most active sediment is derived from reworked deposits rather then sub-aerially eroded bedrock. Therefore, we make the simplifying assumption that most bedrock erosion occurs during glaciation. A deeply buried till that has experienced no nuclide production since deglaciation has a ¹⁰Be concentration of 6.810 x 10³ atoms/g qtz and allows us to estimate how much of the ¹⁰Be in basin-wide samples was produced since deglaciation. A glacial ice thickness of only 2 m would shield all ¹⁰Be production; therefore we assume that no ¹⁰Be production occurred during glacial periods and that all production occurs during interglacial periods. Using 100 ka as the approximate duration of a Pleistocene glacial-interglacial cycle, comprised of a 15 ka interglacial period and an 85 ka glacial period, and the aforementioned assumptions we have derived a numerical model to calculate basin-wide glacial erosion rates in NW Scotland.