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Chances and challenges of reconstructing dynamic palaeo ice sheets

Christopher Lüthgens1
1 Freie Universität Berlin, Physische Geographie; Universität für Bodenkultur Wien, Angewandte Geologie

Key Note 1.7 in Fortschritte der Quartärstratigraphie

17.09.2012, 14:00-14:30, H8

In the late 19th century, the ‘glacial theory’ started to replace the by then established ‘drift theory’. Throughout research history, it has since then experienced a number of key changes, which include the introduction of the concept of polyglaciation, as well as the differentiation of repeated phases of ice advances and ice decay within a single glaciation. Both concepts rely on the identification of morphostratigraphically defined ice marginal positions. With the absence of dating techniques in the early days of Quaternary research, rather static, relative chronologies were developed. Here, the glacial landscape of northern Germany, representing the south-western part of the Scandinavian Ice Sheet (SIS), serves as a good example, with its glacial research history reaching back almost 140 years.

 

Throughout the last decades, numerical dating techniques have been developed which finally enable direct dating of glacigenic deposits. As a consequence, established chronologies have been confronted with a growing number of geochronometrical data. The two most commonly applied dating techniques for the dating of glacigenic deposits are Optically Stimulated Luminescence (OSL) dating and Surface Exposure Dating (SED) of erratic boulders. However, it needs to be pointed out that the ages derived from these different methods must be interpreted with respect to the specific processes in landscape development which are actually dated. When applied to glacial sediments associated with the same ice marginal position, the resulting ages of the two methods are strongly dependent on the sampling position within the geomorphological and stratigraphical framework and theoretically cannot be the same. This is illustrated by the following example: OSL enables the determination of depositional ages of sediments. By applying OSL to glaciofluvial sediments of outwash plains, the process of sediment aggradation linked to meltwater discharge from an ice margin is directly dated. In contrast, the application of SED to erratic boulders deposited upon an associated terminal moraine determines the age of the final stabilisation of the sampled boulders at the landscape surface after the downmelting of stagnant ice, landscape transformation under periglacial conditions and the melting of buried dead ice. These processes may cause a significant time lag between the initial deglaciation of the area and the final stabilisation and exposure of the dated boulder. However, given that a process-based interpretation for numerical ages from OSL and surface exposure dating in glacigenic landscapes is applied, the combination of both dating methods may result in a more detailed reconstruction of regional deglaciation patterns.

 

The fact that it is now possible to date varying geomorphological processes by applying different dating methods to glacigenic deposits, allows a differentiated reflection of morphostratigraphically based chronologies against the background of the modern concepts of dynamic ice sheets. Such concepts and models directly imply asynchronous ice sheet behaviour, with individual ice lobes representing the terrestrial extensions of large scale ice streams. This to a certain degree contradicts the traditional morphostratigraphical definition of ice marginal positions. Consequently, a time based definition should be considered, because it would offer the possibility of a more detailed reconstruction of ice sheet dynamics at a certain point in time.



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Letzte Änderung 19.07.2012