fine particle translocation in soils developed on glacial deposits, southern baffin island, n.w.t.,...

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Fine Particle Translocation in Soils Developed on Glacial Deposits, Southern Baffin Island,N.W.T., CanadaAuthor(s): William W. LockeSource: Arctic and Alpine Research, Vol. 18, No. 1 (Feb., 1986), pp. 33-43Published by: INSTAAR, University of ColoradoStable URL: http://www.jstor.org/stable/1551212 .

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Arctic and Alpine Research, Vol. 18, No. 1, 1986, pp. 33-43

FINE PARTICLE TRANSLOCATION IN SOILS DEVELOPED ON GLACIAL DEPOSITS, SOUTHERN BAFFIN ISLAND, N.W.T., CANADA

WILLIAM W. LOCKE Department of Earth Sciences, Montana State University

Bozeman, Montana 59717, U.S.A.

ABSTRACT

Accumulations of fine particles have been observed in coarse-grained, well-drained soils in Arctic areas, and have also been reported in alpine and temperate areas. Both silt caps on cobbles and silt-enriched horizons have been explained as the result of frost sorting. Detailed particle-size analysis of soils from the eastern Canadian Arctic reveals trends which are incon- sistent with the frost-sorting hypothesis. In the soil matrix, all particles finer than fine sand show surpluses at some depth. The depth of maximum surplus increases from 15 cm for very fine sand to 55 cm or more for fine clay. The excess clay is offset by deficits higher in the profile, implying redistribution of primary clay. Increases over parent material silt content occur throughout the profiles, implying addition of silt-sized particles to the soil. This surplus of silt is consistent with the addition and infiltration of loess. Both the depths of maximum fine particle accumulation and the amount of accumulated material increase with age, but appear to reach finite limits.

Analysis of silt caps reveals surpluses in all size fractions finer than fine sand at all depths. Excess silt is greatest in silt caps in the upper 20 cm, clay increases are greatest between 40 and 60 cm, and silt caps trend toward parent material composition at depths greater than 60 cm. The distribution of sand within the silt caps also differs from parent material.

The mechanism suggested for particle translocation is illuviation by rain. The interstitial pores in the coarse, noncemented matrix are sufficiently large to allow movement of the observed particle sizes, and rainfalls sufficient to saturate the soil to the depth of coarse silt accumulation have been observed in a 15-yr record. The apparent redistribution of material downward from the surface is the most compelling evidence in favor of illuviation.

INTRODUCTION

THE PROBLEM The preferential migration of coarse soil particles rela-

tive to the soil matrix is well documented in some periglacial environments (Washburn, 1973). Frost sorting has been experimentally verified as an operative mechanism by Corte (1966), but is limited only to the movement of coarse particles through a relatively fine matrix under saturated or nearly saturated conditions. This paper examines the problem of the migration of fine particles

through a relatively coarse matrix, under conditions which seldom include saturation.

The phenomena of interest include "silt caps" (Bock- heim, 1979), which cover the upper surfaces of clasts, reaching thicknesses of up to 5 cm. They also include silt- enriched horizons ("isons," Fitzpatrick, 1956, 1971) within the soil profile. Both of these features are dominantly silt, but include appreciable clay as well. The terms cited above will be used throughout this paper, but should be con-

?1986 Regents of the University of Colorado W. W. LOCKE / 33



sidered to include both silt and finer particles. Their re- corded occurrences are summarized by Forman and Miller (1984).

The origin of these features is debatable. Fitzpatrick (1956, 1971) favors a permafrost origin; however, several of the characteristics by which he defines isons (Fitz- patrick, 1956: 248-250) are inaccurate or incompatible with a permafrost origin. Specifically, Fitzpatrick (1956: 249) states that silt accumulations are always found in an area which is known or thought to have had a periglacial climate, occur in freely drained soils, and have sharply defined upper surfaces. Other workers, however, have found features which appear to be analogous in the southeastern (Olson and Hole, 1968: 175) and southwestern United States, outside of the range of other reported periglacial phenomena. Corte (1966) demonstrated that the efficiency of frost sorting decreases as the water supply decreases. Although well-developed silt accumulations may have clearly defined upper surfaces, such does not appear to be the case in the initial stages of indura- tion (below). Thus, isons in excessively drained soils may be explained better by some other process.

Down-profile illuviation of clay is a well-recognized process (U.S. Soil Survey Staff, 1975). Illuviation of silt is less well recognized, although both field study (Allan and Hole, 1968; Forman and Miller, 1984) and laboratory experiments (Wright and Foss, 1968) suggest that it does occur. One factor which makes the recognition of silt migration difficult is the lack of sufficiently detailed data. In this study, particle size analysis at 1-4 increments and sampling at 10-cm depth increments yielded data which support an infiltration origin for silt enrichment in well-drained Arctic soils.

THE FIELD AREA The soils discussed in this paper are located on eastern

Cumberland Peninsula, Baffin Island, N.W.T., Canada (Figure 1). The study sites are about 200 km north and east of those examined by Bockheim (1979), but are affected by a similar climate. The mean annual rainfall varies from about 4 cm at Broughton Island to 11 cm at Cape Dyer and 12 cm at Pangnirtung; the mean annual temperature (-10?C) varies little across the peninsula. The climate of this region is not known in detail, thus the climate at the sample sites may vary significantly from the regional averages.

Cumberland Peninsula is a tilted granitic block, pos- sibly a horst (van der Linden, 1975), with accordant sum- mits at about 1500 m asl and a fringing plateau at about 700 m asl (Bird, 1967; Andrews et al., 1970). The block is dissected by troughs, most probably fault-controlled, which have been scoured or modified by extensive gla- ciation (Dyke, 1979). Glacial and glaciomarine deposits, which are largely cryoturbated, are the result of a series of glacial advances. Recent work (Nelson, 1980; Locke, 1979, 1980; Miller et al., 1977) suggests that significant advances of local ice occurred about 50-350, 3200, 8500, 70,000, and 100,000(?) yr BP and many earlier times.

FIGURE 1. Location of the field area. Dots indicate sample sites, squares indicate weather stations.

THE SAMPLE SITES The original purpose of the soil studies on Cumber-

land Peninsula was to use the extent of soil development as an indicator of relative age of glacial deposits (Andrews and Miller, 1972; Birkeland, 1978; Bockheim, 1979; Locke, 1979). To isolate the time variable the other soil- forming variables (parent material, topography, climate, and biological activity; Jenny, 1941) were held approxi- mately constant through sampling. All sample sites were located on the level crests of moraines or glaciofluvial deposits. Vegetation consisted of a 70% or less cover of saxicolous lichens and a 5% or less cover of vascular plants. Sites with obvious evidence of cryoturbation (e.g., sorted circles) were not sampled and ice-cemented permafrost, although present in finer-grained sediments, was not observed in any of the soil pits. In such well-drained, coarse-grained soils the depth of the active layer may exceed 1.5 m (Evans and Cameron, 1979; Bockheim, pers. comm., 1976).

METHOD Soil samples were collected as channel samples by hori-

zon in preliminary sampling, and at 10-cm depth increments in later sampling. In addition, the silt caps were scraped from cobble tops at various depths in the profile. Some contamination of silt caps with matrix material may have occurred, but the boundary betweeen the

34 / ARCTIC AND ALPINE RESEARCH



two was generally abrupt, thus contamination should be was then analyzed by sieve and pipette methods (modi- negligible. fied from Day, 1965) to yield the particle-size distribu-

The soil samples were air-dried and sieved to remove tion in 1-4 increments. the >2-mm fraction. The fine-earth fraction (<2-mm)

RESULTS

The results described here are those from five profiles sampled by horizon in preliminary work and one profile sampled at 10-cm intervals in a more detailed study. Al- though the raw data' do show trends of particle reloca- tion, there are two problems involved with such data: the constraint that all particle classes must total to 100% (thus a change in one affects all), and the variability of two orders of magnitude in the presence of some size classes (i.e., 20%7o medium sand, 0.2% medium clay), thus emphasizing the coarser particles. In order to correct for these two problems, I have recalculated the percentages to a presumed parent material (PPM) and have presented the data as percentage of that presumed parent material.

The definition of a PPM is more difficult, as a range of particle-size distributions may be present depending on the process(es) of deposition (Vorren, 1977). For the preliminary work the PPM was estimated using both analyses of recent (< 350 yr) till and supraglacial debris and the weighted means of well-sampled (n > 4) profiles (from Locke, 1979). Because the results (Table 1) were comparable, the recent till was held to represent parent material. These results are similar to those determined by Birkeland (1978), Bockheim (1979), and Andrews (1985). Anderson (1978) found Neoglacial tills to be markedly coarser than have other workers; however, his sampling was restricted to the uppermost 5 cm of soil, thus his samples may have been affected by wind action or infiltration with rainwater. For the more detailed profile the PPM was estimated as the average composition at > 75 cm depth in the profile. It should be noted that the PPM does not need to be precisely known in order for the trends shown below to be valid. A different esti- mate of PPM would show the same trends, but with different absolute values.

The standard used to normalize the analyses to PPM is the entire sand class (2000 to 62 Lim). It was assumed that the original parent material had a particle-size distribution similar to that in Table 1 and that the sand fraction was immobile while the finer particle classes migrated. The strength of this assumption is shown by the uniformity of sand distribution within each profile (Figures 2, 3, and 4). Between-profile variation will be discussed below.

The recalculation of each sample proceeds as follows (see example: Table 2):

'These data are available from the Editor, Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colo- rado 80309-0450, U.S.A.

(1) The sand fraction percent is divided by that of the PPM, yielding a correction factor to be applied to the rest of the data.

(2) The data are multiplied by the correction factor in order to approximate the particle-size distribution as modified by particle translocation.

(3) The recalculated data are divided by the expected value for each size fraction, yielding percent deviation from PPM. The discussion will be placed on the percent deviation figures. The raw particle-size data' is a sufficiently large data set as to be difficult to interpret, accordingly, I have taken a graphical approach to the pre- sentation of the data.

PRELIMINARY DATA -SOIL MATRIX

Thirty-five profiles were analyzed in preliminary work. Of these, 28 showed evidence of fine-particle translocation. The results are shown (Figure 2) from five soil profiles which had three or more samples, showed no evidence of cryoturbation, and range in age from ca. 8000 yr to ca. 100,000(?) yr. The data show the effects of parent material variability, fine particle migration, and time.

Each profile shows a similar distribution of sand throughout, with variability between horizons generally less than 10% (of PPM). The variability between profiles is more marked, with profiles C and E showing deficits of coarse fractions and surpluses of fine fractions relative to PPM. This difference indicates better sorting in those two profiles than in PPM. The absolute amounts of translocated fines in these profiles cannot be estimated because of the difference in parent material, but the trends in particle-size distribution are still valid. Those trends involve both depth in the profile and particle size.

The general trend with depth in the profile is best dis- played by profile A. Near the surface there is a deficit relative to PPM in all size classes finer than coarse silt (31 gzm). At greater depth there is a surplus in those same particle sizes, and at still greater depth this surplus decreases to near zero. In other words, the particle-size distribution approaches that of PPM at depth.

The remainder of the profiles illustrate the details of this relationship. In all cases the deficit in fines is most marked near the surface. At depths of as little as 13 cm (profile B), surpluses may be noted in the coarse and medium silt fractions. A similar accumulation is present at 18 cm in profile E, 30 cm in profile C, and 39 cm in profile D. Excesses over parent material in finer fractions (fine silt to coarse clay) are found at depths of 25 to 60 cm. A surplus in even finer particles (<2 2im) is only observed in the soil matrix at depths of 40 cm or more

W. W. LOCKE / 35



(profile D). It is apparent that the translocation of particles within the soil is in part a function of their size, with sand immobile, coarse silt nearly so, medium silt mobile, and fine silt very mobile. Particles of clay size are not as readily removed from the surface horizons as are coarser particles, but once moved, they accumulate at the greatest depth.

There is also a relationship between particle migration and soil age. The older soils show surface depletion of nearly 100%o of the 4- to 31-nm fraction, whereas profile A shows only a 60%7o loss. Profile A is indistinguishable from PPM at a depth of 54 cm, whereas the older profiles have marked accumulations of fines at depths to 85 cm (profile D). It is apparent that the particle-size dis-

tribution within these excessively well-drained soils changes, albeit slowly, with time.

PRELIMINARY DATA -SILT CAPS In the course of the preliminary work a single silt cap

was sampled at 50 cm in profile E (Figure 2). The particle size distribution within the silt cap can be compared to that within the adjacent soil matrix. Both analyses show sand components markedly different from that of PPM. The silt cap shows a greater deficit in medium sand than does the matrix, implying further sorting during the formation of the cap. Most noteworthy, however, is the surplus in all silt and clay size classes within the cap, relative to both PPM and the adjacent soil matrix. A "silt"

TABLE 1 Particle-size distribution in soil parent materiala

Size class (0tm)

2000 1000 500 250 125 62 31 16 8 4 2

A. Recent Tills and Supraglacial Material (n = 7)

Mean (std. dev.)

11.9 9.4 13.8 19.3 13.4 12.2 6.3 2.4 1.8 1.0 1.8 1.0 1.7 1.0

3.8 2.5 1.8 5.8 0.5 0.4 0.7 1.0

B. Reconstructed Tills (n= 5)

Mean (std. dev.)

13.6 11.9 17.3 21.5 12.5 8.4 3.8 2.7 1.8 2.4 1.2 0.2 1.7 0.6 1.5 1.1 0.7 0.4

2.0 0.2

4.8 0.8

C. Comparison with Other Results (Particle Sizeb)

Worker

Locke (this paper) Anderson (1978) Birkeland (1978) Andrews (1985) Bockheim (1979)C

Sand

71.4 94.2 72.0 70.5 85.0

Silt

21.3 4.6

22.2 22.9 12.3

Clay

7.2 1.2 5.8 6.6 2.7

aValues in percent of the fine-earth fraction. bOn the Wentworth scale. COn the U.S.D.A. scale.

TABLE 2 Example of recalculation of particle-size data

Sample 2000 1000

Size class (/m)

500 250 125 62 31 16 8 4 2

Profile D (40 cm) 7.4 9.3 15.7 19.6 11.9 8.1 5.5 5.4 4.6 5.1

PPM 11.9 9.4 13.8 19.3 13.4 12.2 6.3 3.8 2.5 1.8 (Normalize to a total sand fraction of 67.8% from measured 63.9% by multiplying each fraction by 67.8/63.9

Profile D (40 cm) 7.9 9.9 16.7 20.8 12.6

(Divide by PPM to find relative abundance in eo% of PPM.)

Profile D (40 cm) 66

(Compare with Figure 2.)

8.6 5.8 5.7 4.9 5.4 8.0

105 121 108 94 70 92 150 196 300 138


7.5 5.8 1.06.)



cap, although composed to a large part of excess silt, may include significant clay as well.

DETAILED DATA- SOIL MATRIX The detailed profile analysis was undertaken specifi-

cally to examine the phenomenon of fine particle migration. Sampling at 10-cm increments allowed improved resolution of particle-size changes with depth (profile F, Figure 3). The sample site was comparable to the other sites, the level crest of a moraine ca. 70,000 yr old. PPM for this profile was defined as the weighted average of the C horizon samples (the two deepest samples).

The sand fraction percentages are similar to PPM throughout, suggesting uniformity of the parent material. In contrast to the preliminary work, the surficial deficits involve only clays. There are surpluses of fines throughout the profile, with the maximum surplus occur-

ca 8000 yr

A

ca 70,000 yr

ring at about 20 cm depth for very fine sand and coarse silt, 30 cm for medium silt, 40 cm for fine silt to coarse clay, and 50 to 60 cm for medium and finer clay.

The trends of particle migration shown in profile F are similar to those of the other profiles, with surface deficits matched by subsurface surpluses, and the depth to maximum surplus increasing with decreasing particle size. In contrast to those profiles sampled in preliminary work, however, profile F shows a net surplus of fine particles. This net surplus is best explained by addition of eolian material (below).

DETAILED DATA - SILT CAPS In addition to the soil matrix samples, six silt cap

samples were collected at different depths within the profile (Figure 4). Trends in these data are evident within the sand, silt, and clay sizes. In the sand fraction, with the

ca 100,000

M 73

.::185cml, ..:-

C D B

_ I _1 ?I-~~~~~~ PARTICLE

SIZE (um)

o o -0 COr200

CM^ % of

~_1~100 go0 PPM

PPM uncertainty (? 1 S.D.)

indicated by shading

SILT CAP i

E

FIGURE 2. Particle-size distribution as a function of time and depth in the profile (preliminary work). Amount of material in each 1 ( increment is shown as a percentage of presumed parent material. Horizontal axes are located at depth of sample. Shaded areas above the horizontal axes represent surpluses relative to PPM; shaded areas below the axes indicate deficits. See text for discussion.

W. W. LOCKE / 37



Profile F

?L

104-

204-

304-'-

4 0 0-*- w-'

Soil Matrix

Profile F

10

Depth

50^

(cm)

30,

Depth

40 4-.--

_L 7 7 b- ~

Presumed

Parent

Material

^) Scale ;600

?OOu4000 % of 000U~ r-' c ? OCM, <

. cO co cs,t- 200 PPM PART I I S I I I ( um ,

PARTICLE SIZE (jum)'?

(cm)

50 4---

60s o=,-~

701

80 Scale 600

Oo 400

C,000011) ^- r ^200 o oo... ( . c -.. c .- 200

Pt f I I I I t I t( *

PARTICLE SIZE (um)'O

FIGURE 3. Particle-size distribution in the soil matrix as a function of depth in the profile (detailed study). Symbols as in Figure 2.

FIGURE 4. Particle-size distribution in silt caps as a function of depth in the profile (detailed study). Symbols as in Figure 2.


Caps

20t__ J

60^

704

80.

901 ----

10C

% of PPM

- -~~~~~~~~~~

I . _ ..

.:.:: ..... ...

k-~~~~~~~Ei;



exception of the cap at 40 cm, there are deficits in the very coarse and coarse sand fractions and surpluses in the fine and very fine sand fractions. Either a different parent material occurs above cobbles or some sorting has taken place. In either case, the comparison to PPM is no longer valid, and only trends, not precise amounts, can be expected from the finer fractions.

Within the silt and clay fractions there are surpluses

throughout the profile. In the shallow caps (5 and 20 cm) the maximum percentage increase occurs in the 16 to 31 itm fraction. At 40-cm depth, this maximum occurs in the 8 to 16 Am class, and at 50- and 60-cm depths it occurs in the 2 to 4 Am fraction. The deepest cap (80 cm) shows the least cumulative surplus of fines, with the mode again in the 16 to 31 zm class. These trends are consistent with the data from profile E.

DISCUSSION

SOIL MATRIX Frost action has been credited with most features of

arctic soils and is unquestionably the dominant factor in the genesis of soils in poorly drained areas. Several lines of evidence suggest that frost sorting is not the dominant agent in soils such as are studied here. First, there was no evidence of frost-heaved blocks or sorted circles or other periglacial phenomena associated with these soils. Neither is there any evidence of sorting within the sand fraction of these soils. Both theory and experimental data (Corte, 1961, 1962, 1963; Kaplar, 1965) document the coarser size fractions as the most susceptible to frost sorting. Finally, Corte (1966: 233) states that frost sorting is most effective in saturated soils. The soils studied here, dominantly bouldery sands and loamy sands with some openwork structure, show no evidence of saturation sufficient to allow sorting. Winter snows are redistributed from the moraine crests by wind, thus even in the spring saturation is unlikely.

Additional mechanisms which could be invoked to explain variations in particle size with depth in a soil include geologic layering, pedogenic destruction of coarse particles and formation of fines, eolian addition or removal of fines at the soil surface, particle sorting by thermal and moisture fluctuations, and redistribution by percolating rainwater. The uniformity within the sand fraction in each profile and the agreement with PPM in many profiles make geologic layering an unlikely explanation for the observed phenomenon. The pedogenic formation of clay, although important in nonarctic environments, is not considered significant in the Arctic (see summary in Locke, 1985). In any case, the formation of pedogenic clay should be indicated by surpluses in clay and deficits in coarser particles in a Bt horizon and in the profile as a whole, which is not the case.

The remaining mechanisms for variation in particle size distribution within a soil include eolian removal and addition and illuviation. Deflation of fines cannot be proven for these soils, although the results of Anderson (1978) indicate that deflation of surface fines has affected recent tills. Such deflation does not, however, explain the distribution of particles with depth in the studied soils.

Addition of fine particles by wind is likely, particularly to profile F. If the surplus material (over PPM) in the upper 75 cm of this profile is considered to represent additional sediment (Figure 5), it has a maximum particle size of fine sand, a mode of coarse silt, and is fine-skewed.

These characteristics are identical to those of loess (Swineford and Frye, 1945) (Figure 6). Assuming a bulk density of 1.5 g cm-3 for loess (Watkins, 1945), the surplus of fines in profile F would represent about 16 cm of loess without infiltration.

The most probably mechanism for infiltration of both loessial sediment and primary silt and clay is through transportation by percolating rainfall. This is the only hypothesis which explains the lack of sorting of sand-sized particles, the distribution of fines with depth, and the occurrence of these features in well-drained sediments in many different climates. In order for such transportation to occur, four criteria must be met:

(1) There must be material available to be moved. (2) The material must be free to move. (3) There must be pathways along which movement can

take place. (4) There must be a transporting agent. The material required is present, both in the ca. 25%o

of PPM which is silt or clay and in the loessial influx. Better sorted sediment, such as fluvial gravels, would be unlikely to show migration of fines because of a lack of fine particles to migrate.

Movement of fine particles would be hampered by cementation of any sort. All of the profiles sampled in this study are acidic (5 <pH <7) and no carbonate was observed. Organic matter is minimal (0 < OM < 1 %), thus most of the soil is primary mineral matter. Adhesion of fine clay to itself (flocculation) or to larger particles may occur, and may explain the smaller percent depletion of fine clay compared to slightly coarser size grades in profiles B, C, D, and E (Figure 2).

Pathways through the soil matrix are required for particle translocation. Bockheim (1979) suggests that desic- cation cracks and/or micropores are necessary to allow movement. The coarse nature of the sampled sediment, however, makes such features unnecessary. About 75% of the fine earth fraction (which represents less than 50%o of the sediment) is sand. If this sand is viewed as uni- form spheres with diameters of 500 tm, the tightest theoretical packing would allow particles finer than 80/m to pass between the sand grains (Pettijohn, 1975). This is in excellent agreement with the observations that some particles of 62 to 125 /m diameter move short distances (profile F) and that many particles finer than that are transported. This phenomenon of translocation of particles about one-eighth the size of the modal size class

W. W. LOCKE / 39



125

1C

7~

Weight (g) i 1 P

110.< I - 5L__< r--3 i

WEIqHT

5' r --_ ' 2 2!

I SURPLUS 0 4 I I I I I I o -...w~~~~~~

)0

5

ercent

0

5

-25

FIGURE 5. Particle-size distribution of surplus and deficit material (by weight and relative to presumed parent material) in profile F. The 6-g deficit in coarse sand across the profile represents less than 5% of PPM, thus can be ignored.

FIGURE 6. Cumulative curves of presumed parent material and excess fines from profile F and dust and loess (Swineford and Frye, 1945).


I

- - -

I L



has been experimentally verified for both clay (Crone, 1975) and silt (Wright and Foss, 1968). Note that this hypothesis is valid only for sediments with a relatively small proportion of fines (< 10% in the whole sediment). More fines would tend to fill the interstices between framework grains, thus reducing particle migration.

The final requirement is for a transporting agent. Rec- ords from the Broughton Island DEW Line site 40 km north of profile F indicate a maximum storm rainfall in 15 yr of about 8 cm. Assuming a desiccated soil before rainfall and tight packing, such a storm would allow saturation to a depth of about 20 cm, in the zone of coarse silt surplus but far above the maximum depth of particle redistribution. Four hypotheses are possible: that saturation is not required, only throughflow; that maximum particle migration occurs when several storms occur without significant intervening evaporation; that events with recurrence intervals of hundreds of years are required to move particles to the greatest observed depths; or that some other transporting agent is active. Profile A, with only minor redistribution at a depth of 25 cm after ca. 8000 yr, may indicate that infrequent events are important in this process.

The strongest evidence for fine particle illuviation in the soil matrix comes from the distribution of translocated particles as a function of their size. The progres- sively greater depths to which finer particles have been moved are best explained by a surface-related process, as is the necessity to move added loess into the profile. No aspect of frost penetration into well-drained soil has been shown or theorized to cause this kind of movement.

The evolution of the soil matrix in sediments meeting the four criteria above may occur as follows. Primary fines are mobilized in heavy rains, and are carried into the profile. Unless they are replaced from above (as by wind), a deficit occurs near the surface and a surplus

occurs at depth. The depth to which they penetrate may depend on the presence of an impermeable substrate (such as permafrost) at depth, or may be a function of the amount of rainfall and preexisting moisture within the soil. Deposition of fines would tend to block the interstices between matrix particles (Bodman and Harradine, 1938), thus inhibiting further transport past that point. This might, given time, lead to the formation of an impermeable horizon above which all particles in transit would be deposited (profile D, 40 cm). In periglacial climates, the formation of such an impermeable layer should increase the probability of cryoturbation, thus ex- treme examples of such particle migration might not be preserved. Eolian material might be added to the primary fines, provided it was not added so rapidly as to absorb all incoming moisture or block passages near the surface of the sediment.

SILT CAPS The silt caps observed in this study (Figure 4) support

the conclusions above. Fine particle surpluses are evident in these caps, and the particle sizes in surplus decrease with increasing depth. Below a depth of about 80 cm (profile F), the magnitude of the surpluses decreases.

The silt caps also show evidence of sorting within the sand-size fraction. In most caps there is less coarse sand than in the adjacent soil matrix. This phenomenon cannot be explained through infiltration of rainwater. It may represent the collection of water in the excess fines near the surface, then frost sorting on a millimeter scale above boulders, or it may be the result of infilling of voids forming above the boulders through frost action (Mackay, 1984). The latter hypothesis is unlikely due to the lack of evidence for frost action on a profile scale in these soils.

IMPLICATIONS

Silt caps and silt-enriched layers (isons) have been reported from most arctic and alpine areas of the world, including Canada, Spitsbergen, Norway, Sweden, Colo- rado, and New Zealand (see Forman and Miller, 1984, for reference). In such areas, where conventional soil development (leaching, formation of pedogenic clays) is slow (Locke, 1985), translocation of fine particles may be the most rapid mechanism of modification of the soil matrix.

Where translocation has proceeded long enough, the accumulation of fines may fulfill the requirements for an argillic horizon (U.S. Soil Survey Staff, 1975). The horizon between 35 and 65 cm depth in profile F is an example of such an accumulation. The classification of such a soil (U.S. Soil Survey Staff, 1975) is problematical. It would clearly not qualify as a Histosol, Spodosol, Oxisol, Verti- sol, Ultisol, or Mollisol. The presence of an argillic horizon rules out Inceptisols and Entisols, leaving Alfisols or Aridisols. However, Alfisols are not permitted with

an arid soil moisture regime and Aridisols are not defined in a pergelic temperature regime. In view of the low annual rainfall, organic matter content, and coarse tex- ture, such a soil might be best defined as a Pergelic Arenic Borollic Haplargid. Such soils have not been recognized by the U.S. Soil Survey Staff (1975).

Many uses of soils for relative age dating purposes involve the use of the <0.25-mm fraction either directly (quartz-plagioclase ratios by X-ray; Isherwood, 1975; clay mineral determination; Birkeland, 1984) or indirectly (such as soil color and exchangeable cations) because of surface area effects. Caution should be used in the interpretation of such data if migration of fines is suspected.

The eastern Canadian Arctic has a periglacial climate, which may have existed in some areas for the last 65,000 yr (Locke, 1979). One objection to interpretation of a long periglacial period has been the fact that loess, which is normally present in periglacial environments (Wash- burn, 1973), has rarely been reported from that area. The

W. W. LOCKE / 41



results of this study imply that infiltration may incor- porate loess into coarse-grained soils. On fine-grained soils, cryoturbation would accomplish the same end. Loess will be preserved as a blanket only on rock, or

where it has been deposited sufficiently rapidly to pre- vent infiltration, or on well-drained soils with a modal particle size in the fine-earth fraction of medium sand or finer.

CONCLUSIONS

Silt caps and silt-enriched horizons have been observed to occur commonly in periglacial areas, and in areas which experienced a periglacial environment during the late Cenozoic. Similar features have been reported rarely from areas not affected by Pleistocene periglaciation, such as coastal plain alluvium from the southeast United States (Olson and Hole, 1968). These features have generally been ascribed to frost sorting (Fitzpatrick, 1956) without, however, observational or theoretical support- ing evidence.

Detailed particle-size analyses of soils with silt accumulations indicate that sorting has taken place. This sorting is surface related, with fine particles (clay) being removed from the uppermost 50 cm and accumulating between 50 and 75 cm below the surface. Coarse particles (coarse silt) may be depleted at the surface and enriched at depths of 10-20 cm. All silt and clay particle sizes are found in surplus as caps atop cobbles and boulders within 75 cm of the surface, with coarser fractions predominat-

ing near the surface and finer fractions at depth. The redistributed fine particles may have originated as part of the sediment or have been added as loess. All of these factors require that a surface-related process causes downward migration and sorting of fine particles. The most probable such process is illuviation by rainwater.

ACKNOWLEDGMENTS

This work was a by-product of research at the University of Colorado under NSF Grants EAR 74-01857 (to Drs. J. T. Andrews, P. W. Birkeland, G. H. Miller, and E. E. Larson) and EAR 77-24555 (to Andrews and Miller). It has profited from discussion over the last five years with many individuals, including Steven Mabee and Drs. P. W. Birkeland, Scott Burns, and Clifford Montagne. Formal reviews by Burns and a num- ber of anonymous reviewers immensely improved the final product. The opinions expressed herein, however, are solely those of the author.

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fine particle translocation in soils developed on glacial deposits, southern baffin island, n.w.t.,...

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