Land ice studies
Land ice studies Transient behavior of ice stream B and Crary Ice Rise, West Antarctica R.A. BINDSCHADLER National Aeronautics and Space Administration Goddard Space Flight Center Greenbelt, Maryland 20771
D.R. MACAYEAL Department of Geophysical Sciences University of Chicago Chicago, Illinois 60637
S.N. STEPHENSON and P.L. VORNBERGER Science Applications Research Lanham, Maryland 20706
E. P. ROBERTS Department of Geology University of Maryland College Park, Maryland 20742
Analysis of data collected during three field seasons (1983-1986) in the region where ice stream B discharges into the Ross Ice Shelf indicates three examples of transient ice flow: • a positive mass balance in the region surrounding Crary Ice Rise, • separation of a large "raft" of ice from the main ice rise, and • a 20 percent slowing of ice in the mouth of ice stream B. Surface velocity data from this project and the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS) (Thomas et al. 1984) were combined with radar measurements of ice thickness to calculate the volume fluxes across the boundaries of a region of the Ross Ice Shelf which includes the Crary Ice Rise (figure 1) (MacAyeal et al. in press). Combining the excess mass flux with the total surface accumulation flux (assuming negligible basal melting or accretion) gives an area-averaged value of 0.44 ± 0.06 meters per year for the thickening within this region. Additional data on the Crary Ice Rise include aerial photography gathered by the U.S. Geological Survey in January 1985. From a mosaic of these photographs, surface features such as crevasses and surface undulations were identified and mapped. Figure 2 shows the crevasses seen on the photographs in the vicinity of the ice rise. Adjacent to the main ice rise, which is free of crevasses, are other smaller crevasse-free regions. We term these smaller regions rafts. The largest of the rafts, labeled R in 66
figure 2, seems to have separated from the main body of the ice rise because of the intense shear crevasses between it and the main ice rise. The orientation of these crevasses and the azimuth of a velocity measurement obtained near this raft indicate that the raft is now moving as part of the ice shelf. Calculations of when this separation occurred depend on assumptions of how far it has moved and how it has accelerated; the most probable estimates for these values suggest that the separation process began during the last hundred years. Details of this analysis can be found in Bindschadler et al. (in preparation). Comparison of velocities measured during RIGGS with those of the current program show that the ice upstream of Crary Ice Rise has slowed. This comparison was accomplished using current strain rates to extrapolate velocities measured during this field program to the locations of measured velocities during RIGGS. Three RIGGS stations were used and all appear to have slowed approximately 20 percent. This work is being prepared for publication by Stephenson and Bindschadler. Additional field measurements are being made to confirm this trend. These examples of transient behavior may be interrelated. The thickening of the area around Crary Ice Rise would increase the back pressure on ice stream B (MacAyeal et al. in press), slowing the ice upstream of the ice rise. The separation of a large piece of the ice rise might result from a failure of the bed which could occur after the ice rise exceeded some critical ice thickness or surface slope. Tension between the ice shelf and the ice rise would also increase as the ice thickness increased. This type of process may be similar to the incorporation of ice into the head of an ice stream as discussed by Whillans et al. (in press).
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Figure 1. Region around the Crary Ice Rise Complex used by MacAyeal et al. (in press) to calculate the mass balance.
ANTARCTIC JOURNAL
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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
D.R. MACAYEAL Department of Geophysical Sciences University of Chicago Chicago, Illinois 60637
S.N. STEPHENSON and P.L. VORNBERGER Science Applications Research Lanham, Maryland 20706
E. P. ROBERTS Department of Geology University of Maryland College Park, Maryland 20742
Analysis of data collected during three field seasons (1983-1986) in the region where ice stream B discharges into the Ross Ice Shelf indicates three examples of transient ice flow: • a positive mass balance in the region surrounding Crary Ice Rise, • separation of a large "raft" of ice from the main ice rise, and • a 20 percent slowing of ice in the mouth of ice stream B. Surface velocity data from this project and the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS) (Thomas et al. 1984) were combined with radar measurements of ice thickness to calculate the volume fluxes across the boundaries of a region of the Ross Ice Shelf which includes the Crary Ice Rise (figure 1) (MacAyeal et al. in press). Combining the excess mass flux with the total surface accumulation flux (assuming negligible basal melting or accretion) gives an area-averaged value of 0.44 ± 0.06 meters per year for the thickening within this region. Additional data on the Crary Ice Rise include aerial photography gathered by the U.S. Geological Survey in January 1985. From a mosaic of these photographs, surface features such as crevasses and surface undulations were identified and mapped. Figure 2 shows the crevasses seen on the photographs in the vicinity of the ice rise. Adjacent to the main ice rise, which is free of crevasses, are other smaller crevasse-free regions. We term these smaller regions rafts. The largest of the rafts, labeled R in 66
figure 2, seems to have separated from the main body of the ice rise because of the intense shear crevasses between it and the main ice rise. The orientation of these crevasses and the azimuth of a velocity measurement obtained near this raft indicate that the raft is now moving as part of the ice shelf. Calculations of when this separation occurred depend on assumptions of how far it has moved and how it has accelerated; the most probable estimates for these values suggest that the separation process began during the last hundred years. Details of this analysis can be found in Bindschadler et al. (in preparation). Comparison of velocities measured during RIGGS with those of the current program show that the ice upstream of Crary Ice Rise has slowed. This comparison was accomplished using current strain rates to extrapolate velocities measured during this field program to the locations of measured velocities during RIGGS. Three RIGGS stations were used and all appear to have slowed approximately 20 percent. This work is being prepared for publication by Stephenson and Bindschadler. Additional field measurements are being made to confirm this trend. These examples of transient behavior may be interrelated. The thickening of the area around Crary Ice Rise would increase the back pressure on ice stream B (MacAyeal et al. in press), slowing the ice upstream of the ice rise. The separation of a large piece of the ice rise might result from a failure of the bed which could occur after the ice rise exceeded some critical ice thickness or surface slope. Tension between the ice shelf and the ice rise would also increase as the ice thickness increased. This type of process may be similar to the incorporation of ice into the head of an ice stream as discussed by Whillans et al. (in press).
hi
IE
Figure 1. Region around the Crary Ice Rise Complex used by MacAyeal et al. (in press) to calculate the mass balance.
ANTARCTIC JOURNAL
7
0°30'W
0 1-
0
Ji
(a
J3 +
J2
OOcIO;;
-- •LP1.(110) +
-
E4
L
\ \/ ) lcoz
•
(H9) Ki
. ':
K2
I'
I U) 10
II I I I I I
0 10 20 30 40 50 DISTANCE (km) 0c
0
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
