Loch Lomond Stadial glaciers in North Harris, Outer Hebrides, North-West Scotland: glacier reconstruction and palaeoclimatic implications

Abstract

Geomorphological mapping of North Harris provides evidence for the former existence of 10 glaciers with a total area of ca 35km2. A Loch Lomond (Younger Dryas) Stadial age (ca 12.9–11.5kyrBP) for this glacial episode is inferred from glacier configuration, landsystems dominated by hummocky recessional moraines, and relationships with Lateglacial periglacial phenomena. Equilibrium line altitudes (ELAs) of 150–289m were calculated for individual glaciers. ELA variability mainly reflects differences in snow-contributing area. The area-weighted mean ELA (204m) is consistent with a northwards decline in ELAs along the western seaboard of the British Isles of 69.5m (100km)−1, equivalent to a northwards ablation-season temperature decrease of 0.42°C (100km)−1. This latitudinal temperature gradient implies a mean July sea-level temperature of ca 7.2°C for the coldest part of the stade, roughly 6°C lower than at present. Sea-level precipitation at the time of the LLS glacial maximum is inferred to have been between ca 1970±200 and 2350±200mmyr−1, implying that LLS precipitation was up to 25% greater than now. Patterns of recessional moraines indicate that the glaciers remained close to climatic equilibrium as they retreated to their sources, though moraine belts implying near-stationary or readvancing ice margins on flat valley floors are separated by moraine-free zones indicating uninterrupted retreat. Calculation of ELAs for ‘residual’ glaciers in former source areas suggests that summer warming of ⩽1.0°C would have resulted in shrinkage of the glaciers to their sources.