Antarctic ice stream B: conditions controlling its motion and interactions with the climate system

Transcription

1 Glaciers-Ocean-Atmosphere interactions (Proceedings of the International Symposium held at St Petersburg, September 1990). IAHS Publ. no. 208, Antarctic ice stream B: conditions controlling its motion and interactions with the climate system B. KAMB & H. ENGELHARDT Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA ABSTRACT A program of borehole geophysical measurements at the base of West Antarctic ice streams has thus far obtained from Ice Stream B some observations of temperature, basal water pressure, hydraulic parameters of the basal water system, properties of subglacial till including minimum thickness, porosity, lithology, strength, and hydraulic conductivity, and incomplete observations of basal sliding and in situ till deformation. The base of Ice Stream B is at the melting point, the basal water pressure is within about 1 bar of the overburden pressure, and deformable subglacial till at least 3 m thick is present under ice. These conditions favor rapid basal sliding and rapid bed deformation as ice-stream flow mechanisms. A quantitative understanding of them is needed to consider the interaction of the ice streams with the climate system. INTRODUCTION The ice streams of the West Antarctic Ice Sheet (Bentley, 1978) present to the science of glaciology the challenge of explaining an important, large-scale glacier flow phenomenon, which was so largely unknown ten years ago that it was mentioned in only two sentences in the excellent review of glacier physics by Paterson (1981, pp. 162, 178). How can a mass of ice some 50 km wide and 500 km long within the ice sheet, in which the normal flow velocity is ~ 10 m year", move at speeds approaching 1000 m year"? What controls the high speed and the areal extent of the rapidly moving mass? How do the rapid motions start and stop? In the last ten years, much progress has been made in revealing by field observation the detailed configuration and surface flow features of West Antarctic Ice Streams B and C. Flow motions have been documented by Whillans et al. (1987) and by Stephenson & Bindschadler (1989), surface and basal topography by Shabtaie e_t al. (1987) and Shabtaie & Bentley (1988), mass balance by Whillans & 145

2 B. Kamb & H. Engelhardt 146 Bindschadler (1988) and by Shabtaie e_t al. (1988), and ice structure and crevassing by Vornberger & Whillans (1986). Explanations of the ice-stream phenomenon have also flourished. To the original proposals of rapid flow by means of ice superplasticity (Hughes, 1977) and by means of basal sliding (Rose, 1979) has been added the proposal that the rapid motion is lubricated by the shear deformation of a subglacial layer of soft, water saturated till (Alley e_t al., 1986, 1987), whose existence and properties were inferred from seismic reflection data (Blankenship e_t al_^, 1986, 1987). All three proposed mechanisms of ice-stream motion involve processes operation at or near the base of the ice. To seek direct evidence as to these processes we have undertaken to gain access to the basal zone by drilling, and we have made several borehole-geophysical observations that bear on the ice stream flow mechanism (Engelhardt, 1990b). The results are summarized below. In addition to their importance for fundamental glaciology, the ice streams also have a potential role of practical importance in the possible instability of the West Antarctic Ice Sheet and its response to climatic change, a phenomenon of glacier-ocean-atmospheric interaction. Most of the discussion of possible collapse of the ice sheet has been based on the concept of marine ice sheet instability (Hughes, 1973; Weertman, 1974, 1976; Mercer, 1978; Thomas, 1979, 1984; Thomas et al., 1979; Paterson, 1981, p. 179; Bentley, 1984; Fastook, 1984; van der Veen, 1985), but the possibly important role of ice streams in a collapse process has become increasingly recognized (Hughes, 1977, p. 44; Weertman & Birchfield, 1982; van der Veen, 1987, p. 8; Lingle & Brown, 1987; Shabtaie et al., 1988; Bindschadler, 1990). To assess reliably the role of ice streaming flow in ice sheet instability and collapse, and to develop reliable models of such extreme behavior, requires a sound quantitative understanding of ice stream mechanics founded on an observational basis that includes observations and measurements of the actual ice-stream flow process and its controlling variables. DRILLING PROGRAM We have drilled eleven boreholes to the bottom of Ice Stream B in the vicinity of camp Upstream B (83.5 S, 138 W). Five of these were drilled close to the camp in the field season, and reached bottom at depths of m. The other six were drilled about 0.5 km north of the camp in the season and bottomed at m. These depths correspond well with the depth 1070 m indicated at Upstream B in the ice-thickness map of Shabtaie & Bentley (1988), based on radio echo sounding. The boreholes were drilled by the hot-water drilling method, with equipment similar to that used by Engelhardt

3 147 Antarctic Ice Stream B & Determann (1987) in drilling through the Ronne Ice Shelf. The results from the eleven boreholes provide information of thermal and mechanical conditions at the base of the ice stream and on the characteristics of the subglacial material. Preliminary results are reported by Engelhardt et al. (1990a, 1990b) and Kamb (1990). CONDITIONS AT THE ICE-STREAM BED A temperature profile measured to within 100 meters of the bottom in (Engelhardt et al a, Fig. 2), supplemented by temperature measurements in the lowermost 160 meters in January 1990, indicate that the temperature at the base of the ice is at the melting point. This conclusion is confirmed by recovery of unfrozen cores of subglacial till, by electrical conductivity measurements, and by the behavior of borehole water levels (see below). The thermal and mechanical conditions (heat conduction and heat generation by basal deformation or sliding) give a basal melting rate of about 2.5 cm year. The subglacial zone has high electrical conductivity by comparison with ice, indicating that the ice stream is wet-based. The d.c. resistance measured between electrodes at the bottom of two boreholes 20 m apart was 500 ohms, and between two holes 600 meters apart 7000 ohms, much lower than the megohm resistance level that would be encountered if the sub-basal material was frozen. During borehole drilling, borehole water levels stood at a depth of about 25 meters, near the firn-ice transition. When the drill reached the bed, as indicated by abrupt cessation of drill advance, the water level dropped rapidly (within two minutes) to about 105 meters below the surface. The drop in water level shows that the boreholes are connected to a basal water conduit system, which is possible only if the basal zone is unfrozen. Several other observation and experiments provide information that bears on the nature of the basal water system. The immediacy of the water-level drop implies that conduits are abundant at the bed. However, in one borehole (1988 no. 2), the water-level drop began nine hours after the bottom was reached in drilling, which shows that the conduits are in some sense discrete and do not completely pervade the bed. The conduit system is capable of accepting or delivering a seemingly unlimited quantity of water from or to the boreholes without appreciable change of borehole water level, as indicated by pumping experiments. The rapid exit of water from the borehole upon connection to the basal water system provides a measure of the size of the conduits. An independent measure is provided by the downglacier water transport velocity, which was found to be 7 mm s by tracking with electrical conductivity measurements the motion of a salt water injection into the basal water system. These two types of observation indicate that if the water moves in a

4 B. Kamb & H. Engelhardt 148 gap at the ice-till interface the thickness of this gap is about 1 or 2 ram, which compares roughly with the figure of about 5 mm calculated by Alley (1989, p. 131) on the basis of the water flux required to balance water input from basal melting upstream. The depth to which the water level dropped on connection to the basal water system, 107±10 meters (varying from hole to hole), is near but in general slightly below the flotation level (the level at which the basal water pressure would equal the ice overburden pressure), which is at depth 99 meters for ice 1035 meters thick or 101 meters for ice 1057 meters thick. The effective pressure (overburden pressure minus basal water pressure) ranges from +1.6 to -0.4 bars in different holes. This compares with 0.5±0.4 bar estimated by Blankenship et al. (1987, p. 8910) by inference from the seismic data. The low effective pressures represent conditions favorable for basal sliding (Weertman, 1964; Lliboutry, 1968; Budd e_t al., 1979; Iken, 1981; Kamb et al ; Fowler, 1987) and also for subglacial till deformation (Boulton & Jones, 1979; Boulton, 1986, Boulton & Hindmarsh, 1987; Alley e_ al ). The ±1 bar variation in basal water pressure from hole to hole in the small area studied (~ 200 m x 500 m) is difficult to reconcile with a conduit model of the basal water system and poses a significant problem of interpretation. Further problems are posed by the patterns of time variation in basal water pressure that have been observed (e.g. Engelhardt e_t al., 1990, Fig. 3). These observations suggest that the basal water system has complex features that are not yet understood. SUBGLACIAL TILL Four methods were used for sampling of subglacial material : (a) jet-drill sampler; (b) adhesion to a penetrometer and other instruments; (c) split-tube corer; (d) piston corer. The results of (a) and (b), with mention of (d), are reported by Engelhardt et al. (1990). The best samples are the piston cores: three cores two meters long, and one core three meters long, all of 5 cm diameter. Method (c) gives cores about 0.2 meters long. Examination of the cores carried out so far is only preliminary, but the general character of the material is clear: it is a glacial till - a pebbly, sandy, silty clay with the wide range of grain sizes typical of till, and lacking bedding structure. Clasts up to 5 cm in size are present; they are mainly granitic (to gabbroic), but metamorphic lithologies (such as slate and gneiss) are also present. Scarce shell fragments are visible macroscopically, and microscopic organic remains (sponge spicules, diatoms, etc.) are

5 149 Antarctic Ice Stream B present though not abundant. R. Scherer {1989a,b) of Ohio State University has identified diatoms of the following ages: Eocene-Oligocene, Miocene (lower, middle, and upper), Pliocene (lower and upper), and possibly Pleistocene. The till is therefore derived at least in part from open-water marine sediments. ~ 1 1 Ù O o ce 1 ce o 8 i SHEAR STRESS (bar) FIG. 1 Direct shear test data (shear displacement rate vs. shear stress) for till from the base of Ice stream B at Upstream B. Data are for a single specimen from a depth of 10 cm below the top of the till. The specimen was tested repeatedly, at shear stresses of and bar, alternately. Open circles are the averages shear rate during the first 15 seconds after stress application, and small arrows indicate the direction of transient change of the shear rate. In the case of downward-pointing arrows, the_shear displacement rate decreased to =s 1 m day in two or three minutes; in case of upward-pointing arrows, the rate increased to a 2 m day" in 15 seconds or less. Large, open arrows are lower limits for the shear rate in tests in which the initial shear rate was so high that the moving part of the shear box hit the stops before the end of the first 15 seconds. The wide mixture of ages indicates that a wide variety of sedimentary source material has been mixed together, as can be expected in a till. The directly measured porosity is 0.40, which agrees with the seismically inferred value (Blankenship e_t al., 1987). This value is high for a till, and probably indicates that the till has been dilated by recent shear (Alley e_t al., 1987, p. 8921). A hydraulic

6 B. Kamb & H. Engelhardt conductivity of 2-10 m s was measured on a reconstituted sample; the low value, typical of tills, precludes hydraulic conduction via the till layer as a significant contributor to the water balance at the base of the ice stream. Preliminary mechanical tests have been carried out on the till (Fig. 1). They show that as sampled the till has a shear strength of about 0.02 bar and behaves to a good approximation as a perfectly plastic material with this failure strength. At shear stresses slightly below failure the till shows decelerating transient creep. Because of the low hydraulic conductivity, the strength measured on the core samples is probably a good approximation to the strength of the till in situ under the ice stream. The strength is so low that there is little doubt that the till should be deforming in situ if the basal shear stress approximates the regional average value of 0.2 bar. The low strength raises a new problem: the till is by a wide margin too weak to support the average basal shear stress, which calls into question the mechanical support and stability of the ice stream. The cores show that the basal till is at least three meters thick. The seismic data indicate a thickness of 6.5 meters at Upstream B (Rooney e_t al ). With the jet action of the hot-water drill, we penetrated five meters into the till, without being able to sense a solid bottom (Fig. 2). TILL DEFORMATION VS. BASAL SLIDING Three borehole experiments to determine how much of the ice stream motion is due to shear deformation in till and how much to basal sliding gave uncertain and conflicting results. A "tethered stake" experiment attempted to detect basal sliding by anchoring a steel cable to the bed with a heavy stake and observing the pull-in of the cable into the hole as a result of sliding. It indicated a sliding velocity of 1.5 m day". Since this is larger than the total surface motion of 1.2 m day measured in (Whillans e_t al ), the result is subject to question, but on face value it indicates that basal sliding strongly predominates. Two experiments were made to observe the basal and sub-basal profile of relative horizontal velocity by placing a bendable tube across the ice-till boundary and into the till, so as to record by tube bending any vertical variations in horizontal shear displacement. Small amounts of bending (1-2 cm) were recorded during periods of 3 to 6 hours. Bending was maximum at the level of the bottom of the ice. The pattern of bending would on the face of it contradict rapid basal sliding, except for the possibility that the low strength of the till may have allowed the till to move horizontally past the tube without causing enough drag to bend it strongly.

7 151 Antarctic Ice Stream B o CO LU CO o DC *~ down l"'-\ / - >. 5nn 'V-N i i 1130 i '. DEPTH COUNTER READING (m) FIG. 2 Data from attempt to drill through the basal layer at Upstream B on 18 December The tension in the drilling hose is plotted against the counter reading (nominal depth to the drill stem), which does not take into account the effect of stretching or unstretching of the hose as its tension varies. The nominal depth also differs from the true depth by a zero offset. The bottom of the hole is encountered by the descending drill stem ("down") at nominal depth 1123 m (true depth 1057 m). The hose tension decreases as the stem drills down into the till and encounters more and more resistance. At nominal depth m the drilling is terminated by hauling up on the hose. Between positions m and m on the "up" track the drill stem if stuck in the till and the hose tension rises along the "elastic slope" as the hose is hauled up. At position m the drill stem comes unstuck and jumps upward, the hose tension dropping from 440 lb. to the free-hanging value 385 lb., with some oscillations. At this point the drill stem has returned to near the position where it initially encountered the top of the till. Total penetration of the drill stem into the till is the distance from the point of contact with the bottom at 1123 m to the point at the same hose tension (385 lb.) on the "up" track, at 1128 m. If a hard bottom were encountered, the hose tension would drop along the elastic slope as hose is fed down the hole.

8 B. Kamb & H. Engelhardt 152 CONCLUSIONS The results of our geophysical work at Upstream B show that the physical conditions at the base of the ice stream are favorable for subglacial sediment deformation and for basal sliding, but they do not positively identify one process or the other as predominant in the flow mechanism of the ice stream. Till of the general type inferred from seismic data by the University of Wisconsin group is present under the ice and is probably deforming. Its strength appears to be too low to support the regional average basal shear stress of 0.2 bar, implying that some other basal drag mechanism must also play a role in the ice stream motion. A basal water conduit system is present; if it consists of an areally extensive gap between the base of the ice and the till, the thickness of the gap is 1 or 2 mm. Downglacier transport of water in the conduit system at a speed of 7 mm s~ is observed. We acknowledge the support of the U.S. National Science Foundation, which made this work possible. REFERENCES Alley, R.B. (1989) Water-pressure coupling of sliding and bed deformation: II. Velocity-depth profiles. J. Glaciol Alley, R.B., Blankenship, D.D., Bentley, C.R. & Rooney, S.T. (1986) Deformation of till beneath Ice Stream B, West Antarctica. Nature 322, Alley, R.B., Blankenship, D.D., Bentley, C.R. & Rooney, S.T. (1987) Till beneath Ice Stream B. 3. Till deformation: Evidence and implications. J^_ Geophys. Res Bentley, C.R. (1984) Some aspects of the cryosphere and its role in climatic change. Geoph. Mono Bentley, C.R. (1987) Antarctic ice streams: A review. J. Geophys. Res Bindschadler, R.A. (1990) SeaRISE: a Multidisciolinary Research Initiative to Predict Rapid Changes in Global Sea Level Caused by Collapse of Marine Ice Sheets. NASA Conference Publication Preprint, Goddard Space Flight Center, Greenbelt, MD. Blankenship, D.D., Bentley, C.R., Rooney, S.T. & Alley, R.B. (1986) Seismic measurements reveal a saturated, porous layer beneath an active Antarctic ice stream. Nature 322, Blankenship, D.D., Bentley, C.R., Rooney, S.T. & Alley, R.B. (1987) Till beneath Ice Stream B. 1. Properties derived from seismic travel times. JL. Geophys. Res. 12., Boulton, G.S. (1986) A paradigm shift in glaciology? Nature 322, 18. Boulton, G.S. & Hindmarsh, R.C.A. (1987) Sediment deformation beneath glaciers: rheology and geological

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