Introduction Cryostratigraphic and hydrochemical characteristics of permafrost have long been used to infer the nature and origin of ground ice (Mackay, 1971; Harry and French, 1988; Murton and French, 1994). While these techniques have been applied to the study of ground ice within raised lacustrine sediments (Burn et al., 1986), few studies have examined lake-bed ground ice beneath modern lakes. Burn (1989; 1990; 1993) has mea- sured lake-bottom frost heave and used detailed level- ing and cores recovered from the lake-bed to monitor the growth of injection-segregation ice beneath lakes in the Mackenzie Delta. However, because natural expo- sures of ground ice do not occur below the water sur- face, no detailed studies have investigated the cryos- tratigraphy of in situ lake-bed ground ice. This study presents the first description of in situ lake-bed ground ice from a modern lake. Ice was exposed by excavations in the bed of a drained lake during construction of an earthwork dike at Water Supply Lake (WSL) located on northeast Baffin Island (Figure 1). The purposes of this paper are to (1) describe strati- graphic and hydrochemical properties of in situ lake- bed ground ice, and to use these data to (2) determine whether the ground ice is recent, having formed by injection-segregation processes (cf. Burn, 1990), or relic having formed either by burial or segregation. Abstract Cryostratigraphic and hydrochemical characteristics are described for in situ lake-bed ground ice exposed by excavations in a drained lake during construction of an earthwork dike at Water Supply Lake on northeastern Baffin Island. Ground ice occurs as ice lenses 5 mm to 0.5 m thick having cryostructures indicative of a segrega- tion origin. The ice is ionically enriched and isotopically lighter than overlying lake water. These data, together with local changes in surficial geology, and dated wood fragments recovered from the site, are used to infer the origin of lake-bed ground ice. This ice is thought to be relic having formed 29 ka BP when the area last emerged from beneath mid-Wisconsinan Eclipse glacier ice. Permafrost aggradation and the growth of segregated ice lenses incorporated isotopically light meltwater that flooded the area during a still-stand of retreating Eclipse ice. Modern dike construction has raised water levels, submerging and thermally degrading the ground ice. James A. Hyatt 487 THE ORIGIN OF LAKE-BED GROUND ICE AT WATER SUPPLY LAKE, POND INLET, NUNAVUT, CANADA. James A. Hyatt Department of Physics, Astronomy, and Geosciences, Valdosta State University, Valdosta, GA, U.S.A. 31698-0055 e-mail: [email protected]Figure 1. Location of (a) study site on northeastern Baffin Island showing (b) the limits of the Eclipse glaciation as proposed by Klassen and Fisher (1988), and (c) the location of Water Supply Lake.
Transcript
Introduction
Cryostratigraphic and hydrochemical characteristicsof permafrost have long been used to infer the natureand origin of ground ice (Mackay, 1971; Harry andFrench, 1988; Murton and French, 1994). While thesetechniques have been applied to the study of ground icewithin raised lacustrine sediments (Burn et al., 1986),few studies have examined lake-bed ground icebeneath modern lakes. Burn (1989; 1990; 1993) has mea-sured lake-bottom frost heave and used detailed level-ing and cores recovered from the lake-bed to monitorthe growth of injection-segregation ice beneath lakes inthe Mackenzie Delta. However, because natural expo-sures of ground ice do not occur below the water sur-face, no detailed studies have investigated the cryos-tratigraphy of in situ lake-bed ground ice. This studypresents the first description of in situ lake-bed groundice from a modern lake. Ice was exposed by excavationsin the bed of a drained lake during construction of anearthwork dike at Water Supply Lake (WSL) located onnortheast Baffin Island (Figure 1).
The purposes of this paper are to (1) describe strati-graphic and hydrochemical properties of in situ lake-bed ground ice, and to use these data to (2) determinewhether the ground ice is recent, having formed byinjection-segregation processes (cf. Burn, 1990), or relichaving formed either by burial or segregation.
Abstract
Cryostratigraphic and hydrochemical characteristics are described for in situ lake-bed ground ice exposed byexcavations in a drained lake during construction of an earthwork dike at Water Supply Lake on northeasternBaffin Island. Ground ice occurs as ice lenses 5 mm to 0.5 m thick having cryostructures indicative of a segrega-tion origin. The ice is ionically enriched and isotopically lighter than overlying lake water. These data, togetherwith local changes in surficial geology, and dated wood fragments recovered from the site, are used to infer theorigin of lake-bed ground ice. This ice is thought to be relic having formed 29 ka BP when the area last emergedfrom beneath mid-Wisconsinan Eclipse glacier ice. Permafrost aggradation and the growth of segregated icelenses incorporated isotopically light meltwater that flooded the area during a still-stand of retreating Eclipseice. Modern dike construction has raised water levels, submerging and thermally degrading the ground ice.
James A. Hyatt 487
THE ORIGIN OF LAKE-BED GROUND ICE AT WATER SUPPLY LAKE, POND INLET, NUNAVUT, CANADA.
James A. Hyatt
Department of Physics, Astronomy, and Geosciences, Valdosta State University, Valdosta, GA, U.S.A. 31698-0055e-mail: [email protected]
Figure 1. Location of (a) study site on northeastern Baffin Island showing (b)the limits of the Eclipse glaciation as proposed by Klassen and Fisher (1988),and (c) the location of Water Supply Lake.
Study site and glacial history
WSL is a small natural lake located 4.5 km inlandfrom Pond Inlet within the zone of continuous per-mafrost on the northeastern tip of Baffin Island (Figure 1). Mean annual air temperature at Pond Inlet is-15.2¡C, and approximately 230 mm of precipitationfalls annually, 59% as snow (Maxwell, 1980). Water le-vels at WSL have been raised artificially on severaloccasions to increase water storage capacity (Figure 2a). In 1979 a sand and gravel dike was cons-
tructed across the lake outflow channel raising waterlevels by 2 m (Dusseault and Elkin, 1983). Seepagethrough this dike and growing demand for fresh waterprompted the construction of a new lined dike in 1989-90, raising water levels by an additional 1 m. The 1990dike was keyed into a trench that was excavated in thedrained bed of WSL in the summer of 1990 (Figure 2b).Ground ice exposed in the lake-bed was mapped beforethe trench was filled with silty sand and water levelswere raised.
Northern Baffin Island has been glaciated severaltimes, most recently by Laurentide ice during the
Eclipse glaciation (>43 ka BP, Klassen, 1981; 1985;Klassen and Fisher, 1988). Ice flowed northwardthrough Milne Inlet (Figure 1b) overriding a portion ofthe northern coastline of Baffin Island, depositing"Eclipse" drift near WSL and creating moraines farthereast (Figure 1c). Although isostatic depression associa-ted with this glaciation and subsequent rebound pro-duced numerous raised marine deltas and beach ridgesto elevations of 65 m asl (Hodgson and Hasleton, 1974),marine incursions did not inundate WSL (96 m asl).Glacial deposits near WSL have been mapped as plainsof channelized hummocky moraine composed of sand-and boulder-size material with some silt (Hodgson andHasleton, 1974). A distinct change in terrain beside WSL(marked with a dashed line in Figure 3) is thought tomark the edge of a prolonged still-stand of retreatingEclipse ice. Streams flowing from the southeast deeplydissect areas north and east of the boundary (dd inFigure 3). Many of these streams flow parallel to thegeneral topographic gradient, suggesting that they orig-inally formed next to a retreating ice margin. Terrainsouth and west of the boundary, which at the time ofthe still-stand would have been ice-covered and pro-tected from the erosive effects of meltwater, is darker incolor, less densely dissected and is composed primarilyof finer-grained colluvium (c in Figure 3).
Results
CRYOSTRATIGRAPHY
Permafrost exposures were examined in the lake-bedtrench to characterize sediment texture, and determinethe ionic and isotopic composition of ground ice melt-water. Sampling methods and analytical procedures aredescribed in detail by Hyatt (1993). The locations offour mapped sections in the trench with respect to theoriginal, and the 1979 water line, are critical to subse-quent interpretation (inset in Figure 4). Section 1 is theonly site located inside the limit of the original lake.Section 2 is located just outside, and sections 3 and 4 arelocated just inside the 1979 shoreline.
Detailed descriptions for these sections, as summa-rized in Figure 4, support the following interpretations.Section 1 consists of pebble- to cobble-sized fluviallydeposited gravels located in the lakes original outflowchannel and overlain by recently deposited silty-sandthat was introduced to the lake in 1986 to reduce see-page through the old dike (Harris and Smith, 1986).Wood recovered from below the gravel-sand contactyields a conventional radiocarbon date of 29 000 ±1500BP (BSG 1368). This wood is located within imbricatedchannel-fill gravels and indicates that the outflow forWSL may have been established as early as 29 ka BP.Section 2 exposes a portion of the 1979 dike and under-lying pre-existing permafrost. The latter consists oflake-marginal to littoral-zone interbedded silty-sand
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Figure 2. Map showing (a) the limits of WSL in relation to earthwork dikes,the lake-bed trench, and frost blisters. Cross section (b) illustrates stratigra-phy and the location of ice referred to in the text (portions of which are modi-fied from Savigny and Smith, 1988).
and thin ice lenses that are wavy, discontinuous andvary from < 5 mm to 20 mm thick.
Ground ice is most abundant at sections 3 and 4where basal ice-rich silty sands, also interpreted as lake-marginal to littoral-zone deposits, are overlain by ice-poor silty sand and disturbed lake-bed sediment. Basalice lenses are thin (<10 mm thick), horizontally discon-tinuous, and have wavy, non-parallel, lenticularcryostructures (Figure 5a) (cf. Murton and French,1994). They are overlain by a 0.15 to 0.4 m thick ice lenscomposed of clear ice with very few gas inclusions, andblocky crystals 30 to 50 mm in diameter. The lower con-tact of this ice lens is gradational with sediment extend-ing upwards into the ice, whereas the upper contact iswavy but sharp, with protrusions of sediment penetra-ting downward into the ice (Figure 5b). Thin ice lensessimilar to those occurring at the bottom of the section,overlie the thick ice lens at section 3, but not at section4. These ice lenses form an abrupt but wavy contactwith overlying ice-poor silty sand lake-bed sediments.
HYDROCHEMISTRY
Major cation and isotopic concentrations for lake-bedground ice, several small frost blisters located west ofWSL (included for comparative purposes and shown inFigure 2), and water samples from WSL and nearbySalmon Creek are illustrated in Figure 6. Total cation
concentrations for lake-bed ice are more than five timeshigher than water from either WSL or Salmon Creek, orfrom the frost blister ice. The relative proportions ofindividual cations, however, are similar for lake-bedground ice and water from WSL. Mean d18O concentra-tions for lake-bed ice is isotopically lighter (-29.1 ä)than water from WSL (-21.4 ä, single sample value),Salmon Creek (-21.8 ä), or ice from the frost blisters (-21.5 ä). Although fewer results are available, d2Hconcentrations are also isotopically lighter for lake-bedice (-205 ä) than near-surface frost blister ice (-170 ä).d18O concentrations in Figure 4 remain nearly uniformwith depth in the lake-bed ground ice and do not shiftbetween small and thick ice lenses suggesting a similarwater source.
Discussion
Ice within the lake-bed may be recent, having formedby injection-segregation processes (cf. Burn, 1990) afterlake levels were raised by the construction of the firstdike in 1979. Alternatively, ice may be relic havingformed adjacent to the original shoreline (i.e., beforewater levels were raised in 1979) either by burial ofsnow or lake ice, or by segregation processes when per-mafrost last aggraded at the site.
Lake-bed injection-segregation ice forms in responseto top-down freezing below grounded lake ice duringepisodes of mid-winter discharge into the lake.Discharge beneath lake ice increases hydrostatic pres-sure in lake-bed sediments and lifts grounded lake-iceand adjoining frozen sediments. Under these conditionsinjection-segregation ice can form near the outer edgeof grounded lake ice as lake water is injected into free-zing sediment (Burn, 1989; 1990; 1993). If ice within thelake-bed trench at WSL formed by these processes (1)there should be evidence of winter discharge beneaththe lake ice, (2) ice lenses should have hydrochemicalcompositions that are similar to lake water, and (3) theice must have formed after submergence in 1979.
Winter discharge from the small (0.28 km2) drainagebasin surrounding WSL into the lake is likely verysmall because all tributaries entering the lake freeze
completely in winter, and no known groundwatersprings exist at the lake. Furthermore, drinking water iswithdrawn daily throughout the winter, and substantialseepage has occurred through the 1979 dike near thelakes original outflow channel (Harris and Smith, 1986).Given these water losses, it seems highly unlikely thatwinter discharge into WSL has been sufficient to pro-duce injection ice.
While ionic data are somewhat equivocal, d18O dataclearly indicate that ice within the lake-bed trench andunder the 1979 dike have not incorporated modern lakewater. Major cation concentrations in the lake-bed iceare enriched but proportionally similar to WSL water asmight be expected for intrusive ice because of soluterejection on freezing. However, solute rejection associa-ted with a segregation origin would also have elevatedionic concentrations within relic ice. Stable isotopesprovide a more conservative indication of water source
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Figure 4. Cryostratigraphic logs for sections in the lake-bed trench (inset), showing textural classes of sediment, d18O values for ice samples, and the location ofa drift wood sample dated at 29 000 ( 1500 BP (BSG 1368). Ice depths are less than true depth below the lake-bed because a thin but unknown thickness of sedi-ment was removed from the site prior to mapping.
(Michel, 1986). d18O values for lake-bed ice are muchlighter than overlying lake water indicating a differentand likely colder water source for the ice.
Finally, while definitive radiometric dates for the lake-bed ice are lacking, relative dating based on relation-ships between lake-bed ground ice and ice outside thelimits of the 1979 lake do not support a recent injection-segregation origin. Thin interbedded ice lenses beneaththe old dike (section 2) have wavy cryostructures, ele-vated ionic concentrations, and highly negative (18Ovalues that are virtually identical to the characteristics
of the lake-bed ground ice. Although ice lenses at thesetwo locations may have different origins, their proximi-ty, similar elevations, and nearly identical hydroche-mistry suggest otherwise. Furthermore, seepagethrough the old dike, which has occurred near the out-flow channel (section 1), cannot be responsible for theformation of ice at section 2 because d18O values for theice lenses are not consistent with a lake water source.Consequently, ice lenses beneath the old dike at section2 are believed to pre-date dike construction, and arethought to have formed contemporaneously withground ice in the lake-bed trench. Because ice beneaththe old dike has never been submerged, it is reasonableto conclude that the lake-bed ground ice is relic, havingformed prior to submergence in 1979.
RELIC GROUND ICE
If the lake-bed ice is relic it may have originated eitherby burial of snow or lake ice, or by segregation whenpermafrost last aggraded into the ground. A buried ori-gin is rejected because the lake-bed ice is largely bub-ble-free, whereas snowbank ice is bubble-rich (Pollard,1990), the ice was not candled as would be expected formany kinds of lake ice, and an effective burial mecha-nism is difficult to imagine in the low-relief terrainbeside the lake.
A segregation origin for ice in the lake-bed trench isprobable because thin ice lenses have cryostructurestypical of segregation ice (Figure 5a), and the gradatio-nal lower contact for the large ice lens is also consistentwith a segregation origin (Mackay, 1989). In addition,0.1 to 0.2 m of ice-free silts overlie the large ice lens atsection 4 and sediments penetrate downward intounderlying ice (Figure 5b) indicating that some thermaldegradation of the ice has occurred. Presumably thisreflects increased thaw depths in the vicinity of thelake-bed ice following submergence in 1979.
Highly negative d18O values for lake-bed ground ice(-29 to Ð31 ä.) are similar to values reported forWisconsinan glacier ice (Koerner, 1989), and indicate acold water source. The presence of ice-marginal melt-water channels near WSL together with indications of astill-stand by retreating Eclipse ice adjacent to the studysite (Figure 3) imply that WSL was submerged duringdeglaciation. Furthermore, sediments submerged byfresh meltwater (temperature > 0¡C) were likelyunfrozen and saturated with isotopically light waterprior to emergence. Once Eclipse ice retreated from thestill-stand position in Figure 3, meltwater would havedrained allowing the outflow channel for WSL to form(ca. 29,000 BP) exposing the lake-margin to cold ambi-ent air temperatures. Ground ice, formed by segrega-tion processes as permafrost aggraded into the ground,incorporated isotopically light mid-Wisconsinan melt-water within the silty sands next to WSL.
James A. Hyatt 491
Figure 5. Cryostructures observed at sections 3 and 4 showing (a) wavy,non-parallel, lenticular ice lenses at the base of section 3, and (b) an upperthaw contact for a 0.4 m thick ice lens at section 4. Note (1) large and (2)small protrusion of sediments downwards into ice.
Conclusions
Lake-bed ice lenses have cryostructures indicative of asegregation origin, and hydrochemistries that are ioni-cally enriched but isotopically depleted in d18O relativeto overlying lake water. These data, together with datedwood fragments (29,000 ±1500 BP; BSG-1368), andchanges in terrain at the site are used to infer a historyof ground ice development. Neither cryostratigraphicobservations nor the ionic composition of ice in isola-tion can distinguish between a modern or a relic originfor the ice. However, associations based largely oncryostructures and supported by isotopic similaritiesbetween different ice bodies help to constrain the ti-ming of ice formation and rule out a modern ice origin.Lake-bed ground ice is believed to have formed by seg-regation when permafrost last aggraded followingemergence from beneath retreating Eclipse glacier ice 29ka BP. Cold isotopically light meltwater incorporatedinto the ice recently has begun to thaw in response to
submergence following the construction of earthworkdikes.
Acknowledgments
This paper draws from the author's doctoral disserta-tion, and was conducted with grants to Robert Gilbertfrom NSERC, and contracts with the Government of theNorthwest Territories. Additional financial support wasprovided by Queen's University, NSERC, RCGS, andthe ACUNS Trust Fund. Field assistance by R. Cambell,J. Hartling, I. Mathers, and Papakyriakou and logisticalsupport provided by B. Boon (Reid Crowther andPartners Ltd.) and B. Smith (Thurber Consultants Ltd.)is greatly appreciated. Conference travel support wasprovided by Valdosta State University FacultyDevelopment and Internationalization funds. Criticalreviews by T. Bell and an anonymous referee haveimproved this paper.
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