A Reconstruction of the Last Glacial Maximum Ice Cap in the Western Alps and its Paleocircluation Significance

The ice-surface elevation and flow directions within relatively small ice caps, such as those occurring in the Alps during the Last Glacial Maximum (LGM), are primarily affected by regional climate. Therefore, a reconstruction of the LGM Alpine accumulation zone should reflect the principal areas of precipitation and catchment, and thus yield information about paleocirculation. Detailed mapping of glacial trimlines and other erosional features allows an interpretation of the ice-surface elevation and flow directions within the LGM Alpine accumulation zone. Paleocirculation interpretations are derived based on the location of high-elevation "domes" of the ice-cap surface and the direction of ice flow over mountain passes. A three-dimensional model of the LGM western Alpine ice cap, created using a Geographical Information System, is based on mapped erosional features. This digital reconstruction shows two principal ice domes from which ice flowed outward in all directions. The Matterhorn ice dome existed in the southern Valais and was the highest dome of the Alpine ice cap. The large mass of ice in this region, most of which flowed north into the Rhone Valley, explains the presence of far-traveled erratic boulders, which occur on the northern Alpine foreland. Another lower ice dome existed over Mont Blanc. Trimline elevations imply that the ice surface on the southern side of this dome was approx. 200-300 m higher than on the northern side. Ice flow over high passes to the east of Mont Blanc was from south to north. Previous work indicates that southerly airflow was dominant during LGM ice accumulation in the central Alps (Florineth and Schluechter, QR, v. 54, 2000). Such paleocirculation, which is different from the present prevalent (north)westerly storm pattern, is interpreted to have resulted from an intensified jet stream which crossed the Atlantic south of the LGM Polar Front (44 N Latitude). The surface geometry of the Mont Blanc ice dome suggests that ice accumulation in the western Alps was due to a greater influence of precipitation from the south. The location of the Matterhorn ice dome is consistent with a pattern of dominant southerly circulation, however, ice flow in this southern Valais region was controlled to a large degree by the high land-surface topography. A projection of the LGM ice domes mapped in the central and western Alps shows that the ice domes decrease in thickness and areal extent from southeast to northwest, also indicating that the prevalent paleocirculation which led to the build-up of the LGM Alpine ice cap was southerly. Important questions exist with respect to the ages of the glacial erosional features. Without absolute age dating, all features are assumed to have formed during the global LGM (21,000 yr B.P.). Preliminary results from cosmogenic exposure dating of surfaces in the western Alps show that the erosional features are LGM or younger in age. More samples from the southern Valais are currently in preparation. These new data may provide critical information about the LGM deglaciation of the Alps.