Laboratory observations of sediment entrainment by freezing supercooled water

Debris in basal ice produced by glaciohydraulic supercooling is typically characterized by high proportions of silt. A prominent hypothesis for this silt-dominance is that frazil ice growing in supercooled water preferentially traps silt from sediment-laden water percolating through it. It has therefore been suggested that silt-dominance may be diagnostic of glaciohydraulic supercooling. The aim of our work is to test this hypothesis that freezing sediment-laden supercooled water necessarily produces ice dominated by silt. We do this by simulating two freezing processes under laboratory conditions: (1) percolation of sediment-laden water through frazil ice; (2) turbulent supercooling and subsequent freezing of sediment-laden water. In experiments repeated using different particle sizes (ssilt and clay in individual experiments) both processes entrained sand most effectively and silt least effectively. In experiments using a sediment mixture dominated by medium to coarse silt, both processes produced ice facies dominated by particle sizes between fine sand and coarse silt. These results suggest that silt-dominance should therefore not be expected for supercooled freeze-on, and is not a reliable diagnostic signature for supercooling. The silt-dominated character of basal ice types associated with supercooling may result from other controls such as a silt-dominated sediment supply or subglacial water flow rates, rather than the freezing process.