PREFERENTIAL INCORPORATION OF COARSE SEDIMENT DURING NEEDLE- GROWTH: A PRELIMINARY ANALYSIS

Julia Branson

GeoData Institute University of Southampton Southampton SO17 1BJ e-mail: [email protected]

Abstract

Soils affected by permafrost and periglacial activity commonly exhibit distinctive sorting patterns, either at a macroscopic or microscopic scale, in which coarse material is preferentially lifted towards, or across, the surface. Earlier laboratory investigations have indicated no difference between the grain-size distribution of the bulk soil sample and the sediment included within needle-. The present study, however, based on laboratory and field data, shows that the included material is significantly coarser than the host material. The results suggest that, during the migration of a freezing front, fine particles are ÒpushedÓ ahead of it while ÒcoarseÓ particles are incorporated within the ice.

Introduction between the results of this study and an earlier study (Meentemeyer and Zippin, 1981). The contribution Ground affected by action processes such as nee- which this may make to the frost sorting of material dle-ice growth, ice lens formation and frost heave often and to determining the relative frost susceptibility of exhibits distinctive patterns in which sediments may be soil is also discussed. sorted. Differences in particle characteristics can occur either horizontally across the soil surface or vertically Method within the soil profile. Features produced by sorting can be macroscopic (e.g., nets, circles, polygons, stripes) This study forms part of a wider investigation into the or microscopic (comprising small-scale variations in the processes that control the growth of needle ice and the soil profile). In some instances the net movement of disturbance and transport of sediment by needle-ice coarse particles to the soil surface has been observed; crystals. It was conducted primarily in a laboratory, this causes problems in some agricultural areas during allowing the conditions under which needle ice grows severe winters when stones appear at the soil surface to be closely controlled and monitored. The experimen- and fence posts are heaved out of the soil (Corte, 1961; tal design and instrumentation used are described in Jahn, 1985). Branson et al. (1996). A limited number of observations of needle-ice growth in natural conditions were also It is generally accepted that needle ice is important in made at river bank and garden sites close to initiating the development of miniature soil patterns. Birmingham, UK. For example, Ballantyne (1996) determined that the dif- ferential growth of needle ice was the principal process The apparatus consisted of a split perspex box (50 cm in the relatively rapid formation of a miniature sorted long x 30 cm wide x 40 cm deep), the top two-thirds of net in Scotland. Regardless of whether needle ice is a which contained the soil sample and the bottom a factor causing sediment sorting, it is regarded as a bath which ensured that there was a constant supply of significant factor of soil disturbance on bare slopes and water to the soil sample. river banks (e.g., Lawler, 1993). A series of experiments used both remoulded samples This paper investigates the processes by which gro- of soil and undisturbed blocks. The undisturbed blocks wing needle ice may preferentially incorporate sedi- were excavated from a river bank where needle ice had ment that is coarser than the material from which the been observed. In the perspex box, the stream-facing sediment was lifted, and explores the difference side of the block (i.e., that which was subject to needle-

Julia Branson 75 ice growth in the field) formed the upper surface of the reduced by remoulding, but similar handling of a sample box. In using undisturbed blocks of soil, this coarse sample may increase its frost-susceptibility due series of experiments differs from most other laboratory to the degradation of particles and manufacture of studies which have used disturbed soil samples which additional fines. As the current experiments were con- have been sieved and distributed homogeneously in the cerned with the effects that needle-ice growth may have sample box (e.g., Higashi and Corte, 1971; on sediment lifted by the needle-ice crystals, and com- Meentemeyer and Zippin, 1981; Soons and Greenland, parisons are made with natural soil samples, results are 1970). presented from an experiment that used an undisturbed block. Arguments for and against the use of 'remoulded' samples have been presented. Freezing and thawing Platinum resistance thermometers and nylon soil action cause irreversible changes to properties such as moisture blocks were placed at 1, 3, 5 and 10 cm below the bulk density of the soil, and thus will be affec- the soil surface. Linear vertical displacement transdu- ted differently by each freeze-thaw cycle (e.g., Polar cers (LVDTs) were placed on the soil surface to monitor Research Board, 1984). Thus it may be preferable to use the growth of the ice crystals from the surface. The elec- remoulded samples so that a new soil is produced that trical output from the thermometers, soil moisture ele- has not previously been affected by frost action. ments and the LVDTs was scanned at 3 s intervals by a Experiments would thus commence with soils with datalogger. similar characteristics. Although this method means that experiments should be reproducible, properties The box was placed into a cold chamber and soil sur- such as compaction and frost-susceptibility may be dif- face cooling rate was controlled using a microcomputer ferent to that of the original soil under field conditions. which was connected to the thermometer at the soil For example, the frost-susceptibility of fine soil may be surface and a heating coil placed above the soil surface.

Figure 1. Grain-size distribution of parent and sediment incorporated into needle ice: laboratory sample.

76 The 7th International Permafrost Conference Figure 2. Grain-size distribution of parent and sediment incorporated into needle ice: Field site 1. Prescribed time series of temperature were pro- mixing a soil sample, dividing it into four equal weight grammed into the computer which then compared the sub-samples and then analysing each sub-sample as actual soil surface temperature at a given time with the described above. No significant difference in grain-size prescribed temperature in the program. A heating coil distribution determined for each sub-sample was above the soil surface was switched on if the actual found. temperature was below the required temperature (described further in Branson et al., 1996). This method, Results although quite crude, allowed soil surface temperature to be controlled within +/- 0.2¡C. A number of cooling In a number of the laboratory experiments and field curves with different rates of surface cooling were used. observations, sediment was found to have been lifted from the ground surface by the needle-ice crystals. The Prior to the start of a series of experiments, a sample sediment was lifted in several different forms: either of the soil block was taken for particle-size analysis. pushed on top of the crystals, forming a soil 'cap' Following needle-ice growth, the ice needles and sedi- (which could either be frozen or unfrozen) or incorpo- ment lifted by them were collected and the ice melted. rated within the ice crystals. Sediment was incorpora- The organic matter was removed from the sediment ted in two patterns: either as distinct layers of sediment using hydrogen peroxide and then the sediment was within otherwise crystals or as particles dis- wet sieved through a 63 mm sieve. The proportion of the persed throughout the crystals, giving the ice a brown sample > 63 mm was wet-sieved through a bank of appearance. sieves at 0.5 phi intervals - the coarsest being 2 mm. The fine fraction (< 63 mm) was analysed using the pipette Figures 1, 2 and 3 show the grain-size distribution of method (e.g., Akroyd, 1969; Head, 1980). This method the parent material. The granulometric composition of of analysis was tested for reproducibility by thoroughly material that was included within the needle ice from a

Julia Branson 77 Figure 3. Grain-size distribution of parent and sediment incorporated into needle ice: Field site 2. representative laboratory experiment is also shown in sizes 0.8 to 0.017 mm, and the bulk was coarser than the Figure 1. The granulometric composition of material incorporated material for particles > 0.8 mm and < 0.17 from samples of needle ice which grew under natural mm (Figure 1). The incorporated and parent materials conditions on the river bank (from which the laboratory for each sample were compared statistically using a T- sample was taken) is shown in Figure 2. That for sam- test. For each sample it was concluded that the bulk ples from a garden nearby is shown in Figure 3. The and incorporated material had significantly different sediment was principally lifted as distinct layers within particle-size distributions. the needle-ice crystals and dispersed throughout the ice crystals. Field data were collected to test the possibility Figure 4 shows a plot of the difference in the percent- that some aspect of the laboratory procedures had age finer than of the bulk and incorporated material for influenced the results of the study. Only sediment finer each of the soil samples. A positive difference indicates than 2 mm was included in the analysis to ensure com- that at that particular grain size the incorporated sam- parability with the procedures used by Meentemeyer ple was relatively coarser than the bulk sample, and a and Zippin (1981). negative difference that the bulk sample was coarser than the incorporated sample. The profiles for the three samples are distinct. The incorporated sediment from field site 1 was coarser Discussion than the bulk sediment for all particle sizes, while at field site 2 the incorporated sample was coarser than The results from the present study contradict the find- the bulk only for particle sizes less than 0.5 mm ings of Meentemeyer and Zippin (1981), the only pub- (although the difference between the two samples for lished study that compares the characteristics of the particles > 0.5 mm is only 0.07 to 0.3%). The profile for material lifted by needle ice with those of the host the laboratory sample indicates that the incorporated material. From their experiments, in which needle ice sediment was coarser than the bulk sediment for grain was grown in a laboratory using remoulded soils as a

78 The 7th International Permafrost Conference Figure 4. Percentage by which included sediment is coarser than the parent material. growth media, they determined that there was no sig- will occur in the vicinity of the stone. nificant difference between the grain-size distribution This process is thought to occur principally where the of the bulk soil sample and the sediment included with- thermal conductivity of the stone is greater, and its in the needle-ice crystals. Thus, they concluded that porosity lower, than that of the surrounding soil matrix needle-ice growth does not selectively entrain particu- (Van Vliet-Lanše, 1985). lar particle sizes. Washburn (1979) suggested that the upfreezing of The difference in the results of the experiments by stones can occur by either one of two processes: frost Meentemeyer and Zippin and the current study may be push or frost pull. The former process is similar to that a result of differences in the experimental design of the described above - ice forms beneath coarser material two studies. This is discussed below following a review and forces it up. During frost pull, the freezing boun- of studies which have investigated the preferential lift dary descends to the top of the particle, and the frozen of coarse particles by frost action. sediment adheres to it. The stone is then lifted by heave of the freezing layer during the period taken for the PREFERENTIAL SORTING BY FROST ACTION: A REVIEW freezing boundary to descend from top to bottom of the The frost sorting of material is thought primarily to stone. occur within particles in the centimetre size range. Van Vliet-Lanše (1985) described a process by which coarse Results from an earlier series of laboratory experi- material becomes uplifted from the surrounding soil ments by Corte (1961, 1962a, 1962b, 1963, 1965, 1966), (based on the work of Kaplar, 1965), in which ice nucle- subsequently confirmed by Rowell and Dillon (1972), ation and segregation occur first at the base of the stone which used sediment particles in a water column that and then push the stone up through the soil matrix. was frozen at different rates, indicate that coarser parti- This initial growth of ice beneath the stone depletes the cles are preferentially incorporated into ice during local water supply and thus it is less likely that further freezing. These experiments show that fine particles

Julia Branson 79 migrate ahead of the freezing line, whilst coarser parti- to understand the processes by which particles may cles are trapped in the ice. Particles become incorporat- become incorporated during freezing. Results from this ed into the frozen layer when they cannot maintain a study show that sediment is lifted by needle ice when water film between them and the ice interface. Whether there is an imbalance in the heat and water fluxes to / a particle is incorporated is thus dependent on its size, from the freezing front. This can occur when moisture shape and density (Corte, 1962a, 1962b); larger particles supply is restricted or there is over-rapid heat loss from require a greater volume of water in their thin films, the ground surface. This imbalance causes the freezing and this may become depleted if there is limited mois- front, which is required to be stable to maintain needle- ture availability or surface temperatures are very low. ice growth, to descend. Sediment is then frozen within Different size particles will become incorporated there- the ice (Branson et al., 1996). The descent of the freezing fore at different freezing rates. Rowell and Dillon (1972) front seems to occur at different spatial and temporal analysed the data from Corte's (1962b) experiments and scales, and depending on the scale involved, results in determined that particles of 0.2 mm diameter were the formation of either multi-tiered needle ice, needle- pushed ahead of the freezing front at freezing rates of ice crystals with sediment dispersed throughout the < 2 mm hr-1 per hour, while particles with a diameter crystal, or frozen soil caps. of 10 mm moved at freezing rates of < 1 cm hr-1. The lat- Sediment becomes incorporated within needle ice as a ter figures were of the same magnitude as those from result of the migration of the freezing front. It is proba- Rowell and Dillon's own experiments which showed bly this migration process, similar to that described by that particles 5-10 mm, and occasionally down to 0.1 mm Corte (1961, 1962a, 1962b) and Hallet (1990), which -1 became frozen at freezing rates of 1 cm hr . The expe- caused the preferential incorporation of coarser sedi- riments also indicated that clay particles became aggre- ment into the ice. When the freezing front descends into gated during the freezing process (Rowell and Dillon, the soil profile, the coarser particles are more likely to 1972). become frozen in situ; the reduction in the rate of mois- ture flow to the freezing front which causes needle-ice Hallet (1990) described a similar process by which growth to cease means that the moisture film around fine particles are rejected from ice lenses within soils the ÒcoarserÓ particles is depleted. The fine particles when the freezing front propagates upwards towards have a very thin absorbed layer of water around them, the soil surface (rather than downwards when needle- which is more apt to remain unfrozen. These fines may ice growth ceases). He suggests that, during the propa- then be ÒpushedÓ ahead of the migrating freezing front gation of the freezing front, small non-buoyant particles (although some fines may be ÒattachedÓ to the coarser within the soil water are rejected from the freezing particles and thus are also incorporated into the ice). If water. The size of ÒsmallÓ particles is not defined, ice segregation then recommences, the in situ frozen although as suggested by Corte, the relative size of the (relatively coarser) material is incorporated between particles that are rejected from, or incorporated into, the two layers of needle ice. Thus it is expected that sedi- ice is probably related to moisture and temperature ment layers or dispersed particles within the ice crys- conditions. This rejection process is less effective for tals or lifted as a frozen soil cap, can have a higher frac- ÒlargerÓ particles because the distance that the water tion of coarser particles than the soil from which it orig- must travel (a function of particle circumference) is inated. The critical size of the ÒcoarseÓ and ÒfineÓ sedi- increased, and thus they are preferentially frozen into ment is dependent on the rate of freezing which in turn the ice when the water in their thin adhered films is not is affected by soil moisture content and soil surface tem- replaced. perature.

Studies by PŽrez (e.g., 1991), Hastentrath (1977) and INFLUENCE OF SOIL MOISTURE ON SEDIMENT INCORPORATION Ballantyne (1996) have concentrated on the sorting of The plots of the difference between the bulk and coarse material by needle ice, and have concluded that needle-ice entrained material (Figure 4) show that there recurrent needle ice lifts stones on or near the soil sur- is a variation in the texture of the different soil samples, face, which then migrate into surface indentations by particularly between the field samples and the laborato- creep and rolling when the crystals melt. This process ry sample. Part of the difference between samples is of sorting is related to needle-ice ablation rather than caused by natural variations in the parent sediment, formation and consequently is not applicable to the and thus the particle sizes available to be lifted and/or preferential lift of sediment during needle-ice growth. entrained. It is suggested, however, that soil moisture availability, and its influence on freezing rate, is also PREFERENTIAL INCORPORATION OF COARSE PARTICLES DURING NEE- significant. DLE-ICE GROWTH To explain why coarser sediment may be preferential- Temperature and soil moisture data were not avail- ly incorporated into needle-ice crystals it is important able for the field sites and the rate of freezing during

80 The 7th International Permafrost Conference the growth of the needle ice cannot therefore be deter- fines down the soil profile. This could influence the rel- mined. At the river bank site (grain-size distribution, ative grain size of the incorporated soil in one of two Figure 2), however, where soil moisture supply was ways: unlimited, it is likely that the freezing rate was not suf- ficiently rapid to incorporate the smaller particles, and 1. If the soil has been affected by needle-ice growth on thus the % coarser of the included sediment is greater a number of occasions, then the soil surface may be co- than the % coarser of the bulk sample throughout the vered with loose, relatively coarser particles that were profile. Similarly, the needle ice at field site 2 grew lifted towards the soil surface by the previous events. shortly after a period of heavy rain, and thus moisture Thus the soil cap will contain particles that are relative- was probably not limited. ly coarser than the bulk soil.

In the laboratory sample it is unclear, however, why 2. Alternatively, if the surface material has been the incorporated sediment was dominated by a finer removed by a process such as slope wash or aeolian fraction than that of the bulk material at grain sizes erosion prior to needle-ice growth, then the material < 0.017 mm. During the laboratory experiments, in lifted will have a similar grain-size distribution to that which ice containing layers of sediment were formed, of the host soil. the moisture and soil surface temperature conditions were manipulated in an attempt to cause sediment Meentemeyer and Zippin (1981) did not state how the incorporation (moisture supply was limited and surface material was lifted in their experiments (i.e., as layers or cooling rate increased). This caused the freezing rate to as a soil cap), but as the soil sample was progressively increase until it was too rapid to sustain ice segregation depleted in moisture over the series of experiments it is (and an associated stable freezing front), and conse- likely that the sediment was lifted as an unfrozen soil quently the freezing front migrated into the soil profile cap. Thus, as the soil had been remoulded prior to the and sediment incorporation occurred. Given the limita- experiments, the sediment lifted would have a similar tions in soil moisture it is probable that the freezing texture to the parent sample as described in situation rates were higher than those which occurred in the (2) above. This could explain why the results of the cur- field. Typically, freezing rates in the laboratory were rent experiment, in which the sediment was lifted by a between c. 4 and 14 mm hr-1 (this was determined indi- process which involved the migration of the freezing rectly from the thickness of the sediment layer pro- front, produced different results to those of duced and the profile of heave at the soil surface). Meentemeyer and Zippin. These rates are of the same order of magnitude at which Rowell and Dillon (1972) observed that particles IMPLICATIONS OF PREFERENTIAL INCORPORATION less than 0.01 mm were incorporated into the ice lenses. This study has only analysed sediment less than 2 mm This can explain why relatively more finer particles diameter, and differences in grain-size composition below 0.017 mm were incorporated into the laboratory between the bulk and included material were only evi- samples than the field samples (25% of incorporated dent for all samples at grain sizes less than approx 0.5 sediment was finer than 0.017 mm in the laboratory mm. This suggests that the geomorphic implications of compared to 18% and 20% in field samples 1 and 2 the process in this case are not likely to be very evident. respectively). It does not explain, however, why the Nevertheless, a concentration of coarser particles at the incorporated material of the laboratory sample was soil surface may affect the future frost-susceptibility of finer than the bulk, since, if the freezing rates increase the soil. This is similar to the process described by and finer sediment was incorporated, there should be Ballantyne (1996), whereby the lift and drop of stones minimal difference between the bulk and incorporated by needle-ice melt increases the textural contrasts sample. This is an area which requires further laborato- between the coarse borders and fine cells of frost nets. ry testing, with a variety of soils and controlled freezing Patterns initiated by this process could then be devel- rates. In particular, micromorphological analysis of oped by other processes such as differential frost heave frozen soil profiles could be used to analyse the pat- and the growth of ice lenses within fine soil. For sedi- terns of particle size in relation to the freezing front. ment incorporated within needle ice the differences in particle size will be of a smaller magnitude.

SEDIMENT LIFTED IN AN UNFROZEN SOIL CAP When soil is lifted as an unfrozen soil cap, ice nucle- Conclusion ation occurs below the soil surface at a distance deter- mined by the availability of moisture, ice segregation This preliminary analysis of sediment from a small commences immediately, and thus the position of the number of observations has shown that particles lifted freezing front relative to the ground surface remains by needle ice are generally coarser than the soil from stationary (as long as there is a sufficient flow of mois- which they are lifted. It is considered unlikely that the ture to the freezing front). Thus there is no ÒpushingÓ of processes controlling the preferential inclusion of coars-

Julia Branson 81 er particles observed in this study are related to the in front of a migrating front, unlike the coarse particles processes which form . Nevertheless, which become frozen. Further investigations should preferential uplift may cause small-scale variations in focus upon the critical freezing rate thresholds at which grain-size distribution through the soil profile, with particles of different sizes are entrained within the coarser particles being ÒpushedÓ up the profile. In this needle ice. study preferential incorporation was thought to occur by fine particles being ÒpushedÓ down the soil profile

References

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82 The 7th International Permafrost Conference