From morphostratigraphy to geochronology – on the dating of ice marginal positions

This study focuses on the glacial landscape of northern Germany as a part of the southwestern sector of the Scandinavian Ice sheet (SIS), as it has a research history reaching back about 130 years. Since the ‘glacial theory’ replaced the ‘drift theory’ in the late 19th century it has experienced a number of key changes including the introduction of the concept of polyglaciation or the differentiation of ice advances within one glaciation, which is closely linked to the morphostratigraphical definition of ice marginal positions. With the development of numerical dating methods the rather static, morphostratigraphically based chronologies which are deeply rooted in research history, have been confronted with a growing number of geochronometrical data obtained from glacigenic deposits in order to date ice marginal positions. The two most commonly used dating methods for that purpose are Optically Stimulated Luminescence (OSL) and surface exposure dating of erratic boulders. This study highlights the dependency of the significance of results from different dating methods on the sampling position within the geomorphological framework. Results from OSL and surface exposure dating for the same ice marginal position in theory cannot be the same, as the methods date different processes in the development of the glacial landscape. OSL dating of fluvioglacial sandur sediments (dating the process of active sedimentation at the ice front) and surface exposure dating of boulders on end moraines (dating the process of stabilisation of end moraines after the retreat of the ice front) often qualify as the most useful combination of techniques in order to define a time interval for the formation of the landforms at a specific ice marginal position. Methodological drawbacks which might occur unfortunately tend to widen the process-induced offset between OSL and surface exposure ages. Incomplete bleaching of fluvioglacial sediments might cause an offset towards a maximum OSL age for an ice marginal position, whereas delayed stabilisation or post-exposure relocation of boulders might cause a shift towards a minimum age. Of key importance are the influence of periglacial processes and the effects of melting of buried ice on landform stabilisation which remain to be quantified (a possible solution to this problem, combining OSL and surface exposure dating is suggested in this study). Nevertheless, the possibility exists to date different geomorphological processes by applying different dating methods to glacigenic deposits. This allows for a differentiated reflection of morphostratigraphically based chronologies against the background of the modern concept of dynamic ice sheets. In contrast to the traditional morphostratigraphical definition of ice marginal positions, a time based definition is proposed, because it offers the possibility of a more detailed reconstruction of ice sheet dynamics at a certain point in time.