extent and dynamics of the west antarctic ice sheet on the outer continental shelf of pine island...

(2006) 53–72www.elsevier.com/locate/margeo

Marine Geology 230

Extent and dynamics of the West Antarctic Ice Sheet on the outercontinental shelf of Pine Island Bay during the last glaciation

Jeffrey Evans a,⁎, Julian A. Dowdeswell a, Colm Ó Cofaigh b,Toby J. Benham a, John B. Anderson c

a Scott Polar Research Institute, University of Cambridge, Lensfield Road, Cambridge, CB2 1ER, UKb Department of Geography, Durham University, Durham, DH1 3LE, UK

c Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005, USA

Received 26 April 2005; received in revised form 2 March 2006; accepted 2 April 2006

Abstract

Swath bathymetry and TOPAS sub-bottom profiler acoustic data reveal the presence of a major cross-shelf bathymetric troughcontaining streamlined subglacial bedforms formed in soft till that extends to the continental shelf edge of outer Pine Island Bay,Amundsen Sea, West Antarctica. The new data indicate that the West Antarctic Ice Sheet (WAIS) extended to the shelf edge of PineIsland Bay during the last glaciation, and was drained by a grounded, fast-flowing palaeo-ice stream through this outer-shelftrough. Topography and subglacial geology are likely to have exerted a strong control on the development and location of palaeo-ice streams in Pine Island Bay. TOPAS records show that mega-scale glacial lineations are formed in an acoustically transparentsediment layer interpreted to be soft till and are inferred to be the product of subglacial sediment deformation beneath the palaeo-ice stream. The flat to irregular nature of the basal reflector of the acoustically transparent layer of soft till implies that acombination of groove-ploughing of the substrate and subglacial sediment deformation were important processes beneath thepalaeo-ice stream in the outer trough of Pine Island Bay, as well as palaeo-ice streams elsewhere in Antarctica. Therefore, theformation of the soft till layer, and possibly associated subglacial bedforms, are likely to be a function of these palaeo-ice streamprocesses. In a regional context, the WAIS extended to the continental shelf edge of the Bellingshausen, Amundsen and Ross Seas,and was drained by a number of palaeo-ice streams during the last glaciation.© 2006 Elsevier B.V. All rights reserved.

Keywords: West Antarctic Ice Sheet; palaeo-ice stream; groove-ploughing; subglacial sediment deformation; iceberg scouring

1. Introduction

The West Antarctic Ice Sheet (WAIS) comprises anumber of major fast-flowing ice streams that dischargesignificant volumes of ice and sediment into the

⁎ Corresponding author. Tel.: +44 1223 336570; fax: +44 1223336540.E-mail address: [email protected] (J. Evans).

0025-3227/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.margeo.2006.04.001

surrounding Antarctic seas. Major ice streams such asPine Island Glacier and Thwaites Glacier, which drainapproximately 300,000 km2 of WAIS into Pine IslandBay (Fig. 1) (Rignot, 2001; Vaughan et al., 2001), haverecently experienced basal melting and thinning,together with grounding-line retreat (Jenkins et al.,1997; Wingham et al., 1998; Rignot, 1998; Shepherd etal., 2001; Rignot, 2001; Rignot et al., 2002). The ice-sheet basins that drain ice into Pine Island Bay are

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grounded mainly below sea level by up to 2500 m andare not restrained by buttressing ice shelves (Vaughan etal., 2001). They are, therefore, potentially unstable andcould undergo partial collapse with a correspondingeffect on global sea level (e.g. Hughes, 1981; Bindscha-dler, 1998). A total collapse of the present-day WAISwould result in a rise of global eustatic sea level by 5–6 m. The Pine Island Bay region is, therefore,environmentally significant globally and, consequently,it is important to provide an historical context to recentchanges by understanding the Late Quaternary WAIS inthis region.

Reconstruction of the configuration and dynamics ofthe WAIS during the last glaciation is central to theaccurate modelling of its contribution to post-glacialglobal sea-level rise (e.g. Nakada and Lambeck, 1988;Huybrechts, 1990; Tushingham and Peltier, 1991;Bentley, 1999; Nakada et al., 2000). Constraints onpost-glacial sea level rise predicted by these models hasbeen provided by marine geophysical and geologicalinvestigations that have reconstructed the configurationof the Antarctic Ice Sheet during the last glaciation (e.g.Shipp et al., 1999; Domack et al., 1999; Canals et al.,2000; Anderson et al., 2002a,b; Ó Cofaigh et al., 2002;Evans et al., 2004, 2005; Ó Cofaigh et al., 2005a,b).Previous reconstructions of Pine Island Bay during thelast glaciation have shown that the WAIS grounded to atleast the middle-outer shelf, and was drained by apalaeo-ice stream (Kellogg and Kellogg, 1987a,b,c;Lowe and Anderson, 2002; Anderson et al., 2002b;Lowe and Anderson, 2003). Direct glacial geomorpho-logical evidence for ice sheet extent across the outershelf was absent in this sparse data acquired from onlythe eastern side of outer Pine Island Bay. Ice sheet extentto the shelf edge has been inferred based on only indirectevidence in the form of a gully–channel network on theadjoining continental slope which were thought to beproduced in front of an ice margin during full glacials(Lowe and Anderson, 2002; Dowdeswell et al., 2006).Therefore, our understanding of the extent and flow-dynamics of the WAIS on the outer shelf of the PineIsland Bay region of the Amundsen Sea margin duringthe last glaciation remains incomplete.

In this paper, we describe the marine sedimentary andgeomorphological record in outer Pine Island Bay using

Fig. 1. Location map of Pine Island Bay, West Antarctica, showing the trackregional glaciology, geographical names referred to in the text and the locatiofrom the continental shelf presented in this paper were acquired along the reAnderson (2002, 2003) and Anderson et al. (2002a) were acquired along the yalong the blue track lines across the Pine Island Bay continental slope are pNBP9902-PC39 is marked.

sub-bottom profiler and multi-beam swath bathymetricdata in order to determine: (1) the extent and drainagepattern of the WAIS on the outer continental shelf ofPine Island Bay during the last glaciation; (2) flow-dynamics of the last glacial WAIS; (3) subglacialprocesses of the palaeo-ice stream bed; and (4) thenature of iceberg scouring across the shelf and itsimplications for iceberg production during the LateQuaternary. Our data are combined with earlierinvestigations of Late Quaternary glaciation in innerPine Island Bay (Lowe and Anderson, 2002, 2003) inorder to provide a reconstruction of the WAIS on theAmundsen Sea continental margin at the LGM.

2. Regional setting and background

2.1. Regional setting

Pine Island Bay is located in the Amundsen Seaalong the southern Pacific margin of West Antarctica(Fig. 1). The distance from the modern West AntarcticIce Sheet terminus to the shelf edge is almost 500 km.Pine Island Bay deepens inshore due to isostatic loadingand glacial erosion by the WAIS over successiveglaciations (cf. Anderson, 1999; Lowe and Anderson,2002, 2003). The bathymetry of inner Pine Island Bayand the eastern region of outer Pine Island Bay has beendetermined by multi-beam swath bathymetry (cf. Loweand Anderson, 2002, 2003). Inner Pine Island Bay isrugged, with water depths over 1000 m. A cross-shelfbathymetric trough extends NW across the shelf of PineIsland Bay parallel to∼ 108° W, fed by narrow and deeptributaries emerging from Thwaites and Pine IslandGlaciers. The sea floor across the middle/outer shelf inthe eastern region of outer Pine Island Bay is smooth,and water depths are 400 to 500 m. The AntarcticCoastal Current flows westward along the AmundsenSea margin (Gordon, 1971), and Circumpolar DeepWater (CDW) penetrates Pine Island Bay via cross-shelfbathymetric troughs (Jacobs et al., 1996).

The WAIS is drained into Pine Island Bay by severalmajor fast-flowing ice streams; Pine Island Glacier,Thwaites Glacier and Smith Glacier (Fig. 1) (e.g.Ferrigno et al., 1993). Pine Island and Thwaites glacierseach drain an ice-sheet basin of around 165,000 km2,

lines of the RRS James Clark Ross during JR84 (red and blue lines),ns of subsequent figures. Swath and acoustic sub-bottom profiler datad track lines. Geophysical data associated with the study by Lowe andellow track lines. Swath and acoustic sub-bottom profiler data acquiredublished in Dowdeswell et al. (2006). The location of sediment core


and Pine Island Glacier itself drains 4% of the entireAntarctic Ice Sheet (Bamber and Bindschadler, 1997;Vaughan et al., 2001). The balance flux of these basinshas been estimated at ca. 70–80 km3 yr−1 (Vaughan etal., 1999, 2001). The drainage basins of both Pine Islandand Thwaites Glaciers had negative surface elevationchanges between 1992 and 1996 (Wingham et al.,1998). In addition, Pine Island Glacier thinned at up to1.6 m yr−1 between 1992 and 1996 (Shepherd et al.,2001). Similarly, the grounding line of Thwaites Glacierhas retreated by 1.4 km at accelerating rates between1992 and 1996, which implies glacier thinning of up to3.2 m yr−1 (Rignot, 1998, 2001). Such changes in bothglaciers together contributed an estimated 0.08±0.03 mm yr−1 to global sea-level rise in 2000 (Rignotet al., 2002). Relatively warm Circumpolar Deep Waterfloods the bathymetric troughs of Pine Island Bay, and itis thought that these waters interact with the base of thefloating margin of Pine Island Glacier contributing tohigh melt rates (∼10 to 12 m yr−1) equating to half theglacier's mass loss and rapid recession of the glacier(Jacobs et al., 1996; Jenkins et al., 1997; Hellmer et al.,1998).

2.2. Previous work in Pine Island Bay

Diamictic sediments recovered from the middle-outer shelf of Pine Island Bay, just north of Burke Islandduring Deep Freeze cruises 81 and 85, together withmicromorphological analysis of these sediments, sug-gests that an ice sheet was grounded on the shelf duringthe last glaciation (Kellogg and Kellogg, 1987a,b,c;Hiemstra, 2001). Reflection seismic data in the PineIsland Bay region show that the slope at 104° W hasundergone both progradation and aggradation since themid-Miocene with the most recent phase dominated byaggradation. In addition, erosional unconformities arepresent on the outer shelf, implying the former presenceof a grounded ice sheet (cf. Yamaguchi et al., 1988;Nitsche et al., 1997, 2000). Similarly, reflection seismicdata from the outer shelf and upper slope at ∼108° Wshow mainly shelf aggradation and only minor shelfedge and slope progradation (Lowe and Anderson,2002).

Recent investigation of the marine sedimentaryrecord and sea-floor geomorphology of Pine IslandBay has enabled reconstruction of the former behav-iour of the WAIS on the inner-middle shelf during thelast glaciation (Lowe and Anderson, 2002, 2003). Fourdistinct zones were identified on the basis of sea-floormorphology and distribution of glacial landforms;Zones 1 and 2 comprised meltwater-derived tunnel

valleys, channels and cavities, together with drumlinsand P-forms, all formed in crystalline bedrock on theinner shelf; Zone 3 comprised mega-scale glaciallineations developed across sedimentary strata andunconsolidated sediments on the middle shelf; andZone 4 comprised iceberg furrows in water depthsshallower than 700 m on the outer shelf of eastern PineIsland Bay. The subglacial bedforms record theexpansion of a grounded WAIS to at least middle-outer Pine Island Bay during the LGM, and possibly tothe shelf edge, based on indirect evidence in the formof upper slope gullies and a glacially-eroded uncon-formity present in reflection seismic data from theouter shelf. Ice sheet flow across crystalline bedrockon the inner shelf occurred by basal sliding aided bylarge amounts of subglacial meltwater within achannelised drainage system. Ice sheet flow acrossthe middle shelf was in the form of an ice streamcoupled to a deformable sedimentary bed with anacceleration in flow velocity occurring across thetransition from crystalline to sedimentary substrate.Initially during deglaciation, the ice sheet marginretreated gradually, and temporarily grounded in inner-middle Pine Island Bay by ∼16,000 14C yr BP toproduce a prominent grounding-line wedge. Subse-quent ice retreat to a present-day position wascompleted by ∼10,000 14C yr BP.

3. Materials and methods

Geophysical and bathymetrical data were acquiredfrom the continental shelf of Pine Island Bay duringcruise JR84 of the RRS James Clark Ross in 2003. Ourdata were all collected from the outer shelf, north of73°S, due to adverse sea-ice conditions in the inner bay.Two hull-mounted systems were used; a Kongsberg-Simrad EM120 multi-beam swath-bathymetry systemand Topographic Parametric Sonar (TOPAS) sub-bottom profiler.

The EM120 swath bathymetric system emits 191beams, each with a frequency of 12 kHz and a maximumport- and starboard-side angle of 75°. This gives, forexample, a total swath width of about 3–4 km in a waterdepth of 500 m. Swath data were processed through theremoval of anomalous pings and gridded at cell sizes of30–50 m using Kongsberg-Simrad NEPTUNE soft-ware. The TOPAS system uses parametric interferencebetween primary waves to produce a secondary acousticbeam of narrow width and a frequency range of 0.5 to5 kHz. It is used to profile the sub-sea floor at highvertical resolution (i.e. to <1 m). Navigation data wereacquired using differential GPS.


4. Results

4.1. Bathymetry of outer Pine Island Bay

Swath bathymetry from outer Pine Island Bayindicates the presence of a prominent cross-shelfbathymetric trough (55–95 km in width) that trendsNW–SE, and extends inshore from the shelf edge at114° W to at least the middle shelf (Fig. 2). Waterdepths are 490 to 570 m on the middle shelf,increasing to 550 to 640 m on the outer shelf wherethe trough opens directly to the upper continental slope(Fig. 2). The trough has a steep eastern side butshallows progressively towards the west. It is likelythat this trough connects to a narrow bathymetricdepression (∼ 20 km in width; water depths of 450to 520 m) that is present on a single swathbathymetry line across the middle shelf at 72° 50′ Sand 109° 30′ W (Fig. 2). A swath bathymetry lineparallel to 108° W on the shelf east and inshore of our

Fig. 2. Colour bathymetry map showing the EM120 swath data acquired

main outer shelf coverage indicates the presence of abroad bathymetric depression (water depths of 470 to580 m). It is likely that this bathymetric depressioncorresponds to the trough mapped by Lowe andAnderson (2002) on the inner-middle shelf of PineIsland Bay. Outside the deep bathymetric regions,middle and outer Pine Island Bay is smooth and waterdepths are mainly 400 to 500 m.

4.2. Swath bathymetry of the outer Pine Island Bayshelf

The sea floor of the outer Pine Island Bay trough ischaracterised by streamlined sedimentary bedforms thatvary in form and dimension, and are orientated in a SE–NW direction (Figs. 3 and 4). Subtle lineations andmega-scale glacial lineations occur in the outer troughand are distributed to the shelf edge (Fig. 3). Theselineations range between 3500 and >6000 m in lengthand 140 to 180 m in width, with length to width ratios

from the outer continental shelf of Pine Island Bay during JR84.

Fig. 3. EM120 multi-beam swath bathymetry image of the outer trough of Pine Island Bay close to the shelf edge (located in Figs. 1 and 2). The image illustrates the presence of subtle mega-scaleglacial lineations (MSGL) and lineations in the main trough and iceberg scours across the bounding flanks outside the trough. Grid cell size=30×30 m.

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Fig. 4. (a) EM120 multi-beam swath bathymetry image of the sedimentary bedforms in the outer trough further in from the shelf edge (located in Figs. 1 and 2). The image illustrates the presence ofwell-developed MSGL in the main trough and iceberg scours across the shallower sea floor outside the trough. Note the steep eastern trough flank and the progressive overprinting of the MSGL byiceberg scours across the western flank. MSGL are also illustrated in more detail in the large-scale insets (b) and (c). Grid cell size=50×50 m.

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(elongation ratio) from 23:1 to 35:1. Bedforms in theouter trough up to∼70 km further inshore from the shelfedge comprise mega-scale glacial lineations (cf. Clark,1993; Stokes and Clark, 1999, 2001, 2002) that rangefrom >6800 m to about 10,000 m in length and 160 m to420 m in width with elongation ratios between >18:1and >60:1 (Fig. 4a–c). Streamlined subglacial bedformsorientated NW–SE are also present in the narrowbathymetric depression at 72° 50′ S and 109° 30′ Wonthe middle shelf.

Bedforms become increasingly subtle in appearanceand are overprinted by randomly orientated grooves (upto 15 m deep) towards the western side of the trough andacross the trough flanks, corresponding to a progressivedecrease in water depth from 570 m in the trough to<480 m on the trough flank (Fig. 4a–b). Swathbathymetric records across the sea floor of the middle-outer shelf outside the bathymetric trough also show adominant presence of irregular grooves that range fromrandom to orientated SE–NW and NE–SW, and aremetres to tens of metres deep (Figs. 3 and 4). In addition,the sea floor of the broad deep bathymetric depressionon the middle shelf parallel to 108° W is dominated byrandomly orientated grooves.

4.3. Acoustic stratigraphy of the Outer Pine Island Bayshelf

4.3.1. Outer shelf troughStreamlined sedimentary bedforms are present in the

surface of a thin, acoustically transparent, sedimentlayer (<10 m thick) that appears to be thickest along theeastern side of the outer trough (Figs. 5 and 6). Theacoustically transparent sediment layer is, for the mostpart, homogeneous internally, and is present throughoutthe outer trough up to the shelf edge. It appears to varyfrom very thin and laterally continuous betweenbedforms in some regions to discontinuous and patchyin others, particularly in the outermost trough and closeto the shelf edge (Fig. 5). It is possible that the layer isspatially continuous but very thin (<0.5 m) in theseregions and is below the minimum thickness that can beresolved by TOPAS. Locally along the eastern margin ofthe outer trough several sub-bottom reflectors appear tocross-cut one another, each with a similar discreteacoustically transparent sediment layer above (Fig. 5b).There is no discernable surface drape of sedimentoverlying the bedforms and acoustically transparentsediment layer that can be resolved on TOPAS records(Figs. 5 and 6).

The acoustically transparent sediment layer has acontinuous and prominent sub-bottom basal reflector

(Figs. 5 and 6). A highly irregular (grooved) andhummocky basal reflector is widespread in the outer-most trough, separated locally by regions where it isrelatively smooth and horizontal (Fig. 5b). In contrast,the basal reflector in the outer trough further inshore ismainly smooth and horizontal (or gently undulating) butcan be very hummocky or highly irregular in someregions (Fig. 6). Sediments underlying this basalreflector were only resolved in very local regions ofthe outer trough. Here, a wavy or inclined sub-bottomreflector is imaged beneath, and in some cases cross-cutby, a very hummocky basal reflector of the surfaceacoustically transparent sediment layer (Fig. 6). Thismarks a lower sedimentary unit comprising acousticallytransparent sediment that is organised into either a layeror dome-shaped lenses (Fig. 6).

4.3.2. Other outer shelf regionsThe outer Pine Island Bay sea floor outside the main

bathymetric troughs (water depths <400–500 m), and inthe broad bathymetric depression on the middle shelfparallel to 108° W corresponding to the trough mappedby Lowe and Anderson (2002), is characterised by ahighly irregular reflector with little or no acoustic sub-sea floor structure (Fig. 7).The highly irregular reflectorcomprises a paired peak-and-trough morphology of lowamplitude. A unit of acoustically transparent sedimentextends over a distance of >30 km (and possibly>50 km) on acoustic records acquired parallel to ∼ 108°20′ W, and lacks any internal acoustic structure (Figs. 1and 7). The unit occurs on the shelf at the outer end andin front of the Pine Island Bay bathymetric troughmapped by Lowe and Anderson (2002). It wedges outtowards the north at a prominent steep scarp (20 m high)within 30 km of the shelf edge (Fig. 7). However, it isuncertain whether or not the unit also wedges out in thedirection of the middle-shelf. This layer is up to ∼ 20 mthick and is underlain by a distinct basal reflector andoverlain by a highly irregular sea floor reflector (Fig. 7).

5. Interpretation and discussion

5.1. Extent of the WAIS during the last glaciation

The streamlined subglacial bedforms observed onswath bathymetric imagery from the outer trough ofPine Island Bay are interpreted to have formed beneath agrounded ice sheet (Figs. 3 and 4). The subglacialbedforms are similar in form and dimensions to themega-scale glacial lineations of Clark (1993), whichhave now been reported from a number of formerly ice-filled Antarctic and Arctic cross-shelf troughs (e.g.

Fig. 5. (a and b) TOPAS sub-bottom profiler records showing submarine glacial bedforms and shallow acoustic stratigraphy from the outer trough of Pine Island Bay close to the shelf edge (located inFig. 3). MSGL are formed in an acoustically transparent sediment layer with a horizontal to highly irregular or hummocky basal reflector. Note the cross-cutting reflectors, each with an identicalacoustically transparent layer above, along the trough margin in (b).

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Fig. 6. TOPAS sub-bottom profiler record from the outer trough of Pine Island Bay further in from the shelf edge (located in Fig. 4). MSGL are formed in an acoustically transparent sediment layer,which sometimes overlie dome-shaped lenses of acoustically transparent sediment interpreted as buried subglacial bedforms. The hummocky basal reflector of the surface acoustically transparentsediment layer occasionally incises underlying sub-bottom reflectors.

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Fig. 7. TOPAS sub-bottom profiler record acquired parallel to ∼ 108° 20′ W from the outer continental shelf on the eastern side of Pine Isl Bay (Fig. 1 illustrates the location of the profile). Adistinct wedge of acoustically transparent sediment with an iceberg scoured sea floor reflector extends from the outer end of the bathymetric t gh mapped by Lowe and Anderson (2002) across theouter shelf beyond, and terminates ∼ 30 km from the shelf edge.

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androu


Canals et al., 2000; Ó Cofaigh et al., 2002, 2005a,b;Evans et al., 2005; Ottesen et al., 2005). Thesestreamlined subglacial bedforms are distributedthroughout the outer trough, and extend to the shelfedge (Figs. 3, 4 and 8). The bedforms provide directevidence for the extension of a grounded West AntarcticIce Sheet to the shelf edge of Pine Island Bay during aprevious glaciation. This interpretation is supported bythe presence of a gully–channel system and outwardbulging bathymetric contours on the adjoining conti-nental slope (Figs. 3 and 8), that were inferred to be theproduct of downslope transfer of glacigenic sediment bysediment gravity flows and sediment progradation infront of the trough mouth derived directly from the icesheet margin at the shelf edge during glacial periods(Dowdeswell et al., 2006).

The streamlined subglacial bedforms are freshlypreserved and represent the most recent glacialdeposits. They have no discernable surface drape ofoverlying glacimarine sediment (Figs. 3 and 6). Basedon this we interpret them to be the product of the mostrecent ice sheet advance to the shelf edge. On the basisof the character and distribution of the subglacialbedforms, the simplest and most likely explanation isthat this advance occurred during the last glaciation andthat the WAIS was therefore grounded to the shelf edgeduring the LGM. A further supporting argument for alast glacial age comes from a radiocarbon date of15,800±3900 14C years obtained on foraminifera shellsfrom glacimarine sediment (pebbly sand, sand and mudfacies) immediately overlying MSGL (and associatedtill) on the inner-middle shelf (Lowe and Anderson,2002). However, formation of the streamlined bedformson the outer shelf during an older glaciation cannot betotally ruled out in the absence of absolute chronolog-ical control. However, our new data improve signifi-cantly on the earlier ice sheet reconstruction ofAnderson et al. (2002a,b) and Lowe and Anderson(2002, 2003) that could only infer ice sheet extension tothe outer shelf on the basis of indirect evidence (cf.Section 2.2).

5.2. Flow-dynamics of the West Antarctic Ice Sheet inouter Pine Island Bay

The orientation of the subglacial bedforms showsthat the WAIS flowed NW through the outer shelftrough to the shelf edge. Ice on the outer shelf was fedby N to NW flow through the main trough andtributaries on the middle shelf, and may have beenderived in part from the Smith and Thwaites glacierregion (Figs. 1(, 3, 4 and 8)). In the inner Pine Island

Bay trough, a palaeo-ice stream drained the WAIS to theNW–NNW, and was derived from the Pine Island andThwaites glacier region (Fig. 7) (Lowe and Anderson,2002). The absence of cross-cutting bedforms on swathbathymetric records implies that deglaciation of WAISthrough the outer trough on the western side of PineIsland Bay at least was relatively rapid (Figs. 3 and 4).This contrasts with a gradual retreat of the ice sheet tothe middle shelf on the eastern side of Pine Island Bay(Lowe and Anderson, 2002).

The mega-scale glacial lineations and streamlinedsubglacial bedforms imaged in this study provide thefirst direct glacial geomorphological evidence for theadvance of the WAIS to the continental shelf edge of thePine Island Bay continental margin (between 110°–115°W and 71° 15′–72° 10′ S) during the last glaciation. Iceflow within the trough is inferred to have been in theform of a fast-flowing ice stream based on the followinggeomorphological and sedimentological evidence thatmatch criteria for the identification of palaeo-ice streams(cf. Stokes and Clark, 1999, 2001):

1. The presence of mega-scale glacial lineations andstreamlined subglacial bedforms with elongationratios >10:1 in the middle-outer trough are a keycriterion indicating the former presence of agrounded glacier ice, and in particular, fast flowingpalaeo-ice streams in other formerly ice sheetinfluenced regions (cf. Clark, 1993; Stokes andClark, 1999, 2001, 2002). The MSGL recordstreaming flow and fast flow velocities along thepalaeo-ice stream track (Wellner et al., 2001; ÓCofaigh et al., 2002).

2. The streamlined subglacial bedforms are formed in athin, acoustically transparent sediment layer in theouter trough (Figs. 5 and 6). The acoustic propertiesof this layer indicate that it comprises soft, poroussediment. The acoustically transparent layer isidentical to sediment layers described from bathy-metric troughs on the Antarctic Peninsula continentalmargin in which MSGL were formed in associationwith palaeo-ice streams (Ó Cofaigh et al., 2002;Dowdeswell et al., 2004; Ó Cofaigh et al., 2005a,b;Evans et al., 2005). Sediment cores from these layerscharacteristically show them to comprise highlyporous, weak, massive diamicton interpreted as softtill formed by subglacial sediment deformation(Dowdeswell et al., 2004; Ó Cofaigh et al., 2005a;Evans et al., 2005). Furthermore, cores show that thedistinct basal reflector of the acoustically transparentsediment layer represents the boundary between thesoft, deformation till and the underlying substratum


of stiff, low porosity till. Based on the similarity inacoustic properties and close association with mega-scale glacial lineations, we also interpret theacoustically transparent sediment as a soft till that

Fig. 8. Map of the Pine Island Bay continental margin summarising the mainlast glaciation and postglacial iceberg scours mapped both in this study, and bthe outer trough in outer Pine Island Bay, and the 600 m contour of the bathymgully–channel system on the continental slope is mapped on the basis of the

may, at least in part, be the product of subglacialsediment deformation. This interpretation is sup-ported by the recovery of weak, massive diamictonwithin sediment core NBP9902-PC39 from an area

subglacial geomorphic features produced beneath the WAIS during they Lowe and Anderson (2002, 2003). Note the 500 m contour markingetric trough mapped by Lowe and Anderson (2002). The location of thestudy by Dowdeswell et al. (2006).


of mega-scale glacial lineations (no iceberg scouringpresent) in 650 m of water in the mid-shelf trough ofPine Island Bay, southwest of Burke Island (72° 11′12″ S, 105° 40′ 14″W; Fig. 1) (Lowe and Anderson,2002). The massive diamicton is poorly sorted withlow shear strengths of 0.16–0.2 kg cm−2, and has ahigh water content, mineralogical homogeneity anduniform magnetic susceptibility. It is interpreted as asubglacial till and the product of sediment deforma-tion beneath the palaeo-ice stream (Lowe andAnderson, 2002, 2003).

3. The bedforms are distributed within a cross-shelfbathymetric trough (Figs. 2, 3, 4 and 8). Modern andQuaternary ice streams and fast-flowing outletglaciers drain ice sheets through such topographical-ly constrained regions which acts to facilitate strainheating and fast ice-flow (Dowdeswell et al., 1996;Stokes and Clark, 1999; Vaughan et al., 2001;Bennett, 2003; Evans et al., 2004, 2005; Ottesen etal., 2005; Ó Cofaigh et al., 2005a,b).

Further evidence supporting the presence of a palaeo-ice stream in the outer trough is provided by the outwardbulge in bathymetric contours associated with prograda-tion of the continental slope in front of the outer trough(slope gradient of 4° and 6–7° elsewhere along the PineIsland Bay margin), and the presence of a gully–channelsystem and coarse-grained sediment gravity flowdeposits across the slope (Dowdeswell et al., 2006).These features are interpreted to be related to thedownslope transfer by sediment gravity flows ofglacigenic sediment delivered directly to the shelfedge from an expanded WAIS during recent glacials,with larger volumes of deforming bed till fed to thetrough mouth by a palaeo-ice stream compared to theslope in front of intra-trough regions fed by slower-moving ice (cf. Alley et al., 1986; Dowdeswell et al.,1996, 2006; Vorren et al., 1998; Kamb, 2001; Ó Cofaighet al., 2003).

Localised cross-cutting of several reflectors, eachwith a similar discrete acoustically transparent sedimentunit above and stacked vertically in an offset pattern(Fig. 5b), occurs along the margin of the outer shelftrough. This pattern is interpreted to reflect possiblemigration of the lateral margin of the ice stream.

The base of the acoustically transparent layer of softtill, in which the streamlined subglacial bedforms areformed, is hummocky in the outer trough (Fig. 5).Sediments underlying this basal reflector have beenresolved on TOPAS records from several regions of theouter trough. These sediments are also acousticallytransparent and occur as either dome-shaped lenses (in

cross-section) or continuous layers (Fig. 6). Both linesof evidence are interpreted to record the presence ofburied subglacial bedforms immediately beneath themore recent MSGL, with the hummocky reflector whichseparates them marking the top-surface of the lower(buried) bedforms. Therefore, two generations ofsubglacial bedforms are imaged by TOPAS recordsfrom the outer trough. An earlier phase of ice-sheet flow(possibly streaming) through the outer trough producedthe larger-scale bedforms and was followed by the mostrecent phase of ice streaming to produce the MSGL. It isuncertain whether the two sets of subglacial bedformswere produced during a single or multiple glaciations.

Modern and Quaternary ice streams commonly occurover areas of soft and easily erodable sediments andbedrock (cf. Anandakrishnan et al., 1998; Studinger etal., 2001). In these locations, fast ice-flow occurs bysubglacial sediment deformation and/or basal slidingover this soft bed (cf. Alley et al., 1986; Engelhardt andKamb, 1998; Kamb, 2001). Ice streams also occur intopographically constrained regions such as bathymetrictroughs and mountain valleys (cf. Stokes and Clark,1999; Bennett, 2003). The apparent confinement ofMSGL within the outer trough of Pine Island Bayimplies that the location and development of the palaeo-ice stream may have been, in part, topographicallycontrolled, similar to many of the LGM palaeo-icestreams draining the Antarctic Ice Sheet (e.g. Andersonet al., 2002a,b; Ó Cofaigh et al., 2002; Evans et al.,2004;Ó Cofaigh et al., 2005a,b; Evans et al., 2005). Theapparent lack of MSGL on the middle-outer shelfoutside the trough probably reflects an absence ofstreaming flow across unconfined, shallow shelfregions. Alternatively, iceberg scouring across theseareas (Figs. 3, 4 and 8) could have removed subglacialbedforms recording fast ice-flow that may have beenpresent.

A control by subglacial geology on the location ofthe palaeo-ice stream on the outer shelf is also likelygiven that: (1) the MSGL and, hence, fast flowvelocities, occur together with the soft till layer in theouter trough; and (2) the transition from crystallinebedrock in inshore regions to easily erodable sedimen-tary bedrock and unconsolidated sediments furtheroffshore occurs on the inner-middle shelf of Pine IslandBay (cf. Lowe and Anderson, 2002).

The question remains as to the spatial and temporalrelationship between the streamlined subglacial bed-forms and inferred palaeo-ice stream in the outer shelftrough that we document here, and the subglacialgeomorphic features and palaeo-ice stream in the innershelf trough as documented by Lowe and Anderson


(2002, 2003). Fig. 8 summarises the distribution andlocation of glacial geomorphic and sedimentary featuresmapped in Pine Island Bay. The outer shelf troughcontains streamlined subglacial bedforms that indicatepalaeo-ice stream flow to the NW, and those in the inner-middle shelf trough record streaming flow to the N–NNW (cf. Lowe and Anderson, 2002).

One explanation for the distribution of bedforms anddeformable sediments is that there are two discretecross-shelf troughs through which the WAIS wasdrained by two separate palaeo-ice streams to the outershelf at ∼ 108° W and ∼ 114° W, respectively, duringthe last glaciation (Fig. 8). However, this reconstructionappears unlikely as swath bathymetric data indicatesthat the largest progradation of the continental marginoffshore of Pine Island Bay has taken place in front ofthe outer trough at ∼ 114° W, with only minor slopeprogradation elsewhere (Dowdeswell et al., 2006). Thissuggests that only one palaeo-ice stream drained theWAIS through Pine Island Bay and delivered significantvolumes of glacigenic sediment to the shelf edge, at∼ 114° W, at least during recent glacial periods (Fig. 7).Furthermore, MSGL indicating a second palaeo-icestream extending onto the outer shelf at ∼ 108° W (cf.Lowe and Anderson, 2002) are absent, although thiscould reflect overprinting by iceberg furrows. Thepossibility that the palaeo-ice stream trunk split intotwo filaments down-flow in order to explain thedistribution and orientation of bedforms in the troughsof Pine Island Bay is thought unlikely given weacknowledge that modern ice streams rarely showbifurcation (cf. Bennett, 2003). However, this does notmean that ice streams cannot bifurcate and have notdone so in the past.

The simplest and more likely explanation for therelationship between the two sets of data is that thetroughs on the inner and outer shelf are connected andform a single cross-shelf trough. Thus, the streamlinedsubglacial bedforms record a single major palaeo-icestream that drained the Pine Island, Thwaites and Smithglacier region of the WAIS to the shelf edge during theLGM. It is possible that the two sets of bedforms werenot formed synchronously but rather formed time-transgressively during deglaciation and retreat of thepalaeo-ice stream. A distinct wedge of acousticallytransparent sediment was imaged at the outer end, and onthe shelf in front of the Pine Island Bay bathymetrictrough (parallel to ∼ 108° 20′W) (Lowe and Anderson,2002). The wedge terminates within 30 km of the shelfedge (Fig. 8), and is interpreted to represent a grounding-zone wedge of soft till. It is similar to other grounding-zone wedges described from the Antarctic continental

margin (Anderson, 1997; Shipp et al., 1999; Anderson etal., 2002a,b; Howat and Domack, 2003). The groundingzone wedge is possibly a last deglaciation featureproduced by a retreating palaeo-ice stream undergoinga stillstand ∼ 30 km from the shelf edge, with soft tilladvected beneath the ice stream to a till terminus.However, an older age for the grounding zone wedgecannot be ruled out in the absence of any chronologicalcontrol.

The data presented in this paper, in conjunction withthe reconstruction of Lowe and Anderson (2002),provide evidence for an extensive grounded WAIS onthe Pine Island Bay continental margin of the AmundsenSea during the LGM (Fig. 8). In a wider context, our icesheet reconstruction for the Pine Island Bay margin (Fig.8), together with other published marine geophysicalinvestigations of the WAIS at the LGM, shows thatduring the last glacial cycle the ice sheet was groundedto the mid/outer shelf or shelf edge in the AmundsenSea, the Bellinghausen Sea and Ross Sea sectors (e.g.Anderson et al., 1984, 1992; Shipp et al., 1999; Domacket al., 1999; Licht et al., 1999; Wellner et al., 2001;Anderson et al., 2002a,b; Howat and Domack, 2003; ÓCofaigh et al., 2005b). These studies also showed thatthe LGM WAIS was drained via a number of palaeo-icestreams through cross-shelf bathymetric troughs, withice stream location and development being a function oftopography and subglacial geology.

5.3. Subglacial processes beneath the West AntarcticIce sheet

Subglacial processes occurring beneath the palaeo-ice stream in the outer Pine Island Bay trough can beinvoked on the basis of the morphology and nature ofthe acoustically transparent layer of soft till and itsdistinct basal reflector on TOPAS sub-bottom profilerrecords (Figs. 3–6). The distinct reflector at the base ofthe acoustically transparent layer of soft till representsthe boundary with the underlying sedimentary substra-tum, and sediment cores from Antarctic Peninsulapalaeo-ice stream troughs show that this corresponds toa transition form soft, porous till above to stiff, low-porosity till below (Ó Cofaigh et al., 2002, 2005a;Evans et al., 2005). The nature of the basal reflector ofthe soft till layer in Pine Island Bay indicates thatgroove-ploughing of the subglacial substrate occurredbeneath the palaeo-ice stream in the outer shelf troughduring the last glaciation (cf. Tulaczyk et al., 2001;Clark et al., 2003; Evans et al., 2005). The evidence insupport of this interpretation includes: (1) an irregularand grooved reflector at the base of the acoustically


transparent till layer in the outer trough (Figs. 5 and 6);(2) the acoustically transparent till layer is sometimesdistributed across the flanks and crests of the groovesin the outermost trough (Fig. 5b). Conceptually, thiscould result from remobilisation and deformation of thesubglacial substrate during the groove-ploughingprocess where the ice-keels erode sediment anddisplace it to either side as well as forward. Thissediment then becomes plastered to the flanks andcrests of the grooves, with remobilisation of some softtill back into the groove troughs; (3) groove-ploughingof sedimentary strata as recorded by the incision ofunderlying sub-bottom sediment units and acousticreflectors by the irregular reflector forming the base ofthe uppermost acoustically transparent, palaeo-icestream till layer (Fig. 6).

Elsewhere in the outer trough, the basal reflector ofthe acoustically transparent till layer is mainly smoothand horizontal (Figs. 5 and 6). Previous studies fromMarguerite Trough and the northern Larsen shelf on thewest and east sides of the Antarctic Peninsula suggestthat such a smooth reflector are compatible with erosionof the underlying substratum by a mobile layer ofdeforming till beneath a palaeo-ice stream (cf. Cuffeyand Alley, 1996; Ó Cofaigh et al., 2005a; Evans et al.,2005). It is possible that erosion by this deforming tilllayer could also contribute to formation of thehummocky base of the soft till layer from the outertrough (cf. Rooney et al., 1986) (Figs. 5 and 6).

The features preserved on TOPAS records indicate acombination of groove-ploughing of the substrate by icekeels and subglacial sediment deformation associatedwith a mobile till layer beneath the palaeo-ice stream inthe outer Pine Island Bay trough, and that the formationof the layer of soft till, and glacial lineations and mega-scale glacial lineations, were a function of these twoprocesses (Alley et al., 1986; Tulaczyk et al., 2001;Clark et al., 2003; Dowdeswell et al., 2004; Ó Cofaigh etal., 2005a; Evans et al., 2005). Geological evidencefrom the Antarctic Peninsula and Pine Island Baysuggest that groove ploughing, together with subglacialsediment deformation, are important processes occur-ring beneath Antarctic palaeo-ice streams.

Well-developed meltwater channels, such as thoseformed in crystalline bedrock in inner Pine Island Bay(Fig. 8) (cf. Lowe and Anderson, 2002, 2003), were notobserved on high-resolution swath bathymetric records inthe outer shelf trough (Figs. 3 and 4). The widespreadpresence of the acoustically transparent sediment layer ofsoft till suggests that subglacial meltwater evacuationmay have been accomplished by Darcian through-flowwithin the till layer (cf. Wellner et al., 2001; Lowe and

Anderson, 2002, 2003). It is possible that the broad,shallow ‘canals’ hypothesised for deformable beds(Walder and Fowler, 1994), and reportedly present indeforming till beneath the Rutford Ice Stream, Antarctica(King et al., 2004), are present in the outer trough buttheir scale cannot be resolved in our geophysical data.

5.4. Timing of iceberg scouring in Pine Island Bay

In water depths of <570 m in outer Pine Island Bay,the irregular sea floor reflector and lack of sub-seafloor sedimentary structure on TOPAS records, com-bined with the highly irregular pattern of linear grooveson swath bathymetric images, are interpreted to resultfrom iceberg scouring (Figs. 3, 4 and 7). Iceberg scourswere also imaged in water depths down to 700 m in theinner and middle part of the Pine Island Bay trough(Lowe and Anderson, 2002). The question remains asto whether this iceberg scouring was produced byicebergs calved from the retreating WAIS during thelast deglaciation or occurred during postglacial timesby calving from the floating margins of Pine Island andThwaites Glaciers.

The thickness of the floating margins of Pine Islandand Thwaites Glaciers is available in the BEDMAP datacompilation of Antarctic ice thickness (Lythe et al.,2000), and from other published work (e.g. Lucchitta etal., 1995; Rignot, 2001). These ice thickness data,illustrated in Fig. 9, indicate that the floating part of PineIsland Glacier is ∼ 450 to 590 m thick, and extendsbetween 420 and 530 m below present-day sea level.Total ice thickness increases rapidly to 900–1200 mclose to the grounding line (Fig. 9a). In comparison, thefloating tongue of Thwaites Glacier has a total thicknessof 160 to 450 m, with 140 to 400 m below sea level (Fig.9b). Total ice thickness increases rapidly to >950 mclose to the grounding line.

The present-day margins of Pine Island and ThwaitesGlaciers therefore produce deep-keeled icebergs up toabout 500 m thick below the water line (Fig. 9).However, it is conceivable that the depth to which theseiceberg keels extend could increase to >770 m if therewas a total collapse of the floating sections of theseglaciers, which would release ice close to the groundingline. There has been little change in the mean position ofthe Pine Island Glacier front over the last 50 years,although the ice tongues have experienced thinning inrecent times (Rignot, 2001, 2002). There are no palaeo-records available in order to reconstruct glacier fluctua-tions prior to 50 years ago. On the basis of floating-icethickness, the iceberg scours observed across the outershelf in water depths of∼ 400 to 560 m could have been

Fig. 9. Ice thickness of the main glacier-fed ice shelves fringing Pine Island Bay. (a) Location of survey lines and ice thickness profiles for the floatingice shelf of Pine Island Glacier; and (b) location of survey lines and ice thickness profiles for the floating ice shelf of Thwaites Glacier. Data are fromthe BEDMAP compilation (Lythe et al., 2000).


produced by deep-keeled icebergs of up to 530 m ormore, calved from the interglacial (Holocene) floatingmargins of the Pine Island and Thwaites Glaciers.However, the deepest scours on the continental shelf ofPine Island Bay occur in water depths down to ∼ 770 m(cf. Lowe and Anderson, 2002). These scours are morelikely to have been produced by icebergs calved from athicker retreating margin of the West Antarctic Ice Sheetduring the last deglaciation, remembering that eustaticsea level would have been 120 m lower than that oftoday. Thus the iceberg scours observed on the swath

bathymetry probably record both Holocene and degla-cial iceberg calving and ploughing.

6. Conclusions

• Multibeam swath bathymetric data (Figs. 2–4) revealthe presence of mega-scale glacial lineations(MSGL) within a prominent bathymetric troughextending across outer Pine Island Bay to thecontinental shelf edge (between 111° and 115° W,71° 15′–72° 30′ S; Figs. 2 and 8). The distribution of


these streamlined subglacial bedforms providesdirect glacial evidence in support of a groundedWest Antarctic Ice Sheet (WAIS) extending to thecontinental shelf edge as a fast-flowing palaeo-icestream during the last glaciation.

• TOPAS sub-bottom profiler records (Figs. 5 and 6)show that the MSGL and lineations are formed in thesurface of an acoustically transparent unit of soft till,inferred to be the product of subglacial sedimentdeformation. The MSGL and soft till layer record theformer presence of a palaeo-ice stream in the outershelf trough. Ice stream location and development onthe outer shelf was a function of topography andsubglacial geology.

• The smooth, horizontal to irregular basal reflector tothe acoustically transparent unit of soft till in TOPASrecords (Figs. 5 and 6) indicates that both groove-ploughing of the substrate by ice keels and subglacialsediment deformation occurred beneath the palaeo-ice stream in outer Pine Island Bay, and that the softtill layer, and possibly subglacial bedforms, were afunction of these processes. The Pine Island Baydata, in conjunction with published work from theAntarctic Peninsula, implies that these two processesare important mechanisms occurring beneath Ant-arctic palaeo-ice streams.

• In a wider context, our data alongside other publishedice sheet reconstructions show that during the lastglaciation the WAIS was regionally extensive andgrounded to the shelf edge in the Amundsen Sea (Fig.8), Bellingshausen Sea and Ross Sea sectors of WestAntarctica.

Acknowledgements

This work was funded by UK NERC grant NERC/T/5/2000/00986 as part of the Autosub Under IceThematic Programme. We thank the officers and crewof the RRS James Clark Ross for their cooperationduring cruise JR84 to the Antarctic Peninsula and PineIsland Bay. We thank David Vaughan (British AntarcticSurvey) and Jon Copley (Southampton OceanographyCentre) for assisting with data acquisition, and AndrewShepherd (Scott Polar Research Institute) for advice onthe thickness of floating margins of glaciers draining theWAIS into Pine Island Bay.

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extent and dynamics of the west antarctic ice sheet on the outer continental shelf of pine island...

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