The Late Weichselian glacial maximum on western Spitsbergen inferred from offshore sediment cores

Marine Geoh~gy, 104 (1992) I - 17 Elsevier Science Publishers B.V., Amsterdam

The Late Weichselian glacial maximum on western Spitsbergen inferred from offshore sediment cores John lnge Svendsen", Jan Mangerud", Anders Elverhoi b, Anders Solheim c and Ruud T.E. Schiittenhelm d 'Department q/'Geoh~gy, Sect. B, University of Bergen, dllbgt. 41, N-5007 Bergen, Norwa.; bDepartment oj'Geology, University o/Oslo, Box 1047, 0316 Osh~ 3, Norwal, ~Norwegian Polar Research hlstitute, Box 158, N-1330 Oslo Lufthavn, Norway dR(jks Geoh~gische Dienst, Hot!fiiq['deling Ondiepe Ondergrond, Spaarne 17, 2001 CD Haarlem, The Netherlands (Received April I I, 1991; revision accepted August 5, 1991)

A BSTRACT Svendsen, J.l., Mangerud, J., Elverhm, A., Solheim, A. and Schiillenhelm, R.T.E., 1992. The Late Weichselian glacial maximum on western Spitsbergen inferred from offshore sediment cores. Mar. Geol., 104: I-17. The Late Weichselian glacial history of the continental shelf offweslern Spitsbergen is discussed, based on acoustic sub-bottom records and sediment cores. The outer part of Isl.)orden and the inner shelf to the west of this I]ord are characterized by a thin veneer ( 10-20 m) of glacigenic sediments and absence of ice-marginal features. Towards the outer shelf the sediment thickness increases sigmficantly, and exceeds 500 m at the shelf edge. Possible moraine complexes were identified in this outer part. Sediment cores from the inner shelf sampled a firm diamicton, interpreted as till, beneath soft glaciomarine sediments. Radiocarbon dates on shells from the clay resting directly on the till, suggest an age of around 12,500 yrs B.P. for the base of the marine sequence. We argue that grounded ice covered the sites shortly before. In contrast to suggestions that the I]ords and coast were partly tee free during the Late Weichselian, we conclude that the ice must have reached out onto Ihe continental shell'.

Introduction

The nature and timing of the most recent glaciation in the Svalbard region remains controversial in spite of comprehensive investigations during the last decade. Some field studies suggest that the western coast of Spitsbergen in part remained ice free during the Late Weichselian (Boulton, 1979. 1990; Salvigsen and Nydal, 1981; Boulton et al., 1982; Forman, 1989: Miller et al., 1989). Other reconstructions depict an extensive Barents Sea ice sheet covering the entire archipelago of Svalbard (Grosswald, 1980; Denton and Hughes, 1981). Understanding the glacial history of lsfjorden, the largest and most central fjord system on Spitsbergen (Fig. I), is crucial for addressing questions concerning the Late Weichselian glaciation of Svalbard. Mangerud et al. (1987) presented a 0025-3227/92/$05.00

minimum model for the extent of the glacier(s) in western Spitsbergen showing that the entire coastal area around this fjord was ice covered, and that the ice margin was probably located offshore. This is further documented by Mangerud et al. (1991). To determine the extent of the Late Weichselian glacial maximum and to establish a deglaciation chronology, it was essential to map the seafloor sediments in the fjord and on the continental shelf. Investigations of the fjord sediments (EIverhoi et al., 1983) showed limited thickness (10-15 m) above the bedrock and it was evident that important stratigraphic information could be obtained by conventional gravity/piston coring. In this paper we describe acoustic sub-bottom profiles and sediment cores raised from the floor of Isfjorden and the adjacent shelf area (Fig I). Important objectives of the field work were to

(¢~') 1992 -- Elsevier Science Publishers B.V. All rights reserved

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LATE WEICHS~LIAN ICE LIMIT m m e m m M a l l m l , l m Iii'~I llllnll

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Fig. I. Bathymetric map of Isl~orden and the adjacent continental shell', western Sp;t:bergen. The map shows the location of subbottom acoustic profiles and cores obtained in 1987 and 1988. Selected sections shown in Figs. 2 and 3 are also indicated. A minimum and maximum limit for the position of the ice front during the Late Weichselian glacial maximum is indicated in the map. The inset maps show the location of the study area on Svalbard and in the Norwegian Sea.

locate possible ice-marginal features, and to obtain sediment sequences dating back to the last glaciation.

Setting Isfjorden is about 100 km long and 20 km wide, with a maximum depth of 400 m in the central part (Fig. !). The fjord continues as a submarine trough onto the continental shelf. There are no major sills, but several transverse bedrock ridges separate sub-basins. The present hydrographic condition is influenced by both the West Spitsbergeh Current, which supplies Atlantic water to the shelf area, and by a cold coastal current which originates from the Barents Sea (Loeng, 1988).

Approximately 40% of the catchment area around Isfjorden is covered by glaciers. Subpolar valley glaciers terminate in the sea along the northern shore and at the head of the main fjord. The maximum extension of the glaciers during the Holocene is marked by prominent end moraines formed during the Little Ice Age. The glaciers have been generally retreating during the last century (Liest~d, 1988). Methods

Data acquisition The data base for the present study was acquired during two cruises, one in 1987 with the Norwegian

L,A'TE WEICHSELIAN GLACIAL M A X I M L I M ON WESTERN SPITSBERGEN

3

Polar Research Institute, and the other in 1988 with the Geological Survey of the Netherlands (Fig. I). During the 1987 cruise, seismic profiles were obtained ,,sing a I kJ Hartley H M L sparker system with a 9-electrode array, and analogue recording via a Benthos Mod. 25/50 P single channel seismic streamer with a 50 element, 7.5 m active section. Band pass filter width was set to 80-500 Hz. An O.R.E. 3.5 kHz hull-mounted echosounder (PDR} with a Mod. 140 transceiver unit was used continuously, giving high resolution sub-bottom records of the upper soft sediment layers. In 1988, seismic profiling was done using a 12-channel streamer, and a 15 in 3 sleeve air gun as a source (see profile II, Fig. I). The coring sites (Fig. I) were chosen from the acoustic records, and coring was conducted in 1987 with a 6 m long, I I0 mm diameter gravity corer (core nos. 137-144) and in 1988 with a 6 m long 80ram diameter piston corer (core rips. 01-04). Navigation with an accuracy of better than 100 m was ohtained by means of a GPS system combined with Loran C and a rubidium oscillator. The 3.5 kHz system was used continuously also during coring operations, and was an important tool in accurately re-locating the chosen coring sites.

berg Laboratory in Uppsala and two conventional radiocarbon dates were provided by the Radiological Dating Laboratory in Tronoheim (Tabie i). All shell dates are corrected for a marine reservoir age of 440 yrs (Mangerud and Gulliksen, 1975).

Lahoratorr anal l,ses

All laboratory analys~:s and core descriptions were done at the Department o~' Geology, Sect. B, University of Bergen, Norway. The cores were first X-rayed for detailed structural studies, and were then split lengthwise. Undrained shear strength was measured, when possible, by means of a falling cone penetrometer. Grain siz,: distribution was determined by pipette analyses o1" the grades finer than 0.063 mm and wet sievink, of the coarser fractions. The carbonate and organic carbon content has been determined with an EC 12 Leco carbon analyzer. Radiocarbon dales

Fifteen accelerator mass spectrometry (AMS) dates of small shells were obLained from the Sved-

Sediment distribution and acoustic stratigraphy In general, the outer fjord and inner shelf are characterized by a relatively thin (10-20 m) veneer of glacigenic sediments above the underlying bedrock (Figs. 2, 3 and 4). The boundary between the sediments and the bedrock is defined by a distinct upper regional unconformity (URU) (Fig. 2), which is a typical feature for glaciated shelves (Solheim and Kristoffersen, 1984; King and Fader, 1986; Vorren et al., 1988). Locally, sediment thicknesses of up to 60 m are found in minor subbasins, whereas less than l0 m are found on bathymetric highs (Figs. 3 and 4). Svenskesunddjupet, a larger depression near the fjord mouth (Fig. I), contains 100-150 m of glacigenic sediments. Steep slopes to the north and fault features in the sediments, combined with a hummocky sea floor topography suggest, slides/slumps as an important source for this sediment accumulation (cf. Syvitski et al., 1987). An acoustically transparent character is typical for the fjord and fjord mouth sediments, whereas more acoustically opaque sediments are found on the shelf. In general, three nternal reflectors are seen in the sediments above the bedrock on the 3,5 k Hz echosounding profiles from the fjord (Fig. 4). The shallowest reflec:c,r (A) is found on most of the profiles at the 2-4 m sediment depth. Sediment cores which penetrate this level demonstrafe that this reflector occurs within the Holocene marine sediment sequence, and in some cores a higher pebble content is observed at this level. Reflector B is generally found at the 5- l0 m depth. One core (87-144) from the central Oord which penetrated this reflector demonstrates that the underlying unit represents pebbly glaciomarine deposits from a period p:mr to 10,000 yrs B.P. when extensive glaciers still occupied the inner fjord branches (Mangerud et al., 1991). The lowermost reflector, C on the 3.5 kHz profiles, is generally 5-10 m above the assumed sediment/

4

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i.-: 1.0 Fig. 2. Seismic prohle (sleeve air gun) across the West Spitsbergen shell'. See Fig. I for location of profile (la and Ib). This profile was recorded during the Dutch cruise in 1988. Note the thick wedge o1" sediments above bedrock on the outer shelf. The i,neven topography on Ihe outer 35 km is interpreted as a moraine complex.

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bedrock boundary as inferred from the sparker profiles (Figs. 3 and 4). This reflector is suggested to represent the surface of a firm diamicton, essentially opaque to the 3.5 kHz signal. At core station 01, 02 and 04 on the inner shelf (Fig. I), these sediments were cored at a depth of less than 2.5 m, demonstrating a more condensed transparent sediment sequence than found in the I~ord. Towards the outer shelf the sediment thickness increases significantly, and exceeds 500 m at the shelf edge (Fig. 2). A broad uneven ridge-like feature on the southern side of the Isfjorden trough may be part of a moraine complex, as also inferred by Ohta (1982) from the shelf bathymetry. Their glacial origin is interpreted on the basis of the acoustic character on the seismic profiles showing a uniform dense pattern of incoherent reflections. With the possible exception of this ridge complex, the high resolution acoustic data do not show any

LATE WEICHSE, LIAN GLACIAL MAXIMLIM ON WESTERN SPI'IISBERGEN

5

TABLE I '4C datings Core no.

Position Lat.

88"01 88"01 88"01 88"02 88"02 88"03 88"03 88"04 88"04 88"04 87"144 87"144 87"144 87-137 87" 137 87" 137 87" 138

78"02.8" 78"02.8' 78"02.8' 78'02.8' 78"02.8' 78"01.0' 78"01.0' 78'01.0' 78'01.0' 78"01.0' 78"16.6' 78"16.6' 78 '16.6' 78"02.3' 78"02.Y 78"02.3' 78"05.7'

Long.

2"59.3' 2"59.3' 2"59.3' 2"59.3' 2"59.Y 1"41.4" 1"41.4' 1"39.9' 1"39.9'

•

1"39.9' 5"15.6' 5"15.6' 5"15.6' 2' 52.8' 2"52.8' 2 '52.8' 3"04.7'

Waler depl h

Core depl h

(m)

(cm)

270 270 270 271 271 234 234 232 232 232. 228 228 228 271 271 271 271

88 136 204 140 220 26 105 83 142

210 10 217 362 150 296 373 55

Daled species

t4C-Age ( B.P. )

Lab. ref.

Nuc,da telmis Nucula tenuis Astarte ellipth'a

12,080 _+935 11,605 _.+325 11,675 + 180 10,810_+ 115 12,545 + 145 1(I,365_.+155 I 1,605 + 180 10,235 + 260 40,000 40,000 560 -t- 195 6435 _.+305 10,395_.+ 140 7000 + 195 10,070 + 90 10,105 + 140

TUa-38 TUa-39 Tua-40 TUa-41 TUa-42 TUa-43 TUa-44 TUa-45 TUa-46 TUa-47 Ua- 1001 Ua-1047 Ua-757 Ua- 1002 T-8182 Ua-756 TI8183

Shell, unident. Nucula temds Lepeta cocoa Nucula tenuis Acmaea rubecula ?

Shell fragment Shell fragmenl Yoldiella lenticula Nuculana mimaa

Shell, unident. Yohliella h,nticula Neptuna ,h,nseliruta Nuculaua permda Mra trum'ata

1980 ..t-50

Radiocarbon dates from cores. All daled saml"les are marine molluscs. The dates are conJecled for isolopic fractionation to - 25% PDB. A reservoir age of 440 yrs is sublracted (Mangerud and Gulliksen, 1975). Thus Ihe dales should be directly comparabne with dales from terrestrial plants. Fifteen accelerator dates (AMS) were oblained by Ihe Svedberg Laboralory, Uppsala (TUa/Ua). (Samples which are lettered TUa were prelrealed at the Trondheim Radiocarbon Laboratory). Two convenlional dales were obtained by the Trondheim Radiocarbon Laboratory.

evidence o f ice marginal features in the main I]ord and inner shell'.

Litho~tratigraphy A total o f 12 cores was obtained from the shell" and the central part o f Isfjorden (Fig. I). None o f the cores taken in 1987 penetrated the entire marine sequence (Fig. 5). However, three cores (nos. 01, 02 and 04) from 1988 penetrated into a firm diamicton. The sediments have been subdivided into four informal lithostratigraphic formations (Fig. 5): (I) firm diamicton, (2) grey mud, ( 3 ) l a m i n a t e d mud and (4) olive grey mud. This subdivision is partly based on colour, but the units are also distinguished by other parameters, such as grain size, undrained shear strength, organic carbon and carbonate content. A description of each lithostratigraphic unit is given below.

Firm diamicton

Three cores penetrate into this diamicton (Fig. 5). Cores 01 and 02 were collected less than 200 m apart, 6 km west of the mouth o f Isljorden (Fig. I). The third core (04) was taken ca. 40 km farther west. The diamicton is light grey (munsell soil colour chart), and poorly sorted (Figs. 6-8). It is entirely homogeneous and is characterized by a firm and compact consistency. The coarse material consists mainly of sub-angular clasts, several which are striated. The relatively low ( 1 0 - 1 5 % o f wet weight) water content, and undrained shear strength values of about 50 kPa (at 2 m sediment depth in core 04), indicate a slight over-consolidation (Fig. 8). Reliable measurements by fall cone penetrometers were impossible to carry out in the two other cores due to the high pebble content. The diamicton is characterized by a relatively high carbonate content, which is somewhat higher in core 01 ( 1 8 - 2 2 % ) as c o m p a r e d to core 04

J I SVENDSEN ET AL

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(10- 15%) {Figs. 7 and 8). The carbonate is detrital, and biogenic carbonate is rarely found in the sediments. Likely source rocks for the de trital carbonate are Carboniferous-Permian limestones and dolomttes which occurs both in the outer and inner parts of the fjord system ( Flood et al., 1971 ). No systematic identification of the rock types in the sand and gravel fraction was completed, but a few samples from core 01 showed more Ilhan 50% local Precambrian (Hecla Hook) phyllites, which are exposed onshore along the western coast of Spitsbergen (Flood et al., 1971; Winsnes, 1988) and are thought to subcrop extensively offshore. in addition, unmetamorphic silt/sandstones were frequently identified as well as some carbonate clasts. Two accelerator radiocarbon dates (TUa-46 and TUa-47) on small shell fragments were obtained from this diamicton in core 04 (Table I, Fig. 8), both yielding infinite (more than 40,000 yrs B.P.) ages. Sediments with texture and geotechnical proper-

ties similar to those found for the firm diamicton are commonly observed on formerly glaciated continental shelves (Rokoengen et al., 1979; Kravitz, 1983; Elverhoi et al., 1990). The origins of these are widely discussed. We cannot rule out entirely that the sediment was produced by intensive icerafting, but we consider this possibility very unlikely. As to the formation of this 'slightly' overcompacted diamicton in these cores Ihere are only two feasible interpretations - - intensively iceberg gouged glaciomarine sediments or a subglacial formation. Possible iceberg ploughmarks on the shelf were observed from the acoustic records and locally the seafloor sediments may have been reworked by grounded icebergs. However, considering that the firm diamicton is completely homogeneous throughout the formation in all cores with a distinctly different iithology as compared to the soft grey mud above, iceberg ploughing is not a very plausible explanation. The poor grain size sorting, the almost complete absence of microfossils and the fact that the sedin~ent is slightly overcomp:'cted also argue against the iceberg ploughing hypothesis. In the Barents Sea, sediments like the firm diamicton seem to correspond to a glacially fluted surface (Solheim et al., 1990), i.e. the sediments represent subglacial deposition. Furthermore, the diamicton's textures, lack of structures and general appearance make it comparable to the tills we have studied o:~. the adiacent land (Mangerud et al., 1991). We therefore conclude that the ,hrm diamicton on the, Svalbard shelf represents subglacial deposits, i.e. a till.

Grey mud This unit rests directi.~ on top of the basal diamicton~ in core nos. 01, 02 and 04 (Figs. 5-8). In core 137 (Fig. 9) and 138 the grey mud represents the lowest obtained unit. The lower boundary is sharp and, unlike the firm diamict.on below, the grey mud is soft and has only scattered dropstones. The water content is 20-30% of wet weight and undrained shear strength is around 5 kPa (Figs. 7 andt 8). The sediments are mostly massive, due to bi~turbation, but a faint horizontal zonation is distinguishable from the X-ray pictures and by variation in colour shades when fresh sediment is viewed.

LATE WEICHSELIAN GLACIAL MAXIMLIM ON WESTERN SPI'I"SBE',RGEN

7

Inner Shelf

Fjord

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WATER DEPTH 232 0 , ~-~,~

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871137 87/138 871139 87/140 271 ?.71 247 272

87/141 337

87/142 212

87/143 235

87/144 228

General stratigraphy

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Ftg. 5. Composite lithostruligraphy and sediment cores from lsl.]orden and the ad.iacenl shell'. The sedlmenls have been subdivided into Ibur mlbrmal lilhoslratigraphic formations as indicaled by the column to Ihe righl. All =adiocarbon dates are indicaled according to core deplh. The core locations are shown in Fig. I.

Well-defined pebbly diamictons are interbedded with the grey mud in cores 01 and 02 (Figs. 5, 6 .and 8). The diamictic bed in core 01 resembles the lower diamicton; it is compact, poorly sorted and r=as a similar light grey colour. However, as ogposed to the lower diamicton the grain size becomes finer upwards and it contains a few welldefined mud laminae. The carbonate content is also significantly lower. The diamictic bed in core 02 from the same station is only 8-10 cm thick (Fig. 5), occurs almost at the top of the grey mud and is significantly different from the lower diamicton. It has a reddish brown, clayey matrix. Eight radiocarbon dates from this unit yielded ages between 12,500 and 10,070 yrs B.P. (Fig. 5, Table I). The entire formation is interpreted as a glaciomarine sediment which was deposited during glacial withdrawal. The distinct diamicton beds in cores 01 and 02 most likely represent single dumping events, probably from ice bergs.

Lamhul/ed mud This formation wa:: recognized only in core I~,~ where it represents the lowermost lithostr,".tigraphic unit (Figs. l0 and I I). It is correlated with a similar type of sediment at the 6 - l 0 m depth in a l0 m long core from the central part of Esfjorden (EIverhoi e t a ! , 1983). The upper boundary for this unit corresponds to reflector B on the subbottom profiles (Fig. 4). The sediment contains a considerable amount of pebbles embedded in a laminated clayey matrix (Fig. I I). There are some well-defined gravelly beds up to a few centimetres in thickness. The sediment matrix is characterized by a high content (9-18%) of detrital carbonate, which most likely is derived from the Carboniferous-Permian limestones in the inner I~ord area (Flood et al., 1971; Lauritzen et al., 1989). The laminated sediments with frequent droppstones is interpreted as deposited in a glaciomarine environment and thus implies that large glaciers

J I SVENDSEN E l AL

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Fig. 6. Photos of cores 88-01 (A) and 88-04 (B) showing the transition IYomfirmdiamiclon(till) to soft, grey(glaciomarine) mud, above. terminated in the fjord. A radiocarbon date from the base of the laminated mud in core 144 yie,lded the age !0,395+_ 140 yrs B.P. (Ua-757), whereas a shell san~ple from near the upper boundary for this forrnation gave the unexpected age of 6435+_305 yrs B.P. (Ua-1047)(Fig. I I). The latter date is considered to be at least 3000 yrs too young for that level, since other studies have demonstrated that the inner fjord branches were deglaciated shortly after 10,000 yrs B.P. (Mangerud et al., 1991). Additional support for a Late WeichselJan age of the entire laminated sequence is also provided by oxygen isotope analyses which show that foraminifera from this unit predate termination IB at around the Younger Dryas/Holocene boundary (Eystein Jansen, pers. commun., 1990). We suggest that the dated shell, which was collected less than 5cm below the olive grey mud, had burrowed down from this overlying unit. We therefore conclude that the laminated mud is older than 9700 yrs B.P. and that there is a hiatus of 3000 years between the laminated mud and the olive grey mud in this core (Fig. I I).

This unit represents the uppermost formation in all cores (Fig. 5) and is distinguished from the sediments below by an olive grey colour when exposed to air, and also by a lower clay content (Fig. 12). A similar type of sediments occurs in the same stratigraphic position in a number of cores from the Barents Sea (EIverhoi and Solheim, 1983; EIverhoi et al., 1989). The sediments are generally homogeneous, but the content of pebbles and stones is highly variable. In the cores from the I~ord there are several welldefined pebbly beds interbedded in a more or less stone free mud. However, a detailed correlaticn between different cores was not possible. The exception is the core tops which are generally enriched in stones and gravel, most likely as a result of increased ice-rafting during the Little Ice Age. in some of the cores (137, 138 and 04), two well-defined sub-units can be recognized within the olive grey mud (Figs. 5, 8 and 9). The lower subunit is light grey in colour when fresh sediment is viewed whereas the upper one is heavily stained by monosulphides causing a mottled appearance. This upper subunit is also distinguished by a significantly higher content of organic carbon as compared to the sediments 5elow, and is ofrLen intensively bioturbated by Polychaetes. In core 1137 and 138 there is a 15-20cm thick, well-defined sandy silt bed between the two muddy subunits (Fig. 9). The lower subunit was not recognized in core 01 or 02, which may indicate a hiatus between the grey mud and the olive grey mud in these cores. The lateral distribution of this sub-unit is therefore unknown. The lower boundary for the olive grey mud is dated to about I0,000 yrs B.P. (Fig. 5). A similar age was also suggested for the corresponding lithostratigraphic boundary in the Barents Sea (Elverhoi and Solheim, 1983; Elverhoi et al., 1989). The available radiocarbon dates indicate that the light grey subunit was deposited during the early Holocene whereas the upper sulphide stained subunit appears to be younger than ca 7000 yrs B.P. Sedimentation rates

The sedimentation rate for the grey (pre Holocene) mud in cores 01 and 02 is in the order of

LATE W E I C H S E L I A N G L A C I A L M A X I M U M O N W E S ' I E R N S P I T S B E R G E N

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I

Compact, pebbly diamicton (interpreted as till)

I,~,~&'~'~AI Poorly sorted diamicton, I~~1 probably ice rafted

I C,ayey mud

Fig. 7. Description o1" lilhostraligraphy in core 88-01 showing radiocarbon dales, gram size dislribulton, waler conlenl, organic carbon and c;,rbonale conlenls.

0.5 m/1000 yrs as compared to 0.1 m/1000 yrs for

the olive grey (Holocene) mud. Similar rates have also been observed in outer Kongsl]orden (Elverhoi et al., 1983). With the possible exception of core 04 (Fig. 8), all cores show a marked decrease in sedimentation rate through time. Depositiomzl en virotmwnl

The fine grained nature o1" the entire soft sequence above the firm diamicton (reflector C)

shows that deposition was predominantly from suspension. The grey mud and the laminated mud were both deposited during glacial withdrawal, and the clayey matrix suggests a turbid sea water loaded with glacially derived suspended sediments. The content of gravel is probably mainly due to iceberg rafting (Dowdeswell and Dowdeswell, 1989) even though part of it might have been transported by sea ice. The proximity of glaciers is considered to be one of the most important factors influencing sedimentation in glaciomarine

10

J I, SVENDSENE1 AL 88-04

Pos=tion • 78°1.0'N, 11°39.9'E

0 o

Grain size distribution Undrained shear Water~lravel sand sdt clay o g~,~ I:---~1 I strength content kPa o 2 0 0 063 0 002mm -=_ 50 20% 20 40 60 80 % In

"D

•

I

1

•

:___=____

I

I I I I I

I

I~

Organic carbon (TOC) l

I I~l~

tO .D h-Ill

o

2%

10%

7l i

I

"l

100" ~ ' ~

t

L &&&

°

( -N

E

"

200- .~_

"7

Ik&& &&& &&& &&& &&A &&&

U.

&&& i~A A

Z

I ~.',~, ,r:,.~"~"Grey, pebbly mud

I .orn eneous. (oxydized) ,Compact., pebbly diamicton (inlerpreleo as tdl)

Fig. 8. Description o1" hthostraligraphy in core 88-04 showing radiocarbon dales, grain sized dislribulion, waler contenl, undrained shear sirenglh (kPa), organic carbon and carbonale conlenl.

environments (Powell, 1981; Andrews and Matsch, 1983" Eyles et al., 1985; Syvitski, 1989). This probably also applies to the sedimentation environment on Svalbard during deglaciation (Boulton, 1990). The nearly massive and uniform lithology and the relatively low content of ice rafted detritus suggests that the grey mud was deposited in an ice-distal e,,wironment whereas the laminated and pebbly mud represents a more ice-proximal fjord facies. The Holocene sequence differs from the Late Weichseliar, :;ediments by lower clay content, and lower sedimentation rates (Fig. 7). 1 his is probab!:~, due to a longer distance to glaciers. In addition, the sites were closer to the sediment source, i.e. the glacier margin, during the deglaciation period as compared to the Holocene. Pfirman and Solheim (1989) have showed that up to 90% of the suspt:nded sediments are deposited within 5 km of the present day meltwater outlet at the Austfonna ice cap in Nordaustlandet, Svalbard. Similar results

were reported for Arctic Canadian fjords by Syvitski (1989). There is no evidence of increased current activity during deglaciation, but the assumed hiati in cores 01 and 144 (Figs. 8 and I I) and also the sandy mud layers in cores 137 and 138 (Figs. 5 and 9) indicate periods of erosion and winnowing around 6000-7000 yrs B.P. This may be the result of increased current activity in the early/mid Holocene. Also in the Barents Sea, bank areas have been exposed to bottom current erosion during the Holocene leaving a lag deposit (Bjorlykke et al., 1978; Vorren et al., 1984). Along the western margin of ~pitsberk, e;~ !ong-~helf currents produce reworking and scouring on the mid- and oute. shelf and it is suggested that the current activity might have been even stronger during the early Holocene due to a lower relative sea level (Boullon, 1990). However, the sea level at the fjord mouth around 6000-7000 yrs B.P. was close to the present level (Landvik el al., 1987), and the increased

LATF WFICHS~-LIAN GLACIAL MAXIMUM ON WESTERN SPITSBERGEN

II

87-137 Positron • 78u2.3'N. 12°52.8'E

~

Core

"= o=~ ~,..,, ~° .~ ~ ~ G r a i n ~ s tdistribution ze

~" ~ ~ 6 a _ ~ ~'.

OrganiCcarbon -ER

20 00G30002mm (TOC) t 40t 60t 80%1 I ~ t

-~ ..3

depth

(D

(crn~

m ~/0llO

3~ I-"

100-

~.

3"'! m,

ID e,,,,i, (D e,J

~. p.-e-. ~' . 200- ~

:.--:--8- -._

=1

eQ.

- i

. . . .

A

.<

3E O 0

300"

, o._ o.-'~,., ~

400"

'" - Z. - Z . -.Z -. Z.- ~. - -. - - ' Z " . . . . . . . .

0 O'} ÷1 O r'-

°°o u

,-o

I

,< ID

j

r

.......

i',i',iii ,iii

ci

I I ~ ' ~ / ~ sullpdestained' silty mud

I

"

-_-- '_-- "_-" -Z- -- --- -------.---

I

I

3~

Sandy mud

I H°m°ge e°us' s''t' ,ox, ze )

Fig. q Descriptton of hlhostr~ligr~phy in core 87-137 ,,howmg radiocarbon dales, grain size distributton, orgamc carbon and

Core 144 Fig 10. P h o l o s h o w i n g the l a m i n a t e d m u d in core 87-144. See Fig. I I for description o f h l h o s t r a t l g r a p h y .

¢ a r b o n a l e conlenl.

current activity in this particular area can not have been due to shallower water depths on the shell'. Changes in the depositional environment in the I]ord and inner shelf might also in part be related to the general oceanogr;.phic condition. Thermophilous mollusc taxa frot ~central Spitsbt rgen indicate warmer coastal wat::r between 950G and 5000 yrs B.P. (Feyling-Hans~'n, 1955; Feyling-Hansen and Olsson, 1960; Salv,gsc,I et al., 1990) and diatoms from the Norwegian Sea indicate a stronger influx of 'warm' Atlantic water at this time (Koc Karpuz and !';chrader, 1990). A stronger influx of Atlantic wdi,:'r into the Norwegian Sea might have speeded Ul:: the West Spitsbergen Cur-

rent at this time, and thus increased scouring of the shelf areas along the western margin of Spitsbergen.

Glacial history The age

of the last ghzeiation of is/'l'orch'n

As discussed above, the firm diamicton most likely represents ;~ till. The sequence o;" ~iii i'ollowed by soft glaciomarine sediments suggests that grounded ice extended more than 49 km off the coast (Fig. I). However, as only infinite radiocarbon ages have been obtained from the assumed

12

J.I S V E N D S E N E'T' AL,

Core depth

87 - 144 Position • 78016.6'N, 15°15.6'E m

z" ~" t't

Grain size distribution

~..~

2.

°

=1~. (,~ ..~ ~ = " -" ~ --,i, I

2.0 .0 063 00=mm

~o ,,o 6o 60./° t Pc ~'~"

Organ,c carbon Carbonate (TOC) content I

I 2% I

-- _-....-:

I

I

10 20°A

l,

Lr

(cm~

I

I

I

'

0 -r< OtD 5"©

1o0- ~

160

;

. . . . . . . . -" _ --..~-

--'---"

3O0- ~

C

Q.

<

~,

~

m3

-==:::::=_-=:

-h~ ¢-rs.r , ,

,

,

,,

Core 04 Fig. 12. Photo showing the olive grey mud in core 88-04. See

Fig. 8 I%~rdescription of lithoslraligraphy. ~

Sulfide stained, silty mud

I

I Laminated, pebbly mud

Fig. I I. Descrtpltono1"lithostraligraphyin core 87-144showing radiocarbon dates, grain size dislributton,organic carbon and carbonate content. till, the timing of this glacial advance is not known. The sequence can most easily be interpreted in two ways: Aiternative i The lower part of the soft glaciomarine sediments were deposited from the retreating glacier that previously deposited the till. Thus, the radiocarbon dates from the grey mud show that the last deglaciation occurred 12,000-13,000 yrs B.P. A similar young age for the acoustic reflector C is obtained by extrapolation of the sedimentation rate for the lower parts ofcores 137 and 144 (Figs. 9 and I I). According to this interpretation the fjord mouth and the inner shelf musl have been glaciated just prior to 12,500 yrs B.P. As argued below, we favour this interpretation. The oldest date (12,545 + 145; Core 2, Fig. 5)is insignificantly older than the oldest date Mangerud and Svendsen

(Iq90b) obtained from basal marine sediments in Linn6vatnet ( 12,315_ 190 yrs B.P.). This suggests a fast deglaciation, or that we did not obtain the oldest fauna from the shell'. Alternative il There is a major hiatus between the till and the sediments above, and the last glaciation of the region predates the Late Weichselian. No evidence was found that might indicate a major hiatus. Lag deposits or other signs of winnowing or erosion at the transition between the till and the marine sediments were not observed. Nor is there any evidence for slides or turbidities. Also, the flat topography and the undisturbed character of the entire marine sequence make these processes less likely. The water depths at the cored sites (01/02: 270 m and 04: 232m) imply that the seafloor throughout the last ice-free period would have been covered by more than a hundred metres of water if it was ice-free. The sediments have therefore never been exposed to a subaerial or shallow water environment. Interruption of sedimentation or disturbances of older marine depomts

LATE WEICHSELIAN GLACIAL M A X I M U M ON WESTERN SPI'TSBE,RGF'N

13

caused by iceberg ploughing cannot be ruled out. However, considering the relatively thin (less than 5-10 m) drape of acoustically transparent sediments over the entire inner shelf, we find it difficult to understand that the area sh..,uld have remained ice free throughout the Middle and Late Weichselian. A frozen base ice sheet ending as a tide water front may have contributed little sediment to the marine enviru~unenl during the Late Weichselian cold stage and theoretically it ts possible to imagine a very slow sedimentation on the shelf during glacial retreat (Powell, 1984). However, from studies of emerged sediments it is evident that partly open conditions existed during the period prior to the Late Wechselian glacial maximum (cf. Miller et al., 1989; Mangerud et al., 1991) and that large volumes of glacially derived mud was carried to Isl~orden at this time (Lonne and Mangerud, 1991 ). Thus, a much thicker sequence of glacimarine sediments should be expected to have accumulated on the sea floor if not overridden or removed by a younger glacier advance. We therefore argue in favour of a Late Weichselian age for the last 0ord and shelf glaciation. This conclusion is supported by the fact that the sediment thickness in Isl.~orden is similar to that of Van Mijenl.iorden to the south (Elverh~i et al., !983) which was deglaciated around 10,000 yrs B.P. (Mangerud et al., 1991). Data for further studies were recently collected during a cruist, to Svalbard in 1990 (Solheim et al., 1991) as part of the European project "Polar North Atlantic Margins, Late Cenozoic Evolution" (PONAM). A dense grid of seismic profiles were recorded along the western margin of Spitsbergen and a largc number of cores were collected along a transect from the deep sea to the head of the Ijords. A sediment stratigraphy very similar to that described above were recognized, and preliminary interpretations of the data obtained during this cruise support the main conclusion presented in this paper: that a major glacier(s) reached on to the continental shelf during the Late Weichselian.

were not identified on the sub-bottom profiles. Rather there seems to be an even drape of sediments over the bedrock, which indicates no substantial standstill of the glacier. This contrasts with the moraine complexes identified on the outer shelf (Fig. 2). From the lithostratigraphy we conclude that the outermost core sites (nos. 03/04) are silualed inside the ice margin, and it seems a very likely possibility that the moraine complexes identified on the outer shelf may represent the maximum extent of the ice sheet during the Late Weichselia n. A grounded glacier that occupied the entire lsl]ord basin requires ice drainage I¥om a major ice sheet rather than more local glaciers. This would have been an outlet glacier connected to the Barents Ice Sheet which covered the greater part of the shelf area to the east and south of Svalbard (Elverhoi and Solheim, 1983, 1987; Solhelm and Kristoffersen, 1984; Vorren et al., 1987, 1988; Solheim et al., 1988, 1090; Elverhoi et al., 1990; S~ettem, 1990). Studies of marine sediments interbedded with tills in coastal sections at the head of Is0orden (Boulton, 1979; Mangerud and Salvigsen, 1984; Mangerud and Svendsen, 1990a) show that the entire 0ord was ice free at least twice during the Weichselian prior to the last glaciation. Thus, the last glacial maximum on Svalbard represents a major glacier advance which probably inundated the greater part of the archipelago during the Late Weichselian. A major glaciation during the Late Weichselian agrees with the evidence from the mouth of Is0orden and Van Mijen0orden (Mangerud and Svendsen, 1990; Mangerud et al., 1991). On the other hand, this model fits rather poorly with the glacial chronology proposed by Miller et al. (1989) for the Bmggerhalvoya peninsula, 100 km to the north of Isl]orden (Fig. I). From studies of sediments in coastal cliffs they maintain that the last glacial advance across Bmggerhalvoya occurred some 70,000 years ago. This would imply that the seafloor outside this peninsula should contain a record dating back to the Early Weichselian. Judged from available sub-bottom profiles the sediment thickness in Kongs0orden outside Bmggerhalvoya is in the same order of magnitude as outside Is0orden (Elverht~i et al., 1983). There-

The Late Weichselian ghicktl Ihnit Ice front deposits or substantial accumulations of sediments across the main I]ord or inner shelf

14

fore, it ~eems likely that the marine sequences in both areas represent a roughly simiiar time span. This assumption is strongly supported by new core evidence from Kongsl]orden indicating that the entire I]ord was glaciated during the Late WetchselJan (Lehman and Forman, 1991 ). Even though we can not rule out that ice free coastal areas may have existed between separate ice tongues extending far onto the shell', we find it difficult to combine the observations from lsl~orden (and Kongsfjorden) with the land based evidence on Brgggerhal.. voya. A fuller discussion of the ice extent during the Late Weichselian glacial maximum is given by Mangerud et al. (1991), who concluded that the ice sheet reached the continental shell" in the entire area from Van Mijenfjorden to Kongs0orden.

Timing o.f 1he th,glaciulion in a glaciated trough like Isf.'jorden, we would expect that glaciomarine sedimentation started immediately after deglaciation. Thus, the lowermost marine sediments above the till should date back to a'hen the sites became ice fYee. Unfortunately we h"ve no dates from the basal part of the outern st core (04). Two dates from near the base of he mari~e sequence in cores 01 and 02 yielded the somewhat diverging ages of 11,675+ 180 (TU;:-40) and 12,545+ 145 yrs B.P. (TUa-42), respectively {Fig. 5}. The first date is most likely slightly too young for that level when compared with evidence from the coastal area. The 12,500 date is similar to that obtained from the base of the marine sequence in the nearby lake basin of Linnt~vatnet (Mangerud and Svendsen, 1990b). It is also in general agreement with deglaciarian chronologies established elsewhere along the west coast of Spitsbergen, which show that the outer coast was ice free around 12,000- 13,000 yrs B.P. at the latest (Salvigsen, 1976; Boulton et al., 1982: Forman et al., 1987:. Forman, 1989:, Mangerud and Svendsen, 1990b, Mangerud et al., 1991:, Lehman and Forman, 1991). This is '~ignificantly later than the incipient disintegration of the Barents Ice Sheet at 15,000 yrs B.P. as suggested by Jones and Keigwin (1988), Their conclusion was based on an apparent meltwater spike inferred from the oxygen isotope stratigraphy or a deep

J I SVENDSEN El' AL

sea core from the Fram Strait to the west of Spitsbergen. If correct, this may indicate that the glacial retreat from the western margin of Spitsbergen began at a relatively late stage of the deglaciation. Radiocarbon datings of samples recovered from the southern part of the Barents Sea indicate that the ice recession started before 13,000 yrs B.P. in that area (Vorren and Kristoffersen, 1986). An early deglaciation of the southern part of the Barents Sea is also concluded from the fact that there are no shorelines above the present sea level on Bjornoya (Smtersmoen and Hoyden, 1984; Hyvfirinen, 1968). The Barents Ice Sheet had completely disappeared before 10,000 yrs B.P. (Salvigsen, 1981:, Elverhoi et al., 1990:, Mangerud et al., 1991). Typical ice-proximal lithofacies (Andrews and Matsch, 1983: Powell, 1981, 1984: Eyles et al., 1985) such as stratified gravel and sand facies, were not Ibund at the base of the marine sequence. In the outermost core (04) a weak sulphide lamination was ~toted at the base of the marine sediments whereas in core 01 and 02 there is an abrupt bounc:ary between the basal till and bioturbated clayey mud above (Figs. 6-8). This is similar to the succ~':.sion of marine litholacies in Linn~vatnet (Mangerud and Svendsen, 1990b). We conclude that this general lack of stratified and more coarse grained lithofacies is due to a rapid glacial retreat, which is also supported by the radiocarbon dates for degluciation. The stratigraphic evidence indicates the existence of large glaciers in the inner fjord branches until about 10,000 yrs B.P., when a major change in the depositional environment occurred. The frequent pebbles in the laminated mud in core 144 (Figs. 10 and I I) is evidence of intense ice rafting in the central 0ord area during the Younger Dryas. This requires calving glaciers of a much larger size than those at present. This is in general agreement with radiocarbon dates from exposed marine sediments which show that the inner branches of Is0orden were occupied by glaciers until around 10,000 yrs B.P. (Boulton, 1979: Mangerud et al., 1991: Salvigsen, 1984: Salvigsen et al., 1990). On the other hand, along the western coast of Spitsbergen, the local glaciers seem to have been even smnller during Younger Dryas than during the Little Ice

LATE WEICHSEL, IAN GLACIAL M A X I M U M ON WE,C,'T'E'RN SPITSBERGEN

Age (Salvigsen, 1979; Mangerud and Svendsen, 1990b).

Conclusions (I) Sediment cores from the continental shelf to the west of isfjorden sampled a firm diamicton, interpreted as till. (2) Above the firm diamicton follows soft glacimarine mud which probably started to accumulate shortly after the shelf became ice free. Radiocarbon dated molluscs from the base of the mud indicate that deglaciation occurred around 12,500 yrs B.P. (3) The ice front during the Late Weichselian glacial maximum reached beyond the cored sites, more than 40 km off the coast. (4) The ice front probably reached all the way to the shelf' edge during the Late Weichselian glacier advance.

Acknowledgement This project was funded by grants from the Norwegian Research Council for Science and the Humanities (NAVF) and Statoil. The study forms part of the ongoing European Science Foundation project '~Polar North Atlantic Margins, Late Cenozoic Evolution" (PONAM). The data pres2nted in this paper were collected in 1987 with the research vessel M/S Lance, Norwegian Polar Research Institute, and in 1988 as part or a dutch cruise to Svalbard with the vessel Hr. Ms. 7"ydema,, Geological Survey or the Netherlands. The laboratory analysis were conducted by KarI-Johan Karlsen, University of Bergen. Jane Ellingsen and Masaoki Adachi made most of the drawings. Elizabeth Jan Warren corrected the Englisn. To all colleagues and institutions who have contributed to the carrying out or this study we offer our sincere thanks.

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15

ago and the problem ol'a Barents Shelf ice sheet. Boreas, 8: 31-57. Boulton, G.S., 1990. Sedimentary and sea level changes during glacial cycles and their control on glacimarine facies archileclure. In: J.A. Dowdeswell and J.D. Scourse (Editors), Glacimarine Environments: Processes and Sediments. Geol. Soc. London Spec. Publ., 53: 15-52. Boullon, G.S., Baldwin, C.T., Peacock, J.D., McCabe, A.M., Miller, G.H., Jarvis, J., Horsefield, B., Worsley, P., Eyles, N., Chroston, P.N., Day, T.E., Hare, P.E. and Von Brunn, V., 1982. Glacioisostatic facies model and amino acid stratigraphy for late Quaternary events in Spitsbergen and the Arctic. Nature, 298: 437-441. Denton, G H. and Hughes, T.J., 1981. The Arctic Ice Sheet: An outrageous hypothesis. In: G.H. Denton and T.J. Hughes (Editors), The Last Great Ice Sheets. Wiley, New York, pp. 437-467. Dowdeswell, J.A. and Dowdeswell, E.K., 1989. Debris in icebergs and rates of glacl-marine sedimentation: Observations from Spitsbergen and a simple model. J. Gc'ol., 97: 221-231. Elverhoi., A. and Solheim, A., 1983. The Barenis Sea Ice Sheet--a sedimentologlcal discussion. Polar Res., I: 23-42. Elverht,,, A. and Solheim, A., 1987. Late Weichselian glaciation of the northern Barents Sea--a discussion. Polar Res., 5: 285-287. Elverhoi, A., Lonne, O. and Seland, R., 1983. Glaciomarine sedimentation in a modern fjord environment, Spitsbergen. Polar Res., I: 127-149. Elverhm, A., Pfirman, S, Solheim, A. and Larssen, B.B., 1989. Glaciomarine sedimenlalion and processes on High Arctic epiconlinental seas--exemplified by the northern Barent,; Sea. Mar. Geol., 85: 225-250. EIverhoi, A., Nyl,',nd-flerg, M., Russwurm, L. and Solh~im, A., 1990. Lale Weichselian ice recession in the central Barents Sea. In: LI. Pleil and J. Thiede (Editors), Geological Hislory of the P~~lar Oceans. Arctic Versus Antarctic. Kluwer Academic Publishers, Dordrecht, pp. 280-307. Eyles, C.H., k,yles, N. and Mlall, A.D., 1985 Models ol glaciomarme sedimentation and their application to Ih= interpretatior of ancient ~,lacial sequences. Palaeogeogr., Palaeoclimatol., Palaeoecol., 51: 15-84. Feyling-Hansen, R.W., i955. Stratigraphy of the marine Late Pleistocene o1" Billefjorden, Vestspitsbergen. Nor. Polarinsl. Skr., 107, 108 pp. Feyling-Hansen, R.W. and OIsson, W.J., 1960. Five radiocarbon datings of posiglacial shorelines in central Spitsbergen. Nor. Geogr. Tidsskr., 17: 122-131. Flood, B., Nagy, J. and Winsnes, T.S., 1971. Geological map. Svalbard 1:500 000. Sheel IG Spitsb,.rgen, southern part. Nor. Polarinst. Skr., 154 A. Forman, S.L., Mann, D.H. and Miller, G.H., 1987. Late Weichsehan and Holocene relative sea-level history of Brog£¢' r.alw,'Jya, Spitsbergen. Quat. Res., 27:41-50. Forman, S.T., 1989. Late Weichse!ian glaciation and deglacialion ol" Forlandsundet area, western Spitsbergen, Svalbard. Boreas, 18: 51-60. Grosswald, M.G., 1980. Late Weichselian ice sheets of northern Eurasia. Qual. Res., 13: 1-32.

]6

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