NONLINEAR CLIMATIC RESPONSE OF CALVING GLACIERS - A CASE-STUDY OF PIO-XI GLACIER, CHILEAN PATAGONIA

Pio XI (or Bruggen) Glacier is probably the only glacier in the world currently at its Neoglacial maximum. Its recent fluctuations provide a striking example of non-climatic glacier behaviour. A rapid 3 km adva nce in the 1920s was followed by retreat in the 1930s. Between 1945 an d 1983 the glacier advanced 10 km at a mean rate of 263 m a-1, since w hen it has oscillated around a quasi-stable position. In 1992 the term inus was advancing over large accumulations of proglacial sediment and the western margin was advancing into mature forest composed of south ern beech (Nothofagus spp.) and Pilgerodendron uviferum. The proglacia l delta has aggraded to sea level along two thirds of the front and th e maximum water depth is 22 m, permitting only small-scale calving. Th e advance has blocked a tributary valley, forming a 240 km2 ice-dammed lake into which large-scale, frequent calving is taking place. This p attern of behaviour cannot be explained with reference to climate alon e. A simple descriptive model combining calving dynamics, sediment bud get, fjord topography and climate explains the main features of recent behaviour. Many other Patagonian calving glaciers have also responded indirectly to climate this century. Since the established Late Glacia l and Holocene chronology of Patagonian glacier oscillations is largel y based on records from sites at calving glaciers, its validity and it s uncritical use in inter-hemispheric comparative studies is questiona ble.