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

Links between short-termvelocity variations and the subglacial hydrology ofa predominantly cold polythermal

Luke COPLAND,1 Martin J. SHARP,1 PeterW. NIENOW2 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.Thesurface velocityof apredominantlycold polythermal glacier (John EvansGlacier ,Ellesmere Island,Canada) varies significantly on bothseasonal and short- ertime-scales. Seasonalvariations reflect the penetrationof supraglacial water to the glacierbed throughsignificant thicknesses ofcold ice. Shorter-term events areassociated withperiods of rapidly increasing water inputs tothe subglacialdrainage system. Early- seasonshort-term events immediatelyfollow the establishment ofadrainageconnection betweenglacier surface andglacier bed, and coincide with the onset ofsubglacial outflow atthe terminus. Amid-season short-term eventoccurred as surface meltingresumed fol- lowingcold weather ,andmay have been facilitated by partial closure of subglacial chan- nels duringthis coldperiod. There is acloseassociation between the timingand spatial distributionof horizontaland verticalvelocity anomalies, the temporalpattern ofsurface waterinput to the glacier,andthe formation,seasonal evolution and distribution of sub- glacialdrainage pathways. These factors presumably control the occurrence ofhigh- water-pressure events andwater storage at the glacierbed. The observed coupling betweensurface waterinputs andglacier velocity may allow predominantly cold poly- thermal glaciersto respond rapidly to climate-induced changes in surface melting.

1.INTRODUCTION because:(a) water flow along intergranular vein networks is largelyabsent in cold ice, and (b) andmoulins, whichintroduce large-scale permeability to glacier ice, are Studies oftempera te andpredominantly warm polythermal rare onpredominantly cold due to low rates ofice de- glaciers(polythermal structure types bandc ofBlatter and formationand the refreezingof thatdrains into cre- Hutter,1991,fig.1)haveclearly demonstrated that there is an vasses.If this werethe case,the influenceof surface melting on intricate couplingbetween the subglacialhydrology and flow the flowof predominantly cold polythermal glaciers would be dynamicsof such glaciers(Iken, 1981;Bindschadler,1983;Iken limited. andothers, 1983;Iken and Bindschadler ,1986;Kam b,1987; Severalstudies, however,suggestthat the surfacevelocities Jansson,1 995,1996;Raymond and others, 1995;Harbor and ofsuch glaciersvary on a seasonalbasis (M u « ller andIken,1 973; others, 1997;Kavanaughand Clarke, 200 1;Mairand others, Iken,1974;Andreasen, 1985;Rabus and Echelm eyer,1997).This 2001,2002).Thiscoupling is especiallyimportant when surface is alsothe caseon the Greenlandice sheet, wheremelt -induced meltwater is ableto penetrate tothe glacierbed, and it is seasonalvariations in surface velocity have been observed in a fundamentalto such dynamicphenomena as glacier surges, regionwhere cold ice is 41200m thick(Zwally and others, seasonalvelocity variations and short-term high-velocity 2002).Thesestudies implythat surfacem eltwaters canand do events. Whether andhow this couplingaffects the dynamics penetrate tothe glacierbed through significant thicknesses of ofglaciers composed predominantly of ice atsub-freezing tem- ice atsu b-freezingtemperatu res, contraryto the suggestionof peratures is less clear.Thegoal of this paperis therefore toin- Hodgkins( 1997).Theyalso suggest that coupling between sur - vestigatethe relationshipsbetween surface melt, subglacial facemelting and the flowof predominantl ycold,but warm - hydrologyand the flowof a predominantlycold polythermal based,ice masses mayprovidea mechanismby which such ice glacierin the CanadianHigh Arctic. masses canrespond rapidly to changes in surface weather and Predominantlycold polythermal glaciers (polythermal climate (Zwallyand others, 2002).Givenmodel predictions structure types dande ofBlatter andH utter,1991,fig.1)are thatanthropogenicclimate warmingwill be m ost markedin characterizedby a thickmantle of cold ice overlyinga limited northernhigh latitudes (Manabeand others, 1991),andthe areaof temperate ice at,and immediately above, the glacier potentialcontri butionof Arctic glaciersto global sea level, bedin the ablationarea. Exam ples includeWhite Glacier,Axel understandingof this couplingis ahighscientific priority. HeibergIsland, Canada (Blatter ,1987),andMcCall Glacier , Alaska,U .S.A.(Rabus andEchelmeyer ,1997).Arecent review 1.1.Study site (Hodgkins,1 997)suggests thatpenetration of surface-derived tothe beds ofsuch glaciersis limited. Thisis Thestudy was conducted at J ohnEvans Glacier ,a 165 km2 ¹ 337 Copland and others:Velocity variations and subglacial hydrology of apolythermal glacier

ablationarea where the velocitymeasurements weremade (Coplandand Sharp, 200 1).Forthe period1 997^99,the mean annualair temperature at820 m a.s.l.was ^1 5.2³C. Highbed reflection powers in radio-echo sounding records indicatewarm ice atthe bedthroughout most of the lowerablation zone, except along the glaciermargins andwhere the ice isthin(Copland and Sharp ,2001).Acon- tinuousinternal reflecting horizon over the centre ofthe lowerterminus suggests that the warmbasal ice reaches an averagethickness of20 m there. Inthe accumulationand upperablation areas, low bed reflection powers and 1 5m boreholetemperatures of^9.5 to ^1 5.1³C suggestthat the ice iscoldthroughout. Themelt seasontypically occurs betweenearly Juneand earlyAugust. At the start ofthe melt season,meltwater either pondson the glaciersurface orisrouteddirectly to the ice margins,and is unableto access the glacierinterior . Once englacialdrainage of supraglacially derived melt- waters is initiated,however ,approximately25% of the glaciersurface area drains into moulins in a field atthe topof the terminus (Fig.2).Later inthe melt season, Fig.1.Bed topography (ma.s.l.)andlocation ofJohn Evans meltwater fromup to 40% of the surface areaof the glacier Glacier. drainsto the glacierbed viathese moulinsand a secondcre- vasse field 6kmfurther upstream. Dye tracer experiments ¹ confirmthe linkbetween these waterinput locations and a predominantlycold polythermal glacier on the east majordrainage portal at the terminus (Binghamand coastof Ellesmere Island,N unavut,Canada ( 79³40 ’ N, others, 2003).Largeincreases inthe suspended-sediment 74³30’ W;Fig.1).Theglacier ranges in elevation from 1 00to content andelectrical conductivity(EC) ofthe wateras it 1500m a.s.l.,with the long-termequilibrium line at 750^ passes throughthe glaciersuggest that it is routedsub- ¹ 850m a.s.l.Ice depths reacha maximumof 400 m inthe glacially(Skidmore and Sharp, 1999). upperablation area, and average 1 00^250m inthe lower Inthe earlysummer ,the meltwater is trappedat the glacier

Fig.2.Landsat 7image of John Evans Glacier (path049 ,row 002,10 July1999 )showing the location of the velocity stakes (black dots),geophones (whitedots ;LG lower geophone,MG middle geophone,UG upper geophone),surveying base ˆ ˆ ˆ stations (whitesquares) ,stream gauging stations (triangles) and artesian fountain observed in1998 ( ).Crevasse field indi- cates location where most supraglacial meltwater reaches the glacier bed. Lower weather station is located next to the middle geophone.Black box indicates horizontal extent of Figures 4and7. 338 Copland and others:Velocity variations and subglacial hydrologyofapolythermal glacier

wasused toconvert the displacements tovelocities in cm d ^1 (24hours) .Velocitypatterns wereinterpolated over the entire terminus regionfrom the pointmeasurements atthe stakes.Velocities wereset to0 aroundthe glacieredge, which isreasonablegiven that the glacierappears to be frozento its bedat the margins(Copland and Sharp, 200 1).Allinterpo- lationswere completed with the ``v4’’interpolationroutine inMatlab, which is basedon biharmonicspline interpola- tion(Sandwell, 1 987).Interpretations ofvelocity patterns aremade only for areas where stakes arepresent. Forthe purposes ofour discussion, the lowerterminus isdefinedas the areasouth of the uppergeophone (UG inFig .2),andthe upperterminus is definedas the areato the northand northwest ofthis geophone.

2.2.Surface velocity errors Fig.3.Artesian fountain observed on the lower terminus From the Geodimeter technicalspecifications, uncertainties between days 180 and186,1998; see Figure 2for location. indistance measurements amountto (2 mm 3 ppm). § ‡ Thisequates to 5mm overa typicalsurvey distance of § bedbehind the frozenterminus (Skidmoreand Sharp, 1 999). 1km.Angle measurements weremade to a resolutionof 2 ’’, Theinitial release ofsubglaciallyrouted meltwater fromthe whichequates toa maximumpotential error of 4.8 mm § glaciertypically occurs viaan artesian fountain on the overa distanceof 1 km.Instrument drift duringsurveys glaciersurface (Fig.3)and/ orupwellingthrough sediments wascorrected forby resurveying the positionof reference afewmetres infrontof the snout.Since observations began markers after everyseven or fewer stake measurements and in1 994,dates ofinitiationof subglacialoutflow have varied assumingthat drift waslinear between surveys. In the fol- between22 J uneand 1 1July(days 1 73^192).Asoutflowcon- lowingdiscussion, three types oferror areevaluated: tinues, the upwellingmigrates towardsthe snout,and is (i) Position error ( cm):this isthe error indetermining the eventuallyreplaced by the formationof anice-walledchan- § locationof astakeduring a singlesurvey . nelat the glacierbed (the majordrainage portal referred to inthe previousparagraph) .Thefirst waterto be released (ii) Displacement error ( cm):this is the error indetermin- § hashigh total solute concentrations(EC 4 300 ms cm^1), is ingthe displacement ofastakebetween two surveys .It somewhatturbid and, relative to supraglacial runoff, con- iscalculatedby summing the positionerrors fromthe tains elevatedconcentrations of ionicspecies (Na +, K+ and twosurveys and/ orfrom two or more independent Si)that areproducts of silicate weathering(Skidmore and measurements ofthe displacement ofastake. Sharp,1 999).Within afewdays, the waterbecomes more (iii) Velocity error ( cm d^1):this isthe displacement error dilute,although solute concentrationsare still much higher § ^1 ^1 adjustedfor the time betweensurveys. As the measure- (4100 ms cm )thanfor supraglacial water ( 510 ms cm ). ment periodincreases, the velocityerror decreases as Theseobservations suggest that the first waterto be released the displacement error isdividedby a greatertime in- hasbeen stored fora longperiod (possibly over winter) , terval. whilelater waters havebeen transmitted more rapidly throughthe englacialand subglacial drainage system. In1 998,all stakes weresurveyed independently from eachof twostations located 100mapart.F oreachmeas- ¹ urement period,this providedtwo measures ofthe displace- 2.MEASUREMENTS ment ofeach stake, and the displacement error was calculatedfrom the difference betweenthese andthe mean 2.1.Surface velocity displacement ofthe stakefor that survey.Separatevelocity error calculationswere then madefor each stake for each Twenty-onevelocity stakes wereestablished overthe lower- 2daymeasurement period.These are displayed in the most 2kmofJohnEvans Glacier in1998.An additional1 3 velocityfigures discussed later.Forthe entire1998measure- stakes wereadded in 1 999to extend the network1 km ment period,mean errors forall stakes are 1.1cm d^1 in ^1 § further up-glacier(Fig .2).Theterminus regionwas chosen horizontalvelocity and 0.6 cm d inverticalvelocity . § forstudy as it iseasilyaccessible, there aresurrounding cliffs In1 999,the displacement ofeachstake was determined that allowsurvey stations tobe locatedon bedrock,and sub- oncefor each measurement periodfrom surveys at a single glacialwater flow appears to be present (Skidmoreand station.T oevaluateerrors, the locationof each stake was Sharp,1 999;Copland and Sharp, 200 1).Reflectingprisms measured atleast twice fromthe same stationduring each weremounted on 3mlongstakes drilledand subsequently surveyin J ulyand August. On agivenday ,the difference frozeninto the ice surface.Thesewere periodically redrilled betweenthe twoor more measurements ofa stakeand its orreplacedbefore surface meltingmade them unstable. meanlocation provides a positionerror .Thepositionerrors Theposition of each prism wasmeasured daily(weather fromthe surveysat the start andend of a measurement permitting) inthe summers of1 998and 1 999with a Geo- periodthen providethe displacement error,whichin turn dimeter 540total station theodolite. T oreduce errors, the providesthe velocityerror .Thevelocity errors werecalcu- datawere subsampled to determine stakedisplacements latedseparately for each stake for each 2 daymeasurement overperiods of 2daysor longer.Thetime betweensurveys period.F orsummer1999,the meanerrors forall stakes were 339 Copland and others:Velocity variations and subglacial hydrology of apolythermal glacier

0.6 cm d^1 inhorizontalvelocity and 0.2 cm d^1 in verti- tionand erosion around the sensor andthe needto move the § § calvelocity . sensor becauseof channelmigration. The sensors werenot Thefollowing discussions focusmainly on the 1999 calibratedto discharge, so each record was rendered dimen- measurements dueto the lowervelocity errors, improved sionless byscaling it betweenthe highestand lowest water spatialcoverage, and better records fromsupporting instru- levelsover the periodof interest. ments. Inthese discussions, the stated start andend times of velocity` `events’’areapproximate due to the 2dayresolution 2.6.Subglacial flow routing ofthe measurements. Thislikely leads to underestimation of the true magnitudeof velocitychanges since events willlikely Coplandand Sharp ( 2000)reconstructed subglacialflow spanmore thanone measurement period,or be shorter than routingat J ohnEvans Glacier fromthe formof the sub- ameasurement period. glacialhydraulic potential surface usingthe methodof Shreve( 1972).Reconstructions weremade for assumed basal 2.3.Geophones waterpressures at0^1 00%of ice overburdenpressure, but nosignificant differences werefound in predicted flowrout- In1 997,three geophoneswere installed at 5 mdepthalong the ing.This indicates that subglacialtopography ,rather than centre ofthe terminus: alowergeophone 0.3kmup-glacier ¹ variationsin ice thickness, providesthe dominantcontrol fromthe snout,a middlegeophone 1kmfromthe snout,and ¹ onbasalwaterflow at JohnEvans Glacier .Thedrainagepat- anupper geophone 2kmfrom the snout(Fig .2).The ¹ tern presented here isforbasal water pressures at1 00%of 4.5Hz geophoneswere interfaced to a CampbellScientific ice overburdenpressure. datalogger ,whichcounted the number ofseismic events abovea definedthreshold and output a totalevery hour (in 2.7.Residualvertical velocity (cavity opening) 1998)or2 hours(in 1 999).Geophonesensitivity wasadjusted toa levelthat provideda goodcompromise betweensensitiv- Todetermine the importanceof basalprocesses inaccount- ityand noise rejection in studies atT rapridgeGlacier , ingfor the measured verticalvelocities, the residualvertical Canada(Kavanaugh and Clarke, 200 1).Thegeophone velocity (wc)wascalculated for each survey .Thisis the ver- records didnot allow for accurate location of individual seis- ticalvelocity remaining after verticalstrain andthe hori- mic events becauseit wasnot possible to distinguish a weak zontalmovement along a slopingbed have been accounted localevent from a strongdistant one,although it waspossible for,andis commonlyattributed tobasal cavity formation todetermine generalpositioning by noting which geophones (Ikenand others, 1983;Hooke and others, 1989).Inreality , recordedactivity . it maybe causedby any unaccounted-for basal process, such Giventhe lackof volcanicactivity and of activefaults in asachangein the dilatancyof subglacialtill. wc was calcu- this area,it is assumed that most detected events hada latedfrom (Hooke and others, 1989): glacialorigin. Previous studies atother glaciers have shown w w u tan ­ "_ h ; 1 strongcorrelations between ice velocity,waterdischarge c ˆ s ¡ b ¡ zz … † andseismicity (Ikenand Bindschadler ,1986;Raymond where ws is measured Lagrangianvertical velocity at the andothers, 1995;Kavanaugh and Clarke, 200 1).Thisis surface, ub ishorizontalbasal velocity , ­ isbedslope, "_zz is becauseice fracturingcommonly occurs duringperiods of depth-averagedvertical strain rate, and h is ice thickness. rapidice velocitydue to changes in the internalstress distri- Forthese calculations,measured valuesof us (horizontal bution.In addition, basal sliding may produce seismic surface velocity)interpolated to a regular1 00m gridwere events asthe ice movesacross the glacierbed. used insteadof ub,since basalvelocity was not measured directly.Since u u ,ingeneral, this willproduce a min- b µ s 2.4.W eatherstations imum estimate ofthe rate ofcavityopening .Icethickness wasdetermined byradio-echo sounding (Copland and Threeautomatic weather stations wereinstalled on John Sharp,200 1),andinterpolated to the same 100mgrid.Bed EvansGlacier inMay 1996.Thelowerstation is locatednext slopewas calculated from the changein bedelevationacross tothe middlegeophone in the centre ofthe terminus at 200 ¹ the fourgridpoints to the north,east, southand west ofthe ma.s.l.(Fig .2).Hourlymeasurements ofair temperature and pointof interest. surface albedofrom this stationwere used toquantify InEquation ( 1), "_zz s (the measured verticalstrain rate at j changesin weather and ice surface conditions.Rates ofsur- the surface) wasinitially used asanapproximation to "_zz. By facelowering recorded by an ultrasonic depth gauge assumingincompressibility , "_zz s wasdetermined fromthe sur- (UDG) providean indicationof relativerates ofmeltwater j facehorizontal strain rates ( "_zz s "_xx s "_yy s),whichwere j ˆ ¡ j ¡ j production.These records havenot been corrected fordif- calculatedfrom the gradientsin surface horizontal velocity ferences insurface densitybetween ice andsnow ,soprovide betweenadjacent gridpoints. In reality , "_zz s isunlikelyto be onlya generalmeasure ofsurface melt rates. j the same as "_zz atdepth (Balise and Raymond, 1 985;Hooke andothers, 1989),soit isnecessaryto assess howvertical vari- 2.5.Discharge records ations in "_zz wouldaffect calculations of wc. If it is assumed that there isnocavityopening ( w 0), Toprovidean indicationof the meltwater inputs tothe sub- c ˆ u u glacialdrainage system in1 999,pressure sensors were and that 0 5 b 5 s,then fora givenice thickness it is pos- sible todetermine whetherand how "_ must varywith installedupstream ofthe maincrevasse fieldin an ice-mar- zz depth(Hooke and others, 1989;Mair andothers, 2002).For ginallake and at two locations in a supraglacialstream negativevalues of ­ (i.e. whenthe glacieris flowingdown- drainingfrom the lake(Fig .2).Tomonitorthe flowof water hill),three cases arepossible: fromthe glacier,apressure sensor wasinstalled in the main subglacialstream shortlyafter it exitedthe snout.This pro- (a) ws "_zz sh ws us tan ­ ; "_zz is notrequired … † µ … j † µ … ¡ † glacialrecord is approximatedue to frequent bedaggrada- tochange with depth relative to "_zz s j 340 Copland and others:Velocity variations and subglacial hydrologyofapolythermal glacier

Fig.4.(a)Winter 1999/2000 horizontal velocity contours. Each arrow indicates the direction and velocity of astake. Squares indicate stakesgrouped into factor1in principal com- Fig.5.Summer 1999 mean horizontal velocities (solidblack ponent analysis (PCA;see endofsection 3.1forexplanation); line),winter 1999/2000 mean horizontal velocities (dashed circles indicate stakesgrouped intofactor 2.(b)Winter 1999/ line),errors in horizontal velocity (vertical bars),and standard ^1 2000vertical velocity contours.Allvelocities in cm d . deviation in horizontal velocity (grey lines):(a)f or allstakes ; (b)f orstakesgrouped byfactor1in PCA(i.e .lower terminus); and (c)for stakesgrouped by factor 2in PCA(i.e.upper termi- (b) "_zz sh > ws us tan ­ ; "_zz is required todecrease nus).(d)F actorscores for the twofactors (i.e.the standardized … j † … ¡ † withdepth relative to "_zz s j PCAscores on the factorsover the measurement period). (c) "_zz sh < ws ; "_zz is required toincrease withdepth … j † … † relative to "_zz s. j 3. RESULTS Theseequations are reversed inthe more unusualcase when ­ ispositive(i.e. whenthe glacieris flowinguphill) . 3.1.Long-term velocities Basalvertical displacement is unlikelyto have occurred if the surface verticalvelocity can be explainedby a combin- Velocities werecalculated for winter 1998/99(day205, 1998 ationof basalsliding and vertical strain rates that areeither today 1 46,1 999)and 1 999/2000(day 2 14,1 999to day 1 61, constantor decreasing with depth (i.e. scenariosa andb 2000)to provide a measure ofthe ``background’’velocities above).Basalvertical displacement is most likelyto have againstwhich short-term variationscan be compared.Only occurredif the surfacevertical velocity is higherthan can the winter1 999/2000velocities are presented here, asthe beplausiblyexplained by an increase in "_zz withdepth rela- records covera largerarea and are very similar tothe tive to "_zz s (scenarioc above).Classificationof the glacier 1998/99records (Fig.4).Forthe stakes that weremeasured j intoareas where each of these cases holdsfor each velocity inboth winters, differences average0. 14cm d ^1 inhorizontal eventtherefore helps indefining areas where basal vertical velocityand 0. 57³in horizontal direction. displacement islikelyto have occurred. Hooke and others Themean horizontal velocity for all stakes was ^1 ^1 (1989)argued that avalueof "_zz sh that is 3 mm d less 3.5 cm d forwinter 1 999/2000,compared to a meanhori- j ¹ ^1 than ws (when ­ is negative)is likelyindicative of cavity zontalvelocity of 5.3cm d forsummer 1999(days 1 75.50^ openingunder ice 100mthick.T oensure that areasof 204.88,1999)(Fig .5).Horizontalvelocities for summer 1998 ¹ basalvertical displacement areconservatively identified at were also 50%higher than winter values, which suggests ¹ JohnEvans Glacier ,discussion is limited toareas where the that basalsliding is animportantprocess duringat least the inferred rate ofcavityopening is atleast 1cm d ^1. summer atJ ohnEvans Glacier .Thisseasonal increase in 341 Copland and others:Velocity variations and subglacial hydrology of apolythermal glacier

Table1.Eigenvalues andexplained variance for the principal component factors with an eigenvalue 41from aPCAof 2day horizontal velocities over days 164.71^211.98,1999

Factor Eigenvalue Explainedvariance %

1 17.13 50.4 2 4.60 13.5 3 2.43 7.1 4 2.10 6.2 5 1.71 5.0 6 1.46 4.3 7 1.32 3.9 8 1.07 3.2

formedon the 1998velocity data due to the relativelysmall number ofstakes inthat year.

3.2.1999 short-term velocity events

3.2.1.Event 1/99:days184.60^189.04 Thisevent encompasses twomeasurement periods,days 184.60^186.94and days 1 86.94^189.04,during which mean horizontalvelocity was almost 1 00%above winter levelsat 6.7 cm d^1 (Fig.5a).Therewas little changein either mean Fig.6.Summer 1999mean vertical velocities (solidblack line), horizontalor meanvertical velocity between these periods winter 1999/2000 mean vertical velocities (dashed line),errors (Figs5 and6) ,sothe meanvelocities from the twoperiods in vertical velocity (vertical bars),and standarddeviation in combinedare presented inFigure 7a^d. vertical velocity (grey lines):(a)for allstakes ;(b)f or stakes Therelative increases inhorizontal velocity during event 1/99werespatially non-uniform: the meanvelocity of stakes grouped by factor 1in PCA(i.e .lower terminus);and (c)f or inthe lowerterminus increasedby 1 10%relativeto winter stakesgrouped by factor 2in PCA(i .e.upper terminus). levels,while that ofstakes inthe upperterminus increased by7 5%(Fig .5band c) .Comparedto winter ,the central horizontalvelocity is notdistributed evenly,however,butis regionof highestvelocities broadened and there wasa con- concentratedduring 2^4 dayhigh-velocity events, which sequent narrowingof the marginalshear zones, particularly providethe focusof the discussion below.Vertical velocities overthe lowerterminus andclose to the snout(Figs 4a and arelow during the winter,reachinga maximumof approxi- 7aandb) .Thelargest absolute velocity anomalies of up to mately^0.5 cm d ^1 inan areaof steep surfaceslopes between 6 cm d^1 occurredover the upperterminus closeto where the upperand lower terminus (Fig.4b).Meanvertical wintervelocities were highest, and close to the predicted sub- velocitiesgenerally vary little betweenwinter and summer , glacialdrainage pathway there (Fig.7b).Over the centre of althoughsignificant variations do occur during some high- the lowerterminus, horizontalvelocities were an almostcon- horizontal-velocityevents (Fig.6) . stant 3 cm d^1 abovewinter levels (Fig .7b),andthere wasa Toassess the spatialscale of forcing mechanisms and markedrotation of the surfacevelocity vectors (relativeto howthese changeover time, principalcomponent analysis the wintervectors) towardsthe west (Fig.7a;cf. Fig .4a). (PCA)wasused todefine groups of stakes withcoherent Therotation of the velocityvectors increasedtowards the patterns ofhorizontal velocity variation (J ohnston,1 978). glaciersnout, where it reached 445³at some stakes. From analysisof all2 dayhorizontal velocity data over days Therewas little changein vertical velocity from winter 164.71^211.98,1999,PCA identifiedeight principal compon- levels,with most verticalvelocities close to 0 cm d ^1 (Figs 6 ent factorswith eigenvalues 41(Table1 ).When the 34 and7c) .Residualvertical velocity was also generally low , velocitystakes aregrouped based on their loadingson these withonly a limited areaover the lowerterminus where factors,two main clusters areidentified: factor 1 groups cavityopening is suggestedby the apparentneed for "_zz to stakes fromthe lowerterminus, whilefactor 2 groupsstakes increase withdepth relative to "_zz s (Fig.7d). fromthe upperterminus (Fig.4a) .Groupingusing more Event1/99occurredat the endjof the first weekof intense thanthe first twofactors subdivides these twoclusters into summer melting(Fig .8a).Asindicatedby the albedoand smaller groupsthat arenot as spatiallycontiguous. In addi- UDG records fromthe lowerweather station, surface melt- tion,the percentageof varianceexplained by the other fac- inghad been occurring for approximately 1 monthprior to tors is generallylow (T able1 ).Sincemost ofthe variancein event1 /99(Fig.8b andc) ,althoughmuch ofthis melt may horizontalvelocity is explainedby the first twofactors haverefrozen in the coldsnowpack. By the start ofthe event, (63.9%),andthe loadingson these factorscluster stakes into the albedohad fallen to 40% (Fig .8b),whichdefines the populationsthat actdifferently during high-velocity events approximateboundary between a snow-and ice-covered (see belowand Fig .5d),these groupingsare used indiscus- surface (Paterson,1 994,p. 59).Thesurface loweringrate sionof the 1999short-term velocitydata. PCA wasnot per- wascontinuously positive during the event(Fig .8d).The 342 Copland and others:Velocity variations and subglacial hydrologyofapolythermal glacier

Fig.7.Velocities in cmd ^1forevent1/99(days184.60^189.04,1999),pre-event 2/99(days193.92^196.60,1999),event 2/99(days196.60^ 198.88,1999)and event 1/98 (days 182.70^184.70,1998),respectively:(a,e ,i,m)horizontal;(b,f,j,n)horizontal anomalies (i.e.dif- ference from winter 1999/2000;black lines mark reconstructed subglacial drainage pathways);(c,g,k, o)vertical ;(d,h, l,p)residual vertical. Cavity opening is only likely where the assumption that wc 0 requires "_zz to increase with depth relative to "_zz s. ˆ j ice-marginallake drained rapidly during event 1 /99(Fig.9) western side (Fig.7j).Over the upperterminus, the largest (althoughit is notknown when this drainageevent started) , velocityincreases ofthe yearoccurred on the western side, andthe first majordischarges from the snoutwere observed withvelocity anomalies up to 1 8cm d ^1, or 400%, above ¹ duringthe overnightperiod between days 1 85and 1 86.The winter levels(Fig .7j).Theseanomalies were localized above largestgeophone activity of the summer wasrecorded at the the predicted areasof subglacialwater flow ,anddecreased middleand lower geophones on day1 86(Fig .8fandg) .A rapidlytowards the glaciermargins. In addition, there was further three smaller periodsof activity were recorded at astrongrotation in horizontal velocity vectors towardsthe eachof these geophonesbefore the endof event1 /99,while glaciermargin in this areacompared to both the winter and nounusual counts were recorded at the uppergeophone precedingmeasurement periods(Figs 4a and7e andi) .The duringthis time (Fig.8e). rotationwas greatest forstakes closest tothe glacieredge which,in this region,consists ofunsupportedice wallstens 3.2.2.Event 2/99:days 196.60^198.88 ofmetres high. Event2/ 99saw large horizontal velocity increases atmost Therewere rapid and dramatic changes in vertical stakes relativeto both the precedingmeasurement period velocityduring and prior to event 2/ 99,particularly over (days1 93.92^196.60)andwinter levels(Figs 4a, 5and7e, f, the upperterminus (Fig.6).Therewas strong vertical uplift iandj) .Thehorizontal velocity increases werehighly spa- atmost stakes duringthe event,compared to significant ver- tiallyvariable, as the increase invelocity of the stakes was ticallowering in the precedingmeasurement period(Figs 6 much higherover the upperthan over the lowerterminus and7g and k) .Forthe lower-terminus stakes, vertical (Fig.5bandc) .On alocalscale, the horizontalvelocity in- velocitiesaveraged ^1 .4cm d ^1 inthe precedingperiod and creases overthe lowerterminus werefocused on the eastern +1.0 cm d^1 duringevent 2 /99(Fig.6b).Forthe upper-termi- side, withvelocities only a little abovewinter levelson the nus stakes, meanvertical velocities were ^1 .1cm d ^1 in the 343 Copland and others:Velocity variations and subglacial hydrology of apolythermal glacier

Fig.9.Water-pressurerecords for summer 1999 (see Fig.2for sensor locations).The measurements are standardized between the highest and lowest values from each sensor over the period of interest.The proglacial sensor is repositioned during breaks.

precedingperiod and 3.5 cm d^1 duringthe event(Fig .6c). ‡ Aswithhorizontal velocity ,these patterns arehighly spa- tiallyvariable, with the largestchanges occurring over the western partof the upperterminus (Fig.7gand k) .At severalstakes inthis region,the changein rate ofvertical velocityrelative to the precedingperiod was 410 cm d^1. Cavityopening would have been possible over only a limited areain the periodprior to event 2/ 99(Fig.7h),but couldhave occurred over the entire lowerterminus andthe western partof the upperterminus duringthe event(Fig . 7l).Inparticular ,the residualvertical velocity of 415 cm d^1 alongthe axisof predicted subglacialdrainage overthe western upperterminus is much higherthan can be explainedby changes in vertical strain rate withdepth, and makesit likelythat cavityopening occurred in this area. Duringthe 2daysprior to event 2/ 99,a storm system reducedair temperatures tobelow freezing and produced newsnowfall (Fig .8a).Thealbedo sensor records the increase inalbedo due to the newsnowfall (Fig .8b),and the UDG records net accumulationduring this period (Fig.8candd) .Temperatures increasedrapidly on day1 97, andstayed well above freezing for the remainder ofthe event(Fig .8a).Surfacemelting resumed asthe weather warmed,and the highestsustained surface loweringrates ofthe summer occurredthroughout event 2/ 99(Fig.8d). Allthe water-levelsensors showeda clearresponse to these weatherchanges (Fig .9).Water levelswere low at all locationsduring the coolperiod prior to event 2 /99,anddiur- nalvariability disappeared due to the reductionin meltwater supply.Asairtemperature andsurface melt increasedat the onset ofevent 2 /99,waterlevels rose. Surfacelowering rates peakedon day 1 98.1,waterlevels in the ice-marginallake andsupraglacial streams peakedbetween days 1 98.7and 198.8,while the waterlevel in the proglacialstream peaked onday 1 99.1(Figs8d and9 ).Theupper geophone recorded the largestnumber ofevents ofthe summer onday 198during Fig.8.(a^d)Summer 1999 lower-weather-station records: event2 /99(Fig.8e).Smallernum bers ofevents, withcounts (a)air temperature;(b)albedo ;(c)surface height measured wellbelow those duringevent 1 /99,werealso recorded at the from the start of the melt season; and (d)surf ace melt rate. middleand lower geophones (Fig .8fandg) . Positive values indicate ,negative values indicate accu- mulation. (e^g)Summer 1999 geophone records:(e)upper 3.3.1998 short-term velocity event geophone;(f)middle geophone;and (g)lower geophone.Note that onlythe relative magnitude ofevents iscomparablebetween 3.3.1.Event 1/98:days 180.71^184.71 geophones,not the actual number of counts. Onlyone clear event was recorded in summer 1998(Fig .10). 344 Copland and others:Velocity variations and subglacial hydrologyofapolythermal glacier

Fig.10.Summer 1998 horizontal (a)and vertical (b)mean velocity (solid black line),winter 1999/2000 mean velocity (dashed line),errors in velocity (vertical bars),and standard deviation in velocity (grey lines).Sixmore stakes were added to the original network of15 from day185 onwards.

Thisevent covers two measurement periodsfrom days 180.71^182.71and1 82.71^184.71,withthe largestand most significanthorizontal velocity increases duringthe second halfof the event(Fig .10a).Consequently,onlythe velocity patterns fromthis secondperiod are plotted (Fig .7m^p). Themean horizontal velocity for the 15stakes present duringevent 1 /98was 5 .0cm d ^1 duringthe first halfof the event,and 8.2 cm d ^1 duringthe secondhalf (Fig. 10a).This comparesto a meanwinter velocity for these stakes of 3.3 cm d^1.Thehigh standard deviation in horizontal velocity( 3.6cmd ^1)duringevent 1 /98implies either that the increases werenot spatially uniform, or that there is largespatial variability in measurement errors (Fig.10a). Thehighest velocity increases (upto 1 4cm d ^1) occurred overthe eastern partof the lowerterminus (Fig.7n).This areaof high horizontal velocities is definedby at least four stakes, andthe velocityincreases arewell above error esti- mates. Asinevent 1 /99,there wasa rotationin the velocity vectors towardsthe west comparedto winter patterns, and this wasgreatest closeto the glaciersnout (Figs 4a and7m) . Vertical velocitiesvaried little andwere close to winter valuesduring event 1 /98(Figs7o and1 0b).Of the 15stakes measured duringthe event,most hadvertical velocities of 0 Fig.11.Same as Figure 8,but for summer 1998. to ^1cm d^1.Theresidual vertical velocity averaged 0 cm d^1,andthere werefew areas where cavity opening ¹ mayhave occurred (Fig .7p). Event1 /98coincidedwith the initiationof subglacial out- Aswithevent 1 /99,event 1 /98occurredduring a period flowfrom the terminus. Thisoccurred primarily via an ofrapidmelting approximately 30 days after the start ofthe artesianfountain on the lowereastern terminus onday1 80 melt season.Air temperatures werehigher than at any pre- (Fig.3),whichreached its peakheight during days 1 82^184, vioustime in1 998,and remained well above freezing beforedeclining around day 1 86.The fountain reached throughout(Fig .11a).Approximately95 cm ofsurface low- heightsof 5 m, andbrought large volumes of relativelyturbid, eringoccurred in the monthprior to event 1 /98asthe sur- high-ECwater to the glaciersurface through an ice thickness facesnow cover melted (Fig.11c),andmelt rates reached of 70m (measured byradio-echo sounding; Copland and ¹ their highestsustained levelsof the summer duringthe Sharp,2001).Thissuggests asubglacialorigin for the water, event(Fig .11d). andindicates that basal water pressures were 120% of ice ¹ 345 Copland and others:Velocity variations and subglacial hydrology of apolythermal glacier overburdenpressure overat least partof the lowerterminus. flowand the artesianfountain (Fig .7n;see Fig.2forlocation Longitudinalfracturing of the ice surfacewas observed ofartesian fountain) .Thissuggests thatthe hydrologicalfor - aroundthe artesianfountain, with most fracturingoccurring cingresponsible for enhanced basal velocity may have been whenthe fountainfirst emerged.Thisfracturing was recorded strongest inthe vicinityof major su bglacialdrainage axes. bya small number ofevents atthe middlegeophone around day1 80.25,andby a largenumber ofevents atthe lowergeo- 4.2.Mid-season event phonearound day 1 80.46(Fig .11fandg) .Nosignificantactiv- itywas recorded at the uppergeophone at this time (Fig.11e). Event2 /99occurred after aprolongedcold period during whichthere wassnowfall, surface melt ceased(Figs 8a, b andd) ,andwater levels dropped in all monitored streams 4.DISCUSSION (Fig.9).Itislikelythat subglacialwater pressures droppedat From these measurements, it is evidentthat short-term this time asmeltwater inputs tothe glacierinterior decreased. velocityevents areassociated with periods of rapidly increas- Dependingon the subglacialdrainage configuration, this ingmeltwater inputs tothe subglacialdrainage system. wouldhave allowed closure of basal cavities or subglacial Similarrelationships have also been observed during short- channelsby ice and/orbasal sediment deformation,and/ or term high-velocityevents ontemperate andpredominantly compactionof the glacierbed due to a reductionin the poros- warmpolythermal glaciers (e.g. Iken and Bindschadler , ityof basal sediment. Allof these processes wouldresult in 1986;J ansson,1 995;Mair andothers, 2001)andon the net surfacelowering, as was observed in areas close to the Greenlandice sheet (Zwallyand others, 2002).AtJ ohn subglacialdrainage pathways in the periodprior to event 2 / EvansGlacier ,the first velocityevent of the melt season 99(Fig .7g).Thehighest rates oflowering during this period typicallyoccurs about1 monthafter the onset ofmelt, when occurredin the northwestpart of the upperterminus where the connectionbetween the supraglacialand subglacial the ice thicknesses aregreatest (200^250m) (Coplandand drainagesystems isfirst established andsubglacial outflow Sharp,2001).Theinferred reductionin basal water pressure begins(events 1/98and1 /99).Asubsequent eventoccurred wouldhave also increased basal drag ,accountingfor the later inthe melt season,when melt resumed after apro- relativelylow horizontal velocities at this time (Fig.7f). longedcold spell (event 2/99). Melt resumed asthe weatherwarmed at the start ofevent 2/99(Fig .8aand d) ,witha rapidrise inwater levels at all 4.1.Early-season events monitoringlocations (Fig .9).Atthis time, waterlevels in the supraglacialstreams reachedtheir highestlevels since event1/ Risingwater inputs duringearly-season events aredue to a 99,indicating that alargemeltwater pulse entered the sub- combinationof warm weather ,highrates ofsurface melt, glacialdrainage system viathe crevasse fieldat the topof andthe drainageof supraglacial and ice-marginal lakes with- the terminus.Event2 /99was characterized by extremely inand at the upstream ends ofthe supraglacialchannel sys- highhorizontal and vertical velocities in the areaimmedi- tems (Figs8a andd, 9 and1 1aandd) .Outflowof su bglacially atelyabove the predicted locationof the subglacialdrainage routedwaters atthe glacierterminus typicallybegins within axisbeneath the western partof the upperterminus (Fig. 24hours of surfacewaters startingto drain into the glacier 7i^k).Thelarge number ofcounts registered atthe upper viathe crevasse fieldat the topof the terminus. Thisis prob- geophoneat this time (Fig.8e),togetherwith the highPCA ablybecause the newwater input pressurizes the existingsub- factor2 scores (Fig.5d),supports the factthat event2 /99was glacialreservoir beneaththe lowerterminus, drivingoutlet centred overthe upperterminus. developmentat the snout.Evidence for pressures aboveice Thevery pronounced velocity response inthe northwest overburdenlevels is providedby the formationof the artesian regionof the terminus atthis time suggests that basalwater fountain,while the highEC of the first waters tobe released is pressures mayhave risen tovery high levels in this areawhen indicativeof long subglacial residence times. Fracturingasso- meltwaters werereintroduced to the bedfollowing the cold ciatedwith outlet developmentlikely contributes tothe high period.The inferred rates ofcavity opening in this areasug- countsrecorded at the lowergeophone during events 1/99and gest that extensiveice^bed decoupling may have resulted 1/98(Figs 8g and1 1g). (Fig.7l).Thisbehaviour was likely a response tothe contrac- Highhorizontal velocities at this time arepresumably tionof drainagechannels beneath the thickice inthis area linkedto high basal water pressures (andpossibly increasing duringthe precedingcold period, which impeded meltwater subglacialwater storage) ,whichreduce basalfriction and drainageacross this section ofthe glacierbed. The strong enhancebasal velocity .ThePCA scores forthe factor1stakes southwardrotation of the velocityvectors inthis area(Fig . reachedtheir highestvalues during event 1/99(Fig.5d),asthe 7i;cf. Fig .7e)implies that normalglacier flow towards the largesthorizontal velocity changes of the yearoccurred over east-northeast wasimpeded at this time. Thiscan be the lowerterminus (Fig.7aand b) .Thissuggests thatthe explainedby a downflowincrease inbasal drag arising from frozensnout provided the greatest hydraulicresistance toout- adownstreamcollapse or reductionin size ofthe subglacial flowof subglacialwaters atthis stageof the melt season.The drainagechannels. increasinglylarge westward rotation of the horizontal Over the lowerterminus, horizontaland vertical velocitytowards the snoutcompared to winter patterns (Figs velocitiesduring event 2 /99were lower than in the western 4aand 7a and m) suggests thatthe frozenmargin also partof the upperterminus, butthey reached peak values impededthe flowof the warm-basedice upstream. This abovethe eastern drainageaxis (Fig .7jand k) .Inferred resulted incompression and left-lateral shearingin the region rates ofbasalcavity opening were also highest in this area, ofthe warm^coldtransition at the glacierbed. Within the althoughcavity opening appears to have occurred across warm-basedregion, there wasa closeassociation between the wholelower terminus region(Fig .7l).Thisis somewhat the areaof highest horizontal velocity anomalies during event problematicgiven the argumentabove that the large 1/98and the locationsof both the predicted subglacialwater velocityresponse inthe upperterminus regionis attribut- 346 Copland and others:Velocity variations and subglacial hydrologyofapolythermal glacier ableto closure ofmajor drainage channels downstream From these observationsit appearsthat there is both fromthis region.Such closure, if complete, wouldpresum- interannualand intra-annual variability in the pathwayby ablyhave prevented transfer ofsubglacialwater to the lower whichmeltwater whichpenetrates tothe glacierbed drains terminus. Thereare two possible resolutions to this: (i) tothe glacierterminus. Itappearsthat waterdraining sub- incompleteclosure allowedsome basalwater flow to the glaciallyfrom the upperterminus can,at different times, lowerterminus, butstill causedbacking up and high water connectto either orboth of the eastern andwestern drain- pressure belowthe upperterminus; (ii) the reopeningof ageaxes beneath the lowerterminus. Thelikely location of channelsled to a delayedwater input to the lowerterminus, the connectionbetween the twoaxes is apparentfrom the causingthe velocityevent there tolag the onein the upper formof the contoursin Figure7j. terminus. Unfortunately,the resolutionof the velocity measurements is toocoarse to show whether the velocity events inthe upperand lowerterminus regionswere exactly 5.CONCLUSIONS synchronous. Measurements over2 yearsdemonstrate that,in summer ,sur- facemeltwater penetrates significantthicknesses ( 4200 m) of 4.3.Comparisons between events coldice toreach the glacierbed and affect the horizontaland verticalsurface velocities of J ohnEvans Glacier .Horizontal Acomparisonof the horizontaland vertical velocities asso- surfacevelocities are higher in summer thanin winter across ciatedwith events 1/98and1 /99revealssome interesting dif- the entire glacierterminus. High-velocityevents lasting ferences. In1 998,the largesthorizontal velocity anomalies 2^4days occur during periods of rapidly increasing meltwater wereassociated with the eastern subglacialdrainage axis inputto the glacierinterior .Horizontalvelocities during these (Fig.7n),whilein 1 999they were less pronouncedand more events reachup to 400% of winter velocities. widelydistributed (Fig.7b).Vertical velocitiesduring events Early-summer velocityevents arelinked to the initiation 1/98and1 /99weregenerally low (Fig .7cando) ,withcavity ofsupraglacial water inputs tothe glacierbed. These initial openingrestricted toa small areaon the western side ofthe inputs includethe drainageof significant volumes of water lowerterminus duringevent 1 /99(Fig.7dandp) .Thesmal- previouslystored insupraglacial and ice-marginal lakes. A ler magnitudeof the event1 /99horizontalvelocity anoma- mid-summer eventoccurred during a periodwhen melt rates lies suggests that basalwater pressures didnot rise ashighas increasedrapidly following cold weather and new snowfall. inevent 1 /98,which is consistent withthe observationthat Allevents areprobably a result ofhigh, and rapidly rising, noartesianfountain formed on the glaciersurface in1999. subglacialwater pressures and/orwater storage. Thehorizon- One possibleexplanation is that largesubglacial channels taland vertical velocity anomalies that result differsignifi- formedduring the exceptionallywarm summer of1998did cantlybetween events. Thisis probablya result ofseasonal notclose completely during winter 1 998/99,andthat they andinterannual changes in the routingof meltwater across allowedmore efficient drainageof the initialwater inputs the bedand in the natureof the subglacialdrainage system. tothe glacierbed in early summer 1999.Thiscould explain Early-seasonevents areinitiated within a predominantlydis- the limited areaof cavity opening during event 1 /99, tributed subglacialdrainage system, whilethe mid-summer althoughthe generallysmall verticalvelocity response eventappears to have been initiated within a channelized duringthe early-seasonevents makesit likelythat sub- subglacialdrainage system that hadundergone significant glacialdrainage was mainly distributed atthis time. closureimmediately prior to the eventduring a coldperiod Theform of the horizontalvelocity anomalies over the withlimited meltwater drainageto the bed. lowerterminus duringevent 2/ 99provides some evidence Theresults ofthis studyindicate a strongcoupling forthe reorganizationof subglacial drainage (Fig .7j).The betweenthe surface velocityof apredominantlycold poly- anomaliessuggest that the hydrologicalforcing for this thermal glacier,the magnitudeand distribution of surface eventwas associated with the eastern drainageaxis, and waterinputs tothe englacial/subglacialdrainage system, thus different fromthe more widespreadforcing that seems andthe distribution,formation and seasonal evolution of tohave been associated with event 1 /99(Fig .7b).Asdis- subglacialdrainage pathways. The surface velocityof the cussed above,event 1 /98also appears to have been forced glacieris extremely sensitive torapid increases inthe deliv- fromthe eastern drainageaxis (Fig .7n),althoughthere are eryof surface waters tothe glacierbed. This behaviour is significantdifferences inthe ice velocityresponses associated qualitativelycomparable to that seen onboth temperate withthe twoevents inthe lowerterminus region.Horizontal andwarm polythermal glaciers (Iken and Bindschadler , velocityanomalies were higher and more widespreadin event 1986;J ansson,1 995;Kavanaugh and Clarke, 200 1;Mair 1/98(Fig .7jandn) ,whereascavity opening was more wide- andothers, 2001)andon the Greenlandice sheet (Zwally spreadduring event 2 /99,witha peakalong the eastern andothers, 2002).Anumber ofcharacteristics that maybe drainageaxis (Fig .7land p) .Thesedifferences mayreflect typicalof predominantly cold polythermal glaciers influ- differences inthe amountsof water entering the glacierand ence the details ofthe behaviourobserved. These include inthe characterof the subglacialdrainage system between the thermal barrier tothe flowof ice andbasal water at the the twoevents. Event1/98wasassociated with the first input snoutand margins of the glacier,the suddenonset ofsub- ofsurfacewater to the glacierbed during summer 1998,and glacialdrainage in the earlysummer ,andthe importance occurredwhen much of the glacierupstream fromthe cre- ofwaterinputs fromice-marginal and supraglacial lakes. vassefield was still snow-covered.Event 2 /99,bycontrast, occurredwhen glacier ice wasexposed over much ofthis area.Given that airtemperatures reachedsimilar high ACKNOWLEDGEMENTS valuesduring both events (Figs8a and1 1a),melt production inthe areadraining into the crevasse fieldwas likely higher Fundingwas provided by the NaturalSciences andEngin- duringevent 2 /99. eeringResearch Councilof Canada, the U.K. Natural 347 Copland and others:Velocity variations and subglacial hydrology of apolythermal glacier

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MSreceived 21 August 2001and accepted inrevised form 31 March 2003

348