1 INTRODUCTION

Cryostratigraphic and hydrochemical characteristicsof permafrost have long been used to evaluate thenature and origin of ice within the ground (Mackay1983, French & Harry 1990, Mackay & Dallimore1992, Moorman et al. 1998). A recurrent problem inthis field concerns the ability to distinguish segregationice formed in periglacial environments from ice formedsubglacially. In both cases ice may form in associationwith top-down freezing; it may be similar in appear-ance, and potentially, it may have similar hydrochem-ical properties. In North America, field investigationshave primarily examined massive ice within fine-grained sediments in the western Arctic (e.g. Mackay &Dallimore 1992), the Queen Elizabeth Islands (e.g.Lorrain & Demeur 1985) and the High Arctic (e.g.Pollard & Bell 1998). By comparison, relatively fewfield-based studies have examined ground ice withinpredominantly coarse-grained sediments on BaffinIsland (Hyatt 1992, 1993, 1998), although Moorman &Michel (2000) have recently examined natural expo-sures on nearby Bylot Island.

In this paper, we describe natural permafrost expo-sures of coarse-grained diamicton that contain icelenses up to 0.5 m thick located in a moraine on south-eastern Baffin Island (Fig. 1). We use crysotratigraphicand hydrochemical data to test whether ground ice inthis periglacial setting originated (1) by the poolingand freezing of surface water within permafrost, or(2) by conventional segregation processes followingdeglaciation of the site, or in a subglacial setting

(3) by flow obstruction regelation processes, or (4) bysubglacial segregation and subsequent preservationbeneath insulating till.

2 STUDY SITE AND GLACIAL HISTORY

Pangnirtung experiences a mean annual air temperatureof �8.9°C and receives nearly 400 mm of precipitation

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Recognition of subglacial regelation ice near Pangnirtung, Baffin Island, Canada

J.A. HyattEnvironmental Earth Science, Eastern Connecticut State University, Willimantic, Connecticut, USA

F.A. MichelEarth Sciences, Carleton University, Ottawa, Ontario, Canada

R. GilbertDepartment of Geography, Queen’s University, Kingston, Ontario, Canada

ABSTRACT: The cryostratigraphy and hydrochemistry of ice-rich sediments in the core of a moraine are usedto test whether ice lenses formed in a periglacial environment as pool ice or segregation ice, or in a subglacialenvironment as regelation ice. A well defined hydrochemical discontinuity, representing maximum post-glacialdepth of thaw, separates the ice-rich core from overlying cryoturbated diamicton. The core contains stackedsequences of curved gravel layers and ice lenses up to 0.5 m thick with petrofabrics inclined to internal sedimentlayering. Ionic and isotopic ('18O and '2H) characteristics indicate that ice lenses have not incorporated modernsurface water, and that source waters were colder than expected for a post-glacial source making a periglacialorigin unlikely. Co-isotopic slopes in the core are similar to the meteoric water line. This together with the curvednature of ice lenses, which suggests deformation by compressive flow, is most consistent with subglacial freezingand subsequent preservation beneath an insulating cover of diamicton.

Permafrost, Phillips, Springman & Arenson (eds)© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

Figure 1. Location of study site on southeast Baffin Islandin relation to Pangnirtung Fiord and Duval River valley wherestagnant ice topography is common. Contours in m fromNTS map 26-I/4 (1:50,000). Paleo ice flow down fiord andagainst topography (white arrows) from Dyke (1979).

annually. Mean annual ground temperatures varyfrom �8°C to �9°C, and active layer thickness rangesfrom 0.8 to 1.6 m (Hyatt 1993). Glaciological stud-ies indicate that climate has varied throughout theQuaternary (Andrews 1985), although continuous permafrost conditions likely have prevailed sincedeglaciation.

The last major glaciation on Baffin Island, referredto as the “Foxe”, spans 120 to 5 ka BP. Exposuresdescribed in this paper occur within the core of a cross-valley ground moraine (Dyke et al. 1982) locatedbetween early-Foxe and late-Foxe glacial limits and atleast 40 m above the local marine limit. The extent ofice cover and timing of deglaciation of the Pangnirtungarea is still debated (Bierman et al. 2001, Wolfe et al.2001). Conventional views (Dyke 1979), supported byweathering zone chronologies, paleolimnological dataand some cosmogenic nuclide data suggest that uplandsurfaces have long been ice free (Wolfe et al. 2001)and imply that our study site likely was deglaciated 30 to 60 ka BP. Other cosmogenic data (Bierman et al.1999) and marine sedimentology studies (Jennings1993), however, support retreat of ice from the Pangnirtung area between 13.4 and 9.0 ka BP.

Exposures described below occur within diamictondeposited by a small re-entrant glacier driven south-ward against the local topographic gradient by a largemass of ice moving down Pangnirtung Fiord (Dyke1979). The study site, located within the limits of thisre-entrant lobe, is characterized by stagnant hummockyterrain, surface and subsurface drainage, and a seriesof relict ice-marginal streams (Hyatt 1992). There is noevidence to suggest that the valley was glaciated fol-lowing melting of the re-entrant glacier.

3 METHODS

Ice and frozen sediments were collected from freshlyexposed surfaces after removing a minimum of 5 cm offrozen surface material. Samples were initially placedin double thickness polyethylene bags; air was forcedout before the bags were sealed, and the samples wereallowed to thaw at 5°C. After melting, excess pore waterwas transferred to airtight polyethylene bottles and the remaining saturated sediments were resealed fortransport. Where necessary, additional pore water wasextracted from the thawed sediments using a centrifuge.Meltwater samples were analyzed for major cationconcentrations using atomic absorption spectropho-tometry, while '18O and '2H concentrations weredetermined by mass spectrometry at the Stable IsotopeLaboratory of the Ottawa-Carleton Geoscience Center.Particle size distributions, gravimetric moisture con-tents, and ice petrofabrics were determined as describedby Hyatt (1993).

4 CRYOSTRATIGRAPHY AND HYDROCHEMISTRY

Channelized runoff following an intense rainstorm inAugust 1984 (51 mm in 19 h) exposed ice-rich per-mafrost at sites where streams passed beneath portionsof the moraine. Slumping covered these exposures,although subsequent mass movements have periodicallyexposed new sections of the moraine’s ice-rich core.Slumping in 1990 re-exposed the core at the inflow andoutflow of a small subsurface channel. The inflowexposure (Fig. 2) is parallel to, while the outflowexposure located 30 m downslope, is transverse to paleoice-flow. Two cryostratigraphic units are exposed (Fig.3), including a lower ice-rich, shell-bearing diamicton(Unit A), which is subdivided by a well-developedhydrochemical discontinuity, and an overlying ice-poor, cryoturbated and shell-free diamicton (Unit B).

The outflow section exposes 2 m of dense grayishbrown (2.5 Y 5/4) sandy diamicton (Unit A) thatcontains shell fragments dated at 52 460 � 1460 (TO-2196) corrected to a 13C �25%. Unit A has an aver-age moisture content of 33%, contains small lenticularpockets of stratified sand and gravel, and is capped bya 0.3 m thick ice lens, and 0.5 m of sandy diamict.Unit B is a coarse, strong brown (7.5 YR 4/6), ice-poor diamicton with a much lower moisture content(13%), and abundant cryoturbation structures. A gravellayer, averaging 5 cm thick, separates Units A and B.

An oriented sample of a 0.3 m thick ice lens, recov-ered from Unit A (outflow section in Fig. 3) was exam-ined under plain and cross polarized light. The ice is

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Figure 2. (a) NE facing inflow exposure oriented parallel topaleo ice flow with (b) detailed view of a thin gravel layer, troughcross-bedded sand, ice lens sequence. Scale in cm.

clear and colorless in transmitted light, and containsabundant sediment inclusions but few bubbles. Sedi-ment inclusions occur as both discontinuous and con-tinuous bands, and as individual grains and grainaggregates suspended between the bands. Rare sphe-roidal to dish-shaped bubbles (�1 mm diameter) occuralong fracture planes and sediment bands. Ice crystalsare anhedral with some elongation parallel to thesediment bands, and average 2.5 cm2 (range 0.1 cm2 to20.6 cm2). C-axis distributions are loosely groupedabout 2 weak point maxima that dip 25° to 80° fromhorizontal, and are inclined 55° to 65° to the sedimentbanding (Fig. 4).

A similar, albeit more complex, 6 m high sequenceis exposed at the inflow site (Figs 2, 3). Unit A has ahigher ice content (up to 200% by dry weight), fewershell fragments, and three repeated sequences of graveloverlain by thick ice lenses and ice-rich diamicton.Gravel layers are laterally continuous, curved, and dip4° to 6° south. Overlying ice lenses in Unit A are 0.02to 0.5 m thick, have either gradational or sharp lowercontacts, and sharp, conformable upper contacts (Fig. 2b). Sediment infilling structures (Mackay1989) are not present at the upper contact of these icelenses. The ice-rich diamict contains thin (1 to 7 mmthick) discontinuous ice lenses that commonly extendlaterally underneath larger clasts. Unit A also containsa 0.5 m thick package of stratified sand and gravelwith well developed planar and cross bedding, sharpunconformable upper and lower contacts, and dispersedfragments of modern moss (109 a � 0.67% modern

carbon, TO-2195). These sediments are interpreted ascavity fill deposited by subsurface flow through themoraine (Hyatt 1992). The sequence is capped by1.85 m of cryoturbated, ice-poor and oxidized (10 YR3/4) diamicton (Unit B).

The most important characteristic of both sectionsis a hydrochemical discontinuity at 2.1 m (outflow)and 2.5 m (inflow) depth, just below the contactbetween Units A and B (Fig. 3). Electrical conductivityand total cation concentrations of pore ice increasewith depth to just below the discontinuity, below whichthey become more variable. Ice lens samples below thediscontinuity have lower electrical conductivity andtotal cation concentrations than surrounding pore ice,although cations are present in similar proportions.The concentration of individual cations also differsabove and below the discontinuity. Mean concentra-tions of Na� and Ca�� are 2–10 times greater, Mg��

concentrations are nearly the same, and K� concentra-tions are 30% lower below the thaw unconformity.

Isotopic values also shift across this discontinuity(Figs 3, 5). Above, the mean '18O and '2H values respectively are �17.8% and �125.4%, decreasing to�26.1% and �200% below the discontinuity. T-testcomparisons of slope coefficients for simple linearregression lines fit to '18O and '2H data differ signif-icantly (p � 0.02) above and below the discontinuity.Values above the discontinuity follow a line sloping at5.54 (r2 0.87) while the slope below the discontinuity(8.17, r2 0.96) is similar to the meteoric water line.

5 ORIGIN OF THE ICE LENSES

We interpret the hydrochemical discontinuity as a thawunconformity reflecting the maximum post-glacialdepth of thaw. This interpretation is supported by thechange from oxidized sediment colors above (10 YR)to reduced colors below (2.5 Y), and the first appear-ance of visible ice lenses below the thaw unconfor-mity. As well, the co-isotopic slope above the thawunconformity indicates nonequilibrium fractionationprocesses (Michel 1986) that are common within the active layer, whereas the slope below the thaw

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Figure 3. Section log showing conductivity, ion concentrationand stable isotope ('18O and '2H) values for inflow (top) and out-flow (bottom) sections (HD hydrochemical discontinuity, SSO stratified sands with modern organics, TO-2195 109 � 0.67%modern carbon, TO-2196 52 460 � 1430).

Figure 4. (a) Lower hemispheric Schmidt equal area projectionof crystallographic c-axis orientations contoured at 2% intervals(fp foliation plane defined by sediment bands in ice). (b) Orien-tation of 30 cm thick ice lens at outflow section with front being inthe up paleo-ice direction (see Fig. 1).

unconformity does not. Thus, ice lenses in Unit Ahave been preserved beneath insulating sediments ofUnit B and, therefore, may either have formed by freez-ing in a periglacial environment following deglaciation,or they may be relict, having formed prior to deglacia-tion by freezing in a subglacial environment.

Age considerations alone can not distinguish betweenthese origins because the precise date of deglaciationfor this site is debatable (Wolfe et al. 2001). Our shelldate (52 460 � 1460, TO-2196), while consistent withconventional views of deglaciation, may be infinite,and we have not dated gases trapped within the ice. Accordingly, we test hypotheses of ice lens for-mation by comparing observed cryostratigraphic andhydrochemical characteristics with those expected for ice lenses formed in periglacial and in subglacialenvironments.

5.1 Pool ice formation in a periglacial setting

Observed subsurface pipe flow (Hyatt 1992) togetherwith cavity deposits that contain modern organic frag-ments (Fig. 3) indicate that surface waters have pene-trated parts of the moraine core. Thus, it is possiblethat ice lenses in Unit A are pool ice in which case theyshould contain modern organic fragments or be asso-ciated with cavity fill, and have crystallographic prop-erties characteristic of bulk water freezing. However,ice lenses in Unit A are enclosed by dark, non-stratifiedand organic-free diamicton that is easily distinguishedfrom cavity fill deposits. Furthermore, crystallographicdata are not consistent with a pool ice origin. Columnarice crystals were not present; c-axis distributions (Fig.4) were inclined rather than normal to the upper andlower contacts of the ice lens, and there were no indi-cations of either a fine-grained “chilled margin” or acentral discoloration zone, which often develop inassociation with bulk water freezing (Pollard 1990).

If ice lenses below the thaw unconformity are poolice, their ionic and isotopic composition should be sim-ilar to surface water but different from the surround-ing permafrost. However, we find the opposite. Ionic

concentrations in ice samples below the thaw uncon-formity differ from those in snow and summer precip-itation (Fig. 6). Similarly, '18O values below the thawunconformity (�23.3% to �29.0%) differ from thosein ice in tension cracks at the surface (�16.4%) and inmelt water from snow (�19.3% to �17.8%). Yet, '18Oand '2H values for ice lenses below the thaw uncon-formity (�25.3% to �28.2%) are indistinguishablefrom surrounding pore ice indicating a common watersource. Thus, ice lenses in Unit A cannot be pool ice.

5.2 Segregated ice lens formation in a periglacial setting

While the cryostratigraphy of ice lenses in Unit A havesome characteristics consistent with top-down freez-ing, not all properties are typical of ice lenses formed ina periglacial setting. Ice lenses do contain gradationallower contacts, matched sediment bands and grainaggregates commonly found in segregated ice (Mackay1989). Also, most segregated ice coatings underlyinggravel clasts in Unit A are continuous with both largeand small ice lenses (Fig. 2b). However, some icelenses are thicker (up to 0.5 m) than would be expectedgiven the coarse grained texture of enclosing sedimentsand the absence of an obvious mechanism for elevat-ing pore water pressures in a periglacial setting (aswould be required to grow thick ice lenses in coarsegrained sediments).

More importantly, however, the repeated sequenceof curved gravel layers, overlying pockets of finelycross-bedded sands, ice lenses and ice-rich sedimentsare difficult to reconcile with a periglacial origin.Segregated ice coatings under some of the gravel clastsindicate that the ice lenses formed after the sedimentswere deposited, while delicate cross-beds above thegravel indicate that the sediments were deposited byflowing water and have not been sheared since depo-sition. This implies deposition of sand and gravel byflowing water, subsequent freezing with little or nodeformation of the sands, and the development of icelenses up to 0.5 m thick within predominantly sandysediments. However, for sands to be deposited along acurved dipping surface, water would either have to flowdown or be injected into a crack or opening in themoraine during stagnation. While this could happen,continued stagnation would destroy cross bedding inthe sands. If the surrounding sediments did not thaw(partial stagnation), then the ice lenses should havecharacteristics similar to injection ice including a fine-grained “chilled” margin, petrofabrics more normal toits boundaries, and a hydrochemistry which differssubstantially from pore ice in the surrounding sedi-ments. These characteristics were not observed.

If ice lenses in Unit A formed by segregation theyshould have hydrochemical compositions that are

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Figure 5. Co-isotopic relationships for samples collected above(filled circles) and below (open circles and squares) the hydro-chemical discontinuity.

consistent with freezing of near-surface waters, andisotopic values and co-isotopic ratios indicative of slowfreezing in a semi-closed system. The hydrochemistryof ice below the thaw unconformity, while clearly dif-fering from modern surface water sources, does notconclusively refute a periglacial origin. Electrical con-ductivities are lower than in most types of ground iceformed in periglacial environments, although individ-ual cation concentrations are within the range reportedfor a variety of forms of ground ice (cf. Mackay &Dallimore 1992). Similarly, isotopic values, whileindicating a colder and/or older source water for ice inUnit A, are similar to values reported for ice formed bypre-Holocene permafrost aggradation at other sites inArctic Canada (e.g. Michel & Fritz 1978, Mackay1983, Mackay & Dallimore 1992). However, a co-isotopic slope near 8 for Pangnirtung ice is not charac-teristic of freezing in a semi-closed system, as wouldbe expected for conventional segregation ice, althoughit is worth noting that co-isotopic slopes for segregatedice can vary depending upon the rate of freezing(Michel 1986). Thus, although hydrochemical datasuggest that ice lenses are not typical of segregated ice,cryostratigraphic observations provide more conclusiveevidence that ice lenses within Unit A did not form byconventional segregation in a periglacial environment.

5.3 Regelation ice formed in a subglacial setting

Sediment-rich ice at the base of glaciers forms by avariety of mechanical and thermal mechanisms(Hubbard & Sharp 1989). This includes pressure melt-ing and refreezing of water around sub-glacial flowobstructions creating flow obstruction regelation ice.Alternatively, subglacial regelation ice may form asfreezing temperatures penetrate beneath the glacierincorporating underlying sediments (Weertman 1961,Boulton 1970). This can result in the growth of segre-gated ice lenses in a subglacial environment. For sim-plicity we refer to lenses formed by this mechanism assubglacial segregation ice.

Flow obstruction regelation is a destructive processresulting in a layer-by-layer accretion of sediment andice (Boulton 1970). This would disrupt primary sedi-mentary structures like the fine trough crossbeds insands that underlie ice lenses in Unit A. Alternatively,repeated episodes of basal freezing (cf. Weertman 1961)could incorporate stacked sequences of relativelyclear subglacial segregation ice with intervening layersof ice-rich sediment similar to the repeated sequenceof gravel-sand-ice-diamicton observed in Unit A (Fig.2b). Once incorporated these sediments and ice lenseswould become curved by compressive flow. This wouldnot destroy primary sedimentary structures. In addition,the presence of high subglacial pressures would explainc-axis point maxima in the Pangnirtung ice lenses thatare inclined to the foliation. Similarly, high pressurewould promote pore water migration toward a descend-ing freezing front resulting in the growth of thick icelenses within coarse grained subglacial sediments.

A subglacial origin requires the incorporation of old,cold glacial meltwater with freezing in an open systemthat does not promote isotopic fractionation. Although'18O values in Unit A are less negative than early tomid-Wisconsinan glacial ice, which typically rangefrom �30% to �40% (Michel & Fritz 1978,Moorman et al. 1996), it is not uncommon for basaldebris-rich glacier ice to be 3 to 4% less negative thanoverlying glacier ice. This is due to the contribution ofisotopically enriched surface water to subglacial icelayers (Lawson & Kulla 1978) and the removal ofisotopically depleted basal melt water during freezing(Hooke & Clausen 1982). In addition, the enriched '18Ovalues at Pangnirtung may simply reflect a reduceddistance fractionation effect because of the proximityto Cumberland Sound (Fig. 1). The co-isotopic slopefor samples from Unit A is similar to the meteoricwater line, a trend that is consistent with subglacialfreezing where atmospherically derived melt waterfreezes slowly under equilibrium fractionation condi-tions (Michel 1982). Thus, although we recognize thatsubglacial freezing and associated co-isotopic slopesmay be complex, hydrochemical data in conjunctionwith cryostratigaphic observations indicate that ice lensin Unit A most likely formed by segregation processesin a subglacial setting.

6 CONCLUSIONS

Cryostratigraphic and hydrochemical characteristicsof ice lenses in ground moraine near Pangnirtung aremost consistent with a Weertman (1961) style of sub-glacial segregation that incorporated sediments andice lenses into the bed of overriding active Wisconsinglacial ice. Ice lenses were deformed by compressiveflow and survived stagnation beneath an insulatingcover of ice-poor, cryoturbated diamicton. Alternative

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Figure 6. Plot showing similarity in cation composition for iceabove the hydrochemical discontinuity (open circles) with precip-itation (open squares) and snow cover (diamonds). Cation compo-sition for ice below the discontinuity (filled circles) differsmarkedly. Precipitation and snow cover from Staple (pers. comm.).

hypotheses that include ice lens formation in perigla-cial environments either as pool ice or as conventionalsegregated ice lenses, or as subglacial flow-obstructionregelation ice can only be rejected using both cryos-tratigraphic and hydrochemical arguments.

ACKNOWLEDGMENTS

Funding was provided from research grants to theauthors from the Natural Science and EngineeringResearch Council of Canada, and from Eastern Connecticut State University. Critical reviews by S.Wolfe, S. Dallimore, M. Maxwell have improved thismanuscript. The first author thanks affiliates ofQueen’s University for their assistance.

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