The Tolsta Interstadial, Scotland: correlation with D–O cycles GI-8 to GI-5?

Quaternary Science Reviews 21 (2002) 901–915

The Tolsta Interstadial, Scotland: correlation with D–O cycles GI-8 to GI-5? Graeme Whittington*, Adrian M. Hall Deparment of Geography and Geosciences, University of St Andrews, Irvine Building, North Street, St Andrews, Fife, KY16 9AL, Scotland, UK

Abstract Organic sediments, buried by till, at Tolsta Head, in north Lewis in the Scottish Outer Hebridean islands have been reinvestigated. Radiocarbon dates of 32–26 kyr BP indicate correlation with the Denekamp Interstadial of The Netherlands and the ( Sandnes/Alesund Interstadial of Norway. The organic sediments contain an unusually detailed record of the dominantly open grassland vegetation during the interstadial. Variations in palaeotemperature inferred from the pollen assemblages and sedimentology are provisionally matched with the d18O record of the GISP2 ice core and correlation with the Dansgaard– Oeschger cycles GI-8–GI-5 is proposed. The interstadial starts at 38.1k cal yr and 32.8k radiocarbon years. The termination of the interstadial and the build-up of the Late Devensian (Weichselian) ice sheets in Scotland and North West Europe dates from 32k cal yr and 28.7k radiocarbon years. Whether the Tolsta and other contemporaneous episodes are intervals or interstadials is also considered. r 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction This paper examines the terrestrial environments in north Lewis in the Outer Hebrides (Fig. 1) at ca 30 k 14C yr BP. The Tolsta Interstadial is formally defined for the first time in Scotland from the type site at Tolsta Head (National Grid Reference: NB 557468). There, a sequence of organic deposits rests on bedrock and lies beneath diamicton (Gordon and Sutherland, 1993). The site was first described by von Weymarn and Edwards (1973) and further details of this sedimentary sequence have been given by Flinn (1978a) and Edwards (1979) with additional analyses being provided by Birnie (1983). A radiocarbon date of 27,3337240 (SRR-87) was obtained from the topmost 15 cm of the deposit (von Weymarn and Edwards, 1973). A re-examination of the Tolsta Head deposits was considered to be desirable for a number of reasons. Due to glacial erosion, there are very few sites in North West Europe within the Late Devensian (Late Weichselian) ice limits where organic sediments provide a clear picture of the climatic and vegetational conditions during this key episode. Secondly, the reliability of the *Corresponding author. Tel.: +44-1334-463915; fax: +44-1334463949. E-mail address: [email protected] (G. Whittington).

radiocarbon date has been uncertain as other dates for sites in Scotland originally thought to be of comparable age to the Tolsta deposits, notably Teindland (Hall et al., 1995) and Crossbrae (Whittington et al., 1998), have proved subsequently to be beyond the range of radiocarbon dating. Thirdly, due to the hazardous location of the Tolsta organic deposits on a precipitous cliff-face (Fig. 2), the difficulty in obtaining a satisfactory sequence of samples had led to only a rudimentary examination of the physical nature of the sediments and their subfossil pollen and diatom content (von Weymarn and Edwards, 1973; Birnie, 1983). The collection of continuous, overlapping monoliths, covering all but the basal 3 cm of the whole sequence of deposits, has now enabled a programme of closely sampled pollen analyses and radiocarbon dating to be undertaken. This has allowed the age of the interstadial represented at Tolsta to be more tightly constrained and has also provided a detailed definition of the nature of and changes in the interstadial environment. The Tolsta sediments also need to be seen in the wider context of interstadial environments in North West Europe around 30k 14C yr BP, particularly with reference to the Greenland ice core data (Dansgaard et al., 1993). Variations in the pollen record at Tolsta are correlated provisionally with Greenland Interstadials (GI) 8–5 in the GRIP ice core record. This allows the

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Fig. 1. The locations of Tolsta Head, Lewis, Outer Hebrides and other sites named in the text and on Table 4.

in an age frame where minor contamination will produce large errors (Caspers and Freund, 2001). It is unclear if the interstadial is merely the end phase of a long period of fluctuating, but episodically elevated temperatures extending back to ca 40k 14C yr BP (McCabe, 1987; Ran, 1990; Caspers and Freund, 2001), or a discrete, relatively warm interval (Jardine et al., 1988). One approach to an improved understanding of the timing and nature of temperature changes in the Middle to Late Weichselian has been to make comparisons with the Greenland ice core record (Clapperton, 1997). Yet, although palaeoenvironmental analyses for the Lateglacial period in North West Europe (Mayle et al., 1999; Brooks and Birks, 2000) increasingly suggest that a close correlation exists with d18O variations in the ice core data (Dansgaard et al., 1993), a recent survey of Weichselian vegetation and climate concluded that ‘correlation of the terrestrially defined interstadials and intervals with the d18O values of the GRIP ice corey is very uncertain for the Weichselian pleniglacial’ (Caspers and Freund, 2001, p. 45). This fundamental tension is examined here from the perspective of vegetation change on the maritime fringe of North West Europe at the close of the Middle Weichselian (Middle Pleniglacial).

2. Tolsta head 2.1. Stratigraphy The Site is a cliff-top section, backed by low-angled slopes, on the north side of Tolsta Head. The sediment sequence (Fig. 3) sits in a shallow gully trending to the north–north-east. Beneath 10–20 cm of peaty soil there are around 100 cm of frost-disturbed, brown, clastsupported diamicton, with angular pebbles of local Lewisian gneiss in a silty sand matrix. The upper 10 cm

Fig. 2. The 60 m high cliffs of Tolsta Head: the site of the interstadial organic deposits. (Photo courtesy of M.J.C. Walker).

timing of the Tolsta Interstadial and the subsequent onset of full glacial conditions to be closely constrained for the first time. An interstadial period dated to ca 30k 14C yr BP is widely recognized in North West Europe (Ran, 1990; Valen et al., 1996). This period, equivalent to the Denekamp Interstadial, is of particular interest as it immediately predates the advance of the last major ice sheets in North West Europe. Yet the timing of the beginning and ending of this interstadial is poorly constrained, with a heavy reliance on radiocarbon dates

Fig. 3. Generalized stratigraphy at Tolsta Head.

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of this horizon show a prominent stone layer, and clasts below this are preferentially aligned towards the east, features consistent with solifluction and frost sorting. This unit grades downwards into up to 140 cm of brown, matrix-supported, silty, sandy diamicton, with crude subhorizontal structure (the Port Beag Member; Sutherland, 1999). This unit has been interpreted as lodgement till (von Weymarn and Edwards, 1973) but the direction of ice movement associated with till deposition at Tolsta Head is not resolved. Clasts are dominantly angular to subrounded pebbles and cobbles of Lewisian gneiss but numerous brown sandstones are present. These siliceous sandstone clasts have been regarded as Torridonian in age (von Weymarn and Edwards, 1973) and derived ultimately from the Scottish mainland. It is possible that they are Permo-Triassic in age and derived from the North Lewis Basin, where red sandstone occurs offshore from Tolsta Head (Stoker et al., 1993), although this sandstone tends to be more calcareous (Peacock, 1991). Von Weymarn and Edwards (1973) reported that till fabric measurements showed a dominant N501W clast orientation and inferred deposition by ice moving from the south-east. Flinn (1978b) found fragments of phyllonite in the diamicton, apparently derived from the south-west corner of Tolsta Head. This is inconsistent with the eastward trend of striae in the area inland of Tolsta Head (Peacock, 1981). The shelly diamictons found south-west of Tolsta Head (Dougal, 1928) and in north Lewis were deposited by ice moving onshore from the Minch in the Late Devensian (Hall, 1996). The till incorporates organic sediment in its base (Fig. 3), showing that the underlying organic material is locally truncated by erosion (Gordon and Sutherland, 1993). This Tolsta Head Member (Sutherland, 1999) consists of horizontally bedded organic sands and silt, up to 81 cm thick. No evidence of glaciotectonic shearing has been noted within the Member and it is regarded as essentially undisturbed. Sutherland (1999) recognizes an upper bed of organic detritus with sand lenses and a lower bed of fine sand with a low organic content and iron staining. In the lower sand, Birnie (1983) found trilete spores and a few saccate pollen grains, probably reworked from Permo-Triassic to Tertiary sediments in the North Minch Basin. The sequence rests on Lewisian gneiss. 2.2. Sedimentological and pollen analyses 2.2.1. Methods From 1 cm above the base of the overlying diamicton to within 3 cm of the basement rock, five overlapping monoliths were obtained. The uppermost was 50
11
6 cm and the other four were 10
5
3 cm. Sedimentological analyses consisted of loss on ignition (LOI) measurements (4 h at 5501C), particle size

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determination on LOI residues by sieving for material >63 mm (63–250 mm, fine sand; 250–500 mm, medium sand; >500 mm, coarse sand) and Coulter Counter measurements for material in the silt- and clay-size ranges (o63 mm). Sedimentological data are given on Figs. 4 and 5. Pollen samples, at 1 cm intervals, were prepared by standard NaOH, HF and acetolysis methods (Faegri and Iversen, 1989) except for those below 60 cm where heavy liquid separation was used following the method of Nakagawa et al. (1998); in this case Lithium heteropolytungstate (LST) was substituted for KI and CdI2. The addition of tablets containing spores of Lycopodium enabled estimates of absolute palynomorph counts to be made and these showed that below 65.5 cm no meaningful pollen counts were feasible. Samples were mounted in silicone fluid and a minimum counting sum of 500 total land pollen (TLP) was aimed for, although near the base the samples were too impoverished of pollen to allow such a total to be achieved. Pollen type nomenclature follows Stace (1997), amended after Bennett et al. (1994). Pollen preservation status was assessed on the basis of four hierarchical categoriesFperfect, folded/crumpled, broken, pitted/thinned. Data from this exercise are shown on Fig. 5. The determination of local pollen zones and subzones (LPAZ) was aided by a stratigraphically constrained cluster analysis (CONISS; Grimm, 1987) on taxa reaching 3% in at least one sample. This was undertaken with the computer programs TILIA and TILIA  GRAPH (Grimm, 1991) which were also used for the production of the pollen diagram (Fig. 4). 2.2.2. The pollen results All pollen and spore types are expressed as percentages of TLP. The pollen profile (Fig. 4) has been divided into 7 LPAZs of which the basal and uppermost have been separated into 3 subzones, while LPAZ TH-4 is divided into two subzones. The main features of the pollen profile are given in Table 1. 2.3. Discussion 2.3.1. Previous investigations at Tolsta Head The previous explorations of this site (von Weymarn and Edwards, 1973; Birnie, 1983), despite their limited nature, allowed certain suggestions to be made as to the interstadial environment. Vegetationally, the site during the interstadial was dominated by Poaceae and Cyperaceae. Other ground cover was provided by Asteraceae and Ranunculaceae. In the upper 10–20 cm Salix became an important constituent, reaching B30% in the uppermost level (von Weymarn and Edwards, 1973). Both investigations reported a presence of Nymphaea in the upper 5–10 cm. The major difference between the two pollen reports lies in the recording of Juniperus. Von

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Fig. 4. Percentage pollen diagram for Tolsta Head. Where percentages are o2, one cross represents one pollen grain.

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Fig. 4. (continued)

Weymarn and Edwards recorded that taxon in the top 30 cm with a peak of B15%, accompanying the maximum in Salix at the top of the organic deposits. Birnie found only a scattered representation but commented on her unfamiliarity with Juniperus pollen. Her examination, however, did reveal that the deposits needed to be considered as consisting of two distinct units. 2.3.2. The current investigation at Tolsta Head The availability of monoliths providing a continuous section through the Tolsta Head sediments gave the opportunity to investigate the interstadial environment to a finer degree than previously possible. The 1973 (von Weymarn and Edwards, 1973) investigation had been confined to nine levels through 53 cm of sediment. Birnie in 1983 only had eleven 5 cm bulk samples which would tend to hide any sharp and restricted variations that took place in the local environment. Furthermore, the current investigations explored a greater depth of 81 cm of deposits and at 1 cm intervals. The differences in results from the three examinations are highlighted by the fact that the 1973 analyses produced pollen spectra which consisted of 20 pollen and spore taxa while the current analysis recorded 64. A comparison with the 1983 record is not possible as no pollen diagram accompanied the report. All three investigations confirm the open nature of the landscape. At all times in this interstadial record, Poaceae and Cyperaceae (Fig. 4a) were the dominant pollen taxa. There is a strong difference, however, produced by the current analysis. Birnie noted the contrast between the upper and lower parts of the sediments which was also marked by levels of up to 25% of old palynomorphs in the lower part. While this latter feature was not encountered in the current analysis, a pollen record was established below 40 cm, which stands in strong contrast to that from the sediments above that level. In LPAZ TH-1 there are pollen taxa which are either restricted to that zone or appear in greater percentages than subsequently. Thus Betula, Pinus, Picea, Calluna, Ericales undiff. and Plantago spp. occur virtually continuously and in some cases achieve levels of 2% and more. This divergence between the upper and lower parts of the sediments is well brought out by the cumulative pollen diagram in Fig. 4. As the majority of these taxa are dispersed by wind, their presence suggests that a climatic shift might have taken place between the emplacement of the lower and upper sediments. A different atmospheric circulation could have been operating, either resulting in a cessation of northerly pollen transport and/or the confining of these taxa to more distant southerly latitudes. This presupposes that these pollen taxa are from an allochthonous source. In most cases the low percentages suggest that this is so. Plantago lanceolata, however, does not fit into that

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Fig. 5. Pollen preservation status at Tolsta Head. The extremely jagged nature of the Plantago lanceolata and Pinus sylvestris curves at some levels is due to very low pollen totals at those levels.

Table 1 Main features of the Tolsta Head pollen profile. Taxa values are generalised % TLP. Pollen concentrations in grains
103 cm3 LPAZ

Depth (cm)

Taxa

TH-1 TH-1a TH-1b TH-1c TH-2 TH-3 TH-4 TH-4a TH-4b TH-5 TH-6 TH-7 TH-7a TH-7b TH-7c

65.5–41.0 65.5–64.0 64.0–55.0 55.0–41.0 41.0–37.0 37.0–34.0 34.0–23.0 34.0–30.0 30.0–23.0 23.0–17.0 17.0–15.0 15.0–1.5 15.0–8.0 8.0–4.0 4.0–1.5

B40 taxa represented Poaceae dominant 40–80 Plantago lanceolata 20; Pinus sylvestris 15 Some thermophilous woodland Plantago lanceolata o2; Pinus sylvestris o2 Plantago lanceolata 20; Pinus sylvestris 15 B23 taxa represented Cyperaceae B29 taxa represented Poaceae 75–20; Cyperaceae 22–70 Woodland virtually disappears B35 taxa represented Cyperaceae dominant 55 Poaceae 50; Cyperaceae 45 Koenigia islandica Poaceae 35; Cyperaceae 30; Salix 10 Koenigia islandica B24 taxa represented Poaceae 45; Cyperaceae 30; Salix 10 Juniperus communis >5 B31 taxa represented Cyperaceae 50; Poaceae 20; Artemisia 25 B40 taxa represented Poaceae dominant Salix 10 Salix 20 Salix 5

category in that it reaches over 20% TLP in LPAZ TH1b and is also accompanied by other Plantago spp (Fig. 4b), all of which are known constituents of north

Pollen concentration 10 16–73 8–25 13–17 35–59 32 141 53–22 9 18–1200 30 95

Atlantic island maritime vegetation communities in the present (Hansen, 1966; Kristinsson, 1987; Pankhurst and Mullin, 1991) as well as the past (Walker, 1984;

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Hansom and Briggs, 1989–90). The pollen preservation status of P. lanceolata is shown on Fig. 5 and, like the Poaceae, shows a high percentage of grains which either have perfectly preserved exine or are merely folded/ crumpled. This may suggest that long-distance travel is not involved and that Plantago spp were part of the local flora. It is also possible that at least some of the Betula pollen is derived from locally growing B. nana. The opening of LPAZ TH-2 goes further to confirm the suggestion by Birnie (1983) that two distinct periods of sediment accumulation are present at Tolsta Head. Not only do several of the taxa in LPAZ TH-1 decline abruptly but there is a striking jump in the representation of Cyperaceae (from B20% to B80%). There is also an increase in LOI values and a change from ironstained sand to sand with organic streaks (Fig. 5). This is suggestive of an erosional event having occurred, the organic streaks being due to the break up of terrestrial organic horizons prior to sediment re-deposition. Further support for such an erosional event appears to exist in the preservation status of the two major pollen taxaFPoaceae and Cyperaceae (Fig. 5). At this level both taxa show an increase in pitted/thinned grains, which suggests that they have experienced extended residence in a soil. This is particularly marked in the case of Poaceae. Birnie (1983) thought that there was considerable instability in environmental conditions at Tolsta Head in the period covered by the sediments above 40 cm. An examination of the LOI values appears to bear this out (Fig. 4). There are, however, periods when greater stability seems to have occurred during which pollen concentrations achieve their highest values and the number of unidentifiable grains falls to low values (Fig. 4). Even during periods of instability, the energy status of the environment never appears to have been high judging by the evidence presented when the nature of pollen preservation of Poaceae and Cyperaceae is considered (Fig. 5). Both taxa show high values (at least 40–60%) of grains that have merely undergone folding or crumpling. For Poaceae this condition is a normal occurrence. In the case of the more fragile Cyperaceae, if deposition had been part of a high energy environment, the folded/crumpled category might be expected to have been greatly diminished and the broken category considerably enhanced. It is noticeable that the Poaceae pollen shows the greater amount of pitted/thinned grains and that category also increases for Cyperaceae during periods of lower LOI values. This suggests that Cyperaceae was growing nearer to or even on the sediment depositional area while Poaceae occurred in the areas supplying the sediment. The nature of Juniperus representation was discrepant in the two previous investigations at Tolsta Head. The current analysis confirms the taxon’s presence (Fig. 4a) but at lower percentages than those recorded by von

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Weymarn and Edwards (1973). The latter also commented that it was strange that this warmth-loving taxon appeared and rose to its maximum representation just before the onset of glacial conditions. This generated the comment that erosion had perhaps truncated the deposit. It is noticeable, however, that in their record Juniperus and Artemisia occur together which, if the Juniperus is local, seems an unlikely ecological arrangement. An alternative explanation for this feature, while not ruling out truncation of the deposit, is that the sediment became mixed at that location during the emplacement of the diamicton. This possibility arises from the new data presented here. Fig. 4a shows the major representation of Juniperus confined to LPAZ TH-5. The lowest two levels of LPAZ TH-6 show the only two occurrences of Juniperus in that zone at values of 0.2% and 1.4%, but that zone is marked by a B20% peak of Artemisia (Fig. 4b). In this case the Juniperus precedes the major advent of Artemisia. There may be a climatic and successional significance in this arrangement with the Juniperus being present during a warmer, wetter period until the arrival of colder, drier conditions which favoured Artemisia. During LPAZ TH-6, it is also noticeable that there is a reduction in the presence of the wetland taxon Caltha palustris (Fig. 4c). In LPAZ TH-7a and 7c representation of Caltha palustris recovers suggesting that wetter conditions returned to the site. Juniperus does not become reestablished; only one level displays the taxon, at 0.6%, and that is confined to the start of the zone. This might indicate that temperatures remained low except that it is at this level that the two previous investigations recorded an impressive presence of Nymphaea alba. This taxon today occurs in the more temperate areas and is not recorded from the Faeroes or Iceland. The major occurrence of Nymphaea in the two previous investigations, but not in the current one, raises the question of the nature of standing water at Tolsta Head. Von Weymarn and Edwards record Nymphaea, Potamogeton and Alisma sporadically through the profile but only Nymphaea ever exceeds 2%. It is not known if Birnie had similar occurrences but her investigation included diatom analysis which showed that freshwater conditions existed in the area through at least her upper division of the sediments. It was also suggested from the sample covering 10–15 cm, following upon the discovery of the diatom Cymbella and Ranunculus seeds, believed to derive from R. flammula, that shallow water existed but involving moderate to swift water flow. This was believed to contrast with the conditions in her lower division where seeds of R. fluitans were recovered, a taxon which prefers swifter water flow; it should be noted, however, that R. flammula can occur in any wet area. The low percentages of obligate aquatic pollen, other than

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Nymphaea, in the only previous pollen diagram published tends to negate the existence of any extensive water body at or in the vicinity of the sampling site. This seems to be confirmed by the present examination which has sporadic representation of Nymphaea, Myriophyllum alterniflorum and Potamogetonaceae (Fig. 4c). The existence of Caltha palustris pollen and some spores of Equisetum and Sphagnum is suggestive of wetland, probably on the edge of a fluctuating pool which may once have been more extensive, perhaps before the advent of cliff recession. The presence of Hydrocotyle vulgaris, Lychnis flos-cuculi, Succisa pratensis, Montia fontana, Koenigia islandica, Thalictrum and Filipendula (Fig. 4) also bear witness to the continuing wetness of the site. 2.3.3. New radiocarbon dates from Tolsta Head Nine radiocarbon dates, from 1 cm thick sediment samples, were obtained by the AMS method and seven have been calibrated using the formula provided by Bard et al, 1998 (Table 2). The single date obtained in the original investigation by von Weymarn and Edwards (1973) conforms with the new dates. As might be expected the uppermost date of 26,1507280 14C yr BP is younger than the existing one of 27,3337240 BP, obtained from the 15 cm bulk sample in the 1973 investigation. The range of dates takes the emplacement of the sediments back to 31,7007600 14C yr BP at 33 cm. The two lowest horizons dated (42 and 48 cm) provided anomalous values of 6140748 and 9040769 BP. Later fieldwork has shown that there is a significant seepage of water from the section at the points where these two dates were obtained. No attempt at dating below 48 cm could be made. The incoherence of the lower sediments and the restricted size of the monoliths, forced by the physical difficulties imposed by the site in obtaining any material, did not permit satisfactory sampling once the pollen material had been abstracted.

3. Vegetational records from North West Europe ca 30 kyr BP These new radiocarbon dates indicate that the Tolsta Head sediments and their vegetational record belong to the period ca 30 kyr BP. Palynological analyses have also been undertaken at sites elsewhere in North West Europe which have similar dates, e.g. Sourlie and Hirta in Scotland, Derryvree in Northern Ireland, Four Ashes ( in England, Alesund in Norway and a whole suite of locations in The Netherlands. There are problems in trying to reconstruct a regional vegetation for this period. The sites represent a wide range of maritime to continental environments. For sites within the limits of the last ice sheet, pollen records are often poor. Above

Table 2 Radiocarbon dates from Tolsta Head Laboratory code

AA37778 AA37779 AA37780 AA37781 AA37782 AA37783 AA37784 AA37785 AA37786

Depth (cm)

2.5–3.5 6.5–7.5 11.5–12.5 16.0–17.0 20.5–21.5 26.5–27.5 32.5–33.5 41.5–42.5 47.5–48.5

d13CPDB%

Age (14C BP)

(cal BP)

26,1507280 29,0707360 28,8507350 28,6807340 24,5307590 30,0607440 31,7007600 6140748 9040769

30,650 33,930 33,690 33,500 28,810 35,070 36,840 7000 8260

25.90 28.20 28.40 30.40 26.50 28.50 28.70 27.00 26.40

all, the reliability of radiocarbon dates is often uncertain, with determinations produced by many different laboratories over a period of 30 yr, involving a range of organic materials and for a period around 30 kyr BP when minor contamination by exotic carbon will produce large dating errors (Caspers and Freund, 2001). In Scotland, the pollen report for Sourlie (Jardine et al., 1988) only mentions four taxa (Poaceae, Cyperaceae, Caryophyllaceae and Myriophyllum). The investigations at Hirta, St Kilda (Sutherland et al., 1984) yielded 10 pollen spectra from 30 cm of sediment. In ( Norway, at the Alesund sites (Mangerud et al., 1981) pollen recovery was confined to 113 grains at Longva. At Derryvree, Northern Ireland (Colhoun et al., 1972), site A was palynologically the most important. Of the 100 grains identified from there, 76 were Poaceae but Alnus, Betula, Corylus, Piceae, and Ulmus were also found collectively over the three samples examined. Nevertheless those three sites have provided important information. Of the possible Middle Devensian sites in the English Midlands, the pollen analysis at Four Ashes (West, 1977), for the period dated to ca 30 k 14C yr BP, produced two pollen spectra. They are dominated by Poaceae and Cyperaceae with ca 10% Betula, some Pinus and Salix. Other herbaceous taxa are very similar to those recovered at other British sites. Some sites in The Netherlands, for which dates of the Denekamp Interstadial have been obtained, have provided detailed pollen records of a shrub tundra vegetation (Wiggers, 1955; Cnossen and Zandstra, 1965; van der Hammen et al., 1967; van der Hammen, 1971; Zagwijn, 1974; Kolstrup and Wijmstra, 1977; Vandenberghe, 1985; Ran, 1990) but often over restricted depths of sediment. As has been shown, however, for Tolsta Head a detailed and extended series of pollen spectra exists. The pollen taxa present at all of the sites accord with mean July temperatures during the warmest phases of at least 81C, as defined by reference to Climatic Indicator Species (Kolstrup, 1979; Isarin and Bohncke, 1999). At Tolsta Head the recovery of Nymphaea pollen indicates

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that minimum July temperatures of 12–131C were achieved at some stage. The vegetation is dominantly an open grassland, while Cyperaceae, probably representing on-site vegetation, frequently provides the major pollen component. The report from Hirta suggests that the grassland was species-poor and that is supported ( also by the pollen recovered from Alesund. This is contrary to the evidence from Tolsta Head and several of the Denekamp Interstadial sites where many herbaceous taxa are represented. This difference is probably no more than a reflection of the investigative method used at the various sites. A total yield of 113 pollen grains ( from till at Alesund is unlikely to provide very much floristic detail and at Hirta pollen counts were restricted to 100 grains at each level, whereas, in contrast, the Tolsta Head investigation involved, except at the lowest levels, 500 TLP at 1 cm intervals over 65 cm of sediment. However, even at Derryvree, where only low numbers of pollen were counted, macroscopic plant remains also show that the grassland was not species-poor. The richness of the herbaceous flora at Tolsta Head is matched at the nearby earliest Holocene site of Skigersta (Whittington et al., in press). At Tolsta Head, LPAZ TH-1 is marked by the presence of woodland taxa (Fig. 4a), some of which are absent from subsequent LPAZ while others appear as ( scattered grains in low numbers. The Alesund, Derryvree, Hirta and some Denekamp sites show a woodland pollen presence which is akin to that from the earlier LPAZ at Tolsta Head. The Norwegian workers considered that their records marked the start of the interstadial and it is possible that the sediments in LPAZ TH-1 could also belong to an early stage. If the presence of woodland pollen is a diagnostic feature, it would also suggest that the Hirta radiocarbon date is much too young. The origin of the woodland pollen is, however, controversial. The Pinus and thermophilous woodland taxa at Tolsta Head, Derryvree and Hirta are considered to be derived from long-distance wind transport. The pollen records from the Middle Weichselian Interstadials in the Netherlands also list such taxa and their provenance and meaning were discussed by Zagwijn (1974). Pointing out that these taxa increased in the minerogenic deposits, especially in the sandy beds, he considered that the pollen was not of local origin but was derived from the reworking of peat beds and soils from preceding interglacials. It may be that the pollen is not related to vegetation change at the site, but the suggestion that the pollen has survived all the vagaries of the climatic oscillations since at least the Ipswichian/ Eemian period is difficult to accept. Fig. 5 shows the pollen preservation status of the Pinus pollen in LPAZ TH-1. The high percentage of perfectly preserved and folded/crumpled grains does not seem conceivable for pollen that is at least 90k radiocarbon yr old. Transfer of

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pollen from the European continent during the interstadial seems more probable, with perhaps South West Europe being the most likely source. This arboreal pollen could be an important factor in a consideration of vegetational succession in the interstadial.

4. Correlation of climatic fluctuations ca 30 kyr BP with a Bond cycle A Bond cycle (Bond et al., 1993), consisting of the four Dansgaard–Oeschger (D–O) cycles GI-8–GI-5 has been demonstrated for the period 35–28 kyr to which the ( Denekamp, Derryvree and Sandnes/Alesund Interstadials belong. Due to their fragmentary palynological and sedimentary records, it has not been possible to tie down from which part of the Bond cycle they originate. A similar situation exists, due to a variety of reasons, for the Scottish sites other than Tolsta Head. The latter site has collectively many important attributes not possessed by other sites dated to ca 30k 14 C yr BP: an extensive and detailed pollen diagram, analyses of the nature of the sediments, seven radiocarbon dates over a vertical extent of 35 cm above a further 45 cm of material which remain undated. Thus it seems possible, although problematic, to attempt for the first time a correlation between the pollen record at an interstadial site and the oscillations of the Bond cycle as revealed by the Greenland ice core data. The Denekamp Interstadial probably opens with GI-8 (Fig. 6) at 38.1k GRIP yr. The D–O cycle lasted

Fig. 6. Tolsta Head and the Middle Weichselian d18O record from the GRIP Greenland ice core and percentage presence of Neogloboquadrina pachyderma (s.) from deep sea core V23-81.

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ca 2500 yr and the sediments at Tolsta Head above the oldest date (36.8k GRIP yr) may be correlated with the latest part of the cycle. There is, however, some 45 cm of sediment below that date of which the lowest 10 cm yielded no meaningful pollen data. This sediment contains LPAZ TH-1 which, as discussed above, is notable for the occurrence of 10% or more of arboreal pollen, including some thermophilous taxa, suggesting that pollen transfer was taking place from warmer areas at a time when there was a possible northwards colonization by woodland species. The reason for the abrupt warming at the opening of this D–O cycle has been seen as due to an invasion of warmer water into the North Atlantic and the northward migration of the Polar Front. Such a migration would be accompanied by winds from the south-west which could account for the presence of the arboreal pollen. It is conceivable, therefore, that the basal 40 cm of sediment at Tolsta Head accumulated during the warmest part of GI-8. Towards the end of LPAZ TH-1, the volume of arboreal pollen decreases and then ceases abruptly at the opening of LPAZ TH-2 (Fig. 4a). The sediment in this LPAZ consists mainly of sand with streaks and lenses of comminuted organic material, which probably account for the rise in LOI values, and low pollen concentrations. It is noticeable that there is a marked increase in Salix pollen, (leaves of S. herbacea are present) and a sharp decline in Poaceae and the number of herbaceous taxa. The LPAZ also reveals the highest percentages of unidentifiable pollen and of pitted/ thinned grains of Poaceae and Cyperaceae (Fig. 5). These latter features, together with the comminuted organic material, suggest erosion of soils developed around the pollen site. Collectively such erosion and the evidence from the pollen analyses suggest that LPAZ TH-2 results from climatic change to colder conditions at some stage during or at the end of GI-8, this being represented in the GRIP ice core record by the downwards curve. The Bond cycle continues at the close of GI-8 with three warmer oscillations (GI-7–GI-5) separated by cold periods (Fig. 6), one of which, after GI-6, has the lowest d18O value (43.05%) for the whole cycle and lower than any achieved in the period of the Younger Dryas. The question arises as to whether these oscillations can be discerned in the record from Tolsta Head. All three warmer periods show the sharp rises in temperature associated with a D–O cycle and it might, therefore, be expected that the pollen record would reveal evidence of such rises. The lithology from the monoliths at Tolsta Head above 35 cm, i.e. above LPAZ TH-2, contains three distinct strata of brown organic material separated by sands of low LOI status, some of which incorporate suggestive erosional traits of organic streaks and lenses. An examination of the pollen record shows that the

Table 3 Calibrated BP dates of the organic strata from Tolsta Head related to the dates of the peaks GI-7–GI-5 in the GRIP and GISP2 ice core records D–O cycle

GISP2 date

Tolsta Head date (cal BP)

GI-7 GI-6 GI-5

ca 35,147 ca 33,455 ca 32,123

post 36,847 post 33,937 ca 30,657

strongly organic strata are dominated by Poaceae and have decreasing or negligible representations of Salix. They also have high concentrations of pollen. The sandy parts of the lithology reveal the reverse of these features and also have pollen preservation conditions which are inferior to those displayed in the organic strata. It is also very noticeable that the LPAZs do correspond very largely with the lithology (Fig. 4). Thus, it is possible to come to tentative conclusions about the sediments and vegetational record of LPAZs TH-3–TH-7. They appear to relate to D–O cycles within the Bond cycle. TH-4a might represent GI-7, the upper part of TH-7a relating to GI-6 and a truncated TH-7c being the response to the climatic features of GI-5. There is some support for this from the radiocarbon dates from Tolsta Head and the dates for GI-7–GI-5 in the GRIP ice-core (Table 3). Both, of course, could well suffer from defects: the former from contamination of the small AMS samples and the latter from the potential 10% error in the GRIP age calculations, apparently confirmed by recent radiocarbon calendar calibrations for the period after 30k 14C yr (Kitagawa and van der Plicht, 2000). Nevertheless, the relationships appear to provide the first possible evidence for the identification of D–O cycles and, overall, for a Bond cycle in a Scottish Interstadial prior to the Lateglacial. ( 5. Denekamp, Sandnes/Alesund and Tolsta Interstadials: comparison with Greenland ice core data ( The radiocarbon dates for the Sandnes/Alesund, Denekamp and Tolsta Interstadials place them in time within that Bond cycle which is immediately prior to the last major expansion of the Northern Hemisphere ice sheets. Having recognized that only minor contamination can lead to significant errors in radiocarbon dates in this age range, it is, nevertheless, still feasible to attempt to relate these dates to the d18O fluctuations in the GISP2 ice core. In this process the timing of the start and end of the interstadial can be constrained with greater precision. The Greenland Interstadials were defined first in the GRIP ice-core (Dansgaard et al., 1993). There is a close

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match between the GRIP and GISP2 ice-core records (Johnsen et al., 2001) for the period under study. The GISP2 record is used here as its chronology is more securely based, being based on the counting of annual layers back to 37.9 kyr, and its resolution is greater (Grootes et al., 1993). Comparisons between chironomid-inferred Lateglacial Interstadial air temperatures in south-east Scotland and the GRIP ice core record shows a close parallelism in timing and a general one in warming and cooling trends (Brooks and Birks, 2000). Hence, it is reasonable to assume that changes in the GISP2 d18O record for the preceding interstadial at around 30 kyr BP also reflect contemporaneous shifts in temperature in Scotland and western Norway. The GISP2 d18O record, in conjunction with that from the foraminifera evidence in deep sea core V23-81 (Bond et al., 1993; Fig. 6), indicates that the period under consideration was a time of rapid and marked temperature shifts. Within the period equivalent to 32– 28 k radiocarbon yr BP there are major fluctuations, with d18O peaks identified as short-lived, distinct interstadials in Greenland (Johnsen et al., 1995). Using the inferred temperatures of Brooks and Birks (2000), values of o40 d18O % suggest interstadial mean July temperatures below 91C, equivalent to those in the Loch Lomond Stade, and values of >37.5 d18O% suggest temperatures above 111C in south-eastern Scotland. Using similar comparisons with the Loch Lomond Stade, Clapperton (1997) calculated that a 1% change in d18O in the GRIP ice core is roughly equivalent to a temperature fall of 1.51C in the Scottish Highlands. By analogy, the temperature maxima during the interstadial around 30 kyr would be ca 111C, although the presence of Nymphaea suggests higher temperatures were achieved on occasions at Tolsta Head. Greenland Interstadial 8 (GI-8) opens with an abrupt warming at 38.1 cal kyr (Fig. 6), equivalent to 32.8k 14 C yr. GI-8 can be taken to mark the start of the ( Denekamp, Sandnes/Alesund and Tolsta Interstadials. ( The Sandnes/Alesund Interstadial is a period of ice-free conditions recognized along the south-west coast of Norway (Larsen et al., 2000). Unlike the Tolsta and Denekamp Interstadials, it is not defined on the basis of pollen records but on the existence of ice-free conditions along the present coast. These are demonstrated by the presence of glaciomarine and marine clays at elevations of up to 200 m and lying beneath tills of the Late ( Weichselian ice sheet. In the Alesund area, the ice-free period was first dated, using marine shells reworked from tills, to 28–38k 14C yr (Mangerud et al., 1981). However, the oldest radiocarbon date on bones from the cave Skjonghelleren is 34.9k 14C yr (Valen et al., 1996) and 31.9k 14C yr from Hamnsundhelleren (Valen et al., 1996). Further south at Sandnes, the equivalent interval was represented by marine clays (part of the Hogjæren

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Clay; Larsen et al., 2000). Dates on shells ranged from 29–32k 14C yr at Foss–Eigeland to 27–38k 14C yr at Sandnes (Andersen et al., 1981). Recent AMS age determinations on foraminifera from Elgane are 33–35k 14 C yr (Larsen et al., 2000). Radiocarbon age determinations for the period 35–38 kyr come solely from marine shells. These age determinations may simply be in error due to slight contamination. Radiocarbon dating over the period before 33 kyr is also complicated by elevated concentrations of 14C in the atmosphere (Beck et al., 2001). Alternatively, such dates may relate to shells from glaciomarine sediments (Larsen et al., 2000) associated with fluctuations of the Norwegian Channel Ice Stream, rather than to interstadial marine sediments. The ( Sandnes/Alesund Interstadial is now more narrowly assigned to the younger period of 35–29k 14C yr (Andersen et al., 1987; Sejrup et al., 1994; Larsen et al., 2000). Correlation with the start of GI-8 implies further restrictions, with a start at 32.8k 14C yr. It is noteworthy that in south-west Norway, the oldest date from Skjonghelleren of 34.9k 14C yr stands alone (Valen et al., 1996) and the oldest of the multiple dates in the main study is 32.8k 14C yr (Larsen et al., 1987). After GI-8, temperatures declined but recovered in three brief phases equivalent to GI-7, GI-6 and GI-5. The last phase closes soon after 32.1k cal yr, equivalent to 28.7k 14C yr, and was succeeded by 3000 yr of stadial conditions. The termination of GI-5 appears to represent the end of the interstadial. The youngest dates from caves in western Norway are 28,900 and 27,580 (71225) 14 C yr BP, respectively (Larsen et al., 1987; Valen et al., 1996). Ice-advance past the present Norwegian coastline occurred soon after the onset of the Lake Mungo/Mono Lake palaeomagnetic excursion at 29 kyr. The end of the Tolsta and Denekamp Interstadials is therefore taken to be 32.1 and 28.7k 14C yr. Radiocarbon dates on polleniferous organic muds and sands which appear to be younger than 28.7 kyr are very likely to be in error as temperatures thereafter dropped too low for the widespread establishment of vegetation. Such dates include the uppermost age determination of 26.1 kyr at Tolsta Head and that of >24.7 kyr from Hirta. Radiocarbon dates on reindeer remains from the caves at Inchnadamph, northwest Scotland, of 22.3, 24.6 and 25.3 kyr (Murray et al., 1993) are, however, almost certainly too young, particularly as the cave lies only ca 5 km from the limit of the Loch Lomond Stade glaciers in this area. An exception is the dates on bones of Phoca sp. and Plautus alle from the cave Hamnsundhelleren at ca 24.5k 14C yr, representing a brief period of ice retreat, the Hamnsundhelleren Interstadial (Valen et al., 1996). Reference to the ice core record indicates that this period should correlate with GI-4 (28.9k cal yr) or GI-3 (27.7k cal yr).

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( 6. Denekamp, Sandnes/Alesund and Tolsta: interstadial or interval?

Glasgow at ca 33 14C kyr BP (Jardine et al., 1988) and probably at a similar date at Tolsta Head. Numerous radiocarbon dates for the wider interval of 36–26 kyr BP exist on land in Scotland for transported marine shells in glacial deposits (Sutherland and Walker, 1984), for mammalian remains (Rolfe, 1966; Lawson, 1984), and for organic muds and sands (Sutherland et al., 1984) but the integrity of these age determinations is often uncertain. In the English Midlands several sites have yielded radiocarbon dates of ca 28–32 kyr (Coope and Sands, 1966; Coope, 1968; Morgan, 1969). At Four Ashes, over 50 lenses of organic sediment occur within thin fluvial gravels overlain by till (Morgan, 1973; Andrew and West, 1977). The organic remains have given radiocarbon dates spanning the period 42.5– 30.5 kyr BP, but the two youngest dates on the uppermost organic materials are some 6000 radiocarbon years younger than those on the underlying organic material. Hence, there are strong grounds for regarding the period at around 30 kyr BP in the British Isles as a true interstadial, with a discrete period of vegetation and peat growth. In south-west Norway, available dates indicate that ( the Sandnes/Alesund Interstadial spanned the period from 35 or 33 to 29k 14C yr. No coherent vegetation records exist for this time in this area. The Interstadial was essentially an ice-free period when distal glaciomarine conditions prevailed offshore (Larsen et al., 2000). The arctic fauna preserved in caves is typical of that found on Svalbard at the present time (Valen et al., 1996). There are factors other than the radiocarbon dating which led Caspers and Freund (2001) to consider that

In The Netherlands, Belgium and north-west Germany, radiocarbon dates for peat reach maximum frequency about 32 and 29 kyr BP but many also span the periods before and after (Ran, 1990). Although the high atmospheric 14C concentrations prior to 35 kyr make problematic the interpretation of radiocarbon dates for this period (Beck et al., 2001), the apparent spread of dates has led to uncertainty (Huijzer and Vandenberghe, 1998) as to the status of the Denekamp Interstadial as a discrete, relatively warm phase, with a vegetation signature distinctive from that found in overand under-lying units (Behre, 1989). Caspers and Freund (2001, p. 44) refer to an interstadial as ‘a climatic amelioration that facilitates vegetation succession that can be recognized in the pollen record.’ They suggest that the Denekamp phase shows insufficient evidence of vegetation change and so should be termed an interval rather than an interstadial (Caspers and Freund, 2001; Bos et al., 2001). The maritime margin of North West Europe provides a contrasting perspective. Table 4 lists and Fig. 1 locates selected sites in the British Isles and Norway which have been dated to the period between 36 and 26k radiocarbon years BP. In the British Isles, there is a clustering of radiocarbon dates in the interval 32–28 k radiocarbon years. In Northern Ireland, organic silts lying between two tills are radiocarbon dated to ca 30.5 kyr at Derryvree (Colhoun et al., 1972) and at Greenagho (Dardis et al., 1985) to ca 32.5 kyr. In Scotland, peat formation began on a till surface at Sourlie, near

Table 4 Radiocarbon ages from selected sites in the British Isles and Norway dated to between 36–28 kyr BP Country

Site

Material

Age

Laboratory no.

Reference

England England England England England England England England N. Ireland N. Ireland Norway Norway Norway Norway Scotland Scotland Scotland Scotland Scotland

Brandon Brandon

Twigs & leaves Plant debris Plant debris Sandy detritus peat Sandy detritus peat Peat Plant debris Leaves & rootlets Moss-rich detritus Organic detritus

28,2007500 30; 766þ537 520 32; 270þ1029 921 30,5007440 30; 655þ765 700 28,2257330 34; 730þ440 420 32; 160þ1670 1390 30; 500þ1170 1030 32,4607270

(NPL-87) (Birm-27) (Birm-10) (Birm-195) (Birm-25) (Birm-75) (SRR-2302) (NPL-55) (Birm-166) (SRR-2065)

Coope, 1968 Coope, 1968 Coope, 1968 Morgan, 1973 Morgan, 1973 Morgan, 1969 Seddon and Holyoak, 1985 Coope and Sands, 1966 Colhoun et al., 1972 Dardis et al., 1985

Bones Bones Bones Collagen Organic sand Plant debris Clay/silt Clay/silt

31,905 32,8007800 34,900 27; 550þ1370 1680 >24,710 29,2907350 30,2307280 33,2707370

(TUa-802) (T-5593) (TUa-238) (Geochron. Lab. Inc) (SRR-1809) (SRR-3146) (SRR-3147) (SRR-3149)

Valen et al., 1996 Mangerud et al., 1981 Valen et al., 1996 Rolfe, 1966 Sutherland et al., 1984 Jardine et al., 1988 Jardine et al., 1988 Jardine et al., 1988

Four Ashes Four Ashes Great Billing Stanton Harcourt Tame Valley Derryvree Greehagho ( Alesund Hamnsundhelleren Skjonghelleren Skjonghelleren Bishopbriggs Hirta Sourlie Sourlie Sourlie

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interval is a better designation than interstadial (but see Gibbard and West, 2000) for the Denekamp and contemporaneous periods in the vegetational record. One concerned the fact that the sites involved, unlike those from the Brrup and Odderade Interstadials, showed no vegetational succession within the pollen record. This is perhaps not surprising given the often fragmentary, disturbed and vertically restricted nature of the sediments involved. At Tolsta Head, however, there appears to be a more complete record and one which reveals elements of succession. The lowest sediments there, in common with some other sites, contain Betula and Pinus pollen (Fig. 4a) and, while in the latter case no claim is made for the taxon’s presence at the site, this suggests that woodland advance was occurring further south in western Europe. At Grande Pile, the high resolution pollen analysis shows three peaks in arboreal pollen dated to 30.8, 29.7 and 29.1k 14 C yr, respectively (Woillard and Mook, 1982). Thus, only short-lived phases of warming and tree colonization are indicated. That such colonization was not achieved at sites on the maritime fringes of North West Europe might be explained by the strongly oscillatory nature of the climate of the GI-8–GI-5 Bond cycle when compared with the equivalent record for e.g. the Odderade Interstadial (Stuiver and Grootes, 2000). The argument for the recognition of the Tolsta Head and other sites as distinct interstadials rests to a large degree upon the matching of pollen and sedimentary records with oscillations in the Greenland ice core. Caspers and Freund (2001) argue correctly that peaks in the d18O record do not necessarily mean rises in temperature (Stuiver and Grootes, 2000). They also point to the deep sea core record (Martinson et al., 1987) as not showing convincingly oscillations equivalent to those for the GI-8–GI-5 period in the Greenland ice core. Evidence from the chironomid analyses is beginning to show that there is a temperature relevance in the ice core peaks but deep sea core evidence, other than that already quoted, is more compelling. The Martinson et al. (1987) core is not really appropriate in this discussion as it is an analysis of benthic conditions. A more appropriate analysis is found in Core V23-81 from the North Atlantic (Bond et al. 1993). It shows that there is a strong relationship between the abundance of the planktonic foraminifera Neogloboquadrina pachyderma (s.) and the D–O cycles of the GI-8–GI-5 peaks in the GRIP ice core.

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pollen assemblages and sedimentology are provisionally matched with the d18O record of the GISP2 ice core and correlation with the Dansgaard–Oeschger cycles GI-8– GI-5 is proposed. Such correlation implies that, as in the Lateglacial, variations in the oxygen isotope stratigraphy of the Greenland ice cores are matched by palaeotemperature fluctuations in Scotland. This conclusion needs to be supported by other interstadials in North West Europe, using palaeotemperatures inferred from pollen, coleoptera and chironomids. Nonetheless, the potential is clear for cross-calibration between the Greenland ice core stratigraphy, the marine record and and local sequences. The d18O record of the GRIP and GISP2 ice cores indicates that the Tolsta Head Interstadial and its equivalents in North West Europe were periods of markedly fluctuating temperatures. These rapid temperature changes are reflected in the peaks of arboreal pollen at Grande Pile (Woillard and Mook, 1982). Moreover, the strongly oscillatory nature of the climate of the GI-8 to GI-5 Bond cycle may account for the lack of woodland in North West Europe as phases of temperature sufficient to allow its advance and establishment were too short (cf Coope, 2000). It seems that the vegetation signature of the Denekamp Interstadial and its contemporaneous equivalents is more clearly seen on the maritime fringe of North West Europe where relatively elevated temperatures allowed species-diverse grassland to develop. The initial warming is equivalent to that recorded in Greenland Interstadial 8 (GI-8) at 38.1k cal yr. Termination is equivalent to the close of GI-5, indicating that build-up of the last ice sheets in Scotland and North West Europe dates from 32k cal yr and 28.7k radiocarbon years.

Acknowledgements Thanks are due to Colin Cameron for the pollen preparations, to Graeme Sandeman for Figs. 1,3 and 6 and to the referees, Douglas Peacock and Michael Walker, for their valuable suggestions. The radiocarbon dating was funded by the NERC Radiocarbon Laboratory, East Kilbride and supervised by Dr C. Bryant. The extracts from the Greenland ice core and N. pachyderma (s.) records are reproduced by permission from Nature 365 copyright 1993 Macmillan Magazines Ltd.

References 7. Conclusion The organic strata at Tolsta Head contain an unusually detailed record of the dominantly open grassland vegetation on north Lewis at ca 30 kyr BP. Variations in palaeotemperature inferred from the

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