Helena Alexanderson

The Late Pleistocene cataclysmic releases of glacial meltwater left distinct footprint across the Central European Lowland. Deeply incised tunnel valleys, oversized ice-marginal spillways, fields of mega-dunes, and glacial curvilineations carved by rapid drainage events occur along the southern margin of the last (Weichselian) Scandinavian Ice Sheet margin. These meltwater releases contributed significantly to continental-scale sediment re-distribution and landscape re-shaping and may have influenced climate by introducing large volumes of freshwater into the oceans.
Here we present the first record of a major proglacial flood in western Denmark. The flood occurred at the margin of the Weichselian ice sheet during its early deglaciation and retreat from the outermost position around 21,000 years ago. The meltwater, fed through several prominent tunnel valleys, was funneled at the ice forefield and drained along the ice margin to the North Sea generating a field of mega-dunes clearly discernible on LiDAR images. The 47 mapped mega-dunes reach heights of up to 2.2 m (mean 1.1 m) and widths of up to 320 m (mean 138 m) covering an area of approximately 18 km2. Excavations, Ground Penetrating Radar surveys and luminescence dating reveal that the mega-dunes consist of poorly bleached parallel- and cross-bedded meltwater sand and gravel, with occasional large-scale foresets indicating palaeoflow directions perpendicular to the dune crests. Estimates of flow conditions utilizing different empirical approaches suggest water depths between 9 and 21 m, flow velocities between 1.5 and 14.5 m/s and water discharges between 82,000 and 1,400,000 m3/s.
Our findings underline the importance of glacial meltwater as a land-shaping and sedimentological agent acting along the margin of a retreating ice sheet.