.~j l/llals OJ Glaciolog), 22 1996

tf:, Internati o na l G lacio logica l Soc ie ty

Morainal-bank sedirn.ent budgets and their influence on the stability of tidewater terrn.ini of valley glaciers

entering Glacier Bay, Alaska, V.S.A.

L E\\'lS E. H UNTER ,

C.S. ArnD' Cold Regions Research alld Engil/eering LaboralolY, 72 L.yme Road, Hallover, N H 03755, U.S.A.

Ross D . P O \\'ELL,

DejJarlml'lll oJ Geology, Northern llIillois Ul1lVersi~)I, Dek alb, IL 60115, U.S.A.

D A\, IEL E. LA\\, Oi\

[i.S. Arn~JI Cold Regions Research and Engil/eeril/g Laboralol]I, 72 L.JllIle Road. Hanover, .VH 03755, U.S.A.

ABSTRACT. r nvestiga ti ons of g round ing-line sedimen tation in front of tid ewa te r term ini of tempera te \'a ll ey g laciers demo nstra te that sedimen t yields a nd d ynamics provide a second-order control on g lac ier stab ility by influ encing water depth a t th e gro unding li ne. Sediment is del i\'e red (0 th e g rounding lin e by two ro utes : ( I ) d ebris tra nsported in , on a nd beneath the g lac ier , a nd (2) sedim ent transported in g lac ia l out\\'as h streams. G lacia l streams in G lac ier Bay , Alas ka, D.S.A ., d eli ve r l OG lO 107 m 3

yea r 1 of sed iment to the g rounding lin es . The g lacia l d eb ri s nux transports 105 to 106 m 3 yea r 1 of debris to the ice cliffs, where approx im a telv 10% is re leased at th e ground ing linp , the rem a ind er being tra nsported downfJ ord by iceberg-rafting . An add itiona l 10" m ol yea r 1 of sedim ent may be tra nsported to th e grounding line by shea ring a nd ach 'ec rion o [ a deformab le bed.

INTRODUCTION

Process monilOring In front of tide\\'ate r termin i o f temperate \'a ll ey g laciers has been ongo ing in G lac ier Bay (Fi g. I ), Alaska, D.S.A. , since Powe ll ( 1980, 198 1)

bega n defining mod ern sed im entary facies and process

re lat io nships. The we ll-known g lacia l history of G lac ier

Bay (Fie ld , 1947; PO\\'ell , 1984; Go ldthwait, 1987; Hunter a nd POII'e ll , 1995b) p ro\'id es a framelVork for g lac ier be ha\' io r that, us ing th e res ults oC modern process studi es (e.g. ~Iacki e\l' i cz an d othe rs, 1984; Powel l, 199 1; Cowa n , 1992 ), enab les us to e\'a lu ate re lationships between

sed im ent dynamics a nd th e beha\'ior of g lac ier termini

( Powel l, 199 1; Hu nter a nd POII'ell , 1995a) . T he dynam ics of marine-end ing g lac iers res ult from a

ba la nce a mo ng glac ia l, ma rin e a nd sedim en tary processes a t th e grounding line. Brow n a nd o th ers ( 1982 ) noted a

re la ti ons hip between ground ing-line \I'ater depth and

ca h 'ing speed of Alaskan glac iers \I'ith tidewater termini.

Alley (l 991b ) a nd P owell ( 199 1) sugges t th a t sedim ent dynamics may reg ul ate g rounding-lin e wa ter depth. Several processes (T a bl e I ; Fig. 2) interact to regulate th e growth and coll a pse of sedim ent pil es, o r mora ina l

banks, which accumu la te a t th e g rounding lin e. In this

paper, sedim ent-budge t d ata from three mo rainal banks In G lac ier Bay a re presented to provide insig ht into th e

magnitudes of processes affect ing sediment d yna mics in

front of tempera te tidewater term ini in south eas t Alas ka.

SAMPLING METHODOLOGY FOR SEDIMENT­BUDGET ANALYSES

Our ill\'est iga ti o n focused o n d efinin g th e re lat i\'e importa nce of grounding-line processes at Grand Pac i­fi c, M a rge ri e a nd Muir G laciers ( Fig. I ) . A br ief summ ary 0 (' the d a ta co ll ec t ion stra tegy is g ive n below .

Debris dis tribution

T idewater te rmini a re idea l [or th e stud y or debris distributio n w ithin a g lac ier, since th e ice cliff represents a nea r- ve rti ca l, often transverse c ross -sec ti on . I cebe rg

caking introduces ice from a ll positions of the ice cliff to

the fj o rd. By reco rding the loca tion from w hi ch each iceberg o ri g in a ted in th e ice cliff, a ll representa ti ve ice facies can be sampl ed se lec ti ve ly, in acco rd an ce with an ice-fac ies cl ass ifi cation sc hem e based on th a t of L awso n ( 1979; Fig . 3 ) . I t was poss ible (0 d etermine th e d ebris

distribution from d ebris concentra tio ns calculated for 282

iceberg a nd 139 g lacier iee samples (Hun ter a nd others, 1996 ).

2 11

HUllter alld others: .\lomilla/-ballk sedilll flll budgels

~;~ ....... . ~i: ....

Glacier Bay ·····\ National Park .... & Preserve ..... D Glacier ......... .

_ .. Internat ional bound~';~i""" " '" ..... Park boundary ........ .

o 20 .-""- Streams ~ \ .

138 137'

Fig. 1 . . Ih l/) 0/ Glacier B(O' • \ 'a Liolla/ Park alld PreSflTe showillg tlte locations (j[ ( I) u/JPer .\fuir IlIlet alld .Ill/ir GLacier, alld ( 2) lIjJjJer Tan IlIlet witlt Grand PaciJic alld .\fargerie Glaciers.

Basa l ice laye rs a t Gra nd P ac ifi c, ;\farge ri e a nd !\Juir Glac ie rs a re di scha rged in to fj o rd s bc lo ll' sea le\·e l. such that th csc layc rs arc mos t oft en o bsc J"\ 'Cd in ice bergs . Fo rtun a te ly, basa ll y d e ri\'ed ice bc rgs te nd to ri se \'e rticall y a nd th e ir loca ti on of ori g in ca n be inferred.

Sa mpling o f th ese ice bergs pro\'id es a \'a lua ble constra int

on basa l la\'e r thi ckn ess a nd d ebris concentra tion . Supraglac ia l d ebri s thi ckn ess lI'as es timatcd a long

tra nsec ts nca r te rmini . l\i o ra ine thi ckn ess on ?\/la rge ri e a nd Gra nd Pacifi c Gl ac ie rs ra nged from < I mm to 1. 5 m ,

but ra rely exceed ed th e 0 .08 m a\'e rage estim a te of

G o ttl e r ( 1992 ). D ebri s CO \ 'CrS of Imm a re suffi c ientl y

thick to disco lo r th e surface, lI·hereas a thi ckn ess of 1- 2 cm produ ces a cover th a t appea rs to be nea rl y comple tc on ae ri a l ph otog ra phs. Thicker mo ra in e co\'c rs (0 .5- 1 m ) fill surrace cre\'asses and fo rm d ebris rid ges a nd morc-o r-I ess

continuo us g ra \'e l surfaces .

Bathytnetric tnonitoring of glacifluvial seditnent flux

:'\loored lines with sedim ent tra ps werc deployed in bo th

:VJuir a nd T a rr Inlets to mo ni to r th e spa ti a l pa tterns o f

susp ension se ttli ng in th ese inlets (Cai, 1994; Hu n te r ,

1 99 '~ ) . Tra ps suspend ed 1- 6 m a bove the sea fl oo r a re used to represent suspcnd ed nLl\·ia l sedim cnt nux to th e sca noo r. \' 01 umes or d epos i ted sed i ment we re d e term i ned by plo ttin g a nd conto urin g se ttlin g-ra te d a ta di\'ided into

plume set tling (th e tota l th a t acc umul a ted on mora ina l­

ba nk a nd flu v ia l d e poce nte rs) a nd p lume b y-pass (sed im ent th a t became d eposited d ownfj ord from th e g rounding-line sys tem ; Fig . 2) .

Fj o rd ba th ), m e try be twee n 1988 a nd 199 1 \\'as reco rd ed ni ne tim es lI'i th i n I km or ~ l u i r G lacier a nd

seven tim es within 2 km o f Gra nd Pacifi c Gl ac ie r.

Ba th ym etri c mon itoring ena bles mo nito ring of bed load

dumping, squeeze/push a nd mass mO\ 'ements th a t canno t be meas ured direc tly in th e ice-prox ima l ell\·ironmenl. Sedim c n t \'01 u m e con tri bu ted b y th ese processes is de termined indirec tl y by subtrac tin g contributions from

2 12

T able I . Grolllldillg-lille jJrocesses

Process .I l oraillal- Difill itioll ballk cOlltri-bulio ll

GLacier debris .flux: Ice-cliff melt-out Addition

Ca l\'e dumping Addition

Ice berg ra ftin g

Clacijlllvia/ sediment .flu\: Bedload dumping Add ition

Plume se ttlin g Addi ti on

Plume by-pass

Subglacia/ alld ice margillal: F reeze-ree),c l i ng R e-cycling

Squ eeze/ p ush R ecyc ling

:-'lass movem ents R emO\'aI

D efo rming bed Addi ti on

Rel ease o f d eb ri s by surface melting a t the

te rminus

Dumping of supra g lacia l d ebri s during cah 'ing C\ 'e n ts Tra nsport or d ebri s in ice bergs beyond th e

mo ra in a l-ba nk toe

R a pid de posi ti on o r coa rse bed load a t strca m a nd conduit m o uths

Suspension se ttlin g from

overfl ow plumes o n to th c m ora ina l ba nk Fi ne-pa rticle tra nsport in o \'e rn ow plume di sta l 0 (' m o raina l-ba nk toc

Loca li zed subg lac ia l fi 'ecze-on a nd tra nsport to th e ground ing lin e­Scdim ent d eforma ti o n

ca used by grounding­

line flu ctuations

Slides, slumps a nd secli­ment-gra \ 'ity fl ows ge n­c ra ted on th e mo ra in a l ba nk

D own -g lac ie r ach-ec ti on

of so ft sediment belO\\'

th e g lac ier so le

(not drawn to scale)

Plume by-pass

........ 1----Morainal bank ---t.~

Fig. 2. Prilll([J)' sedimental)' jlrocesses at a tidewater termillus.

Fig, 3, Genera/ ice-Jacie,1 dislribllli()11 al a lidnl'aler lenllill/o ,

plume sc ttling, ca k c dum p ing a nd ice-clin' me lt-o ut f,'o lll o bsen'Cd sp a ti a l a nd tcmpo ra l c h a nges in sea -fl oo r

sed im ent \ 'o lum e, G lac illu\' ia l dumping a t po int-sourcc

de pocentcrs is illustra ted o n isop ac h ma ps b \' mo un ds o r

pil es ( Fig, +: Po\\'e ll , 199 1) , \\'he reas m o ra ina l-ba nk g ro \\'th a way I'ro m nu\·ia l so urccs is a ttribut ed to squ eeze/ push m O\'C m ents a nd th e ad \'l'C ti o n o f' sedim e nt in a d c{o rmin g bed , S imil a rl y. m ass-mo\'em e nt p rocesscs

a re reco rd cd by d ep ressio ns o n isopach m a ps, Th e re fo re,

isopach maps based o n re pea ted ba th\'l11 e tri c sun'Cys

\\'(' re used to mo nit o r \'o lumc tri c c ha ng;es in th e m o ra in a l ba nk caused b l' ide ntificd scdim enta ry proccsses (Figs 2 a nd 4 ),

a C.I. =20m b o 1 , ,

km 1------.,

Margerie Glacier Margerie Glacier

Fig , -I , E \'{/177/)/e oJ It Oll ' sedimen/nI)' /)1'0('('.1' ,1('.1' are mOllllored IIsillg ba/I~) ' 177e / ric /JrojlleJ alld iso/l({ch III a/ls , (a j The Oilier filllil riflhe morailla/ ballk oj' Jlargerie and

Gmlld Pacific Glaciers ill Tan flllel 011 29 J lIne 1990 is

delillealed ~J I lite fl/eII ! ~/ the acllz'e de/Josilioll slope (> 10 j , SlIblllarille conlollrs are Slt OII'1I wi//z all inlerml rif 20m , ( b) ,lfolli/ored c/zallgeJ ill 1II0railla/-ballk geome//)' Jar lite Jieriod 29 J"I.)' 1989 /0 29 J ll ll e /990 are ob/ailled 1I,Iillg all iSO/Hldl ma/) , ,·lggradalioll i:, classified as eilher

dellaic ill origill. ll'here lowlerl illjiolll oJ glacial oll/ll'ash

slrealll,l , or frOIl7 sqllee;.e //)lI sh /JI'oCfsses awa,l' from SlIell SOllr('e.l', Large ;,olles 0/ (o//ol)se ~1 1 mass-ill 0 l' fllleI) !

/no((';"les are .rhOl l 'lI ill Jroll/ oJ .I1argerie alld Grand Pacific (; /({Cier. I , The /ill/i! rif /he /lIorailla/ ballk Oil 29 ] II{JI 1989 is ShOll'lI ~v a dashed line alld !hal oJ 29 Jllll e 1990 kJI a soLid lilll' ,

HUll/er alld olher.l: "forailla/-brlll/'- ,Iedimm/ blldge/,I

DAT A ASSESSMENT

Our d a ta re present a first a ttempt to asscss qu a ntit a ti \'{' ly

th e rela ti\,(, impo rta nce o f' \'a ri o us sedim c nt ,H\' p rocesses

ill bo th d eli \'C ring sedime n t to a nd re l1l O\' ill ~ it f,'o m an in" ch'n a mi call y c h a n~in g m o ra in a l ba nks, Th e erro rs in c lud ed in th ese C's tim a tes , 'a ry d e pending o n th e processes m o ni to red , but a re es tim a ted to be \\-ithin a

factor o ('t\\'O, This is a n acce pta ble le\ 'e l o f' acc urac)' since

o ur goa l \\ 'as to produ ce a n ord e r-o f-m ag'nitud c m od el.

Suspensio n-se ttling ra te's ha lT a na tura l \ 'aria bilit y 0 ['

less th a n 8°/., u s in ~ tra ps \\' ith g rca te r th a n 95 % e fTi cieney Co\\'an, 1988 ), a nd \ \'C acco rdin g '" es tim a te a n e rro r of

\\ ' ithin 10% , J\!. eas ure l11 ents o f d e bri s co ncentra ti o ns in icc

d cmo nstra ted th a t th e d e br is cont e nt \\-ith in a n ice f~t c i es

ca n \'iu )' b\' a fac to r o['as m uch as 1,3 Hunte r a nd o th crs.

1996 ) , whic h g rca th ' e:-; ceed s th c sa mpling a nd a na h-ti ca l er ro rs or 5- 10 'Yo, Beca use o f' na tura l haza rds in thi s em 'iro nm ent , the processes o f bed load du m ping. sq ueeze/ push a nd m ass mOH'm en ts canno t be mo ni to red direc tl y ,

I ndi \' idu a l m easure m c nts m ad e fi'o m bat hym et ri c pro files

arc cst im atecl to be \\'ithin th e 90 % co nfid ence li m it.

H O\\'C\'e r. su bj eCl i\ 'e con to urin g a nd plo ning o f iso pac h maps in('l'ea ses th e like lih ood o f e rror. \\'e es tim a te th a t erro rs m a\- he as hi g h as 20 30'!!.) , \\ 'C ll \\ 'ithin ra nge fo r factor o f t\\'O acc uracy ,

RESULTS

The samplin g d es('l'ibed a bo\'e has produced a d a ta sc t th at a ll o \\ 's us to e\'a lu a te th e mo ra in a l-ba nk sedime nt

budge t. PO\\'e ll 199 1 a nd Hun te r a nd Po \\'e ll 1995 b

ha\'c repo rt ed dra m a ti c ba th ym e tri c cha nges of se\ 'C ra l

tens o f' m e tcrs a nd up to 100 m in a sing le fi e ld season, Suc h c ha nges indi cate th a t sedim e nt yields in G lac ie r Ba:' a rc the hi g hes t d oc um ented ro r bo th g lac ie ri zcd a nd no n­g lac ier ized bas i ns (H a ll e t a nd o th e rs, 1996) ,

\ 'o lum e tri c cha nges in rh e m o ra ina l ba nk (t::"B ) a rc

th e sum o r sedim e nt \ 'o lum es res ultin g ri'o m recyc ling

(R I ), inputs (N ), and resedim c nta ti o n p rocesses (111) ac ti \'l' a t a sit e rC) r a g i\'C n tim e per iod, as ex pressed b\':

t::"B = R I + N - 1II (1 )

\\'here R I co nsists o r scdim ent \ 'o lum es contributed b\'

freezc-recycling (R f l a nd squ cczc / push (RsI, a ncl N is th e 1"()lum ctri c Sllm o f' ca k e dump ing (D d), icc-c lirr m elt-o ut (D II ) ) , bedload dumping (F:Il, plume se ttlin g (~» ) a nd ach 'Cct io n of' a defo rming bed (B d) to \\'a rd s th c g ro u ndi ng

lin e (T a bl es I a nd 2 ) ,

Th e d e bri s contributi o n rro m g lac ie r ice to th e

m o ra in a l ba nk ca n be cl ne rmin ed ass um ing p lu g noli' nea r th e g lac ic r tcrminus, o nce th e d e bri s di s tributi o n is es tim a led a nd th e ice nu :-; ( Q i I has bee n ca lcula ted using :

(2)

\\'here Ut is th e <1\'C rage \'C loc it y nca r the te rminus, 111 is th e g lac ier \\' id th , a nd h, is th e te rm inus thi c kn ess (T a b le 3 ), Th e d ebri s flu :-; (D )!, ) in basa l a nd e ng lac ia l tra nspo rt is

th e produ c t o f th e ice flu :-; (Qji and th c sum 01' th e d ebris

co ncentra ti o ns (e j 0[' eac h ice rac ies \ \'e ig ht ed b~- th e ir

2 13

H unter and others : M orainal-bank sediment budgets

Ta ble 2. Sediment budgets Jar 1989- 91 in 1(/ nlyear I

Glacier debris flux : I ce-cliff melt-ou t Calve dumping Iceberg raftin g

Tota l

Glacijluvial sediment flux : Bedload dumping Plume se ttling Plume by-pass

T otal

Subglacial and ice-marginal: F reeze-recycl i ng Squeeze /push tvlass movements D eforming bed

lvl uir lvlargerie Glacier Glacier

0.6 0 .4 0.1 0.4 7.2 9 .3

13 .2 ' 10.1

44.9 144.0 11.0 79.5 6.8 40. 1

62.7 263.6

7.2 0.1 t 2.6 283.7

25 .3 500.0 2.2 2.3

Grand Pacific Glacier

0.5 0. 3 9.6

10.4

6 18 .0 98.4 55 .2

77 1.6

6.2 9 1.5

626.0 1. 3

" 105 3 - I· I An additi ona l 5.3 x m yea r IS dumped on to t l e ice-co n tact del ta.

t Based on sing le sample; treated as minimum estim a te.

fractio nal vo lume (Vj ) of the ice in th e ice cliff, such that:

n

Dg = Qi L Cj 1;) . (3) j = l

D ebris flux es calcul a ted using Equ ation (3) ra nge from 1.0 X 106 to 1. 3 X 106 m3 year 1 (Table 2).

The volume of debris released at the grounding line by melt-out is calculated by determining the melting ra te (R ) using th e W eeks a nd Ca mpbell ( 1973) equatio n:

(4)

a nd recalculating Equations (2) a nd (3) after substituting

Table 3. Glacier paramelers during of/lid)), 1988- 91

Variable Symbol Unit Grand

Average velocity v T erminus veloc ity Vt

Calving speed Vc

G lacier width W

A verage tota l h t

cliff heig h t Adva nce rate X

m year

m yea r

m year m m

- I m year

Pacific Glacier

380" 525"

480 "

1770 54-66

20- 24

Ma/gaie M uir Glacier Glacier

679 1700

810 1700

776 1770 1900 880

90 90

10 0

, Values represent portion of glacier fed only by the Fen"is Tributa ry.

2 14

R for Vt. In Equa tion (4 ), v is the bound a ry-l aye r wa ter ve locity, !::J..T is th e tempera ture difference between th e ice and water a nd I is th e leng th of th e ice cl iffin contac t with water a long th e predom in a nt d irect io n or wa ter fl ow (Syv itski , 1989), either buoya nt upwclling a t Gra nd Pac ific a nd Muir G laciers or longitudin a l currents a t

M argerie G lacier . Buoyant upwelli ng is es tim a ted at 0.03 m s 1 (Mathews a nd Quinl a n, 1975; Powe ll a nd Molnia, 1989), a nd longitudin a l currents a ppear to be a ro und 0 .25 m s I bascd on iceberg-drifting ra tes (H unter , 1994). The ice/wa ter temperature difference was mea­sured a t 2.95°C with thermistors on a remotely controlled

submersibl e (R. D . PO\.yell , unpublish ed d ata) . Based on th ese constra ints, calcu lated ice-cliff melti ng rates are 21 m yea r 1 (Gra nd Pac ifi c G lac ier), 3 1 m year 1 (M a r­geri e Glacier) a nd 20 m yea r- I (Muir G lac ier) . Estim a tes of debris released by melting ra nge rrom 3.0 x 10 1 lo 5 .8 x 104 m3 year I (T ab le 2).

The Oux of ice d ischa rged by ca lving is determined using a con tinuity equation:

Vc = VI - R- X (5)

where Vc is the ca lving speed (Brown a nd others, 1982 )

a nd X is th e cha nge in glac ier leng th (positi ve for

advance: M eier a nd others, 1980) . By repea ting th e ca lculations in Equ a tions (2) a nd (3), this time substitut­ing V e for Vt (Table 3), es timates of icebe rg raft ing a re 7.2 x 10:' lo 9 .6 x 105 m3 yea r I (T a ble 2) .

The supraglac ia l debri s flu x is the product o f glac ier

surface velocity (vd , mora ine widths (wm ) a nd surfi cial debris thi ckn ess (t ). D esp ite th e conspicuous appearance of supraglacia l mo raines, the supraglac ia l flu xes of eac h glac ier were rela ti vely low: 1.4 x 10 I to 4. 1 x 104 m 3

year I (T a ble 2). I ti s ass um ed that a ll of thi s debris is released by grav itat ional processes at tidewate r ice cliffs

by ca lve dumping (Fig. 2).

Fluvia l bedload dumping is calcul ated using isopac h m a ps produced from short-term interva ls ( IOd to abo ut lll1 o nth ) tha t reco rd point-source deposition (Hunter a nd Povl'e ll , 1995b). Use o[ sho rt-term d a ta reduces the poss ibi lity that sig nifi cant a mounts of sediment have been

removed by mass-movemen t processes, so tha t a better

und erstanding of th e magnitud e of cha nge is ac hie, ·ed . Hunter (1994) norll1 a li zed these d ata by ca lcul at ing

a ve rage d a il y accu mul a ti on rates tha t were th en ext ra ­pola ted [o r the 4month melt season (cr. Lawso n, 1993 ) . Bedload dumping was then ca lcu la ted by subtracting the

p lume-se ttling component from morainal-bank depocen­

ters indica ted on iso pach maps (e.g. Fig . 4 ). Suspension­se ttling d a ta in T a ble 2 indicate that plume se ttling onto mo ra in a l banks accounts ror 1.1 x 106 to 9.8 x 106 m 3

year I, a nd bed load dumping ra nges II"om 4.9 x 106 to 1.4 x 107 m3 yea r 1 An add i tional 6.8 x to.) to 5.5 x 106

m3 yea r 1 of sediment is transported beyond th e mora ina l

ba nk a nd d eposited downGord by plum e by-pass . jVlass-movement processes occur episodicall y a nd ca n

remove as mu ch as 0 .8 x 106 to 5.4 x 100 m 3 orsedim ent

within a 10- 2 1 d monitoring interva l a nd 2.5 x 107 m:; in

less than a month. The largest movem ents appear to

occur in June a nd dec rease by a lmost an order of mag ni tude by la te Jul y a nd Aug ust between 1989 a nd 199 1, indi ca ting instability early in the melt scason. It is

likely that consid era ble movement of sediment occurred prior to our sampling in June and may continue beyond

th e end of sampling in August. Given these limitations, a conse rva ti\'e es timate of sediment removed by mass­mO\'ement processes may be twice that monito red in the fi eld ,or a bout 2.5 x 106 to6 .3 x 107m3year 1 (T a ble 2).

Sediment transported in a deformabl e bed has bee n roughl y estimated assum ing a 60 cm thi ck deforming

layer (e.g. Humphrey and o thers, 1993) and a linear

velocity profil e (Alley, 1991 a ) . Subglacia l sediments frozen onto basall y d erived icebergs have been observed in front of Gra nd Pacifi c, J ohns Hopkins, M a rgerie, M cBrid e and Muir Glaciers in Glacier Bay, indicating that deformable sediment is present at th e soles of th ese g laciers. In addition, interstadial trees in Muir Inlet exhibit down-va ll ey deform a tion in th eir upper 60- 80 cm , indica tive of su bglacial shea ri ng d u ri ng ove rridi ng . Assuming plug-now conditions a nd average velocity of th e deforming layer of about ha lf of the surface velocity (e.g . Alley, 199 1a), or abo ut 262, 405 and 850myear 1

for Grand Pacific, Margerie a nd Muir Gl aciers, respec­ti vely, we estim a te that 1.3 x 105 to 2.3 x 105 m3 year- 1

of sediment could be transported to the g rounding lines by d eforming laye rs (Table I ) . H owever, if soft-b ed deformation is more locali zed , th e subglacial sediment Ou x will be considerably less.

The processes of freeze-recycling and sq ueeze/push are th e final components of the morainal-bank sys tem that need to be addressed. Hunter a nd others ( 1996 ) es timate th a t the tota l a mount of sedim ent moved by freeze­recycling in Gl ac ier Bay ra nges from 1.0 x 10-} to

- 3 1 7.2 x 10:> m year (Table 2), from measurements of

froz en sediment (th e lowermo t so lid subfacies of Lawson (1979)) carri ed to the fjord surface on basa ll y derived icebergs. Squeeze /push cannot be monitored direc tly, and is th erefore es tima ted by solving Equation ( I ), such that Rs is the onl y unknown. This yields es timates th a t range from 2.6 x 105 to 2.8 X 107 m3 yea r 1 for squeeze/push. Monitoring of th e Margeri e Glacier morainal bank demonstrates tha t, a lthough squeeze/push may be the mos t signifi cant process co ntributing to moraina l-bank d ynamics during the winter, it is overshadowed by mass­mO\'ement removal of sediment in the summer.

DISCUSSION

A process hi erarchy can be es ta blished for th e moraina l­bank environment based on th ese order-of-m agnitud e sedim ent-budge t a nal yses. Firs t-order processes are

g laciflu via l dum pi ng a nd mass mo ve me n ts, wh ich account for the movement of 106 to 107m3year l of sedim ent and a re th e primary controls on morainal-bank growth and coll apse. GlaciOu via l dumping accounts for 50- 80% of th e g lacia l sediment production in a single summer, while mass-movement processes may remove more th an 1.5 tim es the total a nnual sediment produced in yea rs when mOl"a ina l ba nks co ll apse (Table 2) .

Second-order processes in clud e g laciOuvia l plum e se ttling, plume by-pass and ad vec tion by a d eforming bed , which account for 105 to 106 m3year- 1, 7- 29% of the total glacia l sediment yields. Squeeze/push is a lso assigned to second -ord er processes based on the analyses of Grand

Hun/er and others: f'vlorainaL -bank sediment budgets

Pacifi c a nd Muir Glac iers. Freeze-recyc ling, iceberg­raft ing by-pa 's, ca lve dumping and ice-cliff melt-out a re

thi rd-ord er processes, wh ich acco u n t for the loca l redi stribution of 104 to 105 m3 year I « 0. 1% to 9% ) of sediment. Dowd eswell a nd D owd eswell ( \ 989 ) ha\'e obse rved that sedimenta ti on rates fi"om iceberg ra ftin g a re on ly an ord er of magni tude lower th an the total sed imenta tion rates in Spitsbergen. Th e two orders of

magnitude difference observed in Glacier Bay indica tes an increase in the importance ofglaciOu via l ac tivity in the ma ritime clim a te of south eas t Alaska rela ti ve to that in a sub-polar clim a te.

An a na lysis of the behavior of termini in Glacier Bay indica tes that recent advance and retrea t histori es are closely rela ted to sedim ent dynamics. Catastrophic retreat took place in both Muir Inlet and the ma in arm of G lac ier Bay (Fig. I ) foll owing the Neoglacia l maximum (Powell , 1980; Goldthwait, 1987 ). The last phase of retrea t ofM uir Glacier began in the l890s but acce lera ted following the 1899 ea rthqua ke (T a rr a nd Martin , 19 12;

Field , 1947 ), which may have ca used a catas trophic co llapse of its mora in a l bank a nd introd uced its g rounding lin e to deep water.

Qu asi-sta bility and subsequent advance of rvlargerie and Grand Pac ifi c Glaciers in the 20th century coincide with the formation of ice-contact deltas (Hunter and

Powell , 1995a) . Both glac ier. ha ve been adva ncing for nea rly 50 yea rs behind moraina l banks in a way simil a r to the advance o r Crillon Gl acier (Goldthwa it and others, 1963; Powell , 1991 ) and Hubbard Glacier (Mayo , 1988) elsewhere in Alaska. Appa rent ove rriding on th e mora ina l bank by Gra nd Pacifi c Glac ier during the 1970s and ea rl y

1980s resulted in ice advanc ing into deeper wa ter and a n acceleration in g lacier Oow (Hunter and Povvell , 1995a ) . Subsequen t agg radation or g rounding-line sediment has coincided with slowed g lac ier flow (Hunter, 1994) .

Sedim ent d ynamics a re clear ly not th e on ly control on th e behavior of tid ewater termini in Glacier Bay and other parts o f the world. R eid (1892 ) noted th a t termini tended to become pinned at Gord constri c ti ons rela ted to a reducti on in th e cross-sec tiona l a rea exposed to the sea , a notion th a t was supported by Fi eld ( 1947 ) a nd Po. t (1975 ) . H oweve r, Powe ll ( 1980 ) found no sta tisti ca l rel a tionship to support this id ea. R ecent quasi-stability

orthe terminu ofMuir Gl ac ier has coincided with retrea t in to a narrow stretch of Muir Inlet where ice flux can support the ca lving flux (Hunter a nd Powell , 1995b) . R a pid fjord infilling in 1986 following a peri od of quas i­sta bility resulted in grounding-line aggradation to sea level by 1992. Current ly, Muir Glacier terminates as a terres trial glacier a nd is expec ted to adyance since it is no longer ca lving. It is clea r, however , th a t the stability o f tid ewater termini in Glacier Bay can be inOuenced by sedim ent d ynami cs a t the g rounding line. T ermini ca n therefore Ouctuate independen tl y or any cl im a tic forcing .

CONCLUSIONS

D a ta presented in thi s pa pe r sho uld be use ful in e\'a lu a ting models of g lac ier sensiti\'ity to sediment d ynamics (e.g. Alley, 1991b) and eva luat ing process va riations und er different climatic regim es. In Glacier

2 15

!-fulller and a/hers : A1oraillaf-b(lIIk sedill/ eIl l blldgels

Bay, g lacinu\ 'ial sediment produ ction is as mu ch as t\\·o orders of magnitude g rea te r than the d ebri s nux and cOllStitutes 8 98% of th e to ta l sediment yie ld s. FILI\·ia l bed load dumping accounts fo r 54-80% orthe glacinuvial sediment production a nd is th e sing le mos t important

process adding sediment to morain a l ba nks. I nteractions

between th e first-order processes of g laciflu\ 'ia l dumping and mass mo\'C ment primari ly determine morainal-bank growth a nd co ll a pse, a nd modera te grouncl ing-l ine water dep th . Thro ug h ach iev ing a clea rer understand ing of ho\\' sed iment dyn a mics innuence th e sta bili ty of glaciers \\'ith

tidewater te rmini , wc ca n be tter assess th e async h ronous

beha\ 'ior o r such glac iers in Al as ka (e.g. l\ [a nn , 1986; ;"1a yo, 1988; Powell , 1991 ) and o th er reg ions. G lacia l sys tems in southeas t Alas ka a re idea l fo r monitoring sedim ent d ynamics and eva luating process re lationships since their g laciflu\ 'ia l sedim ent yiclds a re th e h ig hest

known on Ea rth , be ing linked to d enudation rates on the

order of' 10- 60 mm year 1 (H a lle t and others , \996 ).

ACKNOWLEDGEMENTS

Funcling was prO\'id ecl by DPP 88-22098 to R .D.P. ; th e

Geo logical Society or America, Sigma Xi a nd th e Department or Geology a t :\forthcrn I llinois Uni\'Crsity to L.E.H.; and USACRR EL ror D.E .L. und er th e Work U nit "Predicting R unoITand Sediment Yi eld 11'0 111 Partly G lacierized Basins". Logistica l su pport \I ' as provided by

the National Pa rk Service at G lac ie r Bay National Park and Presen T and J. Luthy, captain of the 1\ J /\ ' . \ ·lInalak.

The authors \\'ish to thank J. A. Dowd eswc ll , J. Strasser, L. Ga llo a nd one a no nymous re\·iewcr for CO l11mcnts that illlpro\'ed th e tex l. Assistance on the g raph ics was provided by L. Pau lson.

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