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The Amundsen-Scott South Pole Station water well: A new source of micrometeorites

S. Taylor and J. Lever, Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire 03755-1290

R. Harvey, Department of Geological Sciences, Case Western Reserve University, Cleveland, Ohio 44106-7216

J. Govoni, Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire 03755-1290

We have collected thousands of micrometeorites from the bottom of the Amundsen-Scott South Pole Station water well using a collector that we designed, built, and calibrated. The collector suctions and filters the particles on the ice surface as it traverses the well bottom. Preliminary results, based on two of the five samples returned, indicate that we may have the world's largest, best-dated, and best-preserved collection of micrometeorites. Also, because the area suctioned and the depositional age are known, further analysis of this collection will yield a measure of the terrestrial flux of 50-1,000-micron (µm) micrometeorites, the dominant mass contributors to the Earth. The micrometeorites we collected fell to Earth between 1100 and 1500 AD.

Micrometeorites are submillimeter, terrestrially collected extraterrestrial particles. Like meteorites, micrometeorites range from unaltered primordial materials to those that have seen extensive differentiation and alteration. They are the dominant mass contribution to the present-day Earth at about 100 tons each day (Love and Brownlee 1993). Although ubiquitous in terrestrial environments, micrometeorites are difficult to find and collect because they occur in low concentrations and generally weather rapidly. It is therefore necessary to find deposits where they are concentrated and preserved. Antarctica, with its cold climate and lack of terrestrial debris, is an excellent environment in which to look for micrometeorites.

The South Pole water well (SPWW) is a 24-meter (m) diameter by 16-m-deep melt pool 100 m below the snow surface at the South Pole ( figure 1) and supplies drinking water for the Amundsen-Scott South Pole Station. The well was constructed during the 1992-1993 austral summer, and it has melted over 8,000 tons of firn and ice to date. A pump draws water from about 2 m below the water surface. About 10 percent of the water is consumed, and the rest is heated, using waste heat from the station, and returned to the well. This warm water melts more ice, and the well grows with time, primarily downward (i.e., through older ice). Micrometeorites that originally fell on the snow surface are liberated at the melting front and remain on the well bottom to form a lag deposit. The depositional age of these particles is known because the age of the ice is known as a function of depth (Kuivinen et al. 1982).

Our objective was to collect all 50-1,000-µm micrometeorites, without regard to density, shape, or magnetic susceptibility, from a large, known area of the well bottom. Our principal task was to build a collector that met strict operational requirements: it must not threaten water quality, it must descend through a 30-centimeter-diameter well neck and survive a cold soak at -50°C, and it must operate remotely in about 20 m of water at a distance of up to 200 m below the snow surface.

Our collector ( figure 2) suctions and internally filters particles from the ice while traversing the well bottom. We control it from the surface via a waterproof electromechanical cable and use an underwater video system for visual feedback. The main body is a machined and folded sheet of low-density polyethylene that holds a polyester filter fabric (53-µm openings). When it is folded, a 2-millimeter-wide gap remains through which the pump draws water and entrained particles. The water flow is high enough (>1 meter per second) to entrain all particles, and the filter is immediately downstream of the intake slot to minimize particle damage and loss. A thin strip of polyethylene forms a check valve to seal the slot when the pump is off. A waterproof aluminum housing contains the pump, drive motors, and electrical connections. Spiked stainless steel wheels at opposite ends of the collector, powered by independent drive motors, move the collector around the bottom. The camera and light are suspended about 5 meters from the well floor by a split-out from the cable.

Prior to our deployment in December 1995, no information existed on the bottom topography of a water well. We found that the SPWW bottom had a gently curved central plateau (about 17 m2) sculptured at its periphery into fairly steep arcuate dips that were 0.3-0.6 m below the plateau and 1-3 m wide ( figure 3). These dips led to smaller plateaus (2-8 m2). Associated with most sculptured features were visibly dark pockets of particulates, mostly iron-oxide grains derived from the water-supply system. On the plateau areas, particles were visible but not concentrated into pockets. The local surface was quite smooth (perhaps 1-millimeter depressions over 1-5-millimeter scales). Large circulation cells, established by the injected water and free convection along the walls, and local instabilities in these cells are probably responsible for the sculptured features.

During 2 weeks in December 1995, we deployed and retrieved the collector six times; as a result of this effort, we now have five filter bags (one filter was deployed twice) containing a total of about 200 grams of material. The collector maneuvered easily over the well's central plateau, and we devoted one collection (number 3) exclusively to it. We collected from five adjoining areas (about 10 m2 total), including three particle pockets. Areas suctioned were visibly clean and indicate a high-efficiency particle pickup based on our laboratory experience (Taylor et al. in press).

We processed a pocket sample and the central-plateau sample in our field laboratory to assess the collector's performance. The material in the filters was backflushed into a stack of stainless steel sieves, using well water, and sorted into 53-106-µm, 106-250-µm, 250-425-µm, and more than 425-µm size fractions. Most of the materials in the samples were rust grains, derived from the well pump. Wood fragments and copper weld droplets were also found. To assess the meteoritic component, we removed all spherical particles from the 250-425-µm size fraction using a binocular microscope. Cosmic spherules were then separated optically by their surface texture and color.

Upon our return to Hanover, New Hampshire, a subset of these spherules were mounted in epoxy and sectioned. Both their distinctive mineralogy and bulk chemistry indicate they are extraterrestrial. We found the full range of expected morphologies for cosmic spherules from glass spherules to unmelted particles. Silicate spherules having barred olivine textures are the most common type.

About 0.1 percent of the 250-425-µm size fraction of both samples examined were cosmic spherules. If this percentage represents the fraction of meteoritic material in all our samples, we will have about 0.2 grams of micrometeorites. For two reasons, however, we think this value is a minimum. First, we have not counted unmelted micrometeorites. These were abundant in the melted blue antarctic ice (Maurette et al. 1991), and we therefore expect to find many unmelted micrometeorites in our samples. Second, the melted meteoritic content was greater (0.2 percent) in a subsample of the 106-250-µm size fraction, the size fraction that also contains the largest total mass collected. Thus, the five SPWW samples should contain the world's largest collection of micrometeorites. The combination of a large number of micrometeorites of known depositional age makes the SPWW a unique and valuable source of micrometeorites.

We thank John Rand for his invaluable information about the well and Michael Shandrick of Antarctic Support Associates for assisting us at South Pole. This work was funded by the National Science Foundation grant OPP 93-16715; Julie Palais is our project monitor.

References

Kuivinen K.C., B.R. Koci, G.W. Holdsworth, and A.J. Gow. 1982. South Pole ice core drilling, 1981-1982, Antarctic Journal of the U.S., 27(5), 89-91.

Love S.G., and D.E. Brownlee. 1993. A direct measurement of the terrestrial mass accretion rate of cosmic dust. Science, 262, 550-553.

Maurette M., C. Olinger, M. Christophe, G. Kurat, M. Pourchet, F. Brandstatter, and M. Bourot-Denise. 1991. A collection of diverse micrometeorites recovered from 100 tonnes of antarctic blue ice. Nature, 351, 44.

Taylor S., J.H. Lever, R.P. Harvey, and J. Govoni. In press. Collecting micrometeorites from the South Pole water well (CRREL report). Hanover, N.H.: Cold Regions Research and Engineering Laboratory.