Constraining the timing of deglaciation of the High Alps and rates of subglacial erosion with cosmogenic nuclides

Abstract

Many of the characteristic landforms of the Alps have been shaped by glaciers. The evolution of U-shaped troughs, cirques and overdeepenings is driven by the action of the moving ice that once occupied the landforms or still does. In this thesis we address two central aspects that characterize the impact of glaciers on the Alps. One is the aspect of available time how long did big glacier systems occupy the Alps in the past? The other is the aspect of pace when glaciers are present, how fast do they erode their beds? The Last Glacial Maximum (LGM) was the ultimate of several periods of glaciations in the Quaternary during which the Alps and adjacent forelands where occupied by ice. After more than two centuries of ice age research in Europe, the general chronological outline of the LGM is well known. Current studies focus on resolving the details of the deglaciation process after the LGM. In this context, particularly few data are available from the High Alps. In this thesis we report chronological constraints on the onset of deglaciation in three different study areas distributed across the mountain range. The identification of sampling sites as well as the interpretation of results critically depend on a thorough understanding of the landscape. In each study area we mapped glacial erosional marks and trimlines for a local reconstruction of the LGM ice surface and flow patterns. We select exposed ridges below truncated spurs as preferred targets for dating the onset of ice surface lowering. Furthermore, we constrain the ice surface and terminal positions of Lateglacial extents by an analysis of mapping results and numerical glacier models. As an example, the oldest glacial erosional marks at the mouth of Gelmer valley (Central Alps, CH) show ice movement parallel to the main valley which implies a high ice surface therein. In contrast, the youngest striations indicate dispersive flow out of the tributary valley; we thereupon infer that the ice surface in the main valley was below the bedrock step at the time these striations formed. We observe the first sign of ice surface lowering of the Aare glacier in the Oberhasli region (Central Alps, CH) at (23.0 ± 0.8) ka. Chronologies in the Alpine forelands suggest a simultaneous end of the first LGM advance phase of the piedmont lobes. An only slightly lower ice surface of the Aare glacier was presumably attained until (17.7 ± 0.8) ka. We further determine identical ages for the onset of ice surface lowering of (18.5 ± 1.1) ka in the Ferret valley at the Southern side of the Mont Blanc massif (Western Alps, IT) and of (18.4 ± 1.4) ka in Zillertal in the Eastern Alps (AT). The obtained ages suggest a synchronous decay of the LGM glaciers in the accumulation zones of Western, Central and Eastern Alps. The High Alpine ice surface presumably responded slightly delayed to global warming after the LGM in comparison to the downwasting of the glacier tongues in the forelands that was set no later than 19 18 ka. However, numerous rock surfaces on high elevation ridges were exposed for 2 3 ka less than the time since the ice surface lowering began. We assume that this reflects widespread persistance of remnant ice patches in the Lateglacial accumulation zones approximately until the Bølling-Allerød interstadial.