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Past and Future Grounding-Line Retreat of the West Antarctic Ice Sheet

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Past and Future Grounding-Line Retreat of the West Antarctic Ice Sheet R EPORTS primary source of meltwater pulse 1B. This when sampling, greater activity in the past is difficult to 32. H. W. Borns Jr., C. Dorion, P. E. Calkin, G. C. Wiles, D. conclusion is not strongly dependent on the ice rule out. Basalt boulders derived from lava flows tend to Barclay, ibid. XXX, 100 (1995). be highly jointed and disintegrate relatively rapidly sheet model. 33. M. Stuiver and P. J. Reimer, Radiocarbon 25, 215 along these joint surfaces. Subsequent (measured) ero- (1993). sion of the fragments is slow and primarily by wind. 34. F. M. Phillips and M. A. Plummer, ibid. 38, 98 (1996). 23. T. Sowers and M. Bender, Science 269, 210 (1995). References and Notes 24. J. L. Fastook and M. Prentice, J. Glaciol. 40, 167 35. This research was supported by NSF grants OPP 1. J. H. Mercer, Geol. Soc. Am. Bull. 79, 471 (1968); J. G. (1994). 93-18872 (H.W.B. and P.E.C.), OPP 94-18333, and Bockheim, S. C. Wilson, G. H. Denton, Quat. Res. 31, 25. E. J. Steig et al., in preparation. EAR 96-14561 (M.D.K.). The authors thank S. Murphy 229 (1989); G. H. Denton, J. G. Bockheim, S. C. 26. P. Huybrechts, Clim. Dyn. 5, 79 (1990). for assistance in sample preparation, D. Lott and J. Wilson, J. E. Leide, B. G. Anderson, ibid., p. 189. 27. ࿜࿜࿜࿜ , Ann. Glaciol. 5, 115 (1990). Curtice for maintaining the mass spectrometer, and 2. G. H. Denton, J. G. Bockheim, S. C. Wilson, M. Stuiver, 28. E. J. Steig, ibid. 25, 418 (1997). L. Woodward for drafting figures. N. Dunbar provided samples, differential GPS elevations, and valuable Quat. Res. 31, 151 (1989); E. J. Brook, M. D. Kurz, R. P. 29. R. G. Fairbanks, Nature 342, 637 (1989). discussion. G. Wiles, C. Dorion, and T. Redfield assist- Ackert Jr., G. Raisbeck, F. Yiou, Earth Planet. Sci. Lett. 30. A. M. Tushingham and W. R. Peltier, J. Geophys. Res. ed in the fieldwork. This is Woods Hole Oceano- 131, 41 (1995); B. L. Hall and G. H. Denton, Geogr. Ann., 96, 4497 (1991); W. R. Peltier, Science 265, 195 graphic Institution contribution 10026. in press. (1994). 3. J. B. Anderson, S. S. Shipp, L. R. Bartek, D. E. Reid, in 31. N. W. Dunbar, R. P. Esser, W. C. McIntosh, Antarct. Contributions to Antarctic Research III, vol. 57 of J. U.S., in press. 16 July 1999; accepted 9 September 1999 Antarctic Research Series, D. H. Elliot, Ed. (American Geophysical Union, Washington, DC, 1992), pp. 39Ð 62; S. Shipp, J. B. Anderson, E. W. Domack, Geol. Soc. Am. Bull., in press. 4. K. J. Licht, A. E. Jennings, J. T. Andrews, K. M. Williams, Geology 24, 223 (1996); K. J. Licht, N. W. Dunbar, J. T. Past and Future Grounding-Line Andrews, A. E. Jennings, Geol. Soc. Am. Bull. 111,91 (1999); E. W. Domack, E. A. Jacobson, S. Shipp, J. B. Anderson, ibid., in press. Retreat of the West Antarctic 5. G. H. Denton, M. L. Prentice, L. H. Burkle, in The Geology of Antarctica, R. J. Tingey, Ed. (Clarendon, Oxford, 1991), pp. 365Ð419. Ice Sheet 6. P. Martinerie, D. Raynaud, D. M. Etheridge, J. Barnola, 1 2,3 2 1 D. Mazaudier, Earth Planet. Sci. Lett. 112, 1 (1992). H. Conway, B. L. Hall, G. H. Denton, A. M. Gades, 7. D. Raynaud and B. Lebel, Nature 281, 289 (1979); D. E. D. Waddington1 Raynaud and I. M. Whillans, Ann. Glaciol. 3, 269 (1982). 8. D. Jenssen, in The Climate Record in Polar Ice Sheets, The history of deglaciation of the West Antarctic Ice Sheet (WAIS) gives clues G. d. Q. Robin, Ed. (Cambridge Univ. Press, London, about its future. Southward grounding-line migration was dated past three 1983), pp. 138Ð144. locations in the Ross Sea Embayment. Results indicate that most recession 9. P. Grootes and M. Stuiver, Quat. Res. 26, 49 (1986). 10. W. LeMasurier and Y. Kawachi, in Volcanoes of the occurred during the middle to late Holocene in the absence of substantial sea Antarctic Plate and Southern Oceans, vol. 48 of Ant- level or climate forcing. Current grounding-line retreat may reflect ongoing ice arctic Research Series, W. LeMasurier and J. W. recession that has been under way since the early Holocene. If so, the WAIS Thompson, Eds. (American Geophysical Union, Wash- could continue to retreat even in the absence of further external forcing. ington, DC, 1990), pp. 208Ð211; W. LeMasurier and D. C. Rex, J. Geophys. Res. 94, 7223 (1989). 11. Tephra layers erupted onto the surface of the ice The grounding line of the WAIS has retreated Coast, Hatherton Coast, and Roosevelt Is- sheet are exposed in the ice sheet ablation area on nearly 1300 km since the Last Glacial Max- land (Fig. 1), that resolve the Holocene the southwest side of Mount Waesche. The layers imum (LGM) about 20,000 years before deglaciation of the Ross Sea Embayment. include locally derived pyroclastic debris and basalt clasts up to 4 m thick (31). Individual tephra layers present (yr B.P.), when grounded ice in the At the LGM, outlet glaciers that flowed can be traced to the ice-cored moraine, demonstrat- Ross Sea Embayment extended almost to through the Transantarctic Mountains and ing that englacial tephra is the source material for the Coulman Island (1–3) (Fig. 1). Complete col- across the coast thickened substantially ice-cored moraine. 12. Sample locations were recorded with handheld global lapse of the WAIS would cause sea level to where they merged with grounded ice filling positioning systems (GPSs); sample elevations were rise 5 to 6 m. Estimates of the present stabil- the Ross Sea Embayment. Only along the determined by altimeter in relation to a base camp ity of the WAIS are hampered by uncertain- southern Scott Coast adjacent to McMurdo on the ice sheet. Relative uncertainties in elevation between sample sites are estimated to be Ϯ5m.The ties in the overall mass balance (4) and un- Sound, 450 km south of Coulman Island, did relative elevations were tied to geodetic altitude by certainties concerning the dynamic response the Ross Sea ice sheet terminate on land in differential GPSs (Ϯ20 m) at several locations. of the ice sheet to changes in sea level or the mouths of ice-free Taylor Valley and dry 13. F. M. Phillips et al., Science 274, 749 (1996); J. C. climate. It is thought that it would take ϳ104 valleys fronting the Royal Society Range Gosse, J. Klein, E. B. Evenson, B. Lawn, R. Middleton, ibid. 268, 1329 (1995); E. J. Brook et al., Quat. Res. years for the WAIS to reach equilibrium after (Fig. 1). This peculiar situation arose because 39, 11 (1993). a perturbation (5), but accurate assessment only here did East Antarctic ice and alpine 14. M. D. Kurz, Geochim. Cosmochim. Acta 50, 2855 is difficult because the dynamics of the glaciers terminate well inland, leaving the (1986). The inherited (mantle-derived) 3He/4He ratio was determined by crushing the mineral separates in present ice sheet is dominated by ice coast susceptible to incursions of landward- vacuo, which selectively released gases held in fluid streams. Fast-flowing ice streams evacuate flowing grounded ice at the LGM. Over 200 and melt inclusions. The cosmogenic 3He component inland ice rapidly, but field evidence indi- 14C dates of lacustrine algae from proglacial is calculated by subtracting the initial 3He from that released by fusion of the sample powders in vacuo. cates that abrupt changes from fast to slow lakes (3, 7, 8), dammed in these valleys by 15. D. Lal, Earth Planet. Sci. Lett. 104, 424 (1991). flow have occurred in the past (6). We look grounded ice, show that the Ross Sea ice 16. T. E. Cerling and H. Craig, Geochim. Cosmochim. Acta to the deglacial history of the WAIS for sheet was close to its LGM position from at 58, 349 (1994). 17. F. M. Phillips, M. G. Zreda, M. R. Flinsch, Geophys. Res. clues about its future. Below we present least 27,820 to 12,880 calendar yr B.P. (9). Lett. 23, 949 (1996). dates from three locations, southern Scott The grounding line was still north of Mc- 18. J. O. Stone, personal communication. Murdo Sound 9420 yr B.P.; this date corre- 19. B. Liu, F. M. Phillips, J. T. Fabryka-Martin, M. M. Fowler, 1 sponds to the youngest delta of a proglacial W. D. Stone, Water Resour. Res. 30, 3115 (1994). Geophysics Program, University of Washington, 2 20. F. M. Phillips et al., Geol. Soc. Am. Bull. 109, 1453 Seattle, WA 98195, USA. Institute for Quaternary lake dammed in Taylor Valley by grounded (1997). Studies and Department of Geological Sciences, Uni- Ross Sea ice (3, 8). Two thousand years later, 3 21. E. T. Brown et al., Geochim. Cosmochim. Acta 55, versity of Maine, Orono, ME 04469, USA. Depart- the McMurdo Sound region was free of 2269 (1991); K. Nishiizumi, C. P. Kohl, J. R. Arnold, J. ment of Geology and Geophysics, Woods Hole Klein, D. Fink, Earth Planet. Sci. Lett. 104, 440 (1991). Oceanographic Institution, Woods Hole, MA 02543, grounded ice, based on two lines of evidence. 22. Although areas of active cryoturbation were avoided USA. First, molluscs recolonized the area after the 280 8 OCTOBER 1999 VOL 286 SCIENCE www.sciencemag.org R EPORTS grounding line retreated; the oldest shell from present level ϳ6800 yr B.P. Because Ross Sea ness at the summit, H ϭ 744 Ϯ 10 m, and the the region dates to 7550 yr B.P. (Table 1).
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