ICE-STREAM FLOW AND BASAL CON DITIONS oOw

Seismic reOecti on data fro m two lines o n Rut/o rd lee Stream a re presented and are compa red with data alread y publi shed from a third line on the ice stream. The' a mplitude and phase of the rel1ections have been used to investigate the properti es of the sub-ice materi a l. Multiple reOections on long record-l eng th data a ll owed ca librati on of the reOection coeI1ic ient a t the ice-bed interface a nd determination of the aco ustic impedance o f the bed m a teria l. The cha racteri stics of the bed m a teria l \'a ry both a long and ac ross the ice stream. The a \'Crage aco~s tic im.peq a l~ce o f thc bed ma teri a l ac(ross th~ glaCI er at the upstream IIlle IS 3.88 x 10 kg m s . 1 hi s dec reases to 3.19 x 10 kg m s 152 km furth er dow nstI-eam. These \'a ILles a re withill th e J'a nge which is typical of soft sediments. Us ing aco usti c impedance as a n indica tor of sub g lacia l porosit y, som e a reas of the ice-stream bed a re interpreted as dil a tant water-satura ted sediments undergo ing per\'as ive d eform ation. ]n o ther a reas, the bed is not deforming a nd basa l sliding m ay be a more important process. The proportion o f the ice-stream w idth o\Tr which bed deform ati on occurs increases dovvnstream. ICE-STREAM FLOW AND BASAL CON DITIONS oOw Ice stream s dra in as much as 90% of the icc o f the Anta rctic ice shec t (l\ [organ a nd o thers, 1982) a nd 0 0 \\' at w loc ities \\'hi ch a re typica ll y one to two orders of m ag nitude greater tha n tha t o f the surrounding ice shee t. Ice-strea m Oow is large ly controll ed by fri c tion at the sides a nd at the bed , with basa l conditi ons rela ti\ 'ely more importa nt toward s the ce ntre o f the ice stream (Frolich a nd Doake, 1988; Echclmeye r a nd others, 1994). The fas t ice now wh ich is observed at the surface or a n ice stream is belin'ed to be assoc ia ted \\'ith three poss ible processes occurring a t the base: basal sliding; deformation of a so ft subglacia l bed ; a nd deformati on o f a region of so ft ice forming the lower pa rt of the ice colum n. These three processes pres uma bl y occ ur in different pro po rti o ns beneath differelll ice st rea m s a nd glaciers a nd m ay a lso \'a ry spati ally within a n ice strcam. 90 0 W t----;---It~--I-----+---__1____+'--_4 90oE Th ere is littl e fi eld evidence as to whic h of these three basal processes actu a ll y occur benea th present-day ice stream s a nd in what propo rti ons. Two ice streams where some ev ide nce of the basal conditi ons ex ists a re lee Stream B in Wes t Anta rctica andJa kobsha\'lls Isbn! :: in Greenla nd. Seismic sunTys prO\ 'ided evidence for a so ft defo rming bed beneath Ice Stream B (Fig. I) (All ey a nd o thers, 1986, 1987; Bla nkenship a nd others, 1986, 1987b), which was confirmed by subsequeIll drilling (Engclhardt a nd o thers, 1990). In contras t, temperature pro fil es suggest tha t de formation of a thick laye r o f rela ti ve ly so ft basa l ice contributes significa ntl y to the fas t fl ow o f J a kobshav ns 1sbrd' (Tkell a nd others, 1993). Unlike se ismic surveys on Ice Stream B, those on J a ko bshav ns Isbrd' did not detec t a ny clea rl y defin ed subice laye r (C larke and Echelmeyer, 1996). Rutfo rd Ice Stream is a fast-fl owing g lac ier (Fig. I) which dra ins ice into the ROllne Ice Shelf The d yna mics orits ice Fig. 1. Loralion map sliOlC'ing RlltJord Ire Strealll and olher jJ/aces menlioned illlhe 11' \ 1. 11 0 \\' hm'e been desc ribed in a number of pa pers (Stephenson a nd D oa ke, 1982; Doake a nd others, 1987; Froli eh and others. 1987, 1989; Frolieh a nd D oake, 1988). Ice vel ocity is high (300400 m a I; Frolich a nd others, 1989) a nd basa l shea r stress is low (a round 40 kPa; Froli ch a nd D oake, 1988), so th e ice stream is not froze n to the bed a nd the basa l ice is proba bl y close to the pressu re-melting po int. Acti\ 'e seismic surveys we re ca rri ed o ut on Rutfo rd Ice Stream during the 1991 92 a nd 199293 li ekl seasons (Smith , 199+)