The paleo-ice stream in Vestfjorden,north Norway,over the last 35 k.y.:Glacial erosion and sediment yield

摘要

The basal erosion rate and sediment yield of the Vestfjorden paleo-ice stream,a marine-based part of the northwestern Fennoscandian Ice Sheet,were elucidated during a complete advance-retreat cycle using high-resolution seismic data and multi-beam bathymetry.In most of the area,there is a semitransparent,continuous upper seismic unit showing a southwestward(down-fjord)progradational pattern with megas-cale glacial lineations on top,interpreted to represent a subglacial deformable till.The till formed a continuous bed beneath most of the Vestfjorden paleo-ice stream,except for the upstream parts,where ice streaming is inferred to have occurred above bedrock.The till thickness is up to an order of magnitude larger than those reported from modern ice streams due to a more efficient transfer of material from the source area to the ice streambed.The till is draped by glaciomarine and marine sediments.The average sedimentation rate in the paleo-ice stream drainage route during the period from ca.35 to 11 ka was-2.6 m/k.y.This equals an average sediment discharge of 35 X 10~6 t/yr and an average sediment yield of 6.4 X 10~3 t km~(-2)yr~(-1).From this,we estimate an average erosion depth of-40 m or rate of-1.7 mm/yr in the sediment source area,the encircling fjord,and valley landscape.These values are in agreement with the results reported from similar settings in southern Norway,but they are nearly twice the figures reported for arctic ice-stream erosion.They are also significantly higher than the rates estimated from the British Ice Sheet.In conclusion,glacial erosion beneath paleo-ice streams seems to have been more efficient in subarctic regions.