CLIMATIC CONTROLS ON GLACIAL EROSION – INSIGHTS FROM NUMERICAL GLACIAL LANDSCAPE EVOLUTION MODELING

Alpine glacier patterns of erosion are frequently linked to the equilibrium line where the flux of ice is greatest. Large-scale topographic analyses show an agreement between summit elevations along different mountain ranges and the equilibrium line altitude (ELA), and most glacial landscape evolution models suggest that glacial erosion is most efficient around and above the time mean ELA. In some high-latitude regions, however, mountains have attained elevations above the ELA and glacial erosion rates are presumed to be low because the ice is frozen to the bed. These observations suggest that climate is a primary control on glacial erosion rate. We build a glacial landscape evolution model with the Parallel Ice Sheet Model (PISM) to test this hypothesis. PISM uses a hybrid ice dynamics model that combines the non-sliding shallow ice approximation (SIA) with the shallow shelf approximation (SSA). Basal sliding in PISM is modeled by the SSA stress balance and a pseudo-plastic till model for basal resistance. We add a glacial erosion rule to PISM in which erosion is proportional to basal sliding. We conduct a group of numerical experiments to simulate the glaciation and glacial erosion over a synthetic mountain ridge. Model results show that a transition from frozen ice to sliding ice takes place as elevation decreases, and as a result, most glacial erosion occurs in valleys with little erosion at high elevations. Glacial erosion is greatest below ELA, and there is no strong spatial relationship between the ELA and the maximum erosion rate. We compare several cases with similar ELA but different precipitation rates and temperatures. The ice extent is very similar in these cases, but they predict significantly different glacial erosion rates. In a warm and wet climate, glacial erosion is faster than in a cold and dry climate. The transition from non-erosive, frozen ice to erosive, sliding ice occurs at higher elevation in a warm and wet climate. Our results suggest that climate is a stronger control on glacial erosion than the extent of ice cover and the ELA.