Morainal-bank sedirn.ent budgets and their influence on the stability

.~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\\'l S
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 \\'E LL ,
DejJarlml'lll oJ Geology, Northern llIillois
D A\, IEL
[i.S.
Arn~JI
E.
Ul1lVersi~)I,
Dek alb, IL 60115, U.S.A.
LA\\, Oi\
Cold Regions Research and Engil/eeril/g Laboralol]I, 72 L.JllIle Road. Hanover, .VH 03755, U.S.A.
ABSTRACT. rn vestiga ti o ns of g round in g-lin e sedim en tation
in fro nt of tid ewa te r
term ini of tempera te \'a ll ey g laciers demo nstra te that sedim e n t yield s a nd d ynamics
provide a second-o rder co ntro l o n g lac ier stab ility b y influ encing water d epth a t th e
gro undin g li ne. Sed iment is del i\'e red (0 th e g ro undin g lin e by two ro utes : ( I ) d e bris
tra nspo rted in , o n 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 10 7 m 3
yea r 1 of sed iment to the g ro undin g lin es . Th e g lacia l d eb ri s nux transports 10 5 to
3
10 6 m yea r 1 o f debris to th e ice cliffs, w he re app rox im a telv 10% is re leased at th e
gro und ing linp , the rem a ind er bein g tra nsported downfJ o rd b y iceberg -rafting . An
add iti o na l 10" m ol yea r 1 of sedim ent m ay be tra nspo rted to th e gro undin g lin e by
shea rin g a nd ach 'ec ri on o [ a deformab le bed.
INTRODUCTION
Process monilOring In front o f tide\\'ate r termin i o f
tempe rate \'a ll ey g laciers has been on go in g in G lac ier
Bay (Fi g. I ), Alaska, D.S.A. , sin ce P owe ll ( 1980, 198 1)
bega n definin g mod ern sed im e ntary facies and process
re lat io nships. The we ll-kn ow n g lacia l histor y of G lac ier
Bay (Fi e ld , 1947; PO\\'ell , 1984; Go ldt h wait, 1987; Hunter
a nd P OII'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. ~I ac ki e\l' i cz an d ot he rs, 1984; P owel l, 199 1; Cowa n ,
1992 ), en ab les us to e\'a lu ate re latio nships between
sed im e nt 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 in g g lac iers res ult from a
ba la nce a mo ng g lac ia l, ma rin e a nd sedim e n tary processes
a t th e gro undin g lin e. Brow n a nd o th ers ( 1982 ) noted a
re la ti ons hip between ground ing -lin e \I'ater depth and
ca h 'in g speed of Alaskan g lac iers \I'ith tid ewater termini.
Alley (l 991b ) a nd P owell ( 199 1) sugges t th a t sedim ent
dynam ics m ay reg ul ate g ro undin g-lin e wa ter depth.
Several processes (T a bl e I ; Fig. 2 ) interact to reg ulate
th e growt h and coll a pse of sedim ent pil es, o r mora in a l
banks, which accumu la te a t th e g ro undin g lin e. In this
paper, sedim ent-bud ge t d ata from three mo ra inal banks
In G lac ier Bay a re presented to provide in sig ht into th e
mag nitud es of processes affecting sedim e nt d yna mi cs in
fr o nt o f tempe ra te tid ewater term ini in so uth eas t Al as 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 gro undin g -lin e processes at Grand P ac ifi c, M a rge ri e a nd Muir G laciers ( Fi g. I ). A br ief
summ ary 0 (' the d a ta co ll ec t io n 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
distributi o n w ithin a g lac ier, sin ce th e ice cliff represe nts a
nea r- ve rti ca l, often transverse c ross -sec ti on . I cebe rg
ca kin g introduces ice from a ll positions of the ice cliff to
the fj o rd. By reco rdin g the loca tion from w hi c h each
iceberg o ri g in a ted in th e ice cliff, a ll represe nta ti ve ice
facies ca n be sa mpl ed se lec ti ve ly, in acco rd a n ce with an
ice-fac ies cl ass ifi cation sc hem e b ased o n th a t of L awso n
( 1979; Fi g . 3 ). I t was poss ible (0 d etermin e th e d e bris
distribution from d ebris co nce ntra tio ns calc ulated 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
T able I . Grolllldillg-lille jJrocesses
Process
~;~........
..
~i:
Glacier Bay ·····\
National Park . .
& Preserve
.....
D Glacier
..........
_.. Internat ional bound~';~i"""
..... Park boundary
.-""- Streams
138
o
.I l oraillalballk cOlltribulioll
Difill itioll
GLacier debris .flux:
Ice-cliff m e lt-o ut
Additi o n
Ca l\'e dumpin g
Addition
Rel ease o f d eb ri s b y
surface m e ltin g a t t he
te rminu s
Dumping of supra g lacia l
d e bri s durin g ca h 'in g
C\ 'e n ts
Tra nspo rt or d e bri s in
ice bergs beyond th e
mo ra in a l-b a nk toe
" '"
20
.........
~
\.
I ce be rg ra ftin g
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.
Clacijlllvia/ sediment .flu\:
Bedl oad dumpin g Ad d iti o n
Plum e se ttlin g
Basa l ice laye rs a t Gra nd P ac ifi c, ;\f arge ri e a nd !\Juir
Gl ac ie rs a re di sc ha rged in to fj o rd s bc lo ll' sea le\·e l. such
that th csc layc rs arc mos t o ft en o bsc J"\'Cd in ice bergs .
Fo rtun a t e ly, b asa ll y d e ri\'ed ice bc rgs te nd to ri se
\'e rticall y a nd th e ir loca ti o n of ori g in ca n be inferred.
Sa mplin g o f th ese ice be rgs pro\'id es a \'a lu a bl e co nstra int
o n basa l la\'e r thi ckn ess a nd d e bris co nce ntra tio n .
Suprag lac ia l d e bri s thi c kn ess lI'as es timatcd a lo ng
tra nsec ts nca r te rmini . l\i o ra in e thi c kn ess o n ?\/l a rge ri e
a nd Gra nd P acifi c Gl ac ie rs ra nged fr o m < I mm to 1. 5 m ,
but ra rely exceed ed th e 0 .08 m a\'e rage estim a te o f
G o ttl e r ( 1992 ). D e bri s CO \ 'CrS o f Imm a re suffi c ientl y
thick to disco lo r th e surface, lI·hereas a thi c kn ess of 1- 2 cm
produ ces a cover th a t appea rs to be nea rl y co mpl e tc o n
ae ri a l ph otog ra ph s. Thi c ke r mo ra in e co\'c rs (0 .5- 1 m ) fill
surrace cre\'asses a nd fo rm d ebris rid ges a nd m o rc-o r-I ess
co ntinu o us g ra \'e l surfaces .
Bathytnetric tnonitoring of glacifluvial seditnent
flux
:'\loored lin es with sedim ent tra ps we rc d e ployed in bo th
:VJuir a nd T a rr Inlets to mo ni to r th e spa ti a l p a tterns o f
susp ensio n se ttli ng in th ese inlets (Ca i, 1994; Hu n te r,
1 9 9 '~ ) . Tra ps suspend ed 1- 6 m a bove the sea fl oo r a re
used to re prese nt suspcnd ed nLl\·ia l sedim c nt nu x to th e
sc a noo r. \' 01um es or d epos i ted sed i m ent we re d e term i ned
by pl o ttin g a nd conto urin g se ttlin g-ra te d a ta di\'id ed into
plum e set tling (th e tota l th a t acc umul a ted o n mora in a lba nk a nd flu v ia l d e poce nte rs) a nd p lume b y-p ass
(sed im ent th a t beca m e d eposited d ownfj o rd fr o m th e
g ro undin g -lin e sys tem ; Fi g . 2) .
Fj o rd b a 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 la cier a nd
seve n tim es within 2 km o f Gra nd Pacifi c Gl ac ie r.
Ba th ym e tri c m o n itorin g ena bl es mo nito rin g o f bed load
dumpin g, squ eeze/ push a nd m ass m O\'e m ents th a t ca nn o t
be m eas ured direc tl y in th e ice-prox im a l ell\·ironm enl.
Sedim c n t \'01u m e co n tri bu ted b y th ese pro cesses is
d e termin ed indirec tl y b y subtrac tin g contribution s fro m
2 12
Addi ti on
Plum e b y-pass
R a pid de posi ti o n o r
coa rse bed load a t strca m
a nd co nduit m o uth s
Su spensio n se ttlin g fr o m
overfl ow plum es o n to
th c m o ra in a l ba nk
Fi ne-pa rticle tra nspo rt
in o \'e rn ow plum e di sta l
0 (' m o rain a l-b a nk toc
Subglacia/ alld ice margillal:
F reeze-ree),c l i ng
R e-cyclin g
Squ eeze/ p ush
R ecyc ling
:-'lass movem ents
R em O\'aI
D efo rmin g bed
Addi ti o n
Loca li zed subg lac ia l
fi'ecze-o n a nd tra nspo rt
to th e g round ing lin eScdim e nt d efo rm a ti o n
ca used by g ro undin g lin e flu ctu atio ns
Slid es, slumps a nd seclim e nt-g ra \'ity fl ows ge nc ra ted o n th e mo ra in a l
ba nk
D own -g lac ie r ach-ec ti o n
of so ft sedim e nt bel O\\'
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
termill us.
H Ull/er alld olher.l: "forailla/-brlll/'- ,Iedimm/ blldge/,I
DAT A ASSESSMENT
Our d a ta re prese nt a first a tte mpt to asscss qu a ntit a ti \'{' ly
th e rela ti\,(, impo rt a n ce o f' \'a ri o us sedim c nt ,H\' p rocesses
ill b o th d eli \'C rin g sedime n t to a nd r e 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 er ro rs
in c lud ed in th ese C's tim a tes , 'a r y d e p e ndin g 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 bl e le\ 'e l o f' acc ura c)' sin ce
Fig, 3, Genera/ ice-Jacie,1 dislribllli()11 al a lidnl'aler
lenllill/o ,
plum e sc ttlin g, ca k c dum p ing a nd ice-clin' m e lt-o ut f,'o lll
o bsen'Cd sp a ti a l a nd tc mpo ra l c h a nges in sea -fl oo r
sed im e nt \ 'o lum e , G lac illu\' ia l dumpin g a t p o int-so urcc
de p oce nt c rs is illu stra ted o n iso p ac h m a ps b \' mo un ds o r
pil es ( Fi g, +: Po\\'e ll , 199 1) , \\'h e reas m o ra in a l- b a 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 e nts 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-m o\'e m e nt p rocesscs
a re reco rd cd b y d ep ressio ns o n isopac h m a ps, Th e re fo re,
isopa c h maps b ase d o n re pea ted ba th\'l11 e tri c sun 'Cys
\\'(' re used to m o nit o r \'o lum c tri c c ha n g;es in th e m o ra in a l
b a nk ca used b l' ide ntificd scdim e nta ry proccsses (Fi gs 2
a nd 4 ),
a
o,
C.I. =20m
km
1,
b
1------.,
o ur goa l \\'as to produ ce a n o rd e r- o f-m ag' nitud c m od el.
Sus pe n sio n-se ttlin g ra te's h a 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 cie n ey
Co\\'an, 1988 ), a nd \ \'C a cco rdin g '" es tim a te a n e rro r of
\\'ithin 10 % , J\!. eas ure l11 e nts o f d e bri s co nce ntra ti o ns in icc
d c m o nstr a ted th a t th e d e b r is co nt e nt \\-ith in a n ice f~t c i es
ca n \'iu )' b\' a fac to r o['as m u c h as 1,3 Hunte r a nd o th c rs.
1996 ) , whic h g rca th' e:-; ceed s th c sa mplin g a nd a n a h-ti ca l
e r ro rs o r 5- 10 'Yo, Beca use o f' na tura l h aza rds in thi s
e m 'iro nm e nt , the processes o f bed load du m pin g . sq ueeze/
push a nd m ass m OH'm e n ts cann o t b e m o ni to red direc tl y ,
I ndi \' idu a l m eas ure m c nts m a d e fi' o m b at h ym et ri c pro files
arc cst im atecl to b e \\'ithin th e 90 % co nfid e n ce li m it.
H O\ \'C\'e r. su bj eCl i\'e co n to urin g a nd pl o nin g o f iso p ac h
maps in('l'ea ses th e lik e lih ood o f e rror. \\' e es tim a te th a t
er ro 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
Th e samplin g d es('l'ibed a b o \'e ha s produ ced a d a ta sc t
th at a ll o \\'s us to e\'a lu a te th e m o ra in a l-ba nk sedim e nt
bu dge t. P O\\'e ll 199 1 a nd Hun te r a nd Po \\'e ll 1995 b
h a\'c re p o rt ed dra m a ti c b a th ym e tri c c h a nges of se \'C ra l
te ns o f' m e tcrs a nd up to 100 m in a sing le fi e ld seaso n,
Su c h c ha nges indi cate th a t sedim e nt yields in G lac ie r Ba:'
a rc th e hi g hes t d oc um e nted ro r bo th g lac ie ri zcd a nd no ng lac ie r ized b as i ns (H a ll e t a nd o th e rs, 1996 ) ,
\ ' o lum e tri c c h a nges in rh e m o ra in a l b a 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 ), input s (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 p er iod, as ex pressed b\':
t::"B = R I
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 lllle /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 .I 1argerie 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',
+N
- 1II
(1)
\\'h e re R I co nsists o r scdim e nt \'o lum es co ntribut ed b\'
fr eezc-recyclin g (R f l a nd squ cczc / push (R sI, a ncl N is th e
1"()lum c tri c Sllm o f' ca k e dump ing (D d ) , icc-c lirr m elt- o ut
(D II ) ) , bedl oad dumpin g (F:Il, plum e se ttlin g ( ~» ) a nd
ach 'Cct io n of' a defo rmin g b ed (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 co ntributi o n rro m g lac ie r ice to th e
m o ra in a l b a nk ca n b e cl ne rmin ed ass um ing p lu g noli'
n ea r th e g lac ic r tc rminu s, 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 h as bee n ca lcul a ted usin g :
(2)
\\'h e re Ut is th e <1\'C rage \'C loc it y nca r th e te rminu s, 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 e bri s flu :-; ( D )!, ) in b asa l a nd e ng lac ia l tr a nsp o rt is
th e produ c t o f th e ice flu :-; (Qji and th c sum 01' th e d e bris
co nce ntra ti o ns (e j 0[' eac h ice rac ies \\'eig 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
lvl uir
Glacier
R for
I
lvlargerie
Glacier
Grand
Pacific
Glacier
0 .6
0.1
7.2
13 .2 '
0 .4
0.4
9 .3
10.1
0 .5
0. 3
9.6
10.4
44.9
11. 0
6.8
62.7
144.0
79.5
40. 1
263.6
6 18 .0
98.4
55 .2
77 1. 6
7.2
2.6
25 .3
2.2
0.1 t
283.7
500.0
2.3
6.2
9 1. 5
626. 0
1. 3
Glacier debris flux :
I ce-cliff melt-ou t
Calve dumping
Ice berg raftin g
Tota l
Glacijluvial sediment flux :
Bedload dumping
Plum e se ttlin g
Plume by-pass
T otal
Subglacial and ice-marginal:
F reeze-recycl i ng
Squ eeze / push
tvlass m ove m ents
D eformin g bed
" An additi o na l 5.3 x 10 5 m 3 yea r - I·IS dumped o n to t Il e
ice-co n tact d el ta.
t Based o n sing le sa mple; treated as minimum estim a te.
fra ctio nal vo lum e (Vj ) of th e ice in th e ice cliff, such that:
n
Dg = Qi
L C 1;) .
(3)
j
j=l
D ebris flux es calcul a ted using Equ ation (3) ra nge from
1.0 X 10 6 to 1. 3 X 10 6 m 3 yea r 1 (Tab le 2).
Th e volum e of d ebri s released at the gro unding lin e by
melt-out is calculated by d etermining th e m eltin g ra te
(R ) using th e W eeks a nd C a mpbell ( 197 3) equati o n:
(4)
a nd recalculating Equ ations (2) a nd (3) aft e r substituting
T able 3. Glacier paramelers during of/lid)), 1988- 91
Variable
Symbol
Average velocity v
T erminus veloc ity Vt
Calvin g speed
Vc
G lacier width
W
A ve rage to ta l
ht
cliff heig h t
X
Ad va nce rate
Unit
Grand
Pacific
Glacier
m year
m yea r
m yea r
m
m
m yea r
- I
Ma/gaie M uir
Glacier Glacier
380"
525"
480 "
1770
54-66
679
810
776
1900
90
1700
1700
1770
880
90
20- 24
10
0
, Valu es represent po rtion of glacier fed o nl y by th e Fe n"is
Tributa ry.
2 14
Vt. In Equ a tion (4 ), v is th e bo und a ry-l aye r wa ter
ve locity, !::J..T is th e tempe ra ture difference between th e ice
and water a nd I is th e le ng th of th e ice cl iffin co ntac t with
water a lo ng th e pred om in a nt d irect io n or wa te r fl ow
(S yv itski , 1989 ), eith e r buoya nt upwclling a t Gra nd
Pac ific a nd Muir G lacie rs or lo ngitudin a l c urrents a t
M argerie G lacier . Bu oya nt upwelli ng is es tim a ted at
0.0 3 m s 1 (Math ews a nd Quinl a n, 1975; Powe ll a nd
Molnia, 1989 ), a nd long itudin a l currents a ppear to be
a ro und 0 .25 m s I bascd o n ice be rg-driftin g ra tes (H unter ,
1994). Th e ice/wa ter tempe rature difference was m easured a t 2.95 °C with th e rmistors on a rem o tely co ntrolled
sub m ersibl e (R . D . PO\.yell , unpublish ed d ata) . Based on
th ese co nstra ints, calcu lated ice-cliff melti ng rates are
2 1 m yea r 1 (Gra nd P ac ifi c G lac ie r), 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 m elting ra nge rrom 3.0 x 10 1 lo
5 .8 x 10 4 m 3 year I (T ab le 2).
The Ou x of ice d isc ha rged by ca lving is d e termin ed
using a co n tinuity equation:
Vc
=
VI -
R- X
(5)
w here V c is the ca lving speed (Brow n a nd others, 1982 )
a nd X is th e cha nge in glac ier leng th (positi ve fo r
advance: M eie r a nd ot hers, 1980 ) . By repea ting th e
ca lculations in Equ a tions (2) a nd (3), this tim e substituting V e fo r Vt (Tab le 3), es timates of icebe rg rafting a re
7.2 x 10:' lo 9 .6 x 105 m3 yea r I (T a bl e 2) .
The supraglac ia l debri s flu x is the produ c t o f glac ie r
surface velocity (vd , m o ra in e widths (wm ) a nd surfi cial
d e bris thi c kn ess (t ). D esp ite th e conspic uou s appearance
of supraglacia l mo rain es, the supraglac ia l flu xes of eac h
glac ier were rela ti vely lo w: 1.4 x 10 I to 4. 1 x 10 4 m 3
year I (T a ble 2). I ti s ass um ed that a ll of thi s d e bris is
released by grav itational processes at tid ewate r ice cliffs
by ca lve dumpin g (Fig. 2).
Flu via l bedload dumping is calcul ated usin g iso pac h
m a ps produ ced from short-term interva ls ( IOd to abo ut
lll1 o nth ) th a t reco rd point-so urce depositi o n (Hunte r
a nd Povl'ell , 1995b ). Use o[ sho rt-term d a ta redu ces the
poss ibi lity that sig nifi cant a m o unts of sediment have been
re m oved by m ass-movemen t processes, so tha t a be tte r
und e rstanding of th e mag nitud e of c ha nge is ac hi e,·ed .
Hunte r ( 1994 ) no rll1 a li zed these d ata by ca lcul atin g
a ve rage d a il y accu mul a ti o n rates tha t were th en ext ra pol a ted [o r the 4mont h melt season (cr. Lawso n, 1993 ) .
Bedl oad dumpin g was th e n ca lcu la ted by subtractin g the
p lum e-se ttling compon ent from morainal-bank d epocente rs indica ted o n iso pac h maps (e.g. Fi g . 4 ). Su spensio nse ttling d a ta in T a ble 2 indi cate that plume se ttling o nto
m o ra in a l banks acco unts ror 1.1 x 10 6 to 9.8 x 10 6 m 3
year I, a nd bed load dumping ra nges II"om 4 .9 x 10 6 to
1.4 x 10 7 m 3 yea r 1 An add i tional 6.8 x to.) to 5.5 x 10 6
3
m yea r 1 of sedim ent is tran sported beyo nd th e mora in a l
ba nk a nd d eposited downGord by plum e by-pass .
jVlass-m ovem e nt processes occur epi sodicall y a nd ca n
rem ove as mu ch as 0 .8 x 10 6 to 5.4 x 10 0 m 3 orsedim e nt
w ithin a 10- 2 1 d monitoring inte rva l a nd 2.5 x 107 m:; in
less than a month. The largest movem ents appear to
occur in Jun e a nd d ec rease by a lmost an ord e r of
mag ni tud e by la te Jul y a nd A ug ust betwee n 1989 a nd
199 1, indi ca ting instabilit y early in the m elt scason. It is
H un/er and others: f'vlorainaL -bank sedim ent budgets
lik ely that co nsid era ble movement of sediment occurred
prior to our sampling in Jun e and may continu e beyond
th e end of samplin g in Augu st. Given these limitations, a
co nse rva ti\'e es tim ate of sedim ent removed by massmO\'em ent processes may be twi ce th at monito red in th e
fi eld ,o r a bout 2.5 x 106 to6 .3 x 107 m3yea r 1 (T a ble 2).
Sediment tran sported in a d eformabl e bed has bee n
roughl y estimated assum ing a 60 cm thi ck d eforming
laye r (e.g. Humphrey and o th ers, 1993) and a lin ear
velocity profil e (All ey, 1991 a ) . Subglacia l sediments
fr oze n onto basall y d erived icebergs have bee n observed
in front of Gra nd Pacifi c, J ohns Hopkins, M a rgerie,
M cBrid e a nd Muir Glaciers in Glacier Bay, indicatin g
that d eformabl e sediment is present at th e soles of th ese
g laciers. In addition, interstadial trees in Muir Inl et
ex hibit down-va ll ey d eform a tion in th eir upper 60- 80 cm ,
indi ca tive of su bglacial shea ri ng d u ri ng ove rridi ng .
Ass uming plu g-now conditi ons a nd average velocity of
th e d eforming layer of about h a lf of th e surface velocity
(e.g . Alley, 199 1a ), or abo ut 262, 405 and 850myear 1
for Grand Pacific, Margeri e a nd Muir Gl aciers, resp ec1
ti vely, we estim a te th at 1. 3 x 10 5 to 2.3 x 10 5 m 3 yearof sediment co uld be transported to the g rounding lin es
by d eforming laye rs (Tabl e I ). H owever, if so ft-b ed
d eformation is mo re locali zed , th e subg lacial sedim ent
Ou x will be co nsid erabl y less.
The processes of freeze-recycling and sq ueeze/pu sh are
th e final components of the morainal-ba nk 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 mo ved by freezerecycling in Gl ac ier Bay r a nges from 1.0 x 10-} to
3
1
7.2 x 10:> m yea r
(Tabl e 2), from m eas urem ents of
froz en sediment (th e lowerm o t so lid subfacies of Lawso n
(1979 )) carri ed to th e fjord surface on basa ll y d erived
icebergs. Squ eeze /push cannot be monitored direc tly, and
is th erefore es tim a ted by solvin g Equation ( I ), such that
Rs is th e onl y unkn own. Thi s yield s es timates th a t range
7
from 2.6 x 10 5 to 2. 8 X 10 m 3 yea r 1 for squ eeze/push.
Monitoring of th e Margeri e Glacier morainal bank
d emo nstrates th a t, a lth oug h squ eeze/pu sh may be th e
mos t signifi ca nt process co ntributing to morain a l-bank
d yna mics during th e winter, it is overs hadowed b y massmO\'e m ent remova l of sedim ent in the summ er.
DISCUSSION
A process hi erarchy ca n be es ta bli shed for th e morain a lbank environment based on th ese order-of-m ag nitud e
sedim e nt-bud ge t a nal yses. Firs t-ord er processes are
g laciflu v ia l dum pi ng a nd m ass mo ve m e n ts, wh ich
acco unt for th e movem ent of 10 6 to 107m 3 yea r l of
sedim ent and a re th e primary controls on morainal-bank
growth a nd coll apse. GlaciOu via l dumpin g acco unts for
50- 80% of th e g lacia l sedim ent produ ction in a single
summ er , whil e mass-movem ent processes may remove
more th an 1.5 tim es th e total a nnual sediment produced
in yea rs wh en mOl"a ina l ba nks co ll apse (Tabl e 2) .
Seco nd-order processes in clud e g laciOu via l plum e
se ttlin g, plume by-pass and ad vec ti on by a d eforming
bed , whi ch account for 10 5 to 106 m3 yea r- 1, 7- 29% of th e
total glacia l sedim ent yield s. Squ eeze/push is a lso assigned
to second -ord er processes based on the analyses of Grand
P acifi c a nd Muir Glac iers. Freeze-recyc lin g, ice bergraftin g by-p a 's, ca lve dumpin g a nd ice-cliff melt-out a re
thi rd-ord er processes, wh ich acco u n t for the loca l
redi stributi o n of 10 4 to 10 5 m 3 yea r I « 0. 1% to 9% ) of
sediment. D owd eswell a nd D owd eswell ( \ 989 ) h a\'e
obse rved that sedimenta ti on rates fi"om ice berg ra ftin g
a re on ly an ord er of mag ni tude lower th a n the total
sed im enta ti o n rates in Spitsbergen. Th e two orders of
magnitude difference observed in Gl acier Bay indica tes
a n increase in the importance ofglaciOu via l ac ti vity in the
ma ritim e clim a te of so uth eas t Al as ka rela ti ve to that in a
sub-polar clim a te.
An a na lysis of the beh avior of termini in Gl acier Bay
indi ca tes that recent a d va nce a nd retrea t histori es are
closely rela ted to sedim ent d ynami cs. Catastrophic retreat
took place in both Muir Inl et and th e ma in arm of
G lac ier Bay (Fi g . I ) foll owing the Neoglacia l maximum
(Powell , 1980; Goldthwait, 1987 ). Th e last phase of
retrea t ofM uir Gla cier began in the l 890s but acce lera ted
following th e 1899 ea rthqu a ke (T a rr a nd Martin , 19 12;
Fi eld , 1947 ), whi ch m ay have ca used a catas trophi c
co llapse of its mora in a l bank a nd introd u ced its
g ro unding lin e to d eep water.
Qu asi-sta bility and subsequ ent ad va nce of rvlargerie
a nd Grand Pac ifi c Glaciers in th e 20t h century coin cid e
with th e formation of ice-co ntact d eltas (Hunter a nd
Powell , 1995a ) . Both glac ier. ha ve bee n adva ncing for
nea rly 50 yea rs behind morain a l banks in a way simil a r to
th e advance o r Crillon Gl acier (Goldthwa it a nd others,
1963; Powell , 1991 ) and Hub bard Glacier (Mayo , 1988 )
elsewh er e in Alas ka. Appa rent ove rriding on th e mora in a l
ba nk by Gra nd Pacifi c Gl ac ier during th e 1970s a nd ea rl y
1980s res ulted in ice ad vanc in g into d ee per wa ter and a n
acceleration in g lacier Oow (Hunter and Povvell , 1995a ) .
Subseque n t agg radation or g rounding-lin e sedim ent has
coin cid ed with slowed g lac ier flow (Hunter, 1994) .
Sedim ent d ynami cs a re clear ly not th e on ly control on
th e beha vior of tid ewater termini in Gl acier Bay a nd
other parts o f the world. R eid ( 1892 ) noted th a t termini
tend ed to become pinn ed at Gord co nstri cti ons rela ted to
a redu cti on in th e cross-sec ti ona l a rea ex posed to th e 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 ece nt quasi-stability
orth e terminu ofMuir Gl ac ier has coincid ed with retrea t
in to a narrow stretch of Muir Inl et where ice flux ca n
support th e ca lving flux (Hunter a nd Powell , 1995b ) .
R a pid fjord infilling in 1986 foll owing a peri od of qu as ista bility res ulted in grounding-lin e agg radation to sea
level by 1992. Current ly, Muir Gl acier terminates as a
terres trial glacier a nd is exp ec ted to adya nce sin ce it is no
longer ca lvin g . It is clea r, however , th a t th e stability o f
tid ewater termini in Glacier Bay can be inOu enced by
sedim ent d yn a mi cs a t th e g rounding lin e. T ermini ca n
therefore Ou ct uate ind ependen tl y or any cl im a tic forcing .
CONCLUSIONS
D a ta present ed in thi s p a pe r sh o uld be use ful in
e\'a lu a ting models of g lac ier se nsiti\'ity to sedim ent
d yn a mics (e.g. Alley, 1991b ) and eva lu at ing process
va riations und er different climati c regim es. In Glacier
2 15
!-fulller and a/hers : A1oraillaf- b(lIIk sedill/ eIl l blldgels
Ba y, g lacinu\'ial sediment produ c tion is as mu ch as t\\·o
o rders of mag nitud e g rea te r than the d e bri s nux and
cOllStitutes 8 98 % of th e to ta l sedim e nt yie ld s. FILI\·ia l
bed load dump ing accounts fo r 54-80% orthe glacinuvial
sediment production a nd is th e sing le m os t important
process adding sedim e nt to morain a l ba nks. I nte ractions
betwee n th e first-order processes of g laciflu\'ia l dump ing
and mass mo\'C m e nt primari ly dete rmin e morainal-bank
g rowth a nd co ll a pse, a nd modera te grouncl ing-l in e water
d ep th . Thro ug h ach iev ing a cl ea re r understand ing of ho\\'
sed im ent dyn a mics innu ence th e sta bili ty of glaciers \\'ith
tidewater te rmini , wc ca n be tter assess th e async h ronou s
beha\'io r o r suc h glac iers in Al as ka (e.g. l\ [a nn , 1986;
;"1a yo, 1988; Powell , 1991 ) and o th e r reg ions. G lacia l
sys tem s in so uth eas t Al as ka a re idea l fo r monitoring
sedim e nt d ynamics and eva luating process re lation ships
since t heir g laciflu\'ia l sedim e nt yiclds a re th e h ig hest
known o n Ea rth , be ing linked to d e nudation rates on the
order of' 10- 60 mm year 1 (H a lle t and others , \996 ).
Statc L· ni\'l'rsit\·. BYI'd Po lar R esea rch Cct1lcr. 5 16. (i\ l iscellan eo us
Publi cat ion 236. )
(;oldth\\'ait. R . P .. I. C. :\ l c K e llar and C. Cronk . 1963. Flu ctuations o r
C rill o n Cbcicr "s tc m. so utlt eas t .\ Iaska. I . IllS Bill! .. 8 ( 11.62 7·1.
Cot ticI'. P. 1992. Icc- ral'ting at a tcmpera tl' tidc\\,a ter g la c icr, i\ l cBrici c
Iltl ct. Glacier B"y. Ala s ka.
i\ I. Sc. th esis, N ort hern Illin o is
Cni\Tl"sil y.
l-I a ll ( l, B., L. E, Hunter ancl j. Bogen . 1996. R a tes or eros io n and
scdimcnt c\'ac ua ti on by g la c ic rs: a tT\' ie\\' or tlt e ev id ence. Global fllld
Plallelal) ' Challge. 12. 2 13 233.
l-Iulllphrcy. N .. B. K a mb. :\ 1. Fa hn es tock and 11. Engl'ih"rdt. 1993 .
Characll'J'i s tics or thl' bed 0 1' tit" I,mer Culumbia GlacicT .. \ Iask<!. J.
(:fo"hr.\. N I'\'. 98 BI . 837 8·16 .
I-Iunler. L. E. 199~·. Grounding ,lin e sys tems and g lac ier ma ss balan ce or
modern tempe ra le glac ie rs and th eir effect o n g lac ie r slab ility . ( Ph.D.
thesis. \'onhern Illino is Lni\'Crsity. )
H u ntc r. L. E. a nd R . D. Po\\'c1 !' 1995 a. Climati c controls o n g la c ier m ass
",t1ance in Glacier Ba y :\' al io l1al Park and PrescnT. Alaska. 111
Engs trol11. D .. ed. Proceedings 'l/IIie 77,;rrl (;Ia ria lJ(~Jl SriNI(l' .~I'IlII)o\iflm.
.\n chorag-c. AK , LT.S. I) r partment of lhe Intcrio r. .'\ al io llal Pa rk
Set'l'ice. ,1·6 .') I.
Hut1lc r, L. E. and R. D . Pc)\\·cll. 1995b. Elkcts o ri ec ' prox illl a l sediment
clynamics on th e sta bilit\· of' \iuir Glacier. Glacier Ba,', Alaska. III
En,!(st rom. D .. I'd. Proreedillgs oJ Ihe Third (;lacier Br£1' Sriellcf S),III/lOJilllll .
,\Ill'hora gc, AK , U .S. Depa rtm e nt
the i lllcri or. Natiunal Pa rk
Sl·n·irc . 29 37.
1-1 ""tlT. L. E.. R . D. Po\\'e ll and D. E. La \\'soll. 1996. Flu x or debri s
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4 2 1+0 , 110- 122.
or
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
De partm e nt or Geol ogy a t :\forth c rn I llinoi s Uni\'Crsity
to L.E.H.; and USAC RR EL ror D.E .L. und er th e Work
U nit "Predicting R unoITand Sediment Yi e ld 11'0 111 Partly
G lacierized Ba sin s". Logi stica l su pport \I ' as provided b y
t he Nationa l Pa rk Service at G lac ie r Ba y Nationa l Park
and Prese n T and J. Luthy, ca ptain of the 1\ J /\ ' . \ ·lInalak.
Th e authors \\'ish to thank J. A. Dowd eswc ll , J. Strasser,
L. G a llo a nd one a no n ymou s re\·iewcr for CO l11mcnts that
illlpro\'ed th e tex l. Assistance on the g raph ics was
provided by L. Pau lson.
Lt\\'so n, D. E . 1979. ,\ scclim en lo logica l a nah'sis of' the weste rn te rminu s
regio n ol' th e l\ l a tanu ska Glacier .. \ Ia ska . CRREL Rep. 79-9.
La\\'so n. D . E. 1991. Glacio l,,·cl ro logic a lld g lacio h\·dra uli c dfccts o n
rLlIlOn~ and sedimellt ;. icld in g lacicr iz('d basins. CRR!:" .. \!o"o,~'" 93-02.
:\Ia ck ic\\'icz, :\ . E .. R. D. Po\\' ell . P. R . Carlso ll " lid B.F. :\ [ollli". 1 98~.
I n lc ri a nl in at('cI ice - p rox im a l g-Ia c im a rin e seclimcnts in ~ r uir I llle t,
.\Iasb . . I/ llr. Geol .. 57 I 4 1. 11 3 I<}I .
\ 1<1 nn. D. H. 1986. Rcliahi litv or a fjorcl g lac ie r's lIuClu a ti ons 1'01'
pail'ociim'lIic reco nstructi o lt s. QjllIl. Hes .. 25 ( I ), 10 2·f.
\ Iathe,,·s, .J . B. and .\ . \ '. Quinlan. 1975 . Sea so nal c haracteristics or
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Rl'.I. Horll"l/ CIIII .. 32 , 10 ). 1693 1703.
:\ Ia\·o. L. R . 1988 .. \ ch ·ancc o r I-Iubbard Glacier ""d e losurc or Ru ssc1 1
cll\'ironmenta l e ffe c ts a nd h azarcb ill t lt e Ya kutat
Fi ord .. \ Iaska
area. C.S. (:eol. Sill,/" eire. 10 16 . 4 I G.
\ki e r. ~ l. F. alld 7 olhers. 1980. Pred ict ecl timing "f'the disintegrati o n
th e
10111'1'
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C. S. (;eol.
or
SlIn'. O/Jell
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or
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