ABSTRACT. Jakobshavns Isbrre is a large, fast-moving ice stream/outlet glacier in West Greenland which ends at a floating, calving front. It drains about 6.5% of the area of the Greenland ice sheet. Studies of its surface morphology are described in this paper. The surface is relatively steep (0 .01-0.03) and the thickness is large (up to 2600m along the center line (Clarke and Echelmeyer, 1989)), indicating very high driving stresses (200-300 kPa). The ice stream is about 6 km wide and 85-90 km long, all of which is in an area of surface melting. The base of the ice stream, and of much of the drainage area, is below sea level. Marginal crevasse zones have a width on the order of the width of the ice stream itself. Unique surficial features are ice blisters and lakes; the latter have a sequence of ogive-like features on their floating ice cover which can be used to determine velocity. There is a pinning point near the terminus which may act as a stabilizing influence, possibly playing a role in halting, at least temporarily, a recent retreat of the terminus. Ice-thickness estimates at the terminus lead to a flux which is less than previously assumed by others (e.g. Bindschadler, 1984; Pelto and others, 1989) when estimating J akobshavns Isbne's drainage basin to be nearly in balance.The driving stresses on J akobshavns Isbra: are an order of magnitude higher than those of the ice streams of West Antarctica. Its crevasse patterns are much less localized. Its relatively un confined terminus is more comparable to that of relatively unbuttressed ice streams such as Pine Island and Thwaites Glaciers than it is to other West Antarctic ice streams which terminate in large, confined ice shelves.

Abstract. Satellite imagery of the area between ice streams B1 and B2, Antarctica, shows a lineation on the surface of the ice sheet of uncertain origin. Ice motion in the area (2 m yr-•) is 2 orders of magnitude slower than that of the surrounding streams and shows no significant variation that could explain the feature. A low-power, high-resolution radar system was used to image the upper 80 m of the ice sheet between the two ice streams; the survey shows that the lineation is associated with what is likely an abandoned shear margin. The radar data show that a set of chaotic diffractors lies buried beneath two thirds of the area, while the remaining one third is undisturbed to 80 m depth. The chaotic ice ends abruptly along a boundary that is parallel to, but offset 2.5 km from, the surface lineation. Also, isolated linear diffractors are commonly observed in the otherwise undisturbed ice immediately adjacent to the boundary. The depth, location, orientation, and curved form of the diffractors strongly suggest they are the tops of crevasses that were active at the time the chaotic ice was being strained and that they were formed by leftlateral shear. This is the same sense of shear that is presently active in the B2 margin, 6 km away. The depth to the chaotic diffractors suggests that the shear margin abandoned its prior position -190 years B.P.; the burial depth decreases toward the B2 margin and suggests a migration rate of -100 m yr -•. In addition, a separate high-power radar system was used to image the entire thickness of the ice sheet (-1 km) in the same area. These data show numerous linear diffractors near the base of the ice sheet. It is very likely that at least some of these diffractors are entrained morainal debris. Others may be bottom crevasses or zones of wet, reflective ice that developed in the high-strain environment of the now-abandoned shear margin. IntroductionThe ice streams that drain West Antarctica into the Ross Ice Shelf carry 20-25% of the total outflow and thus form an important ingredient to understanding the dynamics and stability of the West Antarctic ice sheet as a whole (Figure 1 Figures 2a and 2b). The interstream ridge is commonly referred to as the "Unicorn" (Figures 1 and 2 Two separate radar systems were used to image the ice sheet. The first was a low-power impulse-type system that provided high-resolution images of the upper 80 m of the ice sheet. The second was a high-power modulated-type system, which provided images of the entire depth of the kilometerthick ice sheet at low resolution. The purpose of the highresolution radar survey was to image both near-surface and 13,409

ABSTRACT. Se ismic-reflecti o n m e thod s \lTIT used to d e termin e th e ice thi ckn ess a nd basa l topography o f J akobs ha \'ns lsbne. a large , fa st-m oving ice strea m/ou tlet g lac ier in Wes t G ree nland. A meth od of data a nalysis \\"as de\Tloped w hi c h im'oh'es th e point\l'ise mi g ratio n of data from a lin ea r se ismi c a rray and a sin g le ex plosi\'e so urce; the me th od y ie ld s th e depth , hori zon ta l positi on a nd slope o f th e basa l refl ec tor. A dee p U -shaped subglacia l trough II'as ro uncl be neath th e entire length o[ th e well-d efin ed ice stream. Th e trough is in cised up to 1500 m in to bedroc k, a nd its base lies 1200-1500 m b elow sea leve l for a t least 70 km inl a nd. Cente r-lin e ice thi c kness a long most of the cha nnel is a bout 2500 m , or a bout 2.5 times that of" th e surroundin g ice shee t. Thi s prominent bedrock trough \\"as not appa ren t in ex istin g rad io -echo-so undin g data. R efl ection coeffi cients ind ica te that mu ch of the basa l inte rface is probab ly und e rl a in by compac ted , non-d efo rmin g sedim ent. Th e large ice thi c kn ess, coupled \\"ith relatilTly steep surface slopes , lC'ad s to hi gh basal sh ea r stresses (200-300 kPa ) a long th e ice stream. Th e large shea r stresses and lack of a d eformable bed impl y th at inte rn a l d eformatio n plays a dominant rol e in th e dynamics of j a kobs h avns lsbrce.

ABSTRACT. Accumulation studies along the lowermost 100 km of Jakobshavns Isbrre show that the local net balance above the equilibrium line (1210 m elevation in 1986) is significantly less than that measured along the EGIG line about 100 km further north. This indicates the presence of a precipitation low in this region which will affect any global mass-balance assessment for the Jakobshavns Isbrce drainage basin. Comparison of the estimated calving and ablation fluxes shows that calving removes about twice as much mass from this drainage basin as does melting. Basal melting over the entire basin accounts for about 20% of the total ice loss by ablation. Temperature measurements at 12 m depth along the same section of the isbrre show the warming effects of refreezing meltwater and cooling effects of severe crevassing. In addition, there is a significant variation in temperature across the fast-moving ice s tream which is probably caused by deformation heating in the shear margins which delineate the ice stream within the ice sheet. This lateral temperature gradient could be important in ice-stream dynamics through its effects on ice rheology. Detailed measurements within the percolation facies show that surface melt can penetrate up to 3 m by piping in cold firn, and, upon refreezing, can cause significant warming at these depths.

ABSTRACT. Se ismic-reflecti o n m e thod s \lTIT used to d e termin e th e ice thi ckn ess a nd basa l topography o f J akobs ha \'ns lsbne. a large , fa st-m oving ice strea m/ou tlet g lac ier in Wes t G ree nland. A meth od of data a nalysis \\"as de\Tloped w hi c h im'oh'es th e point\l'ise mi g ratio n of data from a lin ea r se ismi c a rray and a sin g le ex plosi\'e so urce; the me th od y ie ld s th e depth , hori zon ta l positi on a nd slope o f th e basa l refl ec tor. A dee p U -shaped subglacia l trough II'as ro uncl be neath th e entire length o[ th e well-d efin ed ice stream. Th e trough is in cised up to 1500 m in to bedroc k, a nd its base lies 1200-1500 m b elow sea leve l for a t least 70 km inl a nd. Cente r-lin e ice thi c kness a long most of the cha nnel is a bout 2500 m , or a bout 2.5 times that of" th e surroundin g ice shee t. Thi s prominent bedrock trough \\"as not appa ren t in ex istin g rad io -echo-so undin g data. R efl ection coeffi cients ind ica te that mu ch of the basa l inte rface is probab ly und e rl a in by compac ted , non-d efo rmin g sedim ent. Th e large ice thi c kn ess, coupled \\"ith relatilTly steep surface slopes , lC'ad s to hi gh basal sh ea r stresses (200-300 kPa ) a long th e ice stream. Th e large shea r stresses and lack of a d eformable bed impl y th at inte rn a l d eformatio n plays a dominant rol e in th e dynamics of j a kobs h avns lsbrce.