Dendrochronologische Auswertung fossiler Hölzer zur Rekonstruktion der nacheiszeitlichen Gletschergeschichte | Dendrochronological evaluation of fossil wood to reconstruct holocene history

Around the middle of the 19^th century, alpine glaciers advanced to their last maximum extension within the Holocene (the last 10 000 years). Some of the glaciers, especially the Great Aletsch and Gorner, penetrated deeply into wooded land and destroyed numerous trees. Not only were trees destroyed but also valuable arable land, alpine farm buildings and dwelling houses. Since this last maximum extension the retreat of the glaciers has accelerated revealing,within the glacier forefields, the remainders of trees once buried (trunks, branches and roots). Some of this fossil wood is found in the place where it grew (in situ). The wood often dates back to a time before the last glacier advance, most of it is several thousands of years old because glacial advance and retreat periods occurred repeatedly within the Holocene. This paper shows the characteristics of fossil wood and, with the help of a model, how they can be analysed to reconstruct glacial history. Using two examples, we demonstrate how glacier length variation can be exactly reconstructed with the help of dendrochronology. The first example deals with the advance of the Gorner glacier in the 14^th century. This glacier reached its maximum extension around 1385. The other example deals with the advance of the Great Aletsch glacier between 1581 and 1678, the year when this glacier reached its maximum extension. Thanks to the very exact reconstruction of the change in the glacier length we can estimate the velocities of both the Gorner and Great Aletsch glaciers during the advancement periods in the 14^th and 16^th centuries. They range from between 7–8 and 20 m/p.a. and 7–8 and 30 m/p.a., for the Gorner glacier and the Great Aletsch glacier,respectively.