LATE PLEISTOCENE DEGLACIATION CHRONOLOGY OF THE SCANDINAVIAN ICE SHEET SOUTHERN MARGIN

  Prominent moraines crossing the Baltic region mark the late Pleistocene maximum extent and recessional phases of the southern margin of the Scandinavian Ice Sheet (SIS).  Five moraines present between the advance during the Last Glacial Maximum (LGM) and the Younger-Dryas Salpausselkä Moraines in Finland suggest a millennial-scale signal for this sector of the ice sheet.  However, dating control constraining the age of the LGM and retreat phases of the southern SIS margin in the region is all but lacking.  We have sampled boulders for surface exposure dating with the cosmogenic nuclide ¹⁰Be from moraines along a broad south-to-north transect spanning Poland, Lithuania, Latvia, Belarus, Estonia, and Finland.  Here we report ¹⁰Be concentrations on 138 boulders measured by accelerator mass spectrometry at the Tandetron facility, Gif-sur-Yvette, France.  We calculated exposure ages assuming a production rate of 5.1 ± 0.3 atoms g^-1 yr^-1 at 1013.25 mbar pressure and high latitude, scaled for each site altitude and latitude.  No corrections for snow cover or erosion have been applied.  Eleven boulders from the LGM have a weighted mean age of 18.6 ± 2.8 ¹⁰Be ka.  Forty-one samples from the Pomeranian Moraine have a weighted mean age of 14.0 ± 1.7 ¹⁰Be ka.  Thirty-three samples from the Middle Lithuanian Moraine have a weighted mean age of 13.1 ± 1.6 ¹⁰Be ka.  Nine samples from the North Lithuanian Moraine have a weighted mean age of 13.0± 0.8 ¹⁰Be ka.  A single boulder on the Pandivere Moraine was dated at 13.0 ± 1.1 ¹⁰Be ka.  Eight samples from the Palivere Moraine have a weighted mean age of 10.0 ± 1.3 ¹⁰Be ka.  We interpret this moraine age as being too young since all the boulders were submerged beneath various phases of the Baltic Ice Lake, which would have substantially reduced the production rate for that period of time.  Nine samples from the Younger Dryas Salpausselkä I Formation have a weighted mean of 11.8 ± 0.9 ¹⁰Be ka.  We chose to present here the moraine age standard deviations as the moraine age uncertainties.  Most of the standard deviations are larger than the analytical uncertainties, indicating a geological source of uncertainties in addition to the analytical ones.  This prevented us so far to use the analytical uncertainties as the uncertainty of the moraine ages. These results define a primary time frame for the deglaciation of SIS southern margin in this region.