HR: 09:00h
AN: C31C-04    [Abstracts]
TI: Antarctic Megadunes: Characteristics and Formation
AU: * Scambos, T
EM: teds@nsidc.org
AF: National Snow and Ice Data Center, CIRES, University of Colorado, 449UCB, Boulder, CO 80309 United States
AU: Fahnestock, M
EM: mark.fahnestock@unh.edu
AF: Complex Systems Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, 39 College Road, Durham, NH 03824 United States
AU: Shuman, C
EM: christopher.a.shuman@nasa.gov
AF: Oceans and Ice Branch, NASA Goddard Space Flight Center, Code 971, Greenbelt, MD 20771 United States
AU: Bauer, R
EM: bauerr@nsidc.org
AF: National Snow and Ice Data Center, CIRES, University of Colorado, 449UCB, Boulder, CO 80309 United States
AB: We review field geophysical, meteorological, and remote sensing data covering Antarctica's 'megadune' regions with the purpose of constraining formation models for the features. Megadunes are striped accumulation variations, oriented perpendicular to mean katabatic windflow, with hieghts ranging from 2 to 8 meters, and crest spacing from 2 to 6 km. Crest ridges have lateral extents of up to 100 km. Upwind faces are steeper than downwind faces, and are characterized by very large, eroded sastrugi. Surface 'glazes' of ice, with coarse recrystalized grains in the subsurface, are present in the lee faces. Dunes are widespread across the East Antarctic plateau, although laterally extensive dune fields occur in just a few regions. Strong variations in surface roughness and snow grain size between crest/upwind faces and trough/downwind faces are evident in albedo and radar or visible-light backscatter. Field measurements at a site 400 km southeast of Vostok station (80.78 deg S, 124.5 deg E) provide insight into dune origin, longevity and migration. Detailed surface topography from GPS confirms height and width of dunes inferred earlier using ICESat. Internal layering of megadunes (imaged using ground-penetrating radar) shows sigmoidal layers of higher accumulation along the dune crests and windward faces. Dunes accrete new layers in the upwind direction. Radar layer structures, consisting of a 6- to 15-meter-thick sequence of accumulation layers separated by erosive or very low accretion glaze layers, are visible to at least 70 meters below the surface. Given an estimated mean accumulation of 20 - 30 kg/m2 over the dune region, each dune sequence represents approximately 250 years of time. High accretion occurs over roughly 1/3 of an active dune field surface. This implies that surfaces on the lee-side dune face spend between 150 and 200 years exposed to near-surface air and temperature variations before burial by the next advancing dune face. GPS ice motion measurements indicate an ice flow of 4 m/yr at bearing 130, almost perpendicular to the mean wind direction of 226. Dune winds are dominated by flow in this direction (+/- 15 degrees) at 8 - 12 m/s for 10 months of the year. Several possible models for dune formation are considered. Katabatic wind flow of a near-surface air layer clearly controls snow redistriburtion. Previous studies have shown that dunes form in a narrow range of regional surface slope (0.0010 - 0.0015), implying that a specific range of winds speeds may determine formation. One possibility is that wave-like boundary layer effects in the katabatic flow create the dunes. Compaction effects due to the long period of low accumulation in the glaze regions may amplify dune topography. An 'anti-dune' (a term from fluvial geomorphology) model of dune formation is forwarded.
DE: 9310 Antarctica
DE: 1824 Geomorphology (1625)
DE: 1827 Glaciology (1863)
DE: 1863 Snow and ice (1827)
SC: Cryosphere [C]
MN: 2004 AGU Fall Meeting