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Iceberg Calving from the Amery Ice Shelf, East Antarctica

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Iceberg Calving from the Amery Ice Shelf, East Antarctica Annals of Glaciology 34 2002 # InternationalGlaciological Society Icebergcalving fromthe Amery Ice Shelf, East Antarctica Helen A. Fricker,1 Neal W. Young,2 Ian Allison,2 Richard Coleman3, 4 1Institute of Geophysics andPlanetary Physics,Scripps Institution of Oceanography,University of California SanDiego , LaJolla,CA92093-0225 ,U.S.A. 2Antarctic CRCand Australian Antarctic Division, Hobart,Tasmania 7001,Australia 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 iceberg-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 satellite imageryacquired over the last 14+years.These rifts haveformed at suture zonesin the ice shelf, 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 but alsothe 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 climate change.I ceberg-calvingevents areepisodic in future calving,toassess the likelihoodof large bergs calving natureand may produce icebergs withsize rangingfrom a few fromthis ice shelf inthe nearfuture. Wealsodiscuss features, hundredmetres upto many tens ofkilometres (Youngand observedin recent RADARSATsynthetic aperture radar 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 arepre- upto many decades. In order to determine whethercalving cursors toan iceberg calving event. rates arechanging, it isnecessary tofirst establish the normal calvingrate .Monitoringof iceberg-calving events fromAnt- arctic 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 Glacier km2 inarea calved (Budd, 1 966).Theberg split intotwo tonguein the 1999/2000austral summer (personalcommuni- smaller bergs ayearlater ,whichwere carriedwestwards cationfrom R .A.M assom,200 1),althoughthese icebergs are fromPrydz Bay in the East Wind Drift (EWD),close tothe 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. 03 Oct 2021 at 18:08:52, subject to the Cambridge Core terms of use. Fricker and others:Icebergcalving from the Amery Ice Shelf 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 Prydz Bay ,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 asmaller 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 bergs arevisible 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 that operatesin 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. 03 Oct 2021 at 18:08:52, subject to the Cambridge Core terms of use. Fricker and others:Icebergcalving from the Amery Ice Shelf the marine-ice basallayer is melted offtowards
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