2003 Fall Meeting          
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Cite abstracts as Eos Trans. AGU, 84(46),
Fall Meet. Suppl., Abstract xxxxx-xx, 2003
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HR: 0830h
AN: GC31B-0176
TI: Evaluating the Age of Buried Ice in Antarctica Using Ashfall Deposits: New Insights from Deposit Morphology, Grain Shape, and LA-ICP-MS Trace-Element Geochemistry
AU: * Lewis, A R
EM: alewis@bu.edu
AF: Boston University, Department of Earth Sciences, Boston, MA 02215 United States
AU: Marchant, D R
EM: marchant@bu.edu
AF: Boston University, Department of Earth Sciences, Boston, MA 02215 United States
AB: Dating of buried ice in the western Dry Valleys region relies on $^{40}$Ar/$^{39}$Ar analysis of ashfall deposits within sublimation tills that rest directly on stagnant glacier ice. The oldest ice so dated is $>$8.1 Ma. The fundamental assumption is that dated ashes are in-situ and have not been transported from surface deposits elsewhere in the Dry Valleys region. Given that the surface of sublimation tills shows well-developed patterned ground, the presumption of ground stability and long-term preservation of in-situ ashfall is questioned. As a test of ground stability, we examined ash-deposit morphology, grain shape, and glass-shard trace-element geochemistry from several ashfall deposits used to provide limiting ages on buried ice and tills in the western Dry Valleys. Detailed field analyses show that ashfall that collects in sublimation tills over buried ice occurs in one of three morphologic settings: surface troughs that delineate sand-wedge polygons, void spaces in gravel-and-cobble lags that overlie active sand wedges, and 1 to 2-cm-wide thermal contraction cracks. Post-depositional sublimation of underlying ice may distort initial deposit morphology through uneven surface lowering. Microscopic analyses of concentrated ashfall deposits that lack detrital sand grains show highly angular glass shards that preserve delicate hair-like spires and thin bubble-wall vesicles. Grain edges are sharp with no chipped, fractured, or pitted surfaces. In contrast, ash deposits containing detrital sand grains show subangular to subrounded shard morphologies with concave fractures and pits on grain edges, all of which are suggestive of abrasion during transport. In such deposits, grains preserving delicate bubble walls and hair-like spires are conspicuously absent. Laser ablation-inductively coupled plasma-mass spectrometry shows that glass shards within each ashfall deposit have uniform trace-element geochemical signatures. If ashfall were eroded and transported after initial deposition, then ashes of different ages and geochemical compositions should be found together in individual deposits. An accurate chronology for buried ice in the western Dry Valleys region of Antarctica has implications for research in atmospheric chemistry (Miocene-aged glacier ice may hold pristine samples of ancient atmosphere), geobiology (ancient microbes are preserved in the ice), and planetary geology (buried ice in Antarctica may serve as an analog for buried ice on Mars).
DE: 1065 Trace elements (3670)
DE: 1823 Frozen ground
DE: 8404 Ash deposits
DE: 9310 Antarctica
SC: Global Climate Change [GC]
MN: 2003 Fall Meeting

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