Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussions The Cryosphere 3 , and M. R. Siegfried 2 374 373 ¨ uller (Ward, 1995). During 1998, 2008 and 2009, the , D. J. Quincey 1 , N. F. Glasser 1 er an assessment on its future stability. The spatial and structural changes ff erent time periods (InSAR and optical feature tracking from 1989 to 2009) to docu- ff This discussion paper is/has been under review for the journal The Cryosphere (TC). Please refer to the corresponding final paper in TC if available. Centre for Glaciology, Institute of Geography and Earth Sciences, AberystwythSchool University, of Geography, University of Leeds,Scripps Leeds, Institution LS2 of 9JT, Oceanography, UK UCSD, 9500 Gilman Drive, La Jolla CA, 92093, USA Wilkins Ice Shelf experienced major breakup phases (Braun et al., 2009; Scambos Background In recent years, several Antarctic Peninsulaand (AP) rapid ice retreat shelves have (Cook undergoneShelf and dramatic Vaughan, (Rott 2010) et for al., example,1993), Prince 1996; Larsen Gustav A Cooper, Channel 1997; (Rott Ice Glasser Glasser et and al., et Scambos, 1998; al.,1988; 2008), Doake 2011), Vaughan, Jones et Larsen 1993) (Fox al., Inlet and and 1998), (Skvarca, M Larsen Vaughan, B 2005), (Rott Wordie et (Reynolds, al., 2002; of the ice shelf.is We vulnerable propose to that on-going whilst atmosphericbreakup GVIIS and oceanic along is warming its in and southern no is margin imminent more in susceptible danger ice to of preconditioned collapse, for further it retreat. 1 Introduction and to o of GVIIS (ca. 1973di to ca. 2010)ment are changes mapped in and the surfacebetween ice velocities 2003 shelf’s and are 2008 flow calculated using regime. repeat at fracture track Surface extent ICESat and elevation acquisitions. distribution We changes at note the are anthe south increase recorded ice north in front, ice-shelf and acceleration south towardsthroughout, both ice with fronts greater variations and observed spatially towards varied the negative central surface and southern elevation change regions George VI Ice Shelf (GVIIS)eral is ice located shelves have on undergone the rapid breakup Antarcticwarming. in Peninsula, We response a to use region atmospheric a where andaltimetry oceanic combination sev- (GLAS) of datasets optical to (Landsat), examine radar the response (ERS of 1/2 GVIIS SAR) to and environmental laser change Abstract The Cryosphere Discuss., 7, 373–417,www.the-cryosphere-discuss.net/7/373/2013/ 2013 doi:10.5194/tcd-7-373-2013 © Author(s) 2013. CC Attribution 3.0 License. Speedup and fracturing of GeorgeShelf, VI Antarctic Ice Peninsula T. O. Holt 1 Aberystwyth, SY23 3DB,2 UK 3 Received: 5 December 2012 – Accepted: 8Correspondence January to: 2013 T. – O. Holt Published: ([email protected]) 24 JanuaryPublished 2013 by Copernicus Publications on behalf of the European Geosciences Union. 5 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ects such as decreasing sea-ice ex- ff 376 375 C mean-annual isotherm that, during 2000, stretched from the Wilkins Ice Shelf ◦ Despite the uncertainty surrounding the stability of the remaining AP ice shelves, few It has been shown that an ice shelf acts as a buttress to grounded ice, thus control- Several common glaciological characteristics have been identified on those ice Ice-shelf stability on the AP has been linked to the southward migration of a critical 9 GVIIS is situated insince the the southwest British AP, Graham and Land has been Expedition the (1934–1937) subject (e.g. of Fleming much et research al., 1938; ICESat GLAS data between 2003logical changes and of 2008. GVIIS, We in response useobservations to these in climatic the results and context to oceanic of variation highlight ice-shelf and glacio- place collapse these elsewhere in the Antarctic Peninsula. 2 George VI Ice Shelf 2.1 Overview Mercer (1978) and Vielicurred et well al. in (2007) advancetimeter both of satellite recognised remote breakup that sensing phases. dataof glaciological Here to: GVIIS changes (1) we from oc- assess use ca. theshelf 1973 optical, spatial surface to and from radar ca. ca. structural and 1989 2010, evolution to laser (2) ca. calculate al- 2009, and multi-annual (3) flow analyse speeds surface-elevation change of from the ice- tary glaciers following ice-shelftributaries collapse, can with accelerate between Rottthermore, three et individual and glaciers al. nine have (2004) been timesice-shelf shown illustrating their to collapse that undergo pre-collapse (e.g. enhanced state. such Hulbe thinning Fur- Consequently, following et the al., rate 2008; at Glasser whichand et thus grounded al., their ice 2011; contribution discharges to Berthier into global et sea the al., level ocean 2012). is is amplified. studies increased have considered their long-term structural and dynamic evolution, even though individual ice shelves. ling the dynamics andDe response Angelis time of and the Skvarcaet inland (2003), ice al. Scambos sheet. et (2011) Rott al. and et (2004), al. Rignot Hulbe (2002, et et 2007), al. al. (2008), (2011b) Glasser have all demonstrated a speed-up of tribu- 1983; Reynolds, 1988; Doake and Vaughan, 1991) may also impact the response of embayment geometry (Fox and Vaughan, 2005) and the presence of ice rises (Hughes, ued thinning from atmospheric orPadman, oceanic 2012); warming (3) (Shepherd an et increase al.,Vieli in 2003; flow et Fricker speed al., and (Rack 2007); etand and al., Scambos, (4) 2000; 2008), Rack structural but and also weakening, Rott, transverse-to-flowthe typically due 2004; ice to along changing shelf suture stress (Braun zones regimes et within on (Glasser al., the 2009). ice-shelf It has surfaceditions also acts the been suggested ice as that shelf aalthough extensive the for meltwater driving winter rapid force breakup retreat ina events (MacAyeal precursor of fracture et for the propagation all al., Wilkins that collapse Ice 2003; phases precon- Shelf Scambos (see suggested et Scambos that et al., this al., 2003), is 2009). not Other factors such as the complexity of ocean-ice-atmosphere interactions. shelves that have recently exhibited breakupceded phases; by: ice-shelf collapse (1) is sustained typicallywards ice-front pre- towards its retreat, centre resulting from both in lateral a pinning points frontal (Doake et geometry al., that 1998); (2) bows contin- in- embayment, across Alexander Island anding George along VI Ice the Shelf AP (GVIIS),been to before noted Jason travers- Peninsula that northculation ice of (e.g. shelves Larsen Shepherd C respond etPritchard Ice to al., et Shelf al., 2003; variations (Fig. 2012), Holland 1). in andtents et It through (Yuan oceanic associated has al., and temperature e also 2010; Martinson,iceberg-calving and Bindschadler 2000; rates et Parkinson from cir- ice and al., shelves Cavalieri,specific through 2011; increased 2012), ice wave that shelf propagation. Attributing may changes increase to a single mechanism is still challenging, however, due to to atmospheric (Vaughan et al., 2003) and oceanic (Martinsonatmospheric et al., thermal 2008) boundary warming. by(2003) a remarked number that of the previous timing− studies. of Morris ice-shelf and collapse Vaughan events were closely linked to the et al., 2009; Padman et al., 2012) that highlighted the on-going cryospheric response 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 ; Hum- 2 of ice estimated to 1 and 144 km − (Fig. 1); it is the sec- 2 a 2 3 20 km wide, with the southern ∼ and 46 km 1 − a 3 378 377 , each year (Wager, 1972; Ridley, 1993; Smith 2 5900 km ∼ 75 km from Monteverdi Peninsula to the English Coast. The ice ∼ GVIIS has high basal melt rates (e.g. Jenkins and Jacobs, 2008; Holland et al., Ablation occurs almost entirely as a result of seasonal-frontal calving and basal melt- The ice shelf catchment covers much of the eastern coast of Alexander Island and vected northwards, completing the cycle (Potter and Paran, 1985; Smith et al., 2007). 2010). Warm water intrusionnating from from the the Circumpolar south Deepunderneath east Water Pacific the (CDW) basin, entire current, flows length origi- et onto of the al., GVIIS, continental 1984; contributing shelfHolland Potter significantly and et extends and to al., 2010). basal Paren, This melttic 1985; process Circumpolar (Potter is Talbot, Current thought 1988; to (ACC) be and Lucchitta1993). linked a The and to circulation cross-current involves the bathymetric dense Rosanova, strength saline low ofbeneath 1998; CDW (Klinck water the GVIIS and advecting Antarc- from Smith, via Marguerite Bay itsmelting northern which is ice first front. deflected Upwelling westwards of by warmer the Coriolis water force, instigates and basal subsequently ad- document the fluctuation of theof ice ice-shelf was margins. lost Between from 1947advance the and (Lucchitta northern 2008, and and 1939 km southern Rosanova,studies, ice there 1998; fronts is Cook combined, little with analysis and no of Vaughan, significant the 2010). spatial or Despite temporal these patterns of retreat over time. winter (Reynolds, 1981). Thefrom inland ice glacier shelf systems (Humbert, thus 2007). consists of largelying consolidated (Pearson ice, and fed Rose,with Potter 1983; and Reynolds Paren and (1985)ically before suggesting Hambrey, calving 1988; that along the Lennon(1978), rifts ice et Doake that fronts al., penetrate (1982), of the Lucchitta 1982), GVIISand entire and advance Vaughan depth (2010) Rosanova period- of used (1998), a the Smith combination ice et of shelf. historical al. Mercer accounts (2007) and and satellite imagery Cook to Wintertime snowfall on GVIIS rarelyface lasts melt through rates, the particularly summer in seasonover the due an northern to regions high ice-shelf where sur- areaet extensive meltpools al., of develop 2007) which subsequently refreezes on the ice-shelf surface during the austral 2.2 Ice-shelf mass balance of the ice to GVIISduces (Humbert, stagnation points 2007). This alonggrounding domination the line, of ice but inflow shelf diverges prior from created toBentley Palmer as reaching Land et ice this pro- point al. flows (Reynolds (2005, towards andHolocene, the Hambrey, 2011) George 1988). opposite VI and Sound Roberts was absentis et of still al. considered shelf (2008) vulnerable ice, to and both environmental thusfiguration. change show the despite present that its day during atypical ice dynamic shelf the con- mid- the western margin of Palmerbe Land, flowing with into 12 km GVIIS, respectivelyIsland (Reynolds only and extends Hambrey, a 1988). few Ice kilometres from1998) into Alexander the as ice-shelf system tributary (Lucchitta and glaciersbert, Rosanova, are 2007). generally Glaciers small flowing (between from 54 Palmer km Land are typically larger and supply much Ronne Entrance that isDe interrupted Atley by Island). a Theits distance succession between centreline. of the ice The two risesmargin northern fronts (Eklund measuring is front Islands approximately sits and 450shelf km in along varies a in channel thicknessregion, from before 100 thinning m again atand the Rosanova, towards 1998; northern the Smith southern et ice al., ice front 2007). to front 600 (Talbot, m 1988; in Lucchitta the central Rosanova, 1998; Smith et al.,The 2007; Humbert, ice 2007; LaBarbera shelf andPalmer occupies MacAyeal, Land, 2011). and George covers VIond an largest Sound, area ice of situated approximately shelffront between 24 that remaining 000 km calves Alexander on into Island the Marguerite Bay, and AP. and GVIIS a has southern two ice ice front that fronts, terminates a into northern the ice Wager, 1972; Pearson and Rose, 1983; Reynolds and Hambrey, 1988; Lucchitta and 5 5 25 15 20 10 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 − C, ◦ 1.1 + ) images, + (Corr et al., 1 − and 6.0 ma 1 − Band 4 (0.76–0.9 µm, 30 m), and + , 3.0 ma 1 − 380 379 , 4.1 ma 1 − 80 m), Landsat TM/ETM ∼ ered the best pixel and spectral resolution, for example, Landsat ff Panchromatic Band 8 (0.52–0.9 µm, 15 m). Digitised features included + C warmer than the freezing point at the base of the ice (Talbot, 1988). Digitising was carried out at three main scales for consistency across all satellite Potter et al. (1984) calculated spatially-averaged basal melt rates to be 2.1 ma ◦ and pressure ridges. Otherat features, scales such appropriate aschange to nunataks was their observed, and further ice individual mappingchanges rises, characteristics. was at were carried finer For spatial out mapped areas resolutions. to provide where detail significant of short-term tures (elsewhere termed flowand stripes, ice flow rumples. bands, foliation), pressure ridges, ice rises scenes: (1) 1 : 100 000tures, was (2) used 1 : for 50 large 000and surface was features (3) used such for 1 as fractures, : 25 longitudinal rifts, 000 struc- the was ice front used and for the crevasses, grounding zone, crevassed zones, transverse structures and Braun et al. (2009).ner Features (MSS), were mapped three from Thermatic sixand Mapper Landsat three Multi-Spectral (TM), ERS-2 Scan- nine SARage Enhanced scenes bands TM (see that Plus Supplement).MSS o (ETM Mapping Band was 5 (0.6–0.7 performed µm, usingLandsat im- ETM the location of thefractures, ice front fracture and traces, ice-shelf crevasses grounding and zone crevassed for zones spatial (fields), assessment, longitudinal rifts, struc- 3.1 Structural and spatial assessment Structural and spatial mapping was carriedSystem out in (GIS) ArcMap software 9.3 Geographical following Information similar procedures to Glasser and Scambos (2008) 3 Methods (2007) who modelled ice-shelfdynamic velocities. or Despite structural these investigationssouthern studies, carried region there out of have along GVIIS. been the Thus,dynamic no northern there changes, is ice despite no front, it documentedregion or evidence being and in of being one the any situated of structural both2003) the or near and most in the a thermal intensely warming limit studied Bellingshausen of Sea systems viability (Holland in (Morris et the and al., Vaughan, 2010). Reynolds (1981) and Reynoldsand and structures of Hambrey the (1988)dinal northern assessed structures regions the and of crevasse surfaceflow GVIIS, patterns. features regime concentrating Their fed on analysis by meltpools, Palmer revealedMacAyeal Land longitu- a (2011) glaciers through highly that numerical compressive was modelling later of emphasised structural by development LaBarbera and and Humbert tively) revealing that theance, correlating ice well shelf with the is sustained(2012) thinning currently and rates Pritchard estimated reported et by to Fricker al.spect and be (2012). Padman to Here, in we spatial, negative assess structural surfaceice-shelf mass elevation response and changes bal- to with dynamic recent re- configurations climatic changes. of the ice2.3 shelf to Structural investigate glaciology 3 in order forspatially-averaged GVIIS losses to of 2.8 remain ma2002; in Jenkins and equilibrium; Jacobs, recent 2008; Holland basal-melt et calculations al., 2010; suggest Dinniman et al., 2012, respec- The maximum observed oceanic temperature from within George VI Sound is 5 5 15 20 10 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent image pairs. Next, ff using the fringe-rate visibility at 1 − 30 ma ± 382 381 , for that particular time period. The data points were sub- 1 − . Errors caused by vertical ice-shelf motion (tide, atmospheric 1 ects and baseline estimation errors are likely to be the largest − ff erent image pair acquisitions (28/29 October 1995, 15/16 February 1996), ff This manual feature tracking approach works particularly well where distinct struc- Surface features were selected based on their distinctness between image pairs and Optical datasets were selected for manual feature tracking as the visual quality of (Watson, 1992). tures are densely packedmore rather sparsely distributed. than However, we overresented assume featureless that by terrain any visible where large surfacedeed variation data of structures changes points flow in (e.g. are is these rep- sheartherefore structures zones, suggest over pressure that time buckling) wheresparsely-distributed (fracture and speeds points, development/propagation). in- have the We been truecertainties. derived speed Comparison is from of within interpolation our the ca. between 2007 boundaries (north) of and the ca. stated 2009 un- (south) feature tracking surements in thisthe way normalised permitted displacements data weretotal collation merged displacement between into in di a ma singlesequently interpolated shapefile, representative using of aalgorithms Natural Neighbour as algorithm, it selected performs over equally alternative well with regularly and irregularly distributed data their distribution across thepressure image. ridges Generally, fractures were and alsoages. rifts A tracked were polyline where used, was they digitised although point could from on the be feature’s image identified starting 2and clearly point added on with in to image the an both 1 attribute total to im- solved table the and polyline within similarly same a length added GIS. to (i.e. Theas the centre feature the attribute point interpolation displacement) table for as point. each calculated X The polylineabsolute and displacement was displacement Y measurements re- coordinates were to and then displacement later converted used in from metres per annum. Normalising the mea- ment) were used over timeImages periods were coregistered of to no within more 1estimated pixel than accuracy accuracy three with of mapping years 1 also between pixel, carriedbetween thus image out image resulting pairs. to pairs in an a total maximum uncertainty of 2 pixels window to assess changes in flow speed. In total 40 Landsat scenes (see Supple- the imagery is far superiortemporal to that resolution of was SAR data. extended Furthermore, back by using to Landsat 1986, data the thus creating an approximate 25-yr 3.2.2 Manual optical feature tracking Following the work of Simmonsture tracking and was Rouse used (1984) toture and calculate surface tracking Simmons speeds methods (1986), of over manual GVIIS.in automated fea- We surface opted alternatives for feature due manual scale, to fea- thescenes, the and ability the considerable to capability variations map of tracking displacements features between through fine multiple clouds coincident or atmospheric haze. inherent contributors to uncertaintyno greater in than the 5 ma resultingpressure) are data estimated to (Mohr be etthe no more grounding al., than zone 2003) and butpasses. the are incidence angles of the ascending and descending SAR age pair was used topair construct used an to ascending-pass construct interferogram, a withsurface the descending-pass were other interferogram. then Velocity image resolved fieldsdescending via of the a interferograms, ice-shelf trigonometric assuming approach thattween between the the di the surface ascending dynamics and and had that not flow altered was be- inup the to annual horizontal displacements plane. for Twenty-fourdata. hour comparison Atmospheric velocity with fields e the were manually-derived scaled feature tracking 3.2.1 Interferometric SAR InSAR procedures were carriedERS-1/2 out 24-h in repeat GAMMA image Remote Sensing pairs software over using the two northern ice front for ca. 1995; one im- 3.2 Flow-speed derivation 5 5 25 15 20 10 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ect (IBE, ff ered the best ff the north ice front ff . Datasets produced 1 − ected by the motion of ice-shelf ff of ice calved o 2 ected by clouds by inspection of gain ff 384 383 of ice was lost from the north ice front between 1974 and 2010, 2 latitude. With a surface footprint of 50–70 m, GLAS collected data every 30) is slightly higher than the calculated precision of ICESat data from ◦ = 86 n ± Finally, surface-elevation changes were calculated between comparable campaigns Because we were only interested in elevation changes, the absolute accuracy of the Following the pre-processing of the GLAS data, each track was converted to a Po- Island pinning point remained relativelyIn stable until total, retreating 1255 between km 2001 and 2010. 4 Results 4.1 Ice-front retreat Between 1974 and 1979,(Figs. approximately 2b, 3). 820 km Further retreattween was 1979 recorded and along 1989, the 1989 Palmer Land and grounding 1996, zone and be- from 1996 to 2001, whilst the Alexander of surface elevation changes werechange. subsequently Repeat obtained surface-elevation that measurements reflected were acquired afrom over sub-decadal a 16 ICESat total tracks offrom 4323 these points methods and are are summarisedgiven displayed in in as Table the 1, change Supplement. with in a ma full list of satellite images used Horgan et al. (2011),maps although instead for of ca. manually 2003elevation removing points and data, in here ca. ArcMap the within 2010 200 structural m were of used any distinct to(i.e. surface automatically on feature. select 12-monthly and timescalesshort-term or remove seasonal ablation multiples the and/or of) accumulation. Thespatial to campaigns and that avoid temporal o elevation resolution for changes eachcampaign caused individual elevations were track by not were used selected, in and the whilst final inter- elevation-change assessments, a record shelf thickness from eitherwere surface further or culled, removing basal anysurface accumulation/ablation. undulations points (i.e. Next, potentially fractures, GLAS a rifts, pressure data the ridges, resulting longitudinal structures) surface-elevation so changes that a reflected change a in true elevation position change of rather surface than structure. This process was similarly undertaken by et al. (2010) to ensure that any variations in surface elevation reflected changes in ice- lar Stereographic projectionto to the match ice-shelf other area datasets in in hydrostatic this equilibrium study, using and the then extents calculated subset by Brunt data using crossover pointsshelves over on a the larger AP dataset (Holt,of also 2012) including and Brenner iteratively Bach et removing and outliers al.,deviation, Stange (following 2007). the ice The methods calculatedearly single-shot campaigns precision (Shuman of et 0.153shelves m al., accounts (1 2006), for standard instrumental whichand uncertainty is inter-campaign bias expected as corrections. as well as the uncertainty precision over in ice the IBE, tide, Brunt et al., 2010),corrections, and we inter-campaign filtered biases data (Siegfriedand from et return campaigns al., a energy 2011). valuesad-hoc After reference on track applying a (Brunt et track-by-track al., basis 2010), and which allowed resampled forGLAS each repeat-track data analysis. is track inconsequential. to We determined an a region-specific precision of our GLAS Ice, Cloud, and landbetween Elevation Satellite (ICESat)172 measured m Earth along-track in surfaceanalysis elevations 33-day requires campaigns two accurate,(GLA12, to tide-corrected release 531) three elevation was “retided” times measurements, by addingand per the back Padman, the year. 2006). GOT99.2 GLAS Since tide This correction data dataset ice-shelf (Fricker corrected was for then vertical converted tidal to(Padman a displacements WGS-84 et using ellipsoid the al., and more then 2002; accurate King CATS2008a model and Padman, 2005), the inverse barometer e agreement at both ice fronts. 3.3 Surface-elevation change From 2003 to 2009, the Geoscience Laser Altimeter System (GLAS) on-board NASA’s results with Rignot et al.’s (2011a) InSAR derived velocities (ca. 2007) show good 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . 1 of − 2 was a ) and 2 of ice 2 1 − sensor, 2 a + 2 20 km − and 16.8 km 1 − − a 2 10 km − of ice was lost, with 182 km 2 386 385 ). Retreat was concentrated in the central por- 1 − a the south ice front towards Monteverdi Peninsula 2 ff 25 km from the 1974 ice margin. , almost twice the rate in the previous ( 1 ∼ 15.9 km − − a 20 km that later facilitated iceberg calving between 6 January 2 ∼ 4 km distances) to the northern boundary of Riley Glacier whilst ∼ 28.0 km − In both of these regions, fractures and rifts continued to propagate throughout the Chaotic fracturing and rifting was also observed along the Palmer Land margin of the At the south ice front, approximately 925 km observation period assessed here. During 1986 (Fig. 5c, d), 1991 (Fig. 5e, f), 2003 During 1973 (Fig. 5a,by b), cross-cutting ice fracture feeding traces, the emanatinglund centre from Islands of fractures further the and upstream. riftsice, south The lee-side with ice largest of occasional front rifts the was smallthe Ek- were dominated pools English filled of Coast with open margin unconsolidated immediately water.spread for downstream brash Fractures up of and to ice 20 rifts km rises intoice. also and the formed ice headlands. shelf along These before becoming indistinct from the surrounding 2010 and 29 Januarybut 2010. detailed This observation event isFew was limited fractures captured due were to by observed poor theexception of data along Landsat some quality ETM the small and rifts north thus at ice each not of front shown the4.2.2 here. during grounding zones March (Fig. 2010, Structural 4h). evolution with at the the south ice front of open waterintermittently running along (at theirdecreasing lengths. in These their overall central length.and rifts Rifts 1989 that could were eventually formed also be the visible ice-front traced during back 1979 northern section during 1979, 2001had and developed 2010 along (Fig. the 4). Alexander During Island 2001,across margin, several the distinct the ice rifts largest shelf of for which had propagated During 1974, ice flowing frombe Palmer traced Land towards into the GVIIS north wasice ice heavily front, was fractured becoming observed and increasingly could between chaoticthe (Fig. some north 4a, of b). ice the Brash front, larger large rifts, rifts whereas were filled in with the smooth, central uniform portion ice of with some evidence 4.2.1 Structural evolution at the north ice front only. processes of the main icetween January shelf. 1973 The and south January ice 2010,and front although 24 the became March retreat increasingly 2010 between subsequently concave 15 be- created January two 2010 new slightly-convex4.2 ice fronts. Structural glaciology and structural evolution Mapped structures and surfacea features full (January list 2010) of arein glaciological Table displayed structures, 2. in identifying Here Fig. criteria we 2, describe and with the their structural significance evolution provided of the north and south ice fronts retreat between January 2010into and two March independent 2010 sections, Souththe subsequently Ice Eklund split Front Islands the 1 and main (SIF1)Island. South ice from A Ice Monteverdi front smaller, Peninsula Front third to 2independent ice (SIF2) over front from the between the timescales Eklund De investigated here Islands Atley and to Island is De and largely Atley Spatz disconnected Island to is the always thus recorded betweenshelf January ice 1973 was and capturedbut January not calving included 2010. o in During lossthe 1973, calculations ice as shelf. 390 km it Betweena was 1991 maximum deemed and of to 1996 have there alreadyproceeding was detached time periods an from ( increasetions in of the the rate of southpoints retreat ice along to front, Monteverdi with Peninsula, only De limited Atley retreat Island observed and at Spatz the Island. ice-front The pinning continued the retreat rate of theFurthermore, north a ice concave front fluctuated profile between servation was periods. observed at the north ice front inadvancing each into of the the ob- Ronne Entrance (Figs. 2c, d and 3); a net loss of 743 km with the centre of the ice front retreating 40.6 km into George VI Sound. Post 1979, 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | and mea- 1 − 1 − 38 ma ected by the re- ± ff . 1 − in the centre of the ice front, 1 − 15 ma of ice-shelf area was a 2 ± 388 387 from ca. 1989. The surface speed of all other glaciers 1 − 38 ma ± 214 ∼ Widespread negative surface-elevation change has also been calculated over 1649 In the central section, six GLAS tracks cut across GVIIS, with 1657 repeat measure- Coast of Palmer Land (Fig. 8).treating In grounding total, zone 172 over km a 90 km distance between the eastern boundary of GT04 front, however, the GLASments dataset was of heavily surface filtered,around elevation and the changes Eklund therefore are Islands. direct incomplete measure- towards the4.5 English Coast Grounding-zone and retreat The analysis of sequential satelliteing images zone highlighted a between gradual 1973 retreat and of the 2010 ground- at the southern extent of GVIIS along the English Large negative elevation changes aretral recorded region along of track the 0382, icesurface shelf. which elevation Tracks dissects 1298 changes. cen- and 0316 both illustrate ice-shelf wide negative repeat measurements in thepositive southern elevation changes section, (Fig. interspersed 8). with Due localised to pockets fracturing of and rifting towards the south ice tracks with a total(less of than 1017 repeat our measurements uncertainty)positive examined negative elevation (Fig. change change 8). is interspersed Afew observed between non-significant distinct patterns in areas in the of the data, surface dataset. lowering; with there pockets of are ments analysed (Fig. 8). Awith complex positive pattern changes of noted surface-elevationwards at changes the is the observed, northern input of extents Goodenough of Glacier tracks (track 0197 0063) and and 0263, to- near the input of Kirwen Inlet. imum recorded velocity in the centre of SIF2 was4.4 796 ma Surface-elevation changes Surface elevation change in the northern section of GVIIS was calculated over three zone of GT04, GT05, GT06 and GT07 towards the ice front. During ca. 2009, the max- sured at the ice front. This increase in surface speed is apparent from the grounding The south ice frontsence (Fig. 7) of are the characterised EklundPeninsula by to Islands complex the dynamics that Eklund due Islands, reduceare to surface driven the the speeds by pres- flow are tributary typically speedsspeeds glaciers less here of some did than GVIIS. not 150 390 km significantly ma From increaseThe upstream. Monteverdi or greatest Our decrease changes between results are ca. indicate 1989fed observed by that and between four ca. flow major the 2009. glaciers Eklund (GT04–GT07), Islands with acceleration and of De up to Atley 340 Island, an increase of entering the ice shelf towards theof north the ice observed front time did periods. not change significantly during4.3.2 any South ice fronts 4.3 Variations in ice-front flow 4.3.1 North ice front The flow of the northca. ice front 1989 is principally and controlled ca. byincreased Riley across 2009 Glacier (Fig. the the 6). whole Between surfacesurface ice speeds speed front reached of as a it ice maximum retreated derived back of from upstream. 390 this During particular ca. 2009, glacier treating ice front, with morealong rifts their showing evidence length. of Furthermore, openfirst an water apparent or area in unconsolidated of ice 1991 open (Fig. waterthat 5f), lee connected increased side SIF1 in of size and thetwo from SIF2 Eklund independent 1991 Islands, ice eventually to fronts breaking 2010 terminating with away into the during the ice March Ronne bridge Entrance. 2010, leaving (Fig. 5g, h) and 2010 (Fig. 5i, j) the area of fractured shelf ice increased towards a re- 5 5 25 15 20 10 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 41 km ∼ 390 389 , respectively) is remarkably similar and perhaps illustrates 1 − 40 km to the northern boundary of Riley Glacier. The regularity of frac- ∼ and 1.1 kma 1 − 40 km linear along centreline) to the loss recorded between 1974 and 2010 ( The retreat recorded between 1974 and 1979 in the northern region occurred in an Our results illustrate a two-phase retreat of the north ice front from 1974 to 2010, ∼ fracture and rift propagationley and Glacier, indeed coalescence prompts with the aundergo sealing second longitudinal of tributary compression fractures. glacier after North did initialof of not fracturing. this Ri- occur, It dynamic thus is configuration, ice fractures proposedzone did emanating that are not from able as to Riley a spread Glacier result ern transversely at ice across the the front grounding ice with shelfthe little as large-scale or it retreat no flows observed. towards resistance, the thus north- preconditioning the northern ice front for areas of fractured ice are observedtend between to coalescing flow exhibit units longitudinal (Fig.with 2). extension stress These at areas regimes the becoming more groundingshelf compressive zone, with margin causing increased initial due distance from fracturing, tensile to the regime ice- the to coalescing a of compressive ice-flow stress units. regime within Thus a the limited switch spatial from extent an restricts initial erned by preconditioned andactual calving. longstanding Indeed, active the eventual rifts,1974 1979 developed Landsat and well 1989 MSS ice in imagery frontsthe (Fig. advance were retreat 4), observed of process as thus the rifts north emphasisingin in ice the this front. long-term region, As thus development of of no March 2010, immediate no large-scale ice-shelf-wide calving rifts is existed area anticipated. devoid of significant glaciernorth input ice and front thus is we largely suggest that controlled the by retreat its rate dynamic of configuration. the Elsewhere on GVIIS, longitudinal extension and/or bendinggating stresses across caused the by ice shelf tidal1996 towards motion, and the before north 2001 propa- ice eventually frontrift led (Fig. to development 4b). a this Further large time riftingrather calving between developed than event from from during the Palmer January Land. western 2010, Most margin although of near the Alexander retreat Island at the north ice front was thus gov- features had gradually developed, forming at the grounding zone through flow-induced upstream for tures and rifts in this zone of relatively slow-moving, inactive ice suggests that these ( linear along centreline). The(1.0 rate kma of retreat over thesethe periodic two large-scale longer breakup observation of periods the northern ice front. with one large breakup event2010). (1974–1979) The followed north by several ice smallerthe front phases entire during (1979– channel 1974 width was between heavily Alexander fractured Island and and rifted Palmer across Land, almost extending back between 1947 and 1960, followedprior by to sustained 1947 retreat. are Therifts uncertain, conditions occupied although of the Fleming the et northern iceJeremy al. front sections (Doake, (1938) of 1982) reported (Fig. George that 1). VIextent, sea-ice-filled If then Sound, these the possibly early amount extending observations of to are ice Cape indicative lost of from the GVIIS former between 1936 and 1974 is comparable 5 Discussion 5.1 Glaciological controls on GVIIS retreat 5.1.1 Northern extent of GVIIS Smith et al. (2007) and Cookto and Vaughan that (2010) show of a the north 1974 ice-front position margin similar presented in this study, but also illustrate a frontal advance elsewhere along either Palmersurface Land visibility or of Monteverdi various ice Peninsula,the rises although Eklund also the Islands points reducing from towidespread which a thinning is retreating since it at grounding least inferred zone 1973. that around the southern region has experienced and the western boundary of GT07. There is no evidence of grounding zone retreat 5 5 25 15 20 10 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ect on ff 392 391 Towards the central and southern extents of GVIIS, a stronger surface-lowering sig- The overwhelming pattern of the south ice front is that of steady retreat, concentrated The presence of the Eklund Islands towards the southern ice front is therefore a crit- demonstrated that the removal ofof an former ice-shelf system tributary results glaciers in (De an increase Angelis in and velocity Skvarca, 2003; Dupont and Alley, 2005; further retreat. Therefore, we suggestchanges that in the ice south thickness caused ice by front oceanic in temperature particular variation. 5.2 responds to Glaciological response of GVIIS toAs ice-front a retreat result ofdominated ice-front by the recession, dynamics the of1989, northern ca. Riley 2002 Glacier. region and Feature of ca. trackingan 2007, GVIIS measurements coupled increase from became with in ca. InSAR increasingly flow velocities speed (ca. of 1995) Riley clearly Glacier illustrate over time. Elsewhere on the AP it has been of ice front retreat ofthat Bach short-term, and intermittent Stange oceanic Ice variation can Shelvesof impact (Holt, ice on 2012) shelves. the Subsequently, that a glaciological promotes conditions warmingSea Ronne the Entrance coupled idea portion with of the the inferred Bellingshausen Eklund high Islands concentrations and of a marine-derived structurally ice weak lee-side ice of shelf the preconditioned the south ice fronts for region in particular, theany ice-shelf other draft point of in GVIISthat George tend reaches to VI much exist Sound, closer greater to andThus, depths the thus sea the than bed it thickest at (Jenkins parts and may Jacobs,rates be of 2008; of Holland subjected GVIIS et basal to are al., 2010). accretion warmer subjectedappears to waters that that high results an rates increase in1996 of in a the basal immediately net rate melt follows loss of and atween of retreat lower strong 1989 of ice vertical and the through 1992 mixing south (Holland a within margin et vertical al., between the 2010). column. 1991 Bellingshausen This and It Sea pattern be- in also reflected in the timing treat, displaying a largelyportions concave of profile. the A south combination iceas front of observed pre-conditioned these between for factors iceberg 1973 calving made and and January large continued 2010. retreat nal is measured from which widespread ice-shelf thinning is inferred. In the central southern ice front arguably satisfied Doake et al.’s (1998) criteria for irreversible re- ice rises/rumples essentially act as a buttress to flow. Furthermore, during 1973, the elsewhere been shown to haveaccreted a in higher basal concentration cavities offractures and warmer, with incorporated marine-derived flow into ice, (Fricker the etered al., ice to 2001; shelf be Khazendar through less and brittle thewarmer Jenkins, than resealing (Vieli 2003). meteoric of Whilst et ice consid- (Lui al.,variations and 2007), (Fricker Miller, marine-derived 1979; et Jansen ice al., etregional is 2001). al., surface more 2010), Third, speeds susceptible the the tomeasurements and Eklund from oceanographic thus Islands each the have ofspeeds the a supply down-ice observation profound of of periods e the reveals ice Eklund substantially Islands to slower than flow this those observed region. outside Feature-tracking of this region; the ical component inregular the fracturing retreat and rifting characteristics ofdue of the to GVIIS. ice a shelf First, shift asstream. their Second, it from the flows presence high-compressional composition around of causes stresses (or shelf-ice, over) upstream lee-side such of to features, ice tensile rises stresses and ice down- rumples has with further calving anticipated alongsince the ca. ice-front 1996 rift (Fig. that 5). has progressively developed in the centre of theagery ice that front. this It region isness has inferred calculation a from visual lower from assessment surface Radar2011) of elevation Altimetry confirm 1973 lee-side this. Landsat of (RA) To im- these elevation thebut ice north data heavily rises. and fractured. (e.g. Thick- west Griggs of and the Eklund Bamber, Islands the ice shelf is thicker, At the south ice fronts,substantial Smith ice et al. loss (2007) between indicate ca.a a 1967 small continued and portion retreat 1973. since of From 1947,be the observations with advancing, south in yet ice this even this front study, only area (adjacent appears to to Monteverdi show Peninsula) a was repeated found advance/calving to regime, 5.1.2 Southern extent of GVIIS 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | to 1 − 30 ma ± ectively removed 380 ff ∼ erent processes: (1) ff 394 393 ect of surface meltwater on its structural stability. Furthermore, the towards Millett Glacier, potentially rendering this entire section weak ff . These increases appear to be driven by two di 2 1 − 15 ma ± Surface melting on GVIIS in the northern region has been on-going since at least Our data illustrate that the structural regime at the north ice front is governed by the Between Monteverdi Peninsula and the Eklund Islands, however, an advance of the At the south ice front we propose that the removal of buttressing ice between 1973 780 area of 1200 km and susceptible to large scale retreat as observed between 19741940 (Stephenson and and 1979. Fleming, 1940)has yet the limited compressive the flow regime e ofnorthern the limit ice of shelf surface meltwateryears has always it been has south ofsult, been the a ice shown combination front, to of but structural inter expand weakening recent and towards (longitudinal extension), intensifying a abundant surface-melt retreating meltwa- brought northern on margin. by As warmer a temperatures re- (e.g. Vaughan has a comparatively slower,input. less-active We dynamic propose regimepresence that absent of this of Riley significant area Glacier, glacier turing. is limiting Continued recession longitudinal currently of extension exposed the that northback-stresses to ice ultimately upstream front high restricts and (over any back-stresses encourage frac- timescale) fracturing would by reduce of the the shelf ice across an approximate dynamics of individual tributary glaciersthat that flow where from there Palmer is Land.sence It a of has lack flow-unit been confluence), of shown fractures longitudinal andice rifts and shelf, are transverse capable along of compression which propagating (due acrossare large-scale the to linked calving the to is the ab- initiated. breakuprate The of much short comparatively less bursts “less-active” where of zoneslatter the of rapid situation ice shelf retreat front currently ice, is exists withby (Fig. directly retreat the 6), supplied dynamics with by of fast-flowing iceberg Riley “active” calving Glacier. ice. and The The retreat current rate flow governed configuration south of Riley Glacier propagate allows the anticipationThus of based ice-loss on extent these wellpected historical in from observations, advance the no north of immediate icewill actual large-scale front, most calving. likely as calving be few is governed fractures ex- by or regular, rifts but currently discreet exist. calving Iceberg at calving the ice front. the ice front. The multidecadal recurrence interval over which fractures develop and The retreat history of thebreakup north of ice heavily-fractured ice front since that ca. stretches 1940 for alludes some to distance periodic back large-scale upstream from ice front is drivensignificant by (greater tributary glaciers than somea our result, 150 uncertainty) km the back area variation between upstream.more in the We stable Eklund and flow measure Islands less and no speeds responsive Monteverdi to over Peninsula observed appears environmental time, to and be and glaciological5.3 changes. as The future stability of GVIIS 5.3.1 The north ice front increased extensional pulling from thegrounding zone. ice There front; is and athe clear (2) link increased acceleration between distribution of the and GT07 increase extentmake in from this of flow its particular speed surface over area fractures, time structurally rifts and and and dynamically fracture weak. traces that regime of the north ice front. and 2010 led to increased longitudinalwithin extension through the the ice reduction shelf. of This back-stresses ca. is clearly 1989, reflected ca. in feature 2002Eklund tracking Islands and measurements and between ca. De 2009, AtleyDe Island with Atley over ever-increasing time. Island, surface Indeed,∼ localised between speeds the flow between Eklund speed the Island had and almost doubled from the mass of GVIIS, thea continued buttress, retreat reducing of back-stresses the andvelocity north subsequently at permitted ice the an front northern increase hasGT12 margin. in e and Further surface Transition Glaciers upstream, do flow notfurther show speeds illustrate an of the increase Skinner, or dominance Chapman, decrease and over controlling time, nature and thus of Riley Glacier on the dynamic Glasser et al., 2011; Berthier et al., 2012), and whilst Riley Glacier still contributes to 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | since 1 − rate suggested 2.0 ma 1 ∼ − to 1 − 1.8 ma ∼ erent glaciological regimes. ff cient mass from its base to instigate ffi 396 395 cient surface drainage means that meltwater re- ffi Ice feeding SIF2 has seen the greatest dynamic and structural changes and indeed Towards the Eklund Islands, SIF1 is structurally weak. Large areas are heavily frac- The negative surface-elevation changes observed to date is linked to increasing The stability of SIF1 is governed by its frontal geometry and thickness-driven veloc- Recent modelling studies (e.g. Holland et al., 2010) have suggested that the rate results in these thinner areas being more susceptible to oceanic variation. et al., 2005). Within theice next rises decade will it have is disappeared, probable leaving the that ice the shelf ice pinnedis directly behind most in them. front likely of to these warming undergo and further intermittent retreat. vertical In mixingpositioning of particular, of CDW it as several remains ice noted vulnerable by rises Hollanddownstream, to close et almost to oceanic al. certainly the (2010). grounding create The zone,gions basal in of cavities, addition that to shelf those not ice,feeding further only but SIF2 results also is in likely encourage thinner to basal re- have accretion a of high concentration marine of ice. marine-derived As ice a that result, naturally ice tured and rifted and the recentto retreat further history ice illustrates loss, that particularly these(Holland as are et increasing most basal al., susceptible melting 2010) andfills will reduced the limit basal rifts accretion basal is accumulation. likely to Furthermore, reduce, the and ice with it, melange any that resistance to further propagation (Bassis ity from further upstream,historically particularly shown towards Monteverdi prolonged Peninsula. periods Thisindeed, of portion calving advance is has followed anticipated by inter large-scale the this coming calving, expected years loss and along ofover a ice, much well-developed the rift. of ice Even its front af- it length, would has remain it done relatively is in stable.its previous expected Being dynamics decades convex that are as it driven it would by isgrounding its also lines. seemingly thickness maintain non-responsive gradient to a between retreat the similar patterns; ice speed front as and its tributary basal melting as a2012). result Assuming that of this aticularly warming warming will vulnerable ocean continue, to (Holland the heighteneda southern et reduction retreat region al., of as of in-situ 2010; GVIIS basal a Pritchard accretion. is result et par- of al., increased basal melting and the central portion of theeven ice despite front their consequently close split it proximity, have into distinctly two di individual ice fronts, that 5.3.2 The south ice fronts The south ice frontssponse have to seen climatic by and far oceanic thesustained variability. greatest retreat, Prior change concentrated to in in March recentand the 2010, have decades this central a as high margin portion concentration a underwent that oftal re- marine-accreted we and ice. inferred vertical) This has to sustained ice rendered beand loss large both prone (horizon- portions thin, to of iceberg the calving southern along margins structurally preconditioned weak fractures and rifts. The removal of phases in 2008 and 2009,to and thus a a stable lack of icemelting vertical shelf. regardless change The may of not northern necessarily whethercharacteristic region, point is mass however, its is is unusual lost susceptibletemporal dynamic to retreat/ from regime breakup regular that the patterns. will surface ice ultimately shelf control or the spatial not. and The stabilising by Potter et al.ern (1984) region in of order GVIISwidespread for is thinning. the therefore Whilst ice notaustral surface shelf losing summer, melting to su the remain is absencefreezes in of evident on su equilibrium. the for surface The prolonged ofnegligible. north- the periods ice As of shelf a and the therefore result,ice-shelf the thinning, the actual although mass northern Fricker and lost Padman section isterminated (2012) considered on of comment Wilkins that Ice GVIIS surface-lowering Shelf is approximately 8 perhaps yr prior less to susceptible the two to most recent breakup ment susceptible to atmospheric warming. of basal melt in1980. Whilst the comparatively northern high relative regionPeninsula, to basal increased the melt from other rate ice remains shelves marginally on the less west than Antarctic the 2.1 ma et al., 2003) and a lengthening melt season (Torinesi et al., 2003), creates an environ- 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 398 397 ectively begun to remove the stabilising ice melange ff Surface velocities at the north andcumstances south this ice has fronts increased been over attributed time. to In reduced both back-stresses cir- within the ice shelf northern ice front are atice present further dominated upstream; by the Riley longevityresponses Glacier of of that Riley the buttresses Glacier rest shelf thus of governs the the ice-shelf system glaciological In in the the central northern and region. recorded. We southern link regions this to ofthus enhanced GVIIS basal conclude melting widespread rather that surface thangoing the surface lowering ablation oceanic southern was and warming. extents Coupledice-shelf of with thinning. grounding GVIIS line are retreat, more we susceptible inferRapid significant to disintegration on- of GVIIS is not anticipated, however, our investigation has fractured, and where highWe concentrations suggest of that marine-accreted thetween ice 1973 southern and were ice 2010, inferred. although frontconvex at ice-front was present geometries. both also SIF1 preconditioned and SIF2 for have retreat more stable, be- that permitted enhanced longitudinal extension.De Between Atley the Island Eklund in Islands particular,ing and this and occurred rifting simultaneously that with has increased rendered this fractur- area structurally weak. The dynamics of the shown that significant glaciologicalposed changes atmospheric limit have of taken ice-shelfvulnerable place viability, and to south that oceanic the of warming ice-shelf than the system atmospheric is pro- change. more Spatial and temporal retreat patterns1974-present were observed controlled at by the longstanding and northern widespread ice ice-shelf front fractures. between few channel-wide fractures that haveice previously front, hastened and thus calving we from do the not north anticipateThe immediate retreat large-scale rate retreat. recordedfront, at the although south there icewarm-water is fronts upwelling in evidence was the steadier of Bellingshausen thanfront Sea. increased the was Iceberg focused retreat calving north in at ice rate areas the where south following the ice shelf periods was of inferred to be thinner, was heavily Rapid breakup (1974–1979) was followedrifts by near steady retreat the (1979-present) ice along region front, and was coupled arguably with preconditioned a for concave sustained ice front retreat. profile; At the present, northern there are . erent environmental settings and thus the glaciological controls of, and responses of, Furthermore, the distribution of open rifts and fractures has increased (Fig. 9), many 4. 5. 3. 1. 2. ff Supplementary material related to this article is available online at: http://www.the-cryosphere-discuss.net/7/373/2013/tcd-7-373-2013-supplement. pdf di ice-front retreat vary considerably. Analysis ofsensing optical, datasets radar has and led laser to altimeter the remote following conclusions: in these rifts. As a result,progress future through widening the of ice these shelf riftsincreasing towards is the the almost likelihood certain front, of to increasing rapid occur the breakup as rate they phases. of retreat and also 6 Summary and conclusions Our analysis of the glaciologicalgoing evolution environmental of change GVIIS in has the revealed AP. its The vulnerability north to and on- south ice fronts occupy two very being filled with an icegation melange (e.g. that Bassis elsewhere et has al.,with been visible 2005). shown water Even to has within increased, limit the andto further last as abundant propa- this decade, surface region the melting, is number it outsidement of is of (climatic that open clear and which oceanic) that rifts is has this subjected e is open sea water. 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F.: High-resolution ground-based GPS mea- Simmons, D. A. and Rouse, J. R.: Geomagnetic measurements made on the moving ice shelf Simmons, D. A.: Flow of the Brunt Ice Shelf, Antarctica, derived from Landsat images, 1974– 5 30 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent flow units. Cumulative length is due to the ff Clear indicator of thetime maximum period. ice-shelf Sequential extentice images for margin can to a track develop particular an the understanding fluctuation of ice-front of dynamics. Typically the formed when the ice exceedsthreshold. a Fractures temperature-dependant form perpendicularmum to tension. the direction ofTypically maxi- formed when the ice exceedsthreshold. a Rifts temperature-dependant form perpendiculartension. to the direction of maximum Formed when the stressesold. within the Form ice perpendicular exceedOpen a to crevasses given indicate thresh- the extensional flow. direction of maximumFormed tension. when icemately) to is the compressed orientation of perpendicularly the original (or fracture and/or approxi- rift. Generally indicate regionszones of of faster di ice flow and depict suture May indicate degraded surfacecaused fractures by ice-shelf or buckling surface undericance undulations compressive discussed stresses. on Signif- a case-by-caseFormed basis. by ice straining verticallystresses. under longitudinal compressive Junction between grounded ice andoften floating flexing ice. with A tidal dynamic amplitude. zone Local bedrock high where the ice shelf is grounded. Local bedrock high whereice the flow ice continues. shelf isIndicates partially grounded, surface but ablationslope and orientation can wheretime-series flow suggest of direction ice-shelf surfacecreasing is atmospheric surface meltwater temperatures. apparent. canMay Assessing indicate indicate a link increasing/ between theConsidered de- ice-shelf to surface be and subsurface. locatedto in abundant thin surface and meltwater. weak ice. Formation linked slow decay timescale relative toa the long time distance. required for ice to travel 408 407 erentiate between fracture trace and rift trace without ff 1 km in width but often exceeding tens-to-hundreds km in assessment assessment velocities change < erentiate them from ice-shelf fractures that form in floating ice. ff ter, fast-ice). Often seenfeature. Sea as ice a or icebergs brightactive often calving. sunlit visible close or to dark the edge shaded indicative sub-linear of the fracture penetrates thea distinct entire couplet thickness of of dark/light reflectance the indicating shelf. the Observed fracturethe walls. as principal ice-flow direction. Large riftsa can mixture be of filled refrozen with iceetrates sea melange; the ice entire and depth angular ofwater calved is the ice visible ice blocks. in shelf, A its inferredSurface rift opening. where pen- fractures melange or appearing sea (snow-covered) as linear lines. dark Oftenthe (open form term in or crevasse distinct is water-filled) applied zones.di to In or fractures this originating bright study, onResembles grounded a ice fracture to or riftopening on is the observed. ice-shelf Naturally surface, butpossible form where down to no ice clear of di fracturesprior and knowledge rifts. of Im- itsfracture previous trace. form, thus allLong, such linear features structures are aligned termed Typically parallel with the principallength. flow Observed direction. as dark andinate light from lines the caused by confluencerelief, shaded at of relief. bed Orig- two protuberances or flow in units regions of and high in basal regions friction. of positive Succession of dark andprincipal light flow linear direction. bands Oftentween appearing located coalescing flow near perpendicular units bedrock to Sudden or break ice-rises in surface or slope be- ponds or tend area of to intense form crevassing.gradient. Meltwater at the grounding zone where there isby a pressure ridges change on of ice the rise. stoss-side and/or crevasses in the lee of the by crevasses. Dark, flat areas onMay ice-shelf or surface may either not as form open along pre-existing or structural closed discontinuities. systems. height of the melt season. Term given to sub-linear surfacemation features where exists. Appear no as clear dark method of lines for- on the ice-shelf surface. Ice-shelf features, identifying criteria and significance. Adapted from Glasser and Datasets produced for GVIIS. Ice shelfGeorge VI StructuralNorth/CentralGeorge 1996, VI 2001, 1974, 2010 1979, 1986,South 1996, 1974, 19763, 2001, 1979, 1986, 2010 1989, 1991, Spatial ca. 1973, ca. 1989, 1986, 2002, ca. 1991, ca. 1995, 2007 2003, 2010 ca. Oct 1989, 2003–Oct ca. 2008 2002, Oct 2003–Oct 2008 Surface 1996, 2003, 2010 ca. 2010 Surface-elevation FeatureIce front Identification Sharp transition from ice shelfFracture to open ocean (summer) or sea ice (win- A clearRift opening in the ice-shelf surface. Not possible to infer whether Ice-shelf surfaceCrevasses fracture with a visibleand opening often perpendicularcrevasse to fields Fracture trace Longitudinal surface structures Significance Pressure ridges Grounding zone Ice rises ElevationIce of rumples the ice-shelf surface with disturbance Elevation to of iceSurface the flow ice-shelf indicated surface withmeltwater disturbance to ice flow indicated Ice dolines Large sub-rounded surface hollows often filled with meltwater during the Transverse structures Table 2. Scambos (2008) and Glasser et al. (2009). See Fig. 2 for full structural map of GVIIS. Table 1. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | http: The Antarctic Peninsula region (B) ) are also illustrated: Note in particular B 410 409 Marguerite Bay. C mean annual isotherm across Alexander Island ◦ = 9 − ) except for those labelled “GT##” that were otherwise previously Glaciological structural overview (2010) illustrating the dominance of longitudinal George VI Ice Shelf with localities mentioned in the text and its key tributary glaciers. the large-scale breakup followed byretreat more in steady retreat the at central the section north of ice the front south and ice concentrated front between ca. 1973 and 2010. Fig. 2. (A) structures, surface meltwater andfor ice-shelf a fractures full list towards of theRetreat identifying patterns south criteria of ice the and front northern significance (Ai) of (see and ice-shelf Table southern structures 2 (Aii, and surface features). Fig. 1. (A) The names of tributary glaciers were taken from the Antarctic Place-names Committee ( //www.antarctica.ac.uk/apc/ (source: Morris and Vaughan, 2003).displaying MB localities mentioned in the text, including the embayments of former ice shelves. unnamed. Note the positioning of the Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | from the eastern grounding line (B) 412 411 Total ice loss (bars) and rate of loss (lines) for GVIIS north and GVIIS south ice fronts Structural evolution at the north ice front illustrating widespread fracturing and rifting that that eventually form the frontalwide profile rifts during at 1979 the and northern 1989. ice Post front. 2001 there are few ice-shelf encouraged retreat. Note the long-term development of rifts Fig. 4. between ca. 1973 and 2010.ice Note front the followed major by ice a loss periodregular event of between across comparatively 1974 the steady and retreat. observation 1979 Loss atThere period, at the is although the north south no retreat ice rate relationship frontfronts. doubles is between between more ice 1990 loss and quantity 1995. or timing between the north and south ice Fig. 3. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , ca. (B) . 1 − 15 ma , ca. 1995 ± (A) (D) , 1 − ma ± 15 (C) , 1 − 414 413 35 ma ± (B) , 1 − 30 ma ± illustrating an increase in flow speed of glacier ice flowing from Riley (A) (D) and ca. 2007 Surface-speed calculations at the north ice front for ca. 1989 Structural evolution at the south ice front illustrating widespread fracturing and rifting lee- (C) 2002 Glacier as the iceperiod that front is retreated inferred upstream. toBlack be dots Less buttressed represent active by the ice the centrespeeds. dominant is point Uncertainties: flow of apparent from tracked Riley in features Glacier each that at were observation the used ice to front. interpolate surface Fig. 6. Fig. 5. side of the Eklundbetween Islands the and Eklund the Islands Englishof and Coast. the De largest Increased Atley Eklund Island. fracturing Island During and broke March rifting apart 2010, is leaving a two apparent weak independent ice ice bridge fronts (SIF1 north and SIF2). Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 1 − (B) and (B) 15 ma ± (B) , 1 − , ca. 2002 (A) 30 ma ± (A) 416 415 . 1 . There is a clear increase in speed between the Eklund Islands and De Atley − Surface elevation changes from ICESat’s GLAS repeat measurements, and (C) ma Surface speed calculations at the south ice front for ca. 1989 ± 15 mean annual change per track overthinning the signal associated is observation periods. recorded A within weak,change the non-significant noted northern towards section, the with ice distinct front.change, pockets The of with central positive section elevation the illustrates greatest complexsection, patterns losses widespread of elevation calculated and where significantwith ice-shelf negative an draft elevation observed change is retreatCoast. has thickest. of In been the the calculated, grounding coupled southern zone between GT04 and GT07 along the English (C) Fig. 8. (A) ca. 2010 Island from ca.Eklund 1989 Islands to does ca. not 2010,features change whilst used over thickness-driven for time. flow interpolating Black (Humbert, surface dots speeds. 2007) represent Uncertainties: north the of centre the point of tracked Fig. 7. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2010). Increasing density is noted (D) 2003 and 417 (C) 1986, (B) 1973, (A) Fracture, fracture trace and rift distribution and density (see Fig. 5 for structural maps) both north and west of theIncreased fracturing Eklund Islands between the in Eklund areas Islands whereice-shelf and widespread flow De thinning speeds Atley has recorded Island been is between shown. also ca. linked 1989 to and increasing ca. 2010 (Fig. 7). Fig. 9. over four time periods (