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*Present address: Department of Geology, Portland State University, Portland, Oregon 97207
The McMurdo Dry Valleys, next to McMurdo Sound at 76°30' to 78°30'S and 160° to 164°E ( figure 1), are the largest of the antarctic ice-free regions, which account for 5 percent of the continental area (Drewry, Jordon, and Jankowski 1982). The hydrology of the dry valleys originates from glacial meltwater. During the summer months, from late November and into February, the glaciers melt providing the only source of water for the streams. Snowfall in the valleys does not contribute to the streams or to the general hydrology because it usually sublimates before melting (Chinn 1981). To assess the mass change of the glaciers, we have established surface measurements of mass change on four glaciers in Taylor Valley (Fountain, Vaughn, and Dana 1994). To determine the component of mass loss resulting in meltwater, we are measuring the energy balance of the ice surface (Lewis et al. 1995; Lewis, Fountain, and Langevin, Antarctic Journal, in this issue) and collecting meteorological information on four glaciers (Doran et al. 1995; Dana, Fountain, and Wharton, Antarctic Journal, in this issue). In this article, we summarize the findings of the mass balance measurements for the Commonwealth, Canada, Howard, and Taylor Glaciers (figure 1).
Snowfall in Taylor Valley occurs any time of the year. During our study period, from November 1993 through January 1996, more snow accumulated in the winter period (February through October) than during the summer period (November through January). Measurements of net snow accumulation on the upper Commonwealth and Howard Glaciers show that during summer, accumulation did not exceed 3.3 centimeters (cm) water equivalent, and many measurements were near or below 0 cm. Winter accumulation did not exceed 11.3 cm water equivalent; most values were 3 cm water equivalent or more. These findings contrast with those of Chinn (1985) for the Wright Valley. It is not clear if this difference is due to the span of time and the variation in meteorological conditions or whether it is due to distinctly different climatic regimes between the two valleys. Chinn (1980) suggests that a local lake effect creates snowfall on the adjacent ridges. We do not observe the same phenomenon in Taylor Valley.
Figure 2 illustrates the spatial variation of glacier mass balance. Individual values of measured mass balance are shown for all four glaciers. For any given elevation, a valley gradient exists in which the mass balance is more negative upvalley and further away from the ocean. Taylor Glacier is about 36 km from the ocean, whereas Howard is about 13 km, Canada is 12 km, and Commonwealth is 4 km. Clearly, Taylor Glacier has more negative values than Canada and Howard, and these glaciers have more negative values than Commonwealth. The change in altitude of the equilibrium line reflects this valley gradient. The equilibrium line starts at about 375 meters (m) for Commonwealth and rises to about 1,200 m for the alpine glaciers near the Taylor Glacier. The altitude was determined from the elevation contours on published topographic maps. Taylor Glacier has no equilibrium line in Taylor Valley because it is an outlet glacier of the east antarctic ice sheet, and its extension into Taylor Valley is completely within the ablation zone. Thus, within Taylor Valley a strong climatic gradient exists that significantly affects the glaciers. We conjecture that the gradient is largely controlled by a decrease in precipitation away from the coast, although some evidence suggests warmer air temperatures and stronger winds upvalley. Current efforts are underway to monitor the meteorological environment (Doran et al. 1995).
The magnitudes of ablation and accumulation are small, as expected for polar glaciers. Accumulation does not often exceed 15 cm water equivalent, and ablation does not exceed 30 cm water equivalent. The exception to this characterization is ablation on the ice cliffs that forms the margin of many glaciers in the valley. The ablation of the ice cliff is typically 5-10 times the ablation of the adjacent top surface of the glacier. Measured values of ablation from the ice cliffs, excluding calving, ranges from 5 to 70 cm water equivalent. During our study, no snow has accumulated on the glaciers at elevations below about 200 m, suggesting that these zones ablate all year long. For the two glaciers with mass balance measurements in the accumulation zone (Howard and Commonwealth), the range of net seasonal snow accumulation is about the same, up to about 11 cm water equivalent. Although the sample size is small, we infer that for the same elevation, snow accumulation in the Kukri Hills is about the same as that in the Asgard Range.
The calving component of ablation is small, in agreement with the findings of Bull and Carnein (1970). Based on paired photographs of the glacier cliffs, taken in the beginning of the summer season and at the end of the season, and on on-site observations, we determined that calving blocks are typically 50 cm thick, and calving occurs over widths on the order of 10 to 100 meters. Averaged over the area of the ice cliff, ablation due to calving is about 1-3 cm water equivalent during each season or 2-6 cm for the year. No seasonal trend in calving is observed between summer and winter, indicating that calving results from the stress and strain of the ice movement rather than fatigue caused by daily heating and cooling from solar illumination.
This work was supported by National Science Foundation grant OPP 92-11773.
References
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