Modelling climate, topography and palaeoglacier fluctuations in the chilean andes

A one-dimensional flowline model has been constructed, tested and applied to two formerly glaciated valley basins within the Chilean Lake District. The vertically integrated ice flow model is similar to those used to study historical fluctuations of European Alpine glaciers and includes terms for internal deformation and basal sliding. In addition, longitudinal deviatoric stresses are computed and velocity terms are correspondingly adjusted. The model is driven through a mass balance term forced by a stepped lowering of the equilibrium line altitude (ELA) through time. Experiments, based on generating equilibrium glacier surface profiles corresponding to various ELAs, indicate that a lowering of at least 1000m of the ELA from its present-day position is required to simulate the glacial maximum. Furthermore, the specific geometry of the two valleys provides an important control on the extent of the two glaciers, effectively decoupling them from further climatic deterioration once they have advanced beyond the constraining influence of their valleys into the piedmont zone. The tight nesting of terminal moraine loops provides evidence for this topographical control on palaeoglacier extent. The modelled response and sensitivity of the two palaeoglaciers to climate change differ markedly as a result of contrasting valley geometry. Glaciers resting on steeper gradients tend to have thinner profiles, faster mass turnover times and correspondingly shorter volume time-scales. Puyehue glacier has a response time of c. 1000 years whereas the Rupanco glacier has a response time of c. 2000 years. Hence, Puyehue is more sensitive to climatic fluctuations occurring on a time-scale of 500-1000 years. Furthermore, the Rupanco glacier may lag or even fail to respond at all to climatic fluctuations at these time-scales, a conclusion substantiated by field evidence.