(1) Now at Malin Space Science Systems, P. O. Box 910148, San Diego, CA 92191-0148
Abrasion is ubiquitous in the ice-free areas in southern Victoria Land, Antarctica. It also appears to occur in ice-covered areas, reflecting the abrading power of strong winds moving ice crystals. The amount of abrasion, although highly variable, depends on sediment supply, wind speeds, and turbulence, as well as on heterogeneity of target materials. Abrasion rates determined from single year observations are probably not representative of long term averages, although they do indicate in general the order of magnitude of abrasion. The field values are nearly 2 orders of magnitude lower than laboratory measurements, mostly reflecting the differences between duration and cumulative exposures for the two types of studies.
Abrasion targets and associated sediment were returned from each of eleven sites established in 1983-1984 (Malin, 1985) after approximately one year (minimum exposure: 355 days; maximum exposure: 405 days). The maximum abrasion of basalt was 0.028 gm/cm^2/year, of dolerite 0.018 gm/cm^2/year, and of non-welded volcanic tuff 0.7 gm/cm^2/year. These are equivalent to about 100 micrometers (microns), 50 microns and 500 microns, respectively. Average abrasion was 0.006 gm/cm^2/year (20 microns), 0.005 gm/cm^2/year (15 microns), and 0.135 gm/cm^2/year (100 microns) for basalt, dolerite, and tuff, respectively.
Variability in abrasion is attributed to sample heterogeneity, the stochastic supply of abrading material, and the stochastic nature of turbulent flow across a surface.
Field values for abrasion are between factors of 125 and 250 lower than the laboratory measurements (e.g., Miotke, 1979, 1982). This difference is understandable in that abrasion probably occurs only during brief, sediment-transporting events, which cumulatively last for only hours per year. Laboratory numbers cannot in themselves be used to estimate the age of "ventifacts" without considerable and detailed information about the setting in which they occur.
Areas along the lower flanks of valley walls, and near sediment supplies along fluvial drainages or margins of lakes, are currently much more active (by as much as two orders of magnitude) than are those within the Olympus and Asgard Ranges. Of particular interest is that the most damage occurs where the wind appears to be concentrated by valley topography, consistent with cold, dense, "gravity driven" winds. Contemporary wind modification in the ranges appears relatively small. This may have important implications to conclusions drawn from studies on cryptoendolithic organisms and their role in weathering their rock habitat (e.g., Friedmann and Weed, 1987).
Samples of dolerite, sandstone, and non-welded tuff were exposed to windblown snow and ice at the Allan Hills "blue ice" site for 359 days. The windblown "sediment" was not collected as no convenient mechanism was found to properly sample and preserve these materials. The average mass losses were: dolerite, 0.007 gm/cm^2/year; sandstone,0.01 gm/cm^2/yr, and tuff, 0.03 gm/cm^2/yr, about 1/4 the average mass loss in the Dry Valleys. Although Miotke (1979, 1982) argued against the often cited possibility of ice crystals participating in abrasion, the first year's results from the Allan Hills "blue ice" site suggest that this phenomenon probably does occur.
These results should not be applied directly to Antarctic meteorites because of the following:
1) The abrasion rate peaks at a height of several ten's of centimeters above the surface of the ice, and lower values (about a factor of 10 lower) were measured at 7 cm height; thus meteorites, which protrude only a few centimeters above the ice, will experience much less abrasion (an estimate is about 0.1 cm/1000 yr or less).
2) 1984, the year of exposure, had anomalously high winds and enhanced sediment transport; the 5 yr sample to be returned in 1988-1989 should provide a more valuable guide to average abrasion rates. The average could be as much as one to two orders of magnitude (reducing the affect on meteorites to 0.001 cm/1000 yr), probably more in keeping with other measures of meteorite surface residence times (K. Nishiizumi, personal communication, 1987).
Friedmann, E. I. and R. Weed. 1987. Microbial trace-fossil formation, biogenous, and abiotic weathering in the Antarctic cold desert. Science 236, 703-705.
Malin, M. (1985) Abrasion rate observations in Victoria Valley, Antarctica: 340-day experiment. Antarctic. J. of the U.S. - 1984 Review 19 (5), 14-16.
Miotke, F.-D. (1979) Die formung und formungsgeschwindigkeit von Windkantern in Victoria-Land, Antarktis. Polarforschung 49 (1), 30-43.
Miotke, F.-D. (1982a) Formation and rate of formation of ventifacts in Victoria Land, Antarctica. Polar Geography and Geology 6 (2), 90-113. (English translation of Miotke, 1979).