Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.257m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 257
Other
Scientific paper
In the melt zone of the west Greenland ice sheet, micrometeorites as well as terrestrial wind-borne dust are initially trapped together in mm-sized cocoons of siderobacteria that make up the bulk of the dark sediments ("cryoconite") that we investigated. These sediments were collected at about 30 distinct locations on various ice fields, at the latitudes of Sondrestromfjord, Jakobson, and Port Victor. Two major limitations arise from the tight encapsulation of the grains in cryoconite: (i) they are exposed to biogenic etching, which probably generates some colloidal form of iron hydroxyde that penetrates through porous grains and that adsorbs very efficiently trace elements of terrestrial origin such as REE (Robin, 1988); (ii) they can only be retrieved through a vigorous disaggregation of cryoconite on a stainless steel sieve with mesh of 100 micrometers that eliminates ~95% of the cryoconite, but preferentially destroys friable micrometeorites. Consequently, the Greenland collection is highly biased toward hard, compact (i.e., non porous), crystalline micrometeorites, and trace elements can only be measured in compact grains. Another limitation of this collection is that, at least up to the latitude of Port Victor, the mineral fraction with size smaller than 100 micrometers extracted from cryoconite is too heavily contaminated by wind born dust. Micrometeorites can only be hand-picked in the larger >100- micrometer-sized fraction, which is much less contaminated. In this useful size fraction the concentration of large (>400 micrometers) micrometeorites ("minimeteorites") sharply increases up to values 20% when the margin of the ice field is near the sea shore, as a result of a sharp drop in the transport of wind-borne dust. The ideal collection zone should thus be an ice field very near the sea shore, but at much higher latitudes, where the arctic summer is too short as to allow the growth of extended colonies of siderobacteria . Running waters could be channelled and filtered, yielding cryoconite free sediments in which the concentration of minimeteorites probably reaches several tens of percent. During the Antarctic summer of 1987 and 1991, we melted ~360 tonnes of blue ice near Cap-Prudhomme on the Antarctic ice sheet, at ~6 km from the French station of Dumont d'Urville. The filtering of melt ice water on sieves yielded ~60 g of >25-micrometer-sized sediments (Maurette et al., 1992). In Antarctica the melting and gentle filtering of melt ice water on sieves preserves friable grains. In addition the bulk of terrestrial contamination is related to either coarse moraine debris (size >200 micrometers) or to finer grains (<50 micrometers) released during the corrosion of the steel pipes of the steam generators used to melt the ice. In our next 1994 expedition the amount of corrosion products will be drastically reduced relying on a new all stainless steel micrometeorite "factory." In this case the finest size fraction (from ~100 micrometers down to ~20 micrometers) should be very rich in micrometeorites, with concentration exceeding 20%. But the concentration of "minimeteorites" (>400 micrometers) is expected to be very small (about 20 grains out of ~200 tonnes of water), and excessively diluted in coarse moraine debris. Consequently the Antarctic and Greenland ice sheets are very complementary and inexhaustible collectors of micrometeorites, with Greenland looking particularly promising for the collection of "minimeteorites," that are quite rare in the micrometeorite flux, and Antarctica being the best collector of smaller micrometeorites down to sizes of ~10-20 micrometers. Acknowledgements. We thank C.T. Pillinger for useful comments. REFERENCES. Maurette M., Immel G., Perreau M., and Kurat G. (1992) Lunar Plan. Sci. 23, 859-860. Robin E. (1988) PhD. Thesis, Faculte des Sciences d'Orsay, 131 pp.
Hammer Conny
Immel G.
Maurette Michel
Pourchet M.
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