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Home , Amery Ice Shelf, Iceberg, Ice shelf

Annals of 34 2002 # InternationalGlaciological Society

Icebergcalving fromthe Amery Shelf, East

Helen A. Fricker,1 Neal W. Young,2 Ian Allison,2 Richard Coleman3, 4 1Institute of Geophysics andPlanetary Physics,Scripps Institution of ,University of California SanDiego , LaJolla,CA92093-0225 ,U.S.A. 2Antarctic CRCand Australian Division, Hobart,Tasmania 7001, 3Antarctic CRCand School of Geography andEnvironmental Studies,University ofTasmania, Box 252-80,Hobart,Tasmania 7001,Australia 4CSIROMarine Research, Box 1538,Hobart,Tasmania 7001,Australia

ABSTRACT.Weinvestigatethe -calvingcycle of the AmeryIce Shelf(AIS) , East Antarctica,using evidence acquired between 1 936and 2000. The most recent major iceberg-calvingevent occurred between late 1 963and early 1 964,when a largeberg total- lingabout 1 0000km 2 inarea broke from the ice front.Therate offorward advance of the ice frontis presently 1300^1400m a ^1.Atthis rate ofadvance,based on the present ice- frontposition from recent RADARSATimagery,it wouldtake 20^25 years to attain the 1963(pre-calve) position,suggesting that the AIScalving cycle has a periodof approxi- mately60^70 years. T wolongitudinal (parallel-to-flow) rifts, approximately25 km apart atthe AISfront, are observed in imageryacquired over the last 14+years.These rifts haveformed at suture zonesin the , whereneighbouring flow-bands have separatedin association with transverse spreading.The rifts were 15km(rift A)and 26km (rift B)inlength in September 2000,and will probably become the sides ofalarge tabulariceberg ( 25km 625km) .Atransverse (perpendicular-to-flow)fracture, visibleat the upstream endof rift Ain1 996,had propagated 6 kmtowardsrift BbySeptember 2000;when it meets rift Bthe icebergwill calve. A satellite imageacquired in 1962shows anembaymentof this size inthe AISfront, hence we deduce that this calvingpattern also occurredduring the last calvingcycle, and therefore that the calvingbehaviour of the AIS apparentlyfollows a regularpattern.

1.INTR ODUCTION whichaccounts for 1 .6 6106 km2 ofthe groundedEAIS ( 16% ofits totalarea) .Themass fluxfrom the interior ofthe system, measured alonga traverse approximatelyfollowing the Most ofthe mass loss fromthe Antarcticice sheet takesplace 2500m contourline, is 44Gt a ^1 (Fricker andothers, 2000b). inthe ice shelves andglacier tongues, via iceberg calving from Thisflux, plus additional ice that accumulatesdownstream their fronts orbasal melting from below (J acobsand others, ofthe traverse route,flows towards the coastin a convergent 1992).Largeiceberg-calving events generategreat interest pattern that is focusedthrough the frontof the AIS,which amongstnot only scientists butalso the generalpu blic, accountsfor only 2% ( 200km) ofthe East Antarcticcoast- ¹ becausean increase inthese types ofevents couldbe an indi- line.In this paper,weinvestigate the AISin terms ofpotential catorof .I ceberg-calvingevents areepisodic in future calving,toassess the likelihoodof large bergs calving natureand may produce withsize rangingfrom a few fromthis ice shelf inthe nearfuture. Wealsodiscuss features, hundredmetres upto many tens ofkilometres (Youngand observedin recent RADARSATsynthetic aperture others,1998).Thetime intervalbetween events forany given (SAR) imageryand a EuropeanR emote-sensing Satellite partof the ice marginmay range from one to a fewyears and (ERS-1/-2) tandem-mission SARinterferogram, that are pre- upto many decades. In order to determine whethercalving cursors toan iceberg calving event. rates arechanging, it isnecessaryto first establish the normal calvingrate .Monitoringof iceberg-calving events fromAnt- ice shelves usingsatellite imageryhas becomecommon- 2.AIS ICEBERGS place(e.g .Ferrignoand Gould, 1 987).Theice shelves ofthe West Antarcticice sheet (WAIS)have been subjects ofrecent 2.1.Last major calving event scrutiny since theyhave produced several giant icebergs over the last 2years(Lazzara and others, 1999).Icebergs ofsignifi- Thelast majorcalving event from the AISwas in late 1 963 cantsize that calvedfrom the East Antarcticice sheet (EAIS) orearly1 964,when a massive tabulariceberg about 1 0000 overthe same time intervalcame from the Ninnis km2 inarea calved (Budd, 1 966).Theberg split intotwo tonguein the 1999/2000austral summer (personalcommuni- smaller bergs ayearlater ,whichwere carried westwards cationfrom R .A.M assom,200 1),althoughthese icebergs are fromPrydz Bay in the East Wind Drift (EWD),closeto the smaller thantheir WAIScounterparts. coastof Antarctica.In 1 967one of the bergs (denominated TheAmery I ce Shelf(AIS) is the largestice shelf inEast 1967B,with dimensions 1 10km 675km) collidedwith the Antarctica.I tdrainsthe groundedportion of the Lambert Trolltungaice tonguein the FimbulIce Shelfat 1³ W ,and Glacier^AmeryI ce Shelfsystem (Lambert^Amerysystem) , this collisioninitiated the calvingof the Trolltungaiceberg 241 Downloaded from https://www.cambridge.org/core. 02 Oct 2021 at 21:11:02, subject to the Cambridge Core terms of use. Fricker and others:Icebergcalving from the

Fig.2.DISPimage of the AISfront acquired inMay 1962. The image is severely affected by cloud,but the ice front can be discerned.

Recentlydeclassified Defense IntelligenceSatellite Program(D ISP)satellite photographsshow the AISfrontin Fig.1.AISfr ont positions for 10 epochs between 1936 and May1962,about18monthsbeforethe calvingevent. Although 2000.These locations were obtained using different methods, these imagesare severely affected by thick cloud cover ,it is asoutlined in the legend. Animated version can be seen at possibleto discern the coastlineof M ac.R obertsonLand and http://rai.ucsd.edu/ helen/Annals_2001/Fig1_ANIM.gif. ¹ the frontof the AISin the images(Fig .2).Atthat time, the ice shelf protrudedfar into ,muchfurther thanits pres- (named1 967A),whichwas 1 04km 653km in dimension ent-dayextent. Thereis clearlyan embayment in the ice front, (Swithinbankand others, 1977).Thisis anexample of the suggestingthat a smaller iceberghad calved from this collisiontheory of icebergcalving which was postulated by locationprior to the DISPphotograph being taken. N oice- Swithinbankand others (1977).Arecent exampleof this bergsare visible in Prydz Bay ,soit is notpossible to estimate typeof eventoccurred on the Ross Ice Shelfin September whenthis smaller bergcalved. However ,there is alsoan 2000,when iceberg B 15-A(US NationalIce Center nomen- embayment present inboth the 1955and 1 959locations (Fig .1), clature) dislodgedC 16fromthe ice front.Attenuation of indicatingtha tthe smaller calvingevent took place at least radio-echosounding (RES) echoesobserved over one of 8yearsprior to the majorcalve .Differences inthe shapeof the Amery-derivedicebergs in1 969indicated that there the embaymentin these yearspossibly arise fromerrors wasmarine ice atthe baseof the berg(Swithinbank and associatedwith the surveymethods used: shipbornesurvey others 1977;see section 2.2). usinga sextantin 1 955and airborne survey via dead-reckon- Between1 936and 1 968,the positionof the AISfront was ingin 1 959.From the air,the presence orabsence of fast ice recordedby various survey methods: airborne,shipborne and insidethe embaymentcould have led to interpretation errors. terrestrial (Robertson,1 992).Sincethe early1 970sthe front hasalso been captured in Landsat M ultispectral Scanner 2.2.Green icebergs (MSS) andThematic Mapper (TM) imageryand, more recently,inRAD ARSATSARimagery .Figure1 showsice- TheAIS has been cited asasource oficebergs whichhave an frontpositions surveyed at six epochs between 1 936and 1 968, emeraldgreen appearance (Kipfstuhl andothers, 1992),the in1 974(LandsatMSS) and1 988(Landsat TM) andin 1 997 colourbeing due to the presence of` `marine’’ice inthe ice- and2000 in RAD ARSATSARimagery .Thedifference berg.Marine ice is depositedas aresult ofthe ``ice-pump’’ betweenthe 1963and 1 965fronts showsthe approximatearea (Lewis andPerkin,1 986)mechanism thatoperates in the cavi- ofthe icebergthat calvedin 1 963^64( 140kmtransverse by ties beneaththe ice shelves.Theice pumpis drivenby changes ¹ 70km longitudinal),approximately1 4%ofthe current area inmelting and freeze rates associatedwith the dependenceon ¹ ofthe AIS( 69000 km 2).Thesequence after 1965shows how pressure (depth) ofthe melting-pointtemperature. Itresults the ice shelf propagatesforward after the calving.Asthe front inmelting near the (deep) groundingline and freezing onto advances,it bulgesoutwards in the centre asaresult ofhigher the ice-shelf baseat shallower depths further downstream, velocitiesthere (see section 3.2).(Thewhole sequence from whichredistributes ice underthe ice shelf (Jacobsand others, 1936to 2000 can beviewed as an animation at: http:/ /rai.ucsd. 1992;Jenkinsand Bom bosch,1 995),forminga marine-ice edu/ helen/Annals_2001/Fig1_ANIM.gif.)Zwallyand others layerbeneath the ice shelf. Basalmelt rates underthe AIS ¹ (2002)estimate ameanannual rate offorward advance of arehigh near the groundingline, on the orderof tens of the AISfrontof 1 .03 0.04 kma^1 forthe period1978^95from metres per annum(F ricker andothers, inpress).Thethick- § satellite radaraltimetry . ness ofthe marine-ice layercan be estimated usinga buoy- 242 Downloaded from https://www.cambridge.org/core. 02 Oct 2021 at 21:11:02, subject to the Cambridge Core terms of use. Fricker and others:Icebergcalving from the Amery Ice Shelf

the marine-ice basallayer is melted offtowards the ice front bya warmsub-shelf current (Warrenand others, 1993);there is nosuch current underthe AIS.Therefore, when icebergs calvefrom the western frontof the AIS,they still contain marineice. These icebergs breakinto smaller bergs,and sub- sequent meltingmakes these bergs unstable,causing them to capsizeand reveal their marineice abovethe oceansurface. Thegreen appearance of the marineice derivesfrom the greatabundance of organicphytoplankton blooms in Prydz Bay:organic particles becometrapped in the marineice (Warrenand others,1 993).Theice itself preferentiallyabsorbs visiblesolar radiation at red wavelengths(causing ice to appearblue) ,andthe organicparticles absorbradiation at bluewavelengths so that the colourof marine ice is shifted fromblue to green. I cebergs fromthe AISdrift westwardsin the EWDwhichsurrounds Antarctica, and it hasbeen claimedthat theydrift asfar as the WeddellSea (Swithinbank andothers, 1977;Kipfstuhl andothers, 1992).Green icebergs areoften observed west ofthe AISnear Mawson station.

3.CURRENT STATUSOFAIS

3.1.RAD ARSATAntarcticMapping Missions

SARdata acquired during the first AntarcticM appingMis- sion(AMM -1)ofthe CanadianSpace Agency RAD ARSAT satellite (inSep tember andO ctober1997)wereused tocompile acomplete mosaicof the Antarcticcontinent (J ezek,1999).The secondmission, the ModifiedAntarctic Mapping Mission (MAMM),tookplace between September andN ovember 2000.Figure 3 containsa mosaicof the Lambert^Amery system compiledfrom MAMM byI .Joughinat the Jet Pro- pulsionLaboratory (JPL) ,Pasadena,CA, andindicates the approximatelocation of the system’snewlydefined grounding line(F rickerand others, inpress) .Themain tributary thatflow into the confluenceregion near the groundingline are(from east towest) Lambert, Mellorand Fisher Glaciers. CharybdisGlacier joinsthe western partof the ice shelf down- stream ofBeaver . The boundaries of four of the seven majorflow-bands ha vebeen overlaid on the mosaic,in a man- ner similar tothat of Ham breyand Dowdeswell ( 1994). Atthe frontof the shelf, twolarge longitudinal-to-flow (north^south)rifts (26and 1 5kmlong) are present (wecall Fig.3.RADARSATmosaic of the Lambert^Amery system these rift Aandrift B,respectively) .Theserifts extendfrom compiled by I.Joughin.RAD ARSATdata were collected the ice-shelf surfaceto its base,and are filled with ice me ¨ lange during MAMMand are copyright to the Canadian Space (Rignotand MacA yeal,1 988)consisting of seaice, ice-shelf Agency.White dotted and dashed lines indicate the approximate fragmentsand wind-blown .Thesefeatures most likely extents of Figures 5and 6,respectively. developedover the 1980s,and a Landsatimage from F ebru- ary1 988shows that theywere 1 6km(rift A)and1 1km(rift B)longat that time. Tracingthe flow-bandsback upstream ancyrelation if surface height and ice-thickness information showsthat the rifts originateat the boundariesof major fromRES areavailable. This technique works because the flow-bands:the westernmost (rift A)betweenthe Charybdis RES signaldoes not penetrate the marineice, soan anomaly flow-bandand the bandcontaining ice thatentered the AIS exists inthe difference betweenthe theoreticalice thickness across its western marginhaving passed through the Prince derivedfrom the heightdata and the measured thickness. CharlesM ountains;the easternmost (rift B)betweenthe Fricker andothers (2001)didthis calculationfor the AIS Fisher andM ellorflow-bands. andfound that the marineice is confinedto the northwestern Thislocation of the longitudinalrifts atthe AISfrontsug- quadrant,and is orientedalong two longitudinal bands, gests that this typeof rift formationis relatedto flow-band eachup to 1 90m thick. historyand source, and is aresult ofstress patterns activelong Thepattern ofmarine ice underneaththe AISis signifi- beforeand far upstream ,thatare preserved downstreamas far cantlydifferent tothat beneath the Ross andR onneI ce asthe ice front.The mechanical properties ofice inthe suture Shelves,and similar tothat ofthe FilchnerIce Shelf(FIS; zone,which has in general been su bject tomore deformation, Grosfeld andothers, 1998),inthat it persists allthe wayto aredifferent tothose ofice inbetween sutures, andrepresent a the calvingfront. U nderthe Ross andR onneI ce Shelves, planeof weaknesswithin the ice shelf. Theflow-bands from 243 Downloaded from https://www.cambridge.org/core. 02 Oct 2021 at 21:11:02, subject to the Cambridge Core terms of use. Fricker and others:Icebergcalving from the Amery Ice Shelf

Fig.5.Magnitude of ice velocity overcentral part of AISfront, derived from maximum coherence tracking inapair of SAR images acquired by RADARSATon 28 September and 18 October 1997 (fromYoung and Hyland,2002).

rifts formlater (Lazzaraand others, 1999).Theeast^west Fig.4.RADARSATimages over AISfr ont collected during (transverse-to-flow) rift associatedwith B- 15first appeared AMM-1(1997)andMAMM (2000).Animated version of inthe late1 980s,and grew to over 280 km lengthbetween this plot isonline at http://rai.ucsd.edu/ helen/Annals_2001/ 1992and 1 996.The B- 15calvingevent in March 2000 ¹ Fig4_ANIM.gif. occurredwhen a short north^south(longitudinal-to-flow) rift, whichwas not visible in satellite imageryuntil after 1996,intersected the transverse rift (personalcommuni- the different inputs merge toform the AIS,whichis confined cationfrom D .R. MacAyeal,200 1).However,wedo not formost ofits lengthsuch thatthe ice flowis roughlyparallel. believethat this is the pattern duringlarge calving events However,whenthe AISfrontprotrudes past acriticallimit ofthe AIS,such asthat whichoccurred in 1 963^64. intoPrydz Bay ,the hightransverse strain rates inthe ice On the eastern side ofthe images,a third longrift (ofa causethe streams toseparate from each other ,forminglongi- ``zigzag’’shape)is seen.Theshape of this rift occurs asa result tudinalrifts .Theserifts areprecursors toiceberg-calving ofthe intersecting pattern ofcrevasses present onthe east side events: it appearsthat it isthis``loose-tooth’’section ofice shelf ofthe shelf nearthe front(region C) .Thesecrevasses, around betweenthese tworifts that calvedaway before the main 40^50km long ,formdownstream of Gillock Island, which calvingevent of 1 963^64,leaving the embaymentseen in disturbs the ice flow.Thesecrevasses willultimately lead to Figure2 .Thebreak-up of tabular icebergs canalso occur the formationof atransverse rift that willbecome the calving alongsutures, asoccurred when B- 15split intoB- 15Aand frontfor the nextma jorcalving event. B-15Balongthe suture formedby I ce Stream Denteringthe (Lazzara and others, 1999).Itis likelythat 3.2.Ice-front velocities whenthe gianticeberg that calvedfrom the AISin1 963^64 split ayearafter calving,it wasalong one of the suture zones Ice-frontvelocitieshave been derived for the AISfrontfrom causingthe longitudinalrifts underdiscussion. maximum coherencetracking in SAR imagery ,usingRADAR- RADARSATimagesfrom AMM -1(1997)andMAMM SATimagesfrom AMM -1inSeptember andOctober 1997 (2000)over the AISfront are shown in Figure 4. In the top (Youngand Hyland,2002; Fig .5).TheRAD ARSAT-derived image,a three-wayfissure is seen atthe tip ofrift A,anda datashow that the velocitiesover the loosetooth in the ice short transverse-to-flowfracture (east^west) isseen. Inthe frontare greater than in the surroundingregion, and that bottomimage the three-wayfissure hasopened up in the the western partof the loosetooth moves around 80 m a ^1 3yearinterval between these images,and the transverse faster thanthe ice immediatelyto the west ofrift A.This is fracture isnowwell developed. Theopening of the rift is seen verymuch greater than the differencethat exists purelyas a more clearlyin an animation of these imagesat: http:/ / result ofthe lateralvelocity gradient across the front: rai.ucsd.edu/ helen/Annals_2001/Fig4_ANIM.gif.The trans- velocitiesderived from feature tracking in 1 988Landsat ¹ verse fracture willprobably become a future calvingline, imagery(prior to formation of the transverse fracture) show andthe longitudinalrifts willform the icebergsides once that the velocitydifference across rift Awasonly 1 0ma ^1. the transverse fracture meets rift B.This is different tocalv- BetweenAMM -1andMAMM the centre ofthe front ingon the Filchner^Ronneand R oss Ice Shelves,where the advancedabout 4 km( 3years),the velocitythere presently transverse fracture istypicallypresent first andlongitudinal being1300^1400m a ^1. 244 Downloaded from https://www.cambridge.org/core. 02 Oct 2021 at 21:11:02, subject to the Cambridge Core terms of use. Fricker and others:Icebergcalving from the Amery Ice Shelf

componentof the phasechange was removed using the synthesized phasesignal from a digitalelevation model for the ice shelf derivedfrom ERS- 1satellite radaraltimetry (Frickerand others, 2000a).Theinterferogram contains rela- tivedisplacement informationarising from tidal motion and ice flow.Thedifference in the predicted tidalheights at the ice frontfrom the Circum-Antarctic DataAssimilation model (Padmanand others, 2002)forthese twopasses is only 2cm andis uniformacross the interferogramregion (per- ¹ sonalcommunication from L. Padman,200 1).Therefore,the observedphase change in Figure 6 isprimarilydue to ice flow. Thereare discontinuities in the phaseacross eachof rifts A andB, and across the transverse (perpendicular-to-flow) fracture just beginningto form at the tip ofrift A.Thefringes across the ice betweenthe rifts areorientated differently to thatoutside of the rifts. Wesuggestthat this is becausethe loosetooth moves in a different mannerto the rest ofthe ice shelf, asaresult ofthe spreadingof the fracture atits south- west corner.Theloose tooth is apparentlyrotating clockwise inits embayment,and is hingedat the northeastor southeast corner.Thefringes are further aparton the east side ofrift A, indicatingthat it hasexperienced more relativemotion than the ice onthe west side, whichis confirmedby the higher velocitiesthere (Fig.5).Thisis becausethe ice onthe east side ofrift Ais undergoingadditional motion due to the spreading rate ofthe transverse fracture. Fringesare also observed on the ice me¨ langewithin the rifts, suggestingthat this ice is undergoingdeformation as aresult ofthe relativemotion of Fig.6.Interferogram derived from ERS-1orbit 24526 (24 the shelf ice oneachside ofthe rift. Thiseffect hasalso been March 1996)and ERS-2 04309 (25 March 1996)of observedwithin rifts onFIS by Rignot and MacA yeal( 1998). the eastern part of theAISfr ont.The topographic contribution Theseauthors proposed that me ¨ langemay have sufficient to the phase has been removed using topography provided by a mechanicalstructure that it playsa rolein holding the two digital elevation model derived from ERS-1satellite radar sides ofthe rift together,whichcan delay iceberg detachment. altimetry (Fricker and others,2000a). Thereis athird longitudinalrift, beginningto form just east ofcentre inthe loosetooth. The flowlines in Figure 3 indicatethat this rift isassociatedwith the suture formedat 3.3.SAR interferometry the western boundaryof the Fisher Glacier flow-band.On the eastern side ofthe interferogram,discontinuities inthe Wegenerateda SARinterferogram over the AISfront to phasecan be clearly seen across the (regionC) examinethe ice motionnear the loosetooth. SAR interfer- discussed insection 3.1. ometry(InSAR) is awell-establishedtechnique that has recently revolutionizedour knowledge of many Antarctic ice-shelf andglacier systems (e.g.Rignot and others, 1997; 5.WHENWILLTHEAISNEXT CALVE? Joughinand others, 1999).Thetheory on whichInSAR is basedhas been discussed indetail elsewhere. Interfero- Itisinteresting toconsider when the nextmajor calving event gramsare formed between pairs of images that have been willlikely take place on the AIS.Surface velocity measure- collectedover the same areafrom almost the same satellite ments madein situ in1 995using global positioning system positions,but atdifferent times. Over ice shelves, the phase comparedwell with those madein 1 968using a theodolite changeobserved between the imagepairs is composedof a andelectronic distancemeter (EDM) traverse alongthe relativetopographic component (resulting froma parallax middleof the AIS,suggesting that the ice shelf wasin near- effect becausethe satellite locationis notexactly the same steady-state flowover this 27yearperiod in the mid-shelf foreach pass) anda relativedisplacement component region(Phillips, 1999).Wetherefore assume thatthe velocities becauseeach radar scatterer hasmoved in the intervening atthe ice-shelf frontare also in steady state, andcombine the time betweensatellite passes (dueto both tidal displace- locationof the ice frontin 2000 with the ice velocitiesshown ment andice-shelf creep flow).TheERS- 1and-2 inFigure 6 toestimate whenthe ice shelf willreach the last operatedin a ``tandemmission’ ’,inwhich ERS-2 followed pre-calve( 1963)position,taking into account the ice-shelf ac- ERS-1inits orbit,but 24hours apart, between 2 1March celerationas it movesforward into Prydz Bay and thins. The 1995and 3 June1 996. AISfrontstill hasnot yet advanced half of the requireddis- Figure6 showsan interferogram generated from ERS- 1 tance ( 70km) ,andwe conclude that the AISwill not experi- ¹ orbit24526 and ERS-2 orbit04853 ,repeats alonga descend- ence amajorcalve until the mid-2020sor later .However,from ingtrack which passed over the centralAIS front on 24 and the evidencepresented here it appearsthat the portionof the 25March 1996,respectively.Aphasechange of 2 º represents ice shelf betweenthe longitudinalrifts willcalve around arelativedisplacement of2.8cm inthe radarlook direction 10yearsor more beforethat, perhaps around 20 10^15. (from rightto left inthis image).Therelative topographic TheAI Sis astablesystem that is currentlyundergoing 245 Downloaded from https://www.cambridge.org/core. 02 Oct 2021 at 21:11:02, subject to the Cambridge Core terms of use. Fricker and others:Icebergcalving from the Amery Ice Shelf

changesthat are part of its naturaladvance^calve^advan ce Landsatimagery . Ann.Glaciol. , 20, 401^406. cycle.I tisreasonableto suggest that this cyclemight be repeat- Jacobs,S. S., H.H. Hellmer ,C. S. M. Doake,A. Jenkinsand R. M. Frolich. 1992.Melting of ice shelves and the mass balanceof Antarctica. J. Glaciol., ablewith a periodof 65^70 years. This is incontrast withthe 38(130),375^387. periodgiven in Budd ( 1966)of40 years. Jenkins,A. andA. Bombosch.1995.Modelingthe effects of crystals on thedynamics and thermodynamics of ice shelf plumes. J. Geophys. Res.,100(C4),6967^6981. ACKNOWLEDGEMENTS Jezek,K .C. 1999.Glaciologicalproperties of the Antarctic from RADARSAT-1syntheticaperture radar imagery . Ann.Glaciol. , 29,286^290. Wewouldlike to thank I. J oughin(JPL) andthe RADAR- Joughin, I. and 7 others.1999.T ributaries ofW estAntarctic ice streams SATAntarcticMapping Program team forproviding the revealedby RADARSA Tinterferometry. Science, 286(5438),283^286. Kipfstuhl, J.,G. S. Dieckmann,H. Oerter ,H.Hellmer and W .Graf.1 992. RADARSATimagery.RADARSATdataare ß Canadian The originof green icebergs in Antarctica. J.Geophys. Res. , 97(C12), SpaceAgency 1 997and 2000. W ealsothank K. Watsonand 20,319^20,324. D.Sandwell(Scripps Institution ofOceanography (SI O)) Lazzara,M. A., K.C. Jezek,T.A. Scambos,D .R.MacA yealand C. J.van fortheir helpin generating the interferogramin Figure 6. derVeen.1999.On therecent calving of icebergsfrom the Ross Ice Shelf. Polar Geogr. , 23(3),201^212. ERS datawere provided by the AlaskaSAR F acilitythrough Lewis, E. L. andR. G. Perkin. 1986.Ice pumps andtheir rates. J. Geophys. NASAR esearch Announcementgrant NRA-O ES-99-10and Res., 91(C10),11,756^11,762. areß EuropeanSpace Agency 1 996.Thanksto B .Betts (SIO) Padman,L., H.A. Fricker,R.Coleman, S. Howardand L. Erofeeva.2002. forproducing Figure 1 .Thanksto T .Scambosand C. Hulbe Anew tidemodel for the Antarctic ice shelves and seas. 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246 Downloaded from https://www.cambridge.org/core. 02 Oct 2021 at 21:11:02, subject to the Cambridge Core terms of use.