west antarctic ice sheet change since the last glacial period

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The potential for rapid deglaciation, orcollapse, of the 2-million-square-kilometerWest Antarctic Ice Sheet (WAIS) in responseto climate change is one of the most serious

environmental threats facing mankind. TheWAIS is a marine ice sheet with large partsof its ice grounded below sea level. Com-plete collapse would result in a global sealevel rise of approximately 5 meters, withimmense social, economic, and ecologicalconsequences.

While most experts consider such a col-lapse unlikely within the next few centuries,the Amundsen Sea sector has been identi-fied as the most likely site for initiation ofcollapse, and it alone contains the potentialto raise sea level by approximately 1.5

meters [Vaughan, 2007]. This would result indevastating flooding in many low-lyingcities (e.g., New Orleans, London), agricul-tural areas (e.g., Netherlands, Bangladesh)and atolls (e.g., Maldives).

Glaciers flowing into the Amundsen Seaexhibit the most rapid recent decrease insurface elevation and grounding line retreatin Antarctica [Rignot, 1998; Thomas et al.,2004]. Basal melting of ice shelves resultingfrom flow of relatively warm CircumpolarDeep Water onto the continental shelf mayhave triggered these changes [Jacobs e t al .,

1996;Shepherd et al., 2004]. The marine andterrestrial records of change in the Amund-sen Sea region during the Quaternary (thepast 1.8 million years) are key to under-standing the stability and climate sensitivityof the WAIS, which in turn is essential torefining ice sheet models and thus improv-ing predictions of the contribution from

VOLUME 88 NUMBER 17

24 APRIL 2007

PAGES 189196

Eos, Vol. 88, No. 17, 24 April 2007

EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION

PAGES 189190

West Antarctic Ice Sheet ChangeSince the Last Glacial Period

BYR.D. LARTER, K. GOHL, C.- D. HILLENBRAND, G.

KUHN, T. J. DEEN, R. DIETRICH, G. EAGLES, J. S. JOHN-

SON, R. A. LIVERMORE, F. O. NITSCHE, C. J. PUDSEY,

H.-W. SCHENKE, J. A. SMITH, G. UDINTSEV, ANDG.

UENZELMANN-NEBEN

Fig. 1. (a) Map showing tracks of 2006 research cruises (red, RRS James Clark Ross; blue, R/VPolarstern), seismic lines (green), core sites (black dots and one red dot) and sites sampled for

surface exposure age dating (red stars) in the Amundsen Sea embayment. Color backgroundis regional bathymetry, from a new compilation of echo sounding data [Nitsche et al.,2006].

Black box indicates location of Fig. 2. Grayscale imagery shown over onshore areas is part of theMODIS Mosaic of Antarctica (courtesy of National Snow and Ice Data Center).

(b) Photographs (left-hand column), lithologi-cal and sedimentary structure logs of core

PS69/289-3 collected in Pine Island Bay. Corelocation is shown by red dot in Figure 1a.


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Antarctica to global sea level rise. In the lat-est Intergovernmental Panel on ClimateChange (IPCC) Summary for Policymakers,model projections of sea level rise excludethe effects of future rapid dynamicalchanges in ice flow because a basis inpublished literature is lacking [IPCC, 2007].

Records of changes in the WAIS duringlate Quaternary deglaciations, particularlysince the Last Glacial Maximum (LGM,~20,000 years ago), can be used to test andrefine such models. Extensive new data onthe late Quaternary history of the WAIS werecollected during two research cruises to theAmundsen Sea embayment that took placein JanuaryApril 2006 (Figure 1a). The expe-ditions were closely coordinated by scien-tists from the British Antarctic Survey (BAS)and the Alfred Wegener Institute for Polarand Marine Research (AWI), Germany.

Previous Quaternary Research

Even though about one third of the out-flow from the WAIS is into the Amundsen

Sea, little is known about the history of themajor glacial systems in this area. A recon-struction of ice retreat since the LGM hasbeen proposed for Pine Island Bay (Figure1a), but it is based on a small number ofradiocarbon dates with large uncertainties[Lowe and Anderson, 2002].

Few vessels visit the Amundsen Seaembayment because of its remoteness andpersistent sea ice cover. Multibeam echo-sounding data collected on the onlyresearch cruise to the region in the five

years before the 2006 expeditions revealedstreamlined subglacial bed forms within abathymetric trough on the continental shelf

at 114W, suggesting that the WAIS groundingline advanced to the shelf edge during thelast glaciation [Evans et al. , 2006]. A cross-shelf trough along 107W containing similarbed forms and extending at least as farnorth as 72S has also been described fromearlier data collected on the R/VNathaniel

B. Palmer[Lowe and Anderson , 2002]. Therelationship between the two troughsremains unclear.

New Data

In early 2006, successive research cruiseson the RRSJames Clark Ross(cruise JR141,

JanuaryFebruary) and the R/VPolarstern(expedition ANT-XXIII/4, FebruaryApril) vis-ited the Amundsen Sea embayment. A com-pilation of previous echo-sounding data pro-duced by the Lamont-Doherty EarthObservatory (and subsequently updated toinclude the new data [Nitsche et al. , 2006])proved a valuable aid to planning. A com-prehensive package of data that was trans-ferred from theJames Clark Rossto the

Polarstern facilitated precise planning ofadditional survey work to cover complemen-tary areas. Much of the work on both cruiseswas concentrated in a large polynya (openwater area) in the western embayment andon the outer shelf and slope (Figure 1a).

Guided by helicopter ice reconnaissanceobservations, thePolarstern broke throughsea ice to reach another polynya in PineIsland Bay and along the eastern coast ofthe embayment.

On both cruises, extensive new sonarimagery (multibeam echo sounding of theseafloor and subbottom acoustic profiledata), sediment cores, and high-resolutionseismic reflection profiles were collected toexamine late Quaternary glacial changes inthe Amundsen Sea. Changes in the surfaceelevation of adjacent parts the WAIS sincethe LGM were also investigated by using thehelicopters on thePolarstern to visit ice-freeonshore sites and obtain samples for surfaceexposure age dating. Thus, the data sets col-lected should provide new constraints onchanges in both the extent and surface ele-vation of the WAIS. GPS measurements weremade to determine present-day flow veloci-ties and tidal motion of several ice shelfsites. Oceanographic data collected on bothcruises will provide constraints on the mod-ern water circulation in the embayment.

The new surveys more than double thespatial coverage of detailed bathymetricdata on the continental shelf and revealstreamlined subglacial bed forms (Figure 2),allowing reconstruction of the paleo-icedrainage pattern during the last glaciation.Data collected north of the Dotson and Getzice shelves indicate that three ice streamtributaries converged into a single maintrunk, which then flowed northwestwardacross the shelf. A trough over 1600 metersdeep north of the Getz Ice Shelf is the deep-est yet found anywhere on the West Antarc-tic continental shelf. The axes of the deepest

troughs in this area form a dendritic patternwith meanders and undulating depths, fea-tures that have been interpreted elsewhereas being indicative of subglacial meltwatererosion [e.g.,Lowe and Anderson, 2002].

Multibeam data collected over the outershelf mostly show randomly oriented trailsproduced by iceberg keels plowing throughthe sediments. However, bed forms of likelysubglacial origin, exhibiting a strong prefer-ential alignment in a northeast to north-northeast direction, were observed in twopreviously unmapped troughs on the easternpart of the outer shelf.

The sediment cores collected (locationsin Figure 1a) will provide new insightsregarding past subglacial processes on thecontinental shelf, paleoenvironmentalchanges associated with deglaciation, andthe timing of the last deglaciation. A vibro-corer used on theJames Clark Rossrecov-ered a total of 130 meters of core from 39sites on the continental shelf and upperslope. On thePolarstern, a gravity corerrecovered a total of 92 meters from 24 sites,

including three cores exceeding 9 meters inlength collected from deep troughs in PineIsland Bay (Figure 1b). Cores from the outershelf and the upper slope typically contain asubglacial, deglacial, and Holocene (the past10,000 years of the current postglacialperiod of time) succession from soft, mas-sive diamicton (nonsorted conglomerates ofglacial sediment) at the base, through grav-elly and sandy mud to bioturbated mud withupward increasing contents of planktonicforaminifera. The succession from inner shelfsites is similar, but with diatoms as the domi-nant microfossils.

Fig. 2. Perspective view of multibeam echo-sounding data showing change in seafloor mor-phology, from drumlins (elongated hills formed by glacial activity) on left to megascale glaciallineations on right, along the course of a paleo-ice stream. The front of the block diagram is trans-

parent to show the position of a seismic reflection profile (shaded light gray) running throughthe area. Red lines on the seismic profile mark the top of acoustic basement. The change in sea-floor morphology coincides with a transition in substrate, from acoustic basement to sedimentary

strata. Inset shows data on part of the seismic profile.


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We will use the new marine data to recon-struct the retreat of the ice sheet since theLGM. The timing of deglaciation will be largelydetermined using radiocarbon techniquesonce we have established a regional marinereservoir correction from seafloor sedimentssampled with box corers. The measurement ofradiocarbon ages of calcareous foraminiferafrom the outer shelf sediments will allowassessment of the offset between ages mea-sured on carbonate and on organic carbon.

To fully reconstruct the deglaciation, it isalso important to obtain data on changes inice surface elevations. Samples were col-lected from glacially derived erratic boul-ders (rocks that have been transported to anew location by glaciation) found on bed-rock surfaces at four sites (Figure 1a). Granit-oid erratics were commonly found. Quartzextracted from these samples will be ana-lyzed for concentrations of beryllium-10 andaluminum-26, isotopes produced when cos-mic rays penetrate the boulders, splittingapart atomic nuclei within the rock. The iso-topic abundance indicates how long ago ice

last covered the rock surfaces.High-resolution seismic reflection profiles

(Figure 1a) will allow assessment of the vari-ability of glacial processes on the shelf overseveral glacial cycles, and also allow evalua-tion of the influence of substrate on icedynamics. Profiles across the outer shelf willenable identification of buried troughs thatwere the paths of paleo-ice streams, and showwhether they changed position during suc-cessive glacial periods. Preliminary examina-tion of profiles across the inner shelf alreadyshow a transition from acoustic basement(crystalline or deformed sedimentary rocks)nearshore to undeformed sedimentary strata

farther offshore that broadly correlates with achange in seafloor bed forms. These prelimi-nary observations are consistent with theidea that substrate was an important controlover the type of seafloor morphological fea-tures formed [Wellner et al., 2001].

On thePolarsterncruise, deep-penetrationreflection and refraction/wide-angle seismicdata, as well as gravity and extensive ship-borne and helicopter magnetic data, werealso collected to investigate the crustalstructure of the Amundsen Sea embayment.These data will provide a basis for consider-ing how structural elements influencedgrowth and retreat of the ice sheet throughcontrols on topography and the dispositionof sedimentary basins.

Future Research

Despite the collection of these extensivenew data, much more remains to be donein the Amundsen Sea embayment, particu-larly on the central part of the shelf thatremained covered by pack ice throughoutthe 20052006 austral summer and onshorestudies. Complete reconstruction of LGMpaleo-ice streams will require much moreextensive bathymetric data coverage. Thecontinental shelf in the embayment is

about 100,000 square kilometers in area,comparable in size to the state of Colorado,and the total area covered so far is onlyabout 20% of that.

Acknowledgments

This work was supported by the AWIMarine, Coastal and Polar Systems (MARCOP-OLI) programs MAR2 and POL6 and the BASGlacial Retreat in Antarctica and Deglaciationof the Earth System (GRADES) program. Theauthors thank the captains, officers, crew, heli-copter operations team, technical supportstaff and other scientists who participated in

the two research cruises. The vibrocorer andseismic equipment used on theJames Clark

Ross were loaned and operated by staff fromthe British Geological Survey Marine Opera-tions Group. This is AWI publication awi-n16355.

References

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Nitsche, F. O., S. Jacobs, K. Gohl, S. Gauger,R. D. Larter, and T. Deen (2006), A new regionalbathymetry map of the Amundsen Sea, West Ant-arctica,Eos Trans. AGU, 87(52), Fall Meet. Suppl.,Abstract C41C-0345.

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from West Antarctica,Science, 306, 255258.Vaughan, D. G. (2007), West Antarctic Ice Sheet

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Wellner, J. S., A. L. Lowe, S. S. Shipp, and J. B. Anderson(2001), Distribution of glacial geomorphic featureson the Antarctic continental shelf and correlationwith substrate: Implications for ice behavior,J. Gla-ciol., 47, 397411.

Author Information

Tara Deen,Claus-Dieter Hillenbrand,Joanne John-son,Rob Larter,Roy Livermore,and James Smith,Brit-ish Antarctic Survey,Cambridge,U.K.; E-mail: [email protected]; Graeme Eagles, Karsten Gohl, Gerhard Kuhn,Hans-Werner Schenke,and Gabriele Uenzelmann-

Neben,Alfred Wegener Institute,Bremerhaven,Germa-ny; Frank Nitsche,Lamont-Doherty Earth Observatoryof Columbia University,Palisades,N.Y.; Carol Pudsey,University of Dundee,U.K.; Reinhard Dietrich,TechnicalUniversity Dresden,Germany; and Gleb Udintsev,Ver-nadsky Institute for Geochemistry,Moscow.

west antarctic ice sheet change since the last glacial period

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