WHEN THE ICE HITS THE WATER: REASSESSING THE BEHAVIOR OF GLACIERS TERMINATING IN CLOSED WATER BODIES

The most dynamic but least understood aspects of ice-sheet behavior are the mechanisms that determine the fate of glaciers terminating in large water bodies. Bouancy, iceberg production, and the sensitivity of ice flow to subtle changes in resistance at the glacier bed, may lead to instabilities and non-linear responses. Modeling efforts assume rapid evacuation of icebergs, but some glaciers terminate in closed basins where icebergs may accumulate. Langjökull, second largest of Iceland's ice caps, feeds two outlet glaciers that terminated in glacial lake Hvítárvatn during the Little Ice Age. Here we show how the Langjökull/ Hvítárvatn system, where iceberg production exceeded iceberg melt or evacuation, produced previously unseen outlet glacier behavior. Multibeam bathymetric surveys document a series of radiating narrow troughs extending from the ice margin into the central deep as well as pockmarks and irregular grooves. These patterns suggest both random iceberg motion, and near-parallel flow of narrow, iceberg-stabilized ice fingers that extended ≥1 km beyond the ice margin. Similar flow behavior on a much larger scale may explain long-standing debates about past ice-sheet behavior, including the rapid readvance of the Laurentide Ice Sheet across Lake Superior ca. 11 ka, and the genesis of glacial flutes observed in the central Arctic Ocean basin.