Annals of Glaciology 20 1994 (: International Glaciological Society

A water-piracy hypothesis for the stagnation of Ice Stream C, Antarctica

R. B. ALLEY, S. AXANDAKRISHXAN, Ear/h .~)'stem Science Center and Department or Geosciences, The Pennsyfuania State Unil!ersi~y, Universi~)' Park, PA 16802, U.S.A.

C. R. BENTLEY AND N. LORD Geopl~)!.\icaland Polar Research Center, UnilJersit] or vVisconsin-Aladison, /vfadison, IYJ 53706, U.S.A.

ABSTRACT. \Vatcr piracy by Icc Stream B, West Antarctica, may bave caused neighboring Icc Stream C to stop. The modern hydrologic potentials near the ups'tream cnd of the main trunk o[ Ice Stream C are directing water [rom the C catchment into Ice Stream B. Interruption of water supply from the catchment would have reduced water lubrication on bedrock regions projecting through lubricating basal till and stopped the ice stream in a tCw years or decades, short enough to appear almost instantaneous. This hypothesis explains several new data sets from Ice Stream C and makes predictions that might be testable.

INTRODUCTION large challenge to glaciological research. Tn seeki ng to understand it, we may learn much about the behavior of The stability of the West Antarctic ice sheet is related to ice streams. the behavior of the fast-moving ice streams that drain it. ~luch effort thus has been directed towards under- standing these ice streams. The United States program PREVIOUS HYPOTHESES has especially focused on those ice streams :A E) flowing across the Siple Coast into the Ross Ice Shelf Among the The behavior of Ice Stream C has provoked much most surprising discoveries on the Siple Coast icc streams speculation. Rose (1979) suggested that it is a surging ice has been how rapidly they are changing now and have stream, which cycles between fast and slow flow in thc changed recently (see review by Alley and Whillans (1991;). same way as some mountain glaciers do. This simple :\lany areas have been examined in detail, and most analogy is unlikely because most surging glaciers spend show non-steady behavior, including large changes in ice most of their time in the slow mode, but four of the five ,'elocity and grounding-line position over historical main Siple Coast ice streams are moving rapidly. periods (Stephenson and Bindschadler, 1988; Thomas Attempts to model surges in Antarctic icc streams failed and others, 1988; Bindschadler, 1993) and modern rates of to produce them (Radok and others, 1987). Surging in thickness change approaching or exceeding I m year 1 in mountain glaciers seems to require build-up of ice in a se\'eral areas (MacAyeal and others, 1987, 1989; Shabtaie reservoir area, and surge termination follows draw-down and others, 1988; Whillans and Bindsebadlcr, 1988; of the reservoir, but the ice reservoir (catchment) of Ice Bindschadlcr and otbers, 1989, 1993). Probably the most Stream C is not strongly drawn down and looks like those dramatic change is that Ice Stream C apparently o[ the other Siple Coast ice streams. Surging thus seems stagnated about a century ago (Rose, 1979; Shabtaie unlikely and we discount this hypothesis. However, some and others, 1987; Retzlaff and Bentley, 1993); it has the o[the mechanisms that have been associated with surging radar signature of an active ice stream, but the crevasses of might remain viable under different guises and below we its shear margins and within the ice stream itself are discuss one (switches between channelized and distrib- buried by about a century of snowfall. uted subglacial water drainages). By analogy to its neighbors, lee Stream C probably In discussing surging and changes in catchment areas, was moving several hundred meters per year. No data are Rose (1979) also indirectly suggested the possibility of available on how long it took to slow down, but it piracy-perhaps lee Stream B had beheaded Ice Stream dropped beneath the speed threshold needed to keep C by stealing ice from its catchment area. However, marginal crevasses open almost synchronously (within careful mapping :Shabtaie and others, 1988; Retzlaffand about 30 years) along about 300 km of the ice stream others, 1993) shows that this has not happened, although extending up-glacier from the grounding line (Retzlaff continuation of ongoing trends in thickness change might and Bentley, 1993). cause it to happen in the future. Such a dramatic change in ice-stream behavior poses a In his discussion of Rose's (1979) paper and in

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Thomas and others (1988), Thomas suggested that recent melt to the bed at certain points, there is no reason to grounding near the mouth of Ice Stream C might have expect the basal system of Ice Stream C to change in the created an ice plain or ice rises that generated back- same way as observed beneath Variegated Glacier at the pressure and stopped the ice stream. By analogy to Ice end of its surge. Stream B, this grounding might have been caused by Arguments have been advanced against the possibility rapid sedimentation at the grounding line, possibly of film-to-channel collapse beneath an ice stream (Alley, supplied by a deforming subglacial till (Alley and 1989a). The Walder instability requires a film of more others, 1987a). However, we consider it unlikely that than a few millimeters thickness. Calculations for the this would stop the ice stream, primarily because of likely film thickness at the down-glacier end ofIce Stream analogy to Ice Stream B. There, an extensive ice plain C, following Alley and others (1987b) and including leads to the Crary Ice Rise complex. Crary Ice Rise has a likely uncertainties in the basal shear stress and the large restraining effect on ice flow ("MacAyeal and others, geothermal flux, include within the error limits films thick 1987,1989), yet Ice Stream B curves around it and flows enough to be unstablc and thin enough to be stable. vigorously. On the ice plain, some strain rates are actually Weertman and Birchfield (1983) argued that, although ..extensional along flow ~Bindschadler and others, 1987) the perturbation analysis of vValder (1982) was correct, and not compressional as would be expected if back- development of the infinitesimal perturbations to finite pressure were braking the ice stream. lYrostof the strain size would induce other changes that would prevent rates do show compression along flow (Bindschadler and maintenance of steady-state channels. This question is not others, 1987), but this can be eXplained by the transverse fully solved. extension caused by down-glacier widening of the ice Perhaps more importantly, the \Valder analysis stream without appealing to ice-plain back-pressure; assumed a rigid bed. Soft sediments could creep into Bindschadler and others (1987) neglected gradients in and close an incipient channel, as they have been longitudinal deviatoric stresses in their assessment of the observed to creep into subglacial tunnels (Boulton, force balance of the iee plain of Ice Stream B. 1976; Boulton and Hindmarsh, 1987). An incipient Retzlaff and Bentley (1993) suggested that instability channel would have small sediment-transport capacity, of a distributed subglacial water system may have led to and so would be closed rapidly, preventing growth to a channelization of the water and reduced basal lubric- low-pressure, steady-state configuration (Alley, 1989a). ation, stopping the ice stream. \Vater apparently flows vValder and Fowler (1994) showed that any channels that beneath Ice Stream B in a high-pressure distributed do form over a soft bed will have relatively high water system, which can be modeled as a thin film, and the high pressure owing to the effects of the soft sediment, allowing water pressures allow rapid bed deformation or sliding continuation of some basal lubrication. (Alley and others, 1987b; Alley, 1989b; Kamb, 1991; vVe recognize that the \Veertman and Birchfield Kamb and Engelhardt, 1991). Walder (1982) showed (1983) and Alley (1989a) arguments are based on theory that, if such a distributed water system beeomes thicker and that theory can err. \Ve thus maintain the film-to- than a few millimeters, perturbations in its thickness will channel collapse of Retzlaff and Bentley (1993) as a tend to grow. A film system thus might collapse into a working hypothesis. low-pn:ssure channelizcd system. Channel formation A thick, Japidly deforming glacier bed allows much would be most likely near the grounding line, where the debris transport and thus requires a large debris source. water supply is largest. Once formed, channels might Unconsolidated or poorly consolidated sediments are the grow headward. \Ve know of no accurate estimates of most likely source. Boulton and Jones (1979) pointed out possible rates of headward growth of channels into a that long-term action of ice would remove such sediments. distributed water systcm, or even whether such growth This would cause the deforming-till layer to thin, which could occur. (The vValder analysis is two-dimensional might slow or stop the ice stream. Anandakrishnan and and does not consider variations along flow.) However, if Bentley (1994) havc reported that low seismic-velocity up-glacier growth rates occurred at rates similar to down- (hence poorly consolidated and easily eroded) sediments glacier flow rates of water, just a few years or decades exist beneath the till of Ice Stream C, with thickness would be needed for channels to extend the length of the varying from 400 m at onc sitc to only 100 m nearby. ice strcam (see below). Such large variations suggest the possibility of these Kamb and others (1985) and Kamb (1987) discussed sediments pinching out elsewhere. Because a till layer of the collapse within hours of a distributed, linkcd-cavity variable thickness does seem to exist widely beneath Ice drainage system to a channelized one beneath Variegated Stream C (Atre and Bentley, 1990, 1993), full loss of the Glacier, Alaska, which caused termination of a surge of sediments supplying that till layer cannot have occurred. the glacier. How'ever, this behavior may not tell us much However, sediment loss by erosion could have caused till about Ice Stream C. Variegated Glacier normally has a thinning, which in turn could have caused the growth of channelized drainage systcm maintained by abundant sticky spots of thin or zero till that slowed the ice stream surface meltwater reaching the bed through moulins. (Alley, 1993). Such sticky spots figure prominently in our Disruption of this surface-fed, channelized system water-piracy hypothesis. triggered the surge, and re-establishmcnt of this channe- The possibility of water piracy by Ice Stream B lized system terminated the surge in a wave that combines elements of Rose's ice piracy and Retzlaff and propagated down-glacier. Theory suggests the possibility Bentley's film-to-channel collapse. \Ve began considering of qualitatively different behavior of subglacial water with the water-piracy idea in group meetings at the University point versus distributed sources (e.g. vVeertman, 1972). of Wisconsin during the mid-1980s and it has influenced Because Ice Stream C lacks delivery of abundant surface our field and theoretical studies since then. It has been

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-5

-6 -7 -6 -5 -4 Grid West

Fig. 1. Hydrologic potential map oj the up-glacier parts oj Ice Streams Band C. Potential is calculated as ice-su~face elevation minus 0.1 x bed elevation in m (Paterson, 1981, p. 142), and is equivalent to a head in units of ice thickness; multiply all values by Pig ~ 9000, where Pi is the ice densit}, to obtain the potential in units oj Pa. One degree on the grid scale is 111km. Water:flow divides are indicated by hea~y lines, and show water flow from the limb oj lee Stream C to lee Stream B.

presented briefly by Alley and Whillans (1991). We now The limb of Ice Stream C extending up-glacier may still be believe that enough data and interpretations are active. A short-pulse radar profile across the limb well available to justify presentation to the community in up-glacier shows a significantly more recent stag- greater detail. nation than for the trunk (Retzlaff and Bentley, 1993). Near the true-north side of the limb, open crevasses have been observed from aircraft, indicating active ice CRITICAL OBSERVATIONS flow (Retzlaff and Bentley, 1990, 1993). Active ice flow with many crevasses bridged might occur; Interpretations of several new data sets are critical to our balance velocities (and by analogy, whatever veloc- hypothesis. We summarize these briefly here. We find it ities apply to the current conditions) for the limb are useful to distinguish between the wide, main, down- slower than for the main part of an active ice stream, glacier part of the ice stream, which we call the trunk, so the limb should shear past its margins and open its and the narrow up-glacier extension, which we call the marginal crevasses more slowly than typical for active limb (Fig. 1). ice streams, allowing a greater chance of bridging The ice-stream trunk stopped almost synchronously (Retzlaff those crevasses. No ice-flow velocities have been and Ben dey, 1993). Ice-flo~ veloci ties on the trunk of measured in the limb but, just beside it toward Ice _ Ice Stream C are barely more than measurement error Stream B, Whillans and Van der Veen (1993) observed (Whillans and others, 1987), although slow basal a velocity transitional between values typical of inland- motion continues (Anandakrishnan and Bentley, ice and ice-stream flow, consistent with transitional 1993). The ice stream is delineated on radar records flow there from inland to ice stream. by the marginal crevassing typical of active ice streams, but the crevasses are bridged by many The bed of the trunk is wellluhricated in most places, although meters of snow accumulation. Short-pulse radar with local sticky spots supporting most oj the basal shear stress. measurements of snow-bridge thicknesses combined Micro-earthquake monitoring near Upstream C camp with accumulation-rate measurements show that the on the trunk (Anandakrishnan and Bentley, 1993) trunk stopped nearly synchronously (± 30 years) shows that brittle fracture is much more common about 130years ago. A profile crossing the up-glacier beneath Ice Stream C than beneath Ice Stream B. end of the trunk and the down-glacier end of the limb Quakes are generated along small thrust faults at or shows a slightly more recent shut-down, but the near, and parallel to, the ice-stream bed, with slip in difference is not statistically significant. the direction oflocal ice flow. The rupture surfaces are

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on the order of ]() m across. Stress drops in the quakes The original interpretations of the basal properties are typically on the order o[ I bar (I (i Pal and a of Icc Stream B were based on seismic-velocity analysis quake preferentially sets off others within 400 ± 400 m and on seismic profiling (Blankenship and others, 1987; but extending as far as 1.5 km (Anandakrishnan and Rooney and others, 19871. :\10 velocity analyses ha\·e Bentley, 1993; Anandakrishnan and Alley, 199,1). been conducted on Ice Stream C, but profiling (Atre Stress drops in ordinary earthq uakes arc not and Bentley, 1993) reveals a layer apparently similar expected to relieve all o[ the applied stress. Earlier to that beneath Ice Stream B. Seismic-phase analysis workers (e.g. \Vyss and Brune, 1968; Brune, 1970) shows that extensive parts of this layer have low seismic suggested that perhaps I-10°/" of the applied stress is vclocity similar to values beneath Ice Stream B. Thus, relieved in a quake. "'Jewer estimates are scarce, it is likely that a soft, deformable till exists beneath Ice possibly because of the difficulty of estimating the Stream C. Phase analysis also suggests spatial applied stress. If simple analogy holds and these old variability in the properties of the uppermost part of estimates are approximately accurate, the micro- the till but cannot reveal how large the variations arc earthquakes under Icc Stream C occur on failure (Atre and Bentley, 1990, 1993). planes in sticky-spot regions supporting 100-1000 Imagery of the icc stream shows an "island" of ice times more basal shear stress than the regional average in its center that looks more mottled than its of about 0.1 bar (Anandakrishnan and Alley, 1994). surroundings, and the t10wlines in the surroundings In most earthq uakes, aftershocks are generated in a seem to be deflected around that island Uacobel and region of similar linear dimension to the length of the others, 1993). Radar data show that the island rupture surface (Das, 1992; in a single case, after- coincides with a bedrock high Uacobel and others, shocks have been documented to a distance of 17 times 1993). High spatial variability in imagery, probably the source size; Hill and others, 1993). Again, if simple linked to bedrock highs, is known to occur over analogy holds, the observation o[ aftershocks within regions of enhanced basal drag (sticky spots) on Ice hundreds of meters of a quake suggests a rupture Stream E (MacAyeal, 1992), which strengthens the surface of hundreds of mders, compared to an possibility that localized sticky spots exist beneath Ice estimate of 10 m from the corner fi-equency of the Stream C. energy spectrum of the quakes (Anandakrishnan and Bentley, 1993; Anandakrishnan and Alley, 1994). vVater pira~y has occurred from the catchment area ~fIce The simplest interpretation of these data is that Stream C to the trunk of Ia Stream B. Rose (1979) and regions between sticky spots arc very well lubricated, Shabtaie and Bentley (1988) noted the combination o[ so that stress is transmitted up to 1.5 krn for rupture of steep bedrock topography under flat surface topo- a locked region spanning 10m. A lObar stress on 10m graphy, suggesting important dynamic effects. In long sticky spots between perfectly lubricated areas particular, subglacial water flow is controlled by the under an average basal shear stress of 0.1 bar would ice-surface slope and about 10 times less importantly imply 100 rn spacing of the sticky spots. Non-zero by the bed slope ie.g. Paterson, 1981, chapter 8, strength of the lubricated areas would increase the eq uation 14). In all regions of steep ice-surface slope, spacing we estimate. In comparison, the clustering of ice and 'yater flow are nearly parallel and down the micro-earthq uakes suggests a few hundred meters gradient in icc-surface elevation. But, in regions of spacing (Anandakrishnan and Alley, 1994), reason- steep bed slope and gentle surface slope, water flow ably good agreement given the order-of~magnitude may occur clown the bed-elevation gradient or nature of these calculations. between it and the icc-surface gradient, whereas ice These data suggest that in most places the ice flow will continue along the ice-sur[ace gradient. stream is well lubricated and supports little or no basal Figure 1 shows hydrologic-potential clata and shear stress; local ized sticky spots support most of the drainage divides [or subglacial water, assuming that stress (Anandakrishnan and Alley, 1994). This would the hydrologic potential is controlled by surface ancl concentrate the stress sufficiently to allow brittle bed elevations for constant-density ice, and not by [racture and would allow stress changes associated gradients in basal shear stress (sec below). Surface and with a micro-earthquake to be transmitted long dis- bed elevations are from Retzlaff and others (1993). It tances to other sticky spots. The scarcity of seismicity appears that water generated by basal melting in the beneath Ice Stream B suggests that any sticky spots catchment ofIce Stream C and along the limb is being there arc better lubricated than those beneath Ice diverted to Ice Stream B, near where the limb meets Stream C (Anandakrishnan and Bentley, 1993). the trunk of Ice Stream C. Despite the tight flight-line Active-seismic data, and more recently drilling, spacing (5 or- 10km grid) and high accuracy (±7m have shown that Ice Stream B is underlain by a meters [or the surface and ± 40 m for the bed) of the data, thick, very soft till that probably deforms and lubricates there is some ambiguity in drawing the drainage hasal motion (Blankenship and others, 1987; Kamb divides. This uncertainty increases if we try to appl) and Engelhardt, 1991). Theoretical estimates o[ its the data to 130 years ago, because surface elevations strength (Alley and others, 1987h) and laboratory may have changed in the interim. But, the appearance (Kamb, 1991; Kamb and Engelhardt, 1991) and small- of modern water piracy is strong and the possibility scale in-situ strength tests (personal communication that this has existed for some time must be considered. from H. Engelhardt and B. Kamb) preclude basal models in which much of the bed supports a significant The lubricant for actilJe ice streams is spatia!S)! variahle. The shear stress without deforming rapidly. evidence for large spatial variation in basal drag o[

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activ(' IC(' str('ams is becoming strong (see revIew by stream. Kam b and Engelhardt (1991) and a personal Alley (1993)1. Seismic studies beneath Ice Stream B communication frorn H. Engelhardt and B. Kamb rep- showed lubricating till thinning to below th(' seismic orted data indicating a water system with a transmissivity resolution of 1-2 m in localized areas, which suggested equal to a film averaging tenths ofa millimeter thick near the possibility of till discontinuity (Rooney and others, Upstream B, which is in the up-glacier part of a branch of 19871. If a localized region has a high basal shear Icc Stream B but which receives pirated water from the stress because of thin or absent till, the water pressure catchment of Ice Stream e. in a distributed system will b(' r('duced in that region Th(' turn-off of the water supply would have allowed and wat('r flow will occur preferentially toward it, drainage of the film. That, in turn, would have lowered partially lubricating it (Weertman, 1972; Alley, 1993). the water pressure in the film, decreasing the area The ice stream thus may hav(' wat('r-lubricated sticky occupi('d by th(' film and the int('rconnectedness of the regions surrounded by till-lubricated slippery regions. film until water supply and transmission attained a new The water lubrication will allow much faster motion balance (Alley, 1989a). For the case ofTce Stream C, that for a giv('n shear stress than vvould occur in the absence balance is likely to involve much less water transmission of the water lubrication; a water-lubricated sticky spot than beneath an active ice stream and thus much less is less sticky than if there wer(' no water lubrication. distributed water. For a typical rift-basin heat flow, a steady temperature profile for inactive Ice Stream e Working hypothesis would hav(' close to zero basal melting or fr('('zing (th(' calculation is the same as that far ridge Be given by Alley \\.(' suggest that thc position of the icc-stream head is and Bentley (1988)). With no water supply from up- related to some threshold, down-glacin of which suflicienr glacier and with little or no basal melting and possibly lubricating water and till have been generated to allow even basal freeze-on, the subglacial system of Ice Stream icc-streaming (Alley, 1990). Up-glacier of that head, steep C cannot carry much water in steady state. ice-surfac(' slop('s control water flow in th(' ic('-str('am The fate of the water trapped in subglacial tills is of catchments, causing water flow to parallel ice flow. Along considerable interest. A few meters of dilated till with the ice stream, surface slopes are small enough that bed 40% porosity (Blankenship and others, 19(7) might slop('s ar(' significant in controlling water flow. compact to 30% porosity by expelling a few tens of \\'e hypothesize that, earlier in its history, Ice Stream centimeters of water. The time-scale would depend on the e did not reach the region of steep basal topography into ability of the distributed system b('tw('('n ic(' and till to \\'hich it now extends. \Vhen, perhaps by head ward remove water supplied to it from below. \Ve hypothesize growth, it reached that region, basal meltwater was that the ice stream stopped because the distributed system di\'erred from the catchment of Icc Str('am C into Ice drained and thus lost its ability to transmit much water. If Stream B. Near the up-glacier end of the trunk, most of the ice then regelated into the sediment (Iverson, 1993) or the subglacial wat('r must be produced in the catchm('nt froze to the sediment, dewatering of till beneath the ice- but farther down-glacier production beneath the ice stream trunk would require Darcian flow through the till stream may be significant. Turning off the water at the to the grounding line. This would be a very slow process.

up-glacier ('nd of the trunk would have reduced lubric- The Darcian volumetric flow velocity, 'Ua, ]s ation on sticky spots ncar there, causing the icc velocity -K and the meltwater production there to drop. This, in turn, Ua = --Pg (2) would hav(' reduc('d lubrication on sticky spots farther pg dO\\'l1-glacier and would have stopped the ice-stream trunk in a wave propagating down-glacier. where K is th(' hydraulic conductivity and is p('rhaps The velocity of down-glacier propagation would scale IO-Gms-1 in a soft till (Boulton and Hindmarsh, 1987), p with the now velocity of water in th(' distributed system. is the density of water, 9 is the acceleration of gravity and For a uniform film of thickness d, the depth-averag('d flow Pg is the potential gradient, and is determined primarily

\'elocity, 'U, is by the ice-surface slope along the trunk of the ice stream (along much of th(' trunk, th(' b('d is nearly horizontal; d2lPgI '11,=--- (1) Shabtaie and Bentley, 19881. 127) The ice-stream trunk is approximately 300 km long._ To drain the water released by till dewatering (about whcre IPgl is the magnitude of the potential gradient 0.1 m water per meter thickness of till) from just 100 km of 4 3 driving water now and 7) = 1.8 X ]03 Pa s is the viscosity that length would require that a volume of V = 10 m ofvvater (\Veertman, 1972). Taking d = 1 mm, and Pg = flow through each meter thickness and width of till. I 10 Pa m I, u::::= 15 km aI, water flow would traverse the Taking Pg = 10 Pa m- , the time to drain the water is 5 trunk length of ::::=300km in ::::=20years. This is less than the V/ Uel =:) x 10 a. Clearly, drainage can be fastel' if a uncertainty in the estimated ages of ice-stream stoppage limited ice-contact water system remains, or if drainage and would appear as an instantaneous stoppage of the ice- through sub-till sediments is allowed, and we would stream trunk. Various estimatcs of water-film thickness expect these. However, retaining dilated tills for 102 a is based on theory (e.g. Weertman and Birchfield, 1983; certainly possible. Alley and others, 1987b) yield tenths of millimetns to Th(' h('adward ('xtension of the ice streams postulated millimeters for the film thickness, depending on assump- here may be an expected development. At the end of the tions about geothermal nux, ice-stream side drag and last ice age, the ice sheet experienced a large surfac(' other factors, and depending on position along the ice warming, which now should be causing basal warming

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and increased basal melting (Whillans, 1978). Increased than in the trunk - in our model, water lubrication of basal melting would allow the head ward extension of ice sticky spots should minimize quakes up-glacier but not streams in at least some models (e.g. Alley, 1990). If so, down-glacier. then the region of steep bedrock topography under the 'Ne postulate that the wave of stagnation of the ice heads of Ice Streams Band C would at one time have stream propagated down-glacier following water piracy. been under steep surface slopes, but the surface slopes Retzlaff and Bentley (1993) postulated that the wave of would have become flatter over time, eventually diverting stagnation propagated up-glacier as the subglacial water the water flow and stopping lee Stream C. film collapsed to channels, starting at the grounding line The rapid head ward extension of Ice Stream B where the film was thickest. Further short-pulse radar observed by Shabtaie and others (1988) may be related studies of the burial depth of marginal crevasses on the to this expected slow head ward growth of the icc streams. trunk of Ice Stream C, of the type already reported by It also may have been accelerated significantly by ad- Retzlaff and Bentley (1993), might provide the clearest ditionallubricating water obtained from Ice Stream C. test of these models if the short duration of the wave of stagnation could be resolved. An informal reviewer suggested that, because water PROBLEMS AND TESTS flO\v in Figure I passes beneath ridge BC, between Ice Streams Band C, but ridge BC is not an ice stream, we 'Ne see a few problems with this hypothesis. One is that have a difficulty with our model. \Ve do not consider this we have not quantified it - we do not have a numerical to be a serious problem for three reasons: model running that produces the stoppage of Ice Stream (i) \Vater flow is a necessary but not sufficient C as a natural consequence. That might be a good test of condition for ice streaming in our model, w'hich also our hypothesis, but we do not anticipate it any time soon. invokes till for much of the lubrication. It may be that There now is so much freedom in terms of till-flow laws, conditions are not right for till generation in that part sticky-spot character, water-system geometry and so on, of ridge BC receiving water from the Ice Stream C that we suspect that we can develop a model that either catchment, or that there is a time lag for generating would or would not stop Ice Stream C for the scenario enough till to lubricate fast flow. outlined here. 'Ne thus have not attempted that exercise, although it might be interesting to find out just how wide (ii) As shown in Figure I, water is funneled into the the range of possible behavior is. active regions ofIce Stream B but there is no funneling Given the rapid changes over time observed in ice- of the water crossing ridge BC. Perhaps not enough surface elevations elsewhere on the Siple Coast, a detailed water reaches ridge BC to provide sufficient lubric- modern determination of water-flow divides does not ation for streaming flow. necessarily tell us where they were 130 years ago when the (iii) The maps and calculations of Shabtaie and others trunk of Ice Stream C stopped. For example, we postulate (1988) suggest, although they do not prove, that Ice that the limb remains active while the trunk has stopped. Stream B is growing at the expense of ridge BC in that The ice input fi'om the limb thus would be piling up near region of ridge BC receiving water diverted from Icc where the limb and trunk mect. Suppose the limb is I km Stream C: If so, then perhaps the effects of the water thick and has flowed at 100 m a I for 100 years after the piracy arc just now being manifested. trunk stopped. Suppose, further, that the limb-trunk junction has thickened by 100 m in this time, and the Although all of these are clearly hypothetical, they thickening decreases linearly and symmetrically away also are possible. from the junction up-glacier and down-glacier. Mass continuity would require the thickening to extend 100km along-flow in each direction, with ice-surface slope having SUMMARY decreased up-glacier of the junction and increased down- glacier of the junction. Data reported recently show that the trunk of Ice Stream It is intriguing that the 100km immediately up-glacier C stopped almost synchronously about 130years ago. The of the junction today has a flatter surface slope than the limb extending up-glacier stopped more recently or is still 100km down-glacier (Shabtaie and others, 1988; Retzlaff active. The trunk of the ice stream has a well-lubricated and others, 1993), whereas surface slope generally bed in most places but with local stress concentrations decreases down-glacier on the other ice streams (Shab- suggesting sticky spots. The lubrication is probably taie and others, 1988). However, if we allow such large supplied by a soft till layer. Near where the limb meets thickness changes, and the possibility that icc-surface slope the trunk, the ice stream has very gentle surface slopes changed transverse to the ice stream as well as along it, we overlying steep bed slopes, and a new map of modern cannot reconstruct water-flow paths from 100years ago hydrologic potential indicates that meltwater from the with great confidence. catchment and limb of lee Stream C drains into Ice Our greatest difficulty is that we cannot provide any Stream B rather than into Ice Stream C. unequivocal test of our hypothesis. Some of our assump- We hypothesize that the ice streams have been tions can bc tested (till discontinuity under the trunk of extending slowly into the inland ice in response to Ice Stream C and continued activity of the limb) but increased basal melting there caused by Holocene correctness of these assumptions would not establish warmth. \Vhen the nearly flat ice-stream surfaces correctness of our model. It would be interesting to see reached the steep basal topography now beneath the whether micro-earthquake activity is lower in the limb limb/trunk junction, water piracy occurred with the

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water from the lee Stream C catchment diverted to Ice Blankenship, D. D., C. R. Bentley, S. T. Rooney and R. B. Alley. 1987. Stream B. This caused enhanced headward ,growth of Ice Till beneath Ice Stream B. 1. Properties derived from seismic travel times. ]. Geophys. Res., 92(B9), 8903-89] 1. Stream B. This water diversion reduced lubrication on Boulton, G. S. 1976. The origin of glacially fluted surfaces~obser- sticky spots beneath Ice Stream C where till was thin or vations and theory. J. Ciarlo I., 17(76), 287-309. absent, causing their drag on ice flow to increase and the Boulton. G. S. and R. C. A. Hindmarsh. ]987. Sediment deformation beneath glaciers: rheology and geologica] consequences. J. Geoph}'J. ice stream to stop. As the distributed water system Res., 92(B9), 9059-9082. drained, the soft, dilated till around the sticky spots lost a Boulton. G. S. and A. S. Jones. 1979. Stability of temperate ice caps and ready conduit for water released during compaction, so ice sheets resting on beds of deformable sediment.]. Glaciol., 24(90), dewatering has been slow and the till remains soft. 29-43. If our hypothesis is correct, then the limb of Ice Brune, J. N. 1970. Tectonic stress and the spectra ofseismic shear waves from earthquakes. ]. G'eophys. Res., 75(26), 4997-5009. Stream C is still active, the till bcneath Ice Stream C Das, S. 1992. Reactivation of an oceanic fracture by the Maquarie Ridge should be discontinuous or at least variable in thickness, earthquake of 1989. Nalure, 357(6374). 150-153. and the ice stream should have stopped in a wave Hill. D. P. and 30 others. 1993. Seismicity remotely triggered by the propagating down-glacier from the region of the limb! magnitude 7.3 Landers, California earthquake. Sriellce, 260(.5] 14). 1617-1623. trunk junction. Iverson, N. R. 1993. 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Walder,j. S. and A. Fowler. 199·1. Channelized subglacial drainage over Whillans, 1.;VI., j. Bolzan and S. Shabtaic. 1987. Velocity oflcc Streams a deICmnahle bed. J. Glar;ol., 40( 134i, (i-I.'). Band C, Antarctica. J. Geo/ilt}". Res., 92iB9:. 8895-8902. \Veertman,j. ]972. General theory o/'water (Jaw at the base ofa glacier "'yss, '.f. and j. "I. Brune. ]968. Seismic moment, stress. and source or icc sheet. RI'1'. Geo/Jlqs. Splice PI!rs .. 101 I 287-333. dimensions for earthquakes in the California '\In'ada region. ]. Wcertman, J. and G. E. Birchfield. 1983. Stability of sheet w'ater flow Geopltys. Res .• 73114i. 4681 4694. under a glacier. J. Glar;ol., 29(103:, 374-·(182. \Vhillans. 1. '.f. 1978. Inland icc sheet thinning clue to Holocene warmth. Scicnce, 201:43601. 1014,1016. Whillans. I. M. and R. A. Bindschadlcr. 1988. ;'vIass balance of Ice Stream B. West Antarctica. Ann. Glllr;ol., 11. 187-193. \\'hillans, 1. '.1. and C.]. van der VeeIl. 1993. New and irnprm·ed detcrminatinns of velocity of Ice Streams Band C, \\'est Antarctica. The accuraC)' oj references in the text alld ill this fist is the ]. Glar;ol., 39! 133), 483-490. /'e!,ponsibili£v oj the authors, 10 whom queries should be addressed.

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