J ournal ifClaciology, VO/. 46, No. 152, 2000

Changes in the tnargin of Ice Stream C, Antarctica

R . \Iv. JAC OBE L,i T. A. SCAMBOS,~ N . A. N E RESON,3 C. F. RAYMOND3

iD ejJarlmelll if Ph)'Sics, SI O/cgCoLLege, Norllifield, Jlil1l1esota 55057, US. A. 2Naliol1a/ Snow and Ice Data Cen.ter, CIRES, Univmil)1 qf Colorado, BOlllder, Colorado 8030.9 -044.9, C S. A.

:1 CeojJ/?)lsics Progra 111, Cniversi0' qfll ash il1gton, Sea I tie. , I 'aslzi I1gton .981.95 -1650, Us. 11.

ABSTRACT. Wc present res ult s from satellit e image ry, ice-mo tion sUr\'eys a nd iee­penetrating rad a r studies of pa rt of the north m a rg in ofIce Stream C , one of the ice streams draining the "Vest Antarctic ice shee t to the "Ross Embayment·'. Our studies suggest tha t the shutdown of!ce Stream C about 150 years ago was not a single event, but a sequence invok­ing stagnation of ice and migra tion of the ice-stream boundary. Ground-based studies con­firm the inference from imagery that a se ri es offo rmer shear zo nes ex ist, decrrasing in age towards the ice-stream cent<:T. A region of ice-stream trunk, including a former margin , lies sheared and folded betlVeen the (recent ) inner a nd (older) outer m a rgins of the a rea. !ce­moti on and top ographic surveys give some constra int on the time of shutdolVn of the outer margin. The results prm'ide a forum for disc ussing shutdolV n mecha nisms. Possible causes for the stepwise migration of the north margin ofIce Stream C include a g radual dec rease in ice flux, a reduction in the available water or hydrostatic pressure in the basal till, or a freez­ing of the till layers on the northern side.

INTRODUCTION

Drainage of the West Antarctic ice sheet (WAlS) is cont roll ed by ice stream s which deliver m ost of the mass flux to the bounding ice shelves. In the "R oss Embayment", Ice Streams A- E drain the interior in a dynamic system that h as changed in both flu x a nd configuration over a range of temporal and spatial scales. Four of the ice streams are currentl y acti\'e, while Ice Stream C stagnated approximately 150 years ago (Shabtaie and Bentl ey, 1987; R etz la ff and Bentley, 1993), lead­ing to questions about the conditi ons necessary to sustain rapid ice motion and the rel a tionship of this event to other changes in the drainage configuration from the ice shee t. At the mill ennia l scale, the system h as lost approxim a tely two­thirds of its mass since the end of the last glacial epoch (Bindschadler, 1998). In the nearer term, both ice-stream velocities and ice-stream widths a re observed to fluctuate on a scale of years to decades at ra tes of 1-2% per year for ice speed, and 0.1 % p er year for w idth (e.g. All ey and Whill ans, 1991; Bohlander and Scambos, 1997; Bindschadler and Vorn­berger, 1998). This suggests tha t ice streams may be constantly acljusting to local changes in basal lubrication and evolving at their margins.

Additional evidence for cha nge a t an intermedi ate time­scale has come from the morphology preserved in the ice­sheet surface and the adjacent ice shell: An assessm ent of sur­face features associated with ice flow (e.g. "scars", fl ow bands, a nd a host of shear-related features on the ice sh ell) suggests that both mass flux and the fund a mental configura tion of the ice streams have changed severa l times during the las t few centuri es (Fig. 1; see also Stephenson and Bindschadler, 1988; Casassa and others, 1991; Scambos and Bindschadler, 1991; Scambos and Nereson, 1996; p ersonal communication from M. A. Fahnes tock, 1998). Scar features (m arking for­mer ice-shea r m a rgins) and relic t fl owlines (m arking for­mer ice-stream trunk areas ) are present in several areas

102

tha t are no longe r r apidly fl owing. These morphological fea­tures ha\'e been revealed through the use of satellite im ages, as more sophisticated processing tech niques han ' been appli ed to extrac t the maxilTlUm amount of sur face detail available in digital images (e.g. Orheim and Lucchitta, 1987; Bind­sch adler and Vornberge r, 1990; K\'ara n and others, 1996). The initi a l, image-based interpretati on of the fCatures in se\'e ral a reas has no\\" been confirmed by radio-echo so unding and other data, res ulting in the discover y of the "Fish hook " exten­sio n orIce Stream B (Bentl ey and othe rs, 199+; persona l com­munication from 1. M . Whill ans and G. H amilton, 1999) and a new rel ict ice-stream outlet trunk, "Siple lee Stream" (Jaco­bel a nd others, 1996a). Several areas h ave ye t to be surveyed, but bear simila r features that sugges t past changes in the m a rg ins ofIce Stream B and the southern side on ce Stream C. H owever, not a ll scars represent former shear ma rgins, as recent evidence from Roose\'elt Isla nd indicates (Conway a nd G ades, in press ).

Knowledge of th e evolution of the ice streams is essenti al far understanding the response of the vVAIS to environmen­ta l change and for gauging the overall contribut ion of the 'VAIS to sea-level rise, both in the p ast and in the future. vVe report here on im age- and g round-based studies of a g roup of scar features on the north side of Ice Stream C a nd di scuss their implications for the causes of its shutdown. " re h ave ca lled this a rea the "Duckfoot" because of the splayed pattern of traces present in satelli te imagery (Fig. 2).

GROUND-BASED STUDIES OF THE SIPLE DOME AREA

St Olaf College, the University of\Vashington and the Uni­versity of Colorad o were involved in g round-based studies of the Si pIe Dome a rea in 1994 a nd 1996. These st udies com­bined ice-penetra ting radar and g lobal positioning sys tem

] acobe{ alld o/hen: Changes in lllmgill rif Ice St ream C

ri/i. J. Ol'erl'ifll' Ilf the rell/ral Si/lIe Coas/frolll a mosaic Ill. J I H RR images. Co IIfi r III edfo rill er shear II/mgillJ are sholl'l1 as solid lilies: J(arl o(.lIIs/lec/ed 1I/(IIgilll are S/lOl1' lI a.1 dOlled Lil1e,\. T he sOll/hjlallk o(Si/l/e DOllle alld /he D//Ck/OO/III(lIgill smr,1 described ill this ,\/111)' are higldigh/ed.

(e ps ) m easurelllent s o f s url~l ('e di splace m e nt to cha racter­

ize th e c urrent a nd pas t ice [l Oll' in t he vicini t y o f the sUll1mi t a nd ll a nks o f the dome. R a d a r echoes [ra m "internallayers",

at mospheric depos it s o n th e surface which a rc subsequent ly hu r ied , she)\\' the ice deformati o ll a nd /l o \\' hi story as de­scr ihed in m Ort' de ta il b elo w. This wo rk h as bee n Ill oti\ 'a ted

by two re la ted goal s: a d es ire to unders ta nd th e dyna mi cs a nd hi sto r y 0 1' in' [l 0l \' in th e a rea, a nd to use t his as a n a id in the siting a nd in terpre ta ti on of a core drill ed to th e bed benea th Siple DOll1e in 1997 99. Figure 2 is a deta il of th e

Siple D o m e a rea fro m enha nccd ,\ ch-a nced \ 'cry High Reso­

lution R adi omcter (A\ ' HRR ) image ry showi ng th e bo und­ing ice streams a nd surface features \I'hi ch a rc rela ted to

cha nges in th e ice fl ow disc ussed below. Also shown a re the

locati o n o f thc core, a ndthe rada r/GPS t rm'C rses.

Rada r a nd CPS da ta a t th c summit havc bcc n usccl LO cha r­ac teri ze the geoll1(' try (R ay m ond and others, 1995; .Jarobel and

oth ers, 1996b l and mass ba la nce of Siplc D ome. tllodeling

work based 0 11 these st ud ies has bee n used to dcscribe ice now

in th e \' ic init y 01' the di\'ide, in pa rticul a r to pred ict the age depth rela ti on (Ne reso n a nd o thers, 1996), a nd to inves tigate

the hi story o f' thc di vide locati on ( ~e reso n , 1998; ~ereso n

a nd o th e rs, 1998a, b), Fig ure :-l shows the res ults of the I'ull

rad a r p rofile across Siple D o me a nd acuaeent m a rg ins. D a ta aerns, the cent ra l pa rt o f t he domc arc a t 2 l\ 1Hz cent er li'C­

qu cnc), o f the impul se tra nsmitter, while d a ta a t th e ma rg in s

a re a t a so m ewha t higher resolution with 5 I\IHz ce nter f're-

qucnc),- Siple D o me is approx im a tel y I km thi ck a t th e sum­

mit, a nd ove rli es a bedrock pl a teau. lnt ern a ll aycring is well reso h-ed down to about two -thirds of th e ice thi ckness be ncat h th e do me summit , a nd to len' ls consider a bl y below th at on the south . The layers a rc continuous from o nc side of'

th e dome to the o ther. but th e p a tte rn is interr upted on both

fla nks, to th e north by th e re li c t m a rgin of Sip le Ice Strea m.

a nd to th c so uth by a se ri es or di sruptions a t the Duckloot

di sc usscd in m ore dcta il belo \\'. L aye rs a rc a rched up\l'a rd

directl y bencath th e dome summit due to local no\\' and m ass-ba la ncc a nomalies at th c di\'ide, a nd th e f ~te t that to

first orde r, th e di\·idc loca ti o n has remaincd sta ble, as di s­

cussed by :\'ereson a nd o th e rs (1998a ). The no rth south asymmetr y o f' th e more rccent layers in th c upper ha lf of

the icc thi ckn ess is likely t he res ult o r an acc umulation g ra­

di ent. a lso di sc ussed in i'\ereson and oth ers (1998a ).

R ada r a nd GPS tral'C rs('s o rthe sca r fe a ture to the north

show an a brupt di srupti on o r the dee per interna l layers ( belm\' a bo ut 100 m depth ) which o th erwise tracc co ntinu­

o usly back to the summit (Fig. 3). This disrupti o n is coinci­

dent \I 'ith th e sca r feature a nd a lso a n abrupt cha nge in th e

renection cha rac teri stics o f the bed (not well depicted in the fi g ure) poss ibl y associated with the prese nce o r wa ter in a

till laye r (G ad es, 1998 . These res ults co nfirm th a t th e scar is t he surface ex press ion o f' t he III a rg i n of a reli c t ice st rea m

which \IT ha\ 'C ca ll ed Sipl e lee Stream th at once crossed thi s

na nk of Sip le D ome CJ acobcl a nd othcrs, 1996a ). This paper

103

J Ollrnal ifGlflCiology

f zg, 2. Dela il q/ Si/Jle DOllle and Jll rro ll lldillg l ee SlreamJ c: alld D and Ihe reliel Si/lie lee Slream Jrom enhallced :1 T'HRR imflgfl)'. r I'hile lilies illdicale Ihe JJa llts qj'radar alld GPS Irm'erses, Sfl'eral !if lite scar lraa; hm'e beell confirmed asj im nN iee­slream/llOlgins as described illlhe lexl, RES, radio-echo sOllnding.

focuses on radar, GPS a nd satellite-im agery studi es o f the south (l ank of Siple D ome, a nd in pa rticul a r on feat ures a t the former margin of Ice Stream C.

THE DUCKFOOT AREA

The south (l ank of Si pie Dome termina tes at the ma rg in of Iec Stream C where satellite image ry (Fig, 2 and, in m ore detail, Fig, ..j.) re\'eals a splayed pattern of margin scars a nd flow bands We' ha\ 'e ca ll ed the Duckfool. IIl\'estiga tions of the opposite ma rg in on the south sidc of Ice Stream C during thc late 19805 showed that rapid (lO\\' ccased approxi­m a tely simultaneously along most of the lower trunk as de­termined by the buri a l depth of ClT \ 'aSSes (Retzlaff a nd Bentl ey, 1993), Based on thesc dcpths a nd the known a ccu­mul a tion rate, Retzla fI' a nd Bcntl ey d a ted the shutdown of C as ha\'ing occurred 130 ± 25 years pr ior to thei r stud y, Shallowe r buri a l depths at one location nea r the Gu up­stream end of their sur\Ty indiea ted tha t shutdown th ere was more re'cent.

Satcllit e image ry of the north m a rg in or c: sugges ts a more complex se ries o f e\'ents, Fig ure -J. shows a deta il of the Duckfoot region derived from an enhanccd La ndsat Them a tic ~dappe r ( T~l ) scenc, Se\'e ra l ma rgin sca rs a nd (l ow band s arc rcvealed by low-angle sun il lumination of

104

breaks in slopc, The sca r represcnting th c most reccntmar­gin of Ice Stream C enters Figure i , adj acent to an older ma rg in (bright band ) a t the top of the fi g ure, Towa rd the center o f the image it di\Trges at about 20° from the old ma rg in a ndtravcrses diagonally, becoming less distinct to­ward the b ottom right 0 (' the fi gure, At that point a series of (low ba nds merges with it in a tight "V" p a ttern, 1t'acing these (l o\\, bands upstream , they wra p tightly around a centra l ra ised (bright ) a rea betwcen the older a nd the most recent m a rgin sca rs, An additiona l scar, considrrably broader tha n the other two (up to 6 km wide ), is \'isible in thc enha nced A\,HRR image of Figure 2 cross ing thc southwes t !lank of Siple Dome, It is not appa rent in the Landsa t image of Figurc l because the sun direction is along the feature,

Figure 5 shows the results of'the GPS survey across the lower pa rt of the south (l ank of Sip le Dome ( ~creson , 1998, 2000), Continuous kinema tic data were obta ined in 1996 to defin c th e topographic surface, and fas t-sta tic sun'C)'s of a scries of 29 poles were acquired in 1996 and rcpea ted in 1997 to obta in velociti es, As expcctcd, downslope velociti es ],cla ti ve to the summit pole increase g radually to about 2 m a 1 nea r km 36 as the surface slope increascs, and there­aft er velociti es decrease ra pid ly, as depictcd in Figurc 5, The promine nt middle scar o f the Duckfoot (Fig, 2) is crossed by the survey at km 61, a ft cr which the slope abruptl y dccreases

Jacobel alld others: Challl!,eJ ill III (IIgill q/ Ire SI ream C

SIPLE DOME

"..... 600 -~ > ~ -~ ~ 400 fIl

~ > Q .c 200 ~ fIl ,. ~ .. ~ 0 a --= Q

:c -200 ~ > ~ -~ ~ -400 ~

.~ -600 Q ,.

Q. Q. -< -800

50 25 0 25 50 75

Distance from summit (km) Fig. 3, Radar /ll'Oji'/e a(ml,1 Si/l/e DOIIII' alld bOlllldillg i({'-,It reallllll(llgills I( 'ith (f l'f'lti('(/I elIlggl'rat iOIl q/a/J/ll'O I illl(f le(J' 10, r'ie/(' is a/J/JI'o I illlat('(J' t01l'ard the emt, wi tll Ire SlrewlI (; alld the DUChJoollII(llgill Oil the nj!,hl alld the rl'l ift Si/)II' h e St rea 1/1 011 the lift, Dislallces are re/at ice 10 the Si/lIe DOlllf.lulIllllillocat iOll , Data a{l'O,1,1 the I/l(llgil/ ,I 1!'flf acquirer! at j .1 111::. cfIIll'lj;-eq/ll'll~l' (!/Ihe tmnslIlillerjor ill lTl'(l.Ied resolutioll, Sum depicted ill the ,Iatell ill' illlagl'l)' correl/iOlld 10 brl'Okl ill the ,I/lrfaa ,I{O/)I' (/lid di,lnt/Jlioll q/the illte/'l/ a/ straligmp/~)' al a/J/Jl'Olilllall'(l' klll6'() I/ orth qfthe SlIlIlIlIit (/efi) al/d al ,lel'eml lomtiol/,I b~ Jlolld km 6() to thfJolllh as described ill Ihe II'll ,

to ze ro a nd the \ '(' Ioc it )' drops hel o\\' 0,5 m a I, Be\'ond thi s po int , the \ '(' Ioc it )' rem a ins esse n tia ll y cons ta nt a t a b o ut

0,25 111 a I, a \'a lue which approac hes th e reso luti o n o f' th e

I year \ 'C locities, T he OIT ra11 pa tte rn o Cstra in ra tes is the ]'c­f(l l'C st rongly compr('ss il"t, from km 42 to km 61 oC the t ra­

\ '('I'se, a nd modcl ing studi es by :\c rcso n ICJ9H show th a t \ ,(, Ioc it ies a long th e south [l a nk a rc not consist(' nt \\'ith

stea d y-sta te now; th e icc is thicke ning ra p idl y. 1\ lso, the lac k

of'ob\ 'ious st ra in a no m a lies ac ross th (' 100\T r 25 km indica tes

th a t the sca rs a nd [l O\\' ba nds in th e Duckfoot co uld not h a\'(' bee n created by the plTsent /10\\' r egime, Thus they a rc

clea rl y r('lic t features \\'hich co nt a in du('s about thc shut­

dow n of lee St rea m c:. The lowe r pa ne l of Fig ure G is a d e ta il or th e Duc kf(lOt

reg io n showing surCace e \c\ 'a ti ons obta inecl b) GPS a nd th e rad a r rcsults, The upper pa nel shows pa n 01' the L a ndsat

im agc 01' Fig ure "~ a t th c sa me sca le fo r compa ri so n, The

p ro mi nent eas t wcst- tre nding m a rg in sca r in th e sate llit e im age is coincide n t w ith a break in surface slope a nd a sh a rp

tro ug h in the inte rn a l la \'CI's 01' th e r a d a r imagc a t km 6 1.

D ee p er layC'rs be lo \\' the tro ug h fo ld st rong]y to\\'a rd th e bed , a nd a li ne of' n ea r-bed dilTrac tn rs, probabl y (TeI'aSSes,

runs sub-pa ra llel to the sca r, onse t tOl\'a rd Ire St rea m c: by

a bo ut two icc thi cknesses, T he mech a ni sm respo nsible [c)r

this d o \\'nwarping m ay b e melting nea r the ice bed int e r­fac(' as di sc ussed by R a) m o nd a nd oth ers, (1997 1, The cn in­cickll('e o f' t he surface brea k in slope, th e b ed clilT'rac tors a nd

a no m a lies in th e inte rn a l layer geome tr y st rongly sugges ts tha t the sca r defines a f() rmer ma rg in , \\'e a lso halT d a ta

Crom a second tralTrSC F ig, 2 across th e sca r, parall e l to

this o ne a nd offse t by a pprox ima tely six ice thicknesses, g i\ '­ing so m e indica tio n o f the three-dime nsio na l structu re o f

the a rea, The sha rp tro ug h in th e inte rn a l layers, c\ow n­

wa rpin g d ce per int e rn a l layers a nd bed cre\'asses sce n in the lower pa nel or Fig ure 6 a rc a ll ('I' idem a t a pprox im a tel y

t hc sa m e loca tions in th is t ril\'ers(' as wel I. The nOr! he r n­

most b road sca r seen in Fig ure 2 is crossed by both pro file s

(approx im a tely km 55 in Fig, 6), but o n ly a ba rely d iscc rn­

ibl e brea k in surface slo p e is obsel'\ 'CcI , with no d isrupt io n of the inte rn a l layers o r c ha ng(' in beel rdkc ti\ 'it \',

The re maind('l' of th c radar sect ion in Fig ure 6 depic ts

zones where intern a l laye rs a re well defined (but dcf()I'med ),

separa ted by sections IVhe re th ey a re high ly d isturbed, Pres um­ahly this corresponds to reg io ns 01' the fo rm C'r icc strea m th a t

han' ex per ienced moderate dcformation in contrast to th osc

where la rge shea ring has occ urred, For th e purposes of" the d is­

cussio n wc halT label eclthese regions I ,) in Fig urc 6, sta rtin g at the o ut ermos t area on the nonh d ircctly be nea th the sca r. In

[05

Journal qfGlaciology

106

Fig. 4. Detail qftlze DllCkJool scars alldJlol.{' balldsJrollll1(,o eJllzallced Landsat TJI scenes. Re.wlutioll qflhe image is 28.5 m per jJi\el, al/d Ihe sun azimuth is ajJprorilllatelj' southeast (JrO Il! the lljJjm right ill t/ie image). The lomtion qf the radar and GPS traverse is indicated ~J' the wlzite lil/e.

-E .lI: -G> u c as -III ;;

UJ I

== 'iii g

..J

g c 0

:0: as > G> "ii

8 as 't: :::J en

15 Map view

10 .....

5 ...... ,"--." "'--...... '

0 + , ..... . . . . . -- ........................ .

5

,1 m/a , 10

Cross:.section view 300 .. ,-250 . " ,-

200 - --, ,. 150 - .. .. .. , . . . . . ..

' .. 100

40 45 50 55 60 65 70 75 80 85 Distance south of divide (m)

h g,5. GP.)' resuLts showing SlI lj(lCf tOj)ograjJI!J' alld dOll'lIsLo/le l'flocilies aLollg the lower /)01'1 oJ the sOll th jlallk q/ Si/lIe Dome_ T he onj;ill o/coordillates is the SijJle Do)))1' ,11I1I1IIIil.

reg ion 2, interna l layers a rc di scc rnible fa r approx imatrl y a ll­

o t her 6 km pas t th e- scar/t rough bero re bccom ing lost in th e d i n'raclors, Th is i mpl ies th a t she<l I' was accommoda te-d across

a fa irl y broad ma rg in zone located so m e- wh a t inboa rd rromtil e

surrace sca r a nd th e underl ying downwar p in th e inte rn a l

laye rs. The reaso n ro r thi s onset is not clea r. The surface a bO\'(' region 2 is a broad troug h whic h r ises

to a ridge or a mplitude- cv 15 m nea r km 75, abo ut 11- km

so uth ortil e scaLTh e c res t a nd so uth slope orthi s ridge co r­res po nds to th e brig ht a rea in th e sa te llit e image- be twee n

ro lded now ba nds. Be neath the ridge- in region 3 (km 71 79) is a zone- o f' \\ '(' II-de fin ed inter na l layers be-a ring no sim­

ple relati o nship to those under Sipk D o m e. Derorm a ti o n in

th e lowC'r layCl's within th e zone does no t ma tch we ll with

th a t abO\'(' , The icc/b ed inte rf~lc(' belo w zonc 3 is th e brig ht­est a nywh ere a lo ng the trm'(' rse. Further so uth , a seco nd

bre-a k in the i nt e rn a I la)'C'l's or cv 25 km ( re-gion I) is ro ll owed

by a no th er zone o r we ll-defined inte rn a l layers ( regio n 5,

km82 R+) not co ntinuous with th e first. Difr raClo rs Ixed o­mina te beyond thi s as th e trm'C rse e nl e rs the most re-cc ntl ),

ac t i\'e pa rt o r lee Stre-am c:. The prese nce o r nea r-s urf~lce difTraC'lo rs so uth ward o f'

th e troug h/brea k in slope or reg io n I, \\ 'hich continue a ll

th e way into th e ice stream , indicates C},(,\'<lssing a nd a rg u l's th a t thi s a rca was a ll fo rm erl y pa rt o r Ice Stream C. {\I so,

th e region of'I I'e ll-de fin ed int crn a ll aye- rs (region 3, km 71

79 ) in Fig ure G shO\I-s undul ating int e- rn a ll aycr ing typica l of'

ice-stream icc Uacob e l a nd oth ers, 1993), The seco nci regio n or well-defin ed int e rn a l laye rs (reg io n 5, km 82 8~~) is a lso

di stinct rromthe a reas whi ch surro und it. [f' th ese two areas o f'wcll-delined illle rn a ll ayers \\-e re indeed fc)!'merly par ts o f'

Ice Stream C, th en th e laye r pa tt ern in fi g ure G d e- pi e ts a

shirt o f'th e north m a rg in o riel' Stream e , ultimately lea ding to th e shutdown_

] al'Obe! and olhen: Changes ill margin cif /ee Strf({ 1/7 r:

WHAT MADE THE DUCKFOOT?

\\' ha tC\'C r sce na ri o e- m c rgcs to acco unt fo r th e detailed fea­lures o r th e Duckfao t pa tte rn , it see ms clea r th at th e no rth

ma rg in o f lee Strea m C shirted inwa rd with an acco m­

panying c ha nge in now direc ti o n, As it did so, ice- rrom the a rca b e twee n the two m a rg ins did no t s tagnatc immedi­

a tely, a nd pa rts o f' it werc she-a re- d a nd ro lded by 0011' a lo ng th c new direct ion or m o ti o n berore stag n a ti o n, The lack o f

a ny substa nti a l di rre re nce in th e surrace tex turc of th e two

regio ns s uggests th at the m a rg in shirt did no t apprecia b ly

p rece-cle- th e shutdOlnl o r e, but \\'e infe r th a t th e outer mar­

gin is o lder, because o f th e cl ea r prese nce o r th e inner o nc a nd th e presc nce of shearing be tll'eenlhe two_ Presum a bl y,

a il now traces or Duck roo t wo uld hal 'e I-a ni shed if that a rea

had bee n pa ri or a n ac ti w' ice strea m. Buri a l depths o f the nea r-surrace CTel 'asses w o uld g i\'c a n es tima te or th e time

since- cessa ti on or motio n in e-aeh sec ti o n , but thc crevasses

a re too 5h a 11 0\1- 1'0 1' a ny o f' t h e m to be reso l\'ed with the low­freq uen cy rada r.

Th e ('ompressi\'e stra in ra te- observed across the lowe r

pa rt o f'th c dome Oa nk impli cs th at thi s a rca must be thicke n­ing considerabl y, proba bl y as a direct res ult or th e shutdown

a nd the thickening o r Ire Stream C. 1'\ereso ll (1998) investi­

gates w h a t may be lea rn ed rega rding the timing or the shut­dow n o f' a n ice stream rro m the rate of' thickening o r th e­

acUaee nl r idge-, According to thi s a na lysis, a wa\T or th icke n­ing, w hi c h tra\'rJ s ras te r th a n th e ice, causes smail topo ­

graphic reatures associa ted with th e fo rm e r m a rgin of' a n ice

Slre-am to be li rted o nto th e na nk of th e ridge As th e- do m e

reaches a l1e\\' stead y sta te, ice noli' ca rri es th e reature- down­

slo pe- . \\ ' h e n appli ed to Siple DOl11e, modc ls o r thi s process show tha t th e out e- rm os t DucH oot sca r is within th e initi a l

stage-s o f' thi s ri se, a nd th a t the inlerredthic ke ning-rate pa t­tern from th e hori zont a l GPS noli' fi eld 1'0 1' th e south Il a nk of'

Siple D o m e is most consis tent with a m a rg in shutdown a pprox im a tely 300- 500 yea rs BP ( ~eres()n , 1998,2000),

" ' hil e it is use lul in pl ac ing constra ints on th e timing or

ice-stream shu tdown, the a bmT disc uss io n o f' the Duckroo t

are-a d oes not rull y ex pl a in the pre-se nt-d ay topogra phi c

profile o r the I'e loc itie-s ac ross the DucH oot reature. Instead or be ing p erched on a pl a tea u or gentl y slo ping dome na nk ,

the o ut c rmost, oldes t sca r is a dj acent to a broad trough, with a cv 15 m ri se betll'ee n it a nd th e most rece nt m a rgin o r Ice

Stream c:. :-\t thi s tim C' wc halT no comple tel y sati sfacto r y

ex pla na ti o n fo r thi s pro file , th o ugh se\'(' ra l hypotheses see m reaso na bl e, The ri sC' m ay re present a rem n a nt bulge f'ro m

Oow ove r a sticky spo t; it m ay be due to m e lting or now of'

warme r, so rter <1cUace nt icc as a resu lt o f' shea r during it s

<}c ti \'(' p e ri od ; or it m ay rc prese nt a spa ti a l Y<uiatio n in ae­(' ulllul a t io n across the regio n.

Re turning to Figure C), wc propose a likely scena r io or

wha t h as ha ppl' ll cd in li g ht o r th e preccdin g di scuss ion. I I' th e o ute r sca r is th c fc) rm e r ice-strea m m a rg in, as seems

likely, regio n 3 must ha\'e b ee n pa rt o f' lee Stream C th a t beca m l' p a rti a ll y defc) rnwd as it stagnat ed a ndlhe ma rg in

shi rted sou th to its ne\\' lo(,a ti o n, Instead o f' th e ma rg in

jumping clea nl y rrom the sca r to the (' ITntu a llocation a nd

ical' ing complet ely stagn a ted icc behind , somc moti on with­in thi s regio n bega n a lo ng th e new Oow dircc ti o n, shea rin g

a nd wa rping pa rts o f' th e fixm er ice stream before motion

SLO pped comple te ly. R eg io n 3, ex hibiting weil-defin ed intern a l laye rs, is the Icas t clcfi lrlllCd section or rorm er ice­

stream icc situated be twee n a reas or hi g he r shear und crl y-

107

Journalo/Glaeiolog)'

= 0 .... ...... ~ ~ <lJ -~ <lJ ...... ~

E .... ~ 0 ;.., Q. Q. ~

108

NORTH C MARGIN

200

0

-200

-400

-600

45 50 55 60 65 70 75 80 85 Distance from Summit (km)

Fig. 6. Part rithe Lalldsat image of Figure 4 (lI/J/Jer/)(lI7el ) together with a detail q/the 5 MH;:: radar jJrq/ile across the Durk[oot area (lower jJallel) at the same /lOri::,olltal srale. rertiral smle ill the 101e'er /)allel is aj)/Jrorimate lIleters abol'e mean sea level. Distance is kmji-olll Sij)/e Dome summit and rorresj)ollds to the roordillates ill Figu re 3. , \ illllbers u·itizin the illtemalla.)'er patlem rifer 10 .colles desrribed in the tnt.

90

ing the outer margin to the north ( regions I a nd 2) a nd the shear folds to th e south (region +).

The second zone of well-defin ed internal layers a long the traverse (reg ion 5, km 82 8-1·) li es below the narrowing triang ular region o n the surface in Figure 6, bounded by the most prominent fl owbandto the no rth a nd the recent m a r­g i n orrce Stream c:. In the scena ri o just described, t hi s too is ice which was pa rt of Ice Stream C before the m a rgin shi fted , but it was el·identl y subj ect to less shea r tha n neigh­boring ice direc tl y beneath the shear lo lds. \\'hen this na r­row zone of ice stagnated, the m a rgin transiti o n was complete a nd the n ew shea r m a rgin deve loped to the south.

Fig ure 7 shows a sketch of th e ti m e sequence o f c ha nges in flow which could lead to the obsen ·ed pattern o f fl o\\" traces a nd margin scars of the Duckfool. In thi s scena ri o, cha nge in th e north ma rgin of Ice St ream C was a resp onse to the stagnati on of a region of ice locatednca r the m a rgin, a pproximately centered at wha t is tod ay the Duckfool. This area may have behalTd something like the "sti cky spots" seen today ill ice streams. but located near the ma rg in . lee fl owing towa rd thi s stagnated regio n was di\Trted so me 20" towa rd the cente r of the ice stream .. As ice began to fl ow around the stagna ted area, shea r-folding of fonner fl o\\" ba nds was initi a ted a t the upstream end , creating a dilfuse zone of shea r a nd a new ma rg in (center panel ). Th e icr­strea m bounda r y adjusted to thi s slig htl y enl arged a rea oC stagnation by once morc shi fti ng inboard and creat i ng the na rrowing tri a ngle defin ed by the shea r folds a nd the nas­cent ma rgin (lower pa nel). Va ri a ti o ns Oil this seque nce or el ·ent s, or a lt r rn a tilT expl a na ti o ns, a re of course poss ibk but thi s sre ll1s a t present the mos t pl a usible scena rio linking the im agery, rad a r a nd GPS obse rl·atio l1S.

Our scenario proposes that the surface featu res we see on the north sicle o f lee Stream c: a re the res ult of a w idening stagnati on of a tri a ng ul a r-shaped a rea of ice just inside the o lder ma rgin . Th e root causes of thi s sLagnati on a rc not yet clea r, but wC' consider three possibiliti es: (I) a ge nera l reduc­ti on in water pressure under Ice St ream C II'hi eh causes the la rges t proporti o nal increase in d Teni l·e norm al stress a t the bed in thi s ma rgin a l zone because the ice is thin there; (2) reduced vcloeit y in fe e Stream C I\·hi ch reduces the drag o n thi s ma rg in fi"olll the central parts of the ice stream; (3)

Ice Stream C

\ Shear folds beginning

Fig. 7 Tillle sequel/ re :,/iowillg like~)' i'l 'O!UIIOII oJ l/ie lIorlh IIImgill oj1ce Strea/ll c:

] arobel ([ li d others: Changes illlll(lIgill oj·Ice Stream c:

onse t o f fr rrz ing associa ted with a ge ne ra l thinning o r lee Stream C which will halT the strongest a nd mos t rapid eflcct where the ire is thin.

Basal water-pressure va riati ons ben ea th a n ice shee t a l"e contro lled prima ril y by surface slope ( ~90% ) a nd secon­da rily by bed slope (10'% ). In the I'icinity o f the DucH oot , bed slo p e is considera bl y g reater tha n surface slope ( ~ 1 %, I·S n ea r-zero at the surface ), with the h ed sloping down to­ward th e strea m cente r (Fig. 6). There fore, \\·ater press ure in a connected syste m a t th r brd \\·o uld decrease ra pidl y ri·om the cenLer trunk towa rds the ma rg in, thereby increas­ing the dfectilT norm a l stress. In a regime o f declining basa l wa te r pressure thi s area might be ex pec ted to be the first to stagn a te, as the uncompensated O\·e rburden pressure or ice rest i ng o n th t' bed increased.

H owel·er, rrcent studies rega rd ing th e downwarpecl p a t­tern o f intern al reflectors near ice-stream m a rgin s, a nd cal­cul a ti o ns of the heat production clu C' to shea ring in m a rg in zo nes, hal·e led to the conclusion that ice-stream shear zones m ay ge nerate water by m elting near the bed (Jacobson a nd Ray mond. 1998). Thus, the older ma rg in it se lf \\ as a likely so urce o f\\'atercontriiJuLing to basa l wa ter pressure. rftha t wa te r producti on declined due to a slo wdown of the ice stream , the reduced wa ter productioll could lead to reduced wa te r p ressurC' in the sub-ice hydrologic sys tem, and stagn a­ti o n a lo ng the bed slop e beneath the Duckfoo l.

Basa l freez ing h as a lso bcC' n di sc ussed as a mecha ni sm fo r inwa rd migrati o n o f a n ire-stream m a rgin (Jacobson a nd R aym ond , 1998). In the steady-sta te situation, shear heating a nd basa l melt inside th e ice sll"ea m are ba la nced by (" onducti\·r losses to the surfacc· a nd the influ x of co lder ridge ice fi"om oUlside the ma rgin . Ir th e ice thi ckness a t the m a rg in is rC'duced, or if the marg in mig ra tes to an a rea o f thinne r ice, the resulting increasC' in temperature gradi e nt prOl·ides a way to d rain heat more eflcc ti\Tly from th e bed , a nd rreez ing occ urs. A s m ay be seen fi ·olTl Figure 2, the ice beneath the Duckfoot is quite thin fo r a n ice stream (500-700 m ), so hea t loss a nel rrrez i ng due to cha nges in the geotherm a l gradient sho uld occ ur first here.

The sequence of el 'C nts at the Duckfi)o t a nd the mecha n­ism s wh ich caused it m ay halT count erpa rt s d sewhrre \\·ith­in th e Sipit' Coas t dra inage syste m . Changes a t the doll' nstl'Ca m end of Ridge Ble: (Fig. I) a ppear to ind icat e a simil a r a rea or stag na ti on with a na rrowing of the so uth m a rg in o f" Ice St rea m c:. Bindschadl cr a nd \ 'o rnbel"ge r (1998) halT dC'scribed a recent 1£'iriellillg o r the north m a rg in o f" Ice Stream B in thi s same a rea, fro lll a compa ri son o f newly released U. S. D e pa rtmcnt or D efe nse im age ry from the 1960s a nd more recent Syst(-me p roba toire pour l'ob5er­I·a ti o n el l' la terre (SPOT ) im age ry. \ e t Fig ure I suggests tha t this m a rgin 100 m ay have na rrowed in the more di sta nt pas t, lea\·ing a tong ue o f stagnant ice a t t he downstream cnd o f Ridge Ble:.

Th e poss ible rela ti o nship of changes in the margin 0 [" lee Stream c: at the Duckfc)() t to the shutelown or Siplc Ice Stream a lso desen ·es commr nl. Recent rada r studi C's by th e Uni \'ers it y o["\\'i sconsin a nd the UnilT rsit y of\\'ashing to n pl ace the shutdow n time of Siplc Ice Stream near .J.20 yea rs BP, based o n th e bu rial depth of cii sturbed layers (Smith a nd others, in press; persona l communicati on fro m A. 1\1. G aclcs. 1998). Th e a pprox i mate d a te filr the shi ft in th e nOrlh m a rgin of Ice St ream e: at th e DuckfclOt, based o n m ode l studirs. is on thi s samr order. \\ 'ith the info rm a ti on curre ntl y ant il able wc can say no more a bo ut a possible con-

109

JournalofGlaciolog}!

nection between these events. However, enhanced Landsat imagery shows clearly that motion in the main trunk orrce Stream C postdates the shutdown of Siplc Ice Stream. This is ev ident from the tai ls of flow bands streaming north into Siple Ice Stream which have been shea red and truncated by more recent flow in Ice Stream C Uacobel and others, 1996a, fig. 3).

CONCLUSIONS

The shutdown of Ice Stream C appears to have been a more complex set of events than might hal 'e been foreseen based on early studies of the south margin. Evidencc from the Duck­foot area shows that the position of the north margin of Tee Stream C changed at least once prior to the most recent shut­down of the main trunk rv 150 years BP. Probably 300-500 years BP, the margin shifted southward in a sequence of events that involved stagnation and shear-foldi ng of former pa rts of the ice stream. With the present study wc are unable to place better limits on timing of the changes, but further information could come [i'om a high-frequenc y radar tra­verse to measure the burial depth of crevasses to place better constraints on the ages of the events at the Duckfool.

Considering these changes in a regional context, perhaps as recentl y as 450 years ago ice traversed the northeast flank of Sip le Dome in the relict Siple Ice Stream. The possible r(:'­lat ionsh ip of this shutdown to changes at the Duckfoot is not yet clear, but the estimates of timing are close to overlap. It is clear, however, that motion in the lower trunk of Ice Stream C postdates the shutdown of Sip le Tee Stream, so that at least part of lee Str(:'am C was active after that time.

ACKNOWLEDGEMENTS

\Ne would like to acknowledge C. Shuman and Xin Chen who measured GPS positions ofth(:, strain grid in 1997. This project was supported by grants from the U.S. National Science roundation: OPP 9316338 to St Olaf College, OPP 9316807 to the Unil'ersity of Wa shington and OPP 9317007 to the National Snow and Ice Data Center, University of Colorado, Boulder.

REFERENCES

AIlty. R. B. and 1. Id. \'·hill"ns. 199 1. Changes in the West Antarrtir ice sheet. S{ifllCf. 254 (.103+1,9.19-963.

Bentley, C. R. , P. D. Burkholdcr, T. S. Clarke. C. Liu a nd N. Lord. 199+. Geophysical expe riments on Ridge Bl jB2. Siplc Coast. .inlanl. J [:8., 29 (5), Re\'ie\\, 199cl, 57-59.

Bindschadl er. R . A. 1998. Future of the West Antarctic ice sheet. Sril'llCf,

282 (5388),428-+29. Bindschadl er. R. A. and P. L. Vornberger. 1990. A\'H RR imagery I'('\'eals

Antarctic ice dynamics. EOS, 71 (23).7+1- 742. Bindschad ler, R. and P. Vornberger. 1998. Changes in the West Alllarrtic ice

sheet since 1963 from declassilied sa tel lite photography. Sriflltf. 279 (53511, 689 692.

Bohlander. j. A. and T. A. Srambos. 1997. Ice \'('Iocit\ of Ice Stream D. F. and the Shirasc Coast: nO\\' dynamics and changes. r Ahstract.] EO.)'. 78 (+6 " Fall :\keting Supplement. F252.

Casassa. G., K. C. .Jezek, j. Turner and I. i\1. Whilb ns. 1991. Rel ict no\\' str ipes on the Ross Ice Shrlf .·11/1/. Glacial., 15. 132- 138.

Conway, H. and .\.;\1. Cades. [n prcss. Glacial hi storyof Roosc\·e ltl sland. \\ 'cst Antarctica. In Bindschadlcr. R . A., al. Thl' Il esl . l lIlanlic ia sherl.

Cha/)/I/all COllferellce. /3-18 SI'/Jlembn 1998. 01'0/10 . . I/ail/I'. \\'ash ington. DC. American Geophysical Lnioll.

Gades .. \. J\ 1. 1998. Spatial and tempo ra l \'ariations of hasal cond itiolls Ixncath glac iers and ice sheets inferrcd from radio echo sound ings. (Ph.D. thesis, Uni\Trsit\· of \\·ashington.)

J acobcl, R. \\'., A. i\1. Gades, I} L. Gottschling, S. i\I. Hodge and D. L. \\·right.1993. Interpretation of radar-detected internal layer folding in \\est .\ntarctic ice streams. J (;Ioriol.. 39 (1331.528 .'i37.

J acobcl , R. \\' .. T. A. Scambos, c:. F. R aymond and A. i\1. Gades. 1996a. ChangC's in the configunllion ofiee streanl now from the \ \ 'est Antarctic ice sheet. J Cfophp. R eJ., 101 (B3 1, 5+99 .150+.

J acobel. R. \\' .. A.j. Fisher and :\T. \1. Sundcll. 1996b. [ntcrnal strat igraplw /i'om ~roLlnd·based racia l' studies at Siplc Dom(' summit. Alllarcl. J Cs.. 31 (51. Rrvicw 1996,55- 56.

Jacobson. H . P and C. F. R avmond. 1998. Thermal effects on the location of ice stream margins . .7 Ceaphys. Rn, 103(B6\ 12.111 12,122.

"-"a ran. G., T. Scambos and 1\1. ,\. Fahneslock. 199(i. [mproved AVHRR inl agery oficC' Sh(,C'lS yia data cumu lation - a theoretical and enlpirical

\·alidation. [Ahstract.] EOS. 77 H·61, Fall l\[eet ing Supplement. F193. Ncreson, :\. A. 1998. The (l0\\' history of Si plc Dome and Ice Streams C and

D. \ Vest ;\llt arctica: inferences from geophysicalmcaslIlTlllClllS and ice no\\· modds. I Ph.D. thesis. Unil'Crsity o f\\ 'ashingtoll.1

Nereson, X A. 2000. Elevation of ice st ream margin scars after stagnation . .7 Glaciol .. 46 (152), 111 -118.

Nereson,:\T .. \ .. E. D. \\ 'add ingron, C. F. R aymond and H. PJacobson. 1996. Predicted a~('-depth scales for Siplc Domc and inland \\'r\IS ice COlTS in \\'est ,\Illarn ica. Geo/A)'I. Re!. Lell .. 23 (22), 3163 3166.

Ncrcson, K. r\., C. F. R aymond. E. n. \\'aclcl ington and R. \\'.Jacobcl. 1998a. i\ligration of the Siple Dome ice di\'ide, \\'est .\Ill a rcti ca . .7 G/ariol.. 44 018:,61-3-652.

:'\ereson, K. ,\ ., R. c:. A Hindmarsh ancl C. F. Ravmond. 1998b. Scnsiti\'ity of the di\·idl' position at Siplr Dome, \I 'est Antarctica. to boundary 1'01'­

ring .. ~//I/. Glacial., 27, 207 214. Orheim, 0. ancl B. 1'-. Lucchilla. 1987. Snow a nd ice studies hy Thematic

i\Iapper ancl multispectral scanner L anclsat imagrs .. 11111. Glacial., 9, 109 11 8.

Raymond. C. F .. ~. r\. Nereson, A. \1. Gades, H . Con",ay. R.J acobe1 andT. Scambos. 1995. Geometry and stra ti ~raphy of Siple Dome, , \ nta rctica. f l lllarrl. J CS., 30 (.1), RC\'icw 1995. 91- 93.

R ay mond. C. F. , R. \\'. J acobel, T. A. Scambos, N. A. :'\crcson and H. PJa­cobson. 1997. Energy balance of ice streams and melting a t margins. [Abstract.l EOS, 78 (461, Fall ;\ Iecling Supplemelll, F2+3 F2++

RetzlaO: R. and c:. R. Bentley. 1993. Timing of stagnation of Ice Stream C.

\-\est Antarctica. from short-pu lse radar studies of' buried surface cre\'<lSSCS. J Clal'iol .. 39 1331, :153-561.

Scambos, T. A. a nd R. A. Bindschacller. 199 1. Feature maps of lee Stn'3ms C, D ancl E, West ,\ntarctica . .-IlIlarcl . .7 [ :s. , 26 (.1\.312 31+.

Scambos, T. A. ancl N. A. Ncreson. 1996. Satellite ima~e and GPS stucl" of the morphology of Siple Domc. Antarctica . .l llll/rd. J Cs. , 30 ' 51. Re­\·il'\\· 1995.87 89.

Shabtaie, S. and C. R. Bentley. 1987. \\ 'est Antarctic ice streams draining into the Ross Ice Shelf: configu ration and mass balanc'(' . .7 Geo/JI!)'5. Res .. 92 B21,1311 1336. \ Erratum: J GfojJ/£jI.I. Res., 1987. 92 (B9), 9+.11.)

Smith, B. E .. ~. Lord and C. R. Bomley.ln press. Ground based radarstucl\' of the stagnation orIce Stream C a nd the Siple Ice Stream, \\'est Antarc· tica. i ll Bindschadler, R. A.. ed. The 11 esl . llI lnrrlic ia sheel. ChajJllwlI COllfer­

ellrt. 13- 18 SejJlember 1998. Orallo .. Iinine. \\·ashington. DC. American Geophvsical Union.

Stcphcllson. S. X and R . A. Bindschadler. 1988. OhscnTd \'doe-itv Ouclua· tions on a major Antarctic ice stream. Xalllre, 334 618+.695 697.

illS received 4. Vovember 1.9.98 and aC{fjJted in revisedform 12 August /.9.9.9

110