The Distribution of Basal Motion Beneath a High Arctic Polythermal

Journal of Glaciology , Vol. 49, No.166, 2003

The distribution ofbasal motionbeneath a High Arctic polythermal glacier

Luke COPLAND,1 Martin J. SHARP,1 Peter NIENOW,2 Robert G. BINGHAM2 1Department of Earth and Atmospheric Sciences,University of Alberta,Edmonton, AlbertaT6G 2E3,Canada E-mail:[email protected] 2Department of Geography andTopographic Science,University of Glasgow,Glasgow G12 8QQ,Scotland

ABSTRACT.Thelongitudinal pattern ofsurfacevelocity of alarge,predominantly cold,polythermal glacier (J ohnEvans Glacier ,Ellesmere Island,Canada) was measured oversummer andwinter periods. In the accumulationand upper ablation areas, where ice ispredominantlycold-based, summer velocitieswere slightly higher than overwinter velocities.Predicted velocitiesdue to ice deformationalone in these areasclosely matched these observationsin the winter,withlimited basalmotion likely in the summer.Inthe lowerablation area, where ice is likelywarm-based, measured summer velocitieswere up todouble overwinter velocities. Predicted ice deformationcould not account for all of these measured velocitiesin either summer orwinter .Thissuggests that basalmotion occurs throughoutthe yearover at least partof the lowerablation area. This finding is supported byradio-echo sounding, subglacial drainage reconstructions andanalyses of early-summer meltwater chemistry,whichsuggest that subglacialwater is present throughoutthe yearin this region.In summer ,basalmotion may account for up to 7 5%ofthe totalsurface velocity throughoutthe lowerablation area. Theinferred rate ofbasal motion increases sharplydir- ectlybelow a set ofmoulinsby which most surfacemeltwater reaches the glacierbed.

1.INTRODUCTION beneatha 2kmlong section ofthe lowerglacier ,whereit accountedfor 470%of the totalmotion. Subglacialwater is animportant influence on the basal Transmission ofsurface meltwaters tothe beds ofpre- motionof temperate glaciers,where short-term high- dominantlycold glaciers provides a mechanism bywhich velocityevents oftencorrespond to episodes of largetransi- the flowof such glaciersmay respond rapidly to changes in ent subglacialwater pressures (Ikenand Bindschadler ,1986; surface weatherand climate (Zwallyand others, 2002).Itis Paterson,1 994;Willis, 1995).Bycontrast, the influenceof therefore importantto establish the rangeof environmental subglacialwater on the motionof predominantlycold poly- conditionsunder which this process canoccur .Thispaper thermal glaciers,in which ice onlyreaches the pressure- reports the results ofmeasurements madeon JohnEvans meltingpoint at and immediately above restricted areasof Glacier,Ellesmere Island,Canada. Climatic conditions at the glacierbed, is notwell known. A commonview is that JohnEvans Glacier arecold and dry (mean annual air tem- surface meltwaters havedifficulty penetrating to the bedsof perature approximately^1 5³C, meanannual precipitation such glaciers,and that variationsin surface melt rates there- 50.15 m a^1),andthe summer melt seasontypically lasts forehave little impacton glaciermotion (Hodgkins, 1 997). for only 60days. Outflow of subglacialwaters ceases com- ¹ Thisis because,in cold ice, waterflow along intergranular pletelyin winter,andis restricted toa periodof 2^6weeksin veinnetworks is limited, andlarge-scale permeability asso- summer (Skidmoreand Sharp ,1999).Thegoals of this paper ciatedwith crevasses andmoulins is poorlydeveloped due to areto determine: (a)whether basal motion occurs beneath lowrates ofice deformation. this predominantlycold polythermal glacier ,(b) whether Anumber ofstudies, however,suggestthat substantial basalmotion occurs year-roundor is restricted tothe sum- basalmotion occurs overparts ofthe ablationzones of pre- mer melt season,and (c) howthe spatialdistribution of dominantlycold polythermal glaciers, at least inthe sum- basalmotion compares with basal thermal andhydrological mer.Forexample, Andreasen ( 1985)observedseasonal conditionsinferred fromfield observations and radio-echo velocityfluctuations at Kitdlerssuaq Glacier,West Green- sounding(Copland and Sharp, 200 1). land,and Zwally and others (2002)observed similar vari- Theanalysis presented here isbasedon measurements of ationson the Greenlandice sheet ata locationwhere cold summer andwinter surface velocities at 20 locations along the ice was 41200m thick.Iken ( 1974)founda strongrelation- lengthof J ohnEvans Glacier .Thesemeasurements arecom - shipbetween short-term summer velocityincreases and paredwith ice deformationvelocities predicted frommeas- waterpressures innearby moulins at White Glacier,Axel uredice thicknesses andsurface slopes, taking into account HeibergIsland, Canada. At McCall Glacier ,Alaska, the effects oflongitudinal stress gradients.Basal motion is U.S.A.,Rabus and Echelmeyer (1997)observed summer assumed tooccur where observed surface velocities consist- velocityincreases ofup to 75% above mean winter values, entlyexceed predicted deformationvelocities. The term basal andconcluded that basalmotion occurred year-round motionis used torefer toany motion at the baseof the glacier 407 Copland and others:Distribution of basal motion beneath apolythermal glacier

Fig.1.Location ofJohn Evans Glacier (inset),andthe longitudinal velocity stake network superimposed on aLandsat 7image (path049 ,row 002,10 July1999).Squares indicate stakes measured by GPS,while circles indicate stakes measured by theodolite. LWS,MWSand UWSmark the lower,middle and upper weather stations,respectively.Temperatures mark ice temperatures at 15 m depth. thatis notdue to ice deformation.I ttherefore encompasses andupper ablation areas (stakes 1^9)sinks tothe glacier motionby basal sliding and/ orbasal sediment deformation. bedvia a series ofmoulins located in a crevasse field Toplacelimits onthe possiblerange of rates ofbasal betweenstakes 9and10(Fig.1).Dye tracingshows that the motion,ice deformationrates werecalculated for different sediment-free andlow-electrical-conductivity (low-EC) longitudinalcoupling lengths (Kamb andEchelmeyer , water (510 mS cm^1)that enters the moulinsin summer is 1986).Longitudinalcoupling occurs dueto longitudinal released atthe glacierterminus assediment-laden and stress gradientsthat arise fromalong-glacier variations in high-ECwater ( 4100 mS cm^1)withina periodof hoursto ice thickness andsurface slope.I tmodifiesthe flowat each days(Bingham and others, 2003).Thus,this waterlikely pointon the glacierfrom that whichwould occur if local flowsalong the glacierbed. Subglacial outflow ceases in flowwere determined directlyby the localslope and ice winter,whenthe glacierfreezes toits bedaround the mar- thickness (i.e. accordingto the basicflow theory that gins.Meltwater isapparentlytrapped behind the snoutby appliesin the absenceof longitudinalgradients (Nye, 1952, the thermal damthat results, andis released whenoutflow 1957)).Thelongitudinal coupling length provides the funda- resumes followingthe penetrationof surface waters tothe mentallength scale in the longitudinalaveraging of the glacierbed the followingmelt season(typically in late J une effects ofperturbations in ice thickness andsurface slope toearly J uly).Thefirst meltwaters tobe released fromthe onthe localflow .Itis highlydependent on ice rheology, glacierbed each summer havean EC of 4350 mS cm^1 andthe couplinglength typically increases asice tempera- (Skidmoreand Sharp, 1 999).Inthe latesummer ,moulins ture decreases (Kamb andEchelmeyer ,1986). mayalso open just down-glacierof stake 6 inthe loweraccu- mulationarea and drain meltwater fromthe upperglacier . 2.JOHN EVANSGLACIER 3.MEASUREMENTS JohnEvans Glacier is locatedon the central east coastof Ellesmere Island,N unavut,Canada ( 79³40 ’ N, 74³30’ W; 3.1.Ice temperature Fig.1).Itisapproximately20 km long, ranges in elevation from1 00to 1 500m a.s.l.,and covers an area of approxi- Inthe period1 997^99,the meanannual air temperature mately1 65km 2.Radio-echosounding indicates a meanice measured atthree automaticweather stations locatedat thickness of 150m inthe terminus area,and a maximum the terminus, equilibrium-linealtitude (ELA) andsummit ¹ ice thickness ofalmost400 mcloseto the equilibriumline at ofthe glacierwas ^1 4.8³C (Fig.1).Near-surface ice tempera- 750^850m a.s.l.(Copland and Sharp ,2000). tures weredetermined bydrilling 1 5mdeepboreholes, ¹ Themelt seasontypically extends fromearly J uneto placinga thermistor inthe baseof each and then refilling earlyAugust each year .Duringthis time, most surface run- the boreholewith snow and ice. After allowingseveral weeks offfrom the lowerablation area (stakes 10^20)flowsoff the forthe temperatures tostabilize, the temperatures at1 5m glaciermargins. Most surfacerunoff from the accumulation depthin the accumulationand upper ablation areas ranged 408 Copland and others:Distribution of basal motion beneath apolythermal glacier

Fig.3.Measured summer and winter velocities along the velocity stake network (see Fig.1for stake locations).

2000,andmost summer velocitiesin 1 999.Exceptions(due to pooror missing data)are that summer 2000velocities were used forstakes 1^3,whilethe overwinter1998/99velocity was used forstake 1 5.Where surfacevelocities were recorded at the same stakein different years,they differed by 510%. To enablecomparison of summer andwinter measurements, all ^1 Fig.2.Bedrock topography atJohn Evans Glacierdetermined velocitiesare expressed inm a . from radio-echo sounding (Coplandand Sharp,2001). Errors inthe theodolitemeasurements weredetermined bysurveying each stake at least twice per dailysurvey in July 1999(the originaldataset was subsampled to produce the between^9 .5³and ^1 5.1³C (Fig.1).Radio-echosounding velocitiesreported here).Thedifference between the twoor measurements ofresidualbed reflection power suggest that more measurements ofa stakeand its meanlocation for a ice inthe accumulationarea is coldthroughout, while ice givensurvey provide a meanposition error of 0.6 cm. § reaches the pressure-melting pointat the bedover much of Assumingthat successive positionerrors areof oppositesign, the ablationarea. A layerof warm basal ice 25 m thick ¹ the maximumhorizontal displacement error averages mayexist inthe centre ofthe terminus region,and warm 1.2cm. Thisproduces a meanvelocity error of 0.2 ma^1 § § basalconditions appear to exist inthe overdeepeningsin overa typical20 day summ er measurement period,and 0.02 § the ablationarea (Fig .2).However,the glacieris likely ma^1 overa typical300 day overwinter m easurement period. frozento its bedaround its marginsand over a largebed- Anestimate ofthe errors inthe differentialGPS measure- rockridge in the centre ofthe ablationarea (Copland and ments is provideddirectly by the instrument, andaverages Sharp,200 1). 1.0cm forthe positioningof astake.Thisequates to a max- § imum displacement error of 2.1cm ifsuccessive position 3.2.Surface velocity § errors areassumed tobe ofopposite sign. This equates to a ^1 Twentyvelocity stakes weredrilled and frozen into the ice maximummean velocity error of 0.4 m a over a 20 day § ^1 summer measurement period,and 0.03 m a over a surface alongthe lengthof the glacier(Fig .1).Due tothe § complexflow structure ofJohnEvans Glacier ,andthe diffi- 300day overwinter measurement period.These errors are cultyof workingin some areasdue to crevassing and large much lowerthan the measured velocities(Fig .3)andgive supraglacialstreams, it wasimpossible toestablish astake confidencethat measurements overthese time-scales provide linethat exactlyfollowed a singleflowline from the summit anaccurate picture ofthe surfacevelocity field. I tis also ofthe glacierto the terminus. Thestakes are,however , likelythat some errors arise fromfactors such asthe tilting everywherealigned approximately in a downslopedirec- ofstakes bywind or melt betweensurveys, but these influ- tion,and convergence/ divergenceof the flowlinesis not ences arelikely small incomparison to the true velocity. appreciableon the lengthscale of the longitudinalfiltering discussed later.Stakes1^6 and 9^20 follow single flowlines 3.3.Glacier geometry quite closely,butstakes 7and8 arestaggered between the other twogroups of stakes, andthe stakearray cuts across Adigitalelevation model (DEM) ofthe glaciersurface was flowlinesin this area(Fig .1). derivedfrom 1959stereo aerialphotography (W oodwardand Thestakes weresurveyed regularly to determine surface others, 1997)andverified against the 1999differential GPS velocitiesover both summer andwinter periods .Thelower 9 measurements. On average,the surfaceelevation at the stakes weresurveyed with a Geodimeter 540total station stakes was7 .3mlowerin 1 999than in 1 959,although the theodolitefrom a stationlocated on bedrock. The upper 1 1 elevationchange did not vary in a consistent waywith dis- stakes wereout of rangeof the theodolite,so were surveyed tancealong the glacier,andthe surfaceat some stakes wasat witha differentialglobal positioning system (GPS).Toensure ahigherelevation in 1 999than in 1 959.Comparisonof the consistencybetween measurements, reported summer originalaerial photography with 1 999Landsat 7 imagery velocitiesrelate tothe last 2^3weeks of J uly,whileoverwinter showsthat JohnEvans Glacier haschanged little overthe last velocitiesrepresent the periodbetween the last surveyof one 40years,and suggests thatthe surfaceDEM is representative yearand the first surveyof the next(typically early Augustto ofpresent surfaceconditions. The surface DEM wasinter- lateMay) .Most overwintervelocities were measured in1999/ polatedto a regulargrid with 25 m spacing,and was used in 409 Copland and others:Distribution of basal motion beneath apolythermal glacier the calculationof the surfaceslopes used asinputfor the ice deformationcalculations discussed later.Themean surface slopeat a pointwas determined fromthe changein elevation betweengridpoints 100mtothe north,east, southand west of that point(Zevenbergen and Thorne,1987). Ice thicknesses at3200 locations were determined from ground-basedradio-echo sounding conducted between 1997 and1 999(Copland and Sharp, 2000, 200 1).ADEM ofthe glacierbed was produced by subtracting the ice thickness fromthe surfaceDEM elevationat each measurement location,and interpolating the resulting bedelevationsto a 25m gridthat matchedthe surface DEM (Fig.2).Errors in the bedDEM arise fromerrors inboth the surface DEM andthe ice-thickness calculations,and are estimated at1 0^ 20%of the localice thickness (Coplandand Sharp, 200 1). Errors ofthis magnitudemake little difference tothe ice deformationcalculations discussed later.

4. RESULTS

4.1.Measured surface velocities

Alongthe stakenetwork, velocities are relatively low (518 m a^1)inthe accumulationand upper ablation areas (stakes 1^9),witha slightincrease inthe summer compared tothe winter(Fig .3).Further down-glacier,there is alarge velocityincrease betweenstakes 9and1 0.Thisis particu- larlytrue inthe summer,whenthe velocityincreases from 17.5 m a^1 atstake 9 to32. 1ma ^1 atstake 1 0overa horizontal distanceof 800 m. Thisoccurs inan area that contains numerousmoulins and transverse crevasses bywhich supra- glacialmeltwater canreach the glacierbed (Fig.1).Between stakes 10and1 7,surfacevelocities reach their peakin both winterand summer .Summer velocitiesare on average 62%higher than winter velocitiesin this region.Locally , theymay be up to double overwinter levels (e.g .stake1 1). Towardsthe glacierterminus, velocitydecreases down- glacierof stake1 8,although velocities are still higherin the Fig.4.For the velocity stake network: (a)surf ace and bed summer thanin the winter. topography;(b)ice thickness;(c)shape factor;(d)surface slope angle. 4.2.Predicted ice motion

4.2.1.Predicted ice motion without longitudinal stresses erson,1994).Thissuggests that variationsin ice temperature If longitudinalstresses areignored, the surface velocitydue maycause large spatial variations in ice motionwhen the ice toice deformation( ud)atany point along the centre lineof is non-temperate. Basedon the informationpresented in aglacierflowing in a channelof paraboliccross-section can section 3.1,twoscenarios were created torepresent the likely bedescribed by(Nye, 1965;Rabus and Echelmeyer ,1997): upperand lower limits ofice temperatures atJ ohnEvans 2A n n 1 Glacier: u »gf sin ¬ h ‡ ; 1 d ˆ n 1 … † … † ‡ (a)Warm:depth-weightedmean ice temperatures of^5³ C in where A is the flow-lawparameter , n isthe flow-lawexpo- the accumulationand upper ablation areas (stakes 1^9) nent (3), » isthe densityof ice, g isthe accelerationdue to and^2³ C inthe lowerablation area (stakes 10^20). gravity, f isthe shapefactor and h isice thickness.Toevalu- (b)Cold:depth-weightedmean ice temperatures of^1 0³C in ateEquation ( 1),alongitudinaltransect with100mgridspa- the accumulationand upper ablation areas (stakes 1^9), cingwas defined that passes throughthe DEM gridcellsin and^5³ C inthe lowerablation area (stakes 10^20). whichthe stakes arelocated. The bed and surface topography,togetherwith the valuesof f, h and ¬ ateach velocity Althoughthese scenariosare crude estimates ofthe true stake,are shown in Figure 4. The value of f isbasedon the ice-temperature distribution,they are expected to bracket half-width/depthratio along the velocitystake network the rangeof likely conditions and thus provideupper and (Paterson,1 994),andis generallyclose to 1 dueto the rela- lowerlimits tothe ice deformationcalculations. The values tivelylarge channel width. In all figures, values are shown of A that relate tothese temperature distributions are onlyfor gridcells that containvelocity stakes, asthese are 2.4610^24 Pa^3 s^1 at ^2³C, 1.6610^24 Pa^3 s^1 at ^5³C, and the onlylocations for which velocity measurements andpre- 4.9610^25 Pa^3 s^1 at^1 0³C (Paterson,1994,p. 97). dictionscan be compared. Thesurface velocities predicted byEquation ( 1)forthe two The value of A dependsmainly on ice temperature (Pat- temperature scenariosare shown in Figure 5 ,togetherwith the 410 Copland and others:Distribution of basal motion beneath apolythermal glacier

Fig.5.Comparison between measured and predicted surface Fig.6.Comparison between measured and predicted surface velocities along the velocity stake network from Equation (1), velocities along the velocity stake network from Equations without accounting for longitudinal stresses. (2)and (3)for longitudinal coupling lengths of 2,3and 4 times the local ice thickness.The measured winter velocity at stake 6isused as the datum. Ice temperature variations are not measured velocities.The fit betweenthe predicted andmeas- taken into account. uredvelocity is generallypoor ,withunder-predic tionat virtu- allyall stakes usingthe coldscenario, particularly in the accumulationarea. The fit withthe warmscenario is better inthe accumulationarea. The small difference insurface inthe upperaccumulation area (stakes 1^4),butfluctuates velocitybetween summer andwinter in this regionsuggests betweenlarge over- and under -predictionin the ablationarea that basalmotion is notas important here asit is inthe (stakes 7^20).Thehigh frequency variability in predicted ablationarea (Fig .3).Indeed,simulations that use stakes in velocitiesis attributable tolarge local variations in surface the ablationarea as the datum(stakes 10^20)dramatically slopeand ice thickness, whichare not smoothed out due to over-predict(by two times ormore) velocitiesin the accu- the failureof Equation ( 1)totake longitudinal coupling into mulationarea. In contrast, whenstakes inthe accumulation account.Clearly ,longitudinalstresses needto be included to area(stakes 1^6)are used asthe datum,there isgenerallya improvethe predictionsof surface velocity . goodfit betweenobserved and predicted velocitiesin the upperglacier and an under-predictionin the lowerglacier . 4.2.2.Predicted ice motion with longitudinal stresses Todetermine whichstake was the most suitablechoice as Totakelongitudinal stresses intoaccount in the determin- the datumlocation, a series ofcalculations was performed ationof surface velocity ( us),the approachof Kamb and usingthe measured wintervelocity at each stake in the Echelmeyer (1986)andRabus and Echelmeyer (1997)was accumulationarea in turn asthe datum.The best results adopted.In this method,surface velocities are not predicted wereobtained using stake 6, which produces a generally directlyfrom physical parameters asin Equation ( 1).In- goodfit betweenmeasured andpredicted wintervelocities stead, theyare predicted fromthe differences betweenlocal forstakes 1^9inthe accumulationand upper ablation areas, ice conditionsand those ata datumlocation ( x0) on the gla- andan under-predictionof velocitiesfor stakes 10^20in the cier wherethe surface velocityhas been measured ( uobs): lowerablation area (Fig .6).Thevalues of ¬, f and h are all us x uobs x0 exp B x B x0 ; 2 closeto the longitudinalaverage at stake 6 (Fig.4). … † ˆ … † … … † ¡ … †† … † where x isthe locationwhere us isevaluated,and B is deter- Thevelocities predicted usinglongitudinal coupling mined fromaveraging the effect of ¬, f and h with a tri- lengthsof 2,3 and4 times the localice thickness (withthe angularweighting function of unitvalue: wintervelocity at stake 6 asthe datum) areshown in Figure x 2l 6.I tis clearthat the inclusionof longitudinal coupling ‡ x0 x dampsthe unrealistic high-amplitudevelocity variations B x n ln ¬f n 1 ln h 1 j ¡ j dx0 ; … † ˆ ‰ … † ‡ … ‡ † Š… ¡ 2l † that wereproduced by Equation ( 1)(as shownin Fig .5), xZ2l ¡ andthat the best fit toupper glacier velocities is provided 3 by l 4h.With longitudinalcoupling lengths 44h, local … † ˆ variabilitytends tobe lost andthere isnofurther significant where l is the longitudinalcoupling length, and x0 is the dis- tancebetween x andthe locationat which the integralis being reductionin the magnitudeof the residuals.Kamb and evaluated.F orJohnEvans Glacier ,the integralwas evaluated Echelmeyer (1986)state that l=h is typicallyin the range1^3 fortemperate valleyglaciers and 4^1 0forice sheets. The asa sum ata spacing ¢x0 of1 00m overa totaldistance of 2l § from x.Thetriangular weighting function gives most import- valueof 4therefore seems reasonablefor a largepolythermal anceto the valuesof ¬, f and h at location x,andprogressively ice mass such asJohnEvans Glacier . less importanceto these valuesat sites more distant from x. Thisapproach does not account for possible varia tionsin ice 5.DISCUSSION temperature alongthe glacier,butrather assumes thatthe ice temperature atthe datumlocation is representativeofthe 5.1.Winterbasal motion glacieras a whole.Forpoints beyond the glacierboundary , the ice thickness andsurface slope are set to0 . Theice deformationvelocities predicted byEquation ( 2)are Animportant consideration in the evaluationof Equa- wellbelow the measured summer andwinter surface tion( 2)is the choiceof which stake to use asthe datum velocitiesacross the lowerablation area of JohnEvans Gla- location.Since the aimis topredict velocitiesdue to ice cier (Fig.6) .Asdiscussed above,the largedifference deformation,it makesmost sense tochoose a datumstake betweensummer andwinter surface velocitiessuggests that 411 Copland and others:Distribution of basal motion beneath apolythermal glacier

behindit oncethe subglacialoutlet isclosedby ice deform- ationand freezing after surfacerunoff ceases atthe endof the summer.Theoccurrence oftrappedbasal water is sup- portedby the progressiveincrease inresidual bed reflection powertowards the glacierterminus inradio-echosounding records (Coplandand Sharp, 200 1).Asatstakes 11^14,most ofthese measurements weremade before the onset ofsur- facemelting, implying that basalwater is present throughoutthe winterbeneath this partof the glacier.Further evidencefor the overwinterstorage of subglacial water is providedby water chemistry (Skidmoreand Sharp, 1 999; Heppenstall,2002) .Thefirst waterto be released fromthe glacierafter the thermal damat the snoutbreaks in the sum- Fig.7.Likely range of basal motion atJohn Evans Glacier in mer hasa veryhigh EC ( 4350 mS cm^1),andcontains ionic the summer.Minimum basal motion is calculated from the species such aslithium that areindicative of longwater/ rock difference between the measured summer and winter surface contacttimes andthat are not present atdetectable levels velocities.Maximum basal motion is calculated from the dif- later inthe melt season.In contrast, waterthat is released ference between the measured summer surface velocity and the later inthe summer is more dilute(EC 5 200 mS cm^1), and predicted deformation rate from Equation (2)(except at containsmore suspendedsediment, indicativeof more rapid stakes 8and9,where itis set to the sameasthe minimum basal subglacialwater flow and lower water/ rockcontact times. All motion due tothe over-prediction of deformation rates at these ofthese independentsources ofevidence support the conten- stakes (see Fig.6)). tionthat subglacialwater is stored overwinter behind the glacier snout,and that basalmotion could occur there during the winter. basalmotion occurs inthe summer,atleast betweenstakes 10and20. The question remains, however,asto whether 5.2.Summer basal motion basalmotion also occurs inthis regionduring the winter. Thefact that the predicted velocitiesin this regionare well Thelikely range of basalmotion during the summer canbe belowthe measured wintervelocities suggests that basal defined by: motiondoes occur in winter .However,the failureof Equa- 1. Minimum:the measured wintersurface velocityis tion( 2)to take into account spatial variations in ice tem- assumed tobe entirelydue to ice deformation(which is perature mayalso provide an explanation since ice unlikelygiven the discussion above).Theminimum temperatures canand do vary substantially along the length basalmotion is then definedby the difference between ofpredominantly cold polythermal glaciers (Blatter ,1987; the measured summer andwinter surface velocities. Blatter andKappenberger ,1988). Thisassumes that there areno significant seasonal vari- Thelack of deep borehole temperature measurements ationsin ice deformation. makesit difficultto determine the importanceof variations Maximum inice temperature atJohnEvans Glacier .Asafirst approxi- 2. :Equation( 2)is assumed todescribe accurately mation,however ,the warmand cold temperature scenarios the velocitydue to ice deformation.The maximum basal outlinedin section 4.2.1(i.e. differences of3^5³ C between motionis definedby the differencebetween the measured the ablationand accumulation areas) can be used toassess summer surfacevelocity and the predicted ice deform- their potentialsignificance. Since the effect ofthe valueof ationshown in Figure 6 (fora longitudinalcoupling length of 4h,withwinter velocity at stake 6 asthe datum). A onice deformationin Equation ( 1)is linear,it ispossible toestimate the flowenhancement produced by these Thisignores possi bleflow enhancement caused by warmer assumed temperature differences. Thedetailed results are ice temperatures inthe lowerablation area, which would notpresented here dueto their approximatenature, but they havethe effect ofincreasing deformation rates andredu- indicatethat it is notpossible to account for all of the meas- cinginferred basalmotion. uredsurface wintervelocity between stakes 10and20 by Figure7 showsthe rangeof basal motion predicted by deformationalone unless the velocityat stakes 15and1 6is these scenarios,and indicates the substantialcontribution that substantiallyover-predicted. basalmotion provides to total surface velocity in the lower Thus,it seems likelythat basalmotion is significantover ablationarea (stakes 10^20)inthe summer.On average,basal atleast partof the lowerablation area (stakes 10^20)in the motionaccounts for a minimum of35% and a maximumof winter.Thisis consistent withour current knowledgeof the 81%of total summer surfacevelocity in this region.Locally , basalhydrological system ofthe glacier.Asdiscussed by maximum basalmotion accounts for up to 98% of the total Coplandand Sharp ( 2001),the areabeneath stakes 11^14is summer surfacevelocity in areas close to the snout. overdeepenedby up to 50 m incomparison to surrounding Therapid onset ofbasalmotion between stakes 9and1 0 basaltopography (Fig .2).Highresidual bed reflection inthe summer is consistent withthe observeddelivery of powersfrom radio-echo sounding measurements madein largevolumes of surface waterto the glacierbed viathe cre- Maysuggest that basalwater is present inthis areaprior to vasses andmoulins in this area(Fig .1).Radio-echosound- the initiationof surface melt. Thismay imply that basal ingand meltwater chemistry indicatethat the glacierbed is wateris stored overwinter inthe overdeepening. warmand wet below these moulins.Dye tracingshows that Inaddition, there isevidencethat wateris present atthe subglacialwater passes fromthe moulinsto the snout(a dis- glacierbed throughout the yearbeneath stakes 18^20.The tance of 4km) ina matter ofhours in the latesummer ¹ thermal damat the glaciermargin likely traps basalwater (Binghamand others, 2003).Thethermal damat the snout 412 Copland and others:Distribution of basal motion beneath apolythermal glacier alsoappears to play a rolein producing very high basal Thisclearly suggests that the rate ofbasal motion is strongly waterpressures beneaththe lowerterminus inthe early influencedby the deliveryof surface meltwaters tothe glacier summer.Evidencefor basal water pressures ofat least bed,even though much of the overlyingice is atsub-freezing 120%of ice overburdenpressure isprovidedby the occur- temperatures. Longitudinalstresses seem tobe an important rence ofan artesian fountain 200m up-glacierfrom the influenceon the flowof the glacier.Neglectingthe effect on ¹ snoutin early July1 998(Copland and others, 2003). the flowof possible longitudinal variations in ice tempera- Figure7 alsosuggests that basalmotion may occur over ture, alongitudinalcoupling length of 4times the localice the upperablation area (stakes 7^9)andlower accumulation thickness providesthe best fit betweenobserved and pre- area(stakes 4^6)in the summer.Alongitudinalcoupling dicted surfacevelocities. length of 4h suggests that the summer velocityincreases in the upperablation area may be attributable to the hydro- ACKNOWLEDGEMENTS logicalforcing provided by the moulinsbetween stakes 9 and1 0.However ,this doesnot account for the highersum- Fundingand field logistics were provided by the University mer velocitiesobserved in the loweraccumulation area. A ofAlberta Izaak W altonKillam Memorial Scholarship, the more likelyexplanation relates tothe moulinsthat havebeen NaturalSciences andEngineering R esearch Councilof observedto open just down-glacierof stake6 towardsthe end Canada,the U.K. NaturalEnvironment Research Council ofthe melt season(mid- July).Althoughwater flow into these ARCICEprogramme,the PolarContinental Shelf Project moulinsis short-lived( 2weeks),dischargesinto them can ¹ (contributionN o.0090 1),the GeologicalSociety of besignificant( 5 m3 s^1).Dye tracingindicates that they are ¹ America,and the CanadianCircumpolar Institute ofthe connectedto the glacierterminus viaa partiallydistributed Universityof Alberta. W ethankthe NunavutR esearch drainagesystem (Binghamand others, 2003).Theseobser- Institute andthe communities ofGrise Fiordand R esolute vationssuggest that water input into these uppermoulins Bayfor permission towork at J ohnEvans Glacier .W.Davis, reaches the glacierbed and results inbasal motion in the late D.Glowacki,A. Arendt, T.Wohlleben,J .Barker,C.Stuart, J . summer inthe loweraccumulation and upper ablation areas. Davis,S. Boon and K. Heppenstallprovided invaluable help Asdiscussed insection 3.1,lowresidual bed reflection inthe field.Wearegrateful to T.Jo¨hannesson(Scientific Edi- powersin radio-echo sounding suggest that the glacierice is tor) andtwo anonymous referees fortheir usefulcomments. coldthroughout the accumulationarea (Copland and Sharp , 2001).Thesemeasurements weremade in May ,beforethe start ofthe melt season,so do not preclude the possibilitythat REFERENCES localizedregions of the bedin the accumulationarea reach Andreasen,J.-O .1985.Seasonal surface-velocity variations on a sub-polar the meltingpoint in late summer .Presumably,the transient glacierinW estGreenland. J. Glaciol., 31(109),319^323. waterinputs providedby the openingof the uppermoulins Bingham,R. G., P .W.Nienowand M .J.Sharp.2003.Intra-annualand intra- produceshort-lived subglacial drainage pathways which seasonalflow dynamicsof a HighArctic polythermal valley glacier . Ann. freeze shut overwinter and would not be detected inradio- Glaciol., 37,181^188. Blatter,H.1 987.On thethermal regime of anArcticvalley glacier: a study echosounding measurements inthe spring. ofWhite Glacier,AxelHeiberg Island, N. W.T.,Canada. J. Glaciol., 33(114),200^211. Blatter,H.and G. Kappenberger .1988.Mass balance and thermal regime 6.CONCLUSIONS ofLaika ice cap, Coburg Island, N .W.T.,Canada. J. Glaciol., 34(116), 102^110. Copland,L .andM .Sharp.2000.Radio-echo sounding determination of poly- Interpretation ofseasonallyresolved surface velocitymeas- thermalglacier hydrology . In Noon,D .,G.F.Stickleyand D .Longstaff, eds. urements alongthe lengthof JohnEvans Glacier suggests GPR2000,EighthInternational Conference on Ground P enetratingRadar ,23^26 that basalmotion occurs throughoutthe yearbeneath the May2000 ,Gold Coast,Australia .Bellingham,WA, InternationalSociety of Photo-opticalInstrumen tationEngineers, 59^64. (S PIEProceedings4084. ) lowerablation area of this predominantlycold polythermal Copland,L. andM. Sharp. 2001.Mappingthermal and hydrological condi- glacier.Theinferred distributionof basal motion is broadly tionsbeneath a polythermalglacier with radio-echosounding . J. Glaciol., consistent withmapping of residualbed reflection power by 47(157),232^242. radio-echosounding (Copland and Sharp, 200 1).Thissug- Copland,L., M. Sharp andP .Nienow.2003.Links betweenshort-term vel- ocityvariations and the subglacial hydrology of apredominantlycold gests that ice iswarm-basedin the ablationarea where basal polythermalglacier . J. Glaciol., 49(166),337^348. motionis requiredto account for the observedsurface Heppenstall,K.E. 2002.Chemical weathering in aglaciatedcarbonate velocities,and generally cold-based in the accumulation catchment,Canadian High Arctic: implications for subglacial hydrol- area.Exceptions to this pattern occurin the loweraccumu- ogy.(M.Sc. thesis,U niversityof Alberta.) Hodgkins,R. 1997.Glacierhydrology in Svalbard,Norwegian High Arctic. lationarea in the latesummer ,whenlocalized areas of the Quat.Sci. Rev . , 16(9),957^973. glacierbed appear to reach the pressure-melting pointdue Iken,A.1 974. VelocityfluctuationsofanArctic valleyglacier;astudyoftheWhiteGlacier , tothe penetrationof surface meltwater tothe glacierbed via Axel HeibergIsland, Canadian Arctic Archipelago. Montre¨al,Que., McGill U ni- moulins.Theinference that basalmotion occurs throughout versity.(AxelHeiberg Island R esearchR eportsGlaciology 5 .) Iken,A. andR. A. Bindschadler.1986.Combined measurements of subglacial the winterin the lowerablation area is consistent withinde- waterpressure andsurface velocity of Findelengletscher ,Switzerland: pendentevidence from radio-echo sounding and meltwater conclusionsabout drainage system and sliding mechanism. J. Glaciol., chemistry forsubglacial water storage in this region. 32(110),101^119. Themagnitude of basal motion beneath the lower Kamb,B. andK. A. Echelmeyer.1986.Stress-gradient coupling in glacier flow: I.Longitudinal averaging of theinfluence of icethickness and sur- ablationarea seems toincrease substantiallyduring the face slope. J. Glaciol., 32(111),267^284. summer melt season.The increase is especiallymarked in Nye,J. F .1952.The mechanicsof glacierflow . J. Glaciol., 2(12),82^93. the areadownstream from a groupof crevasses andlarge Nye,J. F .1957.The distribution ofstressand velocity in glaciersand ice- moulinsby which five large supraglacial streams draininto sheets. Proc.R. Soc.London,Ser .A , 239(1216),113^133. Nye,J .F.1965.The flow ofaglacierin achannelof rectangular ,ellipticor the glacierin summer .Together,these streams drainmuch paraboliccross-section. J. Glaciol., 5(41),661^690. ofthe upperablation and accumulation areas of the glacier. Paterson,W.S. B.1994. Thephysics of glaciers.Thirdedition. Oxford,etc., Elsevier . 413 Copland and others:Distribution of basal motion beneath apolythermal glacier

Rabus,B .T.andK. A. Echelmeyer.1997.The flow ofapolythermalglacier: iceformation on the sensitivity of glacier mass balance to climate change. McCallGlacier ,Alaska,U .S.A. J. Glaciol., 43(145),522^536. Ann.Glaciol. , 24,186^190. Skidmore,M. L. andM. J.Sharp. 1999.Drainage system behaviour of a Zevenbergen,L.W .andC. R.Thorne. 1987.Quantitativeanalysis of land High-Arcticpolythermal glacier . Ann.Glaciol. , 28, 209^215. surfacetopography . EarthSurf .Processes Landforms , 12(1), 47^56. Willis, I.C. 1995.Intra-annual variations in glaciermotion: a review. Prog. Zwally,H.J., W .Abdalati,T .Herring,K. Larson, J. Saba and K. Steffen. Phys. Geogr. , 19(1),61^106. 2002.Surface melt-induced acceleration of Greenland ice-sheet flow . Woodward,J .,M.Sharp andA. Arendt.1997.The influenceof superimposed - Science, 297(5579),218^222.

MSreceived 31 July200 1and accepted in revised form 23 May 2003

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