Supercooled meltwater discharge from Skeiðarárjökull and Skaftafellsjökull and associated glacial debris entrainment

Introduction
Glaciers and ice sheets entrain large quantities of sediment and processes by which they do so continue to be debated. Knowledge of glacial sediment entrainment processes is crucial to understanding past and present sediment fluxes and the sedimentary record. Glaciohydraulic supercooling has been identified as an active sediment entrainment mechanism at glaciers in Alaska(e.g. Lawson et al., 1998; Evenson et al., 1999) and was subsequently documented at several temperate Icelandic glaciers (e.g. Roberts et al., 2002; Tweed et al., 2005; Cook et al., 2007, 2010;Larson et al., 2010). It has also been suggested that that rapid discharge of supercooled meltwaterfrom the base of over-deepened ice sheet basins during the Quaternary constitutes a plausible mechanism for entraining extensive amounts of sediment in the Laurentide Ice Sheet (Alley et al.,1998, 1999; Roberts et al., 2002; Larson et al., 2006). There has been a tendency for the process tobe uncritically accepted and the controls on the spatial and temporal variability of glaciohydraulic supercooling have yet to be thoroughly investigated; even at sites that fulfil the physical requirements to enable the process to occur, researchers report inconsistency in the presence offield evidence and the supercooled signature in meltwater temperature measurements (e.g. Tweed et al., 2005). Over the last fifteen years there has been robust debate regarding i) the link between glaciohydraulic supercooling and the formation of thick sequences of basal ice (e.g. Cook et al.,2007; Larson et al., 2010; Cook et al., 2010, 2011), iii) the potential for silt dominance in basal ice sequences to be used as a diagnostic indicator of the process (e.g. Evenson et al., 1999; Larson etal., 2006; Cook et al., 2011, 2012) and iii) the degree to which landform-sediment assemblages can be attributed to super cooling (e.g. Larson et al., 2006; Evans, 2009). Some of these issues are considered elsewhere in this volume. Here we summarise evidence of super cooling from Skeiðarárjökull and Skaftafellsjökull during a range of hydrological conditions and present evidence of associated glacial sediment entrainment.