Laboratory observations of sediment entrainment by freezing supercooled water

Basal ice produced by glaciohydraulic supercooling is typically characterised by high proportions of silt, and has previously been attributed to the preferential filtering of silt from sediment-laden water percolating through frazil ice. Silt-dominance has come to be considered as diagnostic of supercooling. We test the hypothesis that freezing sediment-laden supercooled water necessarily produces ice that is dominated by silt by simulating two freezing processes simultaneously under laboratory conditions: (1) percolation of sediment-laden water through frazil ice; and (2) turbulent supercooling and subsequent freezing of sediment-laden water. In experiments using unmixed sand, silt or clay particle sizes 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. Our results indicate that silt-dominance should therefore not be expected for supercooled freeze-on, and is not a diagnostic signature for supercooling. The silt-dominated character of basal ice types associated with supercooling may owe their silt-dominance to a silt-dominated sediment supply, rather than to the freezing process.