Included Debris Characteristics

Reference CorteCorte (1962) conducted a series of laboratory freezing experiments in which a horizontal, planar freezing front moved up through a cylindrical chamber. During the process, Corte seeded the advancing front with particles of various densities, sizes and shapes, and subsequently analysed the ice cores for evidence of preferential clast rejection in terms of the debris character and the freezing rates involved. He reported a crude inverse relationship between the freezing rate and the maximum size of particles forced ahead of the ice, and also that the shape of any particle, through its influence on the ratio of mass-to-contact area, played a role in the process such that “increasing the contact area of the particle while the size is constant causes the migration to increase” (Reference CorteCorte, 1962). Theory and modelling of the forces which result in individual particle rejection at the freezing interface have validated these observations. For example, Reference WilcoxWilcox (1980) has equated the hydrodynamic force tending to push a spherical particle into an advancing freezing front with the disjoining pressure of the liquid film which exists between the ice and that particle. The author concluded that, for a particle being lifted against gravity, the critical freezing rate above which the particle will be entrapped varies inversely with the radius of the particle squared or cubed, depending on its size. Similar conclusions have been reached by Reference Uhlmann, Chalmers and JacksonUhlmann and others (1964) from a series of experiments where particulate suspensions were frozen between two slides. Above a minimum size of c.15 pm, a critical velocity for entrapment was identified as being inversely proportional to the diameter of the particle squared.

Stable-Isotope Chemistry

More recently, (Reference Souchez, Tison and JouzelSouchez and others 1987, Reference Souchez, Lorrain, Tison and J1988b) have dealt with the control exerted by freezing rate on the iso topic composition of ice. These authors pointed out that equilibrium fractionation represents only one extreme of a range of situations and that, in reality, the preferential incorporation of heavy isotopes into the ice (¹⁸O and D[²H]) at the interface is controlled by molecular diffusion across a thin “boundary layer”, the thickness of which reflects the amount of mixing in the reservoir. Thus, in general, the faster the freezing rate the further the preferential heavy-isotope incorporation from that described by the equilibrium-fractionation coefficient, corres-ponding to a Rayleigh-type distribution in the ice. In the experiments carried out by Reference ArnasonArnason (1969), in order to determine empirically the fractionation coefficient for deuterium as water froze to ice, the author identified a rate of 2 mmh’^-1 as being the threshold above which equilibrium fractionation did not occur. While this figure is specific to the particular apparatus used, quantitative assessment of the eAect has come from Reference Souchez, Tison and JouzelSouchez and others (1987), who used a box-diffusion model to analyse the migration of isotopes through the boundary layer. These authors found that each combination of boundary-layer thickness and freezing rate resulted in a unique isotopic distribution within the refrozen ice. Support for the model came from the analysis of lake-ice samples where theoretically determined freezing rates were in good agreement with those derived from known temperature fluctuations during the period of ice formation. Similarly, satisfactory results have been obtained for sea-ice growth rates in a core from Breid Bay, Antarctica (Reference Souchez, Tison and JouzelSouchez and others, 1988b).

Gas Nucleation

The thermodynamics of gas precipitation from solution and consequent debris entrapment in freezing liquids may also be intimately associated with the rate of advance of the freezing front. Controlled laboratory experiments carried out by Reference Rowell and DillonRowell and Dillon (1972) demonstrated that, where 50 ml cylinders of various electrolytes containing clay suspensions were frozen from the base at c. 10 mmh^-1 and from the sides at c. 1 mmh^-1, gas bubbles in the resulting frozen cores were observed to have been preferentially precipitated in thin horizontal bands separated by relatively clear ice containing occasional vertical bubble lineations, parallel to the direction of freezing. Additionally, it was observed that the ice which had experienced slow freezing around the outside of the samples was completely free of bubbles. The formation of this bubble-free ice is consistent with processes of efficient gas rejection at a slowly advancing freezing front as described by Reference HalletHallet (1976), yet the horizontal layering through the rest of the sample requires further explanation. The authors stated that, as the solution freezes and both air and electrolyte are displaced into the liquid ahead of the interface, the concentrations of these increase in the liquid until saturation or supersaturation is reached and air nucleates to form gas bubbles. At this point, the sudden reduction in the concentration of dissolved gas at the interface causes the freezing point of the liquid around the bubbles to rise and results in the rapid freezing of a layer of liquid, with insufficient time for the bubbles to migrate away from the front. Support for this theory came from observations that fewer bubbles, in layers further apart, were observed in ice formed from freshly boiled water than in ice formed from distilled water. Reference CorteCorte (1961) froze 5 mm thick layers of water between glass slides and reported the concentration of trapped air bubbles (radius R) to be proportional to R^{1 -7}, while a threshold existed at a freezing rate of c. 17μms^-1 (61 mmh^-1) with bubbles forming as cylinders parallel to the growth direction below this, while bubbles were egg-shaped (with their narrow ends aligned towards the interface) at rates faster than this. In the freezing experiments of Reference Bari and HallettBari and Hallet (1974), a similar threshold was identified, along with another at 2-4 μms^-1 (7.2-14.4 mmh^-1) below which no bubbles formed at all. However, any quantitative analysis of bubble nucleation is problematic as the initial gas and debris content of the water must also be taken into account. These authors also observed a crude bubble layering parallel to the freezing front similar to that reported by Reference Rowell and DillonRowell and Dillon (1972), which they explained in terms of non-linear variations in the relationship between gas saturation at the interface and phases of nucleation.

Reference Rowell and DillonRowell and Dillon (1972) made the additional observation that the formation of bands of air bubbles parallel to the freezing front also played a central role in the incorporation of clay-rich layers in those cases where solutions containing suspended, dispersed clays were frozen. Similar to both the electrolyte and the dissolved gas, and in accordance with the work of Reference CorteCorte (1962) and Reference WilcoxWilcox (1980) cited above, dispersed day particles were observed to migrate in front of the advancing freezing front, but only until gas precipitation occurred. At this point, debris was trapped within the bubbles and thereby incorporated into the ice. The hypothesized enhanced freezing rate through these bubbly zones would also be conducive to debris incorporation, particularly if, during freezing in the presence of high electrolyte concentrations, some of the clays had formed into small aggregates, as was observed by the authors upon thawing the cores.

In some more recent work, Reference Clayton, Reimnitz, Payne and KempemaClayton and others (1990) have documented debris disposition in ice formed from lateral freezing through a cylinder containing an aqueous suspension of clays. With a homogeneous distribution at the onset of freezing, the authors reported not only that particles had been pushed towards the centre of the container (resulting in c. 2 cm of clear ice around the outside of the core) but also that segregation by size had occurred within the interior zone such that finer particles were moved more effectively by the freezing front (according with the experimental results of Corte and the theoretical treatment by Wilcox outlined above). Significantly, such preferential exclusion resulted in debris banding where “the concentric rings approximately halfway into the cross-section are comprised mostly of larger particles, while smaller, less distinct particles cause the ‘turbid’ appearance at the centre of the cross section”. This sorting process resulted in corresponding variations in debris concentration through the core (personal communication from J. R. Clayton).

Ice Crystallography

Ice crystallography (grain-size, shape and fabric) may also be diagnostic of unidirectonal adfreezing. As in the solidification of metamorphic rocks, grain-size is inversely related to the rate of freezing while crystal shape and fabric may be strongly controlled by the direction of the migration of the freezing front through the liquid. Reference ShumskiyShumskiy (1964) analysed the process of geometric selection during orthotropic crystallization whereby more favourably oriented grains capture less effective competitors, usually within 50-100 times the mean diameter of the crystals in the initial layer. During this process the orientations of the main crystal axes can align themselves either normal to the surface of freezing or parallel to it (Reference ShumskiyShumskiy, 1964) while more rapid cooling results in stronger fabrics (more uniform orientations) through reducing the thickness of the supercooled layer immediately adjacent to the ice crystals, Reference Tison and LorrainTison and Lorrain (1987) have analysed the crystallography of ice from a cavity beneath Glacier de Tsanfleuron, Switzerland, at three stages as it progressed from an undeformcd floor-ice coating to full incorporation within the basal layer of the glacier. In horizontal section, the initial fabric of the floor-ice layer was largely equatorial with a certain number of randomly orientated grains. In addition, crystal shape was anhedral perpendicular to the direction of freezing and grains were elongated in the direction of freezing. As this ice was picked up by the glacier and incorporated into the basal layer, the fabric evolved progressively to a bimodal form under the influence of the dominant pure shear-stress field.

Strong evidence therefore exists not only to support the argument that ice formed by unidirectional net adfreezing should be identifiable in terms of its physical characteristics but also that the rate of freezing involved may exert a significant influence over the detail of those characteristics, particularly the debris, gas and isotopic content of the ice. In certain cases, the distribution of gas bubbles and fine debris in the ice, and the freezing rate may be interdependent and linked in a complex manner.

A series of controlled laboratory freezing experiments has been carried out in order to investigate such relationships further.

Ice Crystallography

(a) Crystal Size

Thin sections were cut both from three horizontal sections through both cores 1 and 2, and as a single vertical section through each and analysed in the cold room using a universal stage. Data presented in Table 2 demonstrate a strong tendency for the ice crystals to grow in size parallel to the direction of the migration of the freezing front such that, in experiment 1 where freezing was downwards and the suspension stirred, the mean grain diameter increases continuously from 3.5 mm at a depth of 15-15.1 mm at approximately 85 mm from the top of the core. This represents the product of two complementary effects: first, the tendency for ice crystals to compete and “capture” or die out as the front propagates, and secondly, a tendency towards the growth of larger individual crystals at slower rates of formation.

Table 2. Mean crystal diameter by thin section (“a” and “b” are the longest and shortest axes, respectively)

Grain-size, however, is not only the product of the freezing rate and grain capture but may also be influenced by the debris content of the ice concerned (e.g. Reference BakerBaker, 1978). During freezing, ice-crystal growth is inhibited by the presence of debris such that particulate concentration and grain-size are inversely related. This effect was borne out in all experiments, where it was found that the crystal size was very sensitive to the concentration of fine debris. Ice-crystal size in the field will additionally reflect the effects of strain rate, ice temperature and time since formation (through enhancing recrystallization), though these factors can usually be regarded as “constant” at a sufficiently local scale.

(b) Crystal Fabric

c-axis orientations were measured on each of the horizontal sections through cores 1 and 2, and hemispheric projections of the data are presented in Figure 2. Optic axes are aligned strongly in sub-horizontal positions, broadly parallel to the freezing front and perpendicular to the direction of its migration. This strong monoclinic distribution is in accordance with the work of Reference ShumskiyShumskiy (1964) and Reference Tison and LorrainTison and Lorrain (1987). The strikes of the crystal axes appear to be randomly distributed, as might be expected given the lack of any lateral component of freezing during grain formation. As a result of the highly variable sample numbers concerned, no relation can be determined between the freezing rate and the fabric strength.

Fig. 2. Ice-crystal fabrics from cores I and 2 plotted on a Schmidt (equal area) net. H is the depth below the upper surface.

Sedimentology

(a) Debris Disposition

In both experiments 1 and 2, where particle grain-size was less than 125 pm and exclusion at the interface was assisted by gravitational effects, the major part of the debris was rejected at the freezing front throughout the formation of the cores. In core 1, which was stirred, a very small amount of debris was incorporated both at the beginning of the experiment, when the freezing rate was high, and at the end when the debris concentration had risen such that clast-to-clast contacts may have prevented efficient migration away from the freezing front. Debris concen-trations in six bands down the core are given in Table 3. Throughout the core, a small amount of fine debris was present on the walls of gas bubbles as a result of incorporation at the interface at those locations where bubbles were engulfed by the advancing ice front. In situations where bubbles were not forming, the fines at the interface were pushed ahead of the advancing ice front. In experiments 3 and 5, where the propagation of the freezing front was upwards and debris up to 1 mm in diameter was supplied to the interface during the experiments, the freezing front passed through the debris but pushed a certain amount of the finer fraction ahead of it, lifting it out from the seeded band into the ice above (Fig. 3), In experiment 4, a saturated diamict from Glacier de Tsijiore Nouve, Switzerland, was frozen through from the base. As the freezing front migrated through the top of the diamict, it carried fine particles with it which were subsequently frozen into the ice above the debris (Fig. 4).

Table 3. Included debris concentration as measured in six bands down core 1

Fig. 3. Vertical section through core 5 demonstrating debris translocation from the seeded bands into crude layers in the overlying clearer ice.

Fig. 4. Fine debris elevated from a diamict from Glacier de Tsijiore Mouve (Switzerland) and frozen into the ice above. The contrast between the grain-size in this elevated band and the parent debris is highlighted in Figure 5a.

Fig. 5. Grain-size distributions of initial debris and elevated debris at various height intervals in core 4(a) and core 5(b).

The debris carried out from the frozen bands in experiments 3 and 5 is clearly visible in cross-section (Fig. 3) and is not deposited evenly in the clearer ice above but as crude, low-concentration layers each up to 10 mm thick. These are created in the absence of associated bubble banding and may form as a result of the crossing of an internal threshold involving the gradual build-up of debris at the interface until the overburden resistance to the disjoining pressure is sufficient to prevent the rejection of the debris in contact with the ice. At this point, debris is incorporated into the growing ice, relieving the overburden pressure and once again facilitating the efficient rejection of all debris at the interface. The process will repeat until there is insufficient debris in the water to allow build-up above the interface to the extent required to overcome the forces of rejection. It appears, therefore, that debris banding may develop within the ice either in association with bubble nucleation or in its absence.

Gas Distribution

In general, freezing rates were not fast enough to result in the production of the egg-shaped bubbles reported by Reference Bari and HallettBari and Hallet (1974), yet clouds of fine bubbles and cylindrical bubbles, both occasionally in association with entrapped silt, were observed in the cores. The top 20 mm of core 1 was composed of clear ice, reflecting low gas concentrations in the initial water, even though the freezing rate was high and rejection therefore relatively ineffective. Beneath this layer, cylindrical bubbles formed, aligned perpendicular to the freezing front and thereafter, as the front migrated through the reservoir, these cylindrical bubbles could be observed extending from the freezing front up into the ice. Any debris settling into the opening of these bubbles was subsequently incorporated on the wall of the growing cylinder while complete debris rejection was facilitated at the planar interface over the rest of the core. Such cylinders, occasionally widening and constricting to form linked bubble trains, were predominant through much of the core while, towards the base, the bubbles were observed to aggregate under the influence of the agitation due to the stirrer. This resulted in the production of a complex formation of bubbles of various dimensions including a large, flat bubble aligned in the plane of the freezing front of dimensions 40 mm x 30 mm x 5 mm.

A band of bubble cylinders, each elongated perpendicular to the freezing front, was observed to form in experiment 3 between heights of 5.3 and 5.6 cm at a freezing rate of under c. 1.9 mmh^-1 (0.5 μm s^-1). The ice in this zone was a faint amber colour and closer inspection indicated again that fine particles had been incorporated on to the walls of these bubbles. While the formation of cylinders rather than bubble trains is consistent with low freezing rates, the actual observed rate here is significantly lower than the threshold identified by Reference Bari and HallettBari and Hallet (1974) below which no bubbles formed in their experiments (2-4 μm s^-1). This effect probably arises from the design of the freezing chambers utilized: in the present study, the upper surface of the freezing reservoir was partially sealed during upward-freezing experiments, with only a thin (c. 5 mm) layer of air present to accommodate expansion. In this case, the gas pressure was not atmospheric at the upper interface and diffusion away from the freezing front would have been correspondingly impeded, thereby inducing nucléation at lower freezing rates than in the open-system experiments of Reference Bari and HallettBari and Hallet (1974).