Post-glacial catchment denudation rates from 14C concentrations in Glen Feshie, Scotland

Abstract

Inferring erosion rates from 10Be or 26Al concentrations in stream sediments has become standard practice in geomorphology. In formerly glaciated landscapes, however, this technique is problematic because repeated phases of shielding and exposure during the past glacial and interglacial periods can lead to 10Be or 26Al concentrations that are difficult to interpret. Cosmogenic in-situ 14C has a short half-life (~5,730 years) that means 14C atoms in stream sediments cannot be inherited from before the glacial period and inferred erosion rates will reflect post-glacial, Holocene erosion. Using cosmogenic in-situ 14C, we report the first millennial-scale erosion rates in the post-glacial landscapes of Glen Feshie, within the Cairngorm mountains of Scotland.The River Feshie contains active gravel reaches that cut through glacial outwash terraces. We counterintuitively find the lowest inferred erosion rates (0.06 mm/yr) in the steepest side tributary and the highest inferred erosion rates at the low-relief outlet of Glen Feshie near the confluence with the River Spey (0.21 mm/yr). Based on field observations, we interpret that hillslopes have been largely inactive and contributed limited sediment fluxes. To provide further insight into the highest erosion rate documented furthest downstream, we consider the hypothesis that sediment from the hillslopes with higher concentrations of 14C has been diluted with lower concentration material from the terraces. Further, we hypothesise that if terraces that border the channel increase in height downstream, their incision could have remobilised an increasing amount of sediment with lower 14C concentrations downstream, leading to increased dilution and the observed concentrations. Results show terrace height above the channels does not increase downstream and averages approximately 2 meters. We therefore suggest terrace height does not account for higher erosion rates, and present a cosmogenic-nuclide mixing model to explore the degree to which the input of sediment that has been shielded from cosmic rays in terraces can explain the observed concentrations in stream sediments.