Asynchronous dynamics of the last Scandinavian Ice Sheet along the Pomeranian ice-marginal belt: A new scenario inferred from surface exposure 10Be dating

Abstract

We present a new set of 25 10Be surface exposure ages of boulders located on the Pomeranian moraines and of erratic boulders located directly upstream of the Pomeranian moraines in northern Poland. Together with recalculated 10Be surface exposure ages along the Pomeranian ice-marginal belt from Denmark to the west to Lithuania and Belarus to the east, the full data set (n = 86) enabled us to constrain the timing of the ice front standstill and its subsequent retreat. The investigated area consists of geomorphological record along ∼2000 km of the ice margin associated with the ice sheet limit correlated so far with the Late Weichselian Pomeranian Phase (Bælthav in Denmark, Baltija in Lithuania and Braslav in Belarus).We constrained deposition of the Pomeranian ice-marginal formations to discrete intervals: (1) ∼20–19 ka in the area of the Baltic Ice Stream, (2) ∼19–18 ka in the area of the Odra Ice Stream, (3) ∼20–19 ka in the interstream zone between the Odra and the Vistula Ice Streams, (4) ∼19–18 ka in the area of the Vistula Ice Stream, (5) ∼18–17 ka in the area of the Mazury Ice Stream, (6) ∼17–16 ka in the area of the Riga Ice Stream, and (7) ∼16–15 ka in the area of the Novgorod Ice Stream. Our best age estimates are based on: (1) a minimum age of the ice margin retreat inferred from new and recalculated 10Be ages of boulders as well as interpretation of available radiocarbon ages from organic deposits and OSL ages from sediments overlying tills, and (2) a maximum age of the ice margin stillstand and retreat inferred from interpretation of available OSL ages from sandur sediments deposited in front of the ice sheet. The asynchrony of the ice margin positions along the Pomeranian ice-marginal belt shows about 3–5 ka difference between the Bælthav ice margin in Denmark and the Braslav ice margin in Belarus. We propose a new scenario of the ice sheet evolution and a time-slice reconstruction of the last Scandinavian Ice Sheet's southern fringe for the period ∼20–15 ka. We suggest that the term “Pomeranian Phase” should not be used but rather that the terms “Pomeranian moraines” or “Pomeranian ice-marginal belt” are adequate to describe geomorphological record of the palaeo-ice margin positions, which occurred during various phases of the last deglaciation.