Firn-Ice Transition-Zone Features of Four Polythermal Glaciers in Svalbard Seen by Ground-Penetrating Radar

Annals of Glaciology 37 2003 # InternationalGlaciological Society

Firn^ice transition-zone featuresof four polythermal glaciers in Svalbard seen by ground-penetrating radar

Anja PA« LLI,1, 2 John C. MOORE,1 Cecilie ROLSTAD3 1Arctic Centre,University of Lapland,Box 122,FIN-96101Rovaniemi, Finland E-mail: [email protected] 2Department of Geophysics,Box 3000,University of Oulu,FIN-900 14 Oulu,Finland 3Norwegian Polar Institute,Polar Environmental Centre,N-9296 TromsÖ, Norway

ABSTRACT.Detailed ground-penetratingradar (GPR) surveyson the firn^icetrans- itionzones of Kongsvegen,N ordenskjo « ldbreen,Hansbreen and W erenskioldbreen,Sval- bard,are presented. Wediscuss features seen inGPR radargramsin allpolythermal glaciersdown-glacier of the firnline. The firn line of a polythermalglacier in GPR datais easilydetected withboth 50 and 200 MHz radar.Bycomparison with satellite images,characteristic patterns ofreflection are shown to be causedby superimposed ice,but the reflec- tionsfrom the interfacebetween the superimposed ice andthe underlyingglacier ice areof unknownorigin. Layered reflections withinthe superimposed ice andfirn area are likely fromlayers of ice lenses andrelatively low-density firn in the firnarea, and from bubble-rich andbubble-poor ice inthe superimposed-ice zone.Dipping foliations in the glacierice down-glacierof the firnline show that the layeredreflections fromthe firnline are pre- served,though in much weaker form, even when the firnhas been transformed tosolid ice.

INTRODUCTION surface velocities.During the course ofseveral field seasons onSvalbard,we have mapped many glaciers using ground- On polythermalglaciers, where superimposed ice repre- penetratingradar (GPR) ,includingseveral larger ones with sents asignificantcomponent of glacier mass balance,the muchmore typicalflow regimes andlarger firn areas than firn-line (i.e. snow-line)position and the equilibriumline those studied byBjo « rnsson andothers (1996).Wehavenoticed arenot the same. Thefirn line, unlike the equilibriumline, thatthe firnline and the structures associatedwith it have isthe physicallyrecognizable boundary between firn and manycommon features, whilealso varying quite consider- ice onthe glaciersurface at the endof the melt season ablybetween the glaciers. (Paterson,1994).On polythermalglaciers the firnline marks Our objectivesare to interpret features ofthe radargrams the transitionzone to down-glacier cold ice atthe surface. interms ofphysical structures associatedwith the firnîce Thetransition from water-permeable firn to impermeable transitionand to postulate reasons for their differences coldice evolvesas the snowpackin the upperfirn part of betweenthe glaciers.Wepresent detailedGPR datafrom the the glacierbecomes wet withmeltwater inthe summer firnîcetransition zone of four medium andlarge poly- (e.g. Mu« ller,1962).Thewater that percolatesdown and partlyrefreezes inthe deeper layersraises the temperature ofthe firnto the meltingpoint. However ,down-glacierof the firnline the ice becomes impermeable,preventing pene- trationof meltwater exceptvia crevasses ormoulins.Theice isthen cooledbelow the meltingpoint during the winter. Thistransitionis therefore complex,starting inthe low-per- colation(soaked) zone and finishing in the superimposed- ice zone(Paterson, 1 994,p. 10).Thetransition starts with the formationof ice lenses andice layersin firn that contains much meltwater insummer .Someof this waterrefreezes, oftenafter percolatingalong pipes tocolder depths, and eventuallyspreads horizontallyto form ice lenses. Radaris anexcellent tool for studying the firnîce transitionof a glacierbecause of the largedifference in dielectric permittivity betweenwater ,firnand ice. Bjo « rnsson andothers (1996)notedthe verysteep contactline between the coldand temperate ice withinfour polythermal glaciers inSvalbard. Only two of the glaciersstudied, Kongsvegen andU ve©rsbreen, werelarge enough to have a significantfirn Fig.1.Map of Svalbard. Locations of Kongsvegen, Norden- areatypical of Svalbard glaciers, and all have very low skjo« ldbreen,Hansbreen andWerenskioldbreen. 298 Downloaded from https://www.cambridge.org/core. 28 Sep 2021 at 20:35:24, subject to the Cambridge Core terms of use. Pa«lliand others:Firnîce transition zones of polythermal glaciers

Table 1.Some physical properties of Konsvegen, Norden- Table 2.Measurement parameters for each GPRsurvey skjo«ldbreen, Hansbreen andWereskioldbreen

Glacier Survey Antenna Numberof Time Stackingon Glacier year frequency samples/ window collection KongsvegenN orden- HansbreenW eren- traces skjo«ldbreen skioldbreen MHz ms

2 Area (km ) 102 242 57 27.4 Kongsvegen1 99650 2048 5.240 No ELA (m a.s.l.) 520*660* 320^370 470 199750, 200 2048, 1024 * 5.570,0.299 * No Surfaceslope 0.5^2.5³1.6^1 .9³1.8³ 2.2³ Nordenskjo« ld- 1999 50 2048 5.110 No Bed slope 1.2³ ^ 1.3³ 4.0³ breen Surfacevelocity at 2.31^3.59y 150 30 9.5^11 * * ^1 Hansbreen1 99750, 200 2048,1 024 5.304,0 .692 2 thefirn line(m a ) 199850, 200 2048,1 024 * 5.090,0.989 * No Typicalaccumulation 0.7 0.5 1 ^ * * ^1 Werenskiold- 199850, 200 2048,1 024 3.069,1.201 No rate (m a ) breen Last surge 1948 ^ ^ ^

Orientation southeast^ northeast^ northwest^ east^west * northwest west south Measuredwith 200MHz antennas. Length (km) 25.8 26.5 16 9.5 Width (km) 2^5 4^6 3^4 2^3 Icethickness at the 275 300 320 225 centre lineclose to the equilibriumline on N ordenskjo « ldbreen ¹ firn line(m) suggesthigh ice fluxes(Isaksson and others, 2001). Warmice slope at the 34³ 11³ 10³ 15³ firn linefrom down- Hansbreen glacierdirection Hansbreenis atidewaterglacier situated inthe Hornsund Note:ELA, equilibrium-line altitude. fjordin southern Spitsbergen (Fig .1).Theglacier extends *Pinglotand others ( 1999). yMelvoldand Hagen ( 1998). fromsea level to 600 m a.s.l.Themaximum thickness isabout 400m andthe ablationarea has a 20^90m thickcold-ice layer.Mass-balancestudies havebeen made since 1989.Mean thermal glaciers:K ongsvegen,N ordenskjo « ldbreen,Hansb- winteraccumulation in 2000 was 0. 93m w.e.and mean sum- reen andWerenskioldbreen(Fig .1),eachof which can be con- mer balance^1 .14mw.e.,so the totalnet balancein 2000 was sidered typicalof glaciers in Svalbard, although they range ^0.21mw.e.(personal communication from J .Janiaand P . widelyin their physicalproperties (Table1 ).Manyof the Glowacki,2002) .Hansbreenhas a measured speed of radarfeatures arepreviously unreported, but seem tobe pres- 30 m a^1 atthe equilibriumline. The glacier accelerates sig- ent invarying degrees inall polythermal glaciers. nificantlytowards the terminal ice cliff,exceeding 2 10ma ^1 nearthe terminus (Janiaand others, 1996). STUDY AREA Werenskioldbreen Kongsvegen Werenskioldbreenis aland-basedvalley glacier next to Kongsvegenis alargetidewater glacier situated inthe inner Hansbreenbut flowing from east towest (Fig.1).Itisdivided partof K ongsfjorden(Fig .1).Theglacier has a shallowsurface bya massive moraineridge into W erenskioldbreento the slopeand it extendsfrom sea level to 800 m a.s.l.K ongsvegen southand Skilryggbreen to the north.Theglacier is situated hasa largefirn zone and volume of temperate ice inits higher at0^600 ma.s.l.The glacier is veryshallow; the thickest basin.In the ablationarea the coldice layeris 50^160m thick parts are1 00^140monthe central southernside andin the (Bjo« rnsson andothers, 1996)andthe glacieris frozento the uppernorthern parts ofthe glacier.Thesnout is 550 m bedalong the mountainsides(LiestÖ l, 1 988;Bam ber,1989; thickand the glacieris frozento bedrock for 700 m^1 km Bjo« rnsson andothers, 1996).Mass-balancemeasurements upstream fromthe terminus. Thecold-ice layer thickness werestarted in1 987.Themean annual net balancewith an variesfrom 50 to 1 00m. Boththe sides andsome areasin ^1 estimated calvingrate of0. 05m w.e.a is slightlypositive, the upperparts, wherethe glacieris 550m thick,have cold +0.11mw.e.(Lefauconnier and others, 1999).Theice-flow ice downto the bed.Mass balancewas measured forthe ^1 velocity is 54 m a alongthe entire basin,and the surplus 1993/94and 1 998/99glaciological years, and both net bal- inaccumulation is nottransported to the ablationarea. The anceswere negative: ^0. 36and ^0 .66mw.e.,respectively glacierseems tobe building up towards a surge(M elvoldand (personalcommunication from P .Glowacki,2002) . Hagen,1998;Lefauconnier and others, 1999).

Nordenskjo«ldbreen EQUIPMENT ANDMETHODS

Nordenskjo« ldbreenis oneof the majordrainage glaciers of Allsurveys were done in the springseasons (April and May) Lomonosovfonna,one of the highestice fields onSpitsbergen, between1 996and 2000, when the wholeglacier was covered withthe summit at1 250m a.s.l.The maximum depth is esti- bya layerof winter snow ,andin air temperatures around mated tobe about 500 m (personalcommunication from V . ^10³C, with no evidence of any near-surface water .Weused Pohjola,2002 ).Thereare no mass-balance data available. aRamacGPR (MalÔGeoscience) with50 and 200 MHz Theaccumulation rate variesalong the longitudinalprofile antennas.The profile lines weredriven by snowmobile with ofN ordenskjo « ldbreenfrom 0 .38at the summit to0 .52^ positionfixed by global positioning system receivers.The 0.78m w.e at1 044and 1 173m a.s.l.(Pinglot and others, 1999; radarantennas were mounted on a non-metallicsledge Pa« lliand others, 2002).Ice-velocitymeasurements atthe pulled7 mbehindthe snowmobile.Post-processing used the 299 Downloaded from https://www.cambridge.org/core. 28 Sep 2021 at 20:35:24, subject to the Cambridge Core terms of use. Pa«lliand others:Firn^ice transition zones of polythermal glaciers

Fig.2.50MHz radar profile from Werenskioldbreen. Dipping foliations are seen clearly in the cold ice,and astrong hyperbolic diffraction indicating alikely water body.Much radar clutter is seen in the temperate ice underneath the cold-ice layer.The region ofsuperimposed ice,estimated by the presence of weakersub-parallel layering and sporadic hyperbolic diffractions down-glacierof the firn line,is delimited by the white line and, where itishard to determine,by the dashed line.The numberof firn layers indicates that the oldest firn is 10^15years old.

Haescanprogram (R oadscannersOy) .More detailedsurvey andothers 1993;Pa « lli,1 998)weremeasured usingthe com- informationis presented inTable2 . mon-depth-pointtechnique. W eobservelarge differences in radarvelocity due, as expected, to the different density Time^depthscale profilesin the ice inthe regiondown-glacier of the firnline, ascompared with the areaup-glacier ( 173and 2 10m ms^1), Thepropagation velocities of the radarwaves for snow and andlarge variations over short lateraldistances dueto vari- ^1 firn,cold ice andtemperate ice onHansbreen (Macheret ationsin water content of the glacierice (146^160m ms ; Mooreand others, 1999).Thisvariability precludes the use of simple time migration,which requires aspecific permittivity valueand that hyperbolicdiffraction patterns cannotbe easilyinterpreted. Therefore,in the figures,we present data interms oftwo-wayradar-wave travel times rather thanreal depth;differences inglacier topography are best assessed usingTable1,orestimated usingthe velocitiesquoted.

COMMON FEATURES ANDDIFFERENCES INTHE FIRN^ICE TRANSITION

Thefirn line in GPR profilesappears similar everywhere.As onegoes up-glacier towards the firnline, near -surface layer- ingstarts increasingin thickness. Atthe firnline, strong reflections fromalternating firn/ ice layersdip downwards up-glacier,the cold-icelayer disappears and the wholethickness ofice becomes temperate. However,the lengthof the firn^icetransition zone down-glacier from the firnline varies from300^400 m onHansbreen, Werenskioldbreen(Fig .2)and Fig.3.50 MHz radar profile from Nordenskjo«ldbreen.The firn Nordenskjo« ldbreen(Fig .3)toabout 1 .5km on K ongsvegen line is hard to locate,as there seems to be awet area extending (Fig.4).Thecomplex zone below the firnline is wheremost of down-glacier to about 1350 mof the intersection of the upward- the differences betweenthe glaciersare seen, asresponses to dipping layers,which would intersect the surface around 1410m superimposed-iceformation,ablation and runoff and changes distance. infirn-line position all contribute features tothe radargrams. 300 Downloaded from https://www.cambridge.org/core. 28 Sep 2021 at 20:35:24, subject to the Cambridge Core terms of use. Pa«lliand others:Firn^ice transition zones of polythermal glaciers

Fig.4.The firn^ice transition in 50MHz GPRdata of Kongsvegen.The firn line fromthe GPRdata and the satellite image (labelled as``sat- firn line’’;see Fig.5)ismarked,along with the location ofstake 7.The layering in the firn area is easilyseen.We interpret the hyperbolic diffractions on top of warm ice and just before the line as water bodies.Superimposed ice marked by sporadic reflections around 200nsandvariable-strength sub-parallel reflections are delimited by the white line.

Dip angle fromfirn and relatively bubble-free ice formedby freezing of water-saturatedlayers. Thesteepness ofdipangle of the firn^icetransition within the glacierappears to vary between the glaciers(T able1 ).In Superimposedice asteadystate the anglemust represent abalancebetween the advectionrate oftemperate ice andthe freezingrate of Superimposedice forms whenmeltwater refreezes ontoa waterin the ice.Thismust bedetermined bythe glacierflow sub-freezingglacier surface. I tcanbe distinguished from rate andthe verticalvelocity at the firnline. Thus we may glacierice byits different air-bubblecontent (Ko « nig and expectslow-moving glaciers with low downward vertical others, 2002).Superimposedice hasbeen thought to be diffi- velocityat the firnline to have very steep anglesof dip(cf. cult toobserve with GPR, becauselayers and ice lenses Kongsvegen),whilefast-flowing glaciers with rapid accu- merge intoa continuousmass andwould show neither as mulationat the firnline may have much shallowerangles pointreflectors noras clearlayering in the data.However , (cf. Nordenskjoldbreen).Dataon verticalvelocity at the firn high-resolution500 MHz radardata (personal communi- lineare in general lacking on most glaciers,and in any case cationfrom J .Kohler,2002)show an easilydistinguishable the factthat many(if notall) of these glaciersare surge-type interface betweensuperimposed andenglacial ice on probablyprecludes asteadystate. Kongsvegenthat variesbetween the winter snowbase and Foliations 412mindepth. In our 50 MHz profileson Kongsvegen, sporadichyperbolas and places of enhancedscattering were Down-glacierof the firnline, foliations are often seen seen atdepths ofabout 5^20 m (ortravel times ofup to throughoutthe englacialice. However,crevasses orvery wet 200ns) ,perhapsevery 50^1 00minan irregular area temperate ice candestroy the foliation(Moore and others, down-glacierof the firnline. This region contrasts withthe 1999).On Hansbreen(M ooreand others, 1999)andW eren- engalcialice closeto the terminus ofthe glacier,andthe skioldbreen,foliations (Fig .2)areseen veryclearly because characteristic layeringseen inthe firnarea. ofafairlythick cold-ice layer or low velocity (K ongsvegen; InFigure 5 wemap the three different zonesobserved by Fig.4).On Nordenskjo « ldbreen(Fig .3)the highvelocity of the radar,togetherwith a compositesatellite imageof a the glaciermay partly destroy foliations, which is whythey SPOTinfrared (IR) August1996;Landsat ThematicMapper arenot as numerous,as continuousnor as strongas onHans- (TM) IR August1 993;and European R emote-sensing Satel- breen.Therather weakreflections appearto be causedby the lite-1(ERS-1)synthetic aperture radar(SAR) September densitycontrast betweenlayers of bubble-rich ice formed 1995.Thetime iscloseto the endof the meltingseason for all 301 Downloaded from https://www.cambridge.org/core. 28 Sep 2021 at 20:35:24, subject to the Cambridge Core terms of use. Pa«lliand others:Firn^ice transition zones of polythermal glaciers

Fig.5.Kongsvegen grey-toned RGB colour composite:red: SPOT96; green:TM93;blue:ERS-195.The 50MHz radar profile runs up-glacier,along the centre line fromstake1to 9(about18 km),and across the firn^ice transition in several places.Numbers along the centre line refer toradar scan numbers used in Figure 4.The radar images were interpreted into three classes of ice.The sections shown in black show cold ice only covered by winter snow.The sections which showed largely englacial cold ice,but with features that appear to identifysuperimposed ice,are shown in grey.Sections marked with adashed black line are clearly temperate firn with strong,near-surface layering.The possible superimposed-ice-zone (seen in the SARimage asmedium backscatter) boundaries are marked with thin white solid line.

images.TheSPOT image is froma yearwith moderate melt- inhighly concentrated solution (about 2M ;Lide,1993,p.15^ ing,andthe Landsatimage from a yearwith close to max- 16)consistent withice temperatures of^3 to ^5³ C (tempera- imum melt. Variationsin backscatter in the ERS-1SAR tures seen inHansbreen; J aniaand others, 1996).Itis well imageare due to topography ,andreflectance ofdifferent knownthat duringthe summer melt period,much (perhaps ice types (e.g.Ko « nigand others, 2002).Thezone of sporadic hyperbolicreflections inthe 50MHz profilesfits closelyto the regionthat is betweenthe maximumand minimum ele- vationsof the snowline, the upperlimit beingthe firnline, withthe lowerlimit definedby the regionthat melts downto bareenglacial ice everyyear .Webelievethat this region marksthe superimposed-ice zoneon the glacier,andis rather similar tothe zonationof the SARimagethat Ko « nig andothers (2002)use toidentify superimposed ice.Theclose fit betweenthe GPR zonesand the satellite imageryshows that GPR canbe used toidentify superimposed ice. Acom- parisonbetween the SARimage of Ko « nigand others (2002) andour images of the firnline between stakes 6and7 (Fig. 5)givesa firn-line locationat about scan 4600^4625 in Fig- ures 4and5 .Thereis adifference ofabout1 00^200m. We areunable to identify any equilibrium line in both our satellite imageand that ofKo « nigand others (2002),asthere isno apparentdifference betweensuperimposed ice that isbeing accumulatedand superimposed ice that isbeingablated. Theenhanced GPR scattering features onKongsvegen Fig.6.Hyperbolic reflections (inboxes A,B,C)at 50^100 ns appearto come from depths consistent withthe baseof the (5^10mdepth) 200mdown-glacier of the firn line in ¹ superimposed zone.Wespeculatethat they are caused by 200MHz data of Hansbreen, possibly indicating super- dielectric contrasts dueto enhanced water in the ice: each imposed ice (cf.Figs 2^4).The down-dipping layers in the 1%increase inwater content decreases the radarvelocity by boxes could be due to enhanced water content in the ice,and about2. 5%and also creates alargereflection coefficient the hyperbolic scattering indicates water contents of 1.2%, (Macheret andGlazovsky ,2000).Liquidwater could occur 3.3% and 3.6% for boxes A,Band C,respectively. 302 Downloaded from https://www.cambridge.org/core. 28 Sep 2021 at 20:35:24, subject to the Cambridge Core terms of use. Pa«lliand others:Firn^ice transition zones of polythermal glaciers

90%)of the solubleimpurity in the wintersnow on low- the firnline. They are observed in the top1 0^20m offirn elevationSvalbard glaciers is flushedout (J aniaand others, andincrease inthickness up-glacier.Thesereflecting hori- 1996)andmust travelthrough the superimposed-ice zone. zonsare likely from layers of ice lenses andrelatively low-den- Wealsoobserved sporadic hyperbolas from Hansbreen sity firn.A tthe firnline, reflections fromthe cold-icelayer with200 MHz antennasdown-glacier of the firnline (Fig . disappearand the wholethickness ofice becomes temperate. 6),whichpossibly indicates superimposed ice.Thesefeatures Similarstructures wereobserved in the firnîcetrans- aresimilar tothe superimposed-ice features onKongsvegen itionarea, though with different emphasis oneachglacier . at50 MHz (Fig.4),inthat theyappear to show layering in Foliationswere observed in all the glaciers,but they were the upperparts andthen ahyperbolicdiffraction near the most clearlyseen onHansbreen and W erenskioldbreen, bottomof eachfeature. The down-dipping layers can be ex- whichare relatively slowly flowing and have thick cold-ice plainedby a lowradar-wave velocity region caused by high- layers.Superimposed ice wasclearly detected onKongs- er liquid-watercontent inthe ice, andthe hyperbolacould vegenand Hansbreen. The limited resolutionand the spor- come fromthe more saturatedbase of the wet patch.Fitting adicnature of the reflections means that the overallextent of curves tothe hyperbolasin the boxesof Figure6 giveswater superimposed ice is difficultto measure accurately,though contents of1.2^3.6%inthe superimposed ice withthe low- there is encouragingsimilarity between GPR andsatellite velocity` `pull-down’’layers.The velocities calculated from imageryfor superimposed ice onKongsvegen.The nature the hyperbolasare potentially in error ifthe radarsurvey ofthe reflections inthe superimposed-ice zone,which seem didnot pass verticallyover the source ofthe diffractions. tocome mainly from the interface ofthe superimposed ice Analternative hypothesis is that the features inFigure 6 andthe underlyingglacier ice, is uncertain,but most easily areburied crevasses (Arconeand Delaney ,2000);however, explainableby increased water content ofafewper cent at hyperbolicdiffractions would show a velocityfor firn if they sporadiclocations within the superimposed ice. camefrom the crevasse walls,rather thanthe lowvelocity indicated.Crevasses havenot been observed on this partof ACKNOWLEDGEMENTS glacier,butare seen further down-glacier,wherethey pro- ducereflections withweaker hyperbolas that start nearer Financialsupport cam efromthe FinnishAcadem y,the Fin- the surface,with no ` `pull-down’’layersabove them. nishGradua te Schoolof Snow and I ce andthe ThuleInstitute supportingthe radardevelopment .Wethankthe Institute of Waterbodies Geophysics(P AS)and the PolishPolar Station in Hornsund andthe NorwegianPolar Institute forlogistic support. Our TheGPR datafrom K ongsvegen(Fig .4),Hansbreen(M oore specialgratitude is expressed toJ.Jania,P .Glowacki,J .Kohler, andothers, 1999)andW erenskioldbreen(Fig .2)show that N. Lo« nnruth,K .Michalskiand A. Sinisalo for assistance in these glaciershave occasional hyperbolic diffractions seen the field.The WihuriPhysics Laboratory ,Universityof T urku, onlyin the cold-iceglacier ice orontop of the temperate ice. Finland,kindly allowed the first authorto do her workat their Itseems plausiblethat onlywater bodies could cause such a office.Excellentguidance on the manuscript wasgiven by two largedielectric contrast asto be easily seen withradar . anonymousreviewers. Metre-scale dimensionsare indicated by the point-source reflectionat 50 MHz, andwould remain liquid within ice at ^3³C formany years, even if the waterwere not connected to REFERENCES the glacierhydraulic system. Macheret andGlazovsky ( 2000) Arcone,S. A. andA. 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