Late-glacial pollen diagrams from Hjelm and Draved Mose (Denmark) with a suggestion of the possibility of drought during the earlier Dryas

Late-glacial pollen diagrams from Hjelm and Draved Mose (Denmark) with a suggestion of the possibility of drought during the earlier Dryas

Review o f Palaeobotany and Palynology, 36 (1982): 3 5 - 6 3 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands 35 LATE...

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Review o f Palaeobotany and Palynology, 36 (1982): 3 5 - 6 3 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands




Danmarks Geologiske Undersc~gelse, Thoravej 31, DK 2400 Copenhagen N V (Denmark) (Received November 18, 1980; revised version accepted July 14, 1981)

ABSTRACT Kolstrup, E., 1982. Late-Glacial pollen diagrams from Hjelm and Drayed Mose (Denmark) with a suggestion of the possibility of drought during the Earlier Dryas. Rev. Palaeobot. Palynol., 36 : 35--63. In the first part of this paper Late,Glacial pollen diagrams from Hjelm in eastern Denmark and Drayed Mose in southwestern Denmark are presented. By means of size measurements of the Betula pollen an outline is given of the variation between Betula nana and B. pubescens s.1. This evidence is used for an outline of the local vegetational and environmental developments. It has been demonstrated that parts of eastern Denmark were ice-free from before the B~blling. In the last part of the paper general considerations are given on the climatic conditions during the Earlier Dryas and, rather briefly, the Late Dryas. It is suggested that the Earlier Dryas was probably a relatively dry rather than a relatively cold period; and during the Late Dryas the mean July temperature may have fluctuated around 12°C. INTRODUCTION

In 1942 Iversen (see also Iversen, 1954, 1973) established the presence of the B¢lling and the Earlier Dryas times in the Late-Glacial stratigraphy, and interpreted them as a relatively warm period followed by a cool phase before the AllerCd warming. More recent investigations (Birks, 1973; Coope and Pennington, 1977; Cleveringa et al., 1977; Kolstrup, 1979; Verbruggen, 1979) have queried Iversen's interpretation of the climatic developments of the Late-Glacial, and therefore it seemed desirable to carry out additional investigations of some Danish Late-Glacial deposits. In this paper, two pollen diagrams which are both thought to include the Belling, the Earlier Dryas, and the AllerCd are presented, and an attempt has been made to obtain information on the vegetational development, especially the changes in the birch populations. Investigations in southwestern Sweden reveal that in parts of this area the ice had disappeared before the B~blling (Berglund, 1976) and it seemed reasonable to assume that parts of eastern Denmark might also have been icefree during the B~blling but deposits from the period still needed to be palynologically demonstrated. 0034--6667/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company


Field and laboratory methods In 1955, samples were collected from a bog near Draved (Fig.l) by a team from the Danish Geological Survey under the guidance of J. Iversen, and a preliminary pollen diagram was worked out by S.T. Andersen (Danish Geological Survey). Additional samples from the same cores have been investigated recently in order to obtain a more detailed diagram. All samples have been treated with HC1, followed by KOH, HF, and subsequent acetolysis, and silicone oil has been used as an embedding medium. In 1979 A. Berthelsen (Geological Institute, University of Copenhagen) drew m y attention to a geological profile near Hjelm, in the southeastern part of the island of Men, in eastern Denmark (Fig.l). Samples were collected from an open profile, and in the laboratory they were treated with HC1, followed b y KOH, acetolysis, and subsequent b r o m o f o r m separation. Silicone oil was used as an embedding medium. In some spectra from both localities, the distance between the top of a pore and the face of the opposite wall of all Betula pollen grains seen in polar view was measured.


Fig.1. Map of Denmark showing the localities of Hjelm (X~) and Drayed Mose (X2).

37 In both pollen diagrams the percentages were calculated on m e basis of the sum of arboreal pollen (A.P.) and pollen of non-arboreal plants from dry grounds (dry N.A.P.); the pollen and spores of water plants were thus excluded. The scale of percentage in both d_iagrams is the same for all curves except for Pediastrum, Botryococcus, and secondary pollen and spores in the Hjelm diagram. Black dots in the diagrams indicate percentages of 0.2 and lower, of a pollen t y p e n o t found directly below or above a higher percentage. Size frequency distributions o f the Betula pollen

In Figs.2 and 4 the size frequency distributions of the Betula pollen from Hjelm and Drayed Mose are shown. The vertical scale in these figures is in percent, the horizontal scale in microns, the numbers in the upper left corners indicate the depth of the sample and those in the right-hand corners the number of Betula pollen grains measured. In order to facilitate the survey and the comparison of the curves, the values in Figs.2 and 4 represent the smoothed, weighted percentages calculated according to the formula (n, + 2n2 + n3): 4, where n,, n2, and n3 are the number of Betula pollen grains measured in three subsequent size classes. According to various authors (e.g., Usinger, 1975; Andersen, 1980), the size-distribution of the pollen of a Betula species appears unimodal and symmetric, while populations with two components appear bimodal or skewed. By means of a comparison with other investigations on the size of Betula pollen (e.g., Usinger, 1975; Andersen, 1980), symmetrical size frequency distributions with a m a x i m u m about 19 pm are thought to represent Betula nana, while symmetrical distributions with a maximum in the order of 23 pm are thought to represent Betula pubescens s.1. (including B. tortuosa) (compare Andersen, 1980). A right-skewed curve of distribution with a maximum around 19 p m m a y hence represent a population of B. nana with some treebirch, while a left-skewed distribution with a maximum of 23 pm may represent a dominance of B. pubescens s.1. with an admixture of B. nana, provided of course that only these two c o m p o n e n t s are represented. A representation of Betula verrucosa pollen in the Betula pollen assemblages seems less likely because only B. nana and B. pubescens s.1. are recorded in studies dealing with macro-remains from Late-Glacial deposits in Denmark (Iversen, 1954; Stockmarr, 1975) and northern Germany (Usinger, 1978a). Besides, in Poland, Betula verrucosa certainly first appeared in the early Holocene (RalskaJasiewiczowa, 1966). In the uppermost sample in the Draved sequence the mean size of distribution does n o t fit the assumption of a maximum of 23 p m for Betula pubescens; the change in sediment from gyttja to peat may possibly account for this. In order to obtain a tentative estimate of the proportion between the Betula nana and the B. pubescens percentages, the m e t h o d given in Usinger



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(1975) and Andersen (1980) was used. Standardized calculated curves representing various proportions of the two species, i.e., 0/100, 10/90, 2 0 / 8 0 . . . 100/0, were compared with standardized curves representing the fossil Betula populations. As the pollen sizes in the curves representing the fossil material sometimes deviate from the calculated values, it has in some cases been necessary to displace the "fossil" curves laterally in order to achieve the best fit. It has been a great help for this comparison t h a t the arithmetic mean size and the mode of the fossil material are known and compared to the standard values of 100 and 84 which are the standardized mean sizes for Betula pubescens and B. nana respectively (Andersen, 1980). Where mean and mode are close to 100 (or more) in the :standardized fossil material, Betula pubescens is dominant, while B. nana is d o m i n a n t when the mean size is closer to 84. In Figs.3 and 5 the standardized fossil curves (full lines) are shown together with the best-fitting calculated curves (dashed and dotted lines). On the base lines, mean size and mode of the fossil samples are indicated and the standardized values for the calculated curves are shown on the base line of the lowermost sample. The numbers in the upper left corner of each figure refer to the spectrum in the pollen diagram, and in the upper right-hand corner the deduced proportions between Betula nana and B. pubescens are given. The samples from the Hjelm section fit nicely into the calculated curves. There is a t e n d e n c y t h a t the calculated curves are the steeper ones, but the trends of the compared curves are principally the same. From the lower five samples it can be seen t h a t only a small type, probably representing Betula nana is present. A low percentage of a larger pollen is encountered occasionally, but the presence of this t y p e is probably due to long-distance transport or redeposition..From 1.92 m and upwards the a m o u n t of the small type, viz. Betula nana, gradually decreases as compared to the larger one, probably representing B. pubescens s.l., which is d o m i n a n t in the sample at 1.62 m with 90%. In the sample at 1.54 m the percentage of the small type has again increased a little although the larger type is still dominant. The samples from the Drayed Mose are generally much flatter than the calculated curves, revealing less certain estimates of the relationship between the small and the large type. The reason for this difference is purely speculative, but it might be suggested t h a t storing the samples in the laboratory for several years during which t h e y had completely dried out might be the reason. In the two lowermost samples from this section there is a dominance of a large type whereas in the sample at 8.19 m approximately equal amounts of small and large Betula pollen are present. In the samples from between Fig.2. Betula pollen size--frequency distributions of samples from Hjelm. The percentages are smoothed weighed means. Fig.3. Standardized curves representing Betula populations from Hjelm (full lines) compared with the best fitting standardized calculated curves representing the Betula nana/B. pubescens proportions (dashed and dotted lines).]- indicates mean and i mode of population.



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Fig.4. Betula pollen size--frequency distributions of samples from Drayed Mose. The percentages are smoothed weighed means. Fig.5. Standardized curves representing Betula populations from Draved Mose compared with the best fitting standardized calculated curves representing the Betula nana/B. pubescens proportions (see also text to Fig.3).


8.13 m and 7.62 m, the large type, viz. Betula pubescens, generally dominates, and from 6.95 m to 6.60 m, the small type viz. B. nana, is the dominant type. In the u p p e r m o s t sample a pure B. pubescens population is thought to be represented.

Description and zonation of the pollen diagrams H]elm In a coastal cliff near Hjelm in the southeastern part of M~bn (54°55'19"N, 12°15'56"E, Fig.l) a shallow depression, approximately 45 m wide and ca. 2 m deep, could be seen u p o n till. The sediment in the depression was undisturbed and the lithological sequence was from top to b o t t o m (see also Fig.6): 0.00--1.20 m compact, homogenous, strongly oxidized silty clay 1 . 2 0 - 1 . 3 5 m alternating thin layers of oxidized greenish silty clay with lime and whitegrey lime 1.35--1.37 m white-grey, slightly brownish lime 1.37--1.38 m grey-greenish clay with lime 1.38--1.40 m white-grey, slightly brownish lime 1.40--1.47 m greenish-grey lime with silty clay 1.47--1.93 m alternating layers of white-grey lime and slightly silty greenish-grey clay 1.93--1.96 m layered white-grey lime 1.96--2.04 m greyish-green, layered, clayey silt with lime and thin shells 2.04--2.10 m silty and calcareous fine sand with single gravels and shells

Fig.6. Photograph of the Hjelm section. The visible part of the spade is approximately one meter long.

42 The pollen diagram shown in Fig.7 (pp. 45--48) represents the layered part of the filling in the depression. Secondary pollen, which is by far dominated by pre-Quaternary types, is found t h r o u g h o u t in the pollen diagram, and from the spectrum at 1.89 m and upwards very high percentages are encountered. Although only a small amount of the secondary pollen is constituted b y Quaternary tree-pollen, it is difficult to establish whether all pollen types included in the diagram are primary to this locality, and especially the part between 1.89 m and 1.25 m should consequently be regarded with care. A comparison of the Hjelm diagram with other Late-Glacial pollen diagrams from Denmark and northern Germany (e.g., Brorson-Christensen, 1949; Iversen, 1954; Krog, 1954; DegerbN and Krog, 1959; Fredskild, 1975; Usinger, 1975, 1978a) reveals that the trend of the curves and the association of pollen t y p e s in the present diagram are not very typical, especially not for the part between 1.80 m and 1.25 m. The spectra at 2.06 m and 2.04 m have very low percentages of tree pollen, i.e., of Betula, Pinus, and Salix, while Cyperaceae and Gramineae are abundant. Artemisia is present with a b o u t 2%. This part of the section is due to the low percentages of A.P. pollen, especially Betula, thought to represent pollen zone Ia (see e.g., Iversen, 1954). F r o m 2.02 m to 1.96 m the Betula percentages attain a maximum at the cost of the Cyperaceae, and, to a lesser extent, the Gramineae. Juniperus is represented by low percentages, Artemisia is present, Filipendula and PotentiUa t y p e attain percentages of 1.5 and 8 respectively in the spectrum at 1.96 m, Myriophyllum and Botryococcus are fairly abundant and Pediastrum attains very high values. Armeria maritima is represented by a single pollen grain. Figs.2 and 3 suggest that only Betula nana is represented in the Betula pollen in this part of the diagram. A comparison with the pollen curves and the Betula pollen measurements presented by Usinger (1975, 1978a) suggests that this part of the diagram represents the B~blling time, i.e., pollen zone Ib. In the spectra at ! . 9 4 m and 1.92 m the percentage of Betula decreases and that of Gramineae and Cyperaceae increases. The Betula t y p e represented here is probably Betula nana alone in the spectrum at 1.94 m, whereas in the spectrum at 1.92 m a small amount of a larger pollen type, viz. B. pubescens, is also encountered. A small maximum in the values of Hippopha~ and Batrachium is found, whereas Myriophyllum spicatum/verticillatum, Pediastrum and Botryococcus exhibit lower percentages than in the underlying spectra. The percentages of Filipendula and Artemisia are unchanged as compared to the underlying spectra. The spectra at 1.94 m and 1.92 m are thought to represent the Earlier Dryas time (pollen zone Ic). High percentages of Betula are f o u n d between 1.89 m and 1.76 m. Populus and Empetrum are also present and Juniperus reaches a maximum. The gradual increase in the amount of the larger Betula pollen t y p e as compared to the smaller one from the spectrum at 1.92 m and upwards to 1.62 m, suggests that the species producing this pollen (viz. Betula pubescens s.1.) has gradually become of more importance in the area (compare also Usinger's


investigations from northern Germany: Usinger, 1978a), and furthermore, this gradual increase suggests that reworked Betula pollen has little or no importance for the composition of the Betula pollen curve in this part of the diagram. The presence of the above-mentioned pollen types and the trend and the composition of the Betula pollen curve suggest that this part of the diagram represents a part of the Aller~bd (zone II). The upper part of the pollen diagram is not matched in any other LateGlacial diagram from northwestern Europe and it is only included here for the completeness of the diagram. It has proved impossible to subdivide it into the traditional pollen zones and the only conclusion that can be drawn for the time being is that the uppermost part, with very high percentages of Filipendula and Typha latifolia, probably represents a climate with a fairly high summer temperature.

Draved Mose The locality of the present investigation is situated in the Draved Mose in southwestern Jutland (55°01'17"N, 8°11 , 13"E, Fig.l). The locality is situated in a depression in aeolian sands blown in from the surrounding outwash plains. Underneath the aeolian deposits is a very clayey Saale moraine which is locally found at the surface east of the bog area (B. Aaby, Geological Survey of Denmark, pers. comm., 1980). The lithological sequence is from top to bottom: 6.30--6.40 m 6.40--6.50 m 6.50--7.03 m 7.03--7.30 m 7.30--7.31 m 7.31--7.39 m 7.39--7.46 m 7.46--7.48 m 7.48--7.495 m 7.495--7.51 m 7.51--7.56 m 7.56 m 7.56--7.67 m 7.67--7.79 m 7.79--7.99 m 7.99--8.12 m 8.12--8.18 m 8.18--8.20 m 8.20--8.22 m 8.22--8.23 m 8.23--8.28 m 8.28--8.31 m 8.31 m

brownish-grey gyttja greenish gyttja grey sandy gyttja light grey-green, sandy gyttja with some plant remains brown gyttja light grey-green, sandy gyttja with some plant remains fine sandy brownish gyttja yellow-brown gyttja fine sandy brownish gyttja sandy, green-grey gyttja gyttja with coarse sand erosional contact olive-yellow-brown, weakly sandy gyttja brownish-grey gyttja with thin sand layers brownish*grey gyttja with single sand layers brownish-grey sandy gyttja green-grey sandy gyttja sand with some gyttja fairly coarse sand sand with some gyttja sand grey-green, sandy gyttja sand with some gyttja

The pollen curves in Fig.8 (pp. 49--52) show, generally speaking, relatively small changes throughout the diagram. Secondary pollen is fairly scarce and amounts to approximately 2--3% at most.

44 The three lowermost spectra have Betula percentages around 30% and the size measurements (Figs.4 and 5) show that a large pollen type, probably representing Betula pubescens, is dominant. The majority of the pollen, however, is constituted by Cyperaceae, Gramineae, and herbs, of which Helianthemum, Artemisia, and Hippopha~ are relatively abundant. Juniperus and Potamogeton are present with 2 to 4%, and Myriophyllum spicatum/verticillatum and Hippuris are present with low percentages. A comparison of these spectra (8.35 m to 8.225 m) with those from lake B~blling (Iversen, 1954) and northern Germany (Usinger, 1975) suggests that this part may be interpreted as representing a part of the B~lling (pollen zone Ib). At 8.19 m and 8.17 m the Betula percentages have decreased to between 15 and 20%, and almost equal amounts of Betula nana and B. pubescens are present in the spectrum at 8.19 m. This decrease in the percentages of Betula falls together with a substantial increase in the percentages of Gramineae. Hippopha~ has disappeared, and waterplants are totally absent in the spectrum at 8.19 m, b u t most pollen curves appear virtually unchanged. This part of the section probably represents the Earlier Dryas (pollen zone Ic). From 8.13 m and upwards, a gradual increase in the percentages of Betula is found parallel to a gradual decrease in the percentage of Gramineae. From a b o u t 7.66 m to 7.29 m the values for the curves are fairly constant, only to change backwards again between 7.29 m and 7.09 m. Measurements of the size of Betula pollen show that between 8.13 m and 7.62 m Betula pubescens is probably the generally dominant Betula type as compared to B. nana. Between 8.13 m and 7.09 m a large number of different pollen types are encountered, the m o s t important being Populus, Juniperus, Artemisia, Filipendula, and from a b o u t 7.78 m and upwards Empetrum, and from 7.66 m Sphagnum. In the lower half of the zone Helianthemum and Potamogeton are represented. In the lithological column an erosional contact is seen at 7.56 m, and the possibility of redeposited older Late-Glacial pollen above this level can therefore n o t be excluded. The fairly high percentages of Betula between 8.13 m and 7.09 m, and the presence of Populus and Empetrum together with the slightly higher percentages of Filipendula, suggest an Aller~bd age for this part of the sequence (pollen zone II). Many pollen types found in zone II are also found between 6.95 m and 6.45 m, the most important being Artemisia, Empetrum, Sphagnum, and to a lesser extent Juniperus. The Betula percentages in this part of the diagram are between 35 and 40% and, as stated above, B. nana pollen is found almost exclusively. This zone is interpreted as representing the Late Dryas time (pollen zone III). In the spectra between 6.45 m and 6.30 m, very high percentages of Betula are found and Figs.4 and 5 suggest that only B. pubescens is represented. Corylus is present in the upper spectra and attains 35%, and Populus, Pinus, Ulmus, Quercus, and Alnus are also found. Herbs have become scarcer, and Calluna and Filipendula are n o w the only types of importance. Among the waterplants Myriophyllum, especially M. alterniflorum, is fairly well represented. This evidence points to an early Holocene age.

pp. 45--48

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Parnassia Astragalus .Lotus .Vicia/Lathyrus _~Papilionaeeae Plantago maritlma Plantago major/medla Armeria Polemonium coer. Rumex acetosa t. Rumex acetosella t. It Anemone Ranunculus Thalictrum cf. Trollius , cf. Dryas Filipendula cf. Fragaria



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ntnocarpium •c o p o d i u m annot. •c o p o d i u m clay.

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53 Vegetational and environmental developments Earliest Dryas The Earliest Dryas time is included in the Hjelm diagram and the Weichselian ice must therefore have disappeared from this locality already before the Bdlling. In other words, the ice front as well as large dead ice bodies had, already before the B~,blling, been t o o far from the locality to influence the vegetational and sedimentological developments there, and already from an early stage in the deposition in the depression, a vegetation of Cyperaceae, Gramineae, Salix, and Artemisia was present in the surroundings. This of course does n o t imply that dead ice bodies could not be found in other parts of eastern Denmark during the B~blling, because they may have persisted for a considerable time, especially in large depressions; the ice front, on the other hand, m a y have been at some distance as is suggested b y Swedish investigations (see, e.g., Berglund, 1976 ; BjSrck, 1979). Bc~lling In both pollen diagrams Hippopha~, Helianthemum, and Artemisia are present in the deposits of B~blling age suggesting the presence of a raw soil. In Hjelm Betula nana is the only Betula species encountered during this period, whereas in Draved, Betula pubescens may have been the dominant species (compare Usinger, 1975). In northern Germany a dominance o f B . pubescens is found in areas n o t covered b y the Weichselian ice, while B. nana dominated within the glaciated region (Usinger, 1975, 1978a), and Usinger (1978a) suggested that this difference might be ascribed to differences in the soil. It seems reasonable to assume that raw soil rich in lime was present on M~bn, and it is possible that areas with more stable surfaces where a certain soil development bad taken place were found b e t w e e n active surfaces with raw soil in southwestern Jutland. Usinger's (1978a) hypothesis that a difference in soil is responsible for the difference in the Betula populations can therefore n o t be excluded. F o r the Hjelm locality, however, the migration rate of the tree-birch m a y also have been of importance for the lack of tree-birch during the Belling, because in the Eichholz-Niederung, Heiligenhafen diagram Usinger ( 1978 a) found a Be tula nana/B, pu bescens development similar to the one in Hjelm, only B. pubescens immigrated earlier. Earlier Dryas In b o t h the Drayed and the Hjelm diagrams the Earlier Dryas is characterized b y minimum percentages of Betula. In the Draved diagram the size measurements show that Betula nana becomes relatively more important than B. pubescens, b u t b o t h species are poorly represented (compare also Usinger, 1978a), while in M~bn the decreased percentages of Betula can be attributed to a decrease in Betula nana only. The presence of pioneer species suggests that raw soil and good light conditions were found in b o t h localities, b u t spots with a more developed soil m a y have been present as well, as suggested b y the presence of Filipendula.


A llerc~d In both localities raw soil may have been present locally during part of the Aller~bd as suggested by the presence of Helianthemum, Hippopha~, and maybe also Artemisia. On the other hand, Filipendula is present in both diagrams, suggesting some soil development in places, and the presence of Betula pubescens in Draved may also point in this direction. Tentative percentages of the two Betula species are deduced from the pollen percentages of birch from Hjelm indicated in Fig.7 and the proportions between the two species suggested in Fig.3. In Table I it can be seen that between 1.94 m and 1.76 m the percentage of Betula pubescens amounts to, at the most, 15% of the total pollen included in the sum. If conditions had been suitable for this species, it might have been expected that, as soon as it had immigrated, it would have spread readily and suppressed other plant types. This is obviously not the case. The high amount of secondary pollen and the content of clay and silt in the sediment may point to a washing out of material into the depression, suggestive of unstable soil surface conditions (compare also Degerb~bl and Krog, 1959). The fairly high amount of Artemisia might also point to unstable soil surface conditions (compare Pennington, 1980). Also in the Draved locality the conditions for B. pubescens may have been somewhat undesirable (at least during the early part of the Aller~bd), and the same conclusion can be deduced from other pollen diagrams from northwestern Europe (Van der Hammen, 1951; Iversen, 1954; Usinger, 1978b; Kolstrup and Heyse, 1980). Late Dryas The Betula size measurements made on samples representing the Late Dryas from the Draved locality show that Betula nana has become the dominant Betula species during this period, and furthermore the fairly high percentages of this species in Draved and northern Germany (Usinger, 1975) might indicate that the species has had relatively good growing conditions. TABLE I Betula nana and B. p u b e s c e n s percentages from Hjelm, expressed as percentages of all pollen included in the sum (the figures are deduced from the Betula pollen percentages in

Fig.7 and the proportions between the two species suggested in Fig.3) Depth

B. nana

B. pubescens




154 162 176 180 184 186 189 192 194

9 4 10 16 26 27 34 14 16

20 33 9 15 11 9 7 4 0


Populus has disappeared and the percentages of Juniperus have decreased, but Filipendula is still present. There thus seems to be evidence for a temperature decrease during the Late Dryas as compared to the Aller~bd (see also Iversen, 1954).

Holocene In the Drayed diagram the Holocene is represented by the spectra between 6.425 m and 6.30 m. Tree-birch spread readily in this part and suppressed most herbs except for the shade-tolerant Filipendula, and soon other trees also immigrated. Accordingly, the soil and the climate (including both temperature and moisture conditions) were now favourable for the immigration and settling of these plant types. CLIMATIC CONDITIONS D U R I N G THE E A R L I E R DRYAS AND THE LATE DRYAS

Temperature and moisture conditions during the Earlier Dryas In 1954 Iversen stated for the Earlier (Older) Dryas period: "After the B~blling period the thermophilous phanerophytes (Betula pubescens s.1. and Hippopha~) disappear and again we have a treeless tundra vegetation. We may assume a July temperature below 10°C ''. This assumption has become widely accepted and has found support in the fact that a decrease in the percentages of Betula and Hippopha~ is also found in some other northwestern European diagrams. Brandt (1954), however, found no macro-remains of Betula pubescens in the B~blling zone in Lake B~blling. The present investigation of the deposit in Drayed, where the content of secondary pollen is low seems to support Iversen's assumption because there is a decrease of Betula pubescens and Hippopha~ during the Earlier Dryas (Figs.4, 5, and 8). However, it seems from the low content of both Betula nana and B. pubescens in the Draved locality that neither of these species have had good growing conditions, and in the B~blling locality the pollen percentage of birch is zero when at its minimum (Iversen, 1954). The decrease in the Betula (B. nana) percentages in the Hjelm diagram might be a consequence of a decrease in the mean July temperature. Yet, if this was the case it is hard to explain the small maximum of the rather warmth requiring Hippopha~ in the same spectra (compare also the pollen diagrams from southern Sweden presented by BjSrck, 1979). The fluctuation trends of the Hippopha~ curves in various diagrams differ such that in some cases a maximum percentage of the species is found below the Earlier Dryas (e.g., Drayed Mose diagram; Van der Hammen, 1951; Iversen, 1954; Usinger, 1975; BjSrck, 1979) and in other cases relatively high percentages can be found during the Earlier Dryas (e.g., Hjelm diagram; Van der Hammen, 1951; Usinger, 1975; BjSrck, 1979). The different ages for the Hippopha~ maximum may consequently be explained in terms of migration, soil, and possibly also by competition (compare Vanhoorne and Verbruggen, 1969; Usinger, 1978a). Apart from Hippopha~, other relatively warmth demanding species are

56 found in deposits of Earlier Dryas age such as Myriophyllum spicatum/ verticillatum, Urtica, and Filipendula in Hjelm; Sorbus and Filipendula in Draved; Urtica, Filipendula, Sanguisorba minor, and Nymphaea in the Rabensbergmoor (Usinger, 1975), and some in Dutch deposits (Cleveringa et al., 1977; Kolstrup, 1979). (For a discussion of the temperature indicators see e.g. Kolstrup, 1980.) Within the area glaciated during the Weichselian in Schleswig--Holstein macro-remains of Betula pubescens are found, indicating that tree-birch spread into this area during the Earlier Dryas (Usinger, 1978a). In Flanders in Belgium Verbruggen (1979) found an uninterrupted vegetational development of water plants and helophytes from the B~blling into the AllerCd. During the Earlier Dryas high percentages of Typha latifolia, Nymphaea, Nuphar, and other warmth requiring species were found, and Verbruggen (1979) concluded: "It is fair to say that only the Younger Dryas represented a period of climatic regression" (Verbruggen, 1979, p.139) and he suggested, among other possibilities, that the precipitation ratio should be considered as an explanation for the N.A.P.-expansion during the Earlier Dryas. In Poland Ralska-Jasiewiczowa (1966) found an expansion of communities on dry habitats during the younger part of the pre-AllerCd period but no indications for a low temperature. All in all there seems to be no reason to assume that the summers during the Earlier Dryas were relatively cold. But some change did take place in large parts of Europe during the Earlier Dryas time. It might be suggested that the winter temperature decreased considerably. It is possible that the winter (mean January) temperature dropped during the Earlier Dryas as compared to the Belling, but the presence of Armeria maritima (Iversen, 1954}, which is not found where the mean January temperature is below --8°C (Iversen, 1954), suggests that the winter temperature has probably not been the principal reason for the changes that took place during the Earlier Dryas. In the following some considerations in favour of drought as a possible explanation for the changes in and the differences between various pollen diagrams will be put forward {see also Van Geel and Kolstrup, 1978). Most pollen sequences known till now representing the Late-Glacial are from moist and wet depressions. These depressions may, due to their morphology, have acted as traps for snow transported by wind, and plants growing within the depression may consequently have been protected during the winters, and were thus favoured as compared with those on higher and more level ground where the snow might have blown away. In areas with little or no snow cover, winds may have caused evaporation from those plants or those parts of the plants that were without snow cover, and as the ground and consequently the roots of the plants were frozen, this evaporation was not compensated for by a moisture supply from the roots. The effect may have been a moisture deficit resulting in a dying off of plants without snow cover, and this in turn might explain the decrease in the pollen percentages of e.g. Betula nana.


In areas where dryer conditions prevailed, the effect on taxa preferring a fairly moist ground such as Betula pubescens m a y have been serious'. An alternative explanation for the decrease in tree-birch could be that the soil conditions might have b e c o m e less suited in many places due to instability of the surface layers (compare e.g. Kolstrup and Heyse, 1980). In some localities the predominant part of the pollen influx during the B~blling, the Earlier Dryas, and the Aller~bd might have come from the immediate surroundings, i.e., from within the depression. In pollen diagrams from such depressions only slight changes might be found during the Earlier Dryas, provided that the depression remained moist or wet. If, on the other hand, a substantial part of the pollen influx came from somewhat higher areas which had become dryer or did not receive a sufficient snow cover, the pollen percentages of plants dependent on sufficient soil moisture, such as Betula pubescens, or on snow cover, such as B. nana, would decrease. Therefore, the fact that the decrease in Betula percentages during the Earlier Dryas is hardly or n o t at all discernible in some pollen diagrams (e.g., BjSrck, 1979; see also the survey in Usinger, 1975), whereas in others it is fairly pronounced might be explained as being the result of different hydrological conditions within various areas, especially the depressions. In areas where precipitation may have been relatively abundant during the Earlier Dryas no major changes in the percentages of the various plant types might result. It seems that the Earlier Dryas is less marked in pollen diagrams from certain parts of Great Britain (Birks, 1973; Pennington, 1975, 1977; Watts, 1977) than is generally the case on the continent. If the northwesterly wind direction deduced b y Maarleveld (1964) for the Netherlands during this period was also valid for larger parts of northwestern Europe, the consequence might have been that the western and northern parts of the British Isles received a larger a m o u n t of precipitation than the continent. In some areas drought m a y have resulted in a lowering of the ground water level, causing a lower water level in, or even a drying up of, the depressions. A consequence of this might be that the extension of the growing area of water plants and helophytes would change, and therefore also the pollen percentages. In addition, reduced rainfall and snow cover in an area might result in a drying up of the soil. The drying up effect might be intensified b y evaporation l i t is d i f f i c u l t to p i n p o i n t h o w d r y soil, l o w h u m i d i t y , a n d little s n o w c o v e r e f f e c t this t a x o n . K i h l m a n ( 1 8 9 0 ) described the strong effect o f w i n d a n d lack o f s n o w c o v e r o n b i r c h a t its n o r t h e r n r a n g e in R u s s i a n L a p p l a n d . I n this area t h e b i r c h e s b u i l d u p low tablef o r m e d v e g e t a t i o n s o f w h i c h all p a r t s r e a c h i n g h i g h e r t h a n t h e s n o w c o v e r d r y o u t a n d die off, b u t w h e t h e r t h i s d e v e l o p m e n t c a n b e a p p l i e d t o t h e b i r c h e s in D e n m a r k d u r i n g p a r t s o f t h e Late-Glacial is u n c e r t a i n . T r e e - b i r c h s e e m s t o h a v e b e e n p r e s e n t in p a r t s o f J u t l a n d d u r i n g t h e B~lling a n d b e c a m e scarser d u r i n g t h e Earlier Dryas. As suggested a b o v e , this c h a n g e d o e s n o t seem t o have b e e n caused b y l o w s u m m e r t e m p e r a t u r e s . T h e w i n t e r t e m p e r a t u r e d o e s n o t seem to b e a l i m i t i n g f a c t o r as b i r c h persists in Siberia d u r i n g t h e cold w i n t e r s t h e r e . T h e d e c r e a s e in t r e e - b i r c h c o u l d possibly b e d u e t o a d r y e r e n v i r o n m e n t , i.e., d r y e r soil c o n d i t i o n s , b u t a d e c r e a s e in s n o w c o v e r a n d a t m o s p h e r i c m o i s t u r e c o n t e n t in a w i n d y c l i m a t e m i g h t also h a v e b e e n c o n t r i b u t i n g factors.

58 and sublimation of soil moisture from barren soil surfaces as well as from less water being added to the soil from meltwater. A damaged vegetation cover in combination with a dry soil w o u l d make the soil more susceptible to erosion b y wind and water, and this in turn might explain, for example, w h y wind erosion and deposition could take place during the Earlier Dryas in Belgium, northern Germany, and the Netherlands where the so-called Younger Coversand I was deposited during this time, and w h y an increased a m o u n t of inorganic material is sometimes found in deposits of Earlier Dryas age in depressions. It would be dangerous to give any estimate of the a m o u n t of precipitation for this period. Probably the Late-Glacial as a whole may, generally speaking, have been relatively dry (Iversen, 1954). It can n o t be ruled o u t that the changes in vegetation and soil might merely be the result of stronger or different wind patterns during the Earlier Dryas resulting in a different distribution of the snow, increased evaporation, and rupturing of the vegetation cover and the soil, giving rise to sandstorms and the like. Whether a decreased precipitation or a changed wind pattern, or m a y b e both, m a y have been the essential cause of the change in environment during the Earlier Dryas can, however, n o t be stated b y means of the data available today. In order to test the idea of relatively dry conditions during the Earlier Dryas, additional information m a y be obtained b y investigations of the extension of sediments representing the B~blling, the Earlier Dryas, and the AllerCd separately, within a n u m b e r of localities throughout Europe. If the idea of relatively dry conditions during the Earlier Dryas holds, deposits representing the B~blling and the Aller~bd times would have become deposited over a larger area within the former lake than deposits of Earlier Dryas age. Since organic remains above lake level will tend to disappear it may be difficult to demonstrate former higher water levels. Therefore it may be necessary to combine this s t u d y with palaeobotanical studies including as well microas macro-remains such as it is done b y e.g. Ralska-Jasiewiczowa (1966) and Van Geel et al. (1980/1981).

Mean July temperature conditions during the Late Dryas According to Iversen (1954) Betula pubescens has a July temperature requirement of a b o u t 12°C in Scandinavia today. The decreased pollen percentages of this t y p e in Drayed and in the Rabensbergmoor (Usinger, 1975), which is 20--30 km south of Draved, and the reduced percentages of Pinus, which has a July temperature demand similar to that of Betula pubescens, on Bornholm (Iversen, 1954) during the Late Dryas may consequently point to a mean July temperature decrease to a b o u t 12°C or less. In the northern part of the Netherlands, at a distance of 250 to 300 km southwest of Draved, Casparie and Van Zeist (1960) f o u n d Typha latifolia and Nymphaea in Late Dryas deposits, and Wijmstra and De Vin {1971) found Typha latifolia and Nuphar in the Dinkel valley (compare also Verbruggen, 1979, for Belgium) and this suggests a mean July temperature of 12 ° to 13°C or more, at least

59 periodically, for this period there. Accordingly, it may be suggested that the mean July temperature in southern Denmark during the Late Dryas fluctuated around 12°C.

Comparison with other fields o f study During the beginning of this century Late-Glacial deposits from a number of localities in Denmark were investigated (Hartz, 1902; Madsen, 1903; Johansen, 1904). Although the term "Aller~bd-Gytje" was known, the subdivision of the Late~Glacial into Earliest Dryas, Belling, Earlier Dryas, Aller~bd, Late Dryas was not established at that time. Yet, the descriptions of the fossil content and the lithology of the sequences studied are generally so accurate that there are no problems in recognizing the Aller~bd and the Late Dryas from them. An attempt to subdivide the deposits below the Aller~bd in order to establish the Belling and the Earlier Dryas separately has on the other hand been unsuccessfull, but from the thickness of this part of the sediment (sometimes more than 1.5 m thick) and the floral and faunal developments it seems reasonable to assume that in some localities a fairly long time span is represented, and that consequently deposits from both the Earlier Dryas and the B~blling may be present. Johansen (1904) investigated the present distribution of a number of mollusks and deduced minimum mean July temperature demands for the various species in order to reconstruct the mean July temperature trend for the Late-Glacial and the Holocene. The presence of Anodonta cygnaea L. (minimum mean July temperature demand ca. 13--14°C; Johansen, 1904) commonly found in deposits from well below the Aller~bd and upwards made Johansen (1904; see also Nordmann, 1922) conclude that the mean July temperature was relatively high already some time before the Aller~bd, and Hartz (1902) stated that the above mentioned A nodonta species must have followed "close upon the heels" of the receeding ice. Other warmth requiring mollusks, such as Valvata piscinalis var. antiqua Sow. (min. mean July temp. ca. 15°C) were also found in the clay below the Aller~bd (Madsen, 1903; Johansen, 1904). Johansen (1904, 1906) concluded that the mean July temperature during the Late-Glacial developed from cold to fairly warm (from the floral and faunal developments and the lithologies the warm period is by me thought to include the B~blling, the Earlier Dryas, and the Aller~bd) and back to cold again (Late Dryas). (Johansen's temperature conclusions from 1904 are: 8°C, 8--12°C, 12--14°C, 14--15°C, 12--14°C, 8--12°C, 8°C, 8--12°C.) The highest number of different species was found in the Aller~bd layer, and Johansen (1904) stressed that the association of species found in this layer is nowadays restricted to an area from southern Norway, via the Middle Swedish lake area to the ~land islands. The minimum mean July temperature trend of Johansen (1904, 1906) is indicated in Fig.9. The stratigraphical application of the temperature trend for most of the Aller~bd and the Late Dryas has been made by Johansen himself, whereas the lower part has tentatively been stratigraphically applied by me.






12 16 i




July temperature °C 8 12 16 8 12 16








180 (°/oo) PDB -14 -12 -iO .. ~'1 t






/ / / /

12,0OO :. /

e -

/ l

Fig.9. Tentative mean July temperature trend for southern Denmark (left hand side) together with the mean July temperature trend drawn from Johansen's (1904, 1906) temperature conclusions (second column), and the mean July temperature trend of Coope (1975) (third column). On the right hand side the 1sO curve presented by Eicher and Siegenthaler (1976) is shown.

From some Swiss lakes the palynological records as well as the ~60/~80 relationship have been investigated (Eicher and Siegenthaler, 1976). The curves cover the whole of the Late-Glacial and the ~60/180 curve which is reproduced in Fig.9 shows a gradual decline with only minor fluctuations from between the lower part of the Belling to the top of the AllerCd, suggesting that no major changes took place in the environmental conditions during the Earlier Dryas (Eicher and Siegenthaler, 1976). At the transitions from AllerCd to Late Dryas and Late Dryas to Holocene, changes occur which may point to a lower temperature during the Late Dryas (Eicher and Siegenthaler, 1976). From the fossil Coleopteran assemblages from Late-Glacial deposits in Great Britain, Coope (1970, 1975, 1977) concluded that the highest mean July temperatures of the Late-Glacial were reached before the Aller~l (see Fig.9). During the Earlier Dryas and the Aller~bd a decrease of temperature took place which reached a minimum during the Late Dryas, and rose again before, or at the beginning, of the Holocene. In Fig.9, a minimum mean July temperature curve based on palaeobotanical evidence, mainly from the Netherlands (Kolstrup, 1979), is presented


together with the mean July temperature curve drawn from Johansen's {1904, 1906) estimates based on mollusks, Coope's (1975) July temperature estimates based on Coleoptera, and Eicher and Siegenthaler's (1976) ' 6 0 / ' 8 0 curve. The main trend in the curves is similar in that they all suggest one warm phase covering the B~blling, the Earlier Dryas, and the Aller~bd, and a colder period during the Late Dryas, but they differ in that the maximum temperature conditions fall at different times. Future investigations may show whether the disagreements between the curves are due to a lack of "fossil" data (f.ex. more species on the pollen list might give higher temperature estimates and/or a different temperature trend) or insufficient knowledge of the ecology of the species involved, or some other factor. ACKNOWLEDGEMENTS

The author is very thankful to S.T. Andersen (Copenhagen) and J. Stockmarr (Copenhagen) who critically read the first draft of the manuscript. S.T. Andersen has also been helpful with the statistics. Stimulating discussions with W. Tutin (Leicester) have been of great value to this paper. The work of C. Torres (Copenhagen), who took care of the photographical work, D. Blom (Copenhagen), who improved the English text, and W. Moen (Copenhagen), who typed the manuscript, is also gratefully acknowledged. The investigation was financed by the Danish Natural Science Research Council (S.N.F.). REFERENCES Andersen, S.T., 1980. Early and Late Weichselian chronology and birch assemblages in Denmark. Boreas, 9: 53--69. Berglund, B.E., 1976. The deglaciation of southern Sweden. Presentation of a Research Project and a Tentative Radiocarbon Chronology. Univ. Lund, Dep. Quaternary Geol., Rep. 10, 67 pp. Birks, H.J.B., 1973. Past and Present Vegetation on the Isle of Skye. A Palaeoecological Study. Cambridge University Press, Cambridge, 415 pp. BjSrck, S., 1979. Late Weichselian Stratigraphy of Blekinge, SE Sweden, and Water Level Changes in the Baltic Ice Lake. Univ. Lund, Dep. Quaternary Geol., Thesis 7, 248 pp. Brandt, I., 1954. Late-glacial macroscopic plant remains from BSllings6. Dan. Geol. Unders., II, 80: 156--158. Brorson-Christensen, B., 1949. Pollenanalytisk datering af en rensdyrknogle fra Nebbe Mosse, O. Vemmerl[Sv Socken. K. Human. Vetensk. Lund/~rsber. 1948--1949, V: 121--136. Casparie, W.A. and Van Zeist, W., 1960. A Late-glaciallake deposit near Waskemeer (Prov. of Fries]and). Acta Bot. Neerl., 9: 191--196. Cleveringa, P., de Gans, W., Ko]strup, E. and Paris, F.P., 1977. Vegetational and climatic developments during the Late Glacial and the early Holocene and aeolian sedimentation as recorded in the Uteringsveen (Drente, The Netherlands). Geol. Mijnh., 56: 234--242. Coope, G.R., 1970. Climatic interpretations of Late WeichselianColeoptera from the British Isles. Rev. G6ogr. Phys. G6oL Dyn., (2), XII, 2: 149--155. Coope, G.R., 1975. Climatic fluctuations in northwest Europe since the Last Interglacial, indicated by fossil assemblagesof Coleoptera. In: A.E. Wright and F. Moseley (Editors), Ice Ages: Ancient and Modern. See] House Press, Liverpool, pp.153--168.

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