JournalofClaciology, Vo!. 44, No. 147, 1998

Ice-shelfdynatnics near the front of the Filchner-Ronne Ice Shelf, Antarctica, revealed by SAR interferotnetry

ERIC RIGNOT,1 DOUGLAS R. MACAvEAL2 I Jet Propulsion Laboratory, California Institute cifTechnology, Pasadena, Califomia 91109, USA. 2 Department ofGeojJhysical Sciences, University of Chicago, Chicago, Illinois 60637, USA.

ABSTRACT. Fifteen synthetic aperture radar (SAR) images of the Ronne Ice Shclf (also referred to as the Filchner-Ronne Ice Shelf), Antarctica, obtained by the European remote-sensing satcllites ERS-I and -2, are used to study ice-shelfdynamics near two ends of the iceberg-calving front. Interferograms constructed fr om these SAR images arc used to resolve the ice-shelf displacement along several directions in response to both ocean tide and long-term creep flow. Tidal motion is separated from creep flow using differential interferometry, i.e. two or more interferograms in which fr inge patterns common to all are predominantly associated with creep flow. Creep-flow velocities thus determined com­ pare well with prior ice-shelfvcloeity surveys. Using these data, wc studied the influence of large-scale rifts, ice rises and coastal separation on the ice-shelf flow. �lany of the large rifts that appear to fo rm the boundaries where tabular icebergs may eventually detach from the ice shelf are filled with a melange of sea ice, ice-shelf debris and wi nd-blown snow. The interferograms show that this melange tends to deform coherently in response to the ice-shelf flowand has sufficient strength to trap large tabular ice-shelf fragments fo r several decades before the fragments eventually become icebergs. In many instances, the motion of the tabular fr agments is a rigid-body rotation about a vertical axis that is driven by velocity shear within the melange. The mechanical role of the rift-filling melange may be to bind tabular ice-shelf fragments to the main ice shelf before they calve. This suggests two possible mechanisms by which climate could influence tabular iceberg calving. First, spatial gradients in oceanic and atmospheric temperature may determine where the m('­ lange melts and, thus, the location of the iceberg-calving margin. Second, melting or weakening of ice mclange as a consequence of elimate change could trigger a sudden or widespread release of tabular icebergs and lead to rapid ice-shelf disintegration.

INTRODUCTION Sanderson's (1979) hypothesis is supported by the fact that the Filchner-Ronne, Ross and Amery ice shelves ter­ Among the principal processes that influence Antarctic ice­ minate where the coastline tends to di\·erge. It does not, sheet stability are riflingand tabular iceberg calving along however, address other factors that arc necessary fo r iceberg the seaward margins of the ice shelves. These processes calving, such as the fo rmation of rifts along which tabular determine the location of an ice shclf's seaward terminus icebergs detach from the main ice shel f, tidal flexure, coastal which, in turn, influences the extent of ocean/atmosphere roughness and strain heating. heat exchange surrounding Antarctica, and the degree to We have used synthetic aperture radar (SAR) interfe ro­ which inland ice is buttressed by the ice helr. A variety of metry to study the flow regime at two ends of the calving theoretical treatments of tabular iceberg production have front of the Filchner-Ronne Ice Shelf ( FRI S; Fig. to recti­ been proposed over the years (e.g. Sanderson, 1979). Little I) fy the paucity of observations of iceberg calving and test the progress has been made, however, in the observational study of this subject since calving tends to be a rare, episodic rolc and importance of the above-mentioned fa ctors in ice­ event Oaeobs and others, 1986), and usually occursfrom por­ berg calving. SAR interferometry is a method that measures tions of an ice shelf that are difficult to access by ground ice-shclf surface displacements with a precision and spatial travcl because of cre\'asse hazards. resolution fa r in excess of other, ground-based survey tech­ Theoretical analysis suggests that flow and flexure oc­ niques (Gabriel and others, 1989). \Vc have applied this curring upstream of the calving terminus determines the measurement technique to the calving-front end-points location of the terminus, the frequency of caking and rhe because SAR images of both sites (Figs 2 and 3) reveal open size of icebergs calved. An illustration of this analysis was water along the coasts, suggesting that the ice shelf is indeed performed by Sanderson (1979) who considered an ideal ice separating fr om the coastal boundary. The flow regime in shelf confined within a coastal channel with diverging sides. these regions deviates fr om the ideal geometry considered He proposed that the calving terminus would occur where in Sanderson's (1979) analysis since the easternsite is compli­ the ice shelf separated fr om the walls of the coastal channel cated by Hemmen Ice Rise ( HIR), and the western site where the lateral, cross-channel strain rate in the ice shelf along Lassiter Coast (LC) (and Orville Coast) is compli­ reached its upper bound (i.e. the spreading rate fo r an un­ cated by ice discharge from small inlets. confined ice shclf first determined by Weertman (1957)). To construct our SAR interferograms, wc used 15 passes 405 Journal cifGlaciology

of the European remote-sensing satellites ERS-I and -2 over area extends several km upstream of HlR, and fo rms a the study areas with varying orbital geometry, and with 1, 3, 2 km wide band around HIR that can be traced several tcns and 6 day repeat periods. Our selection was motivated by of km in the lee of the ice rise. the pioneering work of Hartl and others (1994) and K.-H. I n contrast, the ice shelf along the coast ofBl does not Thiel and others (unpublished information, 1996) who appear to be uniformly bright in the SAR amplitude obtained and interpreted SAR interferograms of the study images. The upstream section of ice shelf along El (on the site near HIR, and demonstrated the applicability of SAR far left of the ERS scenes in Figure 2 and upstream of interferometry for the purpose we undertake here. Interfer­ HlR) exhibits smooth modulations in radar brightness. ograms were acquired using SAR passes with both ascend­ These suggest the presence of ice rumples or pressure ridges. ing- and descending-orbit viewing directions to construct The downstream section of ice shelf alongBl, however, is a vector displacements of the ice shelf (a single radar interfer­ radar-bright, chaotic ensemble of chasms and crevasses. The ogram measures the ice-shelf displacement only in one largest and presumably oldest chasms appear to be filled direction), and to help locate the data thus obtained within with sea ice, smaller ice-shelf fr agments and wind-blown snow. These differences in radar brightness and crevassing a precise geographic reference frame. Furthermore, we suggest that the ice shelf is continuous with the grounded compared multiple interferograms to distinguish the effects ice ofBI upstream ofHIR and separates from the grounded of tide and creep flow on the ice-shelf deformation regi me. ice ofBl at a point adjacent to the downstream end of HIR. The observations (SAR interferograms) presented here A 5 km wide strip of ice slightly darker than the ice shelf are the basis of companion papers (Hulbe and others, in appears along the ice fr ont in Figure 2. This is "fast ice", an press; MacAyeal and others, 1998) which apply finite-ele­ apron of sea ice that is rigidly fixed to the ice shelf. Out­ ment modelling methods to investigate further the board of the fa st ice, the We ddell Sea appears in Figure 2 to dynamics of ice shelves. In our presentation, we identify be filled with large, packed sea-ice floes. The interferometric several fe at ures of ice-shelf flow that appear to play impor­ data obtained in that area suggest that the sea-ice cover was tant roles in determining iceberg calving and separation of continuous at the time of observation, with no open leads the ice shelf fr om its confining coast. These fe atures include (phase coherence and fr inge visibility would be lO'vv if open the influence of rifts, the apparent mechanical integrity of water were present). These data also show that fa st ice sea ice that fills rifts, the rigid-body rotation of ice-shelf appears to be pushed ahead of the ice fr ont at the same fragments that appear to be stranded within this sea ice, velocity as the ice shelf. strain heating within velocity shear layers along coastal boundaries, and the vibration modes of the ice shelf in response to tidal forcing.

Physical setting of study areas

Figure I shows the location and satellite-image geometry of two study areas on the eastern and western flanks of the calving fr ont. The complexity of ice flow in these study areas is indicated by the degree ofrifting visible in the annotated SAR images (amplitude images) shown in Figures 2 and 3.

Hemmen Ice Rise The study area on the eastern flankof the calving front (Fig. 2) contains HIR and part ofBerkner Island (BI) which is the largest ice rise in Antarctica (Swithinbank and others, 1988). BI rests on a bed that is 200-300 m below sea level (Hoppe and Thyssen, 1988), and reaches 700 m elevation along the eastern edge of the SAR scene. HlR is an oblong feature less than 100 km2 in size, consisting of stagnant, grounded and crevasse-free ice. The ice shelf surrounding HlR and El has a thickness ranging fr om <200 m at the ice front to about 400 m at the southernlimit of the study area (see Vaughan and others, 1995, fig. 3). North of HlR, the FRIS is broken by large (up to 4 km Fig. 1. Map cifthe FRIS. Black shading denotes open or sea­ wide) rifts presumably fi lIed with a melange of sea ice, ice­ ice covered ocean; white region denotesfloa ting ice she!!; gray shelf debris, wind-blown snow, and small tabular iceberg shading denotes grounded ice sheet or ice-free, sub aerial fragments (Hartl and others, 1994). These rifts seem to form ground. Boxes denote regions covered by SAR imagesfor as­ the boundaries of large tabular ice-shelf fr agments that are cending and descending orbital passes if the ERS sateLLite. tens of km in horizontal span and appear to be precursors of The box near HIR which covers the most open ( or sea-ice cov­ tabular icebergs. The same fe atures, including the melange ered) water represents the image obtained during an ascend­ and the tabular ice-shelf fr agments, are visible in Landsat ing pass. (An orbital pass is termed ascending when the nadir multispectral scanner (MSS) imagery (e.g. Sievers and point below the moving sateLLite moves to more northerly lati­ Bennat, 1989; Hart! and others, 1994). tude) The box near Le which covers the least open (or sea-ice The ERS SAR amplitude images of HIR (Fig. 2) reveal covered) water represents the image obtained during an as­ heavily crevassed areas, or radar-bright zones, that define a cending pass. The other two boxes represent the Images "boundary layer" surrounding HlR. The highly crevassed obtained during descending jJassescif the sateLLite. 406 Rignot and MacAyeal: lee-shelf dynamics near front ofFRIS

I km/a -

Berkner Island 10 km

Fig. 2. ERS-/ SAR images of HIR, collected in February 1992. Upper liftpanel: ascending pass (orbit 3026,frame 5499); upper right panel: descending pass (orbit 3028,frame 5301). Lower right and lower leftpallels: sketch maps showing ice shelf (white shading), grounded ice (gray shading) and open or sea-Ice covered water (black shading). The direction labe/ed R denotes the range direction or line rifsight oflhe radar.The direction labeLed J'idenotes north. Dark regiolZs in the lip/mleft jJone/ correspond to sea ice. Brighl regions correspond 10 crevasses, rifts and areas rifdisturbed ice size!! Vectors denote fm-existing ice-shelf velocity estimotesjrom Vallglzan and otlzers (1995). SAR image data are copyrighted!J.y ESA, /99 6.

Lassiter Coast Tides vs ice-shelf creep Ice-shelf fe atures along LC are displayed in Figure 3, and The southern reaches of the We ddell Sea display a mixed are similar to those pre,·iously described along HIR and diurnal and semi-diurnal tide (Smithson and others, 1996; BL The lack of an ice rise in the Le area is consistent with Robertson and others, in press). Tidal amplitude is greatest thc greater water depths of the vVeddel1 Sea in this area. along the coast ofB! and the interior boundary ofthe FRIS \Vhereas ice influx across the boundary ofB! is negligible, and decays toward two amphidromic poims located at the ice shelf along LC is influenced by ice influx fron1. three approximately the midpoint of the ice from and fa rther inlets: Hansen Inlet to the south, Gardner Inlet in the center north in the \ Veddell Sea, fo r the sem i-di u rnal and diu rnal and Nantucket Inlet to the north. The mountain ranges constituents, rcspectively. The phase of the semi-diurnaltid e along the ice-shelf margin exceed 1000 m in elevation. circulates clockwise around the amphidromic point near Pronounced radar-dark patches signifying large surface the ice front, and progresses from north to south in the study depressions or snow-covered chasms are seen at thejun ction area alongB! and HIR, and vice ,·ersa along Le.The phase between Hansen Inlet and the FRIS, directly in the wake of of the diurnal tide progresses from east to west and shows Dodson Peninsula (Fig. 3). These depressions or snow-cov­ little variation within the study areas. ered chasms increase in size and width downstream from Tidal currents in the Weddell Sea are significantly influ­ Dodson Peninsula, and de,·elop into coastal rifts near where enced by the topographic modifications imposed by the the ice shelf flows past Bowman Peninsula. These rifts initi­ FRIS. Strong tidal currents, up to m s \ are expected ate the rupturing and separation of the ice shelf from the I grounded ice along the coast. The rifts visible in the SAR along the ice-front area in association with the change in imagery are initially shorter than those along HIR, but ex­ water-column thickness and the possible resonant excitation hibit a similar type of regularity. Ta bular ice-shelffragments of topographic vorticity waves by the diu rnaltide (Robcrt­ of varying size up to 10 km in diameter are stranded within son and others, in press; see also MacAyeal, 1984, which dis­ the rifts. As along HIR, the tabular ice-shelf fr agments ap­ cusses similar amplification of currents in the Ross Sea). pear to be bound together by sea ice and wind-blown snow These currents have a strong influencc on the oceanic heat to fo rm a melange that covers the sea surface within the transfer across the ice front (Thyssen, 1988). They may addi­ rifts. tionally have a direct mechanical influence on the ice shelf 407 Journal cifGlaciology

, � - " r ... ,..- . '''�.'�--�

'-, ' ,-- '1,/ IIOkm 1 J----i 10 km // I---l

FRIS

FRIS Dodson Peninsula

Fig. 3. ERS SAR images cifLe, collected inJanumy 1996. Upper liftpanel: ascending pass (orbit 24284,jrame 5589); ujJPer right panel: descending pass (orbit 24044,jrame 5193). Lower panels: sketch maps showing ice she!! (white shading), grounded ice (gray shading) and open or sea-ice covered water (black shading). Vectors denote pre-existing ice-shelf velocity estimatesjrom Vaughan and others (1995). FRIS: SAR image data are copyrighted by ESA, 1996. through fr ictional drag which acts cyclically in the horizon­ METHODS tal plane. The velocity of the ice shelf in the western limit of the Interferogratn.construction I scene containing HIR (Fig. 2) is 1050 ± 20 m a- , with an azi­ muth of 55° (Thyssen,1987) from north. The velocity vector Table I describes the fr ames, orbits and other relevant is within 30-40° of the radar line of sight in both ascending parameters of SAR images used to construct the interfero­ and descending modes (see Table I). Over the 3 day interval grams of the study areas. Each SAR pass over HIR is sepa­ between SAR images used to construct the interferograms rated by 3 days because the data were acquired during the of the HIR area, the ice shelf moves by creep alone approx­ 1992 3 day repeat-pass "ice phase" of ERS-l. Each ascending imately 8.5 m at the extreme eastern limit of the images vs pass was acquired two orbits after the descending pass, almost no motion on the grounded ice. This motion trans­ which is 2 hours later. SAR images of LC were acquired lates into a 3.3 m diff during the 1995-96 ERS "tandem phase", wh en ERS-2 fo l­ erential displacement between the ice­ shelf center and the ice-shelf margin when projected in the lowed ERS-I along the same orbit 1 day later. Hartl and radar line of sight. The tidal displacement over the same others (1994) and K.-H. Thiel and others (unpublished 3 day period is 1 m or less in the vertical (see discussion information, 1996) showed additional interferograms of the below) and is less when resolved into the SAR line-of- sight HIR study area. direction which is 23° off the vertical. The interferograms of The ERS SAR data were combined to produce radar in­ HIR are thus expected to be dominated by creep flow. terferograms at a sample spacing of 40 m x 40 m on the I Ice velocity in the LC scene is about 920 ± 30 m a ,with ground and covering an area 100 km x lOO km in size. Cyclic an azimuth of 33° (Vaughan and others, 1995). Unlike cir­ gray-scale patterns (i.e. fr inges) in the interferograms of cumstances for the HIR region, the time difference between Figures 4 and 5 represent phase difference between initial SAR images used to construct interferograms of the LC and final SAR images separated by a time interval (here region is only I day. Over this short time period the diffe 3 days) over which the reflecting surface of the ice shelf ren­ tial displacement betwcen the margin and the ice-shelf may move toward or away fr om the satellite's vantage point. center due to creep will be about I m along the radar line of Each repetition of the gray-scale cycle is called a "fringe" or sight,which is comparable to the amplitude of the tidal dis­ a "fringe line", and represents a change of 27f in the phase placements. The interferograms of LC are thus expected to difference. The wavclength A of the radar data is 56 mm. reveal an equal mixture of ice-shelf tidal motion and creep An ice-shelf surface displacement of A/2 = 28 mm along flow. the radar range direction (look direction) during the time

408 Rignot and MacAyeal: Ice-shelfdynamics nearfront V FRIS

Table 1. Characteristics qfthe ERS data used in this study. El and E2 mean ERS-l and -2, respectively; "rif" designates a riference scene for all interJerogram. Asc. and Desc. designate ascending-pass and descending-pass geometry, respectively. The B.lis the component qfthe orbital baseline which is meas­ lIred perpendicular 10 the radar line qf sight. For the 6 day

ascending-pass interjerogram, B.l is -40 + 9 = -31 m. (N ega­ tive B.l indicates that the altitude vthe second orbit was lower than that V the firs!.) For tlte 9 da} descending-pass inter­

jerogra171, tlte B.l is 40 - 63 = -23 m. For the ascending-/Jass 171 tidal-displacement interJerogram,the B.l is -40 - 9 = -49

Sile Sal/Orb/Fram. Dale D� Pa»

m

HIR [Satellite altitude: 807 km

Pixel size on the ground: 40 m

I neidence angle at center: 23.r

Ti"ack a/" Ase. pass: 66.07

Ti"ack of Dese. pass: -112.8

Near-range distance: 850 km

Earth radius at nadir: 6357 km J

El 2983-5+99 10 Feb. 1992 -9 Ase.

El 3026-5+99 13 Feb. 1992 re/" Ase.

El 3069-5+99 16 Feb. 1992 -+0 Ase.

El 29+2-.1301 7 Feh 1992 +63 Dese.

El 2985-5301 10 Feh 1992 -82 Dese.

El 3028-5301 13 Feb. 1992 ref Dese.

E1 3071-5301 16 Feb. 1992 +40 Dcsc.

I.e l Satellite altitude: 812 km

Pixel size on the ground: +0 m

I neidcnce angle at eenter: 23.+

Ti'aek o/" Ase. pass: 50A

'lhck o/" Dese. pass: -131.3

i'\ear-rangc distance: 856 kill Earth radius at naclir: 6358 kill] Fig. 4. InterJerograms qf [-lfR_ Upper panel: ascending-pass geometry (orbils 3026-2983, Jrame 5499, 3 del] time inter­

El 235-13-5193 15 Jan. 1996 ref Dese. val). Lower jJaneL: descending-/Jass geometlY (orbits 3028- E2 3870-5193 16.1an. 1996 +238 Dese. 2985, Jimne 5301, 3 da.y time interval). The line qf sight, or

El 2+0 f+-5193 19 Feb. 1996 ref Dcsc. range direction, V the radar Jor ascending and descending or­

E2 +372-.1193 20 Feb. 1996 +222 Dese. bit interferogramsis denoted in Figure 2, lower panels. Figures

El 23783-5589 I Feb. 1996 ref Ase. 8-12 show subsamples qfthe u/JjJer panel(ascending-pass in­

E24110-5589 2 Feb. 1996 +168 Ase. terferogram).

El 2+28+-.1589 7 ;\lar. 1996 ref' Ase. on El). The spatial baselines from the 1992 HIR data wc re E2 +611-5589 8 1996 +181 A se. Idar. typically short (�1O m), which means that the intcrfero­ grams of HIR were weakly innuenced by surface topogra­ interval between the two SAR images induces a two-way phy (the phase varied only by one-tenth of a fringc for a change in the radar travel distance and a 27f phase diITer­ 100 m variation in elevation). The baselines of the ERS cnce. tandem data acquired O\'er Le were large (�100-200 Ill ), but similar in magnitude and sign in the diITerent interfero­ InterferOInetric baseline and differencing grams, so that diffcrelllial interferograms also had a diITer­ ence baseline of only a few meters. The spatial baselines of the interferograms, i.e. the separa­ Phase unwrapping ("unwrapping"denotes the process of tion of the two positions of the satellite when the SAR identifying the absolute phase of each fringe line through images were acquired (1able I), were initially calculated counting fringe lines along paths that lead away from using the precise-orbit information distributed by the locations of known \'e!ocity, such as the zero-\'c1ocity European Space Agency (ESA) (i-olll the German Proces­ regions of HIR and El) was performed usi ng Goldstein sing and Archiving Facility (DPAF ) and subscquently and others' (1988) algorithm in which unwanted areas (e.g. refined using control points of known ice velocity (zero sea-ice noes) and areas too difficult to unwrap (c.g. sC\'crely velocity on HIR) or elevation (prior digital elevation data crevassed regions with sharp discontinuities in phase) were

409 Journal qfGlaci% gy

tion algorithm was applied to both ascending and descend­ ing passes, and the results were compared at the grid points of the original 5 km DEM. It was then assumed that the un­ certainty in geolocation was mostly along track, due to un­ certainties in the timing of the satellite passes. Uncertainties in the positioning of images in the range direction were assumed to be negligible since the echo-time delay of the radar signal is typically measured with great precision. Along-track time delays were adjusted in the least-squares sense so that all gridpoints ofth e DEM fe ll within 1-2 pixels (100 m) of the same location in both passes. Using this com­ bination of ascending and descending passes, wc achieved a geolocation accuracy better than 100 m. All interferograms were subsequently corrected fo r sur­ fa ce topography (influencing mostly grounded ice) using the registered 5 km DEM of Antarctica (Bamber and Bindschadler, 1997). This correction worked well on the ice­ shelf area, BI and HIR. Over the rough topography of grounded ice along Le, the 5 km DEM was not sufficiently accurate and detailed to remove the topographic fr inges. Our main i l1lerest being the ice-shelf area, surface topogra­ phy in the Le scene was removed only on the ice-shelf area. The along-track time delays determined from the as­ cending/descending-pass registration were also used to regis­ ter existing measurements of the ice-shelf velocity (Vaughan and others, 1995) to the interferograms (see Figs 2 and 3 where these velocities arc plotted in both the ascending and descending passes). Thc existing velocity data provided a basis for evaluating the precision of interferometrically derived horizontal ice-shelf velocities once the tidal signal was eliminated using one of the mcthods described in the fo llowing sections of this papeL

RESULTS

A difficult challenge of this study was the qualitative inter­ pretation of the fr inge patterns fo r both HIR and Le. (Fr inge patternsare the graphical representation of an in­ terferogram, as shown in Figures 4 and 5.) As is readily ap­ preciated from the interferograms (Figs 4 and 5; sce also Fig. 5. Interferogmms qf Le. Up/m panel: ascending pass Hartl and others, 1994; K.-H. Thiel and others, unpublished (orbits 24284 and 4611, fr ame 5589, 1 day time interval). information, 1996), fr inge patterns caused by the combina­ Lower panel: descending pass (orbits 24044 and 4371,Jrame tion of creep-flow and tidal displacements, and subject to 5193, 1 day time interval). SAR images llsed to construct the the effects of SAR "look geometry", can seem incomprehen­ interJerograms were acquired using both the ERS-1 and -2 sible. Even considering that the cyclic color (or gray-scale) satellites, which were orbiting in tandem (1 day rejJeat). change in the fr inge pattern represents a "contour map" of ice-shelf displacemelll in a particular, albeit awkward, di­ masked out. The phase values were absolutely referenced rection (i.e. the satellite range direction), it is difficult to using control points on ice rises or rock outcrops where the comprehend the kinematics of the ice shelf from a typical surface velocity is near zero. interferogram. Our interpretation of the interferograms was achieved Cartographic projection by a heuristic process that combined manipulation of the interferogram data and creation of artificial interferograms The ERS radar data were projected into a cartographic using a finite-e lement model of ice-shelf creep. The model­ reference system, here a Lambert equal-area projection. To ling work is presented in companion papers ( Hulbe and help in the process, we utilized a 5 km digital elevation others, in press; MacAyeal and others, 1998). Modelling of model (DEM) of Antarctica (Bamber and Bindschadler, the creep flow and generation of artificial interferograms 1997) which was first registered to the SAR data (meaning fr om model results fo r comparison with the real interfero­ projected into the radar imaging geometry) using precise grams proved to be helpful in guiding us toward a better un­ orbit data, and subsequently illlerpolated to the natural derstanding of the fr inge patterns. However, all the sample spacing of the interfe rograms (40 m). The geoloca­ interpretations presented here, in retrospect, can be sup­ tion accuracy of this process is about 1 km. pOl"ted by reference to the interfe rograms alone. The resuits To improve the geolocation accuracy, the same geoloca- and conclusions presented here, therefore, are not contin- 410 Rignot and Ma cAyeal: !ce-shelfdy namics nearfr ont ofFR IS gent on the accuracy or applicability of assumptions made as part of thc modelling work.

Separation of tidal flexure and creep flow

As mentioncd above, the ice-shelf displacement measured by SAR interferometry is due to the combined effects of ocean tide and creep flow. To differentiate between these two effects, we exploit the fa ct that tidal displacement and creep flow have two widely different time-scales. Displace­ menL associated with tides is periodic, with periods con­ siderably less than the time-span between two SAR scans needed to construct interferograms. Displaccment asso­ ciated with creep flow monotonical1y increases through time, and has a time-scale that is essentially infinite when compared to that of the tides. These differences between tidal displacement and creep flow allow their effects to be separately identified using an interferogram differencing technique (i.e. differenLial interferometry). Differential interferometry, i.e. taking the difference between two inLerferograms of the same scene and in the same orbital geometry, was first illustrated fo r glaciology applications by Hartl and others (1994), and may be used to separate the creep-flow motion of the ice shelffrom the tidal motion. In two interferograms spanning the same temporal baseline, the deformation signal associated with creep flow wi 11 be the same (creep deformation is presumed steady and continuous in time). A differential interferogram produced from these two interferograms will therefore contain no creep-deformation signal but only a tide signal measuring a differential tidal displacemenr between fo ur different epochs (or three epochs mixed together if only three serial images are used to fo rm the two interferograms). T\",o examples of differential interferograms showing tidal motion are shown in Figures 6 and 7 fo r the HIR and Le scene. Within the interior of the ice shelf, the amplitude and spatial \·ariabilit y of tidal motion is consistent with the large-scale pallernsof tidal amplitude and phase predicted fo r diurnaland semi-diurnalti des by Robertson and others (in press; sec also Smithson and others, 1996). Along coasts, howe\·er, there are narrow boundary layers approximately 5 km wide where the ice shelf deforms more precipitously to accommodate the zero tidal range of the grounded ice. The extent of this zone is determined by the flexural rigidity of the ice, which is a fu nction of ice temperature, fa bric and thickness (Holdsworth, 1977). The limit of tidal flexing of the ice shelf defines the hinge line ( Holdsworth, 1969). The grounding line, where the ice detaches from its bed and becomes afloat, typically occu rs downstream from the Fig. 6. Upperand panels: tidal maps oJ hinge line. middle disjJlacement the HIR stu�y area, generatedfrom iffer ncing oftwo Two additional fe atures arc important to notice in the the d e interJerograms (upper panel: ascending-pass geometl), orbits differential interferograms displayed in Figures 6 and 7. 2983-3026-3069, frame 5499; middle panel: descending One is that the fr inge patternass ociated with tidal motion pass geometl), orbits 2985-3028-3071, frame 5301, 2 flOurs is simpler than that displayed in each single interferogram aft er tlte ascending jJasses). Lower panel: creep flow dis­ (Figs 4 and 5). There are fewer regions exhibiting rapid placement map of the HIR study area in a descending-pass increases in fr inge spacing. A second fe ature is that tidal dis­ geometry (orbits 2985-3028-2942, fr ame 5301) generated placement varies from onc differential interferogram to an­ by comparing two interJerograms. other, and the variation is not linear (i.e. onc differential interferogram is not simply an amplified or attenuated the ice shelfat a given epoch. Several methods are possible to version of the other). circumvent this problem. The simplest is to ignore the tidal Separating creep flow from the tidal signal is conversely signal. In the case where the time-span between satellite more difficult because SAR interferometry only measures passes coincides with a dominant period of the tide (semi­ differences in tidal displacement of the ice shelf between diurnal or diurnal), the difTcrential tidal signal should in­ different epochs, and not the absolute tidal di placement of deed be small compared to the horizontal creep motion of 4·1 1 Journal rifGlaciology

along HIR allows synthesis of a new intcrferogram with a 9 day temporal baseline (instead of the nominal 3 days). Over this longer baseline, the interfe rometric signature of the tidal motion will remain more or less the same, while the creep motion of the ice shelf will generate phase varia­ tions 3 times larger than in a 3 day interfe rogram. The tech­ nique therefore reduces tidal cont amination by a quantity proportional to the increase in temporal baseline. The inconvenience of this second method is that it requires a large number of interferograms to effectively re­ duce tidal contamination. Assuming a tidal signal Im in magnitude, and an ice-shelf velocity of 1000 m a \ which I means Im d- in the radar line of sight, at least fo ur scenes separated by 3 days are needed to reduce the tidal signal by a factor 10. A reduction fa ctor of 100 would require 40 scenes. There are fe w places in Antarctica where ERS can provide that many scenes. A third method is to assume that the ice-shelf tidal dis­ placement in a single interferogram is a linearly scaled ver­ sion of the displacement map measured from differential interfe rometry. This technique was utilized to map ice velocities on the floating section of Petermann Gletscher in north Greenland (Rignot, 1996). The same technique was utilized here. The tidal motion measured fr om a di ffe rential interferogram was normalized and the \'alues were reintro­ duced into a single interferogram using a proper scale fa ctor chosen to eliminate the tidal signal. The scale factor was determined in the least-squares sense so that interferometri­ cally derived ice velocities from the single interferogram would match prior-determined ice velocities, i.e. from ground survey or fe ature tracking methods. The control velocities we used were obtained from fe ature tracking on Landsat MSS imagery (Vaughan and others, 1995) and are displayed in Figures 2 and 3. To compare the performance of the three methods des­ cribed above, we used the prior-determined ice-shelf velocity data to predict the creep-flow contribution of the unwrapped phase at particular pixels of the interferograms. These were then compared with the unwrapped phase of the actual interferograms. For each interferogram, we com­ puted an artificial phase corresponding to each point of known velocity using

(1)

where

7r 1/J=-+b-a (2) 2

and where A is the radar wavelength (5.6 cm), v is the creep Fig. 7. Upper and middle panels: tidal displacement maps if velocity magnitude, !J.tis the temporal baseline of the inter­ the Le study area generated by differencing ciftwo intelJero­ fe rograms to which the artificial phase is compared, 'ljJis the grams (upper panel: ascending-p ass geometry,· lower panel: angle between the ice-velocity vector and the cross-track descending-p ass geometry. Black or gray masked areas denote direction, 8i is the local incidence angle of the radar illumi­ where phase could not be unwrapped (i.e. fr inge-line count­ nation (about 23°), b is the satellite track angle from true ing) due to limitations in spatial resolution rifthe data. north, and a is the angle of the ice-velocity vector fr om the ice shelf. We believe this was the case for at least one north. interferogram acquired fo r HIR (orbits 3028 and 2985). In The comparison between the methods for separating most cases, however, the tidal signal will introduce a signifi­ tide fr om creep-flowsignals is summarized in Table 2, com­ cant error in the horizontal ice velocity which varies with paring the first, second and third approaches for HIR, and position on the ice shelf. the first and third fo r Le. The best results are obtained A second method is to combine several interferograms to using the third approach. The resulting estimates of creep synthesize an interferogram with a longer temporal baseline flow are on average within 1-2 fringes of the actual creep than that provided by any single interferogram. Fo r flow observed by independent means, which is probably instance, the addition of fo ur ascending passes acquired within the error level of the control velocities used to per- 412 Rignot and MacAyeal: Ice-shelfdy namics nearfr ont cif FR IS

Table 2. Root-mean-square difference in phase, Dcf;, between The main difference between the two sites is that the zone of jJ hase corrected Jor tide and phase corresponding to pri01'­ ice-shelf rupture in the Le scene develops along the coast, determined ice-shelf velocities (Vaughan and others, 1995). specifically in the wake of Dodson Peninsula, rather than The corresponding uncertaintyin ice velocity is DV. Fo r each in the wake of an ice rise. pass over HIR, we compared single inteiferogramswith no tide correction (298� 3071, 2983, 3069), multiple inteifero­ Discontinuity iffringe sjJacingac ross rifts grams added together (9 day, 6 day ), and intelfirogmms cor­ Riftsand chasms, which penetrate the ice shelf from su rface rectedJor tide (2985 -tide, 307l-tide, 2983 -tide, 3069-tide) to base, are typically associated with "branch-cut" disconti­ nuities in the fr inge pattern. (A branch cut is a type of dis­ continuity commonly encountered in complex analysis Sile IlIlerferogralll bcp bV where a field is continuous in all regions except across a line dcg rad ma which extends semi-infinitelyfr om a single point.) In Figure 8, we show one such branch-cut discontinuity encountered HIR 71 (Asc.) 2985 2,1 1+3 at a rift that extends west from the downstream end of 3071 7.0 555 HIR. The tip of the rift, where the rift terminates within 9da)' 1.5 101 the integrated, undisturbed ice shelf, represents the origin 2985-lidc 0.6 52 of the branch cut. The eastward extension of the rift repre­ 3071-licle 3,2 213 sents the line across which fringe spacing is discontinuous. On the downstream side of the rift, fr inge spacing is wider HIR 24 ( Dcsc.) 2983 6.3 165 than on the upstream side of the rift. The ice shelf on the 3069 4,2 113 downstream side of the riftis less influenced by the drag in­ 6day 4.2 124 duced by HIR. It thus exhibits less \'elocity reduction 2983-lidc 4.1 112 toward the east. The fr inge spacing on the downstream side 3069-tide +. 1 112 of the rift is thus wider, indicating less shear in the flow.The rate at which the rift widens is associated with the strength of the fr inge-line discontinuity across the rift. fo rm the comparison. In terms of ice velocity, the rms error is 75 m a I on average, or 15 % of the mean velocity of the ice Fringe fJat!erns wit hin rifts shelf in the two study areas. Examples of interferograms West of HIR, the fr inge spacing varies abruptly at the trans­ corrected fo r tide using the third method are shown in Fig­ ition between adjacent rift zones. Interferometric fr inges are ures 6, 7 and 12. still visible in the voids filled by what appears to be a me­ There are reasons to believe that this third method pro­ lange of multi-year sea ice, ice-shelf fr agments and wind­ \'ides only a first-order removal of tidal effects, which could blown snow. \Ve term this material "ice melange" to draw at­ probably be summarized to the average tidal amplitude tention to its variable composition. The highly textured and O\'er the whole scene and the patternof tidal bending at the bright radar returnfr om this region is indeed inconsistent junction between ice shelfand grounded ice. As displayed in with the presence of open water. Landsat imagery also indi­ the interferometric examples of tide (Figs 6 and 7), and as cates the presence of an ice mixture in that region, not open predicted by tidal models (Robertson and others, in press), water ( H artl and others, 1994). the tidal deformation regime of a large ice shelf is consider­ The fr inge pattern covering the ice melange appears to ably more complex than e11lertained through our simple lin­ ear model. Despite this limitation, the tidal correction applied to the data was subsequently fo und to be sufIiciently accurate fo r the study undertaken here, which is to qualita­ tively explain the patterns of interferometric fr inges and obtain first-order agreement between a fo rward finite-ele­ ment model simulation of ice-shelf flow and the interfero­ metrically derived creep-flow \'elocities ( H ulbe and others, in press; MacAyeal and others, 1998). [n other words, the results and conclusions of this study would not be changed if tides were removed more accurately by some other means. To achieve a higher lewl of precision in creep-flow \-eloci t y, somewhat comparable to that achie\'able on grounded ice, we would have to take into account the vibra­ .... tional character of the ice shelf. This type of work is the ::J subject ofongoing research. U ...c: u c ro Analysis of fr inge patterns '­ ..0

Figures 4 and 5 display examples of single interferograms - rift made of the HIR and Le study areas using both ascending­ and descending-pass SAR images. In Figures 8- 12, sub­ scenes of the interferograms are shown to better illustrate Fig. 8. Discontinuity ciffringe spacing across rifts ( subsample the interpretation of various fr inge paLlerns. Interfe rograms of Fig.4, upper left panel). The riftshown here is located at the of HIR (Fig. 4) and Le (Fig. 5) show similar fr inge patterns. downstream end cifHIR and extends west.

413 Journal ifClaciology

Fig. 9. ContinuityqfJ ringe patterns within rifts(s ubsample qf Fig. 4, upjmlift pane l). Th e continuousgeo metric patternsign ifies that ice melange, the mixture ofsea ice, ice-shelffragm ents and wind-blown snow, is diformingcoheren tly. be spatially continuous, as illustrated in Figure 9. Fr inges case the thickness and mechanical integrity of ice melange recorded in the melange located close to the ice front are less has not yet been affected by the ice-front conditions, or that distorted than those recorded in the melange filling rifts processes other than basal melting of ice melange are impor­ elose to HIR. The rifts fo und close to the ice front are pre­ tant in the final separation and release of tabular icebergs sumably older than those in the wake of HIR, because most from the ice shelf. Tidal waves, for one, probably have an in­ rifts seem to initiate along the flanks of HIR and subse­ fluence on the stability and mechanical integrity of the rifts quently grow in width and extent as they move seaward and on the ice melange when they get trapped underneath with the ice-shelf flow. the ice shelf proper in the ice-front region. The distinct fringe patterns within rifts imply that ice melange has sufficient mechanical competence to deform Rigid-body rotation in response to differential motion on either side of the rift. The elearest evidence fo r the mechanical integrity of ice This suggests that rifts, some of which ultimately become melange is visible in the fringe patterns downstream of detachment boundaries fo r tabular icebergs, can be filled HIR (Fig. 10) where large tabular ice-shelf fragments are in­ with a material that can bind the two sides of the rift duced into a state of rigid-body rotation by stresses trans­ together and that can temporarily halt tabular-iceberg mitted thruugh the melange that surrounds the fragments. detachment. The character of fringe patternswithin ice melange ap­ pears to evolve with the presumed age of the rift within which the melange is located. Fringe patternsin the immedi­ y ate wake of HIR, where rifts are presumably young, are chaotic, and this suggests that the ice melange in young rifts is thin, mechanically weak and subject to spatially discon­ tinuous deformation. Fringe patterns toward the ice front, where rifts are presumably older, are more continuous than in the immediate wake ofHIR, and this suggests that the ice melange in old rifts is strong. To account fo r the eventual calving of ice-shelf frag­ r ments, we suggest that ice melange which fills rifts may eventually weaken as the rifts propagate toward the ice front ice-shelf fringe where basal melting becomes a significant source of mass fragment pattern loss (Thyssen, 1988). Basal melting of a fe w m a I should ex­ ceed the nat ural growth rate of sea ice due to heat losses into x the atmosphere, reduce its thickness and weaken its me­ chanical competence. Ultimately, basal melting (and sur­ Fig. lO. Eff ect ofrigid-body mtation on thefringe pattern dis­ fa ce melting where possible) will nearly eliminate the ice j)layed by tabular ice-shelffi·ag ment. Component ifvelocity in melange and turn the rifts into detachment boundaries satellite range direction is v = Ox, where n is the angular which liberate tabular icebergs. velocity qf rigid-body rotationabout an axis located at the ori­ Interestingly, however, our interfe rograms do not reveal gin of the x-y plane, and x is the coordinate perpendicular to distorted or chaotic fringes within rifts close to the ice front, the range, or satellite-look, direction. The fringe pattern asso­ as would be expected if ice melange were weakening there. ciated with rigid-body rotation consists of uniformly spaced This result suggests either that our observations were made parallelfr inge lines which are aligned with the range direction well in advance of the next major calving event, in whieh ( after Pe ltzer and others, 1994). 414 Rignot and Ma cAyeal: Ice-shelfdy namics nearjr ont rifFRIS

Rigid-body rotation about an axis that is perpendicular to ofHIR exhibit uniformly spaced fringe lines that are paral­ the horizontal plane is signified in fr inge patterns by uni­ lel to the range direction. This patternis consistent between fo rmly spaced fr inge lines that are aligned with the radar interfe rograms representing motion over different time in­ range direction or, equivalently, that are perpendicular to tervals and suggests that the rigid-body rotation is caused the satell ite night direction ( Peltzer and others, 1994). by the velocity difference between ice-shelf now west of The relationship between fr inge-line geometry and HIR and the stagnant boundary of Bl. In other words, the rigid-body rotation within the horizontal plane is illustrated fragments are being rolled along the boundary of El by in Figure 10. Let us consider an ice-shelffragment in the hor­ fa ster-moving ice shelf that borders them on the west. The izontal, x-y plane that rotates rigidly about a vertical axis at regular fringe paLLerns do not appear in differential interfer­ the origin with angular speed \2. We suppose that the y axis ograms showing tidal motion ( Figs 6 and 7). This result con­ is aligned with the ground-range direction. The \'Clocity, il, firms that the rigid-body rotation is due to motions induced of ice motion at any distance r from the origin is by creep flow and is not a transient phenomenon associated, fo r instance, with tidal-current drag beneath the ice shelf. il=\2xr , (3) Fo r some fragments, most notably those closest to HIR in where D is the angular velocity \'ector, which points along Figure 11, the rotation is clearly driven by coupling with fa st­ the axis of rotation, perpendicular to the horizontal plane, moving ice west ofHIR. The fast-moving ice induces motion and r is the position \'ector, as shown in Figure 12. The SAR in the ice melange, which consequently drags the western interferogram is sensitive only to the component of motion ends of the fr agments scaward. The eastern ends of the fr ag­ in the y direction. The y component ofil, v, is determined by ments are retarded by contact (either direct or through ice­ the angle e between the x axis and the "ector r: melange-filled rifts) with slower-moving ice shelf (or BI ) to v = I il I cos f). ( 4 ) the east. The net effect is to spin the fragments developing on both sides of HIR in a clockwise manner. Substituting Equation (3) into Equation (4), and making use Fo r one ice fr agment shown in Figure 11, there is an am­ of the definition x = r cos e, gives biguity as to whether the rigid-body rotation is driven by ice v = \2x. (5) melange, or dri\'cn by \'orticit y of the shear margin along BI Fr inge lines are therefore contours of motion in the range (i.e. au/ ay - DV/ ax where u and v are the x and y compo­ direction. Equation (5) implies that fr inge lines associated nents of \'elocit y in the x-y plane). In the absence of ice me­ with rigid-body rotation are aligned with the y axis and lange, i.e. open water, vonicity in the shear margin would have a uniform spacing that depends only on n (i.e. be transmitted to the ice-shelf fragment across the narrow av/ ax = \2 ). ice bridgc designated in Figure 11. The stiffness of the ice in As shown in Figure 11, ice-shelf fr agments downstream the ice-shelfrragment would ensure that the \'orticity would

Fig. IJ. Ice-shelfjra gments in rigid-body rotation (subsample rif Fig. 4, upper lift panel). Fo r somejragments, this rotation is driven by coupling ( indicated �Y white circular arrows) between thejr agments and theja st-moving ice shelfwest rifH IR aj forded by the mechanical integrity rif ice melange whichjills the riftsy stem downstream riffIfR . Rotation rifsome largerjr agments (e.g. the one in the center cif the scene) may also be driven �Y vorticity (indicated by black circular arrow) in the shear I11mgin along El which is transmitted to tltei'agments across ice bridgessuch as that shown in the sketch. f 415 Journal qfGlaciology remain uniform within the fragment at the value equal to previously). The magnitude of shear strain rates signified the vorticity at the ice bridge. Uniform vorticity is equiva­ by fringe spacing west of HIR changes conspicuously at a lent to rigid-body rotation. distance of about 5 km from the ice-rise boundary where By counting fringes across an ice raft exhibiting the flow is zero. This width is comparable to that inferred from characteristics of a rigid block rotation about a vertical axis, velocity measurements collected along Crary Ice Rise we can obtain a direct measure of the angular velocity of the (Bindschadler and others, 1988; Bindschadler, 1993), but ice raft, i.e. n, even iflimited to a single viewing angle of the about twice as wide as that suggested by the bright, crevassed

1 I. satellite. In the HIR scenes, n is 2-30 a or 0.01 0 d The zone indicated in the radar amplitude data (see Fig. 2). interferograms are therefore sensitive indicators of rigid The small width of"shear layers along HIR suggests that block rotation. If n exceeded the above values by a factor 2 the ice shelf is more easily deformed in a boundary layer or 3 we would in fact be unable to resolve individual fringes which surrounds HIR. Softening in boundary layer along on the ice raft, which would then appear uncorrelated the margin of fa st-moving ice is observed elsewhere, such as because of phase aliasing. along the margin of Ice Stream B (Echelmeyer and others, On the radar scenes collected along LC (Figs 3 and 5), 1994). Several processes could contribute to modificationof rigid-body rotation of ice rafts is also apparent. The size of the rheological properties at the margi ns of HIR. First, the ice rafts along LC is considerably smaller than along HIR, ice rise is small. It would be less able to slow the surrounding and this suggests that ice melange in the LC scene does not ice shelf than would a larger ice rise like Br, or Roosevelt bind large ice rafts within the general flow of the ice shelf as Island and Crary Ice Rise on the Ross Ice Shelf. Fa ster flow effectively a does the melange in the HIR scene. We suggest past the sides of the small ice rise would undoubtedly pro­ that this diffe rence may be attributed to different mean duce surface and basal crevassing and associated infiltration climate conditions between LC and HIR. Fu rther discus­ of seawater, both of which would have a softeningeff ect on sion of the comparison between HIR and LC is presented the ice shelf. Second, large strain rates would contribute to in a companion paper (Hulbe and others, in press). strain heating and ice-fabric development. The residence time of ice within the boundary layer along HIR is on the Shear layers order of 50 years. This would be sufficient to allow strain­ 3 Figure 12 displays the shear zones on either side ofHIR (see heating rates similar in magnitude to the 10 3 W m- magni­ upper right panel of Figure 12 where tidal flexure effects tude calculated fo r Ice Stream B, where velocity magnitudes have been removed fr om the interferogram, as discussed are similar to those of the HIR flow regime. That amount of

Fig. 12. Narrowfringe spacing along the west coast qf HIR, suggesting ice sriftening (subsample qfFig. 4, upper liftpan el). Upper lift panel: inteiferogram qfscene combining tide and creepflow. Upper right panel: estimate qf creepflow obtained by combining several inteiferograms (orbits 2983 -3026 -3069,frame 5499). Lower right panel: difference between two inteiferograms (orbits 2983-3026-3069,jimne 5499) representing tidalflexure only. Lower Liftpanel: sketch map qfscene. Black masked areas denote wherefringe unwrapp ing could not be pe1formed.

416 Rignot and MaeAyeaL: fee-shelfdy namics nearJront of FRIS heating would warm the ice by several degrees (e.g. Echel­ Spatial gradients in oceanic and atmospheric tempera­ meyer and others, 1994). Third, constant cyclical tidal flex­ ture could determine where ice melange melts or disinteg­ ure along the boundary would allow additional strain rates, thus influencing where tabular ice-shelf fr agments heating, crevassing and brine infiltration. The tidal flexure will detach fr om the ice shelf. Enhanced melting and weak­ zone along HIR (e.g. the area of high fringe-line density ening of ice melange from climate warming could trigger a surrounding HIR shown in Figure 12, lower right panel) ap­ more widespread and sudden release of tabular icebergs. Ice pears to correspond to the zone of highest strain rates in the rafts bound together in the wake of HIR, for instance, may creep flow (e.g. Fig. 12, upper right panel). This correspon­ detach rapidly as a result of ice melange weakening. Rifts dence adds fu rther support to the notion that cyelic tidal would then propagate upstream of the ice rise. Instead of flexure has an influence on the rheological properties that protecting the ice shelf by creating a zone of flow compres­ govern ice-shelf creep flow. sion upstream of the ice rise, the ice rise would start acting as an indenting wedge that would help fracture the ice shelf. This kind of ice-shelf fracturing was observed in the region CONCLUSIONS surrounding Buffer Ice Rise on the floating tongue of Fiem­ ing Glacier during the collapse of the Wordie Ice Shelf Our interferograms suggest that ice-shelf rifting, a long­ (Doake and Va ughan, 1991). term process that culminates in tabular iceberg release, is The Larsen Ice Shelf similarly lost a large fraction of its strongly influenced by sea ice and other types of ice, which area in two nearly simultaneous calving events in 1995, pos­ fi 11 the rift. Indeed, deformation of sea ice within the rifts sibly as a result of its northern location on the Antarctic suggests that the sea ice binds together large tabular ice­ Peninsula, where climate has warmed over the past 50 years shelf fragments. The eventual detachment of these frag­ (Nature, 1995; Va ughan and Doake, 1996). Each event dis­ ments as icebergs thus appears to be determined in part by played a distinct calving style. In the first style, approx­ the dynamics of the melange which fills rifts. imately 25% of Larsen B between Jason Peninsula and Interfe rometry of Antarctic ice shelves provides a highly Robertson Island calved as a single tabular iceberg. The de­ detailed picture of ice flow and tidal deformation and has tachment of this large tabular iceberg is thought to rep­ revealed important aspects of ice-shelf beha\·ior that were resent the "normal" (though episodic) style of calving previously unknown. The most striking aspect of ice-shelf expected in steady-state conditions Uacobs and others, behavior revealed by the interferograms of the FRIS 1986; personal communication from C. L. Hulbe, 1997). Co­ studied here concerns the development of ice-shelf rifts, incident with that calving event, a second style of calving 2 which eventually become tabular iceberg detachment was displayed. Approximately 1500 km of the Larsen Ice boundaries. The ice-shelf rifts presumably develop as a Shelf ( Larsen A) north of Seal Nunataks disintegrated into result of the presence of an ice rise ( HIR) or due to coastal hundreds of small, non-tabular pieces. roughness ( LC). The interferograms of the HIR area, in The suddenness of the second style of calving displayed particular, reveal that the voids within rifts arc filled with by Larsen A in 1995 may also implicate the melting of ice 2 ice, which we call ice melange (a mixture of sea ice, ice-shelf melange. The sudden b,-eak-up of 1500 km of ice shelf into fragments and wind-blown snow which gives a cluttered hundreds of small fr agments is difficult to explain without a SAR amplitude image). Creep flow in the ice shelfis discon­ pre-existing network of fractures that would have acted as tinuous across rifts; however, much of the discontinuity is separation boundaries. Ice-melange filling may have held accOl11modated by deformation of the ice melange within the f,-agments together until weakening by an unusually the rifts. This suggests that ice melange possesses a mechan­ warm summer (suggested by the large number of melt ical integrity and is capable of supporting a spatially contin­ ponds observed on Larsen A prior to break-up) allowed uous velocity field. What the specific rheological properties the ice melange to fa il in a large storm. The interferograms of the ice melange may be (e.g. the strength in shear and of the FRIS demonstrate the mechanical importance ofice tension) remains to be determined through fu rt her study. melange to rift stability near the ice front, and allow us to Nevertheless, the fact that ice melange fills rifts and is cap­ speculate that ice-melange fa ilure as a result of environmen­ able of continuously deforming leads us to speculate that tal conditions was possibly onc of the factors in the break-up melting of the ice melange may be a means by which the of Larsen A. If our speculation is correct, then ice melange oceanic and atmospheric environments can control the which holds the FRIS together near its calving margin may location of Antarctica's ice-shelf calvi ng fronts. A key argu­ be a possible weak link in ice-shelf stability which has here­ ment in support of this speculation is the fa ct that our inter­ tofore not been considered. fc rograms reveal that large ice-shelf fragments that, in the absence of the surrounding ice melange, would be tabular icebergs, are literally rolled along the stagnant margin of ACKNOWLEDGEMENTS BI as a result of motion transmitted through the melange frol11 fa ster-flowi ng ice shelf to the west. This work was performed at theJet Propulsion Laboratory, \ Vhile the presence of ice melange within rifts has been California Institute of Te chnology, under a contract with observed using aerial photographs and Landsat images, the the National Aeronautics and Space Administration mechanical competence of the melange, and the role it may (NASA). Support fo r the University of Chicago component play as a binding agent in heavily fractured ice shelf, is of this project was provided by NASA (NAGW-5005). The revealed fo r the first time by SAR interfe rometry. This ERS data needed fo r this research were provided by the opens the door to speculation about iceberg calving styles ESA ( ERS-I/2 project code: A02.USA.160). Wc thank C. and the significance of the collapse of the Wo rdie Ice Shelf Werner fo r providing a SAR processor to generate the ( Doake and Vaughan, 1991) or recent events on the Larsen SAR interferograms, A. Gabriel fo r help in the project's in­ Ice Shelf (e.g. Vaughan and Doake, 1996). ception, C. Hulbe fo r her assistance in revising the manu- 417 Journal qfClaciology script and fo r her advice in the development of speculations Hulbe, C. L., E. Rignot and D. R. ;\lacAyeal. In press. Comparison of ice­ concerning the calving style ofLarsen A,]. Bamber fo r pro­ shelf creep flow simulations with ice-front motion of the Filchner­ Ronne Ice Shelf, Antarctica, detected by SA R interferometry. Al1n. 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