a 20 km long, 300 m deep trench created beneath

0/ G/aci%gJ' 22 1996
In ternati o na l G lacio logica l Socie ty
Alllla/s
D
Scales and rates of glacial sediment removal: a 20 km long,
300 m deep trench created beneath Breioamerkurjokull
during the Little Ice Age
HELGI B.J0RNSSO .\l
Seiella IlIslilllle, C/liz'ersi~J' of /re/alld, DUllhagi 3, 107 Iceland
ABSTRACT, A 20 km lo n g , 2- 5 km wid e tren ch , w hi c h e:-;te nd s to 300 m be lO\\'
sea le\'e l, is beli eved to h a \'e bee n crea ted b\' remO\'a l o f sed im e nt durin g th e ad"<ln ce
of Breio a m erkLllj b kul1 in th e Little l,ce A g~i Be t\\'ee n 1732 a nd 1890 , th e g l ~c i e r
ach-a n ced by 9 km , co \'ered a n a rea o f 4S km - a nd exca , 'a ted a \'0 1u m e of S x 10' m :l,
eq ui\'alent to a n C\ \'erage o f 110 m Sl \ 'er this a rea , A ve rage sedim ent-rem ond ra te
durin g th e 158 yea rs \I"as 32 x 10() m .l a I o r 0 ,7 m a I km 2 a \'e raged O\'e r the a r ea
co\'e red b y th e ach-a ncin g' g"lac ie r. C a lcula ted OHT th e \\'ho le c1ra in aQ:e a rea o f th eI
eas ts rn bran c h o f th e g laC ier o r 750 km -, th e d enuda tIOn ra te \I'o uld JJe xI I 0 - m a
km - , Flu\'la l processes a r e es tim a ted to ha\'e ea rn ed a b o ut 30 x 10 m a ,a nd th e
sedim ent Ou :-;es w ithin th e ice a nd by th e d efo rmin g sub g lac ia l till a re es tim a ted to be
6
I y , T Ile aye rage secI Im
~ ent co n ce ntra tI,On In
, t Il e g IaC la
' 1
I 0'') a ncI 10 m :J a I , res p ec tl'"e
strea m s \\'o uld h a \'C bee n a bo ut 10 kg III 3 Su ch co n ce ntra tio ns h a \ 'C b ee n m eas ured in
Ice la ndi c ri\ 'C rs durin g j o ku lhl a ups a nd surges, Se\'C ra l surgin g n 'e nts look pl ace
durin g th e a dv a n ce or Breid a m erkurj(ikull , a nd j okulhl a ups dra in reg ul a rl y b en ea th
th e g lacie r from ice-d a mmed m a rgin a ll9- kes , Th e prese nt ra te o f tra nsp o rL, a lth o ug h
3
co nsid era ble, seem s to be a bo ut 10 x lO b m a I, o r whi c h 30 % is tra n sp o rred b y th e
ri\'C r to th e sea a nd 70 % is dump ed int o a proglac ia l la ke ,
•
' :
.......
')
INTRODUCTION
Bre io a m e rkulj o kull is loca ted in th e hig hl y m a ritim e
clima te o f so uth ea st I ce la nd a nd is on e o f th e m os t ac ti ve
o u tl e t g lac iers o r th e ice ca p V a tn aj okull (Fi g , I ), T h e
o u t le t fl ows d ow n to th e coas ta l sa nd ur delta Breioa m e rkursandur, w hi ch is be li e\ 'ed 10 b e co mposed o f a wed ge o f
la le-\\'e ichselia n a nd H o loee n e sedim ents (Bo ul to n a nd
o th ers, 1982 ) , Sin ce th e se ttl em e nt o f'T ce la nd 1100 yea rs
ago , la rge g lac ie r ,'a ri a ti ons a nd m aj or g lac ia ll y indu ced
ch a nges in la nd sca p e h a \'e take n pl ace in th e Bre ioa m erkulj o kull a rea , At t h e tim e of th e se ttl e m ent mu c h la nd
w hi ch is now la rgely eO\'e red b y Breio a m erkurj ii kull was
\'ege ta ted a nd occ upi ed b y se\'C ra l fa rm s, Durin g th e cold
p eri od b e (\\'ee n th e :vliddle A ges a nd th e e nd of' th e las t
ce ntury (th e Littl e I ce A ge ), (h e g lac ie r o utl e t a d\'a need
m ay b e 10- IS km , d ev as ta tin g \'ege ta ti o n a nd ruinin g th e
fa rm s, This ad va n ce a nd its co nsequ e nces a re we ll kn o wn
a nd hi sto ri ca ll y d ocum ented in m edi e\'a l a nn a ls, count)'
r eco rd s, j o urn a ls o f c hurc h es , t r a \T I accounts and
geog r a phi ca l m a ps (H e nd e rso n , 181 8 : Thi e n e m a nn,
1824 ; Pi lsson , 1883, 1945; Th orodd se n , 1892, 1905/06;
Bcl ro a rso n, 1934,; Th 6 ra rin sso n , 1943 , 1974; :\Tb rlund ,
1944; Si g urosso n, 19 78 ) , This saga is prese nt ed h ere ,
During th e re t rea t o f th e g lacier in th e 20 th ce ntur y,
la nd h as r ea pp ea r ed , rn'ea lin g c h a n ges sin ce lh e
se ttl em e n t (see H o w a rrh a nd Pri ce, 1969; Pri ce, 19 71.
1982 ) , Th e m os t o utsta ndin g n e\\' fea ture is a la rge
proglac ia l la ke Ub kul sa rl6 n ) w h er e th ere was fa rml a nd
•
t
LJ
,)
bef'o re th e a d\'a nce , Thi s la ke bas in is co nsid e red to h a \'e
bee n e:-;ea\'a ted b y th e a d va n cin g g lac ier. a nd bea rs
w itn ess to e:-;ce ptio n a ll y hig h ra tes o f' sedim ent rem O\'a l
since a bo ut 1800 (Bo ulto n a nd o th ers, 1982 ) ,
N ew ra di o-ech o so undin gs o f th e g lac ier itse lf show
th a l ice is still hidin g la nclfo rm s crea ted durin g th e
neoglac ia l ad\'a nce, Th ese c ha nges in la nd sca p e g i\'C a n
e\'e n m o re sp ec tac ula r in sig ht in to th e la ndscapin g p owe r
o f th e g lacier: a 20 km lo ng trenc h, 2- S km wid e, w hic h is
up to 300 m below sea b e l. We beli e\'C this trench to ha \'C
bee n crea ted b\' remO\'al of sedim ent durin g th e ach-a nce of
lh e g lac ier durin g th e Littl e I ce Age, i, e, a ft er j-\D 1400,
Thi s pa p er d escr ibes th e evid e nce [o r this statem elll
and di sc usses g la cia l. sub g la cia 1 a nd flU\'ioglac ia l processes r es p o nsibl e for th e exca \'a ti o n o f th e tren ch a nd th e
tra nspo rt o f sedim en t to th e sea,
DESCRIPTION OF BREIDAMERKURJOKULL
AND BREIDAMERKURSANDUR
2
Breio a m erkulJ b kull (to ta l a r ea A = 9 10 km , acc umul a 2
2
ti o n a rea A c= SOOkm , a bla ti o n a rea A ,, =4 10km ) is
b o und ed to th e weS l b y th e hu ge \'o lcan o Orce f'a j o kull
(2 11 9 m a ,s,I. ), to th e no rth by th e nun a ta ks .\l;h 'a b yggoir
a nd Esjuu o li a nd to th e cas t b y \ 'eoura rd a lsuo ll , a ll
reaching ele\'a ti o n s o r 111 0 re th a n 1300 111 (Fig , I ) , Th e
n orth eas tern p a rt o f th e o utl e t dra in s fr o m th e ice d om e
Bre io a bun ga thr o ug h a p ass b e tw ee n Esjuu o ll a nd
1+ 1
BjiimsSIIII : S((llcs {{lid rales oJ glacial sedimelll remo val
- r--~
H
_
1-
- '-
v
"
ice divides
o
_ _ ' ~'-- "_- -
c
mountains
5 km
o
_ I
16° 20'W
L-~~~_
)J
I
- - -' ' --
I
-
Fig. 1, Breioalllerklllj6kuIl alld surroulldillgs. Th e oullel
I S dil'l&d ill lo Ihree brallches ~) I Iwo medial moraill es.
COli lour ft eigltis are giuetl ill melres abore sea Ifl'el .
\' eoura rd a lslj oll. Th e prese nt equilibrium lin e li es a t an
ele\'a ti o n o f 1100 m . Th e c lim ate is hi g hl y m a ritim e \I'ith
n e t m ass b a la n ce o f 4- 7 m a I in th e upperm os t
acc umul a ti o n a rea a nd a bl a ti o n o f abo ut 10 m a I a t th e
terminu s (50 III e lenttion ) wh ere the a nnu a l precip ita ti o n
is a bo ut 2000 mm .
Breio a m erkul:j o kull ca n be di\'ici ed into three lo bes o r
bra nc hes w hi ch a re se pa ra ted b y m edi a l Ill o ra in es that
o ri g in a te from th e nun a ta ks ~ra\ ' a b yggoir a nd Esjuuo ll
a nd rela tecl \ a ll ey g lac ier (Fi gs 1- 3).
5 km
~
~
Fig. 2. Th e bedrock lo/)ogra/)/~)' oJ lheioall1erklllj6kllll,
radio-ccho SOlllldillgS ( Bjiimssoll alld olh ers,
1992) . COlllours ill melres.
s un' ~ ) l ed ~) I
ex tend s below sea !c\'e L An o thcr d epression below sea
le\'e l is loca ted 2 km no rth or Breio a rl o n a nd c1 0es n Ol
reac h th e present glacier m a rgin. A n icc-cl a mm cdla ke b y
Breio a m e rkurfJ a ll , dra in s benea th th e wes tern bra nc h
cl O\ \'ll to Breio a rl o n (Fi g. I ).
Western branch
Th e \\ es tern bra nch (A = 160 kIll ~ , Ac = 60 km 2 , A a = 100
km 2 , a nd 18 km m eas ured a long a ce ntra l fl owline ) fa ll s
stee pl y ('ro m th e no rth eas te rn slo pes o f O nefaj o kull
to \\'a rci s a ca kin g fr ont a t the p roglacia l la ke Breioa rl o n
( 15 m a.d .). The ri\'e r Breio a n O\ \·s fr o l11 th e la ke . Th e
bra nc h is bo und ed to th e north b y the nun a ta ks a nd th e
m ed ia l m o ra in e o f ~ la \·ab yggo ir. Th e bed is i rreg ul a r,
showin g sig ns o f substa n tia l \'a lley eros io n , a nd o nl y
benea th th e o ut erm os t +km o f th e o utl et d oes th e bed
fo rm a sa ndur pl a in below 100 m c1 C'\'a li o n (Fi gs 2 a nd 3;
Bj o rn sso n a nd o th ers , 199 2 ) . N o rth ea s t\\'a rd s Cro m
Breio Arl o n a tre nc h , 7 km lo ng a nd up to I km w id e,
142
Fig . 3. Pcrs/JeClil'f plols a}' 1111' sub -ice l o/}og/{//)/~) I a}'
Breioamerklllj6lwff. " iell' to wards tfle 1I0rlft . ' "ert/m f
naggemlioll len lim es llOri.:olllaf.
BjO'r/l.\soll : Swlfj and ralf,\ ~/gla(ial ,If(lillll'lll relll ol'fll
Central branch
2
2
Th e centra l bran ch (A = 21 0 km , A c = 100 km , A " = 110
km 2 , and 26 km lon g ) is bounded b y th e nunata ks
lVIavabyggo ir and E sjuGo lI and their m ed ial m orain es ,
Most oC the b ed is form ed of g lac ial ly sc u lptured
m o unt a ins, but a 3 km wid e p la in b elow th e 100 m
co nto ur stre tc hes 10 km up- g lac ier ben ea th th e to ng ue,
No ri ve r dra in s fr o m th e fro11l as th e m eltwater is drained
toward s th e easte rn bra nch,
Eastern branch
Th e eas tern branch (A = 540 kn1 2) Ae = 340 kn1 2. Aa = 200
km 2 ) dr a in s fr o m Breio a bun ga ( 1500 m a, s,I. ), 34 km
down to a ca h 'in g front at th e prog lac ia l la kc j o kul s<i. r16 n, The acc umu lation area is situ a ted in th e ce ntra l part
o f V a tnaj 6 ku ll w here th e glac ier bed is abo\'e 500 m a ,s,l. ,
from where the ice flows stee p ly d ow n to a low -lying
a bl a ti on area, About h a lf o fth e bed in th e a bl at ion a rea is
b elow 100 m , a nd thi s COlllo ur ex tend s 25 km up-glac ie r
fr o m th e fron t ( Fi gs 2 and 3 ) ,\\'e sugges t from thi s sub-i ce
inform a ti o n that a sa ndur fi eld (a sedim e nt \\-ed ge ,
reac hin g to th e nunataks E sjuU611 and th e centra l
hi g h la nd n orth eas t o f E sjuG 6 11 , was [orm ed in p os tglacial
tim es (Bo u lto n a nd o th ers, 1982 ) , The prese nt g lac ier bed
in t hi s a rea, however, slopes ra p id ly d o wn a nd fo rm s a
tre nc h w hi c h ex tend s be low sea Ic'T I OITr a di stance o f
20 km d ow n to j okulsarl6n, Th e maximum d epth or this
trench is 300 m below sea b 'e l, 2- 3 km up- g lac ier ri-om
J o kulsarl 6 n " 'here th e ice is 300- 400 m thi c k,
Th e a rea exposed from be n ea th th e re trea tin g glac ier
sin ce th e Littl e I ce Age co nsists o f a co nso lid ated mix ture
or ti ll a nd sa nd , Sei smic refract io n o n Breioamerkursa ndur near th e sn o ut of th e g la cier (Bogad 6 ttir a nd ot he rs,
1986 ) a nd re fl ec ti o n pro fi les in th e la ke J 6 ku lsa rl6 n
(Bou lton a nd o thers, 1982 ) su gges t th a t th e m a rg in of th e
g lac ier a nd th e fo r eland are und erl ai n by g lac ia l drift, In
th e coas ta l a rea the b edro ck is ove r la in b y m o ra in es (40 m
thi c k ) a nd to pped b y sa nd, I n front oC th e la k e
j o kuisa rl 6 n the d epth d own to [he bedrock is 130140 m , a nd th e m aximum d e pth of th e presen t lake is
190 m, f n o th er parts ofBreioam e rkursa ndur th e depth to
[h e b edroc k is 30- 80 m, Th e trenc h is loca ted in a
gla cia ll y erod ed b edrock \'a ll ey w hi ch ex tend s as Breio a m erk u I'd jLIJ) so u th eas t of th e ri , 'e r mou rh of J ok Uis ~l,
60 km to th e ed ge of th e contin ental edge (see Bo ulton
a n d oth ers, 1982, 1989 ) , \ Ve sugges t that th e trench
ben ea th th e g lac ier ma y reach d ow n to b edroc k in th e
d ee p es t parts , but the sid es of th e tren c h co nsist of
u nconso l id a ted sedi ments,
Hydrology
W ater dra ins to th e proglacial la ke J 6 kul sa rl6 n fro m a
2
Th e g lac ier receiYes
b asa l draina ge a rea of 750 km
e no rm o us a m o unts of water thro ug h rainfall, l\ I aximum
ra infa ll intensit y recorded is 230 mm in 24 h a t the
m a rg in, Surf~lce runorr fl ows into m o u lin s d ow n to th e
g lacier base in th e 100N er three-qu a rt ers o f the ablation
a rea , Th e river J 6 kulsa flo\\'s fr o m the lake j ok uls{lrI 6 n,
Th e m ea n a nnu a l disch a rge from the g lac ie r to [h e ri,'e r
J o ku lsa is 130 m 3 Si , but daily peak \'a lues of 750 m 3 s I
ha\'e b ee n reco rd ed, Th e fl OIr of ,,'ater from th e g lac ie r
re aches its hi g h est le,'els b e t"'C'e n jul y a nd midSeptcmbe r. Di sc ha rge m ay (]U ClU<ltc rapidly a nd j o kulhla ups arc fi-cq ul'nt from t"'o lak es lI'hi c h arc damm cd at
th e eastern m argin of Breioal1lerkul jo kull.
Only small a m o unts of supra g la c ia l d ebri s are fo und
on the g lac ie r, and nea r'" a ll th e sedim e nt suppli ed to th e
strea m s appca rs to b e d eri, 'C'c1 ri'o m subg lac ia l drift. \[ os t
of th e sedim ent loa el is prese ntly clump ed into the la ke ,
T y pi ca llv, suspe nd cd -sedim ent loads ca rricd b,' th e ri, 'C' r
J o kul sa a rc 1. 5 kg m :l (Ri st, 19.37; pe rso nal cOl1l muni ca ti o n ri-o m L. S, Z 6p h6niasson , 1995 1, D a ta o n b ed load arc
not a,'ai lable ,
CHANGES OF BREIDAMERKURJOKULL AND
BREIDAMERKURSANDUR IN HISTORICAL
TIMES
Changes of glaciers
During th e c lim a ti ca lly mild period of th e set tl em e nt o r
Ice la nd (,\1) 87+- 930 a nd fo r se' T ral ce nturi es th erea ft e r.
th e g lac ie r o utl e ts of \' atnajoku ll \rere substanria ll :'
sho rte r th a n they h a\T bee n fo r th e las t three to fou r
ce nturi cs (sce Th ora rin sso n , 194,3 l. T () m asson a nd
\ ' ilmuncl a rdottir ( 1967 ) be lic,'C th a t th c o utl e t g lac iers
o f \' a tnaj o kullm a>' h a,'e bec n up to 20 kill further b ac k a t
th e tim c of the se ttl em cnt than th ey nOli' a rc, Th e la nd
a dj ace nt to a nd no'" cO\'ered b:' Breioame rktllj 6 kull ,,'as
se ttied a nd occ upi ed b y f~lrm s a nd co\'e r ed " 'ith
' '('ge tati o n, At th a t tim c sCl'era l ,',t1Ie\' g lac iers ma,' ha,'e
nOIl'eel d o,,'n to Brcio allle rkur,a ndur. but most of th e na t
area be lo\\' th e 100 m con to ur of th e bed ,,'o uld h alT been
free of ice , In rh at case, n o ice-d a mm ed lake ,,'o uld ha\T
existed in \ 'eourardalur, nor ,,'o uld a lake be dammed in
Breioa m e rkurG a ll (Fi g, I ) ,
A c lim a t ic d e tcri o ra ti o n se t in during th e J 2th
ce n tUrl', Fro m a bo ut 1300 to 1900 thc c lim a te \\'as co ld ,
es p ec ia ll y afte r 1600 ,,'hic h ,,'as pres umably th e co ld es t
period in postglacial rimes, Eyth 6 rsso n (see Ahlm a nn,
1953 , p, 3 1) estim a ted that th e rirn lin e o n so uthern
\ 'a tn aj o kull d ec lin ed from a bout 11 00 m arou nd , \J)
1000- 11 00 to a bou t 750m elc,'ation !J,' 1650, Thi s
ca used g lacie r ach'a n ce " 'hi c h g radua ll >' d estro;,ed th e
fa rmin g a rea, Prospc rous f ~\rm s " T re still reported in 13+3
(Ba ro a rso n, 1934, p, 28 ), bu t th e g lac ie r starred to
a d\' ance a fter ,\J) 1400, with accele ra tin g pace in th e
17th ce ntur y until Breioam erkulj 6 kull reac hed its posl g lac ia l maximulll in th e 1890s, '1''''0 [arm s, Fjall and
Bre io{l, o n th e ,,'cs te rn sid c or Breioal11crkurs:\ ndur " T re
a ba ndo n ed in th e 1690s and sho rth' afler ,\LJ 1700,
respeCl i'T ly, b eca use th e g lac ier ach 'anced on'r th e
paslLtres nea r the farm h o uses, On the eas te rn sid e th e
farm Brennh 6la r ,,'as O\'e ITUn by ice in 1753, a nd Fell ,
" 'hi ch " 'as protec ted h,' a large moraill e, ,,'as destroyed in
1865 ,,'hc n the m o ra i ne was breac hed by I h e' g lac ie r
(Th o roddse n , 1892, 1905 /06 ) ,
The , 'a riation s of th e cas te rn bra n c h of Bre io amer kLlIj 6 kull , by th e prese nt ri\ 'C r o utlet of j 6 kul S<1, a rc
s Ull1mari ze d in Fi gu r e 4 , Altoge ther th e g lac ie r
ach 'an ced b,' 9 km b e t\'Te n 1730 a nd 1890, Se\T r a l
r ap id ach 'a nces ha\'e been de sc ribed, lI' hi c h a r e
1+3
Bjtimssoll : Sca{eJ alld rales
~f glacial
sedimenl remaNd
"olum e remm'ccl [i'o m th e tren ch. Th e ma lll pa rt of the
excavatecl m a terial was tran sported by prog lac ia l ri,'ers to
th c sea, building a n o utwash fan bcyo ncl th e mo uth of th e
ri,'er j o kulsa. Fro m 181 7 to 1904 th e coastlinc mm'ed
sea wa rd s b y I J 00 m du e to acc umul a ti o n o f sedim ents fro m
the g lac ier in excess o f what th e lill o ral cu rrents were a bl e
to ca rr y away (Fi g. 5 ) .
10000,------
HI' f' i 6n lIl e rkur jo k ull :
~
Pos llion of
gla c ier margin
8000
aL
Jtlku lsa
I
i 60001
ill
~ 400Jj
.'!'
o
2000l /,/ !
o
'700
~
i
- -,
,
1900
1800
S -"" Surge reponed
I
7000
Year
\
\
Fig . ..f. Changes ill Ih e /)osiliOJl oJ Ihe Jionl oJ
Breioamerkll1}iikuLl al Ihe ] iJkllLsa riuer (w ilh reference
10 Ihe /Jo silioll ill 1735) .
N
~
l
JOKuisar:01'
i Cui rent lake
~
Bre ioamerkur,
sandur
d e finit c ly s ur gcs . In 1794 , th e eas t e rn p a n o f
Breio a m c rkurj o kull a dv a nced 400 m be twee n \\' hitsun
a nd Au g us t, acco rdin g to Pa lsso n ( 1945 , p . 358 ) . Durin g
1820 th e g lac ie r ad\'anccd by I km (in so m e d ays b y 48 m ) according to Thi e ne m a nn ( 1 82 '~ ) , " 'hile large
m asses o f " 'ater pourcd fr o m und crnca th th e g lac ie r ,
whic h s ta rt ed to re trea t sho nl v a f"t e rward s. Inhabita nts
told Thi c nc mann th a t suc h rapid ach 'a nccs occurrcd
a pproxim a te ly evc ry fifth yea r , (o ll owed b y rcccss io n.
Durin g the period 17 94- 18 20 a lt e rn a tin g rap id
ach 'ances a nd slow recession s took p lace (Th 6rarin sso n , 1943 ) . [n 1869 a n ach 'a nce simil a r to th a t or 1820
took pl ace a nd th e g lac ie r a lm os t reached th e sea
( Noro a n(~1r i , 1870 ) . In 1875 a rapid a dvan ce was
rc ported a nd th e inh a bita nts fea red th e g lacie r wou ld
reac h the sea (\ Va tts, 1876 ) . Th e las t a d va nce of thi s
nature " 'a s reported in th e ea rl v yea l's 0[' thi s ce ntur v
(persona l co mmuni ca ti o n from S . Bj o rn sso n , 1995 ).
Changes in the sandur field
L a rge la nd sca pe cha nges hcl\T taken pla ce o n Breio a m e rkursand ur durin g hi sto ri ca l tim es in associati o n with
th e g lacier , ·ari ations. Th e area ha s cha nged ri'o m a
\'cgetated a rca (call ed Breioa m o rk ) La a g lacia l sa ndur
a rea. Th c presence of la rge la kes, likc th e o nes fo und in
th e prcsent prog lac ia l a rca. is neither me ntion cd in
w ri tte n reco rcl s nor show n on ma ps (P:d sso n, 1883, 1945;
Th o rod d sCll , 1892, 1905/06; Th 6 ra rin sso n , 1943; sce
.\!o rlund ( 1944 ) a nd Sigurosson ( 19 78 ) fo r info rm a tion
abo ut the m aps of Kn off (from 1732 ). Pa lsso n (fi'o m
1794) , Gunn laugsson ( fr o m 1834 ) a nd the D a ni sh
Gcodetic Sun'ey (fro m 1903 /04)) . Therefore we co nclud e th a t the trench was excava ted durin g th e g lac ie r
a d nll1ce of th e Little I ce Age.
Th e "olullle of th e proglac ia l Ill ora in es which were
piled up is a negli g ible p ro po rti on o f the ,'o lulll e rem m'ed
ri'o lll th e g la cie r bed when th e trcnc h was fo rm ed. Th e
,'olum e o f th e mora in es is onl y of th e o rd er o f J 5 x 10 1; m:;'
( 15 m hig h, 200 m in th e clown-g la cier clirect ion and a bo ut
5 km wide ac ross th e front of th e trenc h ). Ass uming.
eonsen 'a ti,'cly, th e prese nt porosity ocO .2 fo r this d e pos it ed
ma teri a l and 0.4 [or cl efo rm a bl e till benea th th e g lacier. th c
(;
3
mora ln es acco unt fo r a bou t 20 x 10 m (0.4% ) of th e
144
1817 coast
1989 coast
_ _ _ _ _ _ _~
1904 coast
o~""F===~2~"~;===~4
km
Fig. 5. Changes lI/ Ihe /)osilioll oJ lite coasllille al
Breioamerkurs({ndul" ({[cording 10 majJs oJ 1817 ( I/ze same
({S 011 GllIlIilallg.rsoll III({/) qf 1834; see Sigllrossoll. J978 ) .
190.f. ( D allisl! Geodetic Sun'!!) ,) ({lid 1989 ( k elalldie
Geodetic Surve),).
Th e ri,'e r j o kul sa ( meanin g g lac ia l ri" cr ), first
ment io ned in th e L a nd I nra ke Boo k, becam e grad ua l,,"
sho n eI' as the g lacier ad,'anced, in spite of th e sea " 'a rd
mo,"C me nt o f th e coas tlin e.
Th e prog lacia l la ke J okulsar lo n , first visibl e in 1932,
has expa nd ed durin g continou s retreat of the g lacier. In
1975 a bathymetric sun'ey of th e lake " 'as undertakcn.
Th e lake a rea was 7.9 km 2 a ncl th e es tim a ted vo lum e was
500 x I 0 6 m ~ (Bo ul to n and o th ers, 1982 ) . In 199 1 its a rea
was 10.4 km 2 .
Since th e fo rm atio n o f J oku ls<l rl 6 n in th e 1930s, th e
sho re o ff th e ri,'e r o Ull et has bec n re trea tin g becausc th e
sedim e nt load from th e g lacier has bee n largely dumpcd
into the la ke, a nd onl y a sm a ll amount acldedto th e coas t
b y the r iver. Compariso n of maps fr o m [904 a nd 1989
sho ws that th e coastline has re trea ted ove r a di sta nce of
4 km o n both sides o f" th e ri,'c r outlet (Fig. 5 ). The
m ax imum shorelinc e rosio n h as bce n 700 m in 85 yea rs,
an a ,"C ra ge of 8.5 m al. The retrea l rrom 1945 to 1989
was simil a r (Vikin gsso n , 1991; j o ha nn csso n, 1994) .
REMOVAL OF SEDIMENT BENEATH
BREIDAMERKURJOKULL
SediInent-relD.oval rates
Th e di scm'e ry of" the tre nc h ben ea th the eas te rn branch o f"
Brc io a m erkurj o kull a nd inform a ti o n on th e g lac ier
ad,'a nce m a ke it poss ibl e to eva lu ate rates of excava ti o n
l~jtimJsall:
1732
' 00
-- -~;-ENTS-------- -- - - --- - - - ' --- ------ -- --- -- - -
......
-40D+--~--~--'_-~--~-~--~----1
800,-- - - - - - - - - - - - - - - - - - - - ,
1794
' 00
, - - --- - --- - - -srn.u~------------ --\ _
BEDROCK
- 4 00 +--~--~--,_-~--~-~--~-____<
800 , -- --
-00
··· S(r)'Moos·- -·
BEDROC K
-'OO +--~--_._--c_-_,_--~-~--~-__I
o
~~t
o
(if
;]
km
kl11 "-
I
a" III 1
T ral/J -
D flllld-
/101'1 /{/Ie
alioll /(/Il'
10" m:l a
I
111£1
ICE
SEA
E
I rei/ch
- - - - - - - - - - - - - - - -- ,
1890
,from Ihe easl nll brallch oI B reioamerklllj6kllll
/ '01111111'
ICE
SEA
0-
res p ecri\-e part o f th e tre n c h ( Fi g , 6) , Fo r th e short e r
tim e-s pa ns thi s ass umpti o n is q ues ti o n a bl e , Th e 10 \l-es t
es tim a te fo r th e re m o \ -a l ra te is o bt a in ed if' \I'e ass um e th e
",h o le 20 km lo ng tre n c h \ n1S eXGI\'a ted durin g SOO \ 'Cetr S,
a ft e r th e se lllcm e n t o f Iceland ,
.-l d1'(lIlte ()/giacier
ICE
SEA
surro undin gs o f' th e prese nt tre nc h,
Du ri ng th e pe ri od 17 32- 1890 , th e g lac ie r ach- a n ced b y
9 km , cO \ T r ccl a n a rea or 45 knl:! a nd CXC{) \ ',-1t c cl a \ 'O IUll1C
o f' .5 x I OD m 3 o r 11 0 m a \'e raged O\ T r this a rea , A unifo rm
sedilll e nt-re m o \'a l ra te du r ing th e g lac ie r ach 'a ncc o f 158
C
J
I
I
'I
yea rs \I'as 32 x 10 ' m a o r 0,7 m a km - aHT age d o \-e r
th e a rea cOl-e red by th e ad\ -a ncing g lac ie r. Calcul a ted
OI'C r th e wh o le dra in age area o f th e eas te rn bra nc h o f'
750 km 2 , th e d e nud a ti o n ra te "'o uld be 4x 10 2 11l a I
km 2 Th ese rc m ol'<il ra tes a rc e xce pti o na ll y hi g h b v a Ill'
sta n da rd , Similar ca lcul a ti o ns for o th e r tim e-sp a ns a rc
g i\'C n in Tabl e I , ass umin g th a t th e ach- a n c in g g lac ier
pu shed fOrlr a rd a nd re pl ace d th e \I'ho le \'o lull1 e o f th e
IJeriod
rel/wral
800 ~,---------------------,
a nd tra nspo rt of sedim e nts fi'o m be n ea th th a t g lac ie r
durin g th e Littl e I ce Age , \\'e ass um e that be fo re th e
g lac ie r ach- a nce th e s u r face or th e sa ndur fl e ld \I'h e re th e
tre nc h is nO\I' loca ted h a d a slo p e simil a r to th a t o f th e
T able I, E slill/aled mlll imllm rail'S oI sedimenl lrall s/Jorl
S(aleJ alld rall'J aIgLacial JfdilllclIl
1990
SEA
0
LAK E
_="--------.,0 - - -
______ __- - - -- SEOfI,IENTS
BEDROCK
- , 00 .L-----,---,_-----r--~-__,_---,--_1
15
20
10
o
km
F ig , 6, SclWlllllic sec/iolls {(Io llg a telllralJ111ll'1i1l1' DJ Ihl'
fro l em brallch 0/ B reio alllerkllljii/f lli i II I /)(/rliCII l ar lime,l
1732
1732176511 00
1890
1765
179-11900
1 79 -~ 1965
179+ 1932
158
9
33
1.5
29
5
800 20
17 1
138
~5
7
23
80
5000
800'
3300:1
11 500
500
500
32
2 1')
II -P
14
3
2 ,S
0,7
3,5'
-1- ,9'
0,1 8
0,37"
035 •
* Fro m Bo ult o n a nd o th e rs ( 1982 ),
Bo ult o n a nd o th e rs ( 1982 ) co nc lud ed that th e bas in o r
th e prog lac ia l la ke J o kul s{lr1 o n \I-as crea ted \I-h e n
Bre ioa m e rk u rj o k ull ach oan ced 01,(, 1' Breio a m e rk u rsa nd ur
in th e 18 th a nd 19 th centuri es, H e n ce , th e \'o lum e o f
5 00 x 106 m :1 o f sedim e nts ",as exca \'a ted in 130- 17 5 yea rs
at m os t. Thi s is equi\'a le nt to a laye r o f' 64 m bein g
re m o\'ed , a\'e rage d 0 \'('1' th e \I'h o le la ke are a , a t a
minimum r a te o f 0,37 m a I , Thu s, th e a \Oe rage sedim e nt
load o rth e ri\ '(' r mu st h a \Oe bee n 3 to 4x 10 1; m 3 a I ,
Sedilllent-flux rela tions
H a \Oing d e ri\'ed es tim a tes ro r th e to ta l re m ond rat es \IT
ca n n ow di sc uss th e \Oa ri o us m o d es o f' sedim e lll tra nsp o rt
b y th e ach 'an cing g la ci e r durin g th e excm'a ti o n of the
tre nc h, Th e sedim e nt nux , Ql, thro ug h a c ross-sec ti o n o f
width H'- p er p e ndi c ul a r to th e direc ti o n of ice fl o \l', is th e
s um o f th e flux carri ed \I'ithin th e ice, Qi, th e flu x du e to
d e fo rm a ti o n o f' a subg lac ia l till la\'e r , Q" a nd th e flux
ca rri ed b\- suiJg lae ia l s trea m s, Q\\' :
belweell 1730 alld 1990,
Th e d e bri s flux in th e ice is Qi = C; ll -hhuh wh er e lib is
th e a \'e rage \° cl oe it\, o f th e b asa l ice ,
is th e \I'idth of' th e
n-
g lacie r , hb is th e thi c kn ess o r th e la \n o f seclim e lll-l a cl e n
ict' a nd Ci is th e co n ce n Ira ti o n o f d e bri s g iI'C n as m ass pCI'
\'o lum e o f'i ce , Th e sed im e nt co n ce ntrati o n is mainh' in
th e basa l ice , a nd flux in th e suprag lac ia l a nd e ng lac ial ice
filc ies ca n be neglect ed here , C o nsid e rin g th e eas te rn
bra nc h o r Bre ioa m e rkurj o kull , \loe ca n es tim a te Qi = 10"
'gd
m :> a I , ass ulllln
a se 'Illl e nt co nce ntra ti' o n 0 r' l oo/
/ 0 pe r
n-
=4 km , and
I'o lum e in a basa l laye r o f' hb = I m ,
Ll b = 200 m it I (ass umin g plu g fl ow o f' \lohi e- It \'e loc it\, ",as
es tim a ted fi'om a eri a l ph otogra ph s ).
Th e flu x ca rri ed b y th e d e fo rmin g sub g lac ial till is
Q, = n -h, us \lohe re hs is th e thi c kn ess o r th e defo rmin g
subg la e ial till laye r a nd Us is th e a \ 'e ra ge sp eed 0 (' the till
lave r. Thi s Ilu x is ro ug hl y es tim a ted to be o f' th e o rd e r o f
I Ol; m 3 a I, ass umin g h, = 2 m , a lin ea r \-c loc it y pro fil c in
th e d clo rmin g la ye r (:\ll ey , 199 1), and that the ice is rullY
co upled to a ne! no t slidin g o\-e r th e till.
Th e sedim e nt flu x in th e s ub g lac ial s tre am s is
Q", = Clr q", w he re C", is th e co nce ntra ti o n o f' sedim e nts
in th e \loa te r a nd q,,- is th e disc ha rge o[ \I'a te r drain ed a lo ng
th e glac ie r becl. This tra nspo rt b \' flLl\'ia l processes durin g
th e excit \'a ti o n o f th e tre nc h must ha \Oe bee n se\-e ra l fac to rs
hi g he r th a n th e o th e r tra nspo rt processes : durin g th e
145
Bjiimssoll : Scales and rales
rif glacial sedimenl
removal
pe ri od 17 32- 1890 a bo ut 30x I0 6 m 3 a l of sedim ents
(T a ble I ). Th e present rate of lra nsport , however, seems
to be a bo ut one-third of this; a bout 10 x 10 6 m 3 a I, of
whi ch 30% a rc tra nspo rted by th e ri ve r to th e sea
(qw= 130 m:1 s I, G w = 1.5 kg m 3, d e nsity 2000k g m 3 )
a nd 70 % a re dumped in to the lake. T he acc umula ti o n of
sedim enlS in th e lake over the period 1932- 75 was ob tain ed
from th e interpreta tion of la ke-sedim ent stra tig raphy by
Bou lto n a nd o th ers ( 1982, p. 43 ) .
Du ring the exca va ti o n period 1730- 1890, the ave rage
sedim ent co nce ntra tio n of a river of the sa m e di scha rge as
J ok ulsa ( 130m 3 5 I) wo uld have bee n about 10kg m 3
Suc h co nce ntrations h ave bee n m eas ured in I ce la ndi c
ri vers durin g j b kulhl a u ps a nd surges (Bj brnsson , 1979 ) .
The m a rgin a l ice-d a mm ed la kes eas t of the glac ie r
drain ed a nnu a ll y . During the Littl e Ice Age th e ice
d a m s were thi cker than now, a nd th e disc ha rge of th e
j bkulhl a ups a nd th eir capacity to re move basal sedim ents
wo uld have been mu c h larger th a n today. Furth e r,
seve ral surging e\'ents too k pl ace during the ad va nce of
Breioam erkurjbkull , but this ac tivity te rminated ea rl y in
th e 20th ce ntury . R edu ced sedim ent tran sport would a lso
be ex pec ted now if th e so ft sedim ents have alread y bee n
sco ured away a nd the glacier is mov ing over ha rd
bedrock in th e d eepes t part of th e trenc h. Th e possibility
rem ains th a t depl etion of so ft sedim e nts m ay h ave
termin a ted the surge ac ti vit y.
CONCLU SIONS
A 20 km lo ng, 2- 5 km wide trench, w hi ch is up to 300 m
below sea level is be li eved to have been crea ted by
remova l of sedim ent durin g the ad va nce of Breioa m erkUljbkull in th e Little Ice Age, i. e. fr o m A D 1400 to th e
1890s . Durin g th e pe ri od 1730- 1890 the g lacier advanced
by 9 km , a nd a yolum e of 5 x 10 9 m 3 was excavated ,
impl ying a n ave rage d e nud a tion rate of 0.7 m a- I and a
3
tra n sport ra te ou t of th e trench of 32 x 10 6 m a I. Th e
sedim e nt flu x ca rri ed within the ice a nd by the defo rmin g
subglacia l till m ay have been a bo ut 10 5 and 106 m 3 a I,
resp ec ti ve ly. Thus, the tra nsport by flu via l processes
during th e excava tion of th e trench must h ave been a n
ord er of magnitud e hi g he r than th e o th er transport
processes toget her, abo ut 30 x 106 m 3 a I of sediment. Th e
prese nt rate of transport , however co nsid e ra bl e, see m s to
be about 10x 106 m 3 a I, o f wh ich 30% is tra nsported by
th e ri ve r to th e sea a nd 70% dumped into th e la ke.
H ence, flu vial processes carrying sedim e n t loads three
tim es g rea te r th an a t present operated durin g th e
excava ti o n o f th e tre nc h. Th en th e ave rage sedim ent
conce n trat io n of a ri ve r of th e sa m e size as J ok u isa ( 130
m 3 s I) wou ld have been a bo ut 10kg m 3 . Co ncentra tion s
of th a t m ag nitud e h ave bee n m eas ured o nl y in I ce la ndi c
rivers during jo kulhl a ups a nd surges. S uch ac tiviti es were
fr equ ent during th e excava tio n of th e tren ch ben eath th e
eas tern bra nch of Breioamerkurjoku ll.
\Vhen glac iers a re overriding layers of fin e-gra in ed
sedim e nts which a re eas il y e rodibl e by water, th e suppl y
and distributio n of wate r m ay be th e o nl y limita ti o n o n
the ca pac ity for rem ova l of subglac ia l sediments. At
Breioam e rkurjokull th e suppl y of m eltwa ter and ra in is
g rea t, a nd m a rgin al la kes frequentl y dra in in jbkulhl a ups.
146
vVa te r ge ts easily d ow n to th e base where it no rm a ll y
dra in s ra pidly d own th e tren c h and durin g surges
repeated ly mak es co ntac t with new basal sedim cnts.
REFERENCES
Ah lm a nn , H . \ V. 1953. Glacier variations and climatic/7I1ctllo/ions. New York ,
American Geographical Society.
Alley, R . B., 1991. Defo rming-bed o ri gin fo r so uth ern L au rentide till
sheets? J. (;Iaciol ., 37, 67 76.
B'"'oa rso n, C. C. 1934. Isla nds Cletsc her. Beitr,ige zur K enntni ss der
C letsc hcr beweg ungcn und Schwa nkunge n a uf Cr und a lter Quci lcnsc hriftcn unci ncuCSlcll Forsc hun g. SOCil'trH Sciel1!iarum IJIOlldic(f , 16.
Bjornsson, H . 1979. G laciers in Icela nd. ] akll/!, 29,74--80.
Bjornsson, H ., F. Pit lsso n and ~1. T. Guomundsso n. 1992. Breioamfrkllljiikll/! . . \" iollrsloOllr I:s.sjdnnd!/inga 1991. Rey kj ad k, Un i\Trsit y of
Icela nd . Sc ience Instit u te. (RH-92-1 2. )
Bogad6 ttir, H .. C. S. Bo ul lO n, H. T61l1asso n, and K. Tho rs 1985. T'he
st ruct u re of the sed i men ls bcnea th Brciotll11crku rsa nd ur and l h e fo rm
of the underl ying bedrock. In Sigbjarnaso n, C. , ed. lalaud CorlJtal and
Ril"fr .~)'m/!osillm. Proceedillgs. Rey kj ,l\'ik , \'at io na l Energy Authority,
295- 303.
Bo ul lOn, G. S. , P. W . \ '. H a rris a nd J. J an· is. 1982. Stratigraph y ancl
stru cture of a coas tal sediment wedge of glacia l o rigin infe rred from
spark r r Jllcas ure ments in g lac ial lak e Jokulsa r1 6n in sO Llth easte rn
Iceland. ] oku/!, 32, 37-47.
Bo ulton. C. S., K . Thors and .J.j a ITis. 1989. Dispe rsa l of glac iall y deri "ed
sedim en t ove r part of th e con tinenta l shelf of so ut h Icela nd a nd the
geo me try of th e rcs ulta nt sed im ent bodies. ,\lar. Geology, 83 , 193 223.
H ende rso n. E. 18 18. irelalld: or thejollmol ofa resident ill that island dllrillg the
)lfnrs 1814 alld 1815. 1'01. 1. Edinburg h, O li phant, W a ug h a nd I nn es.
H owart h, P.] . and R .j. Pri ce. 1969. T he p roglac ia l lakes oC Breioamcrkurjokull a nd Fl aajo kull , Icela nd . Geogr.], 135 (4 ), 573 58 1.
J 6hanncssoll , H . 1994. Coas ta l e ros io n nea r the brid ges ac ross J oku lsa a
Brcioamc rkursa ndi in so utheastern Iceland. 111 \,iggoson, G. , ed.
Proceedillgs. IlItematiollal Coastal .»IIII)osilllll ill I-Iojil. Ihe tOU'11 of
HOrtlllfjiirollr . leelalld, 2() 24 ]lIlIe 199-1. Rey kj avi k, The Icela ndi c
H a rbo ur A urho rit y, 405-4 14.
:'\Toroanfari. 1870. J o kulhl a up og Flc ira. Noroanfari, 10 Februar 1870
(6- 7), 14.
N6 rlund , N. E. 1944. Isla nd s kurthcgni ng: en hi stu risk fi·ems tilli ng. Gead.
i m/. Pllbl. VII.
P;dsso n, S. 1883. Ph ysisk. geografis k og hi sto ri sk bcskri velsc af de
isla nd ske Isbjergc ( 179 1 I 79', ). ."' ·orsk Tllristjorellillg.r ..irbok 1882
1883, 1- 63 .
Pa lsso n, S. ( 179 1- 1797. ) 1945. J ok la riti o. III Feroabdk S,'eills Prilssonor.
Dagbd!!.:lIr og ritgeroir 1791 1797. R eykjad k, SmelandsLltgMa n, 423- 552.
Price, R .j . 1969. ;\l ura in es, sa nd ar, ka mes a nd cske rs near BrcioamcrkUlj6kull , Iceland. " ,,/. Br. (;eogr. TrailS. 46 , 17-43.
Pri ce, R .j . 1971 The de"e1opment and destruction of a sandur,
Breioame rkuljokull , Icela nd. Ant. All). Res ., 3 (3 ), 225 237 .
Pri ce, R .J. 1982. Changes in the prog lac ia l area of Breioalllcrkurjbku ll.
so uth ern Iceland : 1890 1980. ] 6kllll, 3 2, 29- 35.
R is!. S. 1957. islellsk I'MII. Rey kj ad k. R afork ulll id as tj6ri.
Sigbjarnaso n, C. 1970. On the recession ofVatnajokul1. J oku/!, 20. 5061.
Sig urosson , H . 1978. ,,'ortasaga islalld... Jid lokulII 16'. aldrll' /il 1848.
Rey kja vik , B6 kaLltg:lf'l M cn ninga rsj6C)s og !)j60\·in a fCl agsins.
Thi cnclllan n, F. A. L. 1824-. Reise illl .Vordell Eurol)(ls, vor<.lIglich ill island il1
dell ]ahml 1820 bis 1821. Lei pzig, Carl H einri ch R eclalll.
Thorarinsso n, S. 1943. Vatn ajokull; scien ti fic resul ts of the S\\"cdis hIcela ndi c investiga tions 1936- 37-38. Chap ter 11. O sc ill a tions of the
Ic ela ndi c glaciers in the last 250 years. Geogr. Allual. , 25 , I 54.
Thora rinsso n. S. 1974. Sam bLlo la nds og los i el lcfu a ldir . III Und a l, S. ,
erl. Sa,~a islauds i . R eykjavi k, Hi o i slcnska B6 kmen ta felag. 29 97.
Thorodd se n , T. 1892. Isla nd s jokier i fortid og nutid . Geogr. Tidsskr.,
11 (5- 6 ), II1 146.
Th oroddsen, T. 1905/06. Island. Grundriss d er G eog raph ic und
Geo logic. .Ifitl ei/ungen ails ]lIslus Pert/us' Geogral)hischer :llIstalt,
32 ( 152jl53 ), 1- 16 1f162 358.
T 6masson, H. a nd E. Vilmund a rd6tt ir. 1967.Th e la kes St6ris j6r and
Langisj6r. ] okll/!, 17, 280 299.
\ 'ikin gsso ll , S. 1991. Suou rslro nci islands brc yt ingar a \egu slranciar
sa lllk\'a:lllt kortum og loftm ynd ulll . R ey kjavik , Orkusto fll un. ( Repo rt
OS-91042 / VOD-07B. )
\\'attS, L. \\'., 1876. Across the l·atlu!jM.II/!; or scenes ill leeland. beillg {/
description of hitherto IIlIkllOI 11 regions. Lond on , Lo nglllans and Co.
l