Denudation rates and Holocene sediment storage dynamics inferred from in situ 14C concentrations in the Feshie basin, Scotland

Scotland's Highlands are tectonically quiescent but have experienced high rates of isostatic uplift in response to deglaciation. To understand the effects of both deglaciation and regional uplift on landscape evolution, we measured the concentration of cosmogenic in situ ¹⁴C in river sands collected in Glen Feshie (Cairngorms). Like other terrestrial cosmogenic radionuclides, in situ ¹⁴C can be used to calculate basin-wide denudation rates over millennial timescales. ¹⁴C has a short half-life relative to other in situ cosmogenic radionuclides, giving it an advantage in post-glacial landscapes: Very little ¹⁴C will be inherited from exposure before glaciation of the landscape, meaning that concentrations will reflect sediment production and transport dominantly in the Holocene. When we calculate denudation rates based on the common assumption of basin-wide homogeneity of erosion, we find no correlation between topographic metrics such as the normalised channel steepness index and inferred denudation rates, which range between 0.175 and 1.356 mm/year. Based on field and remote sensing observations, we suggest that ¹⁴C becomes diluted downstream due to sediment supply from paraglacial terrace material, and develop a mixing model to test this hypothesis. We identify the terraces that are likely to contribute sediment to the channels through flood modelling, geomorphic mapping and remote sensing observations. Our mixing model indicates that the observed distribution of ¹⁴C concentrations can be explained if terrace escarpments have basin-averaged migration distances of 8 to 30 cm during large flood events. This interpretation is consistent with remotely sensed images of channel activity and terrace bank retreat within the catchment. Our results show that paraglacial sediment stores contribute to sediment fluxes in the late Holocene and highlight the on-going glacial legacy on landscape evolution.