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Collaborative Research: Characteristics of Snow Megadunes and their Potential Effects on Ice Core Interpretation

Field study and remote sensing measurements of an area of snow megadunes on the East Antarctic plateau provides a preliminary assessment of dune morphology, firn structure, and layering in the features. Snow megadunes are undulating variations in accumulation and surface texture, with wavelengths of 2 to 5 km and amplitudes up to 5 meters. The features cover 500,000 km2 of the East Antarctic plateau, occurring in areas of moderate regional slope and low accumulation on the flanks of the ice sheet between 2500 and 3800 meters elevation (Fahnestock et al., 2000). Our field area lies in the Byrd Glacier catchment region, near 80.78 degrees S, 124.5 degrees E, at an elevation of 2885 meters. Crest to crest separation near the field site averages 4.2 km. GPS topographic profiles show a 4 to 5 meter surface amplitude on a regional slope of about 0.001. This region contains some of the best examples of the megadune features that could be found in satellite mappings. Landsat images and aerial photography indicate the dunes consist of alternating surfaces of glaze and rough sastrugi with gradational boundaries. This pattern is oriented perpendicular to the mean wind direction, as modeled in katabatic wind studies (and partly confirmed by our brief observations). Glaze surfaces cover the leeward faces and troughs; rough sastrugi cover the windward faces and crests. The megadune pattern is crossed by smooth to eroded wind-parallel longitudinal dunes. Wind-eroded longitudinal dunes form spectacular 1-meter-high sastrugi in nearby areas. Dune structure at depth was profiled by ground-penetrating radar. The profile shows a sub-surface wave structure with amplitude of 5 to 8 m and a shift in the crest/trough pattern with depth consistent with upwind accretionary migration of the rough sastrugi patches. The layer pattern also indicates greatly reduced accumulation in glaze areas. The dune wave pattern is visible to the base of the profile, 40 meters below the surface. Given the very low accumulation rates for the area (1 to 4 cm at Vostok and nearby IGY-era snowpit studies), this thickness may represent the last few thousand years.

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