2000 Fall Meeting          
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Cite abstracts as Eos Trans. AGU, 81 (48),
Fall Meet. Suppl., Abstract xxxxx-xx, 2000
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marchant

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HR: 1330h
AN: H12B-01
TI: Sublimation Losses Computed with Cosmogenic He-3 Depth Profiles, Beacon Valley Relict Glacial Ice, East Antarctica
AU: * Phillips, W M
EM: wmp@geo.ed.ac.uk
AF: Dept of Geography, University of Edinburgh, Edinburgh, EH8 9XP United Kingdom
AU: Lewis, A
AF: Dept of Geological Sciences, University of Maine, Orono, ME 04469 United States
AU: Landis, G P
AF: USGS, Denver Federal Center, Denver, CO 80225-0046 United States
AU: Marchant, D R
AF: Dept of Earth Sciences, Boston University, Boston, MA 02215 United States
AU: Sugden, D E
AF: Dept of Geography, University of Edinburgh, Edinburgh, EH8 9XP United Kingdom
AB: The age of relict glacial ice beneath a thin till in central Beacon Valley is an important but controversial constraint for the climatic history of East Antarctica. Stratigraphic relationships with dated volcanic ash suggest that the ice is $>$8 Ma. However, sublimation rates calculated assuming atmosphere-ice contact permit ice survival for only a few 100 ka. We present new estimates of sublimation losses computed with cosmogenic He-3 data. Two He-3 depth profiles were measured in pits (location 77.84S 160.61E altitude 1377m) dug near the center of raised polygons developed in the till. Profile 1 consists of dolerite cobbles sampled at depths of 0, 14, 21, 59, and 70 cm. Profile 2 cobbles are from depths of 0, 9, 25, and 38 cm. Both profiles exhibit a systematic decrease of He-3 from the surface downward, showing that vertical mixing of clasts by cryoturbation has not occurred. The top of profile 1 has a He-3 surface exposure age of 2.77 $\pm$ 0.26 Ma, while that of profile 2 is 417 $\pm$ 39 ka. The exposure ages (1 $\sigma$ errors) assume zero erosion of surface clasts and do not correct for possible snow shielding. The ages are minimums that do not represent true formation ages of the ice. Subsurface samples in both profiles are grossly deficient in He-3 compared with expected values deduced by extrapolation from the surface. This suggests that the till is a lag deposit created by the progressive removal of relict ice by sublimation. Sublimation losses (computed as per Schaefer et al., EPSL 179, 91-99, 2000) are maximums because they employ a constant till bulk density of 2.00 g/cc. Till density has probably increased over time as ice was lost to sublimation. In profile 1, losses listed in order of decreasing depth are 0, 5.9, 9.2, 16.5, and 25.1 m. In profile 2, losses are 0, 6.0, 12.3, and 16.8 m. A maximum sublimation rate of 9 $\pm$ 0.8 m/Ma is indicated from these data. Our results show that surface stability is variable in Beacon Valley, vertical mixing of till by cryoturbation is not occurring, and that sublimation has proceeded much more slowly than required by atmospherically-determined vapour pressure gradient models.
DE: 9310 Antarctica
DE: 1827 Glaciology (1863)
DE: 1863 Snow and ice (1827)
DE: 1035 Geochronology
SC: H
JN: Eos Trans. AGU, 81 (48), Fall Meet. Suppl., 2000
MN: Fall Meeting 2000


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