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Meteorite recovery and reconnaissance in the Allan Hills-David Glacier and Darwin Glacier regions, 1996-1997

R.P. HARVEY and J.W. SCHUTT, Department of Geological Sciences, Case Western Reserve University, Cleveland, Ohio 44106-7216

The region north and west of the Allan Hills has been a prolific source of meteorites since meteorite concentrations were first discovered there in 1976. Recent search efforts have concentrated on the Elephant Moraine icefield, a large [30X15-kilometer (km)] group of blue-ice areas approximately 40 km west of Reckling Peak (figure 1). These icefields, like those to the east at Reckling Moraine, take the form of a series of steps downward from south to north. As indicated by the presence of local superglacial moraines (not associated with exposed bedrock), these steps probably correspond to local thinning of the icesheet as it passes over submerged barriers.

The exposure of blue ice near Elephant Moraine, although extensive, is not fully continuous, and the concentration of meteorites varies dramatically from place to place. An extensive reconnaissance of the area during the 1987-1988 field season resulted in practical subdivisions of the Elephant Moraine icefield into local, smaller bare ice areas, identifying those with significant meteorite concentrations for future systematic searching (figure 2). One of these areas, an icefield unofficially named "Meteorite City" (76°14'S 156°35'E), was the focus of meteorite recovery efforts during the 1996-1997 season by the Antarctic Search for Meteorites (ANSMET) team. The 1987-1988 reconnaissance, and a subsequent short visit in 1990-1991, indicated that the surface of this icefield contained significant numbers of meteorites. Other local icefields were also to be visited for reconnaissance purposes. Our party was put into the field on 20 November 1996 via LC-130 Hercules aircraft, approximately 12 km west of Griffin Nunatak (figure 1). Group members included Laurie Leshin, Sara Russell, Rene Martinez, Guy Consolmagno, and the authors.

From this put-in site, the field party traversed to the Elephant Moraine region and set up camp on the border of the Meteorite City icefield. After an initial orientation period allowing field party members to become familiarized with the region and proper meteorite identification and collection techniques, systematic searching commenced. After some early successes, search efforts were significantly hampered by abundant new snow and high winds, greatly reducing the effectiveness of searches. Eventually the high winds did remove the new snow and systematic searching once again became profitable. By season's end (27 December), 334 meteorites had been recovered and approximately 50 percent of the Meteorite City icefield had been systematically searched.

Midseason (5 December) two members of our party (Schutt and Russell) were transported via Twin Otter to the Meteorite Hills (79°40'S 155°36'E) at the west end of the Darwin Mountains, in the Darwin Glacier region. This region had been previously visited by a helicopter-based reconnaissance party during the 1978-1979 season, resulting in the recovery of 28 meteorites; however, only a small portion of the icefield was visited and the full extent of the concentration was not known. The 1 996-1997 reconnaissance team spent 7 days traveling across the area on snowmobiles and recovered 38 specimens, confirming the presence of a significant concentration and its geographical extent. Systematic searching of the Meteorite Hills icefield is planned for a future season. Reconnaissance in the Meteorite City area included trips to the Texas Bowl, which had been systematically searched in 1987-1988 and 1990-1991 yielding over 1,200 specimens, and the Shooda Bin, a bowl-shaped icefield that exhibited many promising characteristics when examined remotely. The very small number of specimens found in the large area represented by the Shooda Bin (5 total) illustrates, however, how incomplete our understanding of meteorite concentration mechanisms really is.

By season's end a total of 390 meteorites had been collected; 334 from Meteorite City, 38 from Meteorite Hills, and 18 from other areas of the Elephant Moraine icefields. Although the weather limited our systematic searching efforts, several unusual specimens were recovered, including at least one lunar specimen (table). Future seasons of work in this region will be required to complete systematic searching of this important meteorite concentration. Two additional meteorite specimens were collected by representatives of another science project, from bare ice areas around Mount DeWitt (77°12'S 159°50'E).

At the end of the regular season, two field party members (Harvey and Schutt) made a short series of day trips into the McMurdo Dry Valleys via helicopter. The purpose of these trips was to explore the possibility that ancient bedrock platforms in the region might harbor recognizable meteorite concentrations, mimicking those found in dry deserts from other parts of the world such as the Nullarbor Plain in Australia and the Libyan Desert. Such concentrations typically are found where sediment is actively being removed from a surface, enhancing the relative contribution of meteorites from space; however, erosion must not be so active as to actively destroy the meteorites. Meteorites had previously been recovered from bedrock in Antarctica; the most notable of these were the many Derrick Peak specimens collected in 1979, which represented a shower of iron meteorites. These finds, however, represented isolated random falls rather than a true concentration. Our efforts focused on identifying regions where predominant rock type was sedimentary and light in color to make meteorite recognition easier. Regions of the Asgard and Olympus ranges were overflown, as well as Mount Fleming, the Taylor Valley, the Beacon Valley, and the Miers Valley regions. Landings and extensive searches were conducted at promising locations on the Nussbaum Reigel and Cirque 3 of the Asgard Range, but no specimens were found. A few promising sites remain to be examined; but, in general, it seems regions of suitable age and bedrock composition are simply not big enough to represent a significant collection of meteorites.

This work was supported by National Science Foundation grant number OPP 96-15276 to Ralph P. Harvey. We gratefully acknowledge the contributions of Kim McDonald, Andy Young, and Mary Roach as visiting field party members. We also thank Nelia Dunbar, Richard Esser, and Bill Macintosh for recovery of the Mount DeWitt specimens, and Dave Marchant for guidance during the McMurdo Dry Valleys reconnaissance. Outstanding support from both fixed wing and helicopter teams at Antarctic Support Associates, VXE-6, Petroleum Helicopters Incorporated, and Kenn Borek Air is appreciatively acknowledged