Scales and rates of glacial sediment removal: a 20 km long, 300 m deep trench created beneath Breiðamerkurjökull during the Little Ice Age

Abstract

A 20 km long, 2-5 km wide trench, which extends to 300 m below sea level, is believed to have been created by removal of sediment during the advance of Breiðamerkurjökull in the Little Ice Age. Between 1732 and 1890, the glacier advanced by 9 km, covered an area of 45 km2and excavated a volume of 5 × 109m3, equivalent to an average of 110 m over this area. Average sediment-removal rate during the 158 years was 32 × 106m3a−1or 0.7 m a−1km−2averaged over the area covered by the advancing glacier. Calculated over the whole drainage area of the eastern branch of the glacier of 750 km2, the denudation rate would be 4 × 10−2m a−1km−2. Fluvial processes are estimated to have carried about 30 × 106m3a−1, and the sediment fluxes within the ice and by the deforming subglacial till are estimated to be 105and 106m3a−1, respectively. The average sediment concentration in the glacial streams would have been about 10 kg m−3. Such concentrations have been measured in Icelandic rivers during jökulhlaups and surges. Several surging events took place during the advance of Breiðamerkurjökull, and jökulhlaups drain regularly beneath the glacier from ice-dammed marginal lakes. The present rate of transport, although considerable, seems to be about 10 × 106m3a−1, of which 30% is transported by the river to the sea and 70% is dumped into a proglacial lake.