Late Weichselian Vegetation and Ice-Front ...

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in the Bergen District, Western Norway. Norsk geogr. Tidsskr. .... (Undas 1945). ' ..... less than 50-60 % of all pollen derived from trees and ...... the same fashion.
Late Weichselian Vegetation and Ice-Front Oscillations in the Bergen District, Western Norway Downloaded by [Universitetsbiblioteket i Bergen], [Jan Mangerud] at 01:09 25 September 2015

JAN MANGERUD Mangerud, J. 1970. Late Weichselian Vegetation and Ice-Front Oscillations in the Bergen District, Western Norway. Norsk geogr. Tidsskr. 24, 121-148. The published pollen diagram spans the Late Weichselian and Early Flandrian (Holocene) Age. Grass tundra formed the vegetation during the Older Dryas and Early Allerod. The vegetation of the Later Allerod can be characterised as park tundra, with tree birches and willow (Salix) scrub. The summer temperature of the Late Allerod is assumed to have been 2-2.5° C lower than at the present day. During the climatic deterioration of the Younger Dryas, birch (Betula) suffered a major setback. The ice-front oscillation during the Late Weichselian is clarified, mainly on the basis of fossiliferous till and sediments below till. The ice-front retreated during the mild interstadials, Boiling and Allerod, whilst there were marked advances during the Older and Younger Dryas. The final section deals with the general principles underlying a stratigraphic subdivision of the Late Quaternary. Jan Mangerud, Geologisk Inslilult, avd. B, Universilelcl i Bergen, Bergen

INTRODUCTION

CONTENTS Introduction

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Pollen diagram fron* a peatbog at Dale, BlomSy :....,. The sediments '.. '.*.'.'.....; Pollen and spores Climatic development and datings Ice-front oscillations Bergen area .«.•:.:.. >.'.: Correlations with southerp "^cajujinayia Ci-J-datings

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Palaeogeographical map of southern Norway during the Younger. Dryas Stadial er

Principles for the stratigraphical subdivision of the Quaternary

ik-

Acknowledgements References / — Norsk geogr. Tidsskr.

For some time now, deposits from the Boiling Interstadial, buried beneath lodgement till (Fig. 5), have been known from Blomvag (Fig. 1). 123 Consequently, a glacial advance over Blomoy 124 must have occurred at some subsequent time, 126 though the actual date of that event has re130 mained unknown. Bearing this in mind, I 132 therefore conducted a pollen-analytical investi•f'^ation, and obtained CH-datings for sediments 7 w o r n a bog at Dale (Fig. 1), which overlie f that till stratigraphically. By this method, I 136 dated the glacial advance to the Older Dryas Stadial (Fig. 7). The examination of this bog produced results additional to the desired 139 dating and these will be treated in a separate section. Marine sediments which have been over142 ridden by ice have been noticed for a long time 145 in the Bergen area (Rekstad 1900, C. F. Kolderup 1908, pp. 55-66, Undas 1942, 1963, p. 13, 146 H. Holtedahl 1964, Mangerud 1968, 1970).

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GLACIAl STRIAE Oldest Youngest

6C'N

Fig. 1. Map of the Bergen area, with inset map showing its position in southern Norway. Contour intervals of 200 m for both land and sea. Only a few of the main directions of the glacial striae are indicated. The end moraines from the later part of the Younger Dryas are shown as solid black lines, stippled in places where the position of the ice-front is uncertain.

I have started a systematic investigation of these sediments and a more detailed description of the results, with a fuller discussion, will be published later. Here, I only intend to summarise those results which are of importance in clarifying the oscillations of the icefront (Fig. 7). The present paper deals with the sediments,

their fossils, climatic development, landforms, Ci-i-datings etc. Since in the course of this work I encountered major problems connected with the stratigraphic subdivision and correlation of various events, I shall discuss in the final section the general principles underlying a stratigraphic subdivision of the Late Quaternary.

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Late Weichselian in the Bergen District

1km

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Fig. 2. Vertical air-photo of Blomoy. x — the peatbog at Dale to which the pollen diagram (PI. 1) refers, o — the graveyard at Blomvag (Fig. 5). Westwards, outside the photo, there arc only a few small skerries separating the island from the North Sea. (Photo by Nor-Fly A/S.)

POLLEN DIAGRAM FROM A PEATBOG AT DALE, BLOMOY Topography. Blomoy is situated on the outermost part of the coast bordering the North Sea. The island's highest point is a mere 73 m above sea-level. The dominant geological feature is of surface outcrops of naked crystalline rocks, all around (Fig. 2). The principal soil types are peat and weathering products. In hollows, small pockets of glacial and marine sediments can be found. The Late Weichselian marine limit is approximately 30 m a.s.l. (Undas 1945). ' Regarding the interpretation of the pollen diagram, I should like to point out that because

of the higher sea-level at that time, Blomoy was split up into a number of small islands, the largest being only 1 X 1.5 km in extent. The diagram presumably reflects only the local vegetation on a couple of these islands. The present-day vegetation is limited first and foremost by the scarcity of soil. There are no woods on Blomoy, but this is partly due to human interference. Apart from the cultivated areas, the vegetation is mainly heather moorland of western Norwegian type, strongly influenced by sheep grazing. The peatbog (Fig. 3) from which the pollen profile was collected is quite small, ca. 10 X 30 m, with a catchment area of 0.03 sq. km.

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Fig. 3. The peatbog at Dale (PI. 1). The person is standing beside the bore-hole. The photo was taken in a more or less due N direction.

It lies ca. 36 m a.s.l. I made trial borings in several bogs lying above the marine limit, but this bog proved to be the deepest and, in fact, the only one containing sediments which could be considered likely to be older than the SubAtlantic zone. Sampling procedure. The samples for pollen analysis, as well as some of those used for estimating the loss on ignition, were taken with a Hiller sampler. The samples for the other investigations were taken with a piston sampler. Borings were made on three separate occasions, correlations being made visually lithostratigraphically, since the depths of the borings beneath the bog surface varied. The boundaries between the various deposits were sharp, apart from that between the lower grey gyttja clay and the brownish-grey clay gyttja. This difficulty has led to some uncertainty in correlating the stratigraphy of some of the samples taken for loss on ignition, grain size analysis, and Ci4-dating (T-625).

The sediments Grain-size analyses and mineral content. First of all, organic matter was removed by oxidation in 6 % H 2 O 2 solution. Thereafter grain-size analyses of the mineral matter were carried out by the sieving and pipette method. The samples varied from 2-5 cm in thickness, and their position in the stratigraphy is shown in PI. 1. The grain-size distributions are shown as histograms in PI. 1, and as cumulative curves in Fig. 4. All the samples were composed of poorly sorted clay and silt; the sediments must, therefore, have been transported in suspension. The samples from the upper gray gyttja clay (A6 and A7) are more poorly sorted than the limit given by Selmer-Olsen (1954, Fig. 21) for lacustrine sediments, whilst the remaining samples just fall within the limit. The lake basin from which the samples arc derived measures only ca. 10 X 30 m, however, so that a better degree of sorting was virtually impossible. The

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V-/ittemblagc zone. The zone is characterised by high, though decreasing, NAP values, on account of the rise in Betula and Salix, which are the most important 'AP' components. I have not distinguished between pollen of the different Betula species, mainly because the samples have been treated with HF and pollen size, therefore, cannot be used as a reliable criterion. Sample No. 8, however, has been analysed by Dr. Bjorn Berglund, who has separated Betula nana on the basis of his own criteria (Berglund 1966, p. 37). The result was 14 Vc of B. nana type and 86 % of B. pubescens agg. type. At that time, there was a real dominance of arboreal birch species, but one should not conclude further from this that the same applies in the lower part of the diagram. A species identification was not carried out for Salix either. A series of pollen types were in fact encountered, some of which showed marked morphological differences, and I am in no doubt that there must have been bushtype Salix species in addition to the low-growing types such as S. herbacea, S. polaris etc. Pinus exhibits a small, but pronounced increase between samples 6 and 7. This is certainly pollen derived from long-distance transport, and the increase may be due to augmentation of the supply of Pinus pollen, or to reduced production of local pollen of other kinds. The first alternative would seem the most likely explanation in the upper part of zone B2, whilst the latter perhaps applies to zone B3. Sample No. 8 was analysed by Berglund, as mentioned earlier, and the high Pinus value is probably due to the use of another method of counting damaged pollen grains. The low values for Ericales pollen is rather surprising; not until sample 9 does it reach

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3 7c This may be due to the presence of snowbeds, since Empetrum is chionophobous, or to delayed immigration. Most probably, however, it is due to the lack of leaching of the soil at that time (Iversen 1954, p.-108), since Empetrum is an acidophilous plant.The high values for Pediastrum in zone B2 indicate eutrophic conditions, and it is noticeable how, when Empetrum heath becomes established in the area in zone B4, Pediastrum disappears from the lake, whilst the acidophilous Isoetes increases markedly. Both the species frequency and the large quantity of herb pollen indicate that heliophile plants dominated during the whole zone of B2. In the lower part of the zone' the vegetation was still tundra, later on with admixture of shrubby willows and tree birches, such that the vegetation in the upper part of the zone (Late Allerod) must be characterised as park tundra (Berglund 1966, p. 139). Apart from this general characteristic, it is difficult to draw any safe conclusions about the types of vegetation present. I should like to mention a few interesting points. Planlago maritima indicates that the diagram throughout is influenced by the seashore vegetation, which was present only a few hundred metres away from the lake basin. Fcegri (1940, p. 34) and Hafsten (1963, pp. 333-335), in their interpretation of similar diagrams, have both stressed the importance of snow-beds. Snow-bed com-' munities have indeed played some part on Blomoy too, but in the absence of a closer identification of the species concerned, it is difficult, in the pollen diagram, to separate the snow-bed communities from the other heliophile plant communities. Of all the ecological factors, I would, however, lay particular stress on the edaphic. In Late Weichselian times, when no weathering soils had yet been formed, most of Blomoy was completely naked rock. Soil was present only in the cracks and crevices which seam the bedrock in all directions, and in which there is a continuous variation in scale, from narrow rifts a few cm wide, up to features gouged out by glacial erosin, and being 10-30 m deep relative to the intervening rock ribs. These

depressions contained both the deepest and most extensive accumulations of soil, and, on account of their topographic situation, this was allied to humid conditions and a favourable local climate, wind-shelter in particular. If the vegetational succession is viewed from this standpoint, the marked fall in the curve for Gramineae pollen after zone Bl is explained by the establishment of new plant communities in these depressions. First of all there was a damp meadow, dominated, on account of the humid conditions, more by Cyperaceae (pollen maxima in samples 3,4 and 5) than by Gramineae. This gave way in its turn to shrubby vegetation with some trees, Betula and Salix. The trees (Betula pubescens agg.), and perhaps the willow thickets as well, remained, throughout zone B2, restricted to these habitats, with relatively deep soil and a favourable local climate. On the outskirts of this shrubby vegetation with trees, and in all the minor depressions in which this vegetation did not find a foothold, there was probably little change in the type of vegetation from zone Bl, termed Artemisiagrass vegetation by Berglund (1966, p. 90). Pollen zone B3: Sa\ix-NAP Assemblage zone. This zone is characterised by a marked decline in the Betula curve, with a corresponding increase in NAP. An interpretation of the Betula decrease is strongly hampered by the lack of a species determination analysis. It is quite obvious, however, that the decline is due to a marked retreat of Betula pubescens agg. Whether tree birches disappeared entirely cannot be decided on the basis of the present material, but I consider it very likely. The curves for several herbaceous plants (Lycopodium, Rumex, Artemisia, Ranunculaceae) show a change from zone B2 to zone B3. The most obvious change, however, is the strong increase in Cyperaceae. Following the discussion on vegetational development in zone B2, the natural assumption is that the Cyperaceae, more than the other plants, occupied the ecological niche vacated when the birch woods disappeared.

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Late Weichselian in the Bergen District

As I shall discuss later, under climatostratigraphy, it is clear that these changes in vegetation were due to a climatic deterioration. I have not counted Pediastrum so carefully as the pollen grains, but its decline in zone B3 is significant and indicates a decline in the autochthonous organic matter production of the lake basin, which was probably conditioned by the fall in temperature. Pediastrum increases again slightly in zone B4, but disappears completely thereafter, on account of the change from eutrophic to oligotrophic conditions. Pollen zone B4: Betula-Ericales Assemblage zone. This zone is represented by a single spectrum (sample No. 13), in which Betitla comprises 74 % of the AP and Ericales 62 % of the NAP. The zone bears witness to a radical change in the vegetation compared to zone B3. Salix disappears almost entirely and Betula attains complete dominance among the tree species. There is a sudden and massive increase in heath vegetation, with the result that other herbaceous plants almost entirely vanish. In this zone the heath is an almost pure Empetrum heath (80 % of the Ericales pollen), with some Calluna, whilst later on Calluna becomes the dominant heath species. The zone has a very limited extent, and this, together with the marked changes in comparison with zone B3, makes it but a small step to draw the conclusion that a hiatus exists here. There are no other indications of this, however, so it is perhaps just as likely that it is due to a low rate of sedimentation. Vegetationally this zone may be correlated with Jessen's zone IV, and indicates such a marked climatic amelioration that there is no doubt that it belongs in the Holocene. Pollen zone B5: Corylus Range-zone. The rise in the Corylus takes place between samples 13 and 14 and the subsequent period of time will not be discussed any further in this paper. Therefore I have not bothered to zone the diagram, to any finer degree, above this point.

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Ephedra. Several finds have been made in Scandinavia of pollen of the steppe plants Ephedra distachya and E. strobilacea. There was discussion for a long time as to whether Ephedra had in fact grown in Scandinavia, or whether the pollen was derived from longdistance transport. Now, however, most people seem to consider that it did in fact grow here, both during the Late Weichselian and in the Holocene (e.g. Iversen 1954, pp. 104-105, Hafsten 1956, pp. 51-54, Berglund 1966, p. 148). Ephedra has been found in Holocene sediments in eastern Norway (Hafsten 1956, pp. 51-54) and in Late Weichselian sediments in southern Norway (Hafsten 1963, p. 332). The finds described in the present paper are the first in western Norway. Pollen grains of both E. strobilacea and E. distachya were found (PI. 1). In addition, I have found a single grain of E. distachya in samples from the Corylus maximum in Lepsoyvann, Os (Fig. 1),Ci-i-dated to (T-580) 8380 ± 180 BP. This sample indicated closed woodland, with only 8 % NAP. According to Gams (cited in Iversen 1954, p. 104), Ephedra has no particular temperature demands, 'but it seems to require both climatic and edaphic dryness' (Iversen, op. cit.). It is therefore surprising to find Ephedra pollen in western Norway, which has, at the present day, a very humid climate. The generalised picture of the vegetation (Plate 1), in particular perhaps the large amount of Salix, makes it difficult to assume that the climate can have been especially dry hereabouts during the Late Weichselian. This problem must remain for the present unsolved. After writing this section of the manuscript I became aware of a detailed discussion of the same problem by Danielsen (in press). He refers to a series of accounts of long-distance transport of Ephedra pollen from the present day, and concludes that none of the species of Ephedra have grown in situ in Scandinavia since the last Ice Age. Seen against the background of the material he presents, his conclusion appears quite logical, and if one accepts it, the problem regarding Ephedra in western Norway is automatically solved.

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Salix. In several spectra the Salix curve attains 30 % of the total pollen sum, and in pollen zones Bl, B2 and B3 it makes up no less than 50-60 % of all pollen derived from trees and shrubs. High percentages of Salix are often encountered in Late Weichselian sediments (e.g. Krog 1954, Hafsten 1963, Chanda 1965, Berglund 1966, pollen diagrams II and V), and this is discussed at length by Foegri (1936, pp. 17-18, 1940, p. 35, 1953). Most frequently the curve exhibits a maximum right at the base of the diagram, thereafter declining rapidly upwards. Fasgri (1936, p. 18) points out, however, that the high Salix pollen content must be climatically conditioned, and does not merely represent a pioneer vegetation. In the present diagram the Salix curve rises steadily from the base upward, practically to the top of pollen zone B2, and particularly during the period of time represented by pollen zone B3, Salix species were dominant in the vegetation. Mainly on a basis of macrofossil analyses made by Holmboe (cited in Fjegri 1936, p. 18), Fsegri assumed (1936, pp. 18-19, 1940, p. 34) that it was principally Salix herbacea which provided the lowermost Salix maxima in the diagrams from Eigebakken and Brondmyra. He thereupon interpreted this as representing an extreme, arctic, snow-bed vegetation. Later on (Fasgri 1953, pp. 70-71) he carried out species identification analyses of the Salix pollen in these spectra, and then discovered that, in addition, shrubby species of Salix had played a certain role. He thereupon modified his interpretation, and states (op. cit., p. 71) that the vegetation 'was chionophilous, but hardly quite as extreme as originally supposed'. I have not made any detailed specific identifications for Salix in the present diagram. I have noted, however, that the genus is represented by a variety of pollen types, which are so clearly differentiated morphologically that on this basis alone I have no hesitation in stating that, in addition, bush species of Salix were represented. Taking into consideration the high percentage of Salix in relation to Betula, as well, the above conclusion seems to be the only reasonable one.

In the Norwegian mountain areas, a dense thicket of Salix bushes is usually present, especially on damp ground, both in the subalpine birch belt and in the low-alpine belt above. Some of these species can tolerate a heavy snow cover, but are not such extreme chionophiles as S. herbacea and S. polaris. Without wishing to lay too much stress on the ecology of the various species, I would say that the upper part of pollen zone B2 may well represent the Salix-nch. areas in the upper part of the subalpine birch belt in the Norwegain mountains at the present day, whilst zone B3 is equivalent to the same area above the tree-line for birch. As far as zones Bl and the lower part of B2 are concerned, I would prefer to let the question remain open, because, in this instance, I am uncertain whether tree-birches or bush species of Salix were present.

Climatic development and datings The main features in the climatostratigraphical classification are quite clear (PI. 1) and are supported by the Ci^-datings. I propose, in the following section, to discuss the major boundaries and certain special features of the climatic development. Older DryasjAllerod. The ice withdrew from Blomoy shortly before the bluish-grey clay was laid down. This, the first indication of climatic amelioration, can thereby be dated to a little before (T-672) 12,070 ± 180 B. P. The question which then arises is whether it was a rapid improvement in the climate, such that the vegetational development evidenced in pollen zones Bl and B2 merely reflects an immigration succession and adjustment to that climate, or, whether the vegetational development indicates a more gradual climatic amelioration over a longer period of time. Betula can migrate rapidly, and in the Preboreal it achieved, throughout Scandinavia, an extensive and rapid expansion as soon as it had immigrated. I therefore consider that the steady and tardy increase of Salix and Betula

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Late Wcichsclian in the Bergen District

at the expense of NAP in zone B2 is conditioned by a gradual amelioration of the climate over the course of several centuries, even though this interpretation is somewhat tentative on account of the lack of species determinations for Bcltila. The boundary Older Dryas Stadial/ Allerod Interstadial must therefore be placed at some point within this period of climatic improvement. As mentioned previously, the vegetation of the uppermost part of pollen zone B2, representing the end of the Allerod, consisted for a large part of tree birches (Betula pubesccns agg.)- This provides us with a minimum value for (mean) summer temperature. Aas (1964) has investigated the present-day timber-line for Betula in Norway. This tree-limit decreases strongly in altitude westward towards the coast of western Norway, and if we project the limit westward from his 500 m a.s.l. isohypse, the theoretical limit for Blomoy lies at about 300 m a.s.l., maximum 400 m. This implies that the timber-line for Betula at the end of the Allerod Interstadial probably lay ca. 300 m (maximum 400 m) lower than today. Aas (1964) found that the best climatic correlation with the timber-line was with the mean temperature for the three warmest months (a tritherm), but that the correlations with the wannest, or the two warmest, months were also good. The vertical gradient for atmospheric temperature change along the Norwegian coast is 0.6-0.7° C/ 100 m (Andersen 1968, p. 131). A 300 m (maximum 400 m) lower timber-line for Betula would thus indicate that the tritherm (or, instead, the temperature for the warmest month) was 1.8-2.8° C lower at the end of the Allerod than today. This can also be calculated in another manner. By using the four localities lying closest to Blomoy (Aas 1964), we obtain an average temperature value for the birch timber-line of 11.7° C for the warmest month, and 10.3°C for the tritherm. At Hellisoy lighthouse, ca. 25 km north of Blomoy, but on about the same isotherm, the corresponding temperatures are 14.1° and 13.4° C (Bruun 1962). The differences, therefore, are 2.4° C for the warmest month, and 3.1° C for the tritherm. The assumption is made for both modes of

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reasoning that the climate was more or less as oceanic then as now, since the tritherm for the birch timber-line falls from 10°-H°C in the coastal regions to ca. 8° C in the continental regions of Norway (Aas 1964). On this assumption, the first method of estimation is the more logical, since all four of the localities used in the second method lie much further inland than Blomoy. I draw the conclusion, therefore, that the summer temperature during the Allerod Interstadial was only 2-2.5° C lower than today. This may seem a small amount, but it agrees well with Iversen's results (1954, pp. 97-98) from Denmark. Younger Dryas. The boundary Allerod Interstadial/Younger Dryas Stadial is very sharp and I set this boundary to coincide with the lithostratigraphic boundary brownish-grey clay gyttja/upper, grey clay gyttja. Pollen sample No. 9 is taken at this boundary, but has to be assigned to pollen zone B2. The zonal boundary B2/B3, which also bears evidence of a marked climatic deterioration, must therefore lie a few cm above the lithostratigraphic boundary. The Ci-*-datings for the upper part of the Allerod, (T-624) 10, 940 ± 180, agree well with Danish datings. Both the sediments and the vegetation indicate a very marked climatic deterioration at the boundary Allerod/Younger Dryas, leading to a return of a tundra vegetation. The present material does not, however, provide a basis for a more detailed, quantitative evaluation of the climate during the Younger Dryas. Holocene. The most recent climatostratigraphic boundary that I shall discuss here is the climatic amelioration following the Younger Dryas, which is usually employed as the boundary between the Pleistocene and the Holocene. This, too, is a sharp boundary, resulting probably from a very rapid improvement in climate. The closed woodlands of Betula (and later of Coryhts), which rapidly became established, indicate that the climate, figuratively speaking almost overnight, became better than

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Llthostratigrapriy

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Littoral sand and gravel

OLDER DRYAS SIADIAL

Hard till, rich in boulders cal,5m

Littoral stdimtnts with fossils

ca 1,5m

(wood,bones of reindeer, birds and what*, molluscs)

1-138 12200: 350 a ? (wood)

B3UING INTER SIAOIAL

T - 139 12700 s 350ER (Mylilus tdulis)

OLDEST DRYAS STAOIAL

Fig. 5. The stratigraphy at the graveyard at Blomvag, ca. 20 m above sea level. The Iithostratigraphy and fossils are essentially after Undas (1942). For C14-datings see Table I. that of the Allerod, and quite soon more or less as today. The C^-dating (T-623) 9340 ± 160 B. P. is from a sample some 5 cm in thickness, which according to the pollen analyses represents a time interval of several centuries, yielding, therefore, a date which must be assumed to be appreciably younger than the boundary. ICE-FRONT OSCILLATIONS Bergen area Boiling-Older Dryas. In 1941, during excavations for a graveyard in Blomvag, littoral sediments were discovered beneath lodgement till (Undis 1942) (Fig. 5).

Undas (1942, p. 106) assumed these sediments to be interglacial in age. CH-datings (Nydal 1960, p. 88), however, gave dates ot (T-138) 12,200 ± 350 B. P. and (T-139) 12,700 ± 350 B. P. The glacier front, consequently, must have retreated landward past Blomvag at some time prior to 12,700 ± 350, and then advanced over Blomvag and out into the North Sea once again after 12,200 ± 350. As mentioned during discussion of the pollen diagram, this glacier advance must be older than (T-672) 12,070 ± 180, and must, therefore, have taken place during the Older Dryas Interstadial. At Sandviken in Bergen, during excavations on a building site in 1968, I came across the stratigraphy shown in Fig. 6. The C^-dating (T-750; 12,470 ± 150 B. P.) suggests that the fossiliferous clay is from the Boiling Interstadial. Above the clay are two till beds, separated by a 15 cm thick silt layer, containing lenses and gently undulating lamina. The tills can be interpreted as lodgement till from the Older and Younger Dryas Stadials respectively, whilst the silt layer may be from the Allerod Interstadial. I should perhaps emphasise that no fossils have been found in the silt layer, and that this may possibly be subglacial in origin. The interpretation that the clay dates from the Boiling Interstadial cannot therefore be proved from the stratigraphy, but is supported solely by the Ci^-dating. The sediments from Blomvag and Sandviken are the only ones from the Boiling Interstadial that are known to exist in the Bergen area. They indicate that extensive parts of the area were ice-free at that time, but it is not possible, for the present, to decide whether or not the ice-front had retreated still further inland (Fig. 7). Allerdd-Younger Dryas. Blomoy finally became free of ice just before 12,070 ± 180 B. P. (T-672), and was not subsequently covered by ice. Whether this ice-melt started in the Older Dryas Stadial, or whether it should be wholly referred to the Allerod Interstadial, is really a question of definitions alone. At all events the ice continued its retreat. There is a C ^ dating of shells from a till in Bergen (H. Holtedahl 1964, p. 320), which gave (T-228A)

Late Weichselian in the Bergen District

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Fig. 6. The stratigraphy from an excavation at Sandviken, Bergen, ca. 8-10 m above sea level.

CORRELATION

133 WITH

CLIMATO-STRATIGRAPHY

SiIt. Lamina and lenses ~-~r^^~i

3m

C u -YEARS EC.

BP.

Correlation with Climatostratigraphy

OSCILLATIONS NORTH

BIOMSY

OF

HEROU OS

THE

POSITION OF

THE GLACIER

FRONT BERGEN 30 km

EIKANGESV i G

HOLOCENE 10000-

YOUNGER DRYAS STADIAL

ALLER0D INTERSTADIAL

Fig. 7. Oscillations of the position of ths glacier front during the Late Weichselian in the Bergen area. The curve indicates the position of the ice-front at various times. The Clf-datings which form the basis for this reconstruction of events have been added to the diagram. The profile runs in the direction of movement of the ice, Blomoy-Herdla — a little to the N of Bergen. Os lies S of Bergen (Fig. 1), but the datings from Os have been marked in at Herdla because the end moraines from the Younger Dryas Stadial pass through both Os and Herdla.

134 APPROXIMATE AGE

C 1 4 YEARS

/. Mangenid

Climato. Stratigraphy

VARIATIONS

OF THE

POSITION OF THE GLACIER

FRONT

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YOUNGER ORYAS STAOIAL

AU.ER8JD INTERSTAOIAi.

OLDEST DRYA5

Fig. 8. A schematic correlation between the changes in the position of the ice-front in the Bergen area and in Vastergotland in Sweden, during the Late Weichselian. The curve for the Bergen area is that shown in Fig. 7. The curve for Vastergotland has been constructed from data in Morner (1969, PI. 2) and E. Nilsson (1968, PI. 1). 11,700 ± 150 B. P. (Nydal et. al. 1964, pp. 283284). In addition, I have obtained a dating of shell from a clay from Bergen, for which consolidometer investigations show that the clay must have been pre-consolidated by ice. The dating (T-751) gave 11,400 ± 110 B. P. The ice, therefore, must have retreated inland beyond Bergen early in the Allerod Interstadial (Fig. 7). Quite recently, I have also obtained a CHdating of shells from a till at Eikangervag (Fig. 1), which gave (T-846) 11,930 ± 140 B. P., thus indicating a rapid retreat of the ice during the Allerod. I have no concrete evidence as to how far inland the ice had melted in the Allerod. Skreden (1967), however, describes from the Voss area layers of well-sorted silt and clay lying between two beds of till, and he considers that the former sediments were deposited in an icedammed lake. Skreden (op. cit. pp. 79-80) states that it is possible that this ice-free period was the AIlerod-Younger Dryas, but since no fossils were found it is not possible to say anything definite on this question. Further investigations are now in hand. I find it quite likely, nevertheless, that the ice-front had retreated inland as far as Voss during the Allerod. ' If we accept that Bergen was free from ice in 11,700 ± 150 B. P., and Eikangervag in 11,930 ± 140 B. P., then there were after all 7-800 years still before the onset of the climatic deter-

ioration of the Younger Dryas Stadial. The datings from Os (see below) point to the same conclusion. The end-moraine, which I correlate with the Ra-moraine in eastern and southern Norway, runs through Os, south of Bergen (Fig. 1). On the diagram illustrating the oscillations of the ice-front (Fig. 1),- therefore, the datings from Os are entered together with Herdla, which is assumed to belong to the same system of endmoraines. There are 5 O-1-datings from Os (Table 1), of fossils which have subsequently been overridden by ice. The oldest (T-305) gave 11,700 ± 230 B. P. and since these fossils demand relatively clear water as a habitat (H. Holtedahl 1964, p. 321), the ice must already have retreated quite some way beyond Os.The youngest datings (T-229 and T-304) gave 10,150 ± 300 and 10,050 ± 250 respectively (H. Holtedahl 1964, pp.320-321).They indicate that the glacier did not reach the outermost ice-margin of the Younger Dryas before the very end of this stadial. : Holocene. During the Holocene the ice obviously melted quickly away from the fjords. There is a CU-dating of gyttja (Klovriing & Hafsten 1965, p. 337) from Flam, innermost in the Sognefjord (Fig. 9), of (T-412) 9300 ± 300 B. P., and another, of gyttja from Busnes, innermost in the Hardangerfjord (Fig. 9) of

Late Weichselian in the Bergen District

4° E Greenwich

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Fig. 9. A cartographic comparison of the Collatings which are assumed to provide information on the course of the deglaciation in Norway. Datings shown in parentheses have been made on fossils over which the ice-front has subsequently passed. These therefore provide a minimum age for the last time the locality was covered by ice. Datings not in parentheses, on the other hand, provide a minimum age for when the area became free of ice. Datings which are underlined refer to marine shells, the rest to lacustrine gyttja, wood or charcoal samples. The Ra end-moraine is shown as in Fig. 10. For references, see Appendix, p. 145.

136

J. Mangcrud

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9720 ± 330 B. P. (T-585) (Anundsen & Simonsen 1967, p. 35). Even younger end-moraines can be found up in the mountains, but these will be investigated more closely in the future.

Correlations with southern Scandinavia The main features of the course of deglaciation in Denmark (S. Hansen 1965) and Sweden (S. Lundqvist 1965) are known. There is no unanimity, however, on all datings and correlations of the position of the ice-front. In Fig. 8 I have drawn up a schematic comparison between the oscillations of the icefront in the Bergen area and in Vastergotland in Sweden. The curve for the latter area, relating to the Younger Dryas and Allerod, has been constructed from E. Nilsson's data (1968, PL 1), and the section relating to earlier periods from Morner (1969, PL 2). There are very great moments of uncertainty applying to the time correlations between the two areas, and detailed comparisons have little or no value. The main features are nevertheless clear; in western Norway there were large oscillations in the positions of the ice-front, whilst in Sweden there was a rapid retreat from icemelting during the mild interstadials (Boiling, Allerod) and a slowed retreat or stagnation during the cold stadials (Older Dryas, Younger Dryas). However, in Sweden too, a few minor advances of the ice have been reported (E. Nilsson 1968, p. 14 and pp. 16-17, Morner 1969, p. 128, Hillefors 1969, pp. 252-255). These major differences in the movements of the ice-front in western Norway and Sweden may, in part, be due to differences in the development of the climate. I would assume, however, that the difference was first and foremost conditioned by the large glaciological differences between the two areas. In the eastern parts of the Scandinavian icesheet, there were long distances between the accumulation areas and the actual ice-front, and throughout this region there were divergent directions of ice-flow, such that any excess of accumulation in the central areas became spread out over an extensive ice-front.

In western Norway, in the Late Weichselian, there were but short distances between the areas of accumulation and the ice-front. Outflow from the relatively vast mountainous areas took place through narrow valleys and fjords, a state of affairs which led to a rapid and extensive advance of the ice-front following climatic deterioration. Deep fjords gave rise to calving of the glaciers on a large-scale, and thereby, to a rapid retreat of the ice-front following climatic improvement. C^-DATINGS A series of Cii-datings from Late Weichselian sediments exists from the Bergen area. All the datings were carried out by the Radiological Dating Laboratory at Trondheim. They are presented in Table I. All dates are based on the Libby value, for the half-life, of 5570 ± 30 years, and this has been used throughout in the values quoted in the text. All datings are given in CH-years before the present day (B.P.), using 1950 as the reference year. The datings have been made on pieces of wood, lacustrine gyttjas and marine fossils. In the following sections, I shall discuss the problem of dating marine fossils, since these are dealt with in a great variety of ways in tabulations of dates and in the geological literature. Two factors in particular are very important, namely isotopic fractionation and the apparent age of seawater in the ocean basins. Isotopic fractionation. In the course of many chemical reactions in nature (e.g. photosynthesis) a fractionation occurs between the three carbon isotopes Ci2, C13 and C1-*. Both theoretical and experimental results (Craig 1954, pp. 133134 and 147) indicate that a close relationship exists between C13 and Ci», and that the enrichment factor in the fractionation process is 2. Thus the enrichment of Ci-t in a given compound is twice that of C13. This is particularly important because C1-* is radioactive and the content of this isotope, consequently, starts to decrease as soon as the carbon is removed from equilibrium with atmospheric carbon. By measuring the relationship Ci 3 /Ci2 in a carbon compound, it is possible, however.

Late Weichselian in the Bergen District

137

Table I. C14-dated samples of Late Weichselian age from the Bergen area. Datings for which no reference to a dating list is given have been supplied by Dr. Reidar Nydal (personal comm.). Samples for which the laboratory numbers are italicized were submitted by the author. Sample T-594 was collected by C. F. Kolderup. AH the rest of the italicized samples were collected by me personally. All the localities can be found in Fig. 1.

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Radiocarbon age Years B.P. (1950)

Laboratory numbers and dating lists

Locality

Dated material

Comments. References

12700 ± 350

T-139 Nydal 1960, p.

Blomvag Blomoy

Marine molluscs Mytilus edulis

From fossil-bearing gravel below layers of mud and till, 12 m above sea level (Fig. 5). The dating is discussed by O. Holtedahl (1960, p. 411), H. Holtedahl (1964, p. 322) and Mangerud (1968, p. 465).

12470 ± 150

T-750

Sandviken Bergen

Marine molluscs Chlamys islandicus

From clay below two beds of till (Fig. 6). The date obtained would suggest that the clay belongs to the Boiling Interstadial.

12200 ± 350

T-138 Nydal, 1960, p. 88

Blomvag Blomoy

Wood

From the same bed as T-139, above. The wood was preserved in alcohol for 18 years, and might consequently have been contaminated by younger carbon. Note that the dating 12100 ± 300, used by O. Holtedahl (1960, p. 411) and Mangerud (1968, p. 465), is incorrect.

12070 ± 180

T-672 Nydal et. al. 1970, pp. 210-211

Dale, Blomoy

Gyttja clay

See PI. 1. The dating indicates a minimum age for the deglaciation of Blomoy (Mangerud 1968, p. 465).

11930 ± 140

T-846

Eikangervag

Marine molluscs Fossils present in till. Chlamys islandicus, Saxicava sp.

11700 ± 150

T-228A Nydal et. al. 1964 pp. 283-284

Florida Bergen

Marine molluscs Fossils present in till Chlamys islandicus (H. Holtedahl 1964, p. 320). Mya tnmcata

11700 ±230

T-305 Lundetre, Nydal 1962, p. 172 Os

11500 ±300

T-142 Nydal 1960, p. 89

11400 ± 110

T-751

Bryozoan limestone

Limestone containing Bryozoa deposited on a rockface and subsequently covered by clay and till (H. Holtedahl 1964).

Ulvenvann Os

Marine molluscs Mya tnmcata

Fossils present in till (H. Holtedahl 1964, p. 320, UndSsl963,p. 13, O. Holtedahl 1960, p. 409).

Grieghallen, Bergen

Marine molluscs Occurring in a clay which had Chlamys islandicus been preconsolidated by the pressure of an overriding glacier. Stratigraphy not clear. (Contd. next page)

2 — Norsk geogr. Tidsskr.

/ . Mangerltd

138

(Table I contd.) Radiocarbon age Years B.P. (1950) 11070 ± 190

Laboratory numbers and dating lists T-62S

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Nydal et. al. 1970 pp. 210-211

Locality

Dated material

Comments. References

Dale Blomoy

Clay gyttja

See PI. 1. Clay gyttja deposited during the Allerod Interstadial.

10970 ± 180

T-594 Nydal et. ai. 1970 p. 211

Vinnes Fusa.

Marine molluscs Mya tnmcata

No signs were described of a subsequent ice-advance over the clay in which the fossils occur (C. F. Kolderup 1908, pp. 85 ff.). This problem will now be more closely investigated.

10940 i 180

T-624 Nydal et. al. 1970 pp. 210-211.

Dale Blomoy

Clay gyttja

See PI. 1. The sample dates the uppermost part of the Allerod Interstadial.

10790 i 110

T-752

Osoyri Marine molluscs sentrum, Os. Mya tnmcata Saxicava photos.

10150 ±300

T-229 Osoyri Nydal 1962, p. 171. Os

Marine molluscs Mya tnmcata

10050 ± 250

T-304 Lundetre Nydal 1962, p. 171. Os

Marine fossils Fossils in clay subsequently Balanus porcatus overridden by ice (H. HolteSaxicara arctica dahl 1964).

to find out whether or not isotopic fractionation has taken place, and, under the abovementioned presupposition, to calculate the magnitude of this process for C14. In practice, the C13 content of a sample is expressed as the deviation ((5C13) from a standard value, expressed per mille (Craig 1954, P . 116):

In clayey till, a few cm above the bedrock surface. Fabric analysis shows that the long axes of pebbles are parallel to the youngest glacial striae on that rock surface. Fossils present in till (H. Holtedahl 1964, p. 320).

•= —25.4 0/0Q for ordinary terrestrial plants (1953, p. 69) = —0.2 o/ oo for marine limestones (1953, p. 61) and for marine shells (1954, p. 136) = —13.4 o/ 00 for marine plants (calculated from his Table 4; 1953, p. 63) = —2.2 o/ 00 for ocean water (calculated from his Table 2; 1954, p. 136)

S C 13 == C I3 /C 12 sample — C 13 /C 18 standard C 13 /C 12 standard

1000

Craig gives as average values (Craig 1953 and 1954) relative to the Chicago PDB standard (Ciaig 1957, p. 135): —7 o/OQ for atmospheric CO., (1953, p. 72)

Marine shells are enriched with ca. 25 o/ oo by isotopic fractionation, in comparison with terrestrial plants, to which the Ci4-activity in the standard (95 % of the activity in the NBS oxalic acid) is correlated. Enrichment with C1 4 is twice as much, i.e. ca. 5 c/r, equivalent to ca. 400 CH-years. As indicated in the table above, little or no isotopic fractionation takes place between seawater and shells. Broecker & Olson (1961, p.

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Late Weicliselian in the Bergen District

178) have also found that the CH-activity in 'coastal shells' is more or less the same as in the 'adjacent surface ocean'. The apparent age of seawater. Exchange of carbon between the ocean and the atmosphere, naturally enough, only occurs at the surface contact. Since radioactive decay of C'J takes place at all depths, the seawater assumes an apparent Ci-J-age dependent upon the length of time it has remained in the ocean depths (Broecker et al. 1960, pp. 2903-4). In the Atlantic Ocean this apparent age varies, according to Broecker et. al. (1960, p. 2921), between 300 and 900 years, the greatest age discovered for oceanic deep water. In oceanic regions in which old deep water reappears at the surface, the seawater will show a particularly low Ci-i-activity; e.g. a dating of a freshly killed seal in the Antarctic had an apparent age of 1300 years (Broecker & Olson 1961, pp. 179 and 200).

I

Corrections. The two factors mentioned above are both operative, but in opposing directions. In comparison with terrestrial plants, isotopic fractionation leads to an increase in the CHactivity of marine shells, whilst the apparent age of seawater leads to decrease. In many areas, e.g. the North-Atlantic, it appears that these two factors in surface water cancel each other out, more or less, so that surface water possesses the same CH-activity as recent terrestrial plants. This is why many laboratories (Radiocarbon Measurements: Comprehensive Index, 1950-1965, p. 2) calculate the age of marine shells on a basis of the uncorrected CH-activity, or else corrected for the deviation in

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