Geophysical studies of the Siple Coast area
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 Vornberger (unpublished data, Ohio State University) and Vornberger and Whillans (1986) report on this.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
Blankenship, and Bentley in press). However, the energy released in the faulting is a minuscule fraction of the total strain energy dissipation at the bed (Blankenship et al. 1987a). Polarization-dependent velocities for shear waves stemming from these sources support the crystalline-fabric model previously mentioned. The high flow velocities of ice stream B, which are hard to explain by glacier sliding models (Bentley 1987a, 1987b), appear to arise from deformation of a meters-thick subglacial debris layer (Blankenship et al. 1987b; Alley et al. 1987b). Continued seismic profiling in the Upstream B area has shown that the layer is essentially continuous over at least 12 kilometers parallel to ice flow and 8 kilometers transverse to flow (Rooney et al. 1987b, in press a). We believe that the debris flux from deformation has caused the deposition of "till deltas" or morainal banks tens of meters thick and tens of kilometers long at the grounding line. Glaciological considerations suggest that a drop in sea level would cause erosion on the upglacier side and deposition on the downglacier side of these morainal banks, causing grounding-line advance. We propose that the regional unconformity found in the Ross Sea, which extends to the edge of the continental shelf and is overlain by a meters-thick till layer, is the sedimentological signature of an expanded, till-lubricated west antarctic ice sheet during the Pleistocene (Alley et al. in press a and in press b). There is an angular unconformity beneath the till at Upstream B, which is consistent with a model of steady-state ice stream flow that implies erosion there (Alley et al. 1987a, 1987c). Seismic reflections from deeper beneath the ice stream show lithified sedimentary beds that dip grid northeast about 0.5° and are truncated by the unconformity (Rooney et al. in press b). The compressional-wave velocities in these layers are less than 2.3 kilometers per second which suggests that the sediments are late Oligocene or younger in age. The sedimentary basin beneath ice stream B is at least 1 kilometer thick and possibly is bounded (on one side) by a large normal fault that was revealed by seismic refraction shooting (Rooney et al. 1987a). Our radar data are now recorded digitally (Schultz, Powell, and Bentley 1987); development of techniques for quantitative analysis of echo characteristics is in progress. In the meantime, ice thicknesses have been obtained from analog radar sounding over and around ice streams A, B, and C (Shabtaie, Whillans, and Bentley 1987; Shabtaie and Bentley in press a and in press b). The entire length of ice stream A is marked by a deep subglacial trough. The beds of ridge AB (part of which may be a remanent ice stream) and ice stream 131 also are deep at their headward ends but shoal rapidly downstream. Ice stream B2 is 1,000 meters thinner than ice stream 131 near its head, but much more nearly constant in thickness along its length. Ridge BC is characterized by a smoother bed than ridge AB. Inactive ice stream C is marked by uncorrelated maxima and minima in surface and bed topography. Ice streams A and B have negative overall mass balances whereas inactive ice stream C has a strongly positive balance (Shabtaie and Bentley 1987). Individual sections of ice stream A do not show a significant imbalance, but a section at the heads of ice streams BI and B2 shows a strongly negative net flux that we attribute to lateral and headward expansion of these ice streams (Shabtaie et al. in press). Although there are no statistically significant deviations from zero net flux along the rest of ice stream B, there is an intriguing suggestion of a wave of adjustment moving downstream rather like that on a temperate glacier during the initiation of a surge. 1987 REVIEW
This research was supported by National Science Foundation grant DPP 84-12404. This is contribution number 485 of the University of Wisconsin at Madison, Geophysical and Polar Research Center. References Alley, RB., and C.R. Bentley. 1987. Analysis of Siple Coast firn cores. Antarctic Journal of the U.S., 22(5). Alley, R.B., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. 1987a. Continuous till defornation beneath ice sheets. (IAHS Publication No. 170) Symposium on Physical Basis of Ice Sheet Modelling, Vancouver. Alley, RB., D.D. Blankenship, C.R. Bentley, and S.T. Rooney. 1987b. Till beneath ice stream B. 3. Till deformation: Evidence and implications. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(139), 8921-8929, Alley, R.V., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. 1987c. Till beneath ice stream B. 4. A coupled ice-till flow model. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(139), 8931-8940. Alley, R.B., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. In press a. Sedimentation beneath ice shelves-The view from ice stream B. Proceedings of INQUA Symposium.
Alley, R.B., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. In press b. A glaciological model for the Quaternary sedimentary history of the Ross Embayment (abstract). Proceedings of the Fifth International Symposium on Antarctic Earth Sciences.
Anandakrishnan, S., D.D. Blankenship, and C.R. Bentley. In press. Microearthquake source locations and mechanisms: Ice stream B, Antarctica (abstract). Annals of Glaciology. Anandakrishnan, S., D.D. Blankenship, R.B. Alley, and C.R. Bentley. In press. Density-depth profile determined by seismic refraction studies: Ice stream B, West Antarctica (abstract). Annals of Glaciology. Bentley, C.R. 1987a. Antarctic ice streams: A review. Journal of Geophysical Research, 92(139), 8843-8858. Bentley, C . R. 1987b. Constraints on models in the Ross Embayment, Antarctica. (IAHS Publication No. 170.) Symposium on Physical Basis of Ice Sheet Modelling, Vancouver. Bentley, CR., D.D. Blankenship, D.G. Schultz, S.T. Rooney, and S. Anandakrishnan. 1986. Geophysical program at Upstream B camp, Siple Coast. Antarctic Journal of the U.S., 21(5), 109-110. Bentley, CR., S. Shabtaie, D.D. Blankenship, S.T. Rooney, D.G. Schultz, S. Anandakrishnan, and R. B. Alley. 1987. Remote sensing of the Ross ice streams and adjacent Ross Ice Shelf, Antarctica. Annals of Glaciology, 9, 20-29. Blankenship, D. D., and C. R. Bentley. 1987. Crystalline fabric of polar ice sheets from seismic anistropy. (IAHS Publication No. 170.) Symposium on Physical Basis of Ice Sheet Modelling, Vancouver. Blankenship, D. D., and C. Bentley. In press. Seismic measurement of the gross crystalline structure of a West Antarctic ice stream (abstract). Annals of Glaciology.
Blankenship, D.D., S. Anandakrishnan, J.L. Kempf, and C.R. Bentley. 1987a. Microearthquakes under and alongside ice stream B, detected by a new passive seismic array. Annals of Glaciology, 9, 30-34. Blankenship, D.D., C.R. Bentley, S.T. Rooney, and R.B. Alley. 1987b. Till beneath ice stream B. 1. Properties derived from seismic travel times. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(139), 8903-8911. Kohnen, H., and C.R. Bentley. 1973. Seismic refraction and reflection measurements at "Byrd" Station, Antarctica. Journal of Glaciology, 12(64), 101-111. Robertson, J.D., and C.R. Bentley. 1975. Investigations of polar snow using seismic velocity gradients. Journal of Glaciology, 14(70), 39-48. Rooney, ST., D.D. Blankenship, and C.R. Bentley. 1987a. Seismic refraction measurements of crustal structure in West Antarctica. In G . D. McKenzie (Ed.), Gondwana Six: Structure, tectonics, and geophysics, (Geophysical Monograph 40). Washington, D.C.: American Geophysical Union. Rooney, ST., D.D. Blankenship, R.B. Alley, and C.R. Bentley. 1987b.
69
Till beneath ice stream B. 2. Structure and continuity. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue) 92(139), 8913-8920. Rooney, S.T., D.D. Blankenship, R.B. Alley, and C.R. Bentley. In press a. Seismic reflection profiling of a widespread till beneath ice stream B, West Antarctica (abstract). Annals of Glaciology. Rooney, S.T., D.D. Blankenship, R.B. Alley, and C.R. Bentley. In press b. Seismic reflection profiling of a sediment-filled graben beneath ice stream B, West Antarctica. Proceedings of Fifth International Symposium on Antarctic Earth Sciences.
Schultz, D.G., L.A. Powell, and C.R. Bentley. 1987. A digital recording system for echo studies on ice sheets. Annals of Glaciology, 9, 206-210. Shabtaie, S., and C.R. Bentley. 1987. West Antarctic ice streams draining into the Ross Ice Shelf: Configuration and mass balance. Journal of
Analysis of Siple Coast fir cores R.B. ALLEY and C.R. BENTLEY Geophysical and Polar Research Center University of Wisconsin Madison, Wisconsin 53706
Analyses of firn and ice cores from the Siple Coast of West Antarctica have continued to yield interesting results. The 100meter cores were drilled by W. Boller, B. Koci, K. Kuivinen, and J. Litwak of the Polar Ice Coring Office at the Upstream B camp (UpB) during 1984-1985 and at the Ohio State North Camp on ridge BC ("BC" on figure) during 1985-1986. The UpB core melted partially during shipment, but the BC core is safely in storage in our lab (Alley and Bentley 1985, 1986). Pit studies were conducted in conjunction with the coring. We summarize some of our results here. At densities less than 550 kilograms per cubic meter firn densifies primarily by rearrangement of grains through viscous boundary sliding (Alley 1987a). Rearrangement slows, causing a critical point in depth-density profiles, when a grain comes in contact with about six neighbors (coordination number = 6; Alley 1986) and achieves geometric stability. We now can predict depth-density curves at densities up to 550 kilograms per cubic meter from a physically based model, the mean annual temperature, and measurements made in a 2-meter pit. Grain-growth theory from metallurgy suggests that, for unstrained glacial ice, dissolved impurities (especially sea salt) slow grain growth significantly but that microparticles and porosity usually have little effect (Alley et al. 1986a, 1986b). The theory describes grain growth well in the upper 90 meters at BC and the upper 20 meters at UpB; strain energy causes accelerated grain growth in deeper ice at both sites. Firn in the upper 10-15 meters at both UpB and BC is characterized by strong anisotropy, with vertically elongated grains bonded near their ends into vertical columns. The degree of anisotropy, and several other interesting textural parameters, can be characterized accurately from thin-section measurements (Alley 1987b). Use of such data will allow better modeling 70
Geophysical Research (Chapman Conference on Fast Glacier Flow is-
sue), 92(132), 1311-1336. Shabtaie, S., and C.R. Bentley. In press a. Subglacial topography of West Antarctic ice streams from airborne radar sounding (abstract). Proceedings of Fifth international Symposium on Antarctic Earth Sciences.
Shabtaie, S., and C.R. Bentley. In press b. Radar sounding of the West Antarctic ice streams. Annals of Glaciology. Shabtaie, S., 1. 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 (Chapman Conference on Fast Glacier Flow issue), 92(139), 8865-8883. Shabtaie, S., C.R. Bentley, R.A. Bindschadler, and D.R. MacAyeal. In press. Mass balance studies of ice streams A, B, and C and possible surging behavior of ice stream B. Annals of Glaciology.
of processes in firn and better interpretation of remotely sensed data over firn. Further analyses of the field data and BC core are in progress. We are processing the BC core for oxygen-isotopic analysis by P. Grootes of the University of Washington. Our data (tabulated in Alley 1987c) also are being used in a study with K. Jezek of the U.S. Army Cold Regions Research and Engineering Laboratory to determine the effect of likely inhomogeneities in firn on radar-altimeter data. Development of C-axis fabrics in ice sheets is of special interest both as a record of strain history and as a control on present and future behavior. We now are studying the development of C-axis fabrics in ice sheets as a function of stress state and cumulative strain. We are testing our models against fabric measurements on the BC core and seismic data collected at UpB by D. Blankenship and S. Rooney of the University of Wisconsin, as well as against other published data sets.
V
BIOS
60 W
40
20
0 (180°)
Location map. UpB (triangle) and BC (+) are shown. Modified from Shabtaie and Bentley (1987). ANTARCTIC JOURNAL
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
Blankenship, and Bentley in press). However, the energy released in the faulting is a minuscule fraction of the total strain energy dissipation at the bed (Blankenship et al. 1987a). Polarization-dependent velocities for shear waves stemming from these sources support the crystalline-fabric model previously mentioned. The high flow velocities of ice stream B, which are hard to explain by glacier sliding models (Bentley 1987a, 1987b), appear to arise from deformation of a meters-thick subglacial debris layer (Blankenship et al. 1987b; Alley et al. 1987b). Continued seismic profiling in the Upstream B area has shown that the layer is essentially continuous over at least 12 kilometers parallel to ice flow and 8 kilometers transverse to flow (Rooney et al. 1987b, in press a). We believe that the debris flux from deformation has caused the deposition of "till deltas" or morainal banks tens of meters thick and tens of kilometers long at the grounding line. Glaciological considerations suggest that a drop in sea level would cause erosion on the upglacier side and deposition on the downglacier side of these morainal banks, causing grounding-line advance. We propose that the regional unconformity found in the Ross Sea, which extends to the edge of the continental shelf and is overlain by a meters-thick till layer, is the sedimentological signature of an expanded, till-lubricated west antarctic ice sheet during the Pleistocene (Alley et al. in press a and in press b). There is an angular unconformity beneath the till at Upstream B, which is consistent with a model of steady-state ice stream flow that implies erosion there (Alley et al. 1987a, 1987c). Seismic reflections from deeper beneath the ice stream show lithified sedimentary beds that dip grid northeast about 0.5° and are truncated by the unconformity (Rooney et al. in press b). The compressional-wave velocities in these layers are less than 2.3 kilometers per second which suggests that the sediments are late Oligocene or younger in age. The sedimentary basin beneath ice stream B is at least 1 kilometer thick and possibly is bounded (on one side) by a large normal fault that was revealed by seismic refraction shooting (Rooney et al. 1987a). Our radar data are now recorded digitally (Schultz, Powell, and Bentley 1987); development of techniques for quantitative analysis of echo characteristics is in progress. In the meantime, ice thicknesses have been obtained from analog radar sounding over and around ice streams A, B, and C (Shabtaie, Whillans, and Bentley 1987; Shabtaie and Bentley in press a and in press b). The entire length of ice stream A is marked by a deep subglacial trough. The beds of ridge AB (part of which may be a remanent ice stream) and ice stream 131 also are deep at their headward ends but shoal rapidly downstream. Ice stream B2 is 1,000 meters thinner than ice stream 131 near its head, but much more nearly constant in thickness along its length. Ridge BC is characterized by a smoother bed than ridge AB. Inactive ice stream C is marked by uncorrelated maxima and minima in surface and bed topography. Ice streams A and B have negative overall mass balances whereas inactive ice stream C has a strongly positive balance (Shabtaie and Bentley 1987). Individual sections of ice stream A do not show a significant imbalance, but a section at the heads of ice streams BI and B2 shows a strongly negative net flux that we attribute to lateral and headward expansion of these ice streams (Shabtaie et al. in press). Although there are no statistically significant deviations from zero net flux along the rest of ice stream B, there is an intriguing suggestion of a wave of adjustment moving downstream rather like that on a temperate glacier during the initiation of a surge. 1987 REVIEW
This research was supported by National Science Foundation grant DPP 84-12404. This is contribution number 485 of the University of Wisconsin at Madison, Geophysical and Polar Research Center. References Alley, RB., and C.R. Bentley. 1987. Analysis of Siple Coast firn cores. Antarctic Journal of the U.S., 22(5). Alley, R.B., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. 1987a. Continuous till defornation beneath ice sheets. (IAHS Publication No. 170) Symposium on Physical Basis of Ice Sheet Modelling, Vancouver. Alley, RB., D.D. Blankenship, C.R. Bentley, and S.T. Rooney. 1987b. Till beneath ice stream B. 3. Till deformation: Evidence and implications. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(139), 8921-8929, Alley, R.V., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. 1987c. Till beneath ice stream B. 4. A coupled ice-till flow model. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(139), 8931-8940. Alley, R.B., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. In press a. Sedimentation beneath ice shelves-The view from ice stream B. Proceedings of INQUA Symposium.
Alley, R.B., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. In press b. A glaciological model for the Quaternary sedimentary history of the Ross Embayment (abstract). Proceedings of the Fifth International Symposium on Antarctic Earth Sciences.
Anandakrishnan, S., D.D. Blankenship, and C.R. Bentley. In press. Microearthquake source locations and mechanisms: Ice stream B, Antarctica (abstract). Annals of Glaciology. Anandakrishnan, S., D.D. Blankenship, R.B. Alley, and C.R. Bentley. In press. Density-depth profile determined by seismic refraction studies: Ice stream B, West Antarctica (abstract). Annals of Glaciology. Bentley, C.R. 1987a. Antarctic ice streams: A review. Journal of Geophysical Research, 92(139), 8843-8858. Bentley, C . R. 1987b. Constraints on models in the Ross Embayment, Antarctica. (IAHS Publication No. 170.) Symposium on Physical Basis of Ice Sheet Modelling, Vancouver. Bentley, CR., D.D. Blankenship, D.G. Schultz, S.T. Rooney, and S. Anandakrishnan. 1986. Geophysical program at Upstream B camp, Siple Coast. Antarctic Journal of the U.S., 21(5), 109-110. Bentley, CR., S. Shabtaie, D.D. Blankenship, S.T. Rooney, D.G. Schultz, S. Anandakrishnan, and R. B. Alley. 1987. Remote sensing of the Ross ice streams and adjacent Ross Ice Shelf, Antarctica. Annals of Glaciology, 9, 20-29. Blankenship, D. D., and C. R. Bentley. 1987. Crystalline fabric of polar ice sheets from seismic anistropy. (IAHS Publication No. 170.) Symposium on Physical Basis of Ice Sheet Modelling, Vancouver. Blankenship, D. D., and C. Bentley. In press. Seismic measurement of the gross crystalline structure of a West Antarctic ice stream (abstract). Annals of Glaciology.
Blankenship, D.D., S. Anandakrishnan, J.L. Kempf, and C.R. Bentley. 1987a. Microearthquakes under and alongside ice stream B, detected by a new passive seismic array. Annals of Glaciology, 9, 30-34. Blankenship, D.D., C.R. Bentley, S.T. Rooney, and R.B. Alley. 1987b. Till beneath ice stream B. 1. Properties derived from seismic travel times. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(139), 8903-8911. Kohnen, H., and C.R. Bentley. 1973. Seismic refraction and reflection measurements at "Byrd" Station, Antarctica. Journal of Glaciology, 12(64), 101-111. Robertson, J.D., and C.R. Bentley. 1975. Investigations of polar snow using seismic velocity gradients. Journal of Glaciology, 14(70), 39-48. Rooney, ST., D.D. Blankenship, and C.R. Bentley. 1987a. Seismic refraction measurements of crustal structure in West Antarctica. In G . D. McKenzie (Ed.), Gondwana Six: Structure, tectonics, and geophysics, (Geophysical Monograph 40). Washington, D.C.: American Geophysical Union. Rooney, ST., D.D. Blankenship, R.B. Alley, and C.R. Bentley. 1987b.
69
Till beneath ice stream B. 2. Structure and continuity. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue) 92(139), 8913-8920. Rooney, S.T., D.D. Blankenship, R.B. Alley, and C.R. Bentley. In press a. Seismic reflection profiling of a widespread till beneath ice stream B, West Antarctica (abstract). Annals of Glaciology. Rooney, S.T., D.D. Blankenship, R.B. Alley, and C.R. Bentley. In press b. Seismic reflection profiling of a sediment-filled graben beneath ice stream B, West Antarctica. Proceedings of Fifth International Symposium on Antarctic Earth Sciences.
Schultz, D.G., L.A. Powell, and C.R. Bentley. 1987. A digital recording system for echo studies on ice sheets. Annals of Glaciology, 9, 206-210. Shabtaie, S., and C.R. Bentley. 1987. West Antarctic ice streams draining into the Ross Ice Shelf: Configuration and mass balance. Journal of
Analysis of Siple Coast fir cores R.B. ALLEY and C.R. BENTLEY Geophysical and Polar Research Center University of Wisconsin Madison, Wisconsin 53706
Analyses of firn and ice cores from the Siple Coast of West Antarctica have continued to yield interesting results. The 100meter cores were drilled by W. Boller, B. Koci, K. Kuivinen, and J. Litwak of the Polar Ice Coring Office at the Upstream B camp (UpB) during 1984-1985 and at the Ohio State North Camp on ridge BC ("BC" on figure) during 1985-1986. The UpB core melted partially during shipment, but the BC core is safely in storage in our lab (Alley and Bentley 1985, 1986). Pit studies were conducted in conjunction with the coring. We summarize some of our results here. At densities less than 550 kilograms per cubic meter firn densifies primarily by rearrangement of grains through viscous boundary sliding (Alley 1987a). Rearrangement slows, causing a critical point in depth-density profiles, when a grain comes in contact with about six neighbors (coordination number = 6; Alley 1986) and achieves geometric stability. We now can predict depth-density curves at densities up to 550 kilograms per cubic meter from a physically based model, the mean annual temperature, and measurements made in a 2-meter pit. Grain-growth theory from metallurgy suggests that, for unstrained glacial ice, dissolved impurities (especially sea salt) slow grain growth significantly but that microparticles and porosity usually have little effect (Alley et al. 1986a, 1986b). The theory describes grain growth well in the upper 90 meters at BC and the upper 20 meters at UpB; strain energy causes accelerated grain growth in deeper ice at both sites. Firn in the upper 10-15 meters at both UpB and BC is characterized by strong anisotropy, with vertically elongated grains bonded near their ends into vertical columns. The degree of anisotropy, and several other interesting textural parameters, can be characterized accurately from thin-section measurements (Alley 1987b). Use of such data will allow better modeling 70
Geophysical Research (Chapman Conference on Fast Glacier Flow is-
sue), 92(132), 1311-1336. Shabtaie, S., and C.R. Bentley. In press a. Subglacial topography of West Antarctic ice streams from airborne radar sounding (abstract). Proceedings of Fifth international Symposium on Antarctic Earth Sciences.
Shabtaie, S., and C.R. Bentley. In press b. Radar sounding of the West Antarctic ice streams. Annals of Glaciology. Shabtaie, S., 1. 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 (Chapman Conference on Fast Glacier Flow issue), 92(139), 8865-8883. Shabtaie, S., C.R. Bentley, R.A. Bindschadler, and D.R. MacAyeal. In press. Mass balance studies of ice streams A, B, and C and possible surging behavior of ice stream B. Annals of Glaciology.
of processes in firn and better interpretation of remotely sensed data over firn. Further analyses of the field data and BC core are in progress. We are processing the BC core for oxygen-isotopic analysis by P. Grootes of the University of Washington. Our data (tabulated in Alley 1987c) also are being used in a study with K. Jezek of the U.S. Army Cold Regions Research and Engineering Laboratory to determine the effect of likely inhomogeneities in firn on radar-altimeter data. Development of C-axis fabrics in ice sheets is of special interest both as a record of strain history and as a control on present and future behavior. We now are studying the development of C-axis fabrics in ice sheets as a function of stress state and cumulative strain. We are testing our models against fabric measurements on the BC core and seismic data collected at UpB by D. Blankenship and S. Rooney of the University of Wisconsin, as well as against other published data sets.
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Location map. UpB (triangle) and BC (+) are shown. Modified from Shabtaie and Bentley (1987). ANTARCTIC JOURNAL
