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.~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 ).
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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.
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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
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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
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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
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!-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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