(1) Now at: Malin Space Science Systems, P. O. Box 910148, CA 92191-0148
Computer simulated view of ice-free valleys of Southern Victoria Land, Antarctica
In the absence of contemporary fluvial and other geological and biological processes capable of removing weathered material from rock surfaces, eolian abrasion acts as an important limit on the rate and magnitude of erosion in the Antarctic cold desert. As noted in previous Antarctic Journal of the United States (AJUS) progress reports from this study, the amount of abrasion depends on sediment supply, wind speeds and turbulence, and on the heterogeneity of target materials. These earlier reports were based on single-year observations at the eleven test sites scattered throughout the ice-free valleys and transantarctic ranges in Southern Victoria Land (Malin, 1985,1988). Only one site had more than one single-year of observation. In addition to providing specific numerical values for short-term abrasion, Malin (1988) concluded that: 1) abrasion rates determined from single year observations were probably not representative of long term averages, although they indicated in general the order of magnitude of abrasion, and 2) field values were nearly 2 orders of magnitude lower than laboratory measurements, mostly reflecting the differences between duration and cumulative exposures for the two types of studies.
Samples deployed during the austral summer of 1983-1984 were retrieved during the austral summer of 1988-1989, after exposure for approximately five years. The 5-year sample suite includes three different materials (basalt, dolerite, and non-welded volcanic tuff) arrayed at five different heights above the surface (nominally 7, 14, 21, 35, and 70 cm) facing four orthogonal directions (oriented true north, east, south, and west) at ten sites, plus two materials (dolerite and tuff) at five heights and facing four directions from Site 11, on the "blue ice" adjacent to the Allan Hills. Sand collectors deployed throughout the experiment, facing the same directions and collecting at the same heights, were also sampled; the results of analysis of these specimens will not be reported here.
As the five-year exposure included the effects that occurred during the single-year exposure, two computations were made to assess the magnitude of the longer term average abrasion. First, the five-year exposures were divided by five, weighting the first year equally with subsequent years. Second, the single-year results were subtracted from the five-year results, and divided by four, attempting to separate effects of the first year's abrasion from that which occurred in later years.
On average, a minimum of 67-75% of abrasion measured after five years occurred during the first year of the study. At some sites and in some directions, all the abrasion (to the ability to measure mass changes) occurred during that single year, while at other locations and in other directions the first year's mass loss was less than the average of the remaining years' mass loss. Despite the philosophical problems inherent in averaging such wildly variable data, it appears, averaging all heights, all directions, and all materials, that an order of magnitude more abrasion occurred during 1984 than in each of the subsequent four years. This result is not surprising, as other factors (e.g., the condition of the target racks and sand collectors, the failures of automated weather stations, etc.) had indicated that the winter of 1984 had been anomalous. In addition, simultaneous collection of sediment is consistent with this view; it indicates that significantly more material was in motion during 1984 than in subsequent years. The mass of sediment collected, the susceptibility of the target materials known from both laboratory and field studies, and the magnitude of abrasion also suggests that sediment transport velocities were much higher in 1984 than in the subsequent four years.
Two sedimentological factors appear to play important roles in determining where abrasion occurs: the availability of loose, wind-transportable debris and the small-scale configuration of the surface. As noted in earlier studies (Malin 1984, 1985, 1986, 1988), surfaces mostly or partly covered with loose, wind-transportable material show the most abrasion: Site 1, in the Victoria Valley dunes and Site 7, on the northern flank of central Wright Valley east of Bull Pass and a few hundred meters above the Onyx River floodplain, experienced factors of 2 to 5 more abrasion than other areas. However, Sites 6 (lower Wright Valley near Lake Brownworth) and 9 (upper Taylor Valley on Bonney Riegel above Lake Bonney) also show significant abrasion, and yet have much less freely mobile sediment. Surface configuration at these two sites is likely contributing indirectly to enhanced abrasion in two ways. First, sand saltating across rock-strewn, armored surfaces attains greater kinetic energy (primarily because of higher rebound heights), and even a small amount is more effective in abrading. Second, coarse sand and gravel are often the only wind-transportable particles exposed on such surfaces (owing to previous exportation of finer fractions), which require higher winds to move and, once in motion, have greater kinetic energy owing to both larger mass and higher velocities of transport.
The five year study confirms the conclusion based upon the single-year results that indicated that abrasion of rock targets was occurring at Site 11, despite the fact that the moving sediment there was particulate ice. Although abrasion at the Allan Hills site was the lowest of any site during 1984, the average over five years, despite occasional burial by drifted snow, was comparable to that at Site 5 (eastern Asgard Range), Site 8 (North Fork, Upper Wright Valley), Site 10 (Cirque 4, Asgard Range), and Site 3 (northern Bull Pass). The average amount of abrasion at Site 11 (dolerite ~0.001 gm/cm^2/yr and non-welded tuff ~ 0.01 gm/cm^2/yr or about 3 microns and 80 microns, respectively) is a factor of 5-10 less than the estimate in Malin (1988) based on the single-year measurement, and consistent with codicils attached to that previous estimate. Despite a five-fold increase in exposure, such small values remain near the limit of experimental uncertainty; longer exposures presently underway should continue to reduce this uncertainty.
In many environments, chemical and biochemical weathering of rocks is limited by the ability of the weathered materials to be removed and transported away from the rock. Although slow in comparison to other environments, removal of weathered material from Antarctic rocks in areas subjected to wind-transport of loose sediment appears to be sufficiently rapid to continually replenish fresh surfaces for further weathering. However, in other areas this is not the case, and weathering products and silica and iron-oxide deposition "choke-off" weathering. The rates and amounts of abrasion determined in this study may be combined with numerical and analytical studies of chemical activity to further constrain rates of geomorphic modification in Antarctica.
Dean B. Eppler prepared the materials for, and participated in, their deployment during the 1983-1984 field season. Michael A. Ravine assisted in their recovery during the 1988-1989 season. Ms. Diana Michna performed the pre- and post-deployment measurements. I am especially indebted to numerous VXE-6 helicopter pilots and crew chiefs who enthusiastically supported this effort without substantially adding to the abrasion of the test targets. This work was supported under National Science Foundation grants DPP8206391 and DPP8716505.
Malin, M. C. 1984. Preliminary abrasion rate observations in Victoria Valley, Antarctica. Ant. J. of the U. S. - 1983 Review 18 (5), p. 25-26.
Malin, M. 1985. Abrasion rate observations in Victoria Valley, Antarctica: 340-day experiment. Ant. J. of the U. S.-1984 Review 19 (5), 14-16.
Malin, M. 1986. Rates of geomorphic modification in ice-free areas, southern Victoria Land, Antarctica: Ant. J. of the U. S. - 1985 Review 20 (5), 18-21.
Malin, M. C. (1988) Abrasion in ice-free areas of southern Victoria Land, Antarctica: Ant. J. of the U. S. - 1987 Review 22(5), 38-39.