Pollen stratigraphy of late pleistocene marine sediments at Nørre Lyngby and Skagen, North Denmark

Quaternary Science Reviews, Vol. 17, pp. 839 — 854, 1998 ( 1998 Elsevier Science Ltd. All rights reserved. Printed in Great Britain PII: S0277–3791(98)00021-3 0277—3791/98/$ — See front matter

POLLEN STRATIGRAPHY OF LATE PLEISTOCENE MARINE SEDIMENTS AT NØRRE LYNGBY AND SKAGEN, NORTH DENMARK C.G. GLAISTER and P.L. GIBBARD Godwin Institute for Quaternary Research, Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, U.K. (E-mail: [email protected]) Abstract—Two cores from North Denmark, N+rre Lyngby 2 and Skagen 3, have been investigated palynologically. The sediment examined is correlated with the Eemian interglacial Stage (Oxygen Isotope substage 5e) and part of the Early Weichselian. The Skagen 3 core includes only the latter part of the interglacial. The pollen assemblages in both cores significantly differ from nearby terrestrial records: reflecting the nature of the depositional environment, taphonomic processes and current regime during a time of higher eustatic sea-level. The upper part of the N+rre Lyngby 2 core contains an assemblage which is correlated with the Early Weichselian Br+rup Interstadial. The N+rre Lyngby 2 sequence also includes a characteristic sub-biozone not seen in the Danish terrestrial sequence which represents a climatic deterioration, similar to that observed in the foraminiferal assemblages from the same samples. The pollen sequence appears to have been influenced by changes in source area and surface water circulation, which supports the interpretation of the cooling events given by the foraminiferal record. ( 1998 Elsevier Science Ltd. All rights reserved.

sediments and the presence nearby of well-documented terrestrial Eemian sites, including the parastratotype Hollerup (Andersen, 1964, 1965); the latter currently under reinvestigation by Bjo¨rck et al. (unpublished). The strength of the shallow marine (continental shelf) sequence available is in its comparatively high-resolution representing relatively continuous sedimentation during high sea-level stands. This allows its use as an intermediate point or ‘stepping-stone’ for correlation between the terrestrial and deep water marine environments. Palaeobotanical investigations of sediments from northern Jutland have expanded knowledge of the Eemian history of the Baltic region and are fundamental in that the original zonation scheme for the Last Interglacial was devised in this area by Jessen and Milthers (1928). The establishment of terrestrial—marine correlations using multidisciplinary investigations including micropalaeontological and palynological techniques offers an unrivalled possibility of testing the possible impact of climate fluctuations during the Eemian (Donner, 1995). Such correlations are of particular value in this area because of its proximity to the glacial centres and the original type areas of the climatic events.

INTRODUCTION In North Jutland, a series of boreholes have been put down into the relatively deep water shelf sediments that infill a northwest—southeast trending basin extending from the Skagerrak into the Kattegat (Fig. 1). This basin is thought to result from tensional stress directed perpendicularly to the Fennoscandian shield boundary zone (Lykke-Andersen, 1987). Foraminiferal analyses carried out on sediments from the boreholes indicate that they range from Late Saalian to Middle Weichselian in age, apparently as a continuous sequence (Knudsen, 1984, 1986, 1992; Seidenkrantz et al., 1995). The results presented in this paper are from the two cores N+rre Lyngby 2 (57°25@N; 9°44@E) and Skagen 3 (57°44@N; 10°38@E) (Fig. 1). The borings were 70.5 and 220 m deep respectively, and pollen analysis was concentrated on the section in the two cores thought to represent the Last Interglacial and its transitions into the preceding and following cold stages (approximately 8 and 5 m thick respectively, on the basis of foraminiferal assemblages) (Seidenkrantz et al., 1995). Both core sites are 1.0 m above the present sea level. The cores are of particular value in that they were obtained as coherent sediment with 100% recovery in the analysed interval. Their stratigraphy is illustrated in Figs 2 and 3. The Danish area is considered to have an important role in any future correlation between the deep sea oxygen isotope (OI) record and terrestrial pollen sequences because of both the availability of suitable

METHOD The cores from both sites were subsampled at the University of Aarhus into quarter diameter slices approximately 3 cm thick, and prepared using the 839

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Fig. 1. Location map showing the position of sites in Denmark and SW Sweden correlated with the Eemian interglacial Stage.

standard chemical preparation method with hydrofluoric acid (HF) (West, 1968). The method was modified using an additional stage of sodium pyrophosphate to remove clays (Bates et al., 1978) and nitric acid to remove insoluble sulphides. The large amounts of finely disseminated clay were removed using the method in which samples are passed through a 10 lm nylon monofilament mesh (Heusser and Stock, 1984). The combination of sodium pyrophosphate as a separate stage, followed by sieving, removed sufficient clay particles to provide exceptionally clean samples, and a greater pollen concentration for counting. Samples were initially counted to a main sum of 300 non-aquatic land pollen and spores. However, the very high frequencies of Pinus pollen encountered in all samples (thought to result from the characteristics of the depositional environment) necessitated counts of 300 non-aquatic, non-Pinus grains in order to achieve a statistically valid main sum. This resulted in total pollen sums of ca. 2000 grains in some cases. During processing, tablets containing a known number of exotic ¸ycopodium spores were added to each sample in order to obtain total concentration values. The concentration data is discussed in Glaister (1998). The drafting of the diagram recognises some difficulties in nomenclature and in accurate determination. Pollen of the genus Myrica can be distinguished from that of Corylus in situations where grains are well preserved. The two are normally distinguished by slight differences in the region of the pori, which is

facilitated by using a phase-contrast microscope. This facility was not available during pollen counting; the two taxa were therefore difficult to distinguish, especially in the case of less well-preserved specimens. In this report these pollen taxa are referred to as Coryloid type. In the case of the bisaccate taxa, the large number of broken grains required that individual bladders of Pinus and Picea pollen were counted. Reworked palynomorphs were recognised on the basis of differences in staining, wall thickness and morphology, and were excluded from the main sum. Pollen type conventions used broadly follow Andrew (1970), the plant nomenclature follows Clapham et al. (1987).

TAPHONOMY Pollen assemblages in sediments are known to be biased as a result of taphonomic processes which act on them before, during and after deposition. These processes should be considered when interpreting the assemblages described here, since the relative importance of each factor can vary with both the type of depositional environment and the prevailing climatic conditions. The absence from the area of modern surface samples means the interpretation of the pollen assemblages from N+rre Lyngby and Skagen must be undertaken with caution, because the relative

C.G. Glaister and P.L. Gibbard: Pollen Stratigraphy of Late Pleistocene Marine Sediments

Fig. 2. Sediment log of the N+rre Lyngby 2 sequence. Legend applies to Figs 2 and 3.

Fig. 3. Sediment log of the Skagen 3 sequence. For legend see Fig.2.

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importance of different taphonomic processes cannot be accurately gauged from present day evidence. However, comparison with contemporaneous terrestrial sequences may give some indication of dominant processes. Initial biases such as variations in pollen productivity (Pohl, 1937) and plant physical characteristics (Fægri and Iversen, 1989) are amplified by taphonomic processes, especially if the pollen assemblage was deposited in a comparatively high energy depositional environment or at some distance from the original source area; criteria which are fulfilled by marine environments. Several major north European rivers enter the North Sea and Baltic basin, and in the area of deposition, it is probable that the principal factor controlling pollen deposition is water transport, as discussed by Muller (1959). Although much of the pollen assemblage is likely to consist of pollen from nearby landmasses (northern Denmark, western Norway and southern Sweden), it is probable that a significant proportion is long distance transported from other parts of northern Europe. Similarly, it is possible that, in the marine environment, pollen biozones may have more transitional boundaries as a result of the presence of pollen from both local and distant sources; an assemblage may contain pollen transported from warmer environments within more southerly regions. The mode of transport can have a strong effect on the relative proportions of pollen taxa entering the marine environment. Prentice (1988) points out that large rivers have a broad catchment area, and therefore trap large quantities of relatively well dispersed taxa such as Pinus. At a site with a large catchment area, these taxa will comprise a large proportion of the pollen assemblage, and other, less well dispersed taxa will be correspondingly more rare. The regional signal in an area of broad catchment size may therefore overwhelm that of the local vegetation. In this case, the interpretation will necessarily be based on a regional rather than a local signal. This is preferable for correlation purposes, because it will avoid the local site biases often encountered at terrestrial localities, particularly those with small catchments. The morphological characteristics of different palynomorph taxa have a substantial effect on their representation in the fossil record. This is particularly true in comparatively large catchment areas. Factors such as relative buoyancy and differential settling of pollen and spore taxa can result in a bias towards grains with a high surface area to volume ratio, such as Pinus and Picea pollen. For example, Mudie and McCarthy (1994) found that off the coast of Nova Scotia, Pinus and Picea pollen peak in concentration on the continental margin, and in percentage further offshore. Similar effects have been recognised using other pollen and spore taxa such as Betula and Filicales. These factors will be magnified with greater distance of transport. The Baltic is effectively a ‘settling tank’ for pollen and spores, lacking strong currents at

present; therefore an assemblage which has been transported a long distance might be expected to have lost a large proportion of its pollen through differential settling. Although non-arboreal pollen enters the pollen assemblage in relatively small amounts, Muller (1959) found that, once in the marine environment, it can be transported considerable distances because of its relatively small size; it will therefore increase in proportion offshore. The transport process itself can influence the pollen spectrum, by damaging or destroying certain grains depending on the mode of transport, particularly through chemical deterioration. The preservation potential of spores and pollen depends mostly on the relative sporopollenin content of the exine, which differs between taxa (Traverse, 1988). Generally speaking, grains with a high sporopollenin content, such as Pinus and other conifer pollen, are resistant to damage, whereas those with low sporopollenin such as Equisetum spores and Populus pollen, are not. The pollen assemblage will vary according to the source vegetation and therefore the prevailing climate. A change in climate can be expected to influence the relative effect of different taphonomic processes. As pollen grains are effectively sedimentary particles, and behave as such (Hofmeister, 1954), it follows that they will be subject to the relative biases that can result. A sea-level fall at the beginning of a cold stage and the attendant rejuvenation of river systems will result in an increase in the relative proportion of reworked grains. The increase in influx of siliclastic sediments during a cold stage will result in a relative drop in pollen concentrations, as pollen is deposited with sediment of a similar grain size (Traverse and Ginsburg, 1966). Following a sea-level fall, modifications to currents may cause an attendant change in the provenance of the pollen assemblage. Taphonomy is not, therefore, a static process and consideration should be given to the effect climate change can have on the pollen record not only by changing the source vegetation, but also by influencing the way in which the pollen and spores derived from the vegetation are deposited. The water depth during those parts of the sequences correlated with the interglacial has been interpreted at around 100 m at N+rre Lyngby 2 and 200—300 m at Skagen 3 (Seidenkrantz et al., 1995). A substantial fall in sea-level of 60—70 m at the end of the Eemian has been widely recorded in the area (Knudsen, 1984; Zagwijn, 1977, 1983; Seidenkrantz and Knudsen, 1994). The fall in sea-level may imply that any palynomorphs deposited during the cold stage may be less affected by taphonomic processes such as differential flotation, as a result of the relative proximity of the sites to land, although, as described above, other factors may become more important. The entire pollen sequences from N+rre Lyngby 2 and Skagen 3 are probably dominated by long-distance transported grains. Long distance transported assemblages still bear a relation to the source area vegetation, as shown by Fægri and

Fig. 4. N+rre Lyngby 2: Pollen diagram. Pinus and aquatics are excluded from the main sum. Taxa present at low frequencies are shown with a]10 exaggeration.

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Iversen (1989). Therefore, the obvious disadvantages in vegetational detail are outweighed by the advantage of establishing a robust correlation between sedimentary sequences of markedly different facies, i.e. terrestrial (lacustrine) and shallow marine. These factors must be taken into account when considering the interpretation of the N+rre Lyngby 2 and Skagen 3 pollen records.

To judge from this spectrum it is likely that the Betula was present in isolated scrub-like stands, surrounded by grassland. In those areas with greater moisture, such as along river banks, alder was the dominant tree. Coryloid and Carpinus pollen are present in small quantities in this biozone and are interpreted as being long distance transported or reworked. Indeterminate grains are abundant, suggesting a high proportion of the pollen is long-distance transported from other environments.

RESULTS AND INTERPRETATION The pollen percentage diagram of the main taxa from the N+rre Lyngby 2 and Skagen 3 sediments are shown in Figs 4 and 5 respectively. Single grain or very rare occurrences are combined and included in the diagram as ‘other trees and shrubs’, ‘other herbs’ and ‘other lower plants’, and are shown in greater detail in Glaister (1998). Preservation was generally variable, and is reflected in the curve representing ‘‘total indeterminate’’ grains. It should be noted that, as discussed earlier, the Pinus pollen curve in N+rre Lyngby 2 and Skagen 3 is thought to largely record overrepresentation, and has therefore been deemed unreliable as a palaeoenvironmental indicator. N+rre Lyngby 2 The following sequence of pollen assemblage biozones and sub-biozones is proposed for the N+rre Lyngby 2 sediments, (Fig. 4), using the abbreviation ‘NL’. The biozones were subdivided by recognition of significant changes in the pollen assemblage. Biozone NL-1 69.8–67.5 m This biozone is characterised by a rise in the occurrence of pollen of thermophilous tree taxa such as ºlmus, Quercus, Alnus and Coryloid types. Picea pollen values rise steadily throughout the biozone, and spores of Filicales decline. A relatively high proportion of indeterminate grains are present. The biozone has been divided into two sub-biozones, described below. Sub-biozone NL-1a 69.8–69.45 m This sub-biozone is based on only one count. Although this is not normally a legitimate criterion for subdivision, the assemblage present at this level is thought to be sufficiently distinct from subsequent levels to merit tentative subdivision into a sub-biozone. The earliest spectrum present records the beginning of forest development, with a dominant arboreal component of Betula (23%). The low frequencies of Alnus pollen in comparison to those found in terrestrial sequences is probably because alder is a common component of floodplain environments. The main non-arboreal component is Gramineae pollen (25%), accompanied by minor occurrences of other herbaceous taxa such as Filipendula and Helianthemum.

Sub-biozone NL-1b 69.45–67.5 m The appearance of thermophilous tree taxa in this sub-biozone such as ºlmus, ¹ilia, Quercus and Fraxinus corresponds with a relative decrease in herbaceous taxa, suggesting possible shading or competition. The presence of Picea pollen at frequencies of ca. 20% may result from the tendency of this taxon to become overrepresented as a result of flotation; accordingly it is considered to either be present in the local environment in comparatively small populations, or to be long distance transported from further north and east. Of the non-arboreal taxa, Filicales spores replace Gramineae pollen as the most dominant taxon, suggesting a slightly more moisture-rich environment, confirmed by the presence of Filipendula and Sphagnum. Towards the end of the sub-biozone, many pollen taxa, such as Gramineae, Coryloid types, Carpinus, Fraxinus, ¹ilia and Quercus decrease in frequency, while Filicales spores decrease throughout. The environment can be summarised as mixed oak forest surrounded by boggy or moisture-rich environments, possibly on more exposed uplands. The presence in significant quantities of Alnus, Filicales and Sphagnum suggests the development of a floodplain vegetation in river valleys. Biozone NL-2 67.5–65.1 m This biozone is characterised by a general rise in the proportion of pollen of tree and shrub taxa. Picea pollen remains at relatively constant levels, whilst pollen of the majority of thermophilous taxa is absent or at very low frequencies at the beginning of the biozone, but increase in the latter part, in particular Quercus. The biozone has been divided into two sub-biozones, described below. Sub-biozone NL-2a 67.5–66.45 m This sub-biozone is notable for a substantial drop in frequency of the pollen of many thermophilous taxa in comparison to the previous sub-biozone, and their replacement by Picea pollen (up to 70%). Although Picea is present in the earlier zones, it occurs at relatively low frequencies. This early occurrence is prob ably misleading, resulting from the overrepresentation of Picea through differential flotation. In the case of the

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cores described, it is suggested that this early occurrence of around 20% arboreal pollen (AP) represents a ‘base level’ resulting from the hydrodynamic properties of Picea pollen. West (1961) noted that the pollen record of the Ludham borehole showed that taphonomic factors applying to Picea may result in its appearance even when not present in the local environment. He also notes that in an environment where Picea is present, fluctuations in the curve can still reflect actual vegetational change even where taphonomic factors such as differential flotation have resulted in an artificially high frequency. Thus the change seen in this sub-biozone from a mixed oak woodland to mixed oak woodland and Picea is probably a result of climatic and edaphic changes. The sub-biozone appears to represent continued growth of the mixed oak forest, but a possible climatic deterioration accompanied by increased moisture availability encouraged the development of stands of Picea. The continued presence of mixed oak forest, although reduced, may mean Picea grew preferentially in those areas with a lower risk of drought and more acid soils, hence the decline in Gramineae and Filicales occurrence throughout the sub-biozone.

Sub-biozone NL-2b 66.45–65.1 m This biozone appears to represent a slight climatic amelioration. Pollen of ºlmus, Quercus, ¹ilia, Alnus, Carpinus, Coryloid types and Fraxinus all appear or increase, whilst Picea pollen decreases in frequency. The decrease in Picea pollen is not thought to be taphonomic in origin, and would appear to represent a resurgence in importance of the mixed oak forest. In the early part of the sub-biozone, Gramineae, Compositae Liguliflorae and Chenopodiaceae pollen and Pteridium and Filicales spores also increase in frequency, which in the context of the interpretation given for Picea in the previous sub-biozone, suggests the recolonisation of moist exposed or upland environments in the pollen source area.

Biozone NL-3 65.1–63.55 m Pollen of thermophilous tree taxa decrease to similar levels as seen in sub-biozone NL-2a, with Picea pollen frequency increasing to greater levels. Concentration data, however, shows that the apparent drop in occurrence of thermophilous taxa is largely an artefact caused by the Picea pollen rise, and that the thermophilous taxa would still have been present in the environment. The dominance of Picea pollen in this biozone can be interpreted as near total spruce forest. Mangerud (1981) proposed a figure of 40% arboreal pollen (AP) as indicative of near complete spruce cover (excluding taphonomic factors). In a situation such as this, where differential flotation will lead to overrepresentation, it is proposed that the 80% occurrence seen in this biozone is a more realistic figure. Of the

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other types, only Filicales spores are present in any major proportion, and was most likely present in damper habitats and where the spruce cover thins sufficiently to provide adequate light for growth. Some members of the Filicales may have been present as epiphytes in the spruce forest. Biozone NL-4 63.55–61.0 m The vegetational signal in this biozone is hard to deduce. It is noticeable from the pollen diagram (Fig. 4) that the curves for many of the taxa are erratic, most notably those of Betula, Alnus and Picea. It is highly unlikely that such a pattern is the result of actual climatic variability, and it would seem more probable that it results from variation in the taphonomic factors acting on the pollen during deposition. This may be a result of changes in current and sea level, causing changes in transport distance and (possibly) sedimentation pattern. The result of these changes would be a very mixed, regional pollen spectra, making any interpretation rather unreliable. Arboreal taxa as a whole decrease in frequency throughout the zone, reflected in the increase in frequency of the pollen of herbaceous taxa such as Gramineae, Ericales, Compositae and Chenopodiaceae. In addition, Picea is gradually replaced throughout the biozone by Betula, and marsh species such as Sphagnum and Sparganium increase in frequency. It is possible that this biozone represents a slight climatic deterioration and the development of acidic soils. Biozone NL-5 61.0–56.2 m This biozone is characterised by high frequencies of non-arboreal taxa such as pollen of Ericales and Gramineae. Pollen of arboreal taxa are present at relatively low frequencies, with the exception of Betula, Alnus and Coryloid types. Picea pollen is present at relatively low frequencies in the early part of the zone, but increases in frequency in the latter part. The biozone has been divided into two sub-biozones, described below. Sub-biozone NL-5a 61.0–58.0 m Arboreal pollen remains dominant in this unit, but declines in importance throughout the sub-biozone. Most notable is Picea pollen, which, after an initial increase, decreases to its lowest values of 3%. The presence in this sub-biozone of significant quantities of pollen of Ericales and Sphagnum spores suggests the development of a heathland vegetation on leached, acidic soils, with Ericales more dominant in exposed areas. The increase in pollen frequency of Betula would also support this interpretation, with a wide tolerance of soil conditions, it is known to be an effective constituent of heathland vegetation (Godwin, 1975). The continued presence of Picea pollen in the

Fig. 5. Skagen 3: Pollen diagram. Pinus and aquatics are excluded from the main sum. Taxa present at low frequencies are shown with a]10 exaggeration.

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sub-biozone suggests that spruce was still growing in the area, although as mentioned earlier, it is likely that only small populations were present. The high frequency of herb pollen suggests more open, marshy environments. This is a similar pattern to that seen at Scho¨ningen (Urban et al., 1991a, b; Urban 1992) and several other NW European sites (Veil et al., 1994; Behre and Lade, 1986; Pas sse et al., 1988). Urban (1993) concludes that the spreading of heather plants marks interglacial—glacial transitions, an interpretation that would seem appropriate to this sub-biozone. The presence in this sub-biozone, and the latter part of biozone NL-3, of the pollen of thermophilous tree taxa is not thought to be of any climatic significance. The taxa are present at low, fairly constant levels, and it is probable that they are a result of long distance transport from more temperate environments or reworking caused by base level lowering at the beginning of a glacial period (Traverse, 1988). This is partly confirmed by the curve for indeterminate pollen types, which show a rise at the beginning of the sub-biozone. The sub-biozone also shows a clear rise in Coryloid pollen types, and it is proposed that, in the context of the rest of the interpretation, the majority of Coryloid types in this sub-biozone represent Myrica. Sub-biozone NL-5b 58.0–56.2 m This sub-biozone sees a resurgence in Picea forest, with pollen values of up to 48%. This is seen as a climatic succession, with Picea replacing the Betula development seen in the latter part of the previous sub-biozone, possibly due to a slight cooling event and acidic soils. This would also explain the lack of any development of mixed oak woodland taxa. The comparatively high values of Ericales (up to 27%), and appearance of Cyperaceae and pollen of numerous other herbaceous taxa suggests a landscape of Picea stands surrounded by heathland. The low frequencies of Gramineae pollen and Sphagnum spores suggests that these taxa were out-competed for moisture-rich areas by Picea. Biozone NL-6 56.2–52.75 m This biozone is characterised by very low frequencies of arboreal pollen in the early part, with accompanying high values of Ericales pollen. The latter part of the biozone sees a reverse in this trend, with high values of Picea pollen. The biozone has been divided into two sub-biozones, described below. Sub-biozone NL-6a 56.2–54.4 m This sub-biozone appears to represent a return to similar conditions as those described for sub-biozone NL-5a, albeit more severe. Pollen of Ericales is at its highest frequency in the middle of the zone (73%), and is associated with a dramatic drop in the occurrence of Picea pollen. Comparatively high values of Betula

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pollen, coupled with the Ericales curve suggest a partial recolonisation of the moist, acidic soils previously occupied by Picea. An early peak of Gramineae preceding the main Ericales rise suggests opportunistic recolonisation before Ericales became dominant. The latter part of the sub-biozone sees a drop in the frequency of Ericales coupled with a rise in the pollen of Betula, Alnus, Coryloid types and Filicales, all of which suggests a climatic amelioration with warmer, possibly drier conditions. The frequency occurrence of Pinus drops significantly in this sub-biozone from relatively constant levels in previous biozones. It is possible that with conditions much colder than previously that the representation of this taxa in the marine record reflects a genuine drop in the amount of Pinus on land. Sub-biozone NL-6b 54.4–52.75 m This sub-biozone sees a substantial climatic amelioration in comparison to the early part of the previous sub-biozone, with Picea pollen frequencies rising to 72%, close to the interglacial levels seen in biozone NL-3. The Picea forest almost completely replaces the Betula scrub of the latter part of biozone NL-6a, and is accompanied only by relatively high values of Ericales pollen and Filicales spores, although both drop in frequency compared to the previous sub-biozone. The Ericales was probably present mostly in more exposed areas, whilst Filicales was associated more closely with the Picea forest, possibly as epiphytes. Biozone NL-7 52.75–51.2 m This biozone represents a significant climatic deterioration, with Ericales succeeding Picea pollen as the dominant component. It is likely that this represents direct habitat replacement. A decline in the frequency occurrence of Pinus similar to that in subbiozone NL-6a is also seen. One level in this biozone shows a much higher proportion of the pollen of moisture-loving taxa such as Betula, Alnus and Sphagnum. Interpretation of this is difficult as it relies on only one count, but it may reflect increased precipitation and an increase in boggy environments. Skagen 3 The following sequence of pollen assemblage biozones and sub-biozones is proposed for the Skagen 3 sediments, using the abbreviation ‘‘SK’’. The biozones were subdivided by recognition of significant changes in pollen assemblage. In general the pollen preservation in the Skagen 3 sediments was worse than that in the N+rre Lyngby 2 sediments. Biozone SK-1 186.0–180.40 m This biozone is characterised by a rise in the proportion of pollen of arboreal taxa, with Picea pollen

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dominant. Of non-arboreal taxa, only Pteridium and Filicales spores are present at any notable frequency. The biozone has been divided into three sub-biozones, described below. Sub-biozone SK-1a 186.0–185.7 m As with sub-biozone NL-1a, this sub-biozone is based on only one count. Again, it is thought that the assemblage present here is sufficiently different from subsequent levels to merit status as a sub-biozone. The early pollen record suggests an essentially lateglacial landscape with a background occurrence of Picea. The comparatively high proportions of pollen of non-arboreal taxa such as Cyperaceae, Filicales and Polypodium implies a damp environment with low-lying vegetation and very little, if any, tree cover. The other major constituents of this landscape are Gramineae and Pteridium, with Pteridium on the drier, acid soils, probably also the more exposed areas. Sub-biozone SK-1b 185.7–182.65 m This sub-biozone is similar in character to N+rre Lyngby biozone NL-3. Picea pollen values average ca. 67% and reach 84% in the latter part of the zone, suggesting near total spruce forest. The constant presence of pollen of Betula, Alnus and Coryloid types throughout the sub-biozone indicates their presence as forest components, possibly in damper habitats along watercourses. Pteridium and Filicales also occur; both probably occupy more exposed areas, but Filicales may well be present as a forest understory plant, or as an epiphyte on Picea. It should be noted that this unit contains three levels (between 184.90 and 185.32, Fig. 5) from which no pollen was obtained.

Sub-biozone SK-2a 180.40–178.45 m This unit sees a dramatic rise in the frequency occurrence of pollen of Ericales to 30%, accompanied by the pollen of Gramineae and Cyperaceae, and Filicales spores, and a drop in both frequency and concentration of Picea pollen. This is interpreted as a climatic deterioration of the type described for N+rre Lyngby sub-biozone NL-6a, with vegetation consisting primarily of ericaceous heathland. The high proportion of Filicales spores in the sub-biozone suggests their presence in damper environments, and possibly associated with the remaining Picea forest (Picea pollen is present at a maximum of 32%, probably a nominal presence in the environment considering depositional factors). An increase in Betula pollen suggests that it was present in isolated stands, possibly in damper or less acid soils. Sub-biozone SK-2b 178.45–177.25 m Picea pollen declines further in occurrence to the point where its presence can be interpreted as entirely resulting from long distance transport. Its place is taken by pollen of Ericales, which further increases in occurrence to 39%, whilst Filicales spores stay at a similar level of occurrence, suggesting it is at its maximum extent in the environment. Alnus increases slightly, and is probably present only along watercourses or associated with the stands of Betula, again probably only in damper localities. Numerous other herbs, such as Compositae Liguliflorae, Compositae Tubuliflorae, and Caryophyllaceae are present in small quantities, and are most likely associated with grassier areas.

CORRELATION Sub-biozone SK-1c 182.65–180.40 m The frequency occurrence of Picea pollen decreases throughout this unit from an acme of 85%, almost mirroring the increase in Ericales pollen. This can be thought of as almost direct habitat replacement, probably reflecting climatic deterioration or increasing soil podsolisation. Betula and Alnus pollen values slightly increase and are present throughout the zone. The appearance in the latter part of the zone of pollen of thermophilous tree species such as ºlmus, Quercus and Carpinus is interpreted as being an effect of reworking or long distance transport, together with ¹axus, Taxodiaceae and Ilex. Biozone SK-2 180.40–177.25 m This biozone is characterised by an increase in the pollen of non-arboreal taxa, in particular Ericales, Gramineae and Cyperaceae. Pollen of arboreal taxa are present only at very low frequencies, with the exception of Betula and Alnus. The biozone has been divided into two sub-biozones, described below.

The pollen sequence obtained from the N+rre Lyngby 2 core shows an early development of ºlmus, Quercus, Alnus and Coryloid types, representing mixed oak forest. This is followed by replacement of the thermophilous tree taxa by pollen of Picea, before the sequence becomes dominated by more cold stage taxa such as Ericales. The development of the cold stage taxa is interrupted at two points by a resurgence in the frequency occurrence of Betula and Picea, suggesting climatic amelioration or possibly reworking. The sequence at Skagen 3 shows only the Picea development, followed by the appearance of cold stage vegetation. No climatic amelioration is seen in the part of the core representing the cold stage. The pollen assemblages in the N+rre Lyngby 2 and Skagen 3 cores show significant differences from interglacials, other than the Eemian, that have been recorded in Northern Europe. The absence of Abies and Fagus, the lack of a well marked ¹axus phase (possibly a result of the fragility of the pollen grains) and the rarity of Pterocarya suggests the sediments cannot be

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Fig. 6. Proposed correlation of pollen zonation between the N+rre Lyngby 2 and Skagen 3 sequences. Foraminiferal and Ostracod zonation is also shown (Kristensen, pers. comm.), as is the regional zonation proposed by Andersen (1961, 1975).

correlated with the Holsteinian. This is further confirmed by the lack of exotic Tertiary taxa such as Eucommia, ¹suga and Cedrus. Significant differences between the sediments and sequences correlated with OI Stages 7 and 9 are also seen. In comparison to the Reinsdorf Interglacial of northern Germany (Urban, 1995), thought to represent OI Stage 9, the N+rre Lyngby 2 and Skagen 3 sequences are comparatively lacking in Abies, ¹axus and Fagus. The N+rre Lyngby 2 and Skagen 3 sequences show a different vegetational succession when compared with sequences assigned to OI Stage 7, such as Scho¨ningen (Urban, 1995), Wacken (Menke, 1968) and Do¨mnitz in Germany (Erd, 1973). They also do not show the characteristic Alnus dominance in all the biozones. For these reasons, the N+rre Lyngby 2 and Skagen 3 sequences between 69.4561.0 m and 185.7—180.40 m should be correlated with the Eemian interglacial Stage. The zonation of the Eemian interglacial from terrestrial sequences in Denmark is based on Jessen and Milthers (1928), Zagwijn (1961) and Andersen (1961, 1975, shown in Fig. 6). The broad elements of this scheme can be recognised in the pollen assemblage at N+rre Lyngby 2, sub-biozones NL-1b to NL-4. Subbiozones SK-1a to SK-1c of the Skagen 3 sequence appear to correlate with the latter part of the Eemian. There are, however, several differences from the terrestrial pollen sequence, which may result from the palaeogeographical context of the site. An important point is that the characteristic successional nature of the vegetation, with well marked phases of forest development, is not as obvious from the cores studied as it is in terrestrial sequences such as

the Danish parastratotype sequence at Hollerup (Andersen, 1965) (Fig. 1). At Hollerup, approximately 160 km distant, the pollen sequence is dominated by Quercus (E3 and 4: Andersen, 1961, 1975), before being succeeded by Corylus (zone E4), Carpinus (zone E5) and finally Pinus and Picea (zones E6 and 7), with taxa present in the early part of the interglacial still occurring at relatively low frequencies later in the sequence. Such a succession is most clearly seen at the transition from mixed oak woodland to Picea dominance in biozone NL-3 of the N+rre Lyngby core. The pollen assemblage in N+rre Lyngby 2 parallels that at Hollerup but with less marked biozone boundaries, and with taxa present at lower frequencies, other than bisaccate taxa. This may be a result of one or more factors. (1) The taphonomic processes operating in the marine environment may explain the relatively low frequencies of many of the thermophilous woodland taxa. It is clear that the factors involved can result in the suppression in the record of pollen taxa which are of relatively large size, easily damaged, or are otherwise not susceptible to long distance transport. (2) Bioturbation results in a ‘blurring’ of the zone boundaries, giving rise to a loss of definition. (3) A sedimentary hiatus or erosional event. This is likely at Skagen 3, because only the later, Picea-dominated part of the interglacial is present. However, there appears to be no conclusive evidence in the sedimentary record for such an event. Possible causes include subaerial exposure, the deposit being within the storm wave-base in the early part of the interglacial or alterations in the current system causing erosion. In the

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case of the N+rre Lyngby 2 record, the most notable absence is that of the characteristic Carpinus zone g of Hollerup and other terrestrial Eemian sites. This could result from a non-sequence or erosional event, but it is more likely in this environment to be a result of competition with Picea. Phillips (1974) describes how at some terrestrial sites, Picea pollen depresses and often exceeds Carpinus pollen values. It is likely that such an effect will be exacerbated in the marine environment as a result of taphonomic processes. (4) Pollen and spores may be derived primarily from higher latitudes. This would be reflected in the pollen record by an incomplete development of the mixed oak woodland, and an early arrival, and sustained presence, of Picea-dominated woodland. This is thought to be a major factor influencing the pollen assemblages of the two cores studied, and the pollen assemblages bear similarities in this respect to records from Sweden (Pas sse et al., 1988; Pas sse, 1992). It is, however, unlikely that significant proportions of pollen are derived from north of 64° latitude, as pollen records north of this line show the vegetation to be composed primarily of herbs and shrubs even during the peak of the interglacial (Robertsson, 1991a, b). The higher sea-levels of the Eemian, and a possible White Sea—Baltic connection (Zans, 1936; Gross, 1967; Forsstro¨m et al., 1988; Raukas, 1991; Donner, 1995; Zagwijn, 1996; van Andel and Tzedakis, 1996), the latter potentially driving an increased Baltic circulation, may provide a mechanism by which this transport would be encouraged. This appears to be confirmed by the presence in the foraminiferal record of high percentages of the arctic N. pachyderma (sinistral), and suggests a reversed circulation system (Kristensen et al., 1998). Furthermore, this might explain the seemingly better correlation with the subsequent cold stage pollen assemblage; as ice advanced, cutting off the White Sea - Baltic connection, pollen recruitment might be dominated by more southerly catchments. The White Sea - Baltic connection might, however, have closed before the glaciation as a result of isostasy. The annually laminated sequence at Bispingen, north Germany (approximately 460km south of N+rre Lyngby and Skagen) has provided a chronology for the Eemian (Mu¨ller, 1974; Field et al., 1994). Field et al. (1994) give ca. 9700 years for the duration of the interglacial. Their figures suggest that the main phase of mixed oak forest development (zones E2—E4 of Andersen’s 1975 zonation) spanned 3200 years, with the subsequent Picea-Carpinus phase (zones E5—E7) covering 6500 years. Direct correlation with Skagen 3, assuming both a constant sedimentation rate, and presence of the whole Picea phase, gives a rate of 8.2 cm/100 years for the Eemian part of the core. Similarly, the part of the N+rre Lyngby 2 sequence representing the mixed oak forest (sub-biozones NL-1a to NL-2b), gives a sedimentation rate of 13.6 cm/100 years, while the latter Picea-dominated zone was deposited at 6.3 cm/100 years, i.e. comparable to Skagen 3. In the context of these interpretations, it should be noted that the Car-

pinus and Picea zones at Bispingen are assumed to represent the Picea zone as a whole in both the N+rre Lyngby 2 and Skagen 3 sequences. This is because the Carpinus zone is absent in the N+rre Lyngby 2 and Skagen 3 cores for taphonomic reasons described above. In addition, the sediments in the N+rre Lyngby 2 borehole between 69.8 and 61.0 m, and in the Skagen 3 borehole between 180.25 and 184.9 m, have been independently correlated with the Eemian Stage on the basis of foraminiferal stratigraphy and optically stimulated luminescence measurements (Poolton et al., 1994; Seidenkrantz et al., 1995; Seidenkrantz and Knudsen, 1997; Kristensen et al., 1998). Furthermore, correlation of the Eemian is now virtually universally accepted with OI Stage 5e (Mangerud et al., 1979; Mangerud 1989). Evidence for two possible climate fluctuations in the Eemian (Fig. 6) has been identified by Seidenkrantz et al. (1995) in the foraminiferal record from the sediments in the N+rre Lyngby 2 and Skagen 3 cores. Both sequences show an increase in arctic and subarctic foraminiferal species such as Cassidulina reniforme and Elphidium excavatum at two levels in the sediment correlated with the Last Interglacial, 67.89—66.01 m and 63.44—63.01 m in N+rre Lyngby 2, 183.38— 182.93 m and 181.58—181.10 m in Skagen 3. The N+rre Lyngby 2 and Skagen 3 cooling events have been interpreted by Seidenkrantz et al. (1995) as representing fluctuations in the strength of the North Atlantic surface-water circulation. Sub-biozone NL-2a (between 67.5 and 66.45 m) is interpreted as a climatic deterioration, represented by a decline in the frequency occurrence of the pollen of thermophilous tree taxa and a rise in the occurrence of Picea pollen. It can be tentatively correlated with the cooling event NLI seen in the foraminiferal record between 67.89 and 66.01 m. No evidence is seen in either core for the cooling events NLII (N+rre Lyngby 2) or SKI and II (Skagen 3). The pollen and spore record shows that a cooling event is present in sediment representing the early part of the interglacial, which is correlated with an event (NLI) seen at a similar level (67.89—66.01 m) in the foraminiferal record of the N+rre Lyngby 2 core. The NLI cooling event has been correlated by Seidenkrantz et al. (1995) with the SKI event in Skagen 3 between 183.38 and 182.93 m. Because the pollen and spore evidence demonstrates that the Skagen 3 sequence represents only the latter part of the interglacial, this implies that the correlations given by Seidenkrantz et al. (1995) may be incorrect, and that the foraminiferal record may actually reflect three or more such cooling events. Cooling events have been identified in Eemian terrestrial sites, e.g. Les Echets (1200 km distant in eastern France) (Beaulieu and Reille, 1984, 1989), but those in the North Jutland marine sediments appear more severe. This difference in magnitude may reflect the more northerly position of the Danish sites, or that the Danish sites represent different events altogether. The

C.G. Glaister and P.L. Gibbard: Pollen Stratigraphy of Late Pleistocene Marine Sediments

apparent cooling in the Last Interglacial part of the N+rre Lyngby 2 core is more pronounced than the episodes with which they are correlated (using foraminifera) in Skagen 3. This suggests a climate similar to that of late OI Stage 6 or early OI Substage 5d (Seidenkrantz et al., 1995). The apparent rapidity of the changes from comparatively warm to cool conditions led Seidenkrantz et al. (1995) to suggest that these events were equivalent to those identified in the Greenland GRIP ice core (Dansgaard et al., 1993; GRIP project members, 1993). Although data from the Greenland GISP2 ice core (Boulton, 1993; Grootes et al., 1993; Taylor et al., 1993) and the North Atlantic sea floor (Cortijo et al., 1994; Keigwin et al., 1994; McManus et al., 1994) cast doubt on the validity of the Eemian climate fluctuations and the stratigraphic continuity of the GRIP ice core (Peel, 1995), pollen sequences from central and southern Europe seem to support their existence. Tzedakis et al. (1994) point out that sequences in France (Beaulieu and Reille, 1984, 1989, 1992a, b; Reille and de Beaulieu, 1990), Switzerland (Welten, 1981; Wegmuller, 1986), Italy (Follieri et al., 1988), northern Germany (Field et al., 1994) and Greece (Wijmstra, 1969; Wijmstra and Smit, 1976; Tzedakis, 1993) suggest the presence of minor climatic oscillations within the Eemian. These involve double peaks of Abies or Picea interrupted by expansions of Pinus, and are clearly different in character from the oscillation seen at N+rre Lyngby. More recently, terrestrial—marine correlation has implied the presence of an intra-Eemian cold event at around 122 ka (Maslin and Tzedakis, 1996). The pollen and foraminiferal records from N+rre Lyngby 2 and Skagen 3 are further evidence pointing to the existence of these climate fluctuations. The upper boundary of the interglacial is placed where, as a result of climate cooling, the forest is replaced by predominantly open vegetation, accompanied by soil acidification and erosion, cf. Zagwijn (1960). This is further confirmed by Urban (1993), who notes that, in an interglacial, Pinus pollen dominance accompanied by an expansion of Ericaceae (and often significant Sphagnum spore expansion) marks the transition. The very high proportions of Ericaceae pollen as well as significant amounts of pollen of Cyperaceae and Betula present in the cold stage sections of both cores allows a good correlation with other sites representing the Last Interglacial-Glacial transition (Urban, 1993; Veil et al., 1994; Behre and Lade, 1986; Pas sse et al., 1988; Behre, 1989), which suggests increased continentality in the pollen and spore source area and possible dystrophication (Behre and Lade, 1986). In the case of both N+rre Lyngby 2 and Skagen 3, the interglacial/glacial transition corresponds closely with the foraminiferal evidence (Seidenkrantz et al., 1995) (Fig. 6). Stratigraphical continuity of the sediment implies that the following cold stage is the Weichselian, originally zoned on the basis of pollen assemblages by Andersen (1957).

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The palaeoecological interpretation of the N+rre Lyngby 2 record suggests two periods of climatic amelioration are represented in the cold stage sediments overlying the Eemian. These consist of an early Betula/Alnus/Corylus expansion, followed by Picea forest (with ¸arix in the earlier event), before the vegetation reverted to an open, cold stage type dominated by herbaceous taxa. This compares closely with the description given by Behre (1989) for the Oerel site in southern Lower Saxony. The earlier of the two periods (Zone NL-5b) is thought to represent the Amersfoort (Zagwijn, 1961, and subsequently redefined as the early part of the Br+rup Interstadial by Behre, 1989), whilst the latter period (zone NL-6b) is correlated with the main part of the Br+rup (OI stage 5c, Behre, 1989). The comparative lack of thermophilous tree taxa in the interstadial may result from the persistence of land ice in the north of the continent (Zagwijn, 1989, 1992) (cf. Lagerba¨ck and Robertsson (1988) who reveal the presence of ice free interstadials in Sweden) or a southward displacement of the Gulf Stream (Behre, 1989).

CONCLUSIONS (1) The sediments between 69.45 and 61.0 m in the N+rre Lyngby core are correlated with the Eemian Interglacial Stage (OI substage 5e). The sediments between 185.7 and 180.4 m in the Skagen 3 core are correlated with the latter half of the same period, and represent zones E6 to E7 (Andersen, 1961, 1975). The sediments in the latter part of both cores are correlated with the subsequent Weichselian Stage. It should be noted, however, that discrepancies exist between amino-acid and other dating methods such as those detailed in this paper. (2) The pollen assemblages in the sediments of both cores display significant variation from nearby terrestrial Eemian pollen records such as Hollerup (Andersen, 1961, 1965). This is interpreted as a result of taphonomic processes acting on the pollen before, during and after deposition. Another influencing factor may have been the substantially different palaeogeography of the region, with a possible Baltic—White Sea connection resulting from sea-levels 5—7 m above present and possible isostatic movement (Fairbanks and Matthews, 1978; Chappell and Shackleton, 1986). Such a connection, and the associated alteration in current flow, may have resulted in a primary pollen source area at a higher latitude than might otherwise have been expected. (3) The subsequent cold stage sediments in both cores are correlated with the Early Weichselian on the basis of pollen content and sedimentary continuity. The sediments in the N+rre Lyngby 2 core between 54.4 and 52.75 m contain a pollen assemblage which is correlated with the Br+rup Interstadial (OI Substage 5c), the first time such a relationship has been demonstrated in the area. The earlier climatic amelioration in

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the Weichselian sediments between 58.0 and 56.2 m is correlated with the Amersfoort Interstadial (Zagwijn, 1961) which is now thought to represent the early part of the Br+rup (Behre, 1989), with a climatic deterioration separating the two periods. Such a deterioration cannot always be seen in pollen records, and varies in magnitude throughout NW Europe. (4) Sub-biozone NL-2a is interpreted as representing a period of climatic deterioration, and is correlated with the cooling event NLI recognised in the foraminiferal record, correlated by Seidenkrantz et al. (1995) with event 5e4 in the GRIP ice core (GRIP members, 1993). However, the evidence presented in this paper suggests that the correlation of the Skagen cooling events (not seen in the pollen assemblage) by Seidenkrantz et al. (1995) with the N+rre Lyngby 2 and GRIP records may need to be revised. (5) The N+rre Lyngby 2 and Skagen 3 cores demonstrate the importance of the region in terrestrial - marine correlation, palaeoclimatological and palaeoceanographic studies.

ACKNOWLEDGEMENTS This paper is part of the SHELF project (The northwest European continental shelf over the past 250,000 years: Palaeoclimate, Palaeoceanography, Tectonics and Sea-level change) and is sponsored by the European Community. Many people have contributed helpful discussion to this work, and the following deserve special thanks: S. Boreham for technical support and continued assistance throughout; K.L. Knudsen and P. Kristensen at As rhus University for providing the samples and for valuable discussion, P. Knutz for the sedimentary descriptions; J. Scourse and the rest of the SHELF group for help and advice, and A. ChepstowLusty, A. Davis, J. Dye, J. Fuller, S. Lumley, H. Roe, C. Turner and C. Tzedakis for practical advice and support. The drilling of the Skagen 3 boring was funded by the Danish Natural Science Research Council and the Municipality of Skagen, Denmark and the N+rre Lyngby 2 boring was funded by the European Commission Programme ‘Thermie’ and the Danish Natural Research Council. This project was supported by the European Community. We are grateful to A-M. Robertsson and J. Scourse for their helpful comments on the manuscript.

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