Three physically distinctive ice facies, arranged approximately concentrically within the frozen block, formed during the freezing process (Fig. 92.1). While the turbulence was maintained, frazil ice formed at the surface and around the edges of the tub, and created a network of interlocking plates and needles of ice between which debris was trapped. Continued freezing of the turbulent water-sediment mix developed a debris-poor, bubble-poor 'clear' ice facies with suspended aggregates of debris and with a strongly oriented crystal fabric indicating directional freezing towards the centre of the tub. After the turbulence was switched off a third facies, with a distinctive 'herringbone' crystal structure and intracrystalline bubble/debris lineations, was created nearest to the centre of the tub, with a pool of unfrozen water remaining at the tub centre at the end of the experiment. The physical structure ofthe herringbone ice varied with ambient temperature (Fig. 92.2), with lower temperatures associated with both smaller crystal sizes and finer intracrystal bubble- and debris-lineations. The chemical composition of the three facies was also distinctive. The initial water was spiked with a variety of ionic solutes and the fractionation of the impurities during the experimental freezing is illustrated in Fig. 92.3. Impurities were effectively expelled during the directional freezing of the clear facies but incorporated into the ice during the multidirectional freezing of the frazil ice and, to an even greater extent, of the herringbone facies.

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