Introduction

Supercooling of subglacial water by rapid pressure reduction during flow towards the glacier margin has been widely adopted as a hypothesis to explain the origin of anomalously thick basal ice sequences beneath temperate glaciers, and has even been invoked to explain the debris content of North Atlantic Heinrich layers (e.g. Alley et al., 1998; Lawson et al., 1998; Andrews & Maclean, 2003). However, the application of supercooling theory to the problem of basal ice formation remains controversial. For example, whereas Roberts et al. (2002) suggested that in southern Iceland the entire basal ice sequence of many glaciers draining the ice cap Vatnajokull can be attributed to freezing of supercooled water, Spedding & Evans (2002), working in the same area, concluded that it is premature to attribute thick sequences of basal ice to supercooling, that other mechanisms remained tenable, and that the evidence for supercooling was unclear. One obstacle to our understanding is the lack of diagnostic physical or chemical criteria that relate the supercooling process with identifiable ice facies. As a contribution to clarifying the characteristics that would be expected of basal ice formed by freezing supercooled subglacial water, we have simulated the supercooling process in the low-temperature laboratory and identified specific facies characteristics associated with the process.

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