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Intra-annual and intra-seasonal flow dynamics of a High Arctic

Citation for published version: Bingham, RG, Nienow, PW & Sharp, MJ 2003, 'Intra-annual and intra-seasonal flow dynamics of a High Arctic polythermal valley glacier' Annals of Glaciology, vol 37, no. 1, pp. 181-188., 10.3189/172756403781815762

Digital Object Identifier (DOI): 10.3189/172756403781815762

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Download date: 20. Feb. 2015 Annals of Glaciology 37 2003 # InternationalGlaciological Society

Intra-annual and intra-seasonal flow dynamics ofa High Arcticpolythermal valley glacier

Robert G. BINGHAM,1 PeterW. NIENOW,1 Martin J. SHARP2 1Department of Geography andTopographic Science,University of Glasgow,Glasgow G12 8QQ,Scotland E-mail:[email protected] 2Department of Earth and Atmospheric Sciences,University of Alberta, Edmonton, AlbertaT6G 2E3,Canada

ABSTRACT.Measurements ofsurface dynamicson polythermalJohnEvans Glacier , Nunavut,Canada, over two winter periodsand every 7^1 0daysthroughout two melt seasons (June^July2000, 200 1)providenew insight into spatio-temporal patterns ofHigh Arctic glacierdynamics. In the lowerablation zone, mean annual surface velocitiesare 10^21m a^1,butpeak velocities up to 50% higher are attained during late June/earlyJ uly. Inthe upperablation zone and loweraccumulation zone, mean annual surface velocities aretypically 1 0^18ma ^1,andpeak velocities up to 40% higheroccur during late July.In the upperaccumulation zone, mean annual surface velocitiesare 2^9 ma ^1, and motion inmid- tolate July exceeds this byup to 1 0%.Rapid drainage of pondedsupraglacial waterin the upperablation zone to an initiallydistributed subglacialdrainage system in mid-June mayforce excess surface motionin the warm-basedlower glacier. The data indicatethat the durationof the velocityresponse maybe relatedto the rate ofchanneliz- ationof the basaldrainage, and the velocityresponse maybe transmitted up-glacierby longitudinalcoupling .Anincrease insurface velocitiesin the middleglacier in late J uly occurs inconjunction with the openingof two further moulinsin the accumulationzone.

INTRODUCTION 1997);Finsterwalderbreen,Svalbard (W adhamand others, 2001);andJ ohnEvans Glacier ,Ellesmere Island(Copland Recent modelsof climate changehave identified the High andSharp, 200 1). Arctic (the globalarea north of 75³N) ashighlysensitive to Intra-annualand intra-seasonal variations in the surface predicted globalwarming (Hardy and Bradley ,1996; dynamicsof temperate glaciershave been attributed tothe Houghtonand others, 2001).Theresultant meltingof High spatiallyextensive penetration of supraglacialmeltwaters to Arctic ice cover(comprising an area of 275 000 km2, the glacierbed during each melt season(W illis, 1995;F oun- ¹ excludingGreenland) may contribute to eustatic sea-level tainand W alder,1998).Peaksurface velocitiesare typically rise of0.5^1.5m (Prowse,1990;M unro,2000) .However,it is observedshortly after the onset ofmelt, aslargesurface melt- unclearhow rapidly glaciers and ice capsin the HighArctic waterinputs encountera predominantlydistributed sub- willrespond to climatic warming,due in part to limited glacialdrainage network, inducing high subglacial water knowledgeof the dynamicsof High Arctic glaciers.Largely pressures andenhanced basal motion (Kamb, 1987).Con- dueto a dearthof field data with which to parameterize model tinuedsurface meltwater supplies overthe melt seasonmay inputs,temporal variations in High Arctic ice flow,andcoup- inducechannelization of subglacial drainage (Nienow and lingbetween dynamics and hydrology in High Arctic others,1998),leadingto a fallin water pressures anda slow- glaciers,have been little consideredin recent modelsof cryo- downin surface velocities (Willis, 1995).Aschannels are spheric response toclimate change(Houghton and others, formedfurther up-glacier,inassociation with the retreat of 2001).Byinfluencing mass transfer tolower altitudes, these the transient snowline,a waveof high basal water pressures/ factorsmay provide a critical mechanism forrapid, large- highsurface velocities may move up-glacier as the melt scaleresponses ofglaciersand ice sheets toclimatic warming seasonprogresses. Velocities remainlow outside the melt (Zwallyand others, 2002).Thispaper goes some wayto- seasonas channelsclose due to the influenceof overburden wardsaddressing these issues byinvestigating intra-annual pressures (Fountainand W alder,1998).Whether similar pro- andintra-seasonal variations in glacier motion and hydrol- cesses occurat High Arctic glaciersis unclear,becausesur- ogyovertwo melt seasonsat a HighArctic glacier. faceinputs (moulins/crevasses) tend tobe few and widely Inthis paper,wedefine a ``HighArctic glacier’’asa poly- spaced(due to lower rates ofglacier activity) ,andbasal thermal glaciercomprised almostentirely of cold ice, but motionmay be precludedin many areas by cold-based ice. containinga basallayer of warmice underneaththe ablation Previousobservations of HighArctic glacierflow dynam- zone.Such a thermal regime is characteristic ofmany glaciers ics havedemonstrated that surface motionvaries intra- inthe CanadianHigh Arctic, northernAlaska, Greenland annually,atleast inthe ablationzone, where ice is warm- andSvalbard (Blatter andH utter,1991,fig.1),andis exempli- based (Mu« ller andIken, 1 973;Rabus and Echelmeyer ,1997; fiedby White Glacier,AxelHeiberg Island (Blatter ,1987); Copland,200 1;Zwallyand others, 2002).Peakglacier surface McCallGlacier ,northernAlaska (Rabus andEchelmeyer , velocitiesduring summer melt seasonshave variously been 181 Bingham andothers:Flowdynamics of High Arctic polythermal glacier

Fig.1.(a)Location ofJohn Evans Glacier.(b)Distribution of velocity stakes measured from1999to 2001.Thedistribution of warm-based ice is derived from radio-echo sounding under- taken by Copland and Sharp (2001).Note the locations of two moulin fields:h1^h5are situated over alarge riegel; and h6 and h7 are situated in the accumulation zone above alarge nunatak.These moulin fields bound three sectors of the glacier: the upper glacier (Ustakes;upstream of h6^h7),the middle glacier (Mstakes;between the moulin fields) and the lower glacier (Lstakes;downstream of h1^h5).Rstakes overlie a bedrock riegel,over which ice thins to 40 mand ice is appar- ¹ ently cold-based. (c)Close-up of lower-ablation-zone stake network. Legend asin (b). observedto coincide with: (i) peaksin water levels measured 15kmalong its mainflowline and spans an elevation range of inmoulins (Iken, 1 972);(ii) peaksin supraglacial discharges 100^1500m (Fig.1).Theequilibrium line is locatedat 750^ ¹ enteringmoulins and crevasses (Copland,200 1);and(iii) 850m, nextto a smallnunatak (Fig .1);the 1997^99mean peaksin rates ofsurface melting (Zwally and others, 2002). annualair temperature measured ata meteorologicalstation Theseobservations suggest tha t, asat temperate glaciers, locatednear to the equilibrium-linealtitude (ELA) was hydrologicallyinduced variations in basalmotion are predom - ^15.2³C. I ce thicknesses havebeen determined fromground- inantlyresponsible for varia tionsin surface dynamics. basedradio-echo sounding at 3200 locations across the Thispaper reports onglacier-wide variations in intra- glacier:near to the ELA,ice is almost400 m thick,but under annualand intra-seasonal surface dynamics measured atJohn the accumulationzone and the lower4 kmofthe glacier,ice is EvansGlacier ,Nunavut,Canada, from late J uly1 999to late typically1 00^200m indepth (Copland and Sharp ,2001). July200 1.Themeasurements formedone component of an in- However,ice thins to 40m abovea largebedrock riegel ¹ tegratedstudy of the hydrologyand dynamics of this High 4kmabovethe glaciersnout. ¹ Arctic glacier,the mainhydrological results ofwhich will be Radio-echosounding and 1 5mice temperatures suggest presented elsewhere. Thespecific purposesof this studywere that the glacieris polythermal(Fig .1).Throughoutthe toelucidate: (i) whethersurface motion varies spatially and/ or glacier,15mice temperatures rangefrom ^7³ to ^1 5³C, temporallyover a melt season,(ii) whethermotion anomalies demonstratingthat near-surfaceice remains cold.In the propagatespatially ,and(iii) whatmechanisms areresponsible lowerablation zone, high bed reflection powers (BRP r) forthe observedsurface dynamics . (corrected forice thickness) andan internal reflector indicate the presence ofa basallayer of warm ice upto 40 m thick (Coplandand Sharp, 2001).Thiswarm basallayer results from FIELD SITE pressure meltingunder 200m ofice. F urther up-glacier,high ¹ JohnEvans Glacier isalargevalley glacier situated at79³ 40 ’ N, BRPr occurs inthe absenceof an internal reflector ,suggesting 74³00’ Woneastern Ellesmere Island,N unavut,Canada. The that onlythe basalinterface is warm(Copland and Sharp, glaciercovers 75%ofa 220km 2 catchment, hasa lengthof 2001).Nearto the marginsand terminus, andover the riegel, ¹ 182 Bingham and others:Flowdynamics of High Arctic polythermal glacier ice remains entirelyfrozen due to a steep englacialtempera- ture gradientconducting away geothermal heat (Copland andSharp, 200 1).Inthe accumulationarea, low BRP r has beeninterpreted byCopland and Sharp ( 2001)tosuggest that ice is entirelycold. This interpretation of the thermal regime is consistent withdirect boreholemeasurements ofthe thermal regime ofnearby White Glacier (Blatter,1987)and LaikaI ce Cap(Blatter andKappenberger ,1988). Atthe beginningof each melt season,cold ice atthe terminus mayact as athermal damto subglacial outflow ,leadingto the developmentof atemporarysubglacial reservoir beneaththe lowerablation zone (Skidmore and Sharp, 1999).Subglacialoutflow initiates severalweeks into each melt season(typically late J uneêarlyJ uly),byforcing a pathbeneath, or through fractures in,the ice (Skidmore andSharp, 1 999).Thesource androuting of the subglacial outfloware not well understood. In 1 994,1996and 200 1,subglacialoutflow throughout the melt seasonwas intermittent andpulsed, with channel shifts betweenoutflow events. In 1998,1999and 2000, subglacial outflow rapidly settled intoa diurnalrhythm ( 2^3daysafter initiation)which persisted ¹ throughoutthe melt season,and its magnitudevaried in closerelation with air temperatures andsurface melting, stronglysuggesting a supraglacialorigin. Skidmore and Sharp( 1999)suggestedthat surfacerunoff draining into Fig.2.Breakthrough curves derived from dye injected into moulinslocated in a crevasse fieldoverlying the bedrock moulin h1 and detected in the subglacial outflow during (a) riegel(moulins h1^h5; Fig .1)mightconstitute the originof 2000and (b)2001.Note the difference in x-scales.Dye curves much,if not all, of the subglacialoutflow ,whichmay tem- from 2000 demonstrate clear evolution of the basal drainage porarilyshut downin cooler summers dueto low rates ofsur- system. In 2001,basal drainage remained distributed through facerunoff. Copland ( 2001)demonstrated thatintra-annual earlyJuly,and rationalized in mid-Julyafter the resumption of contrasts betweensummer (July)and winter (August^May) high meltwater inputs. surfacevelocities along the mainflowline were greatest down-glacierof these moulins.Above the riegel,it wascon- cludedthat internaldeformation could account for all sur- dates onwhich each opened are unknown) ,capturinglarge facemotion, although longitudinal stress-gradient coupling supraglacialstreams (combineddischarge 5 m3 s^1) drain- ¹ ensured that summer velocitiesexceeded winter velocities up ingthe accumulationzone. tothe upperaccumulation zone. In2000 and 200 1,subglacialoutflow at the terminus Atthe onset ofthe 2000and 200 1melt seasons,all mou- initiatedon 22J uneand 29 J unerespectively (subglacial out- lins wereinactive, having been sealed by snow and ice flowhaving been completely inactive during winter) .Inboth duringthe precedingwinter .Runoffgenerated in the lower cases, subglacialoutflow initiated 524hours after the sud- ablationzone drained supraglacially throughout each melt dendrainage of the largesupraglacial ponds into the riegel season.(Throughout this paper,``lowerablation zone’ ’refers moulins(h 1^h5) 5kmup-glacier .Rhodamine-WTdye, ¹ tothe areabetween the terminus andthe riegelmoulins h 1^ subsequentlyinjected intomoulins h 1^h4during June^J uly h5(Fig .1).Additionallywe refer tothe `ùpperablation 2000and into h 1duringJ uly200 1,waslater detected inthe area’’,betweenh 1^h5and the ELA;the ``loweraccumu- subglacialoutflow using a StJohnsFluoro- Tec fieldfluorom- lationarea ’’fromthe ELAto the uppermoulins, h6 andh7 ; eter,confirmingthat supraglacialmeltwaters enteringthese andthe `ùpperaccumulation area ’’fromh6 and h7 to the moulinsaccessed the subglacialdrainage system betweenthe glacierhead (Fig .1b).)Surfacemelt generatedin the upper riegeland the terminus. Dye injected intoh6 inlateJ uly2000 ablationarea and throughout the accumulationzone ini- wasalso subsequently detected inthe subglacialoutflow , tiallyrefroze inthe coldsnowpack and subsequently col- demonstratingthat surfacerunoff entering a moulinin the lected inlarge supraglacial ponds which built up along accumulationzone also accessed the subglacialdrainage relict stream networksbetween the nunatakand the riegel. system. Periodicdye injections into h 1throughoutthe 2000 Over 2^3weeks, these pondsbecame connected along a series melt seasonindicated that the subglacialdrainage system be- ofinitially snow-plugged englacial channels .Eachyear ,on21 neaththe lowerablation zone rapidly became channelized June2000 and 28 J une200 1respectively,fivelarge moulins followingthe initiationof subglacial outflow (Fig .2a).How- (h1^h5;Fig .1b)openedsuddenly in acrevasse fieldover the ever,similar experiments in200 1demonstrated that the sub- riegel,and the networkof supraglacial ponds (combined glacialdrainage system remainedlargely distributed, asa capacity 200 000 m3)drainedrapidly (over 24 hours) into result ofcoolweather significantly reducing surface runoff ¹ the englacialsystem. Followingthese largedrainage events, duringmuch ofearly J uly(Fig .2b). supraglacialstreams developedalong the networkof supra- Thehydrological observations from 2000 and 200 1raise glacialpond basins and continued to drain into h 1^h5for severalimportant questions. Firstly ,surface meltwaters the remainderof the melt season.F urther up-glacier,two clearlyaccess the subglacialdrainage system followingtwo additionalmoulins (h6, h7 :Fig.1b) openedbetween 5^1 9 largedrainage events eachyear ,viah 1^h5in mid- tolate Julyand 1 3^21Julyin 2000 and 200 1respectively(the exact Juneand via h6 and h7 in mid- tolate J uly.Dothese 183 Bingham andothers:Flowdynamics of High Arctic polythermal glacier drainageevents producevarying dynamic responses indif- detected bythe GPSsystem, systematic errors duringparts ferent parts ofthe glacier,accordingto location upstream or ofa day.Thismeans that caremust betakennot to rely on downstreamof the lowerand/ orupper moulin fields? Sec- individualsurvey stakes todefine motion trends .) ondly,surfacerunoff may access the subglacialdrainage StakesL3 and L6^L37 each had a reflectingprism system viah6 and h7 where radio-echo sounding suggests attached,and prism positionswere periodically surveyed that ice is cold-based.Could this accountfor surface motion usinga totalstation instrument (Geodimeter System 500). variationsupstream ofthe bedrockriegel? Thirdly,whatis Thetotal station was mounted on tripods onbedrockat ss 1 the effect ofsubglacialdrainage-system evolution on surface tosurvey L 12^L37,andat ss2 tosurvey L3 andL6^L 11(Fig. dynamicsin the lowerablation zone? This paper addresses 1c) (line-of-sight problems necessitated the use oftwo these questions byanalyzing glacier-wide surface dynamics surveystations) .Repeat surveysto three separatereference measured atJ ohnEvans Glacier fromthe endof the 1999 targets, locatedon the surroundingbedrock, allowed for melt seasonthrough to the endof the 2001melt season. determinationof, and compensation for ,errors associated withsetting upthe totalstation on each survey tripod, and withchanges in temperature andpressure duringa survey. METHODS Eachvelocity stake was surveyed at least twice duringa survey; the x, y and z coordinateswere taken as the mean,and Sixtyvelocity stakes weredrilled and frozen into the ice sur- the error associatedwith each coordinate value was calcu- facethroughout the glaciertowards the endof 1999(Fig .1). latedas the standarddeviation of this meanvalue. Theaver- Thestakes werelabelled as follows: ageerror forthe positionof each survey stake from all surveys was 8.61mm. Againassuming that successive (i) Ustakes ösituated inthe upperaccumulation zone; § errors areof opposite sign, this yieldeda maximum (ii) Mstakes ösituated betweenthe bedrockriegel and between-surveydisplacement error of17.22mm, producing moulinsh1^h5, but down-glacier of h6^h7; amaximummean velocity error of0.90ma ^1 over a 7 day (iii) Rstakes öoverlyingthe bedrockriegel, and sur- measurement period.These errors areinsignificant in com- roundingmoulins h 1^h5; parisonwith observed velocities. Errors derivedfrom the totalstation itself wereneglected as theywere significantly (iv)L stakes ösituated inthe warm-basedlower ablation smaller thanmeasurement errors. zone,downstream of all knownsupraglacial meltwater inputs. Stakepositions were measured oncetowards the endof the RESULTS 1999melt season,and every 7^1 0daysduring the 2000and Totest whethersurface flowin different parts ofJohnEvans 2001melt seasons. Glacier respondedto temporal variations in supraglacially ALeicaSystem 500differential global positioning system derivedbasal hydrological forcing, each year was split into (GPS) wasused tomonitor the positionsof all stakes, with the the followingtime periods: exceptionsof L3 and L6^L37 (discussed below).Theproce- dureinvolved leaving a ``reference’’GPSreceiver tofix a (i) ``Winter’’öthe periodbetween the finalsurvey of one knownposition on bedrock throughout each survey ,whilst melt seasonand the initialsurvey of the followingmelt placingthe ``roving’’GPSreceiver adjacentto each survey season(i.e. endJ uly/start Augustto early J une); stakefor up to 40 min toobtain each individual stake posi- (ii) `Èarlyspring’ ’öearlyJ une,from the onset ofsurface tion.Toensure consistencybetween surveys, the GPSreceiver meltingto the onset ofmeltwater drainageinto moulins wasplaced vertically just abovethe ice orsnow surface adja- h1^h5; cent tothe northside ofeach stake, and the distanceto the top ofthe stakefrom the receiver wasmeasured manually.This (iii) ``Springevent’ ’ötime periodsurrounding initial gave the xyz positionof the topof the stakefor each survey , drainageof supraglacial meltwaters intomoulins h 1^h5; reducingerrors associatedwith tilting of the stakes dueto (iv)` `Mid-summer’’öearlyto mid- July,whenmeltwaters windor melt-out, whichwould be accentuated if the receiver continueto drain into h1^h5 and the snowlineretreats wereplaced at the topof the stake.Systematic errors forindi- up-glacier; vidualsurveys were reduced by surveying three fixedsurvey (v) ``Latesummer’ ’ötime periodfollowing drainage of pointson bedrock with the rovingGPS receiver duringeach supraglacialmeltwaters intoup-glacier moulins h6 GPSsurvey .Estimates oferrors associatedwith the GPS and h7. system itself weredirectly calculated by the unititself. Where positionalerrors exceeded5 mm, the result wasclassed as Timeconstraints precludedthe possibilityof surveyingall `àmbiguous’’andwas not considered further inthe analysis. 60stakes onasingleday ,so,for example, the period` èarly Theaverage error forthe positioningof a stakewas spring2000’ ’atstake U3 is slightlyoffset from` èarlyspring 2.04mm, equatingto a maximumbetween-survey displa- 2000’’atstake M8. The exact time periodscovered by § cement error of4.08mm, assumingsuccessive positionerrors winter,earlyspring, etc., foreach stake are detailed in Table areof opposite sign. Over a7daysummer measurement 1.Asthe temporaloffsets aresmall (typically1^2 days period(the smallest measurement intervalin this analysis), betweendifferent stakes) incomparison to the durationof this produceda maximummean velocity error of0.21ma ^1; eachtime period,we assume theyare negligible and that overlonger measurement intervalsthis error wasreduced winter,early-spring,etc., velocitiesare directly comparable further.Tobeconsidered ` `real’’,variationsin velocities betweenall survey stakes. betweenperiods of observation must exceedthese errors. (It Meanannual surface velocitiesfor all stakes onJohn shouldbe noted, however ,that poorsatellite configurations EvansGlacier from1 999to 200 1areplotted in Figure 3a. mayoccasionally lead to apparently unam biguous,i.e. not Alsoprovided are surface velocitiesduring different periods 184 Bingham and others:Flowdynamics of High Arctic polythermal glacier

Table1.Division of specific periods over which velocities were measured at each survey stake

Stakessurveyed Winter 1999/2000 Early spring2000 Springevent 2000 Mid-summer 2000 Late summer 2000

All Ustakes,M 1^M43 Aug.1999^4Jun. 2000 4 Jun.2000^1 4Jun.2000 1 4Jun.2000^5 Jul. 2000 5 Jul.2000^1 9Jul.2000 1 9Jul.2000^30 Jul. 2000 M5^M11,R1^R5, 31Jul.1999^6Jun. 2000 6 Jun.2000^1 4Jun.2000 1 4Jun.2000^8 Jul. 2000 8 Jul.2000^23 Jul. 2000 23 Jul. 2000^1 Aug .2000 L1,L2,L4, L5 L3, L6^L37 30Jul. 1999^10Jun.2000 1 0Jun.2000^1 6Jun.2000 1 6Jun.2000^5 Jul. 2000 5 Jul.2000^20 Jul. 2000 20 Jul. 2000^3 1Jul.2000

Stakessurveyed Winter 2000/2001 Early spring2001 Springevent 2001 Mid-summer 2001 Late summer 2001

All Ustakes,M 1^M430 Jul. 2000^1 Jun. 200 11Jun.200 1^19Jul.200 119Jul.200 1^6Jul.200 16Jul.200 1^21Jul.200 121Jul.200 1^28Jul. 200 1 M5^M11,R1^R5, 1Aug.2000^2Jun. 200 12Jun.200 1^24Jun. 200 124Jun. 200 1^9Jul.200 19Jul.200 1^18Jul.200 118Jul.200 1^29Jul. 200 1 L1,L2,L4, L5 L3, L6^L37 31Jul.2000^8 Jun. 200 18Jun.200 1^23Jun. 200 123Jun. 200 1^8Jul. 200 18Jul.200 1^19Jul.200 119Jul.200 1^26Jul. 200 1

ofeachyear ,shownas percentagesof meanannual motion 2000),surface velocitiesin the lowerablation zone increased (Fig.3b^k).(Notethat these diagramsrely on interpolation byup to 60%, whilst surface velocitiesin the upperablation betweenindividual data points (velocity stakes) andbound- zoneand lower accumulation zone increased by up to 25% aries (glaciermargin) ,sovelocity estimates awayfrom the (Fig.3d).Thedistinct increase insurface velocitiesin the stakelocations may be erroneous and arenot considered in lowerablation zone during the springevent likely resulted the analysis.)Maximumaverage surface velocitiesof 1 5^ fromenhanced basal motion induced by rising subglacial 20 m a^1 aremostly attained throughout the lowerablation waterpressures ina hydraulicallyinefficient basal drainage zone,below the bedrockriegel and all moulins,where ice is system (cf. Kamb,1 987).Thesehigh pressures wereinduced probablywarm-based (Fig .3a).However,relativelyfast- bythe rapid( 24hours) drainage of up to 200 000 m 3 of ¹ movingsurface ice ( 17 m a^1)is alsolocated just down- pondedsupraglacial meltwaters, viah 1^h5,tothe glacier ¹ glacierof the uppermoulins at stakes M2and M3 .The bedon 2 1June2000. Thatthese meltwaters initiallyencoun- slowest-movingstakes aresituated (i) inthe upperaccumu- tered adistributed drainagesystem underneaththe lowerab- lationzone upstream ofallknown moulins (U stakes);(ii) in lationzone was confirmed by a dye-tracer test on25 June locationsperipheral to the mainflowline (M 10,L5 );and 2000,which showed that the subglacialdrainage system (iii) immediatelyup-glacier and down-glacier of the nuna- betweenh 1andthe terminus wasrelatively inefficient tak,which retards flowin these locations(M4, M9).(In shortlyafter the onset ofdrainage into h 1(Fig.2a).The fact,stakes M4and M9 are virtually stationary and are dampedincrease insurface velocities up-glacier of the riegel therefore excludedfrom further analysis.) (Fig.3d)mayhave resulted fromlongitudinal stress-gradient Almost allstakes, regardless oflocation,flowed consider- couplingwith the hydrologicallyforced velocity anomalies ablyfaster duringthe melt seasoncompared with over occurringdown-glacier . winter(Fig. 3 ).Stakesin the lowerablation zone (L stakes) Inmid-summer 2000,surface velocitiesthroughout the experiencedpeak velocities during the springevent in 2000 glacierexceeded mean annual velocities, but motion in the (Fig.3d)and during mid-summer in2001(Fig.3j).In2000, lowerablation zone slowed down relative to the springevent this wasfollowed by a slow-downduring mid-summer (Fig. (Fig.3e).Thismay be explainedby considering the chan- 3e)anda speed-up inlate summer (Fig.3f).In200 1,highsur- gingform of the subglacialdrainage system duringsummer facevelocities were maintained in the lowerablation zone 2000.Dye-tracing experiments frommoulin h 1confirmed throughoutmid- tolate summer (Fig.3jandk) .Velocityvari- thatpersistently highsurface runoff throughout summer ationsover the middleglacier (R andM stakes) mostlyex- 2000led to rapid channelization of the subglacialdrainage periencedthe same annualcycle as stakes inthe lower system beneaththe lowerablation zone during late Juneand glacier,butthe variationswere strongly damped. However , throughoutJ uly(Fig. 2a) .Thisrationalization of the sub- aperiodof high velocities during late summer wassuper- glacialdrainage system inthe lowerablation zone reduced imposedon this signal(Fig .3fandk) .Stakemotion in the subglacialwater pressures inmid-summer ,therebyreducing upperglacier (U stakes) experiencedonly slight fluctuations, basalmotion and leading to the observedreduction in sur- althoughmotion in summer exceededthat inwinter (Fig .3). facevelocities. Inlate summer 2000,surface velocitiesin the lower ablationzone once again increased, but this time incon- DISCUSSION junctionwith peak surface velocitiesrecorded in the upper ablationzone and lower accumulation zone (Fig .3f).This Theobserved intra-annual and intra-seasonal surface late-summer velocityincrease throughoutmuch ofthe dynamicsat J ohnEvans Glacier relate closelyto the spatial glaciermay be attributed tothe openingof h6 and h7 ,in distributionand timing of supraglacialmeltwater inputs to the accumulationzone, in mid- July.Combineddischarges the subglacialdrainage system. Duringwinter 1 999/2000 intothese moulinsthrough late July typically attained (Fig.3b),andearly spring 2000 (Fig .3c),priorto supra- values of 5 m3 s^1.Asstated earlier,highBRP suggest ¹ r glacialdrainage into h 1^h5,surface velocitiesthroughout warm-basedice underlies much ofthe ablationzone up to the glacierdiffered little frommean annual velocities. How- andaround the sides ofthe nunatak(Copland and Sharp , ever,fromlate J uneto early J uly2000 (i.e. springevent 2001),allowingfor the possibilityof subglacial drainage 185 Bingham andothers:Flowdynamics of High Arctic polythermal glacier

Fig.3.(a) Distribution of mean annual velocities (inm a ^1)atJohn Evans Glacier measured from1999to 2001.The black dots represent velocity stakes.Due to the method of interpolation, velocities awayfrom the concentration ofdata points (velocity stakes; margins) may be inappropriate and are not considered inthe analysis.(b^k)Surf ace velocity distributions at different stages of 2000 and 2001,as percentages of mean annual velocity (100% mean annual velocity).Specific periods over which velocities ˆ were measured at each stake are given inTable1. 186 Bingham and others:Flowdynamics of High Arctic polythermal glacier beneathmuch ofthe upperablation zone (Fig .1).Moreover, ofthe abovehypotheses best explainsthe late-summer alate-Julydye injection into h6 demonstrated that melt- motionpeaks is beyondthe scopeof this paper,although waters draininginto h6 travelled to the subglacialoutflow the correlationof the motionpeaks with the onset ofdrain- viaa partiallydistributed drainagesystem. Therefore,the ageinto the uppermoulins in both years suggests that the openingof h6 andh7 inmid- Julylikely led to increased sub- onset ofdrainageinto h6 andh7 issignificant. glacialwater pressures ina distributed basaldrainage Throughoutsummer 2000,the glaciersurface in the system undermuch ofthe upperablation zone, inducing upperaccumulation zone (above moulins h6 and h7 )moved highsurface velocitiesdown to the riegel.Below the riegel, onlyslightly faster thanin winter ( 510%faster thanmean anefficient, channelizeddrainage system underlaythe annualvelocity) (Fig .3eand f) .Thisis consistent withthe lowerablation zone bylate summer 2000(Fig .2a).However, hypothesisthat upstream ofall known moulins small surface asuddenincrease insubglacial discharge of 5 m3 s^1, de- flowvariations might be explicable entirely by longitudinal ¹ rivedfrom supraglacial drainage into h6 andh7 ,mayhave couplingwith hydrologically forced events downstream. ledto a significantrise insubglacial water pressures dueto Thespatio-temporal pattern ofsurfacevelocity variations the inabilityof the channelizedsystem totransmit allofthe throughoutsummer 2001differedslightly from that exhibited enhanceddischarge efficiently ,resulting ina resumption of in2000, but many of the broad-scaleconclusions relating to highsurface velocities(Fig .3f). supraglacialhydrological forcing are similar .Asin2000, sur- Thelate-summer velocitypeaks in the loweraccumu- facevelocities prior to the springevent differed little from lationzone (stakes M1^M3;Fig .3f)are noteworthy ,since meanannual velocities (Fig .3gandh) .Alsoas in the previous theyoverlie ice whichhas been interpreted, fromlow BRP r, year,avelocityincrease ofupto 40% occurred in the lower tobe cold-based(Copland and Sharp, 200 1).Thelate-sum- ablationzone during the springevent, with small increases in mer velocitypeaks at these stakes are30^40% greater than velocity(up to 20%) occurring in the upperablation zone meanannual flow rates (Fig.3f),stronglysuggesting that andlower accumulation zone (Fig .3i).Thismay once again basalmotion forms asignificantcomponent of late-summer beattributed tothe rapiddrainage of 200 000 m3 of ponded ¹ motion.Three possibilities exist toexplain these phenom- supraglacialmeltwaters intoh1^h5 (this time on28 J une) ena.Firstly ,ifthe radardata have been misinterpreted, the inducinghigh subglacial water pressures underthe lower glaciermay in fact be warm-basedunder the loweraccumu- ablationzone; the resultant velocityresponse beingtrans- lationzone. In this case,surface melt penetratingto the base mitted up-glacierby longitudinal coupling . viah6 and h7 could perturb the basaldrainage system In200 1,incontrast to2000, surface velocities in the underlyingthe loweraccumulation zone. This would imply lowerablation zone continued to rise inmid-summer ,in either that lowBRP r measured inthis area(Copland and some locationsreaching almost double the meanannual sur- Sharp,2001)resulted fromenglacial attenuation of the radar facevelocity (Fig .3j).Thiscontrast withthe slow-down signalrather thancold-based conditions, or that the warm observedin mid-summer 2000(Fig .3e)resulted froma con- basalinterface was too thin or patchy to cause strong bed re- trasting subglacialdrainage configuration underneath the flectionpowers. The possibility of warm-based ice underthe lowerablation zone. In 200 1,coldweather from late Juneto accumulationzone was previously discounted, as it is mid-July,almostimmediately following the drainageevent inconsistent withthe directlymeasured thermal regime of intoh 1^h5,precluded the developmentof a hydraulically nearbyWhite Glacier andLaika I ce Cap(Copland and efficient channelizedsubglacial drainage system tothe sub- Sharp,2001).However,direct measurements orthermo- glacialoutflow until late summer .Instead,dye-tracing mechanicalmodelling of the thermal regime ofJ ohnEvans experiments fromh 1confirmthat subglacialdrainage Glacier arerequired to resolve this issue. Asecondpossibility remaineddistributed untillate J uly(Fig .2b),allowinghigh is thatthe glacieris indeedcold-based in the loweraccum u- subglacialwater pressures tomaintain high basal motion lationarea and thatthe surfacemotion peaks result fromlongi- throughoutmid-summer (Fig.3j).Channelizationof the tudinalstress-gradient couplingwith down-glacier motion basaldrainage system onlytook place after mid-July,when anomalies.However ,acomparisonof winter (August^May) warmweather resumed andhigh meltwater volumes andsummer (July)motion by Copland ( 2001)hasdemon- entered h1^h5(Fig .2b). strated thatthe glacierhas a couplinglength scale of only Inlate summer 2001,the surface velocitydistribution 2km,making this optionunlikely .Athird possibilityis (Fig.3k)once again mirrored thatof the previousyear ¹ 3 ^1 that surface runoff(combined discharge 5 m s ) cap- (Fig.3f).Asin2000, peak velocities up to 30% greater than ¹ tured suddenlyby h6 and h7 inmid- tolate Julypropagates meanvelocities (Fig .3k)in the loweraccumulation zone rapidlyvia hydrofracturealong a relict crevasse tothe base, maybe linked with the onset ofdrainage into moulins h6 whereit melts aconduitrapidly down-glacier to connect andh7 inmid-July.Again,this induceda velocityresponse withwarm-based ice downstream.W eertman (1973)and throughoutthe ablationzone (Fig .3k)asthe basaldrainage Scambosand others (2000)have shown that water-filled system attempted toadjust to a rapidincrease inup-glacier- crevasses maypropagate by hydrofracture to the baseof a deriveddischarges. glacier,whileSkidmore and Sharp ( 1999)invoked hydrofracture asameans bywhich subglacial meltwaters initially breachcold ice atthe terminus ofJ ohnEvans Glacier .As CONCLUSIONS longas propagationis rapid,and high water supply is maintained,advection of surface heat,viscous dissipation of heat Observed intra-annualand intra-seasonal variations in sur- andlatent heatgenerated by refreezing may be sufficient to faceflow dynamics atJ ohnEvans Glacier arerelated primar- counteractwholesale freezing of subglacial water flow in ilyto the spatio-temporalpattern ofsupraglacial meltwater localizedareas. During the initialstages ofconduit growth, inputs tothe glacierbase. Specific patterns ofsupraglacial highsubglacial water pressures mayinduce localized high hydrologicalforcing may vary between years, leading to motionin the loweraccumulation zone. Todetermine which contrastingpatterns ofsurface dynamics. Each spring (typi- 187 Bingham andothers:Flowdynamics of High Arctic polythermal glacier callymid- tolate J une),largevolumes of supraglacially Lewis,K .Heppenstall,T .Wohlleben,J .Barkerand L. pondedmeltwaters inthe upperablation zone drain into a Coplandfor field assistance, andthank the NunavutR esearch distributed subglacialdrainage system, inducinghigh sub- Institute andcommunities ofGrise Fiordand R esolute Bay glacialwater pressures andenhanced basal motion in the forpermission towork at J ohnEvans Glacier .WethankD . warm-basedlower ablation zone. This causes surface Swiftfor comments onan earlier draft, P .Holmlundand C. velocitiesthroughout the lowerablation zone to increase by Schneebergerfor helpful reviews, and C. S. Hvidberg for edi- upto 50% in spring (Fig .3dandi) .Throughoutlate Juneand torialwork on the paper. earlyJuly,surfacevelocities in the lowerablation zone exceed meanannual velocities, but the specific velocitydistribution REFERENCES is afunctionof the degreeof channelization (and thus hydraulicefficiency) of the basaldrainage system, which Blatter,H.1 987.On thethermal regime of anArcticvalley glacier: a study influencesthe magnitudeof subglacial water pressures ofWhite Glacier,AxelHeiberg Island, N. W.T.,Canada. J. Glaciol., (Kamb,1987).Inrelatively warm summers withpersistent 33(114),200^211. runoff(e.g .2000),significantchannelization of the basal Blatter,H.and K. Hutter .1991.Polythermalconditions in Arcticglaciers. J. 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InternationalAssociation of ScientificHydrology Publication 95 (Sym- assistants fromNSER C,the Universityof Alberta Canadian posiumat Cambridge 1 969ö Hydrology of Glaciers ),139^145. CircumpolarInstitute, the GeologicalSociety of America Willis, I.C. 1995.Intra-annual variations in glaciermotion: a review. Prog. andthe Northern ScienceT rainingProgramme. Invaluable Phys. Geogr. , 19(1),61^106. Zwally,H.J., W .Abdalati,T .Herring,K. Larson, J. Saba and K. Steffen. fieldsupport and logistics were provided by the PolarContin- 2002.Surface melt-induced acceleration of Greenland ice-sheet flow . entalShelf Project, N aturalR esources Canada(PCSP/ EPCP Science, 297(5579),218^222. Contribution03202 ).Wegratefullyacknowledge S. Boon, D .

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