Short-term variations in the speed of ice stream B, Antarctica
Further data analysis will be a cooperative project involving both institutions. Jerry Bradley (USGS, Denver), Tom Grover (USGS, Denver), Bruce Vaughn (USGS, Tacoma), and Kurt Hayden (Ohio State University) participated at various stages in the development and field work.
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References
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Figure 3. Another section of the same profile shown in figure 2, to the same scale. The bottom is much rougher here, the bed echo has a distinctly different character, and the undulating internal layering can be detected to a greater depth.
Short-term variations in the speed of ice stream B, Antarctica W.D. H1\ui .usoN and
K.A. ECHELMEYER
Geophysical institute University of Alaska Fairbanks, Alaska 99775-0800
A search for variations in the speed, vertical strain rate, and seismicity of ice stream B was begun in late November 1988, near Upstream B camp (83°29'S 138°12'W). This is part of a cooperative program among several institutions to study the dynamics of this and the other rapidly moving ice streams that feed the western side of the Ross Ice Shelf. Interest in these ice streams has been stimulated by several discoveries: • the neighboring ice stream C is stagnant but has been so for a relatively short time (Rose 1979; Whillans and Boizan 1987), • the drainage of ice stream B is out of balance with accumulation (Whillans and Bolzan 1987), and • ice stream B, at least near Upstream B Camp, is underlain by a thin layer of till (Blankenship et al. 1986) whose defor1989 REVIEW
Bentley, C.R. 1987. Antarctic ice streams: A review. Journal of Geophysical Research, 92(B9), 8,843-3,358. Hodge, S.M., D.L. Wright, J.A. Bradley, R.W. Jacobel, N. Skou, and B.H. Vaughn. In preparation. Determination of the surface and bed topography in central Greenland. Journal of Glaciology. 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(B2), 1,311-1,336. Watts, R.D., and A.W. England. 1976. Radio-echo sounding of temperate glaciers: Ice properties and sounder design criteria. Journal of Glaciology, 34(118), 39-48. Wright, D.L., J.A. Bradley, and S.M. Hodge. In press. Use of a new high-speed digital data acquisition system in airborne ice-sounding. IEEE Transactions on Geoscience and Remote Sensing.
Wright, DL., S.M. Hodge, J.A. Bradley, T.P. Grover, and R.W. Jacobel. In preparation. A digital low-frequency, surface-based profiling ice radar system. Journal of Glaqciology.
mation can probably account for most of the observed motion, which is more rapid than can be produced by ice deformation at the existing low basal shear stresses. The ice streams appear to be rapidly changing features that may play a role in the stability of the west antarctic ice sheet. The similarities and differences in the fast flow of the antarctic and Greenland ice streams, and of tide water and surging glaciers, are of considerable interest. Our program consists of two parts, the measurement of ice speed and surface strain rate while personnel are in the field, and the measurement of strain rate and seismicity year round by geophones and strain wires installed in several shallow boreholes near Upstream B Camp (and the California Institute of Technology drill site). At the end of the field season, the strain wires were still responding to transient effects associated with their installation, and there had been little seismicity; however, about 5 weeks of velocity data had been acquired by two methods, both of which determined distance to two ref erence points located on relatively stagnant ice off the ice stream. In the one case, a Del Norte ultra-high-frequency positioning system was used for daily motion studies, in the other, a Wild DI-20 EDM was used for terrestrial surveying, which was typically done several times per day, but sometimes hourly. The latter system is the more sensitive, although its success is weather dependent. Results from the EDM survey are summarized in figure 1. 81
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348 DAY
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Figure 1. Speed of ice stream B at a site near its edge, 1988. The scatter is larger for short measurement intervals than the error limits for the twice-daily velocity measurements, which are shown. (m denotes meter.)
Figure 2. Ice stream motion as determined over intervals of approximately 1 to 2 hours. Error bar for hourly speed is shown. (m denotes meter.)
The speed of the instrument site, which was located near the edge of the ice stream, was determined by two shots, 6 and 9 kilometers long, to two markers off the ice stream. It is 1.0 meter per day. The accuracy is determined by our ability to make atmospheric corrections for the velocity of light. Since there are two shots, there is enough redundancy in the data to calculate this correction, which was also checked by measuring to a point on the ice stream, and by calculation from pressure and temperature data. Analysis of the error in this correction indicates that it can account for all the scatter in figure 1. The large scatter near the center of the record is due to the short time interval of the measurements. The results of these short-period measurements (made over approximately 1-hour intervals) are shown in figure 2. Again, the apparent fluctuations in speed are nearly all within the estimated error limits. The results to date, therefore, do not resolve any short-term variations in the behavior of the ice stream. Measurements carried out with lower resolution, but over a longer period (2 years) by McDonald and Whillans (in preparation), also do not
reveal any secular variations in velocity. These results are interesting in the light of the suspected transient behavior of the ice streams noted earlier. This work is supported by National Science Foundation grant DPP 87-16604.
82
References Blankenship, D.D., C.R. Bentley, S.T. Rooney, and R.B. Alley. 1986. Seismic measurements reveal a saturated, porous layer beneath an active Antarctic Ice Stream. Nature, 322, 54-57. McDonald, J., and I.M. Whillans. In preparation. Search for secular changes in the velocity of Ice Stream "B," West Antarctica. Journal of Glaciology.
Rose, K.E. 1979. Characteristics of ice flow in Marie Byrd Land, Antarctica. Journal of Glaciology, 24(90), 63-74. Whillans, I.M., and J. Boizan. 1987. Velocity of Ice Streams B and C, Antarctica. Journal of Geophysical Research, 92(139), 8,895-8,902.
ANTARCTIC JOURNAL
• ;jiur *f_
References
• •.• :- -1rr1*v-.. - .1. •-ri B—
-
4i
•
•"'-p
'-t
AiL
Figure 3. Another section of the same profile shown in figure 2, to the same scale. The bottom is much rougher here, the bed echo has a distinctly different character, and the undulating internal layering can be detected to a greater depth.
Short-term variations in the speed of ice stream B, Antarctica W.D. H1\ui .usoN and
K.A. ECHELMEYER
Geophysical institute University of Alaska Fairbanks, Alaska 99775-0800
A search for variations in the speed, vertical strain rate, and seismicity of ice stream B was begun in late November 1988, near Upstream B camp (83°29'S 138°12'W). This is part of a cooperative program among several institutions to study the dynamics of this and the other rapidly moving ice streams that feed the western side of the Ross Ice Shelf. Interest in these ice streams has been stimulated by several discoveries: • the neighboring ice stream C is stagnant but has been so for a relatively short time (Rose 1979; Whillans and Boizan 1987), • the drainage of ice stream B is out of balance with accumulation (Whillans and Bolzan 1987), and • ice stream B, at least near Upstream B Camp, is underlain by a thin layer of till (Blankenship et al. 1986) whose defor1989 REVIEW
Bentley, C.R. 1987. Antarctic ice streams: A review. Journal of Geophysical Research, 92(B9), 8,843-3,358. Hodge, S.M., D.L. Wright, J.A. Bradley, R.W. Jacobel, N. Skou, and B.H. Vaughn. In preparation. Determination of the surface and bed topography in central Greenland. Journal of Glaciology. 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(B2), 1,311-1,336. Watts, R.D., and A.W. England. 1976. Radio-echo sounding of temperate glaciers: Ice properties and sounder design criteria. Journal of Glaciology, 34(118), 39-48. Wright, D.L., J.A. Bradley, and S.M. Hodge. In press. Use of a new high-speed digital data acquisition system in airborne ice-sounding. IEEE Transactions on Geoscience and Remote Sensing.
Wright, DL., S.M. Hodge, J.A. Bradley, T.P. Grover, and R.W. Jacobel. In preparation. A digital low-frequency, surface-based profiling ice radar system. Journal of Glaqciology.
mation can probably account for most of the observed motion, which is more rapid than can be produced by ice deformation at the existing low basal shear stresses. The ice streams appear to be rapidly changing features that may play a role in the stability of the west antarctic ice sheet. The similarities and differences in the fast flow of the antarctic and Greenland ice streams, and of tide water and surging glaciers, are of considerable interest. Our program consists of two parts, the measurement of ice speed and surface strain rate while personnel are in the field, and the measurement of strain rate and seismicity year round by geophones and strain wires installed in several shallow boreholes near Upstream B Camp (and the California Institute of Technology drill site). At the end of the field season, the strain wires were still responding to transient effects associated with their installation, and there had been little seismicity; however, about 5 weeks of velocity data had been acquired by two methods, both of which determined distance to two ref erence points located on relatively stagnant ice off the ice stream. In the one case, a Del Norte ultra-high-frequency positioning system was used for daily motion studies, in the other, a Wild DI-20 EDM was used for terrestrial surveying, which was typically done several times per day, but sometimes hourly. The latter system is the more sensitive, although its success is weather dependent. Results from the EDM survey are summarized in figure 1. 81
1.5
1.
>
>. (V V
V E Ui UJ
E
1.0
1.c
Ui Ui
a.
M
U)
U)
0.5
o.
344
348 DAY
DAY
Figure 1. Speed of ice stream B at a site near its edge, 1988. The scatter is larger for short measurement intervals than the error limits for the twice-daily velocity measurements, which are shown. (m denotes meter.)
Figure 2. Ice stream motion as determined over intervals of approximately 1 to 2 hours. Error bar for hourly speed is shown. (m denotes meter.)
The speed of the instrument site, which was located near the edge of the ice stream, was determined by two shots, 6 and 9 kilometers long, to two markers off the ice stream. It is 1.0 meter per day. The accuracy is determined by our ability to make atmospheric corrections for the velocity of light. Since there are two shots, there is enough redundancy in the data to calculate this correction, which was also checked by measuring to a point on the ice stream, and by calculation from pressure and temperature data. Analysis of the error in this correction indicates that it can account for all the scatter in figure 1. The large scatter near the center of the record is due to the short time interval of the measurements. The results of these short-period measurements (made over approximately 1-hour intervals) are shown in figure 2. Again, the apparent fluctuations in speed are nearly all within the estimated error limits. The results to date, therefore, do not resolve any short-term variations in the behavior of the ice stream. Measurements carried out with lower resolution, but over a longer period (2 years) by McDonald and Whillans (in preparation), also do not
reveal any secular variations in velocity. These results are interesting in the light of the suspected transient behavior of the ice streams noted earlier. This work is supported by National Science Foundation grant DPP 87-16604.
82
References Blankenship, D.D., C.R. Bentley, S.T. Rooney, and R.B. Alley. 1986. Seismic measurements reveal a saturated, porous layer beneath an active Antarctic Ice Stream. Nature, 322, 54-57. McDonald, J., and I.M. Whillans. In preparation. Search for secular changes in the velocity of Ice Stream "B," West Antarctica. Journal of Glaciology.
Rose, K.E. 1979. Characteristics of ice flow in Marie Byrd Land, Antarctica. Journal of Glaciology, 24(90), 63-74. Whillans, I.M., and J. Boizan. 1987. Velocity of Ice Streams B and C, Antarctica. Journal of Geophysical Research, 92(139), 8,895-8,902.
ANTARCTIC JOURNAL
