Recognition of Subglacial Regelation Ice Near Pangnirtung, Baffin Island, Canada

Home , Lawson Island, Pangnirtung, Pond Inlet

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

J.A. Hyatt Environmental Earth Science, Eastern Connecticut State University, Willimantic, Connecticut, USA F.A. Michel Earth Sciences, Carleton University, Ottawa, Ontario, Canada R. Gilbert Department of Geography, Queen’s University, Kingston, Ontario, Canada

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

1 INTRODUCTION

Cryostratigraphic and hydrochemical characteristics of permafrost have long been used to evaluate the nature and origin of ice within the ground (Mackay 1983, French & Harry 1990, Mackay & Dallimore 1992, Moorman et al. 1998). A recurrent problem in this field concerns the ability to distinguish segregation ice formed in periglacial environments from ice formed subglacially. In both cases ice may form in association with top-down freezing; it may be similar in appear- ance, and potentially, it may have similar hydrochemical properties. In North America, field investigations have 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 few Figure 1. Location of study site on southeast Baffin Island field-based studies have examined ground ice within in relation to Pangnirtung Fiord and Duval River valley where predominantly coarse-grained sediments on Baffin stagnant ice topography is common. Contours in m from Island (Hyatt 1992, 1993, 1998), although Moorman & NTS map 26-I/4 (1:50,000). Paleo ice flow down fiord and Michel (2000) have recently examined natural expo- against topography (white arrows) from Dyke (1979). sures on nearby Bylot Island. In this paper, we describe natural permafrost expo- (3) by flow obstruction regelation processes, or (4) by sures of coarse-grained diamicton that contain ice subglacial segregation and subsequent preservation lenses up to 0.5 m thick located in a moraine on south- beneath insulating till. eastern Baffin Island (Fig. 1). We use crysotratigraphic and hydrochemical data to test whether ground ice in this periglacial setting originated (1) by the pooling 2 STUDY SITE AND GLACIAL HISTORY and freezing of surface water within permafrost, or (2) by conventional segregation processes following Pangnirtung experiences a mean annual air temperature deglaciation of the site, or in a subglacial setting of 8.9°C and receives nearly 400 mm of precipitation

443 annually. Mean annual ground temperatures vary 4 CRYOSTRATIGRAPHY AND from 8°C to 9°C, and active layer thickness ranges HYDROCHEMISTRY from 0.8 to 1.6 m (Hyatt 1993). Glaciological studies indicate that climate has varied throughout the Channelized runoff following an intense rainstorm in Quaternary (Andrews 1985), although continuous August 1984 (51 mm in 19 h) exposed ice-rich per- permafrost conditions likely have prevailed since mafrost at sites where streams passed beneath portions deglaciation. of the moraine. Slumping covered these exposures, The last major glaciation on Baffin Island, referred although subsequent mass movements have periodically to as the “Foxe”, spans 120 to 5 ka BP. Exposures exposed new sections of the moraine’s ice-rich core. described in this paper occur within the core of a cross- Slumping in 1990 re-exposed the core at the inflow and valley ground moraine (Dyke et al. 1982) located outflow of a small subsurface channel. The inflow between early-Foxe and late-Foxe glacial limits and at exposure (Fig. 2) is parallel to, while the outflow least 40 m above the local marine limit. The extent of exposure located 30 m downslope, is transverse to paleo ice cover and timing of deglaciation of the Pangnirtung ice-flow. Two cryostratigraphic units are exposed (Fig. area is still debated (Bierman et al. 2001, Wolfe et al. 3), including a lower ice-rich, shell-bearing diamicton 2001). Conventional views (Dyke 1979), supported by (Unit A), which is subdivided by a well-developed weathering zone chronologies, paleolimnological data hydrochemical discontinuity, and an overlying ice- and some cosmogenic nuclide data suggest that upland poor, cryoturbated and shell-free diamicton (Unit B). surfaces have long been ice free (Wolfe et al. 2001) The outflow section exposes 2 m of dense grayish and imply that our study site likely was deglaciated brown (2.5 Y 5/4) sandy diamicton (Unit A) that 30 to 60 ka BP. Other cosmogenic data (Bierman et al. contains shell fragments dated at 52 460 1460 (TO- 1999) and marine sedimentology studies (Jennings 2196) corrected to a 13C 25%. Unit A has an aver- 1993), however, support retreat of ice from the age moisture content of 33%, contains small lenticular Pangnirtung area between 13.4 and 9.0 ka BP. pockets of stratified sand and gravel, and is capped by Exposures described below occur within diamicton a 0.3 m thick ice lens, and 0.5 m of sandy diamict. deposited by a small re-entrant glacier driven south- Unit B is a coarse, strong brown (7.5 YR 4/6), ice- ward against the local topographic gradient by a large poor diamicton with a much lower moisture content mass of ice moving down Pangnirtung Fiord (Dyke (13%), and abundant cryoturbation structures. A gravel 1979). The study site, located within the limits of this layer, averaging 5 cm thick, separates Units A and B. re-entrant lobe, is characterized by stagnant hummocky An oriented sample of a 0.3 m thick ice lens, recov- terrain, surface and subsurface drainage, and a series ered from Unit A (outflow section in Fig. 3) was exam- of relict ice-marginal streams (Hyatt 1992). There is no ined under plain and cross polarized light. The ice is evidence to suggest that the valley was glaciated following melting of the re-entrant glacier.

3 METHODS

Ice and frozen sediments were collected from freshly exposed surfaces after removing a minimum of 5 cm of frozen surface material. Samples were initially placed in double thickness polyethylene bags; air was forced out before the bags were sealed, and the samples were allowed to thaw at 5°C. After melting, excess pore water was transferred to airtight polyethylene bottles and the remaining saturated sediments were resealed for transport. Where necessary, additional pore water was extracted from the thawed sediments using a centrifuge. Meltwater samples were analyzed for major cation concentrations using atomic absorption spectropho- tometry, while 18O and 2H concentrations were determined by mass spectrometry at the Stable Isotope Laboratory of the Ottawa-Carleton Geoscience Center. Particle size distributions, gravimetric moisture con- Figure 2. (a) NE facing inflow exposure oriented parallel to tents, and ice petrofabrics were determined as described paleo ice flow with (b) detailed view of a thin gravel layer, trough by Hyatt (1993). cross-bedded sand, ice lens sequence. Scale in cm.

444 Figure 4. (a) Lower hemispheric Schmidt equal area projection of 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 in the up paleo-ice direction (see Fig. 1).

carbon, TO-2195). These sediments are interpreted as cavity fill deposited by subsurface flow through the moraine (Hyatt 1992). The sequence is capped by 1.85 m of cryoturbated, ice-poor and oxidized (10 YR 3/4) diamicton (Unit B). The most important characteristic of both sections is a hydrochemical discontinuity at 2.1 m (outflow) Figure 3. Section log showing conductivity, ion concentration and 2.5 m (inflow) depth, just below the contact and stable isotope (18O and 2H) values for inflow (top) and outflow (bottom) sections (HD hydrochemical discontinuity, SSO between Units A and B (Fig. 3). Electrical conductivity stratified sands with modern organics, TO-2195 109 0.67% and total cation concentrations of pore ice increase modern carbon, TO-2196 52 460 1430). with depth to just below the discontinuity, below which they become more variable. Ice lens samples below the clear and colorless in transmitted light, and contains discontinuity have lower electrical conductivity and abundant sediment inclusions but few bubbles. Sedi- total cation concentrations than surrounding pore ice, ment inclusions occur as both discontinuous and con- although cations are present in similar proportions. tinuous bands, and as individual grains and grain The concentration of individual cations also differs above and below the discontinuity. Mean concentra- aggregates suspended between the bands. Rare sphe- tions of Na and Ca are 2–10 times greater, Mg roidal to dish-shaped bubbles ( 1 mm diameter) occur along fracture planes and sediment bands. Ice crystals concentrations are nearly the same, and K concentra- are anhedral with some elongation parallel to the tions are 30% lower below the thaw unconformity. sediment bands, and average 2.5 cm2 (range 0.1 cm2 to Isotopic values also shift across this discontinuity 18 2 20.6 cm2). C-axis distributions are loosely grouped (Figs 3, 5). Above, the mean O and H values about 2 weak point maxima that dip 25° to 80° from respectively are 17.8% and 125.4%, decreasing to horizontal, and are inclined 55° to 65° to the sediment 26.1% and 200% below the discontinuity. T-test banding (Fig. 4). comparisons of slope coefficients for simple linear 18 2 A similar, albeit more complex, 6 m high sequence regression lines fit to O and H data differ signif- is exposed at the inflow site (Figs 2, 3). Unit A has a icantly (p 0.02) above and below the discontinuity. higher ice content (up to 200% by dry weight), fewer Values above the discontinuity follow a line sloping at 2 shell fragments, and three repeated sequences of gravel 5.54 (r 0.87) while the slope below the discontinuity 2 overlain by thick ice lenses and ice-rich diamicton. (8.17, r 0.96) is similar to the meteoric water line. Gravel layers are laterally continuous, curved, and dip 4° to 6° south. Overlying ice lenses in Unit A are 0.02 5 ORIGIN OF THE ICE LENSES to 0.5 m thick, have either gradational or sharp lower contacts, and sharp, conformable upper contacts We interpret the hydrochemical discontinuity as a thaw (Fig. 2b). Sediment infilling structures (Mackay unconformity reflecting the maximum post-glacial 1989) are not present at the upper contact of these ice depth of thaw. This interpretation is supported by the lenses. The ice-rich diamict contains thin (1 to 7 mm change from oxidized sediment colors above (10 YR) thick) discontinuous ice lenses that commonly extend to reduced colors below (2.5 Y), and the first appear- laterally underneath larger clasts. Unit A also contains ance of visible ice lenses below the thaw unconfor- a 0.5 m thick package of stratified sand and gravel mity. As well, the co-isotopic slope above the thaw with well developed planar and cross bedding, sharp unconformity indicates nonequilibrium fractionation unconformable upper and lower contacts, and dispersed processes (Michel 1986) that are common within fragments of modern moss (109 a 0.67% modern the active layer, whereas the slope below the thaw

445 concentrations in ice samples below the thaw unconformity differ from those in snow and summer precipitation (Fig. 6). Similarly, 18O values below the thaw unconformity (23.3% to 29.0%) differ from those in ice in tension cracks at the surface (16.4%) and in melt water from snow (19.3% to 17.8%). Yet, 18O and 2H values for ice lenses below the thaw unconformity (25.3% to 28.2%) are indistinguishable from surrounding pore ice indicating a common water source. Thus, ice lenses in Unit A cannot be pool ice. Figure 5. Co-isotopic relationships for samples collected above (filled circles) and below (open circles and squares) the hydro- 5.2 Segregated ice lens formation in a chemical discontinuity. periglacial setting unconformity does not. Thus, ice lenses in Unit A While the cryostratigraphy of ice lenses in Unit A have have been preserved beneath insulating sediments of some characteristics consistent with top-down freez- Unit B and, therefore, may either have formed by freezing, not all properties are typical of ice lenses formed in ing in a periglacial environment following deglaciation, a periglacial setting. Ice lenses do contain gradational or they may be relict, having formed prior to deglacia- lower contacts, matched sediment bands and grain tion by freezing in a subglacial environment. aggregates commonly found in segregated ice (Mackay Age considerations alone can not distinguish between 1989). Also, most segregated ice coatings underlying these origins because the precise date of deglaciation gravel clasts in Unit A are continuous with both large for this site is debatable (Wolfe et al. 2001). Our shell and small ice lenses (Fig. 2b). However, some ice date (52 460 1460, TO-2196), while consistent with lenses are thicker (up to 0.5 m) than would be expected conventional views of deglaciation, may be infinite, given the coarse grained texture of enclosing sediments and we have not dated gases trapped within the and the absence of an obvious mechanism for elevat- ice. Accordingly, we test hypotheses of ice lens for- ing pore water pressures in a periglacial setting (as mation by comparing observed cryostratigraphic and would be required to grow thick ice lenses in coarse hydrochemical characteristics with those expected grained sediments). for ice lenses formed in periglacial and in subglacial More importantly, however, the repeated sequence environments. of curved gravel layers, overlying pockets of finely cross-bedded sands, ice lenses and ice-rich sediments 5.1 Pool ice formation in a periglacial setting are difficult to reconcile with a periglacial origin. Segregated ice coatings under some of the gravel clasts Observed subsurface pipe flow (Hyatt 1992) together indicate that the ice lenses formed after the sediments with cavity deposits that contain modern organic frag- were deposited, while delicate cross-beds above the ments (Fig. 3) indicate that surface waters have pene- gravel indicate that the sediments were deposited by trated parts of the moraine core. Thus, it is possible flowing water and have not been sheared since depo- that ice lenses in Unit A are pool ice in which case they sition. This implies deposition of sand and gravel by should contain modern organic fragments or be asso- flowing water, subsequent freezing with little or no ciated with cavity fill, and have crystallographic prop- deformation of the sands, and the development of ice erties characteristic of bulk water freezing. However, lenses up to 0.5 m thick within predominantly sandy ice lenses in Unit A are enclosed by dark, non-stratified sediments. However, for sands to be deposited along a and organic-free diamicton that is easily distinguished curved dipping surface, water would either have to flow from cavity fill deposits. Furthermore, crystallographic down or be injected into a crack or opening in the data are not consistent with a pool ice origin. Columnar moraine during stagnation. While this could happen, ice crystals were not present; c-axis distributions (Fig. continued stagnation would destroy cross bedding in 4) were inclined rather than normal to the upper and the sands. If the surrounding sediments did not thaw lower contacts of the ice lens, and there were no indi- (partial stagnation), then the ice lenses should have cations of either a fine-grained “chilled margin” or a characteristics similar to injection ice including a fine- central discoloration zone, which often develop in grained “chilled” margin, petrofabrics more normal to association with bulk water freezing (Pollard 1990). its boundaries, and a hydrochemistry which differs If ice lenses below the thaw unconformity are pool substantially from pore ice in the surrounding sedi- ice, their ionic and isotopic composition should be sim- ments. These characteristics were not observed. ilar to surface water but different from the surround- If ice lenses in Unit A formed by segregation they ing permafrost. However, we find the opposite. Ionic should have hydrochemical compositions that are

446 Flow obstruction regelation is a destructive process resulting in a layer-by-layer accretion of sediment and ice (Boulton 1970). This would disrupt primary sedimentary structures like the fine trough crossbeds in sands that underlie ice lenses in Unit A. Alternatively, repeated episodes of basal freezing (cf. Weertman 1961) could incorporate stacked sequences of relatively clear subglacial segregation ice with intervening layers of ice-rich sediment similar to the repeated sequence Figure 6. Plot showing similarity in cation composition for ice above the hydrochemical discontinuity (open circles) with precip- of gravel-sand-ice-diamicton observed in Unit A (Fig. itation (open squares) and snow cover (diamonds). Cation compo- 2b). Once incorporated these sediments and ice lenses sition for ice below the discontinuity (filled circles) differs would become curved by compressive flow. This would markedly. Precipitation and snow cover from Staple (pers. comm.). not destroy primary sedimentary structures. In addition, the presence of high subglacial pressures would explain c-axis point maxima in the Pangnirtung ice lenses that consistent with freezing of near-surface waters, and are inclined to the foliation. Similarly, high pressure isotopic values and co-isotopic ratios indicative of slow would promote pore water migration toward a descend- freezing in a semi-closed system. The hydrochemistry ing freezing front resulting in the growth of thick ice of ice below the thaw unconformity, while clearly dif- lenses within coarse grained subglacial sediments. fering from modern surface water sources, does not A subglacial origin requires the incorporation of old, conclusively refute a periglacial origin. Electrical con- cold glacial meltwater with freezing in an open system ductivities are lower than in most types of ground ice that does not promote isotopic fractionation. Although formed in periglacial environments, although individ- 18O values in Unit A are less negative than early to ual cation concentrations are within the range reported mid-Wisconsinan glacial ice, which typically range for a variety of forms of ground ice (cf. Mackay & from 30% to 40% (Michel & Fritz 1978, Dallimore 1992). Similarly, isotopic values, while Moorman et al. 1996), it is not uncommon for basal indicating a colder and/or older source water for ice in debris-rich glacier ice to be 3 to 4% less negative than Unit A, are similar to values reported for ice formed by overlying glacier ice. This is due to the contribution of pre-Holocene permafrost aggradation at other sites in isotopically enriched surface water to subglacial ice Arctic Canada (e.g. Michel & Fritz 1978, Mackay layers (Lawson & Kulla 1978) and the removal of 1983, Mackay & Dallimore 1992). However, a co- isotopically depleted basal melt water during freezing isotopic slope near 8 for Pangnirtung ice is not charac- (Hooke & Clausen 1982). In addition, the enriched 18O teristic of freezing in a semi-closed system, as would values at Pangnirtung may simply reflect a reduced be expected for conventional segregation ice, although distance fractionation effect because of the proximity it is worth noting that co-isotopic slopes for segregated to Cumberland Sound (Fig. 1). The co-isotopic slope ice can vary depending upon the rate of freezing for samples from Unit A is similar to the meteoric (Michel 1986). Thus, although hydrochemical data water line, a trend that is consistent with subglacial suggest that ice lenses are not typical of segregated ice, freezing where atmospherically derived melt water cryostratigraphic observations provide more conclusive freezes slowly under equilibrium fractionation condi- evidence that ice lenses within Unit A did not form by tions (Michel 1982). Thus, although we recognize that conventional segregation in a periglacial environment. subglacial freezing and associated co-isotopic slopes may be complex, hydrochemical data in conjunction 5.3 Regelation ice formed in a subglacial setting with cryostratigaphic observations indicate that ice lens in Unit A most likely formed by segregation processes Sediment-rich ice at the base of glaciers forms by a in a subglacial setting. variety of mechanical and thermal mechanisms (Hubbard & Sharp 1989). This includes pressure melt- 6 CONCLUSIONS ing and refreezing of water around sub-glacial flow obstructions creating flow obstruction regelation ice. Cryostratigraphic and hydrochemical characteristics Alternatively, subglacial regelation ice may form as of ice lenses in ground moraine near Pangnirtung are freezing temperatures penetrate beneath the glacier most consistent with a Weertman (1961) style of sub- incorporating underlying sediments (Weertman 1961, glacial segregation that incorporated sediments and Boulton 1970). This can result in the growth of segre- ice lenses into the bed of overriding active Wisconsin gated ice lenses in a subglacial environment. For sim- glacial ice. Ice lenses were deformed by compressive plicity we refer to lenses formed by this mechanism as flow and survived stagnation beneath an insulating subglacial segregation ice. cover of ice-poor, cryoturbated diamicton. Alternative

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