late quaternary submarine bedforms and ice-sheet flow in gerlache strait and on the adjacent...

JOURNAL OF QUATERNARY SCIENCE (2004) 19(4) 397–407Copyright � 2004 John Wiley & Sons, Ltd.Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.831

Late Quaternary submarine bedforms andice-sheet flow in Gerlache Strait and on theadjacent continental shelf, Antarctic PeninsulaJEFFREY EVANS,* JULIAN A. DOWDESWELL and COLM O COFAIGHScott Polar Research Institute, University of Cambridge, Lensfield Road, Cambridge CB2 1ER, England

Evans, J., Dowdeswell, J. A. and O Cofaigh, C. 2004. Late Quaternary submarine bedforms and ice-sheet flow in Gerlache Strait and on the adjacent continental shelf,Antarctic Peninsula. J. Quaternary Sci., Vol. 19 pp. 397–407. ISSN 0267-8179.

Received 16 September 2003; Revised 18 December 2003; Accepted 23 December 2003

ABSTRACT: Geophysical data from Gerlache Strait, Croker Passage, Bismarck Strait and theadjacent continental shelf reveal streamlined subglacial bedforms that were produced at the bedof the Antarctic Peninsula Ice Sheet (APIS) during the last glaciation. The spatial arrangement andorientation of these bedforms record the former drainage pattern and flow dynamics of an APIS outletup-flow, and feeding into, a palaeo-ice stream in the Western Bransfield Basin. Evidence suggests thattogether, they represent a single ice-flow system that drained the APIS during the last glaciation. Theice-sheet outlet flowed north/northeastwards through Gerlache Strait and Croker Passage and con-verged with a second, more easterly ice-flow tributary on the middle shelf to form the mainpalaeo-ice stream. The dominance of drumlins with low elongation ratios suggests that ice-sheet out-let draining through Gerlache Strait was comparatively slower than the main palaeo-ice stream in theWestern Bransfield Basin, although the low elongation ratios may also partly reflect the lack of sedi-ment. Progressive elongation of drumlins further down-flow indicates that the ice sheet acceleratedthrough Croker Passage and the western tributary trough, and fed into the main zone of streamingflow in the Western Bransfield Basin. Topography would have exerted a strong control on the devel-opment of the palaeo-ice stream system but subglacial geology may also have been significant giventhe transition from crystalline bedrock to sedimentary strata on the inner–mid-shelf. In the broadercontext, the APIS was drained by a number of major fast-flowing outlets through cross-shelf troughsto the outer continental shelf during the last glaciation. Copyright � 2004 John Wiley & Sons, Ltd.

KEYWORDS: Antarctic Peninsula Ice Sheet; subglacial bedforms; ice-flow pattern; onset zone; palaeo-ice stream.

Introduction

A significant volume (up to 90%) of the contemporary Antarcticand Greenland ice sheets are drained by fast-flowing outlet gla-ciers and ice streams. These fast-flowing outlets exert an impor-tant control on the stability and dynamics of ice sheets.Reconstructing the former flow of these systems during the lateQuaternary is important to our understanding of present andfuture behaviour of these ice sheets.

The Antarctic Peninsula Ice Sheet (APIS) was groundedacross the continental shelf during the last glaciation, and com-prised a number of major palaeo-ice streams that drained theice sheet through cross-shelf troughs, transporting largevolumes of ice and sediment to the continental slope (Pudseyet al., 1994; Bentley and Anderson, 1999; Canals et al., 2000,2002; O Cofaigh et al., 2002; Dowdeswell et al., 2004a,b;Evans et al., in press). One such major palaeo-ice streamdrained through a cross-shelf trough in the western BransfieldBasin, immediately north-northeast of Gerlache Strait (Canals

et al., 2000, 2003; Fig. 1). This is termed the Western BransfieldBasin palaeo-ice stream. However, the nature of past ice-flowand associated subglacial geomorphology upstream of this fast-flowing outlet is unknown in the complex of islands aroundGerlache Strait and Croker Passage (Fig. 1) and on the innercontinental shelf. In addition, the controls on the down-flowdevelopment of the Western Bransfield Basin palaeo-ice streamremain unresolved.

In this paper we present submarine geomorphological evi-dence concerning the nature of past APIS flow through Ger-lache Strait and across the inner continental shelf (Fig. 1).Swath-bathymetric data from Gerlache Strait, Neumayer Chan-nel, Croker Passage and the inner continental shelf both eastand west of the Palmer Archipelago, Antarctic Peninsula (Fig.1) document the occurrence of streamlined bedforms on thesea floor. The origin of these bedforms, their implications forpalaeoice-sheet drainage and flow-dynamics, within the wes-tern APIS, and their relationship to a major former ice streamthat drained across the adjacent Antarctic Peninsula shelf(Canals et al., 2000) are discussed. The findings of our studyare placed within a broader context by comparison with exist-ing reconstructions of ice flow during the LGM on the AntarcticPeninsula continental shelf.

* Correspondence to: J. Evans, Scott Polar Research Institute, University ofCambridge, Lensfield Road, Cambridge CB2 1ER, England.E-mail: [email protected]

Study area and background

The Palmer Archipelago is located along the Danco Coast ofthe northwest Antarctic Peninsula, and comprises a group ofislands, of which Anvers and Brabant Islands form the largest(Fig. 1). Gerlache Strait forms a channel, up to 40 km wideand 120 km long, that separates the Danco Coast area of theAntarctic Peninsula from Anvers Island and Brabant Island(Fig. 1). The strait opens on to the continental shelf at its south-western and northeastern ends and via the Schollaert Channel.The region is heavily glaciated today with the margins ofGerlache Strait and its tributary bays and inlets surroundedalmost entirely by tidewater ice-fronts associated with outlet,valley and cirque glaciers, together with piedmont glaciersand ice aprons (Anderson, 1999). The general northeast tosouthwest orientation of relief in the Palmer Archipelago isrelated to structural geology, most notably back-arc extensionover the whole region (Storey and Garrett, 1985; Anderson,1999). The onshore geology of the Palmer Archipelago andthe Danco Coast of the Antarctic Peninsula surroundingGerlache Strait and Croker Passage is dominated by Mesozoic

and early Cenozoic plutonic and volcanic rocks, as well asoutcrops of deformed sedimentary rocks of the TrinityPeninsula Group (British Antarctic Survey, 1979, 1981).

The Gerlache Strait is 300 m deep in the southwest, deepen-ing progressively to 1200 m towards the northeast end of thestrait and between Brabant Island and Two Hummock Island(Fig. 2). Water depth ranges between 600 m and 850 m in theregion where Schollaert Channel (300–500 m water depths)opens into Gerlache Strait (Fig. 1). The shallow flanks of thestrait are typically 300–400 m deep with a fault-bounded wes-tern margin. The tributary bays and fjords feeding intoGerlache Strait reach maximum water depths of 500 m (e.g.Griffith and Anderson, 1989; Domack and Ishman, 1993).Neumayer Channel is 3–4 km wide and <400 m deep.

Water depths on the inner continental shelf west of AnversIsland are>1200 m in Palmer Deep and the main trough ofBismarck Strait, but are shallower (<600 m) outside thesetroughs (Fig. 1). The deep area on the inner shelf connects witha cross-shelf trough (450–600 m water depth) that extendsnorthwards from the innermost continental shelf east of theBiscoe Islands to the shelf break (Fig. 1; Pope and Anderson,1992; Bart and Anderson, 1995; Canals et al., 2003).

Figure 1 Location map of Gerlache Strait and the adjacent Antarctic Peninsula continental shelf with detailed offshore bathymetry shown. The boxedarea marks the area of megascale glacial lineations (Western Bransfield Basin bundle structure) associated with the West Bransfield Basin palaeo-icestream (Canals et al., 2000). Inset shows the location of the study area in Antarctica. Both the western tributary trough (1) and eastern tributary trough(2) are marked. NC¼Neumeyer Channel; BS¼Bismarck Strait; SC¼ Schollaert Channel

398 JOURNAL OF QUATERNARY SCIENCE

Copyright � 2004 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 19(4) 397–407 (2004)

Water depths are 1000–1200 m in Croker Passage where itconnects Gerlache Strait to the inner continental shelf betweenBrabant, Liege, Two Hummock and Hoseason islands (Fig. 1).This part of Croker Passage (termed here the western tributarytrough) continues north to north-northeast across the adjacentinner continental shelf with water depths ca. 800 m, and is sepa-rated by a very shallow (90 m) bank from another trough trendingSSE–NNW across the inner shelf north of Trinity Island (termedthe eastern tributary trough) (Fig. 1). The western and eastern tri-butary troughs merge into one trough system on the middle shelfof the West Bransfield Basin, and continue across the outer con-tinental shelf through Boyd Strait east of Low and Smith islands(Fig. 1; Canals et al., 2000, 2003). A major palaeo-ice streamwithin the APIS, which was at least 100 km in length and about15–20 km wide, drained through this trough during the last gla-ciation depositing the Western Bransfield Basin bundle of mega-scale glacial lineations (Canals et al., 2000, 2003).

The western APIS grounded on to the outer shelf, and possi-bly reached the shelf break during the last glaciation (Bentleyand Anderson, 1998; Anderson et al., 2002). Variations in thethickness of post-glacial drape overlying the West BransfieldBasin bundle has been used to infer that sea-floor decouplingand retreat of the palaeo-ice stream from the mid-outer shelfwas not linear (Willmott et al., 2003). Decoupling occurredfirst in the central region of the palaeo-ice stream followedby its outer and inner regions, but a chronology for ice-streamretreat has yet to be established. Deglaciation of the outer con-tinental shelf of the northwest Antarctic Peninsula began priorto 14 365 yr BP, based on radiocarbon dates obtained from ice-proximal glaciomarine deposits on the outer Trinity Peninsulashelf bordering Bransfield Strait (Banfield and Anderson, 1995).Retreat of the APIS from the inner shelf adjacent to the PalmerArchipelago occurred around 13 000 yr BP, as indicated byradiocarbon dates obtained from glaciomarine sediments over-

lying subglacial till in Palmer Deep (Domack et al., 2001). Thisis supported by dated glaciomarine sediments on the continen-tal shelf adjacent to Anvers Island that suggest deglaciationtook place here before 12 400 yr BP (Pudsey et al., 1994). Ice-sheet retreat from Gerlache Strait began sometime after8000 yr BP, based on a radiocarbon date from a core recoveredin the central part of the strait (Harden et al., 1992). A similarage for deglaciation also comes from dated cores in the tribu-tary bays of the Strait (Anderson 1999).

Data acquisition and methods

Geophysical data were acquired from Gerlache Strait,Neumeyer Channel and the inner shelf west of Anvers Islandduring cruise JR84 of the RRS James Clark Ross in 2003, usinghullmounted Kongsberg-Simrad EM120 multi-beam swath-bathymetry and Topographic Parametric Sonar (TOPAS) sub-bottom profiling systems (Fig. 2). The swath system emits 191beams, each with a frequency of 12 kHz and a maximum port-and starboard-side angle of 75�. This gives, for example, a totalswath width of about 4 km in a water depth of 500 m. Swathdata were reduced and gridded to a resolution of 46 m. A totalarea of about 2500 km2 of swath bathymetry was recorded,including about 1500 km2 from Gerlache Strait itself (Fig. 2).

The TOPAS system uses parametric interference betweenprimary waves to produce a secondary acoustic beam of nar-row width and frequency range of 0.5 to 5 kHz. It is used to pro-file the subsea-floor at high spatial resolution. A total tracklength of about 700 km of TOPAS records was acquired fromthe study area (Fig. 2). Navigation data were acquired using dif-ferential global positioning system (GPS).

Figure 2 Location map of Gerlache Strait and the adjacent Antarctic Peninsula continental shelf showing the JR84 cruise tracks. The locationsof EM120 swath-bathymetric records illustrated in Figs 3–7 are shown by the boxed areas together with the area of megascale glacial lineationsassociated with the West Bransfield Basin palaeo-ice stream. Bathymetric contours are indicated by the dotted lines. The western tributary trough(1) and eastern tributary trough (2) on the inner shelf are marked

LATE QUATERNARY ICE-SHEET FLOW, ANTARCTIC PENINSULA 399


Submarine bedforms and acoustic stratigraphy

Gerlache Strait

Swath-bathymetric imagery shows that there are three maintypes of streamlined bedforms on the sea floor of GerlacheStrait. These comprise drumlins and more crudely streamlinedforms to irregular ‘scalloped’ features (Fig. 3). Elongation ratiohas often been used to define streamlined drumlinoid bedforms(Benn and Evans, 1998). Drumlins are the most common bed-form in this study and range from blunt, irregular forms(<1700 m in length and <600 m in width) to more elongateforms with blunt stoss sides and tapered lee sides (<3000 min length and <800 m in width) (Fig. 3A–C). The length to widthratio (elongation ratio) of the drumlins is mainly 2:1 to 5:1 and,rarely, >7:1. The drumlins are orientated in a north-northeast-erly direction at the very southwestern end of Gerlache Strait,changing to a northeasterly direction through most of the Straitbefore switching to a northerly direction in the very northeast-ern part of the Strait (Fig. 3).

The second type of bedform comprises crudely streamlinedforms (Fig. 3B) that are up to 3400 m long and 640 m wide.They have no clearly identifiable stoss–lee form and are not,therefore, defined as drumlins. The streamlined features areorientated WSW–ENE and are confined to the central part ofGerlache Strait adjacent to Schollaert Channel (Fig. 3B). A thirdfeature comprising irregular, ‘scalloped’-shaped bedforms upto 1200 m long and 600 m wide, are distributed across the east-ern margin of the northeastern end of Gerlache Strait (Fig. 3C).

Two types of sea-floor return are visible on TOPAS sub-bot-tom profiler records from Gerlache Strait (Fig. 4a–c). The first isa highly reflective and irregular surface with little or no acou-stic penetration (Fig. 4a–c). This predominates over most ofGerlache Strait where we have mapped the drumlins and irre-gular ‘scalloped’ bedforms (Fig. 3). Such acoustic returns sug-gest that these bedforms are composed predominantly ofbedrock. A second, diffuse acoustic return is found onlylocally, most notably in the region of crudely streamlined bed-forms, suggesting that these features are formed in a thin, later-ally discontinuous unit of sediment (Figs 3 and 4a).Furthermore, this sediment appears to be internally structure-less on the TOPAS records. The bedforms are draped for themost part by a thin cover (<2 m) of post-glacial glaciomarinesediment (cf. Griffith and Anderson, 1989). However, locally(along the northern margin of Gerlache Strait) this cover thick-ens to 25 m and contains well-developed acoustic stratification(Fig. 4a). Griffith and Anderson (1989) investigated the acousticstratigraphy of Gerlache Strait and tributary bays and inlets ofthe Danco Coast and also noted a generally thin and localisedcover of glacial and post-glacial sediment overlying bedrock.

Croker Passage

Streamlined bedforms are well developed in Croker Passage,and are dominated by drumlins (Fig. 5b). The drumlins areshort and irregular in the southwestern region of Croker Pas-sage (up to 1800 m in length and 600 m in width; elongationratios 2:1 to 7:1), but become more elongate (blunt stoss sidesand tapered lee sides) in the northeastern region (up to 3200 min length and 300 m in width; elongation ratios 7:1 to 16:1)(Fig. 5b). The drumlins are streamlined towards the northeast.The lack of acoustic penetration in TOPAS records and the irre-gular and rough appearance of the drumlins on swath bathy-metric records (Fig. 5b) indicate that they are composedpredominantly of bedrock.

Inner continental shelf

The western and eastern tributary troughs on the inner shelf tothe northeast of Brabant Island contain well-developed stream-lined bedforms (Fig. 5b and c). Bedforms within the western tri-butary trough extending beyond Croker Passage comprisedrumlins with blunt stoss sides and very elongate lee sides upto 4200 m in length and 500 m in width and elongation ratios of8:1 to 18:1 (Fig. 5c). These drumlins have a northeasterly orien-tation that becomes more northerly further out on the shelf andalong the eastern trough margin.

Glacial lineations are present in the eastern tributary troughto the north of Trinity Island. These bedforms are 3000 m to� 4200 m in length and <450 m in width with elongation ratiosof 11:1 to 21:1 (Fig. 5c). The orientation of the lineations rangesbetween SE–NW and E–W. Bedforms are absent across theshallow shelf region separating the two cross-shelf troughs.

In the western tributary trough, the irregular and roughappearance of the drumlins on swath-bathymetric records indi-cate that they are formed in bedrock (Fig. 5c). In contrast, thetapered lee sides of the drumlins have a smooth appearancesuggesting that they are composed mainly of sediment. TheTOPAS sub-bottom profiler records indicate the presence of aunit of acoustically transparent sediment (Fig. 5c). In the east-ern tributary trough, TOPAS records indicate that the lineationsare formed in a thin unit of semicontinuous, acoustically trans-parent sediment that appears to be internally structureless(Fig. 5d). The bedforms in both troughs are overlain by a drapeof sediment, up to 15 m thick, that is interpreted as glaciomar-ine sediment deposited after ice retreat from the shelf and strait.However, the drape is generally thin or absent across the shal-lower trough flanks and shelf banks (Fig. 5d).

To the west of the Palmer Archipelago, crudely developedstreamlined bedforms are present on the inner continental shelfsouthwest of Anvers Island and are orientated SE–NW (Fig. 6).The bedforms are short and irregular (<1500 m in length and<430 m in width) in the inner region of Bismarck Strait, butbecome more elongate (up to 3600 m in length and 550 m inwidth) with distance across the inner continental shelf.

Discussion

Palaeodrainage of the APIS in the Palmer Archipe-lago and on the continental shelf

The streamlined submarine bedforms in Gerlache Strait and theadjacent inner continental shelf are interpreted to have formedat the bed of the former APIS when it last filled the Strait andwas grounded across the continental shelf. The freshly pre-served nature of the bedforms, and the drape of post-glacialglaciomarine sediment overlying them, imply that the sea-floorfeatures were formed during the last glaciation. Bedform orien-tation provides direct evidence for the former flow directionand drainage pattern of the ice sheet in this region of the Ant-arctic Peninsula. The major bedforms and former ice-flow pat-tern in our study area are summarised in Fig. 7.

Ice draining from the interior of the APIS was channelledthrough the tributary bays on the Danco Coast before coales-cing in Gerlache Strait. The size of the glacial catchment areafeeding ice into the ice stream in the west Bransfield Basin hasbeen estimated at about 23 000 km2 at the last glacial maxi-mum (Canals et al., 2000). The prevailing northeasterly orien-tation of the bedforms indicates that palaeo-ice flow wastowards the northeast through Gerlache Strait (Fig. 3). Ice



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draining into southwest Gerlache Strait from as far south as theParadise Bay and Pryde Island region contributed to this flow(Fig. 7).

Bedform orientation in the northeastern region of GerlacheStrait is northerly, rotating to northeasterly through CrokerPassage (Figs 3 and 5). This indicates that the ice sheet flowedthrough the northern end of Gerlache Strait and extended on tothe inner continental shelf via Croker Passage (Figs 1 and 7). Iceflow continued northeastwards through the western tributarytrough and then northwards further out on the shelf in responseto convergence with mainly westerly ice flow through the east-ern tributary trough. The latter ice flow was probably derivedfrom former glacial-drainage basins on the Antarctic Peninsulato the east and southeast of Trinity Island (Figs 5 and 7). Ice flowdownstream of this convergence continued northwards on tothe middle–outer shelf where the Western Bransfield basinpalaeo-ice stream was located during the last glaciation(Canals et al., 2000).

Ice flow through the Bismarck Channel and Palmer Deepand across the inner continental shelf west of Gerlache Straitand Anvers Island was in a westerly to northwesterly direction

(Figs 6 and 7). This ice was derived from the region of thepeninsula south of Paradise Bay (Fig. 1).

Former flow dynamics of the APIS

The presence of highly attenuated glacial bedforms (megascaleglacial lineations; elongation ratios >10:1) is one criterion bywhich fast ice flow and, in particular, palaeo-ice streams havebeen identified in the Late Quaternary geological record (e.g.Clark, 1993; Stokes and Clark, 1999, 2001, 2002). The WesternBransfield basin palaeo-ice stream draining northwards acrossthe continental shelf (including Boyd Strait) was identified bythe presence of a ‘bundle structure’ of megascale glacial linea-tions up to 100 km in length and 14 to 21 km wide on the mid-dle–outer shelf (Canals et al., 2000) (Figs 1, 2 and 7). Theswath-bathymetric study by Canals et al. covered much ofthe outer–middle continental shelf as far south as 63� 400 S(Figs 1, 2 and 7). The streamlined subglacial bedforms thatwe describe from the inner shelf are confined to the western

Figure 4 The TOPAS sub-bottom profiler record from Gerlache Strait showing: (a) (i) diffuse acoustic returns associated with crudely streamlinedforms composed of sediment, (ii) highly reflective and hard sea floor across regions of bedrock-cored drumlins, and (iii) a well-stratified post-glacialglaciomarine drape occurring locally along the northern margin of Gerlache Strait; and (b–c) little or no acoustic penetration of the sea floor in areas ofbedrock drumlins and scallops. The profiles are located in Fig. 3



and eastern tributary troughs (Fig. 5). The megascale glaciallineations on the middle–outer shelf occur in the same cross-shelf trough and are located directly down-flow of the bed-forms of our study (Fig. 7). Therefore, the sets of bedforms arespatially related and were produced along the same flow path-way (i.e. the same ice-sheet outlet).

The type and scale of subglacial bedforms in Gerlache Straitand on the inner shelf are used to infer the past dynamics of theAPIS upstream of the Western Bransfield Basin palaeo-icestream. On the basis of their rough and irregular appearanceon the swath records, and the lack of penetration on TOPAS,subglacial bedforms in Gerlache Strait are inferred to be

Figure 5 (a) EM120 swath-bathymetric record from Croker Passage and the inner continental shelf to the north east of the Palmer Archipelago (Figs 1and 2). Inset images show more detailed swath records of: (b) Croker Passage; (c) the adjacent western tributary trough on the inner shelf; and (d) theeastern tributary trough to the north of Trinity Island. Detailed TOPAS records from both tributary troughs are shown in (c) and (d) which are located bydashed lines on the accompanying swath record



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composed of bedrock (Figs 3 and 4a–c). This implies activebasal sliding of an ice-sheet outlet over the floor of GerlacheStrait. These bedforms have low elongation ratios (mainly 2:1to 5:1) as compared with the megascale glacial lineationsfurther down-flow in the West Bransfield Basin. This suggeststhat the ice-sheet outlet draining through Gerlache Strait wasflowing at comparatively slower velocities than the mainpalaeo-ice stream trunk in the Western Bransfield Basin(cf. Wellner et al., 2001; Lowe and Anderson, 2002), althoughsuch low elongation ratios may also partially reflect a lack ofsediment in Gerlache Strait.

The orientation of the bedforms that we mapped on the innershelf indicates that the Western Bransfield Basin palaeo-icestream (Canals et al., 2000) was fed by the convergence offast-flowing ice channelled along the western and easterntributary troughs from at least two large glacial catchment areasaround Gerlache Strait and east/southeast of Trinity Island(Figs 5 and 7). Drumlins become more elongate along CrokerPassage (elongation ratios increase from 2:1–7:1 to 7:1–16:1)and across the adjacent inner shelf trough (8:1–18:1). The pro-gressive down-flow evolution in the morphology of bedformsfrom drumlins to highly attenuated lineations has been recog-nised in several cross-shelf bathymetric troughs around

Antarctica (e.g. Wellner et al., 2001; Lowe and Anderson,2002; O Cofaigh et al., 2002). This pattern is interpreted tobe consistent with the former presence of ice streams, andrecords a transitional region where grounded ice progressivelyaccelerates to the highest velocities that characterise the ice-stream trunk (e.g. Stokes and Clark, 1999, 2001; Wellneret al., 2001). The bedform pattern suggests that the APIS accel-erated through Croker Passage and the western tributary troughon the inner shelf (ice-stream onset zone), and fed into the mainzone of streaming flow in the Western Bransfield Basin. Thisinterpretation is further supported by the bedform pattern thatindicates strong flow convergence of the eastern and westerntributaries on the inner–middle shelf (cf. Hodge and Doppel-hammer, 1996). Glacial lineations within the eastern tributarytrough on the shelf north of Trinity Island (Figs 1 and 7) alsopoints to rapid ice flow and suggests that the main trunk ofthe ice stream with the fastest flow velocities may haveextended on to this region of the shelf.

It is difficult to determine unequivocally whether or not thebedforms in Gerlache Strait, Croker Passage and on the innershelf were formed synchronously with the megascale glaciallineations of the Western Bransfield Basin palaeo-ice streamon the mid–outer shelf. However, several observations suggest

Figure 7 Interpretation diagram of submarine bedform distribution and former ice-sheet flow directions in the study area. The swath-bathymetricrecords of this study and those of Canals et al. (2000) show that a fast-flowing outlet of the Antarctic Peninsula Ice Sheet drained through GerlacheStrait and Croker Passage and the adjacent cross-shelf trough system (West Bransfield Basin) during the last glaciation. The nature and pattern ofinferred ice dynamics along this flow pathway are indicated



that the two sets of bedforms were probably deposited as part ofa single ice-flow outlet that drained the APIS during the last gla-ciation:

1 bedforms on the inner shelf display parallel conformity andan absence of cross-cutting relationships;

2 bedforms in Gerlache Strait and on the shelf are distributedwithin the same cross-shelf trough, and indicate the sameice-flow trajectory;

3 the progressive elongation of drumlins with distance down-flow implies accelerating ice flow through the western tri-butary trough.

In this case, the region of the main trunk of the palaeo-icestream with the highest flow velocities had a maximum lengthof between 120 and 150 km (maximum length applies if the icestream extended beyond Boyd Strait to the shelf break; Canalset al., 2003). If the outlet system was streaming across the entirearea of sea floor with subglacial bedforms, then the ice streamwould have extended into Gerlache Strait and therefore,exceeded 300 km in length.

The question remains as to whether the development of thepalaeo-ice stream in the Western Bransfield Basin was con-trolled by topography and/or subglacial geology. A strong topo-graphic control is implied by the fact that the subglacialbedforms are confined mainly to the cross-shelf bathymetrictroughs, and are not found on the shallow inter-trough banks.The influence of subglacial geology/sediment availability onthe down-flow evolution of bedform dimensions and ice-flowvelocities cannot be determined unequivocally owing to theabsence of deep reflection seismics in the study area. However,the onshore geology of the islands and Antarctic Peninsula sur-rounding Gerlache Strait and Croker Passage (including TrinityIsland) is composed predominately of crystalline rocks (pluto-nic and volcanic rocks; British Antarctic Survey, 1979, 1981)that probably extend into the offshore. This is supported bythe presence of bedrock-cored drumlins within Gerlache Strait,Croker Passage and on the inner shelf, as well as seismic pro-files from west of Brabant Island that indicate extension of crys-talline bedrock across the inner shelf (Larter et al., 1997). Athick sequence of sedimentary strata underlain by acousticbasement is located along Boyd Strait further offshore wherethe megascale glacial lineations and Western Bransfield Basinpalaeo-ice stream was located (Jin et al., 2002). Therefore, itappears that the transition to the highest flow velocities of themain ice-stream trunk was facilitated by a transition from crys-talline bedrock to a sedimentary substrate (cf. Shipp et al.,1999; Wellner et al., 2001; Lowe and Anderson, 2002;O Cofaigh et al., 2002). However, the exact location of theboundary between crystalline basement and sedimentary stratabeneath the inner shelf is unknown.

A number of other multibeam geophysical studies have beenperformed on the Antarctic Peninsula continental margin and,together with our study, reveal new insights into the flow andextent of the APIS during the last glaciation (Canals et al., 2000,2002, 2003; O Cofaigh et al., 2002, 2003; Dowdeswell et al.,2004a; Evans et al., in press). These studies have documentedthe widespread occurrence of streamlined subglacial bed-forms, indicating that the APIS was drained via a number ofmajor palaeo-ice streams through cross-shelf bathymetrictroughs. Subglacial sediment, bedrock geology and sea-floortopography played important roles in controlling the location,size and velocities of these fast-flowing outlets. The fastestvelocities, implied by the presence of megascale glacial linea-tions, occurred across sedimentary strata on the mid–outershelf, with slower velocities over crystalline bedrock on theinner shelf (e.g. Wellner et al., 2001; O Cofaigh et al., 2002).

Fast-flow was established sometime after advance of the APISacross the continental shelf, either during the LGM or deglacia-tion (O Cofaigh et al., 2002; Dowdeswell et al., 2004a; Evanset al., in press). Geological and geophysical evidence coupledwith the distribution of glacial bedforms in multibeam recordsindicate that grounded ice extended to the outer shelf, and insome cases to the shelf edge, along the Pacific and northwestWeddell margins of the APIS (Pudsey et al., 1994; Banfield andAnderson, 1995; Larter and Vanneste, 1995; Vanneste and Lar-ter, 1995; Canals et al., 2000, 2002, 2003; O Cofaigh et al.,2002; Evans et al., in press).

Conclusions

1 Swath-bathymetric and TOPAS sub-bottom profiler recordsfrom Gerlache Strait, Croker Passage, Bismarck Strait andthe adjacent continental shelf reveal streamlined subglacialbedforms on the sea floor. These bedforms record the drai-nage pattern and flow dynamics of a former major outlet ofthe western APIS, which fed into a palaeo-ice stream in thewest Bransfield Basin during the last glaciation (Figs 3, 5and 6).

2 Bedform orientation supports ice-sheet flow to the northand northeast through Gerlache Strait and Croker Passageand the western tributary trough on the inner shelf, withice derived from as far south as the Paradise Bay andPryde Island region (Fig. 7). The pattern of bedforms onthe inner–middle shelf indicates strong flow convergenceof the eastern and western tributaries, with resultant iceflow northwards on to the middle–outer shelf where theWestern Bransfield Basin palaeo-ice stream was located(Figs 5 and 7).

3 The spatial arrangement and orientation of bedforms in ourstudy area, and those of the Western Bransfield Basinpalaeo-ice stream (Canals et al., 2000), indicate that theywere produced as part of a single ice-flow outlet system thatdrained the APIS during the last glaciation (Fig. 7).

4 The dominance of drumlins with low elongation ratios inGerlache Strait suggests that the ice-sheet outlet was flow-ing at comparatively slower velocities than the mainpalaeo-ice stream trunk in the Western Bransfield Basin,although bedform morphology may possibly also partiallyreflect the lack of sediment in Gerlache Strait. Progressiveelongation of bedforms, coupled to convergent ice flow onthe inner shelf indicates that the APIS outlet acceleratedthrough Croker Passage and the western tributary troughon the inner shelf and fed into the main zone of streamingflow in the Western Bransfield Basin (Fig. 7). The maintrunk of the palaeo-ice stream was fed by convergence ofat least two fast-flowing ice-sheet outlets.

5 The main trunk of the palaeo-ice stream with the highestflow velocities was between 120–150 km in length, andthe entire length of streaming flow may have been longer,possibly exceeding 300 km. Topography imparted a strongcontrol on the development of the main palaeo-icestream in the West Bransfield Basin, and may have beenaugmented by the transition from crystalline bedrock to asedimentary substrate on the inner–mid-shelf. In a widercontext, the APIS was drained via a number of fast-flowingoutlets through cross-shelf troughs to the outermost shelfduring the last glaciation, and appears to have been influ-enced by both topography and the nature of the subglacialsubstrate.



Acknowledgements This work was funded by UK NERC grant NERC/T/5/2000/00986 as part of the Autosub Under Ice Thematic Pro-gramme. We thank the officers and crew of the RRS James Clark Rossfor their cooperation during cruise JR84 to the Antarctic Peninsula andPine Island Bay. The manuscript was improved by the review of Carol J.Pudsey (British Antarctic Survey). We thank Toby Benham for assis-tance with data analysis and graphics, and David Vaughan (BritishAntarctic Survey) and Jon Copley (Southampton Oceanography Centre)for assisting with data acquisition.

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