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Today, the summit region of Siple Dome is a source area of slow-moving ice that contributes little to the mass flux into the Ross Ice Shelf. In the past, however, portions of the dome appear to have been overridden by inland ice draining the west antarctic ice sheet. Evidence in support of a relict ice stream traversing the flank of Siple Dome has resulted from a collaborative study of Siple Dome by St. Olaf College, the University of Washington, and the University of Colorado (Raymond et al. 1995; Jacobel et al. 1996).
In addition to our work on the relict ice stream, we carried out field studies in a region centered on the Siple Dome summit to characterize the ice dynamics and history of the area where a high-resolution climate core will be drilled. During the 1994-1995 field season, over 100 kilometers (km) of ground-based radar traverses were made in a 10-km-square grid centered on the topographic summit. Surface elevations were obtained for 45 points in the summit grid using stop-and-go kinematic global positioning system (GPS) and optical surveying. These data were augmented with continuous elevation measurements along the radar profiles obtained with a pressure transducer.
Ice thickness has been calculated from two-way travel times of the radar echoes, and together with the surface survey data, has been used to make a map of the bed topography (Fisher et al. 1995). Figure 1 shows that the bed beneath the summit region is generally smooth and slightly concave upward beneath approximately 1 km of overlying ice. Elevations in figure 1 are given with respect to the WGS-84 ellipsoid, and thus the bed is some 300 meters below mean sea level. The surface at the summit is largely two-dimensional, more of a ridge than a dome, with the ridge axis running approximately east-west. The greatest ice thickness is slightly south of this ridge, which has shallow surface slopes of 0.003 to 0.004.
Clear internal echoes down to approximately 60 percent of the ice thickness have also been measured from the radar profiles. Because they represent isocrones, they have the potential to reveal important information about ice history and dynamics. Figure 2 shows the topography of the ice surface and two of the more prominent internal layers at elevations of approximately 240 and 340 meters above the WGS-84 ellipsoid. The overall smooth variation in the internal layers shows that Siple Dome has been an area of generally stable accumulation throughout their history of deposition, about 10,000 years (Nereson et al. in press).
In addition to their general shape and smoothness, the internal layers also have two other common features:
To quantify the slope information for the surface and internal layers, we have made least-squares fits with a first-order polynomial to the radar surface and internal echo-depth data. Fits were obtained for the surface and the two internal layers of each half of the five north-south profiles, 30 fits in all. Slopes from these fits were then averaged along the east-west dimension for the five profiles, and the results, together with standard deviations, are shown in the table. The table shows that both internal layers have slope patterns that differ substantially from the modern surface, quantifying what is depicted in figure 2. Also, although both internal layers have similar slopes, significant differences are evident.
Our analysis of the surface slopes and internal layers appears to rule out large changes in flow dynamics during the past 10,000 years, but some time-dependent behavior seems to be required to explain the pattern. Modeling work at the University of Washington is currently underway to test various mechanisms, such as ridge-migration and spatial accumulation rate gradients, which might account for the observations (Nereson and Raymond 1995; Nereson and Raymond, Antarctic Journal, in this issue).
We would like to acknowledge the efforts of our collaborators in the fieldwork: H. Conway, T. Gades, N. Nereson, C. Raymond, and T. Scambos. This work was supported by National Science Foundation grant OPP 93-16338 to St. Olaf College.
References
Fisher, A.M., R.W. Jacobel, N.M. Sundell, and T. Gades. 1995. Bedrock topography and internal stratigraphy of the Siple Dome summit. EOS, Transactions of the American Geophysical Union, 76(46), F194.
Jacobel, R.W., T.A. Scambos, C.R. Raymond, and A.M. Gades. 1996. Changes in the configuration of ice stream flow from the west antarctic ice sheet. Journal of Geophysical Research, 101(B3), 5499-5504.
Nereson, N.A., and C.F. Raymond. 1995. The geometry and stratigraphy of Siple Dome and implication for its history. EOS, Transactions of the American Geophysical Union, 76(46), F194.
Nereson, N.A., and C.F. Raymond. 1996. Recent migration of Siple Dome divide determined from 1994 radio-echo sounding measurements. Antarctic Journal of the U.S., 31(2).
Nereson, N.A., E.D. Waddington, C.F. Raymond, and H.P. Jacobsen. In press. Predicted age-depth scales for Siple Dome and inland WAIS ice cores in West Antarctica. Geophysical Research Letters.
Raymond, C.F. 1983. Deformation in the vicinity of ice divides. Journal of Glaciology, 29(103), 357-373.
Raymond, C.F., N.A. Nereson, A.M. Gades, H. Conway, R.W. Jacobel, and T.A. Scambos. 1995. Geometry and stratigraphy of Siple Dome, Antarctica. Antarctic Journal of the U.S., 30(5), 91-93.