LABORATORY OBSERVATIONS OF SEDIMENT ENTRAINMENT BY FREEZING SUPERCOOLED WATER SIMON J. COOK 1 , PETER G. KNIGHT 2,3 , DEBORAH A. KNIGHT 3 and RICHARD I. WALLER 2,3 1 Centre for Glaciology, Institute of Geography and Earth Sciences, Aberystwyth University, Ceredigion, UK 2 Research Institute for Environment, Physical Sciences and Applied Mathematics (EPSAM), Keele University, Staffordshire, UK 3 School of Physical and Geographical Sciences, Keele University, Staffordshire, UK Cook, S.J., Knight, P.G., Knight, D.A. and Waller, R.I, 2012. Laboratory observations of sediment entrainment by freez- ing supercooled water. Geografiska Annaler: Series A, Physical Geography, 94, 351–362. doi:10.1111/j.1468- 0459.2011.00445.x ABSTRACT. Debris in basal ice produced by glaciohy- draulic 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 perco- lating through it. It has therefore been suggested that silt-dominance may be diagnostic of glaciohydraulic super- cooling. The aim of our work is to test this hypothesis that freezing sediment-laden supercooled water necessarily pro- duces ice dominated by silt. We do this by simulating two freezing processes under laboratory conditions: (1) perco- lation of sediment-laden water through frazil ice; (2) turbulent supercooling and subsequent freezing of sediment-laden water. In experiments repeated using differ- ent particle sizes (sand, silt and clay in individual experiments) both processes entrained sand most effec- tively 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 sig- nature 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 freez- ing process. Key words: basal ice, freezing processes, laboratory experi- ments, sediment entrainment, supercooling Introduction The establishment of diagnostic sedimentary crite- ria by which to recognize the influence of specific glaciological processes is crucial for making inter- pretations about the origin and significance of glacier ice facies and glacigenic sediments (e.g. Bennett et al. 1999; Knight et al. 2002; Evans et al. 2006). Numerous studies at the Matanuska Glacier, Alaska (Strasser et al. 1996; Lawson et al. 1998; Evenson et al. 1999; Larson et al. 2006) and at glaciers in southern Iceland (Cook et al. 2007, 2010, 2011) have demonstrated that ‘stratified facies’ basal ice (debris-rich basal ice with a layered appearance) produced by glaciohydraulic supercooling is characterized by high proportions of silt. This has led to the suggestion that the pref- erential entrainment of silt may be diagnostic of supercooling-related ice facies, and hence that it may be possible to identify the influence of super- cooling at modern and former glacier margins from analysis of sediment texture (Evenson et al. 1999; Larson et al. 2006). We present results of labora- tory experiments designed to test a hypothesis that silt is preferentially entrained by freezing super- cooled water and that silt-dominance is, therefore, a diagnostic indicator of glaciohydraulic super- cooling. Research context Glaciohydraulic supercooling Glaciohydraulic supercooling allows the freeze-on of water and sediment to the base of glaciers, and can occur where the pressure melting point of water ascending the adverse slope of a subglacial overdeepening rises faster than the water is heated by viscous dissipation (Alley et al. 1998; Lawson et al. 1998). This requires that the adverse bed slope is between 20 and 70% steeper than the ice © The authors 2012 Geografiska Annaler: Series A, Physical Geography © 2012 Swedish Society for Anthropology and Geography DOI:10.1111/j.1468-0459.2011.00445.x 351