Paleoclimatic implications of an early Holocene glacier advance on Disko Island, West Greenland OLAFUR INGOLFSSON, POVL FRICH, SVEND FUNDER AND aLE HUMLUM BOREAS Jng6lfsson, 0., Frich, P., Funder, S. & Humlum, O. 19901201: Paleoclimatic implications of an early Holocene glacier advance on Disko Island, West Greenland. Boreas, Vo!. 19, pp. 297-311. Oslo. ISSN 0300--9483. The paper describes studies of glacial deposits and raised beaches on the island of Disko, West Greenland. Two stades in the glacial history are defined, the Godhavn stade, which represents the last major glaciation on Disko, and the Disko stade, which defines a Preboreal readvance. During the Godhavn stade, only eastern and southern Disko were affected by the extended Inland Ice, while glaciation on western and northern Disko was local in character. In connection with the deglaciation, the sea transgressed to the marine limit at c. 9,250--9,000 BP. Immediately before, tentatively culminating around 9,300 BP, a significant readvance of glaciers on eastern Disko occurred. The marine limit rises on a transect from northwest to southeast across Disko from 60 m a.s.!. to 90 m. The paper discusses changes in equilibrium line altitude (ELA) during the deglaciation, and explains the Disko stade readvance in terms of variations in upper-air wind conditions. Olafur Ing6lfsson, Department of Quaternary Geology, Lund University, Solvegatan /3, S-223 62 Lund, Sweden; Povl Frich, Danish Meteorological Instill/te, Lyngbyvej /0O, DK-2/00 Copenhagen @, Denmark; Svend Funder, Geological Museum, @ster Voldgade 5-7, DK-/350 Copenhagen K, Denmark; Die Humlwn, Department of GeomorpllOlogy, Geographical Institute, University of Copenhagen, @ster Voldgade 10, DK-I350 Copenhagen K, Denmark; 20th June, 1990 (revised 14th September, 1990). The island of Disko (8,600 km 2 ) is situated in Disko Bugt, between about 69°15'N - 70"20'N and 51°50'W - 55°00'W (Fig. 1). The shortest distance from Disko to the mainland is across Vaigat in the northeast, about 10 km, but towards the east and south the island is separated from the mainland by a 50-80 km stretch of water. The Inland Ice margin in the Disko Bugt area is usually situated some 25-40 km inside the coast, but a number of actively calving glaciers reach thebaythroughthefjords, notablythe Jakobshavn Isbrre, which is the largest single outlet draining the Inland Ice. The climate on Disko is polar maritime, with moist air masses coming from south and southwest along the Davis Strait during the summer half year, but dominated by continental polar air masses from the Inland Ice during winter. The mean annual air temperature is approximately -4°C, and precipitation ranges from 100 to 500 mm y-l water equivalent at sea level. There are substantial local variations in climate due to topography and exposure. At present, Disko supports considerable local- glaciation (Fig. 6), with about 20% of the island covered by glaciers. The marine climate of the coastal waters of Disko Bugtis the result of a complex hydrography: a branch of the cold and low salinity East Greenland current flows parallel to and partly mixes with the Irminger current, which is a branch of the North Atlantic Drift. After turning around Kap Farvel they join in the West Greenland current, which brings warm Atlantic water to Disko Bugt. Fluctuations in its intensity have in time highly influenced the marine climate along the West Greenland coast (Funder 1989). The position of Disko at the junction of Davis Strait and Baffin Bay, and exposed to continental polar air masses from the Inland Ice, makes its glacial history interesting in terms of regional glacial responses to large-scale climatic changes. Today, Disko Bugt catches a great number of cyclones and accompanying moist maritime air masses (Humlum" 1985, 1986). The resulting orographic precipitation along the borders of the· bay accentuates climatic changes, and the glaciers on Disko respond rapidly to climatic changes in contrast to the Inland Ice which reacts very slowly. Because of the peripheral position of DiskO'with regard to the Inland Ice, the island GEOLOGICAL M·USEUM OF THE UNIVERSITY OF COPENHAGEN I . CONTRIBUTIONS TO GEOLOGY No. 646
Transcript
Paleoclimatic implications of an early Holocene glacieradvance on Disko Island, West Greenland
OLAFUR INGOLFSSON, POVL FRICH, SVEND FUNDER AND aLE HUMLUM
BOREAS
~
Jng6lfsson, 0., Frich, P., Funder, S. & Humlum, O. 19901201: Paleoclimatic implications of an earlyHolocene glacier advance on Disko Island, West Greenland. Boreas, Vo!. 19, pp. 297-311. Oslo. ISSN0300--9483.
The paper describes studies of glacial deposits and raised beaches on the island of Disko, WestGreenland. Two stades in the glacial history are defined, the Godhavn stade, which represents the lastmajor glaciation on Disko, and the Disko stade, which defines a Preboreal readvance. During theGodhavn stade, only eastern and southern Disko were affected by the extended Inland Ice, whileglaciation on western and northern Disko was local in character. In connection with the deglaciation,the sea transgressed to the marine limit at c. 9,250--9,000 BP. Immediately before, tentatively culminatingaround 9,300 BP, a significant readvance of glaciers on eastern Disko occurred. The marine limit riseson a transect from northwest to southeast across Disko from 60 m a.s.!. to 90 m. The paper discusseschanges in equilibrium line altitude (ELA) during the deglaciation, and explains the Disko stadereadvance in terms of variations in upper-air wind conditions.
Olafur Ing6lfsson, Department of Quaternary Geology, Lund University, Solvegatan /3, S-223 62 Lund,Sweden; Povl Frich, Danish Meteorological Instill/te, Lyngbyvej /0O, DK-2/00 Copenhagen @, Denmark;Svend Funder, Geological Museum, @ster Voldgade 5-7, DK-/350 Copenhagen K, Denmark; DieHumlwn, Department of GeomorpllOlogy, Geographical Institute, University of Copenhagen, @sterVoldgade 10, DK-I350 Copenhagen K, Denmark; 20th June, 1990 (revised 14th September, 1990).
The island of Disko (8,600 km2) is situated inDisko Bugt, between about 69°15'N - 70"20'Nand 51°50'W - 55°00'W (Fig. 1). The shortestdistance from Disko to the mainland is acrossVaigat in the northeast, about 10 km, but towardsthe east and south the island is separated fromthe mainland by a 50-80 km stretch of water. TheInland Ice margin in the Disko Bugt area isusually situated some 25-40 km inside the coast,but a number of actively calving glaciers reachthe bay through the fjords, notably the JakobshavnIsbrre, which is the largest single outlet drainingthe Inland Ice. The climate on Disko is polarmaritime, with moist air masses coming fromsouth and southwest along the Davis Strait duringthe summer half year, but dominated bycontinental polar air masses from the Inland Iceduring winter. The mean annual air temperatureis approximately -4°C, and precipitation rangesfrom 100 to 500 mm y-l water equivalent at sealevel. There are substantial local variations inclimate due to topography and exposure. Atpresent, Disko supports considerable localglaciation (Fig. 6), with about 20% of the islandcovered by glaciers.
The marine climate of the coastal waters ofDisko Bugtis the result of a complex hydrography:a branch of the cold and low salinity EastGreenland current flows parallel to and partlymixes with the Irminger current, which is a branchof the North Atlantic Drift. After turning aroundKap Farvel they join in the West Greenlandcurrent, which brings warm Atlantic water toDisko Bugt. Fluctuations in its intensity have intime highly influenced the marine climate alongthe West Greenland coast (Funder 1989).
The position of Disko at the junction of DavisStrait and Baffin Bay, and exposed to continentalpolar air masses from the Inland Ice, makes itsglacial history interesting in terms of regionalglacial responses to large-scale climatic changes.Today, Disko Bugt catches a great number ofcyclones and accompanying moist maritime airmasses (Humlum" 1985, 1986). The resultingorographic precipitation along the borders of the·bay accentuates climatic changes, and the glacierson Disko respond rapidly to climatic changes incontrast to the Inland Ice which reacts veryslowly. Because of the peripheral position ofDiskO'with regard to the Inland Ice, the island
GEOLOGICAL M·USEUM OFTHE UNIVERSITY OF COPENHAGEN
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CONTRIBUTIONS TO GEOLOGY No. 646
298 Olafur Ing6lfsson et al. BOREAS 19 (1990)
"~" DISKO BUGTKronprinsens'
Ejland
68° N
DISKO
... : .. .:.." .
..::·······G~~·HA VN
:.54° W
a
U'"
...... ,>56° W
MellemfjOr~: ., ..... ".
zUJUJer:
"
100 km
KapFarvel
70° N
BaffinBay
Fig. I. Location map.
D 0-500ma.s.1.
o SOO-1000m
• >1000 m
I40 km
-----r-'l>.....IC......:::.- Bloosedal
Fig. 2. The maintopographical features ofDisko and locations referredto in the paper.
BOREAS 19 (1990)
became ice-free at a relatively early stage duringthe last deglaciation. Thus the lithostratigraphicalas well as the biostratigraphical and morphostratigraphical records are longer than corresponding records from the mainland. Theperipheral position of Disko is also clearlyreflected in the landscape development (Sugden1974): while the crystalline rocks of the coastalmainland are characterized by areal scouring byan ice sheet, the Tertiary plateau of Disko isheavily dissected by cirques and valleys (Fig. 2).
Previous investigationsSteenstrup (1883, 1900) recognized that theInland Ice at an earlier stage expanded into DiskoBugt and probably covered Disko. He drew hisconclusions from the pattern of glacial striae inthe coastal areas and from observations of gneisserratics from the mainland found on the westcoast of Disko. He also drew attention tomarine terraces and raised beaches in the fjordsof Disko. Jensen (1942), Harder et al. (1949),Laursen (1950) and Funder & Weidick (in press)investigated the stratigraphy and subfossil faunaof raised marine deposits in the Disko Bugt area.
N-S trending ridges on the banks off WestGreenland, as well as presumed nunatak areas onthe outer coast, have been used for reconstructingthe maximum extent of the Inland Ice (Weidick1976). Repeated glaciations of the outer coasthave been recognized on the basis of interglacialdeposits (Funder & Sfmonarson 1984), but it hasnot been possible to recognize distinct features ofindividual glacial stages. Quaternary deposits andminor landforms in the coastal areas of WestGreenland generally belong to the last majorWisconsinan-Weichselian glacial episode and thesubsequent deglaciation (Funder 1989). There isvery little information on the growth of iCe andits maximum extention, except that the Inland Icetransgressed the present coastline to a position onthe present inner shelf (Ten Brink & Weidick1974; Kelly 1985). Kelly (1985) proposed the ageof its maximum extent as 14,000-11,000 BP andcalled this glacial episode, including the deglaciation phases, the Sisimiut glacial event.
Weidick (1868, 1972, 1976) studied raisedbeaches and stages in the deglaciation of coastalWest Greenland, based on radiocarbon-datedmarine molluscs. He concluded that ice retreatfrom the maximum position began some time
Glaciation on Disko Island 299
prior to 10,000 BP, and that the sea transgressedto the marine limit at c. 80 m in Nordfjord onDisko around 10,000 BP. The Inland Ice marginhad retreated to its present margin by c. 7,000BP, and in the following millennia It retreated upto 20 km behind its present margin (Weidick 1985;Weidick et al. in press).
Donner & Jungner (1975) concluded that theouter coast in the Disko Bugt area, near Egdesminde, where the marine limit lies at 130 m a.s.l.,had become ice-free at c. 11 ,000 BP. Donner(1978) published a chronology for the main Holocene events on the west coast of Disko Islandbased on radiocarbon dates and correlation withDonner & Jungner's (1975) results. He concludedthat the outer coast on Disko became ice-freearound 10,000 BP, and that glaciers had retreatedfrom the large fjords of Nordfjord, Mellemfjordand Diskofjord (Fig. 1) at 8,000 BP. He estimatedthat the marine limit in Diskofjord at 85 m a.s.l.was reached at c. 9,000 BP.
Kelly (1985) and Funder (1989) summarizedthe Quaternary geology of West Greenland. Theretreat of the Sisimiut Inland Ice margin fromDisko was dated to between 11 ,000 BP and10,000 BP, while a major readvance, the Diskostade, took place from a highland ice cap on Diskoaround 9,000 BP, during which outlet glaciersreached the sea. Funder (1989) was not able todemonstrate any contemporaneous change in theadjacent Inland Ice margin, and suggested thatthe Disko stade readvance was a response toincreased precipitation in the coastal areas.
Frich & Ing61fsson (1990) described the lithostratigraphy of a coastal cliff sequence at Igpik,Disko. At the base, a lag unit of crystalline boulders was interpreted to be a wave-washed residualof a till deposit laid down by the Inland Ice duringthe maximum Sisimiut glacial advance. It wasoverlain in the sequence by a glaciomarine diamicton containing abundant shell fragments. Insitu shells at 69 m a.s.l. were dated and used inthe construction of a model for relative sea-levelchanges in the Disko Bugt area. They suggestedthat the southern coast of Disko was ice-freebefore 9,200 BP, and that the marine limit atapproximately 85 m was reached around9,000 BP.
Humlum (1987) discussed the influence of variations in upper-air wind conditions on glacierbehavior with reference to glaciers on Disko. Heconclu,ded that as glacier fluctuations representintegrated responses to complex interactions
300 . Olafur Ing6lfsson et al. BOREAS 19 (1990)
Table 1. Compiled radiocarbon dates from the Disko Bugt area, listed by ascending age.
Age, years BPField OBC
Lab. no. N. lat. W. long. Material all. (m) Cony. Corrl. %oPDB Reference
I Marine reservoir correction made for 14C dated samples normalized to -25%0 PDB "c, according to a recommendation byKrog & Tauber (1974).
2 Samples marked' form the database for the emergence curves in Fig. 7.
between factors that influence glacier behavior,even glaciers within a geographically restrictedarea may display different responses to identicaloverall climatic changes. He continued to showthat high-level snow drift exercised strong influence locally on snow accumulation and the Equilibrium Line Altitude (ELA), and called forcaution to be exercised when attempting to reconstruct past climate from observations on past andpresent glacier cover, ELAs and cirque floorlevels.
The present studyOur study is based on a number of individualinvestigations by the authors during the past 10years. Its purpose is to present new data on theglacial history and Holocene relative sea levelchanges on Disko, and to discuss the paleoclimaticdevelopment in the light of our combined observations.
Our results are based on (a) morphological!lithostratigraphical studies from the Godhavn
area (including Igpik, c. 11 km east of Godhavn),Hammers Dal on the northwestern coast, EnoksHavn in Mellemfjord and the Mudderbugt-Flakkerhuk area (Fig. 2), and (b) a number of newradiocarbon dates with significance to the deglaciation and marine limit (Table 1). We havedefined two stades in the Sisimiut/Holocene glacial history of Disko, the Godhavn stade, whichprobably referred to the last maximum glaciation,and the Disko stade, an early Holocene readvance.
The Godhaun stade
The most impressive feature from this glacialevent is the large moraine, called the Pjetursson'smoraine, that blocis:s the mouth of Blresedal nearGodhavn (Figs. 3 and 4), and was described indetail by Pjetursson (1898). The ridge is 1.5 kmlong and rises up to 75 m above the surroundingvalley floor, reaching c. 220 m a.s.l. at the junctionwith Skarvefjeld at its eastern end. At the base itmeasures c. 600 m in width; the cross section isasyrrmletrical with a gently sloping up-valley side
302 Olafur Ing6lfsson et al. BOREAS 19 (1990)
Fig. 3. Godhavn stade moraine at the mouth of Blresedal; a view towards the east over canyon cut by the stream draining Blresedalto the left. On the left flank of the moraine a boulder shoreline appears as a light zone (thin arrow), on its right side a lake (thickarrow). A shell from glaciomarine sediments below lake sediments in a core from the lake yielded a "c date of 9,250 BP.
LEGEND:
1/:/:/::4 Present ice cover
• Little Ice Age glaciation
[2§] Godhavn stade moraines
110 IDisko stade moraines
1--1 Glacial striae
1•••51Height in m a.s.1.
Contour interval: 100 mroj--r,--.---,o 1 2 3km,
Fig. 4. The extent of glaciersat different times in theGodhavn-Lyngmarksbrreenarea, southern Disko.
'.~...
BOREAS 19 (1990)
and a steeper coastal side. Pjetursson (1898) andSteenstrup (1900) interpreted it as a terminalmoraine deposited in front of a valley glacier inBlresedalen, while Weidick (1968) suggested thatit is a medial moraine formed at the junctionbetween a glacier in Blresedal and a major icetongue in Disko Bugt. However, we have foundno indications that ice moved down Blresedalentowards the south coast. Glacial striae in Blresedalen suggest that at some time prior to theformation of the large moraine, ice moved intothe valley from Disko Bugt. A system of morainesindicates that this ice stream terminated about8 km north of the Pjetursson's moraine (Fig. 4).The configuration of this moraine system suggeststhat local glaciers at this time were comparablein size to the Little Ice Age situation. The asymmetrical cross section of the Pjetursson's moraine,the slope of the crest line from east to west, andthe occurrence of fresh E-W striations at thejunction with Lyngmarksfjeld indicate that themoraine was formed by an ice tongue in DiskoBugt. A boulder shoreline at c. 90 m a.s.l. on theup-valley side of the moraine may have beenformed at this time by an ice dammed lake inlower Blresedalen. A seepage line at c. 130 ma.s.l. running along the coastal side ofthe moraineshows that the basal part of the moraine is madeup of impermeable sediment, probably the homogeneous silt with scattered stones that can be seenin small exposures below this line, covered by athick mantle of slope-wash deposits. This showsthat the moraine may have a complex history, andmay to some extent be composed of water-laidsediments.
Moraines deposited at the margin of an icetongue in Disko Bugt can be traced as far to thewest as the valley Itivneq kitdleq, 15 km west ofGodhavn, where a glacier lobe invaded thevalley and left terminal moraines 5 km up-valleyat c. 110 m a.s.l. We also correlate the Godhavnstade deposits with the crystalline boulder lag inthe Igpik sequence, east of Godhavn, describedby Frich & Ing6lfsson (1990). Crystalline erratics,frequently found on the basalt plateaux ofsouthern and eastern Disko, show that the InlandIce at some time overran even these plateaux.Whether this happened during the last glacialmaximum is not known. The Disko Bugt basinis limited towards the southwest by a markedbedrock ridge extending approximately fromGodhavn over Kronprinsens Ejland to Egdesminde. This submarine threshold probably
Glaciation on Disko Island 303
marked the grounding line for the Disko Bugtglacier during the Sisimiut glaciation maximum.
The geomorphological evidence from the Godhavn area suggests that at the height of the glaciation a major outlet glacier in Disko Bugt enteredBlresedalen from the south. At a later stageBlresedalen was dammed towards the south bythe big Pjetursson's moraine formed against theice tongue in Disko Bugt; the lower part of thevalley was filled by a lake (Fig. 3). A minimumage for the disappearance of this ice tongue andthe deglaciation of Disko Bugt is provided by a14C date of 9,250 BP (sample AAR-5, Table 1)obtained on an in situ mollusc shell from marinesilt immediately below lake sediment in a corefrom a lake on the proximal side of the morainein Blresedalen. This age is in agreement with datesfrom the Igpik section east of Godhavn (samplesK-4567 and Hel-2210, Table 1; Frich & Ing6lfsson1990), and gives the approximate age of the Holocene marine limit in this area, which postdatesthe termination of the Godhavn stade.
The Disko stade
On eastern Disko, moraines of the Godhavn stadeare truncated by younger moraines coming out ofall major valleys (Fig. 6), showing that after thedisappearance of the ice tongue in Disko Bugtthe valleys carried glaciers draining a local ice capon the high mountain plateaux of central Disko.
The moraines from the Disko stade are bestdeveloped at the mouths of Kvandalen and Blabrerdalen-Laksedalen (Figs. 5 and 6), where theyform complexes with up to seven parallel ridges,each up to 5 km long and separated by meltwaterchannels and strings of kettle holes. Terminalmoraines have rarely been preserved because theglaciers terminated in the waters of Disko Bugt.Thus the lateral moraines disappear on the coastalplain at the local marine limit, at c. 90 m a.s.l.(Fig. 6), and lateral outwash plains are graded tothis sea level. On the proximal sides of the moraines beach ridges and delta terraces have beenformed at almost the same level, indicating thatthe glacier re-advqnce was contemporaneous inall the valleys and apparently was a very shortlived event. Furthermore, while these conspicuous and well-developed moraines can befound at the mouths of all valleys on easternDisko, similar features are absent from the valleysof western Disko, as well as from areas along themargirt of the Inland Ice. This implies that the re-
Fig. 5. Disko stade moraines at Mudderbugt with intervening kettle holes. View towards Disko Bugt to the east.
advance during the Disko stade was caused bylocal climatic factors, causing the eastern marginof the local ice cap to expand rapidly while thewestern margin remained stationary.
A minimum age for the termination of theDisko stade is provided by two 14C dates fromthe area inside the moraines at Mudderbugt oneasternmost Disko, where mollusc shellsreflecting sea level higher than 45 m above thepresent gave an age of 8,700 BP (sample K-3667,Table 1); the basal gyttja in a lake sediment corewas dated to 8,950 BP (sample K-3667, Table 1).Reworked shells in the moraines gave a non-finite14C age, and amino acid ratios indicative of a highage (Funder & Sfmonarson 1984).
If it is assumed that the marine limit on easternDisko was attained simultaneously with that50 km further to the west (Godhavn and Igpik),the age of the Disko stade is c. 9,300 BP.
Deglaciation and relative sea level changes
The marine limit on Disko is highest on the southern and southeastern coast, where it reaches toor slightly above 90 m a.s.l., and lowest on thenorthwestern coast where it lies at approximately 60 m. The highest marine limits in theDisko Bugt area are on the outer coast nearEgdesminde, south of Disko, where altitudes of11D-130 m were reported by Donner & Jungner(1975). Western and northern Disko were neverseriously affected by the Inland Ice during the last
glaciation. Crystalline erratics are most frequentlyencountered on the mountain plateaux south andeast of Diskofjord, while they are sparse on northern Disko. This pattern of distribution probablyrelates to a glaciation older than the Godhavnstade.
Two localities, Enoks Havn and Hammers Dal,give information on the deglaciation and sea-levelchanges, as well as on the age of the Disko stadereadvance.
Enoks Haun (Ivisarqut, 69°44'N, 54°50'W). Enoks Havn is a bay on the southern side ofMellemfjord (Akugdlit) (Fig. 2). The marine limitis at about 55 m a.s.l.
A raised delta is situated in front of the mouthof a glaciated valley. The delta is about400 x 300 m and the surface slopes fromabout 42 m down to about 18 m a.s.l. The deltatop surface is reworked by marine processes, andthe marine limit appears to be somewhat youngerthan the delta itself.
Shells of Mya truncata were collected fromexposed bottomsets at about 35 m a.s.l. Thesynchronous topsets lie at 39-40 m a.s.l. Theshells were radiocqrbon dated (sample Lu-3041,Table 1) to 9,070 BP.
In the valley above the delta there is an oldmoraine system, its lower part terminating closeto the upper marine limit. The marine limit canbe traced on the proximal side of old moraineson the opposite side of the main valley, and thuspostda'tes the formation of the moraines. We
BOREAS 19 (1990)
o
Glaciation on Disko Island 305
I40km
Fig. 6. The extent of glaciers on Disko today (black) and during the Disko stade (blue).
---~-------~--------------------------~.
306 Olafur Ing6lfsson et al. BOREAS 19 (1990)
40
50
30
20
10
m a.s.l.
j90
80
70
-60
+SW
oSE
.. NW
oNE
o
8
..
..
o
/c:J' East of Godhavno \
<\
localities, deltaic sediments containing fossilshells were deposited when sea level was at ortransgressing towards the marine limit. The ageof the marine limit according to our interpretationis between 9,250 and 9,000 BP.
For a further study of sea level changes we haveanalysed the relative sea level data from DiskoBugt (altitude of marine limit (Fig. 8) andradiocarbon dates of fossil shells (Table 1)) byconstructing emergence curves by visual interpolation (Fig. 7). We divided the Disko Bugt areainto four sections, with respect to Godhavn onsouthern Disko, to look for different patterns inrelative sea level changes. This results in twocurves, which probably reflect differences in theSisimiut glacial situations between western Diskoand Disko Bugt south and east of Godhavn. Ouremergence curve for Disko Bugt south and eastof Godhavn fits well with the two best fittingcurves of Donner & Jungner (1975) for the areasouth of Disko Bugt. The curve for western Diskoindicates that the sn';aller emergence for that areacompared with the area south of Disko Bugt isreal, a question posed first by Donner (1978).Our curves also give approximate points for theage of the marine limit, based on stratigraphicpositions of radiocarbon-dated samples fromEnoks Havn, Godhavn and Igpik.
5 4 3
14C yrs BP x 10 3
Fig. 7. Emergence curves for the Disko Bugt area. The dataset(Table 1) has been divided into four sectors with respect toGodhavn, southern Disko.
\\
\\
\West of \
Godhavn "/ .... \
" \'\ 0 \'\ \
o .', 0 \
• • '\ QO \, LLtJ \, \
O''O:J ", \o ID 0 0 ,,\
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o ~",., '':;~o 0 ~ .....
o. 0 0 0 ............. _
'----+----j---f---+------jf---+--+--+-=_--+oo
Hammers Dal (70 0 10'N, 54°50'W). - HammersDal is a large glacial valley ending at the coast inthe northwestern part of Disko Island (Fig. 2).North and south of the valley there are extensiveraised beach deposits within a 500-1000 m widezone along the present coast. The marine limit isat about 60 m a.s.l.
The beach deposits overlie an old till sheet,which also covers the gently rolling terrain abovethe marine limit. The till contains many gneissicrocks and is therefore not of local origin. The tillwas most probably deposited by a branch of theInland Ice which has flowed along the VaigatSound northeast of Disko, and covered the northern part of the island. We do not know whetherthe till deposition relates to the WisconsinjSisimiut glaciation or to an older event.
In front of a small valley about 2 km north ofHammers Dal, a raised delta (200 x 300 m) islocated at the marine limit. Shells of Hiatellaarctica were collected from bottomsets 38 m a.s.l.The contact between foresets and topsets is at53 m a.s.l. The shells were radiocarbon dated to9,370 BP (sample Lu-3037, Table 1). Most of thedelta surface appears to be reworked by marineprocesses, and although the delta may be slightlyolder, we consider it to be of almost identical ageas the marine limit.
The river which originally deposited the deltastill drains water from the valley above it. At thepresent shore, however, no delta is being formedtoday. The reason for this is a lack of sufficientbedload, probably due to the non-glacierizedcharacter of the area today.
Small moraine systems on hill slopes belowlocal highest summits (about 900 m a.s.l.) indicatethe former existence of a number of small glaciers,in size ranging from 0.2 km2 to 1 km2 • The deltawas probably synchronous with the existence ofthese small glaciers.
Interpretation of the lithological sequences inEnoks Havn and Hammers Dal, together with theradiocarbon datings, indicates that deglaciation ofcoastal western Disko occurred prior to 9,100 BP,somewhat earlier on the northwestern coast atHammers Dal than in Mellemfjord. At both
interpret the Enoks Havn data as showing that(a) the marine limit at Enoks Havn was probablyreached around 9,000 BP and that (b) the oldmoraines close to the marine limit can be ofsimilar age to the Disko stade moraines, as tentatively dated at Mudderbugt on eastern Disko.
BOREAS 19 (1990) Glaciation on Disko Island 307
100
100
60 200
500400
300
Io
300
500
I40 km
Fig. S. Changes in EI,A and the marine limit. (1) Approximate isolines for the calculated vertical rise of the ELA since the Diskostade; (2) L ELA observation value; (3) approximate isoline for the relative land rise since the sea stood at the marine limit; (4)measure point for the marine limit; (5) present glaciers on Disko.
ConclusionsIn summary, our observations on the glacial history and relative sea level changes are the following:
1. During the Godhavn stade, which representsthe maximum Sisimiut glaciation on Disko, theDisko Bugt was occupied by a large outletglacier from the Inland Ice. It depositedmoraines and till along the southeastern coastof Disko. An absolute minimum age for itsretreat from southeastern Disko is 9,250 BP.
The Inland Ice probably did not seriously affectthe northern and western coasts of Disko.
2. Following the retreat of the Disko Bugt glacierand deglaciation of the coastal areas, the seatransgressed to' the marine limit. Transgressionsequences have been recognized at Igpik, eastof Godhavn, and at Enoks Havn inMellemfjord, and in situ shells from Godhavnrelate to sea level near the marine limit. Wehave radiocarbon-dated its age to between9,250 and 9,000 BP.
308 Olafur Ingolfsson et al.
3. The Disko stade readvance occurred somewhatprior to the stand of sea level at the marinelimit; we suggest that it occurred around9,300 BP. The Disko Stade readvance was pronounced in eastern Disko, but glaciers elsewhere on Disko show little or no contemporaneous change.
DiscussionA map of the Disko stade extent of glaciers onDisko Island (Fig. 6), based on field evidenceand air-photo interpretation, shows that glaciersdraining ice from the eastern part of the Storbrreen (Sermerssuaq) ice cap were large, whileoutlet glaciers from the western part of the icecap were similar to the present size or smaller atthat time. Often paleoglaciological information isconverted into reconstructions of past regionaltemperature and precipitation conditions. However, the large difference in glacier size duringthe Disko stade over such a small area is difficultto explain satisfactorily in terms of temperatureand spatial distribution of precipitation. Thisraises the question concerning which other factorscan affect the equilibrium line altitude (ELA) sodifferently between glaciers facing east and otherglaciers?
A first approximation of the Disko stade ELAhas been obtained by mapping the uppermostoccurrence of Disko stade moraines. These valueswere then compared with modern ELA valuesobtained by studies of aelial photographs (Humlum 1987). The numeric differences between bothsets of ELA values were then considered as a firstapproach toward the vertical shift of ELAs sincethe Disko stade.
As discussed above, the Disko stade is tentatively dated at about 9,300 BP. During the timefollowing this, Disko and the Disko Bugt experienced considerable isostatic and eustatic movements. Today, the marine limit is about 60 m a.s.l.in the northwestern part of the island, and risesto about 90 m in the southeastern and easternparts of the island. That is, the eastern and southeastern parts of the island have risen about 30 mmore than the northwestern part. All other thingsbeing stable, this should itself make modern glaciers in the eastern part of the island larger compared with the Disko stade than in the remainingpart of the island. As can be seen from Fig. 6,
BOREAS 19 (1990)
this is not the case. Local values for relative landrise have thus to be added to the above values forthe ELA shift since the Disko stade, if morerealistic values are to be obtained.
Fig. 8 shows approximate isolines for thecalculated vertical rise of the ELA since the Diskostade. Also shown are isolines for the relativeland rise since the formation of the marine limit.In the easternmost part of the island the verticalshift in ELA is about 400-500 m, while valuesabout 200-300 m characterize most of the remaining parts of the island. In the northwestern partof the island values are as low as about 100 m.
This pattern cannot be the result of regionalchanges in temperature, as these would tend tobe almost uniform over an area as small as thatconsidered (about 100 x 100 km2
). Neither canregional changes in precipitation explain theobservations, especially not when one considersthat the largest absolute variations in precipitationwill normally be associated with areas with largeprecipitation values, that is, in the southwesternpart of the island.
Most likely, this leaves only one explanation;introducing an altered wind regime compared tothat characterizing Disko today.
Today, terrain above 500-600 m a.s.l. (Fig. 2)usually penetrates into a meridional air flow fromthe SSE. This can be seen from radiosonderecordings made at the 850 mb level (radiosondestation at Egdesminde, 65 km south of DiskoIsland, DMI 1967, 1977), and from the fact thatmost large-scale snow drift forms on glaciers andmountain plateaux on Disko are orientatedtoward the NNW. This is in contrast to conditionsat sea level, where air flow is usually from theWSW during summer, and from the ENE duringthe remaining part of the year. The influence ofvariations in the upper level meridional air flowfor the behavior of glaciers on Disko during thelast century has been discussed by Humlum(1987).
To explain the behavior of glaciers on Diskoduring the Disko stade, we suggest that the modern meridional air flow from a southerly direction,characterizing the 850 mb level (usually 9001400 m a.s.l.), did not exist during the Diskostade. Instead, we suggest the existence of a dominant air flow from a westerly direction at the850 mb level during that period. This would havecaused a heavy snow drift from westerly directionson the uppermost parts of the island, leading toincreaselJ accumulation on glaciers facing east.
4
BOREAS 19 (1990)
This effect would be especially important for glaciers with large mountain plateaux west of theiraccumulation areas, acting as source areas fordrift snow, and for ice cap sectors facing east (seediscussion in Humlum 1987).
This hypothesis could explain why only outletglaciers from sectors facing east from the largeStorbneen ice cap advanced considerably duringthe Disko stade to positions much beyond theirlater Little Ice Age moraines, while outlet glaciersfacing differently did not. This explanation wastested on a smaller ice cap, the 125 km2 Lyngmarksbreen north of Godhavn (Fig. 4). Also hereit was found that the outlet glaciers facing eastadvanced more than other outlet glaciers duringthe Disko stade, compared with both their modern size and with their size during the Little IceAge.
What could have caused such a considerabledifference in air flow conditions at the 850 mblevel during the Disko Stage from modern conditions with a meridional air flow from a southerlydirection? Most likely the existence at that timeof the Laurentide Ice Sheet towards the west wasthe direct cause. In a recent study by Shinn &Barron (1989) it was argued that the generalatmospheric circulation during the last glacialmaximum could have been markedly differentfrom that of today over central West Greenland.Due to the presence of the former LaurentideIce Sheet, which is assumed to have penetrated1,200-2,500 m upwards into the westerly air flow,the jetstream would have been split around theice sheet. One branch ofthe jetstream would havestrengthened and gone south of the LaurentideIce Sheet and another branch would have followed the northern margin. The northern branchwould have had a strong westerly componentover central West Greenland. By 9,500 BP theLaurentide Ice Sheet had retreated considerablycompared to its full glacial size, but still had itswestern margin close to the present coast in theBaffin Bay/Davis Strait area (Andrews 1987;Hughes 1987). By 9,000 BP it still covered vastareas in North America, with margins on BaffinIsland and in Quebec (Fuiton 1989). Regional airflow at the 850 mb level can then be envisagedfollowing the northern part of the Laurentide IceSheet from the west towards the east, leading toheavy snow drift from the west on high terrainpenetrating into this air flow system.
An advance of glaciers facing east would thenbe expected to be limited not only to the Disko
Glaciation on Disko Island 309
area, but may also have occurred at other placeswith high terrain in both Greenland and northeastern Canada during this time. In a recentreview by Andrews (1987), considerable readvances are recognized on Baffin Island around10,000 BP and between 8,500 and 8,000 BP, whilethe period 10,000-9,000 BP seems to have beencharacterized mainly by rapid glacial retreat.However, considering topography, few places areprobably as well suited as Disko Island, with itshigh mountain plateaux, for displaying as cleara glaciological response to changes in the windregime as that described in the present paper.
The onset of present-day meridional air flow inWest Greenland may have been contemporaneous with the appearance of the West Greenland Current, dated to c. 9,100 BP by FeylingHanssen & Funder (1990), or it may have beenas late as 7,500 BP, as evidenced by high frequencies of North American pollen in WestGreenland (Fredskild 1984, 1988).
Our results, together with Humlum's (1987)study of the influence of variations in upper-airwind conditions on glacier behavior, also implythat wind conditions in accumulation areas shouldnot be neglected when dealing with climatic variables in paleoclimatic reconstructions. Hoinkes(1957) and Kotlyakov (1973) concluded thatsnowdrift could be as important a factor for themass balance of valley glaciers as precipitation.In a study by Meier (1965) it was concluded thataccumulation due to windblown snow during andafter storms on the Dinwoody and Grinell glaciersin the eastern Rocky Mountains was as much astwice the local annual precipitation. Sissons &Sutherland (1976) tried to assess the effect ofsnowdrift and avalanching on Loch LomondReadvance glaciers in the southeastern Grampians. They calculated that between c. 10% and22% of accumulation on these glaciers had comefrom these sources. A study by 0strem & Tvede(1986) showed that a change in the frequency ofwesterly winds at the 850 mb level had considerably changed the mass balance distributionof the Midtre Folgefonni ice cap, westernNorway. We condude that paleoclimatic reconstructions, where the effects of wind are takeninto consideration, could reveal interestingaspects not only as regards glacier mass balanceand accumulation, but also as regards areas wherethe lack of signs of glaciation has formerly beeninterl(reted as lack of precipitation, or in terms ofbasal temperatures of former ice sheets.
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310 Olafur Ing6lfsson et al.
Acknowledgements. - The field observations reported here arethe results of different projects carried out by the authorsindependently over a period of ten years. However, all theprojects have benefited from the use of facilities at the University of Copenhagen's Arctic Station in Godhavn. We gratefully acknowledge the support given by the board of directorsof the Arctic Station, and thank especially Jakob Broberg,skipper on the Station's vessel 'Porsild' for accompanying us onmany jotlrneys. The 14C dates determined at the RadiocarbonLaboratory of the National Museum and the Geological Surveyof Denmark were obtained by courtesy of the latter institution.S. Funder's field work in 1980 and 1981 and O. Humlum's fieldwork in 1987 were supported by grants from the Danish NaturalScience Research Council. The work of O. Ing6lfsson has beensupported by the Swedish Natural Science Research Council,and funds for field work in 1987 were provided by NordiskaForskarstipendier and Kungliga Fysiografiska Sallskapet i Lund.
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