International Geologiical Congress - Oslo 2008

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CGG-02 Subglacial environments: Processes, sediments, landforms, modelling and experiments

 

SW-Scandinavian ice streams: Triggered by warming and driven by shallow basins?

 

Michael Houmark-Nielsen, University of Copenhagen (Denmark)
 

 

High amplitude climatic changes initiated by rapid warming followed by gradual cooling fingerprint the Weichselian climate. Northwest European lowland sediment records indicate repeated out flow of the Scandinavian Ice Sheet (SIS) from Norwegian, Swedish and Baltic uplands. While the Last Glacial Maximum (LGM) experienced steady sheet flow of SIS, rapid and canalized land based ice streams (IS) fled through shallow basins bordering SIS prior to and immediately after LGM. The latter, Young Baltic IS moved across a substratum of dead LGM-ice and formed arch shaped end moraines. Radiating flow patterns in marginal regions are witnessed by sedimentary features and stream lined ground moraine. During glacier retreat ice stream landforms partly overprinted each other into complex multi scale superimposed glacial landscapes. Landforms of the former, Kattegat IS are erased, but ice stream behaviour is revealed by radiating flow patterns and the confinement to a shallow basin where ice flowed over a muddy substratum, facilitating rapid movement. The regional distribution of former land based ice streams in the circum Baltic lowlands seems closely related to the location of such basins.
Contrary to numerical model predictions on the evolution of the Scandinavian Ice Sheet (SIS), glacial advances occurred between MIS 4 and 2. Fine grained freshwater and marine sediments with interstadial flora and fauna remains were deposited during MIS 3 in ice dammed lakes in the Baltic Basin and narrow fjords in the Kattegat trough. Clast provenance of till indicates advance via these depressions twice and dating of inter till deposits indicate that glacial episodes seem to be out of phase with the global ice volume trend. Stream flow occurred under relatively high glacio-eustatic sea level and low-arctic climate conditions, and ice advance could have been triggered by abrupt climatic warming responsible for ameliorated climate during MIS 3. Rapid flow of SIS reaching North European lowlands could have been initiated by an expanding zone of basal melting beneath a steep gradient ice sheet. Instability enhanced by melt water causing ice-bed decoupling over large areas may eventually have led to marginal collapse and surging across soft, water saturated sediments in basins bordering SIS. Whether each of these episodes correlates in time with specific long time cooling trends, Bond cycles, depend on age constraints. Alternatively, ice streaming could be caused by internal ice sheet dynamics operating independently or out of phase with North Atlantic climate forcing. Thus, the relations between episodes of ice streaming, climate records and the dynamics of SIS in terrestrial regimes are to be further pursued.

 

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