Younger Dryas cirque glaciers in western Spitsbergen: smaller than during the Little Ice Age

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1 Younger Dryas cirque glaciers in western Spitsbergen: smaller than during the Little Ice Age JAN MANGERUD AND JON Y. LANDVIK BOREAS Mangerud, J. & Landvik, J. Y (July): Younger Dryas cirque glaciers in western Spitsbergen: smaller than during the Little Ice Age. Boreas, Vol. 36, pp Oslo. ISSN The outermost moraines in front of the Scottbreen glacier in Spitsbergen date from c. AD These moraines rest on top of a marine shoreline radiocarbon-dated to about C yr BP and demonstrate that the AD-1900 moraines show the maximum glacier extent since late Allerød time. This means that Scottbreen was smaller during the Younger Dryas than at AD 1900, in contrast with glaciers on mainland western Europe, which were all much larger during the Younger Dryas. The explanation is probably starvation of precipitation on western Spitsbergen during the Younger Dryas. In contrast, ice sheets and glaciers in Spitsbergen reacted more or less in concert with glaciers in western Europe, during the global Last Glacial Maximum and the Little Ice Age. Jan Mangerud ( Jan.Mangerud@geo.uib.no), Department of Earth Science and Bjerknes Centre for Climate Research, University of Bergen, Allégt. 41, NO-5007 Bergen, Norway; Jon Y. Landvik ( jon.landvik@ umb.no), Norwegian University of Life Sciences, Department of Plant and Environmental Sciences, P.O. Box 5003, NO-1432 Ås, Norway; received 11th August 2006, accepted 31st October The Younger Dryas (YD) in western Europe, i.e. downwind of the North Atlantic, is characterized by extensive glacier growth, and all glaciers were much larger during the YD than at any time during the Holocene (Gray & Coxon 1991; Andersen et al. 1995; Kerschner et al. 2000; Geirsdottir 2004; Denton et al. 2005). On the other hand, further north along the same seaboard the opposite situation has been reported, i.e. that glaciers on western Spitsbergen were smaller during the YD than during the Little Ice Age (LIA) (Salvigsen 1979; Mangerud & Svendsen 1990; Svendsen & Mangerud 1992). However, this has been documented at only a few sites. During the Last Glacial Maximum (LGM), the Barents ( Svalbard) Ice Sheet reached the shelf edge west of our study site (Fig. 1) (Landvik et al. 1998). The ice margin retreat started about C yr BP and the study area became ice-free at about Cyr BP (Landvik et al. 1992; Mangerud et al. 1992). The deglaciation dates of and C yr BP (T-6000 and Ua-280), performed on sediment-feeding molluscs collected close to Scottbreen, are now considered to be too old (Mangerud et al. 2006). In this paper we present radiocarbon dates demonstrating that Scottbreen, a cirque glacier on the west coast of Spitsbergen in the Svalbard archipelago (Figs 1, 2), was smaller during the YD than during its LIA maximum extent at c. AD Study area and observations Svalbard is located at N. The archipelago is therefore characterized by an arctic climate, and about 60% of the land area is covered by glaciers (Fig. 1) (Hagen et al. 1993). However, because of advection of warm Atlantic water and southwesterly winds, the climate, in particular the winter climate, is exceptionally warm compared with other areas at this latitude. At Isfjord Radio, located on the coast 55 km north of Scottbreen (Fig. 1), the mean annual temperature ( ) is /5.18C (Førland et al. 1997). The means for the coldest and warmest months are /12.48C and /4.88C, respectively. At Sveagruva, situated some 70 km inland, the corresponding temperatures are /7.18C, /17.08C and /5.88C, respectively. Annual precipitation is 480 mm at Isfjord and 260 mm at Sveagruva (Førland et al. 1997). Scottbreen ( breen /glacier) (77833?N, 14822?E) is a 4.4-km long cirque glacier stretching from 700 to 90 m a.s.l. (Hagen et al. 1993). It is located close to the open ocean on the west coast of Spitsbergen (Figs 1, 2). The regional equilibrium line altitude (ELA) for glaciers along this coast is /400 m a.s.l. (Hagen et al. 2003a, b) and the present Scottbreen ELA is estimated to be m a.s.l. (J. O. Hagen, pers. comm. 2006). In front of the glacier there is a belt of up to 5060-m high ice-cored end moraines (Figs 3, 4) that were clearly formed during the LIA. Oblique air photographs from 1936 (Fig. 2) show that the glacier reached the proximal slope of the outermost moraine at that time (Norsk Polarinstitutt, S36, photographs 1694 and 3189). The small glaciers on western Spitsbergen generally reached their maximum LIA position around AD 1900 (Hagen & Liestøl 1990; Lefauconnier & Hagen 1990) and this was probably the case for Scottbreen. The present ice margin is some 800 m behind the LIA moraine, and the lower part of the DOI / # 2007 Taylor & Francis

2 BOREAS 36 (2007) Younger Dryas cirque glaciers in western Spitsbergen 279 Fig. 1. The inset map shows the location of Svalbard. Present-day glaciers are shown in white on the main map. The maximum extent (Last Glacial Maximum/LGM) of the Barents Ice Sheet is shown beyond the west coast (Landvik et al. 1998; Ottesen et al. 2005). The approximate limit of the Younger Dryas ice sheet on Svalbard (modified from Svendsen et al. 2004) is indicated as a line separating deglaciation dates of Younger Dryas and early Holocene ages. We assume errors rarely exceeded 30 km along the west coast. In the eastern areas the ice margin was beyond the coast and cores with relevant dates were only obtained from far out in the Barents Sea. Therefore the line is more generalized here and errors may exceed 100 km. Norwegian Polar Institute, digital map. glacier has a significantly lower surface slope than shown on the air photographs from Most important for the present discussion is that the distal part of the LIA moraine was deposited on top of a well preserved beach terrace of about 57 m a.s.l. (Figs 4, 5). Thus the LIA moraine shows the maximum extent of Scottbreen after the formation of this terrace, which, as we will demonstrate, is of Allerød age.

3 280 Jan Mangerud and Jon Y. Landvik BOREAS 36 (2007) Fig. 2. Oblique air photo of Scottbreen (Fig. 1) taken on 28th July 1936, approximately towards the southwest. The arrow shows the location of Fig. 4. Norwegian Polar Institute, photograph no. S The beach terraces in front of Scottbreen are dissected by meltwater channels from the glacier (Figs 3, 4) that have exposed sandy gravelly beach deposits with shell fragments, predominantly of Mya truncata. Four radiocarbon dates from the mollusc shells have yielded uncorrected ages in the range of /50 to /80 14 C yr BP (Table 1). Assuming a marine reservoir age of 440 years, which is the standard used for Svalbard (Mangerud & Gulliksen 1975), the corrected ages are in the range C yr BP, i.e. a late Allerød age. A new estimate of the present-day reservoir age of 3809/80 (Mangerud et al. 2006) indicates that the ages are slightly older. Precise Allerød YD reservoir ages have not been determined from Svalbard, but three pairs of mollusc shell/driftwood dates from eastern Svalbard covering the period C yr BP gave reservoir ages of about 400 years (J. Mangerud, unpublished). Along the west coast of Norway the reservoir age was also close to the present day during most of the Allerød, and some 300 years higher during parts of the YD (Bondevik et al. 2006). We conclude that the dates indicate a late Allerød or possibly an early YD age for the shells. In theory, the molluscs may have lived at several metres of water depth or they may have been redeposited from higher and older terraces. If so, the 57-m terrace could be younger than the shells. However, the southern flank of the moraine cross-cuts a distinct beach terrace at 61 m a.s.l. as well as beach sediments at 66 m a.s.l. According to a relative sea level curve from the area immediately to the west (Salvigsen et al. 1991), the latter altitude represents the marine limit dated to c C yr BP and the 57-m terrace represents a late Allerød sea level, and thus supports our inference that the shells accurately date the terrace.

4 BOREAS 36 (2007) Younger Dryas cirque glaciers in western Spitsbergen 281 Fig. 3. Scottbreen with the large Little Ice Age moraines (oblique arrows). As seen from Fig. 2, the glacier front reached the foot of this moraine as late as The vertical arrow indicates the dated marine terrace overrun by the glacier (Fig. 4). Younger terraces are seen as horizontal lines below. Photograph taken 27th August Fig. 4. The person is standing on the Allerød-age shoreline, 57 m a.s.l. The dated shells were found by excavating this terrace. The Little Ice Age moraine is seen on top of the terrace to the right of the person, and as the large ridge behind and to the left of the person. The glacier is located to the right of the photograph.

5 282 Jan Mangerud and Jon Y. Landvik BOREAS 36 (2007) Table 1. Radiocarbon dates from the marine terrace in front of the Scottbreen Little Ice Age moraine. All dates were performed on fragments of marine shells, probably Mya truncata. d 13 C was not measured but a value of 1.0 rel. PDB was assumed and used for correction. The reservoir age is according to Mangerud & Gulliksen (1975). Field sample no. Lab. no. 14 Cage Assumed reservoir age Reservoir age-corrected 14 Cage JM a Tua / /80 JM b Tua / /70 JM c Tua / /50 JM d Tua / /50 A B Fig. 5. A. Longitudinal profile of Scottbreen drawn from the 1: map sheet Van Keulenfjorden, which was constructed from air photographs taken in 1936 and has contour intervals of 50 m. Issued by Norsk Polarinstitutt B. Stratigraphical relationship between the dated shoreline and the Little Ice Age moraine. The radiocarbon dates are corrected for a marine reservoir age of 440 years. ± ± ± ± The marine limit was formed during the deglaciation of the BarentsSvalbard Ice Sheet (Landvik et al. 1987; Mangerud et al. 1992), meaning that Scottbreen remained inside the position of the LIA moraines from the deglaciation at about C yr BP until about AD 1900, i.e. also during the YD. Discussion One question arising is whether Scottbreen could have been cold-based during the YD and overrun the terrace without depositing any till. Today, a noticeable part of the bouldery material deposited by the glacier is debris originating from rock falls from the steep mountain slopes onto the glacier surface, a supply that must have existed even if the glacier was cold-based. However, we could not find any such debris on top of the terraces and we consider it unlikely that the glacier overran the terrace without depositing at least some blocks from englacial or supraglacial transport. During the YD, a remnant of the BarentsSvalbard Ice Sheet still covered much of Svalbard further to the east (Fig. 1) (Landvik et al. 1998; Svendsen et al. 2004). In that sense the glacial history of Svalbard is similar to the development in Scandinavia, Scotland and the Alps, where ice sheets or ice caps also existed during the YD. However, there are two major differences in the glacial behaviour. First, prominent YD moraines have not been found on Svalbard, even if a slow-down in glacio-isostatic rebound indicates that the retreat of the ice sheet halted (Landvik et al. 1987, 1998). The YD ice sheet extent in Fig. 1 is mainly mapped as the up-fjord limit of YD shorelines and from the distribution of deglaciation ages in stratigraphical successions (Mangerud et al. 1992). A probable reason for the lack of YD moraines is simply that the ice sheet over Svalbard did not readvance towards the west, in contrast with the Scandinavian, Scottish and Alp ice sheets/ice caps. The last ice sheet over Svalbard comprised fast-flowing ice along the fjords and less dynamic, possibly coldbased, ice between these ice streams (Landvik et al. 2005). Thus any moraines formed during the YD may be confined to the floors of fjords or even to valleys presently occupied by fjord-head glaciers. Some moraines of postulated YD age, and interpreted as the result of a glacial re-advance, have indeed been described recently from the floors of Isfjorden and its tributaries (Forwick & Vorren 2005). Second is the difference demonstrated in this paper and earlier by Salvigsen (1979), Mangerud & Svendsen

6 ELA depression (m) BOREAS 36 (2007) Younger Dryas cirque glaciers in western Spitsbergen 283 (1990) and Svendsen & Mangerud (1992), that glaciers located west of the ice sheet on Svalbard (Fig. 1) were smaller during the YD than the LIA. In contrast, all glaciers in western Europe were, as already mentioned, much larger during the YD than LIA (Fig. 6). The surprisingly small YD glaciers on Svalbard could be the result of warm summers and/or limited precipitation (snowfall). The YD summer insolation at this latitude was about 10% higher than today (Berger & Loutre 1991), but this was also the case for northern Scandinavia and therefore it cannot explain the difference in glacial response between the two areas. Unfortunately, there are no observations on Svalbard that can be used to construct the YD summer temperatures but, according to a compilation by Birks et al. (2005), the summer sea surface temperatures south of Svalbard were 2108C colder than today. It is therefore reasonable to assume that summers were colder on Svalbard too, and we conclude that the glaciers on western Svalbard remained small because of limited snow accumulation. Regional glacier re-advances in northern Norway during the YD (Andersen et al. 1995; Vorren & Plassen 2002) show that the latitudinal boundary between the dry climate favouring glacier starvation and the climate with enough precipitation for glacial advances was located between Svalbard and the northern tip of Norway (Fig. 6). The explanation for the dry climate may be the hypothesis proposed by Birgel & Hass (2004): prevailing easterly winds over Svalbard during the YD. Western Svalbard would then be in the precipitation shadow, whereas there would be more precipitation in eastern Svalbard and the Barents Sea. This pattern is consistent with the snow accumulation postulated from the slow-down of isostatic rebound, as mentioned above. This is also consistent with modelling experiments that produce a YD air pressure pattern giving westerly winds over all of Scandinavia and easterly winds over Svalbard, particularly during winter (Renssen et al. 2001). The YD glaciers in northwest Europe were mainly fed by precipitation brought in with westerly or southerly winds from the Atlantic Ocean and the Nordic seas 4000 km Fig. 6. Estimation of how much lower the equilibrium line altitude (ELA) was during the Little Ice Age and the Younger Dryas relative to the present day ELA. Note the very different situation in Spitsbergen compared with mainland western Europe. Modified from Mangerud & Svendsen (1990) and Svendsen & Mangerud (1992). (Sissons 1980; Larsen et al. 1984). However, it is clear that the glacial re-advances that took place across such a large and climatically diverse region as western Europe and Iceland were mainly caused by cold summers. Precipitation differences would regionally modify the large-scale pattern, as in Scotland where glaciers grew in the west whereas the east was in the precipitation shadow (Gray & Coxon 1991). Benn & Lukas (2006) found that precipitation in northwest Scotland was c. 26% higher during the YD than today, whereas in the mountains further east it was close to present-day levels (Benn & Ballantyne 2005). The Scandinavian Ice Sheet had its largest growth in its southwest sector, as a result of the underlying topography (Mangerud 1980) and, perhaps more importantly, a larger winter precipitation than in other areas (Mangerud 2004). In our interpretation, Svalbard would be almost a mirror image of Scotland: an ice cap in the east and a precipitation shadow in the west. The glaciers on Svalbard and the Barents Sea grew in concert with the Scandinavian, British and Alp glaciers during the period before and around the global LGM (Landvik et al. 1998), in contrast with what we have described for the YD. This could indicate different types of glacial climates between the two cold periods. However, it should be borne in mind that YD was a short period of about 1300 yr, whereas the LGM was almost 10 times longer when the period of ice build-up is included. Conditions for snow accumulation could therefore have varied over time during the LGM. Nevertheless, if precipitation was the main factor causing the difference between mainland Europe and Svalbard during the YD, then less difference in the precipitation pattern during the LGM ice growth would be expected. Open water, at least periodically and during summers, has been described during the LGM in the North Atlantic and the Norwegian Sea as a source for precipitation on the Scandinavian and Barents ice sheets (Hebbeln et al. 1994; Hald et al. 2001). However, open waters have also been reported for the YD (Koç et al. 1993; Birgel & Hass 2004) and were probably required for the ice growth in Scandinavia.

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