Flow of ice streams B and C

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Figure 2. Crevasses in the vicinity of Crary Ice Rise. Station names and locations are given (parenthesized for RIGGS stations). ("R" denotes raft feature discussed in text. Coordinate system corresponds to aeronautical grid system adopted by RIGGS. "km" denotes "kilometer:')

This research was supported by National Science Foundation grants DPI' 82-07320, DPP 84-05287, DPI' 85-14543, and DPP 8509451. References Bindschadler, R.A., D.R. MacAyeal, S.N. Stephenson, P. Vornberger, S. Shabtaie, and E. Roberts. In preparation. Ice shelf flow at the boundary of Crary Ice Rise. Annals of Glaciology, Vol. 10.

Flow of ice streams B and C I.M. WHILLANS

Department of Geology and Mineralogy

and Byrd Polar Research Center Ohio State University Columbus, Ohio 43210

During the 1984-1985 and 1985-1986 field seasons about 40 surface velocities were measured by transit satellite tracking and these have been analyzed to calculate the mass balance of 1987 REVIEW

MacAyeal, D.R., R.A. Bindschadler, S. Shabtaie, S. Stephenson, and CR: Bentley. In press. Force, mass, and energy budgets of the Crary Ice-Rise Complex, Antarctica. Journal of Glaciology. Thomas, R.H., D.R. MacAyeal, D.H. Eilers, and D.R. Gaylord. 1984. Glaciological studies on the Ross Ice Shelf, Antarctica, 1973-1978. In D. Hayes and C.R. Bentley (Eds.), The Ross Ice Shelf: Glaciology and geophysics. Washington, D.C.: American Geophysical Union. Antarctic Research Series, 42, 21-53.

Whillans, I.M., J . Boizan, and S. Shabtaie. In press. Velocities of Ice Streams B and C, Antarctica. Journal of Geophysical Research.

ice streams B and C and the flow pattern of ice stream B (Whillans, Bolzan, and Shabtaie 1987; Whillans and Bindschadler in press). Results to date indicate large mass imbalances for ice streams B and C and inhomogeneities in the flow of ice stream B. Ice stream B has velocities in the 100- to 800-meters-per-year range and is draining the interior at about 1.3 times the rate of replenishment by snow accumulation. Furthermore, at the ice stream head, the flow is very irregular and the simplest interpretation for this is that blocks of stiffer inland ice are being broken out, rafted along in the ice stream, and slowly assimilated. The rafts are some 10 kilometers across, and portions are even traveling in the reverse direction. Although not proved, it is probable that these results are related, in that the mass imbalance may be due to lengthening of the ice stream up-glacier by irregularly consuming and extending into the inland ice. 67

Ice stream C, which is next to ice stream B, is in contrast, virtually stagnant. Its velocity is less than 10 meters per year (McDonald and Whillans, in press), and it must be thickening. The net mass accumulation for the region of ice stream C is about equal to the net mass depletion of ice stream B so that the net effect is near zero. Radio-echo records of crevasses indicate that ice stream C was formerly very active (Rose 1979; Shabtaie and Bentley 1987) and that it must have stopped about 100 years ago. The satellite tracking data also provide control for aerial radioecho sounding of surface elevation and the results have been used to prepare a surface elevation map of the ice streams and ridges between them. Shabtaie, Whillans, and Bentley (1987) present and discuss this map; it exhibits a great many unusual and unexpected features. Crevasse patterns can be used to infer much about glacial dynamics. Vornberger (unpublished data, Ohio State University) and Vornberger and Whillans (1986) report on this. Vornberger developed a numerical model to predict crevasse patterns for given velocity fields and used this to model observed patterns. From this, we have learned of many dynamic features of ice stream flow without having to make detailed velocity measurements. The technique is not universally applicable, because certain crevasse patterns can be developed by a range of velocity fields and unique answers are not always possible. Repeat photography has been obtained for parts of ice stream B and the methods developed by Brecher (1986) for Byrd Glacier are being applied. Ground control is afforded by the satellite tracking. Difficulties, with aircraft navigation, delays in obtaining the prints and diapositives, and scratches on the original negatives have restricted the program, but good quality photogrammetric velocities are being obtained for parts of the ice stream.

Geophysical studies of the Siple Coast area C.R. BENTLEY, R.B. ALLEY, S. ANANDAKRISHNAN, D.D. BLANKENSHIP, S.T. ROONEY, D.G. SCHULTZ, and S. SHABTALE Geophysical and Polar Research Center University of Wisconsin Madison, Wisconsin 53706

In 1986-1987 the Geophysical and Polar Research Center did not send a party to the field. Instead, the year was spent analyzing data previously collected as part of the Siple Coast project. More than 20 papers have been published or presented on the results of those analyses since our last Antarctic Journal report (Bentley et al. 1986); here we will give a summary of these results (see also Bentley et al. 1987). 68

The methods for interpreting the photogrammetric results are being refined using existing data from Byrd Glacier, (Brecher 1986) and Dye-3, Greenland (Whillans et al. 1984). By this method, the velocity field together with surface slopes (also obtained photogrammetrically) and thickness are used to calculate stresses and velocities at depth. These results will show how and where the glacial flow is restrained. This is a first step toward learning what controls ice stream flow and what causes the curious results discussed above. This work is supported by National Science Foundation grants DPP 83-17235 and DPP 85-17590. References

Brecher, H.H. 1986. Surface velocity determination on large polar ice glaciers by aerial photogrammetry. Annals of Glaciology, 8, 22-26. McDonald, J., and I.M. Whillans. In press. Comparison of results from transit satellite tracking. Annals of Glaciology. Rose, K.E. 1979. Characteristics of ice flow in Marie Byrd Land, Antarctica. Journal of Glaciology, 24(90), 63-75. Shabtaie, S., and C.R. Bentley. 1987. West Antarctic ice streams draining into the Ross Ice Shelf: Configuration and mass balance. Journal of Geophysical Research, 92(82), 1311-1336. Shabtaie, S., I.M. Whillans, and C.R. Bentley. 1987. The morphology of ice streams A, B and C, West Antarctica, and their environs. Journal of Geophysical Research, 92(139), 8865-8883. Vornberger, P. L., andl.M. Whillans. 1986. Surface features of ice stream B, Marie Byrd Land, West Antarctica. (it)Annals of Glaciology,(no 8, 168-170. Whillans, I.M., and R. Bindschadler. In press. Mass balance of ice stream B. Annals of Glaciology.

Whillans, I.M.,J. Bolzan, and S. Shabtaie. 1987. Velocity of ice streams B and C, Antarctica. Journal of Geophysical Research, 92(B9), 8895-8902. Whillans, l.M., K.C. Jezek, A.R. Drew, and N. Gundestrup. 1984. Ice flow leading to the deep core hole at Dye 3, Greenland. Annals of Glaciology, 5, 185-190.

A curve of density versus depth calculated from a seismic compressional-wave profile at Upstream B agrees well with densities measured directly on a core obtained nearby (Anandakrishnan et al. in press). Discontinuities in the velocity gradient do not appear at the "critical density" as they did at Byrd Station and elsewhere (Kohnen and Bentley 1973; Robertson and Bentley 1975). Marked differences in velocity between horizontally and vertically polarized shear waves, particularly in the shallow firn, can be explained by a strong vertical shape-andbonding fabric in the shallow firn, such as has been observed in the cores (Alley and Bentley, Antarctic Journal, this issue). But inversion of seismic reflection times observed at Upstream B camp by a procedure developed by Blankenship and Bentley (1987) suggests that most of the ice is characterized by a strong concentration of crystal c-axes in a vertical plane that is transverse to the axis of the ice stream. This fabric is well defined by an absence of apparent compressional-wave anisotropy combined with the presence of distinct shear-wave anisotropy (Blankenship and Bentley in press). Fault-plane solutions applied to a group of microearthquakes from the bed of ice stream B reveal that slip occurs on horizontal planes at or just below the base of the ice (Anandakrishnan, ANTARCTIC JOURNAL