Cosmogenic radionuclide exposure ages from the Enns and Mur Glaciers in the Eastern Alps (Styria/Austria)

Le résumé fourni par la source

The European Alps were covered by a large interconnected system of valley glaciers during the Last Glacial Maximum (LGM). Many of the glaciers advanced into the Alpine Foreland leaving large latero-frontal moraine complexes suitable for (direct) exposure age dating and correlation of the ice extent at different times. In contrast to large parts of the Alps, valley glaciers flowing to the east did not reach the Alpine Foreland resulting in limited preservation of datable relicts. Fortunately, two localities at the margin of the Enns and Mur Glaciers have been found, where the requirements (quartz-rich blocks resting on latero-frontal moraine ridges) for age dating using cosmogenic 10Be and 26Al are met. The Mur Glacier occupied a W-E oriented valley located south of the Niedere Tauern mountain range in Styria. It had several outlets, one of them terminating in the very east at the village Pöls, where a roughly 400 m wide end moraine ridge is preserved. At least two-phases of ice stabilisation are indicated by two to three superimposed ridges. 1.5 m³ Pegmatite-gneiss blocks are embedded in the end moraine ridge, where we collected three samples to determine their exposure age. Age calculation using cosmogenic 10Be and 26Al yields a mean age of 19.6 ± 1.7 ka, whereby the oldest ages were obtained in the outermost part of the ridge following the expected stratigraphic sequence during ice retreat. These ages are in good agreement with other data of end moraines from LGM ice margins around the Alps. More precisely, the age range falls into a second ice re-advance, specified at other locations (especially at the southern alpine rim) but not differentiable at the Mur Glacier.At the Enns Glacier, which extended north of the Niedere Tauern mountain range, subparallel to the Mur Glacier, a multiphase moraine complex is preserved, however almost all of the boulders are limestone or dolomite. We managed to scout few conglomerate/breccia blocks that contain 1-2 cm quartz components in a fine matrix. Three of them are embedded in the termination area and two additional boulders are located further proximal. Mean exposure ages calculated using 10Be range between 14 and 17 ka. Ages calculated from the same samples using 26Al are even more scattered. This is surprising given the similarities in location, valley orientation, geographical location, and altitude between both sample locations. Results from Enns Glacier definitely do not fall into the LGM period. But field evidence such as the location and morphological height of the ridges, strongly suggest that they were formed during the LGM and not in a Late-Glacial phase. Implementation of a snow/forest cover correction only has a minor impact on the calculated age. It is possible that the large spread in the Enns glacier exposure ages is caused by the lithological heterogenity of the sampled boulders. Large quartz clasts resist weathering for a longer duration while the matrix is continuously removed until one clast falls out and results in a discontinuous accumulation of cosmogenic radionuclides at the surface. Discussion at the conference is appreciated.

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