We achieved supercooling by two different methods. The first method was to pressurize water to approximately 206kPa, reduce the water temperature to -0.3°C, and then release the pressure. The second method was to maintain turbulence in the water by means of a bath-recirculating pump, reduce the temperature to -0.3°C, and then switch off the turbulence. Pilot experiments produced similar freezing rates and styles whether the supercooling was achieved by pressure or by turbulence. We adopted the turbulence method for our final experimental procedure because the turbulence also allowed us to keep sediment in suspension within the sample during freezing, which was analogous to the subglacial prototype environment.

Water was supercooled and frozen in 100-L tubs at different ambient laboratory temperatures, with different amounts of clay, silt and fine sand in the water, and in some cases with chemically 'spiked' water. For crystallographic and sedimentological analysis

Figure 92.1 Frazil (left), clear (centre) and herringbone (right) facies created during the freezing experiments. Thick sections (0.5 cm) viewed in natural light. Scale bar is 1 cm in each photograph.

the distinctive facies that typically comprised the frozen blocks were sampled by cutting from the block and thin-sectioning. For chemical analysis, samples were taken of water before freezing, of water remaining unfrozen at the end of the experiment, and of each frozen facies.

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