early holocene pollen-and lithostratigraphy in lakes in the ålesund area, western Norway

Reuiew of Palaeobotany and Palynology, 53 (1988): 185-231 Elsevier Science Publishers B.V., Amsterdam ~ Printed in The Netherlands

185

LATE WEICHSELIAN/EARLY HOLOCENE POLLEN- AND LITHOSTRATIGRAPHY IN LAKES IN THE ALESUND AREA, WESTERN NORWAY INGER LISE KRISTIANSEN’, Department

JAN MANGERUD

of Geology, Sect. B, University of Bergen, (Received

and accepted

and LEIF L0MOl

Allegt. 41, N-5000 Bergen

(Norwa.~)

May 5, 1987)

Abstract Kristiansen, IL., Mangerud, J. and Lsmo, L., 1988. Late Weichselian/Early Holocene pollen- and lithostratigraphy lakes in the Alesund area, western Norway. Rev. Palaeobot. Palynol., 53: 185 231.

in

Three palaeolakes at elevations from 44 m a.s.1. to 30 m a.s.1. all have marine sediments in the lower part of the sequences, with lacustrine sediments above. The highest lake was isolated from the sea approximately 12,400 years ago, and the lowest 11,100 years ago. The lacustrine sedimentary sequence is so similar in these lakes and many other palaeolakes that we propose formal lithostratigraphical units. Climatic changes were the most important cause for changes in the sediment composition. The earliest vegetation was dominated by grass and herbs, especially Rumex and Oxyria. Later in the Allerod the flora was more diversified, but a tree-less vegetation, still covered large areas. Open birch forests were established on favourable localities approx. 11,600 years ago. Tree birch probably survived in the area during the Younger Dryas. Low sedimentation rates during the Younger Dryas suggests slow erosion on land, and a continuous vegetation cover, which was dominated by herbs and Salix. An Older Dryas climatic deterioration was not detected. In the Preboreal a Juniperus maximum occurs after the Beth rise, in contrast to the sequence in Denmark and Germany. Pinus is supposed to have immigrated from the east, through Sweden.

Introduction In this paper we present the lithostratigraphy and palynomorph stratigraphy for three palaeolakes in the Alesund area, Western Norway (Fig.1). Two of the palaeolakes, Torvlomyra and Saudedalsmyra (see Fig.9) are completely filled in with Holocene organic sediments. and are presently bogs. The third palaeolake, Lerstadvatn (see Figs.3 and 12) consists of one deep basin that is still a lake, and one smaller basin that is filled in and appears as a bog at the western end of the lake (see Figs.12 and 13). ‘Present address: Norsk Hydro, Research 4313, N-5028 Bergen (Norway). 0034.6667/88/$03.50

Centre, P. Box

(” 1988 Elsevier Science

Publishers

This study is part of an investigation of the Late Weichselian and Holocene sea-level changes in the Alesund area. The main criteria for selecting these palaeolakes was that they covered the elevation interval (approximately 45530m a.s.1.) that emerged from the sea during the Late Weichselian. Actually there were hardly any alternative basins. The resulting sea-level curve is presented by Lie et al. (1983). The main purpose of the palynological investigation was to provide a biostratigraphical tool for inter-correlation of the strata in the three paleolakes. In this paper we also use the pollen-stratigraphy to deduce the vegetational history of the area. As stated above, the sites were not chosen for that purpose. However, Lerstadvatn B.V

186

meets nearly all the requirements for such a study. In Torvlomyra and Saudedalsmyra a large part of the sequence consists of marine sediments, and these basins are also too small to be optimal for a study of the regional vegetational history. Nevertheless, the two latter palaeolakes provided interesting information, since they are situated in an area with extremely sparse soil-cover on the bedrock, and close to a north-facing mountain slope, while the Lerstadvatn area has richer soils and a warmer local-climate. All geographical names are given on maps; here we will only mention that Alesund is a

city, and we loosely call the surrounding area the Alesund area, Sunnmore is the name of a wider coastal area, stretching from just north of Krdkenes (Fig.1) to north of Alesund. Topography The most characteristic topographic feature of Sunnmore is the sharp relief. The investigated area lies at the coast, but there are mountains up to 900 m a.s.1. close by (Fig.2). The coastal area is also characterized by the strandflat which lies as a rim around the mountains, giving the islands a hat form

_-

_

,,, ,,,

“~

-

,,,

>

-

-

_^

, .

Fig.2 Map of the Alesund area; for location see Fig.1. Contour interval 20 m. The investigated means bog.

,,-

> ,

,T_,

,._

sites are marked with dots. In Norwegian

-vatn means lake and -myra

188

(Holtedahl, 1960). The three investigated localities are situated on the upper, undulating part of the strandflat (Figs.2 and 3).

Mangerud et al., 1979; Sollid and Sorbel, 1979; Larsen et al., 1984) and cirque moraines are present only 3 km west of the sites Torvlomyra and Saudedalsmyra (see Fig.21).

Deglaciation Present vegetation From the area around the investigated sites, we have six radiocarbon datings on shells giving a minimum age for the deglaciation ranging from 12,310+ 130 to 12,630+60 yr B.P. We conclude that the Alesund area was deglaciated somewhere in the interval 12,500&12,300 yr B.P. The Younger Dryas end moraines from the Scandinavian ice sheet are localized approximately 60 km east of Alesund (Fig.1) (Sollid and Serrbel, 1979). Cirque glaciers were frequent in the area outside the ice sheet during Younger Dryas (Reite, 1967;

Outside cultivated areas, the coastal lowland of Sunnmore is dominated by heaths, mires and forests. In the eastern parts there are extensive pine forests. In the western parts Calluna heaths, Atlantic and blanket bogs (Moen, 1973) cover the largest areas, but birch forests are also found, mainly along the mountain slopes and hills. Corylus avellana and Ulmus glabra are occasionally found in favourable localities near the outer coast, but they occur more frequently

Fig.3. Photo towards east, showing the general topography of the area, and location Photo: Svein Skare, Historical Museum, University of Bergen.

of the site Lerstadvatn,

compare Fig.2

in the mixed deciduous forests on south-facing slopes along the fjords. Along rivers and valley slopes Alnus incana thrives. A. glutinosa, however, mainly occupies seashores and wet land. Quercus robur is restricted to a few localities. Tarus baccata occurs occasionally in the pine forests, here being close to its northern limit. Great areas of the most favourable soils in the coastal lowland of Sunnmore are at present cultivated. Methods Field u!ork The three palaeolakes were cored during 1978, 1979 and 1980. Most borings were done with a Russian peat sampler (Tolonen, 1968) from the bog surface, from rafts on the lakes in the summer, and from the ice in the winter. The samples for laboratory investigations were taken with piston samplers with diameter 110 mm, giving up to 2.0 m long PVC cores. One device is a modified Geonor A/S (Grinidammen 10, Oslo) sampler with inner rods connected to the piston. Another device is a modification of Wright’s (1967) square rod sampler with a wire to the piston. Both devices normally work well, the former being more robust whereas the latter is lighter. At the laboratory the cores were sawed longitudinally.

all the analyzed cores. In addition t.he content of organic carbon has been determined with a Leco carbon determinator EC 12 in two cores from Lerstadvatn (502-30-01/2a and 502-30-07). The loss on ignition was approximately twice the content of organic carbon, which was also found by Digerfeldt (1972) and Fimreite (1980). Preparation

of pollen

The samples were taken as a known volume of wet sediment and Lycopodium-tablets were added (Stockmarr, 1971, 1973) to calculate the number of pollen per cm3 and to construct pollen influx diagrams (Birks and Birks, 1980). The preparation procedure was as follows: (1) HF-treatment; (a) Torvlomyra and Saudedalsmyra: plastic bottles standing for 3-4 days in warm sand with shaking once each day, (b) Lerstadvatn: hot HF for 10 min, then cold HF for two days, (2) disaggregation with Na,P20, (Bates et al., 1978) (3) acetolysis (Fzgri and Iversen, 1975) and (4) KOH-treatment and staining, usually in this order. Heavy-liquid separation has not been used because we found that numerous pollen and spores sunk with the minerogenic particles. reducing the number of polien per cm3 even though the relative frequencies of the taxa do not seem to be changed (Bjork et al., 1978: Larsen et al., 1984). Pollen diagrams

Grain size analysis Grain size analysis was carried out where the loss on ignition was below 10%. Only sieving has been done. Accordingly clay and silt are not separated. Organic matter was first removed with H,O,. Loss on ignition and organic carbon Loss on ignition is often used for estimating the organic content in sediments, although there are some disadvantages to this method, especially in clay-rich sediments. Loss on ignition, using 550°C. has been carried out on

Each pollen sample is given a number corresponding to its depth in cm. Apart from the lowermost samples in each diagram, the pollen sum (ZP) is mostly between 300 and 500. This pollen sum, used for calculating the percentages, includes all pollen except the limnophytes and Menyanthes. There are some minor differences in the method of calculation in the relative diagrams. In Torvlomyra (see Fig.19) and Saudedalsmyra (see Fig.20) the palynomorphs which are not included in CP (e.g. Equisetum) are calculated as percent of CP + the actual palynomorph (Equisetum). In Lerstadvatn (see Fig.17) these percentages are

190

based on CP + C of the palynomorph group (e.g. Pteridophyta). In the diagrams from Torvlomyra and Saudedalsmyra, + is used when the palynomorph type does not exceed 0.5%, whereas for Lerstadvatn + indicates observations of palynomorphs during scanning more of the slides after the ordinary analysis. For phytoplankton and dinoflagellate cysts + is used in the Lerstadvatn diagram for values less than 0.2%. The pollen diagram from Lerstadvatn (see Fig.17) is composed from two cores, 502-3001/2a (lower part) and 502-30-03 (upper part), where the first one is from the bog and the latter from the lake. This was done because the Late Weichselian sequence appeared to be best developed in the bog basin, while the Holocene was best in the lake. The cores are easily correlated by means of the Vedde Ash Bed (10,600f60 yr B.P., Mangerud et al., 1984). The diatomite silt above the Vedde Ash is analyzed in both cores. We constructed pollen influx diagrams for all three basins. However, the sedimentation rates in Torvlomyra and Saudedalsmyra changed very fast, mainly due to late isolation from the sea, but also because of the Younger Dryas climatic changes and the small size of the basins. These two diagrams are therefore rarely used in the interpretation, and only the influx diagram for Lerstadvatn (see Fig.18) is included. We have not tried to calculate the total influx of pollen into the basins (Davis et al., 1984). Pollen and spore identification The analyses were carried out by means of a Zeiss microscope with oil immersion phase contrast objectives with 320%1000 times magnification. The pollen identifications are based mainly on Fmgri and Iversen (1975) and the pollen herbarium at the Department of Botany, University of Bergen. Moe (1974) and Eide (1981) have been used to identify the trilete Pteridophyte spores and Rosaceae pollen respectively. Our Potentilla type includes Potentilla spp.,

Sibbaldia, Comarum, and Fragaria. In addition to Anemone sp. the Anemone type seems to include Ranunculus glacialis and R. nivalis among others. Initially, Lycopodium inundatum spores were identified (Lie and Lomo, 1981). Recent samples of L. selago, show that this spore type has an extremely great variability, from the typical L. selago type (Moe, 1974) to spherical, L. inundatum like types. The latter lacks the trilete mark and is probably not a fully developed spore. The “L. inundatum” spores in the fossil material also lack the trilete mark, and they may be not fully developed L. selago spores. According to Fmgri and Iversen (1975) it is possible to distinguish Salk herbacea from the other Salk species (see also Faegri, 1953 and Fredskild, 1973). The most important criterion is “knot-like thickenings” of the muri (columellae) in S. herbacea. Our experience is, however, that corrosion of pollen grains from other Salk species in some cases may result in a similar appearance. Because the Salk pollen in our material were usually corroded and crumpled, we have not distinguished S. herbatea. Measurements

of Betula pollen

The main problem of separating the pollen of different Betula species by size measurements is the different swelling or contraction of pollen grains in sediments and during preparation (Berglund and Digerfeldt, 1970; Kristiansen, 1979; Prentice, 1981). We have attempted to solve this problem by regarding the total populations as consisting of sub-populations with normal distributions (Usinger, 1975; Andersen, 1980; Prentice, 1981) instead of using absolute size (Eneroth, 1951). Birks (1968) found that B.pubescens and B. nana could be distinguished by the ratio grain diameter/pore depth (D/P. see Fig.4). Theoretically, this parameter should not be affected by changes in the absolute dimensions of the pollen grain. In Fig.5 we have plotted the distribution curves for the D/P ratios and for

191

D

I

Fig.4. Pollen grain of Betula. D=diameter,

P=pore

depth.

the grain diameters for one sample processed in four different ways. Sample A is only acetolyzed, while B, C and D are acetolyzed and treated with HF in different ways. The four grain-size distribution curves are quite different. In cases B and D shrinkage is observed compared to sample A. The size range is extended in the two samples treated with HF after acetolysis (B and C). Treatment with HF before acetolysis (D) has caused shrinkage, but hardly any extension of the size range. Fredskild (1975) found that cold 40% HF overnight after acetolysis (most similar to C in our case) hardly causes any shrinkage of the pollen. The changes in the D/P ratios are small

Ratio

Grain _~

diameter A-~.L

(Fig.@, and all four distribution curves would be assigned to almost only B. pubescens according to Birks (1968) and Van Leeuwaarden (1982). It thus seems that the D/P ratio is a better method for distinguishing the Bet&a species if the preparation methods have varied. The grain diameter and the pore depth (Fig.4) were measured in eleven fossil samples from Lerstadvatn. The size units of the measurements were 0.82 pm, (diameter) and 0.41 pm (pore). All triporate, undamaged Betula grains with at least one diameter lying horizontally were measured. Many grains were damaged but at least 30% were measured in each sample. Unfortunately processing with different HF-treatments was used, and therefore only the distribution curves for the DIP ratios are used to distinguish the Betula species. The DIP ratio distribution curves have greater standard deviations for the Late Weichselian samples than for the Holocene (Fig.6). This is due to a larger amount of grains with DIP ratio larger than 9. The D/P ratios for the different Betula species are not well established, but the majority of those with ratios above 9 are probably B. nana (Birks, 1968; Van Leeuwarden, 1982) even though

Grar

diameter

Fig.5. Grain diameter/pore depth (D/P) ratios and grain diameters for Bet& pollen at 925.5 cm in Lerstadvatn with varying HF-treatment. A total of 75 grains were measured in each sample. A: no HF.treatment. B: HF after acetolysis. Warm HF for c. 20 min. C: HF after acetolysis. Warm HF for c. 10 min, then cold HF for two days. D: HF before acetolysis, otherwise identical with C.

Fig.6. Grain diameter/pore depth (D/P) ratios for Betula pollen in Lerstadvatn. Note that the samples are from two different cores The oldest samples, with depths between 500 and 610 cm, are from the bog basin. The rest are from the lake basin. Compare Fig.17.

Birks (1968) found that B.pubescens subsp. tortuosa Nyman has D/P ratios between B. nana and B.pubescens. If this interpretation is correct, B. nana was most frequent in the oldest Allerod sample and during the Younger Dryas. B. nana has hardly contributed to the Holocene curves. The high-ratio tail on the curve of the Boreal sample (900 cm) may be assigned to the appearance of B. uerrucosa which has a DIP ratio slightly greater than B.pubescens (c. 8.5 according to Van Leeuwaarden, 1982). Diatoms Diat.om analysis has been carried out et al. (1983) to determine the isolation basins from the sea. The distribution salinity groups of diatoms are shown relative pollen diagrams. Radiocarbon

by of of in

Lie the the the

dates

The radiocarbon datings (Table I) were carried out at the Radiological Dating Labora-

tory, Trondheim under the supervision of Reidar Nydal and Steinar Gulliksen. The dated material is mainly lacustrine gyttja, and most of the dates were carried out on the NaOH-soluble fraction only (A after the lab no. in Table I). The insoluble fraction (B) was dated on one sample from each basin. In all cases except Lerstadvatn (T-4161) there were extremely good agreements between the soluble and insoluble fractions. The insoluble fraction yielded the youngest age in Lerstadvatn, which is in accordance with the result of Kaland et al. (1984). The total sample (all organic matter) was dated from the Vedde Ash Bed and in one sample from Krdkenes (T-2532). For two samples (T-3956C and T-4116C) from Saudedalsmyra, marine shells were dated. They are corrected for a reservoir age of 440 years (Mangerud and Gulliksen, 1975) and should be directly comparable with the lacustrine gyttja dates. In addition, three samples of marine gyttja were dated from Saudedalsmyra (Table I, T3956 A/B and T-3951A). The composition of the

T-3953 A T-3987 A

T-4586 A

T-2533 T-2533 T-3952 T-4585

T-3959 T-3960 T-3961 T-3962 T-4381

T-2532 T-3954 A T-3955 A

T-3958 A

Pinus rise

Juniperus decline

Base of Hatlen Formation = Bet&a rise

Vedde Ash Bed

Top of Ase Member

Isolation

T-4160 A

T-3951 A

T-3957 A

T-4116 C

Pollenzone boundary Al l/Al 2

Saudedalsmyra 706710 cm

Isolation Lerstadsvatn

Base of form. B Saudedalsmvra

12,650_+ 230

12,050 * 110

T-3553 T-3553 T-3956 T-3956 T-3956

Isolation Torvlovmyra

A B A B C

11,640+ 170 11,570& 110

T-4158 A T-4159 A

Bet&a decline Small Bet& rise

11,130* 150

10.680 + 80

10.340 + 110

9400+ 100

9010 + 100

10,860 * 140 10,330+290

- 26,O

- 28,4 ~ 27,4

- 24.6 - 24.0

- 28,7

T-4161 A T-4161 B

ll,lOOf80

a 10,230+ 300 b 10,870 k 340

10,060 + 100 10,030 * 90

8340 f 130

B.P.

“%yr

6’“C

‘%-yr

B.P.

Lerstadvatn

Krakenes

Bet&a rise in Late Allerod

Saudedalsmyra

T-3988 A

Corylus rise

A B A A

Lab no.

Event

~ 20,6

- 20,4

- 20,3 - 19,5

-21,7 - 17,7

- 20,3

- 26.9

- 25.0

- 27,5

- 28.3

6’3C

11,780+80 11,750* 120

- 23,8 ~ 22,9

- 253

- 27,l - 25,8

a 9170*90 b 10,640 f 70

11,340f 120

- 28,4

61°C

10,430 f 110

“+C-yr B.P.

Torvlomyra

12,310+ 140

*12,8OOi 100

*12.320 + 120 *12;350 z 140 11,960*90

11,130~140

8830+ 130

I‘%-yr B.P.

Saudedalsmyra

+ l,o

- 21.6

-- 21.2 ~ 20;8 + 1,o

~ 23,9

- 29.4

fi’3C

12,300

12,40012,500

12,000

11.900

11,600

11,150

11,000

10,600

10,200 10,300

8900

‘Y-yr

B.P.

Assumed age

Radiocarbon dates from Krikenes (Larsen and Mangerup! 1981) and from the basins presented in this paper. The last column (assumed age) gives our estimated age for the events, based on all the given dates, and in addition on several other shell dates on the deglaciation and for the Vedde Ash Bed (Mangerud et al., 1984). Note that for the assumed age, corrections are also made for the thickness of the samples where they are collected below or above the boundary. Suffix A after laboratory number indicates that the NaOH-soluble fraction is dated; Suffix B, that the insoluble fraction is; and no suffix that all organic material is used. Suffix C indicates dates of marine shells, and these are corrected for the reservoir age (Mangerud and Gulliksen, 1975). The three dates from Saudedalsmyra marked with stars are from organic matter in the marine silts, not corrected for any possible reservoir age. The other dates are on organic matter in lacustrine sediments. For the Vedde ash, a. indicates a sample just above the ash, b. just below, and no letter equal amount of sediment from above and below the ash. 13C is given in the PDB scale. All ages are conventional radiocarbon years.

TABLE I

g

194

marine gyttja has not been examined in detail, but it is a mixture of marine and terrestrial material, as indicated by the 613C values. The gyttja dates should have been corrected for a reservoir age corresponding to their content of marine components. The obtained difference of 375 years between T-3956 A/B and shell fragments from the same level (T3956 C) might be the reservoir age, but the difference appears slightly too large. The third date on marine gyttja (T-3951A) seems to be approximately 500 years too old. The conclusion is that the marine gyttjas in this case gave slightly too high ages, even if they are corrected for a reservoir age, indicating that they contain some redeposited carbon. The dates on lacustrine gyttja and shells are very consistent, and only two samples are rejected because they gave ages deviating more than two standard deviations from ages derived from other samples. In both cases (T3959, Torvlomyra, and T-4161 A and B, Lerstadvatn) the ages were considerably younger than stratigraphically higher samples from the same cores. The reason for the erroneous dates is not known but younger roots or simply mixing of samples (Kaland et al., 1984) may be alternative possibilities.

There are generally several dates of the important events, and the “assumed age” (last column, Table I) is based on an evaluation of all dates, sometimes also from dates above or below the level in question. Correction for the thickness of the samples is included when the sample is collected above or below the dated boundary. The “assumed ages” are used for calculation of the sedimentation rates and the pollen influx, and some of them are commented on below, from the oldest upwards. Lerstadvatn was isolated from the sea soon after the deglaciation, and therefore the lowermost date (T-3957A) should be consistent with other dates of that event. As mentioned before, we have obtained many dates suggesting that deglaciation the took place between 12,500-12,300, probably 12,400-12,300 yr B.P., or approximately one standard deviation younger than the Lerstadvatn date 12,650+230 yr B.P. (T-3957A). For the Ase member in Lerstadvatn, we used a constant sedimentation rate curve (Fig.7), based on an age of 12,500 years for the isolation. The lowermost date in Saudedalsmyra, 12,310 f 140 yr B.P. (T-4116C), is from the base of the marine formation B, and it should be contemporaneous or slightly younger than the

fr

T-4158A

T-4159A +\

\T-4160A

\

Fig.7. The radiocarbon constant sedimentation

dates of the Ase Member rate curve is shown.

in Lerstadvatn

against

T-3957A

depth. Dates with one standard

deviation.

A

195

date discussed above from Lerstadvatn, since Lerstadvatn was isolated from the sea before the deposition of formation B started. The dates are indeed compatible. The dates just above the isolation contact in Torvlomyra yielded 11,780 +80 and 11,750 + 120 yr B.P. (T-3553 A and B). Immediately above the isolation there is a distinct maximum of the fresh water green alga Tetraedron minimum. Apparently this alga was transported out of Torvlomyra by the brook, and the level can be identified as a peak of T. minimum at 600 cm depth in the marine sediment in Saudedalsmyra. The date of marine shells just above this level gave 11,960+90 yr B.P. (T-3956C). Our conclusion is that the age of the isolation of Torvlemyra is 11,900 yr B.P. We have seven dates from the Ase Member in Lerstadvatn (Fig.7), but as mentioned before, samples T-4161 A and B are rejected. The dates T-4158 A and T-4159 A are slightly reversed, but overlap completely within one standard deviation. We therefore use the mean age, 11,600, for the midpoint between them. The top of the Ase member is dated more or less directly in all basins. Saudedalsmyra was isolated from the sea during upper Ase, and we accept the date 11,150 yr B.P. (T-3958A, Table I) for this isolation contact. Depending on the sedimentation rate, this date suggests an age of around 11,000 for the top of the Ase. The sample from Torvlomyra gave 11,340f120 yr. The sedimentation rate curve for Lerstadvatn (Fig.7) suggests an age of 11,100 for the top of Ase, when the midpoint of the uppermost sample is used. Similar calculations (Larsen and Mangerud, 1981) gave 10,900, for Krakenes. These estimates indicate an age very close to 11,000 yr B.P. for the boundary between the Ase and Leirstad Members. The age given for the Vedde Ash Bed, 10,600f50 yr B.P., is from Mangerud et al. (1984). There are several dates available for the base of the Hatlen Formation. Taking sample thickness into account, the dates for Torvlermyra and Lerstadvatn suggest an age of 10,400-10,500 yr B.P., while the dates from

Krakenes suggest an age of 10,100~10,200 yr B.P. The discrepancy could be due to real age differences, due to delayed melting of the cirque glacier at Krakenes. We find it, howis ever, more likely that the discrepancy mainly due to the spread of the datings, caused by contamination, sample thickness, etc. and precision of the measurements. Also taking into account the weighted mean age of 10,600+ 50 yr B.P. for the Vedde Ash Bed, we will at present assume an age of 10,200-10,300 yr B.P. for the Leirstad/Hatlen boundary, stressing that a more precise age is desirable for this important boundary. For the calculations of sedimentation rates and pollen influx, 10,200 is used in Fig.18. Lithostratigraphy, macrofossils, descriptions of the basins Lithostratigraphical considerations

and

units; general

The sediment sequence is very similar in all cored basins, the main difference being that the highest lakes have more lacustrine beds, and the lowest lakes more marine beds. However, all beds fit into a composite sequence (Fig.8). The entire sequence is subdivided into formations and members. We have found it most useful to define informal formations (Hedberg, 1976, p.35) designated with letters for the marine sequence, because the subdivision used here possibly not will be the most useful in the future. For the lacustrine sequence we have defined formal stratigraphic units, because in many lakes and palaeolakes in Western Norway, and also other parts of NW-Europe there is a very consistent sequence of sediments from the Late Weichselian/Holocene. Changes in pollen composition nearly correspond with the lithological changes, although the boundaries do not always coincide. Mainly based on this lithoand pollen stratigraphy, in combination with radiocarbon dating, chronostratigraphical subdivisions are defined, e.g. the Boiling, Older Dryas, Allerod, Younger Dryas chrono-

196

r

.- ! 5 I 2 0 ~ 1 ._ a,

5 0 2 -I

Dryas

- 4 z

Allerad

Older

F

m

Uryas

Balling

L:

___._,

Glacial sediments or bedrock

Fig.8. A schematic presentation of the stratigraphic relationships between the lithostratigraphic units. Note that the vertical scale is time, and not thickness of the units. Sloping lines indicate time-transgressive boundaries. The boundary between the marine and lacustrine sediments is determined by emergence; lakes at high elevations (e.g. Lerstadvatn) have more lacustrine sediments than lakes at lower elevations (e.g. Gjelvatn). The Mehuken Member and formation A are related to the ice-front, and their boundaries are therefore diachronous, depending on the deglaciation. The stippled lines indicate schematically the sequences in the three basins described in this paper, and Krakenesvatn (Larsen et al., 1984) and Gjolvatn (Mangerud et al., 1984).

zones in Nor-den (Mangerud et al., 1974) and climatostratigraphical subdivisions, e.g. the Windermere Interstadial, Loch Lomond Stadial in Britain (Coope and Pennington, 1977) and, proposed for entire NW-Europe: the Lateglacial Interstadial, Younger Dryas Stadial (Lowe and Gray, 1980). However, useful as chronostratigraphical and climatostratigraphical units are, they are inferred units, the boundaries sometimes interpreted to coincide with lithostratigraphical - in other cases with biostratigraphical boundaries, or, often with none of them. We realize that the introduction of a formal lithostratigraphical subdivision produces more names, but with the present resolution of the Late Weichselian stratigraphy we nevertheless believe it to be useful because it is more precise for our discussion. Sediments deposited in a lake are restricted to that body of water. A bed of e.g. lacustrine gyttja can never be mapped physically from one lake to another. However, due to parallel

development, the sediment sequences in many Western Norwegian lakes are so similar that it is practical to extend the formally defined units from one lake to hundreds of others. We introduce two formally defined formations and several members for this area. In our opinion the units may be extended to the rest of Western Norway, and also to lakes in other parts of Northern Europe.

Formation A This informal formation comprises the lowermost marine sediments, the lower boundary normally being to bedrock or glacial sediments. The formation consists mainly of grey silt, with beds of silty sand and clayey silt. The loss on ignition is < 3%, the content of carbon < 1%. Macrofossils are seldom found. The formation is interpreted to be of glaciomarine or cold marine origin, and older than 12,300 yr B.P. It has been described by Lie et al. (1983).

197

Formation B The formation consists of brownish grey to grey gyttja silt. The main difference from formation A is the higher organic content and the brownish colour. The loss on ignition is 4412%). The formation is generally homogenous, but frequently well defined red, redbrown and green lamina occur in the upper (brackish) part. Fragments of marine shells are frequent, except in the upper part. Plant macrofossils occur. The sediments are marine, and, in the top, brackish. The lower boundary of the formation is timetransgressive due to the deglaciation, generally becoming younger from the coast towards the east. The upper boundary is extremely time-transgressive due to emergence, becoming younger from the high to the low-level lakes. The formation has previously been described by Lie et al. (1983). Mangerud et al. (1984, fig.5) combined formation A and B into “marine sediments, mainly silt”. The Lange&g Formation Langevag is the name of the small town west of Torvlomyra and Saudedalsmyra. The Langevag Formation consists mainly of clay/silt with a varying but low content of organic matter. The loss on ignition rarely exceeds 40%, and is generally much lower. The Formation may include more sandy facies. All sediments are of lacustrine origin. The lower boundary stratotype is the lake Krdkenesvatn (Larsen et al., 1984), and is identical to the lower boundary of the Mehuken Member, which is a sharp contact with glacial sediments or bedrock there. In the three basins described in this paper, the Mehuken Member is missing (Figs.10, 11, 14 and 15). The stratotype for the Langevag Formation above the Mehuken Member, and the upper boundary-stratotype is the lake Lerstadvatn. The upper boundary is defined by the sharp increase of organic matter to the overlying Hatlen Formation. The Langevag Formation is subdivided into three formal members (Fig.8): Mehuken, Ase and Leirstad. The Mehuken Member is de-

scribed by Larsen et al. (1984), the two others are described below. The Ase Member Ase is the name of the area surrounding Lerstadvatn. The Member consists mainly of silty gyttja, but includes less organic beds. The unit-stratotype is Lerstadvatn, except that the lower boundary is defined at Krakenes (Larsen et al., 1984). The main characteristic is its much higher organic content than the underlying (and directly overlying) members. The loss on ignition varies in the three described basins between 15 and 40%, highest in the upper part. The colour varies between greyish, greenish and brownish, the brownish colours dominate. Beds with abundant plant remains occur. The Ase Member is entirely lacustrine. In the basins described in this paper, the Ase directly overlies the marine formations A and B, the boundary normally being a smooth transition. Due to the different time of emergence from the sea, the age of the lower boundary is very different in the three basins (Fig.8). The Leirstad Member Leirstad is the name of the farm from which the type locality Lerstadvatn got its name. It may be mentioned that “leir” in Norwegian means clay. The Member consists mainly of silt and diatom frustules, with a varying content of clay and organic matter. At the type locality and the surrounding basins, the diatom frustules are so frequent that the sediment is best described as a diatomite silt, but the Member is defined to include also more minerogenic facies as shown by the correlation with Krakenes (Larsen et al., 1984). The colour is pale grey to yellowish grey. The main difference from the underlying and overlying members is the lower organic content. Loss on ignition is usually less than 10%. The boundaries are easily recognized through changing colours, even though there are frequently transitions covering some few cm. The Vedde Ash Bed with an age of 10,600+60 yr B.P. (Mangerud et al., 1984) is found near the midpoint of the Leirstad

198

Member. Plant macrofossils, mainly mosses, are found through the Member, frequently in some places. The Leirstad is entirely lacustrine, and of Younger Dryas age. The boundaries are probably slightly time-transgressive over long distances, but isochronous within small areas. The Hatlen Formation Hatlen is the name of the settlement SW of Lerstadvatn. The name is derived from a local name for hazel (Corylus). The Hatlen Formation consists in the lower part of a light brown fine detritus gyttja. Upwards the gyttja usually becomes darker and coarser and the dy content increases. The boundary-stratotype for the lower boundary is Lerstadvatn. In lakes the upper boundary is the sediment/water interface. In bogs the formation is overlain by peat, commonly with a very gradual transition between the two sediment types. The Hatlen Formation is of lacustrine origin and mainly of Holocene age. The lowermost part is probably of Younger Dryas age, when using the definition of Mangerud et al. (1974). The strata designated the Hatlen Formation is commonly called Holocene gyttja, brown gyttja, or simply gyttja. Mangerud (1970) introduced the name Blomoy Gyttja Member for corresponding beds, but as he used the name Blomoy also for a lower member we propose to abandon those names. Descriptions

of the basins

Torvkmyra basin The two bogs Torvlomyra and Saudedalsmyra are situated on the undulating strand-flat (lo-50 m a.s.1.) on the NE side of the island Sula (Fig.2). South of the bogs is a mountain reaching 776 m a.s.1. The area around the two bogs is characterized by small hills and ridges up to lo-15 m above the bog surfaces and with small lake and bog basins in between. The vegetation is mainly open pine forest. The bedrock is granodioritic gneiss (Gjelsvik, 1951) and there are only small amounts of Quaternary sediment in the neighbourhood.

Saudedalsmyra

1OOm

Gg.9. A. Detailed map of Saudedalsmyra and Torvlemyra. Contour interval 5 m. For location see Fig.2. B. Reconstruction on a larger scale of the lakes that during the Younger Dryas occupied parts of the present day bogs Torvlemyra and Saudedalsmyra. Crosses mark the coring points shown in Fig.10 and 11. Dots mark other cored points. (In Saudedalsmyra the coring points are shown in Fig.11 connected with a line.)

Both Torvlemyra and Saudedalsmyra are small bogs, just greater than 100 m across, and they are only 50 m apart (Fig.9). The elevation is 35 and 30 m a.s.l., respectively. The inlet to Torvlomyra is a small brook coming from the mountain slope in the south (Fig.9). From Torvlomyra the brook runs through Saudedalsmyra. The most important species in the poor oligotrophic vegetation on the two bogs are Calluna vulgaris, Erica tetralir, Eriophorum spp., Scirpus caespitosus and Narthecium ossifragum. Some small pines are also found. The southern part of Torvlomyra is very shallow, and a continuous Late Weichselian and Early Holocene sequence was found only in the northeastern part of the bog (Figs.9 and 10). The analyzed core is from the deepest part of the basin where the thickness of the Late Weichselian organic sediments was greatest.

I

10

I

I

20

I

30

I

I

40

I

I

50

m

Fig.10. Cross section from Torvlamyra. Depth below bog surface, distance in meters from brook outlet in west. Legend as for Fig.11. The analyzed core shown in Fig.19 was taken near point T-3. That core was taken with different. core equipment at a different time and the measured depths were slightly different.

The lithostratigraphy is as follows, all depths given for the analysed core (see Fig.19): Formation B (750-702 cm) is a marine gyttja silt. The lowermost 28 cm is more sandy, also containing some gravel. Plant remains are more abundant than in Saudedalsmyra, and occur throughout the sequence. The upper 5 cm (brackish) consists of well defined reddish, brownish and green lamina. The Ase Member (702-648 cm) consists of a massive lacustrine silty gyttja, wedging out towards the margins of the basin (Fig.10). A leaf of Salix cf. polaris was found at 674 cm. The Leirstad Member (648-617 cm). The Vedde Ash Bed (Mangerud et al., 1984) is here faintly laminated and constitutes a large part of the member. The thickness of the ash bed is 23 cm near the brook inlet, and decreases to 5 cm only 25 m further east (Fig.10). The remaining part of the Leirstad is a diatomite-silt showing a quite different thickness variation. The diatomite-silt (subtracted the ash) is 16 cm thick in the deepest part of the basin and increases to more than 25 cm towards east and west. This demonstrates that different processes governed the deposition of

ESE *

f$j

Hatlen Formation

E

~~r,S:a,“,,“,e~~~r&d

Q

he

Member

q Formation cl

B

Sand I 30

I ‘lo

m

Fig.11. Cross section from Saudedalsmyra (see Fig.9). Depth below bog surface. The analyzed core shown in Fig.20 was taken near point S-8. That core was taken with different core equipment at a different time and the measured depths were slightly different.

the diatomite-silt and the ash bed. The ash was deposited as a wedge from the brook (Mangerud et al., 1984), while a considerable part of the diatomite-silt was produced in the watercolumn, with diatom frustules “raining” to the bottom. The thinning of the diatomite-silt in the deeper part is, however, difficult to understand, as depth differences are too small to encounter for a considerably differentiated dissolution. The Hatlen Formation (6177560 cm) is in the lowermost part a light brown gyttja with a gradual transition to darker brown and coarser gyttja further upwards. Saudedalsmyra basin In Saudedalsmyra we cored two profiles, one in the W-E direction and one in the N-S

200

direction (Figs.9 and 11). The analyzed core (see Fig.20) was taken from the deepest part near the centre of the basin. The lithostratigraphy is as follows, all depths given for the analysed core (see Fig.20): Formation A (828-764 cm) is a sandy silt with highest sand content below 820 cm. The upper part (7809764 cm) is mainly silt (see Fig.20). There are some pebbles in the uppermost part where the sediment is poorly sorted. Some plant remains and shell fragments have also been found near the top. The sediment is generally grey, whereas the lowermost part has a more blue-grey colour. We did not penetrate into formation A with the Russian sampler, and it is therefore not shown in Fig.11. Formation B (7644519 cm) consists of a relatively homogenous brownish grey gyttja silt. A lighter grey zone in the lower part is found in all cores. At 712 cm there is a 1 cm thick sand bed and the sand content also increases in the uppermost part of the formation. Plant remains are found throughout the entire formation, but in variable amounts. The moss Rhacomitrium lanuginosum has been identified (6299561 cm). Shell fragments are abundant up to 541 cm, with enrichment at 7644740 cm, 712 cm, and 661C629 cm. Mytilus edulis, Littorina littorea and Ralanus balanoides are identified. Two bones from 729 and 708 cm are identified as cod (Gadus sp.) (Rolf Lie, det.) A 1 cm thick reddishbrown brackish laminated zone at the top of the formation is only recognized in the analyzed core. Note that in the present paper the upper boundary of formation B is moved upwards compared to that of Lie et al. (1983) as the brackish gyttja is now included in formation B. The Ase Member (519-511 cm) is only a thin bed of silty gyttja, due to the late isolation from the sea. Equisetum sp. is most important among the plant macrofossils. The Leirstad Member (511-500 cm) consists of a diatomite silt with relatively high loss on ignition, lo-12'/,. Lerstadvatn basin The lake Lerstadvatn is situated 44 m a.s.1. and approximately 6 km NE of Torvlomyra and

Saudedalsmyra (Fig.2). The basin lies near the marine limit for the area (Reite, 1967). The topography around the lake is undulating (Fig.3) with heights up to 130 m a.s.1. The bedrock is gneiss containing some carbonate (Gjelsvik, 1951).A considerable part of the surface is covered with Quaternary sediments, mainly till, favouring a richer vegetation compared to the area around Torvlomyra and Saudedalsmyra. There is however, very little natural vegetation left in this area, only seven km from the city of Alesund, but there are remnants of pine and birch forests. The basin is expected to represent the regional pollen rain quite well as its situation is well protected from the coast, and at some distance from steep mountain slopes. The watershed is only five time the size of the lake, while this ratio for Torvlomyra is 65 (Mangerud et al., 1984, fig.7). The lake is extremely dystrophic at present, probably mainly because of the supply of humus from the farms which until recently were active around the lake. At present there are mainly urban areas. In the summer nearly the whole lake is covered by floating leaves of Nymphaea sp. and Potamogeton sp. The bog at the western end (Fig.12) covers an earlier extension of the lake filled in with lacustrine sediments with peat on top. Beneath the southern part of the bog there is an isolated basin (the bog basin, Fig.13, called shallow basin in Mangerud et al., 1984), separated from the lake basin by a shallow sill. The larger parts of the bog are covered by an oligotrophic mat vegetation with some heather; Calluna vulgaris, Erica tetralix and Myrica gale. Molinia caerulea, Eriophorum angustifolium, E. vaginatum, Scirpus caespitosus, Carex rostrata, Potentilla erecta, and Narthecium ossifragum are the main grasses and herbs. Sphagnum spp., Hylocomium splendens and Pleurozium schreberi, are important in the ground layer (I. Rossberg, pers. comm., 1982). Along the northwestern and western margins of the bog there are also more base-demanding vegetation types with Carex hostiana, Eriophorum latifolium, Drepanocladus revolvens, and Scorpidium scorpioides as important species.

201

Fig.12 A map of the lake Lerstadvatn and its surroundings. For location, see Fig.2. The crosses mark coring points. The different coring points are given letters in the lake. On the bog in the western end the coring points are numbered, and the two profiles are named A and B.

Fig.13. Composite E--W profile of the bog (profile A) and the lake basins at Lerstadvatn. Depth below bog are given on the vertical scale,, and distance in meters from eastern shore on the horizontal. The figures the top of the lake profile show the water depth in meters. Note the break in the horizontal scale; coring basin is situated 50 m south of point E in the lake basin (see Fig.12). The analyzed cores shown in Figs.17 near point C in the lake basin and between point 1 and 6 in the bog basin.

and water surface in parentheses at point 5 in the bog and 18 were taken

202

-

Lelrstad

u

wtth

Member

Vedde

Ash

’

Bed

AseMember

B

w 700 i

0 0 w

I

4 beds

Bed

A,

Gravel

j

I A

Bed

L

I

540

with

I

500

450 m

Fig.14 E-W profile (profile A in Fig.12) from the bog basin at Lerstadvatn, showing details of the Late Weichselian sequence. Depth below bog surface is given on the vertical scale, and distance from the eastern shore on the horizontal. The analyzed core shown in Fig.17 and 18 was taken from the deepest part; between point 1 and 6 and just north of the profile (see also Fig.16).

cmlll10

9

8

1

14

13

12

600-

J 20

4b

50

6b

70

Fig.15. N 3 profile (profile B in Fig.12) from the bog basin at Lerstadvatn. southern border of the present bog.

9’0

100

110

Depth below bog surface,

I”

distance

from the

203

B ! 368.5 4

x622.5

-I

*

c---(

I

Depths the

I” cm for Ase

the

Member

base

of

lsopach

map

for

Thickness

the

Ase Member I” cm.

lsopach

map

for

the

Lerstad

Thickness

I” cm

Member

Fig.16. A paleobathymetric map for the bog basin at Lerstadvatn at the time it was isolated from the sea (base of the Ase Member), and isopach maps for the Ase and Leirstad members. Profile A and B (Fig.12) are marked with lines with crosses for the coring points. The maps are based on 20 cores.

The area of the bog and the lake together is approximately 0.16 km2. The lake basin is much deeper than the bog basin (Fig.13). The formation A is subdivided into two beds, A, and A, (Figs.14 and 15) mainly based on the difference in organic carbon (Fig.17) resulting in a slight difference in colour. The lower bed A, (718-660 cm, Fig.17) is light grey. Loss on ignition is c. 0.5% and organic carbon content 0.1%. Thin gravel beds and sandy beds are abundant. There are relatively great variations in the grain size both vertically and laterally. The transition to bed A, is usually marked with a gravel bed one to a few cm thick. The upper bed A, (660-635 cm) is grey or faintly brownish grey. Loss on ignition is 3% and organic carbon content c. 1%. The bed is from 7.5 to 42.5 cm thick, with the greatest thicknesses in the deepest part of the basins. A leaf of Salk herbacea and a bone of cod (Gadus morrhua L.) (Rolf Lie, det.) were found in bed A,. The upper boundary is smooth but distinct. Lamination at this boundary is only recorded in the 110 mm core from the lake basin (502-30-03). The formation B is missing in Lerstadvatn because the sill of the lake emerged from the sea before the commencement of sedimentation of member B (Fig.8).

The Ase Member (6355565 cm) consists of a silty gyttja with loss on ignition 15-37%, and organic carbon content 5515%. The thickness varies from 5-79 cm in the bog basin with the greatest thicknesses in the deepest parts (Figs.15 and 16). The Member is distinctly thinner in the lake basin. In the bog basin we could distinguish four beds within the Ase Member, informally labelled &e-l to Ase-4, mainly based on variation in colour, organic carbon and macroscopic plant remains. Ase-1 (635-606 cm) is greyish brown, and relatively light. The content of organic carbon is 7-9%. It is more silty than the rest of the Member. The thickness varies from lo-20 cm. The colour gets darker and more brownish upwards. Plant remains occur in the upper part; a leaf of S. herbacea has been found. Usually, there is a distinct upper boundary to Ase-2 which has a pale lower part. The bed Ase1 was identified in all the corings in the bog, in contrast to the overlying beds (Fig.14). Ase-2 (606-585 cm) is greenish brown and has a higher organic content. Plant remains are abundant and often found in distinct beds. Ase2 is found up to 50 cm thick, but there is great variation in thickness. Ase-3 (585-573.5 cm) is light, brownish green.

204

The content of organic carbon is c. 5% lower than in Ase-2. Macroscopic plants remains are almost absent. Ase-4 (573.5-565 cm) is brown, with lots of plant remains, mainly in the lowermost part. Organic carbon content increases to c. 5%. It is up to 10 cm thick, but usually thinner. The Leirstad Member (565-553.5 cm) consists of diatomite-silt with 2-3% organic carbon. The Vedde Ash is one cm thick in the bog basin and somewhat thicker (2-3 cm) in the lake basin (Fig.17). Remains of mosses are usually found both below, in, and above the ash bed; five species have been identified in a core from the bog basin (502-30-07): Drepanocladus sp., D. exannulatus (Bruch, Schimper, Guembel) Warnstorf, Bryum sp., Hygrohypnum ochraceum (Wils) Loeske, and Calliergon trifarium (Web and Mohr) Kindberg (Hans H. Blom, det.). The diatomite-silt is usually more yellowish above than below the ash. The Leirstad Member is from a few up to 20 cm thick. In the bog basin, the Leirstad is thickest east of the maximum thickness of the Ase Member (Fig.lG), partly because the position of the maximum depth of the lake was displaced by the accumulation of the Ase. The influx of minerogenic particles does not seem to have been much higher in the Leirstad than in the Ase. The Hatlen Formation is nearly 7 m thick at the coring point in the lake basin. The lowermost meter is a light brown gyttja with finely dispersed plant remains. Going upwards, the sediment gets gradually darker brown and the dy content increases. The upper 1.5 m is a very loose dy. Genesis of the sediments The fossil content clearly demonstrates the marine origin of the formations A and B. The marine limit in this area is approximately 45 m above sea level, and the topographically lowest beds we have described are 23 m a.s.1. in Saudedalsmyra. The maximum water depth during deposition therefore was 22 m. In all basins the water depth was gradually decreas-

ing during deposition of formations A and B, due to isostatic uplift (Lie et al., 1983). Fine grained marine sediments on land in this area are not found outside the local depressions (basins). This might be due to later erosion. However, we find it more probable that waves and tidal currents primarily transported the fines into local basins which acted as sediment-traps on the shallow sea-floor. This explains the high sedimentation rates (l-2 mm per year) found in formations A and B in Torvlemyra and Saudedalsmyra. The main difference between the two marine formations is the content of organic carbon, resulting from the balance between production and destruction rates of organic matter. In Lerstadvatn formation A was deposited soon after the deglaciation, and we assume the low carbon content is a result of low production. The formation is interpreted as deposited in a cold marine environment, where most of the sediment particles were probably transported from the shore zone, and not directly from glaciers. Formation A is also generally more coarse grained and contains more pebbles than formation B. This may in some cases be the result of the supply of coarser material from a glacier snout, in other cases the dropping of stones from sea-ice. In basins with marine sediments from the Younger Dryas (not described in this paper), we have found a marked increase of pebbles in the sediments, presumably dropped by sea-ice. During the deposition of the upper parts of formation B (in Lerstadvatn, formation A), the studied basins became narrow bays with a shallow sill at the entrance, and with deeper water in the central parts of the basins. Shells are absent in the upper parts of formation B and the conservation of the laminated sequence also indicates that burrowing organisms were absent. This was probably caused by a layering of the water, producing anoxic conditions in the deeper parts of the bays, as known in similar present-day situations along the Norwegian coast. The origin of the strongly coloured laminae is not exactly known, but they are typical for a

205

more or less brackish phase in this type of basins (e.g. Lie et al., 1983; Krzywinski and Stabell, 1984). The diatom composition in three different coloured laminae in Torvlomyra suggests that they are not caused by salinity variations. Probably the lamination is a result of different redox potentials and/or different production of the blue-green and green algae, which constitute a major part of the laminated sediment (P.E. Kaland and K. Krzywinski, pers. comm., 1980). When the bay was isolated from the sea and turned into a lake, the environment changed completely. For the composition of the sediments we assume two factors to be of special importance. First, the autochthonous organic matter, produced within the basin, changed from marine organisms to freshwater organisms. Secondly, the intertidal zone, which we assume was the major source of the minerogenie fraction of the formation B, disappeared, causing the strong relative decrease of the minerogenic component. There was probably also a gradual increase in the supply of terrestrial organic matter, due to increasing vegetation cover on land, but we assume this change to be of much less importance than the decrease in influx of minerogenic particles. The Ase Member is generally thickest in the deepest part of each basin, wedging out towards the shores. This pattern is a result of deposition from suspension and probably resuspension by waves and currents, and has been termed “focusing” (e.g. Davis and Ford, 1982). In the bog basin at Lerstadvatn there is an interesting depositional pattern. The sedimentation rate of the Ase was fastest in the deepest part of the lake, as normal in this type of lake. However, after the lake floor became flat, the sediment continued to accumulate fastest at the same site, thus moving the deepest part of the lake towards the east (Figs.l4,15 and 16). The reason for this may be a larger influx of organic matter from the SW-shore, and obviously that the “distribution mechanisms” not were efficient enough for an even distribution on the lake floor. A further consequence of this was that when the sedimentation regime changed from

the Ase to the Leirstad, a new cycle of focusing started at the new maximum depth (Figs.14 and IS), with the result that the maximum thickness of the Leirstad is offset compared to the Ase. Another interesting feature is that the Ase is much thicker in the shallow bog basin than in the deep lake basin at Lerstadvatn (Fig.13). It has also been observed in other basins that the Allerod organic sediments are thicker in small lakes than larger. This may partly be a result of larger production in shallow lakes because of a higher temperature and better light conditions. In many cases there will also be a larger influx per area of terrestrial organic matter in small lakes compared to larger ones. The composition of the Leirstad Member is very different from the Ase, but probably the depositional processes were similar. In the Leirstad the influx of terrestrial organic matter has decreased while the production of diatoms and the content of minerogenic matter per volume of sediment has increased. However, the sedimentation rates decreased drastically from the Ase to the Leirstad (Fig.18) and the influx of minerogenic particles was therefore probably not significantly higher, suggesting that soil erosion did not increase, as opposed to what is often concluded from other sites. The main part of the Leirstad is deposited from suspension, as is the Ase. Only the Vedde Ash Bed increases in thickness towards the inlet brooks (Mangerud et al., 1984). Pollen stratigraphy history

and vegetational

The two small palaeolakes, Torvlomyra and Saudedalsmyra, reflect mainly the local vegetation, while the larger Lerstadvatn reflects the vegetation in a greater region (Fcegri and Iversen, 1975; Berglund, 1979). Lerstadvatn was isolated from the sea approximately 12,400 yr B.P. (Lie et al., 1983); Torvlomyra 11,900 yr B.P. (Table I) and Saudedalsmyra 11,150 yr B.P. Accordingly, in Saudedalsmyra most of the pre-Y ounger Dryas sediments are of marine origin, and the pollen composition is therefore difficult to relate to vegetation in detail. For

206

these reasons we give greatest weight to Lerstadvatn in the interpretation and discussion of the general vegetation development in the area. To simplify the discussion of the vegetation development, the same informal pollen assemblage-zones are used in the three diagrams (Figs.17, 19 and 20). The zone boundaries are drawn at levels with marked changes in the pollen content. The zones are named after the important taxa and in addition given the prefix Al from Alesund and a number. In Lerstadvatn we have also used subzones, shown by a number. Pinus is, in addition to Bet&a, the only APconstituent with continuous curves through the Late Weichselian (Zones Al l-Al 3) in the diagrams. It is generally assumed that Pinus did not grow in Norway during the Late Weichselian and that the Pinus pollen are long distance transported. The percentages of Pinus are somewhat smaller (mainly less than 5%) in the Alesund diagrams (also in the marine sediments) than in the diagrams from further south in Norway, e.g. Hafsten (1963), Chanda (1965), Mangerud (1970) Anundsen (1978), Thomsen (1982), Krzywinsky and Stabell (1984). The reason is probably the greater distance from Alesund to the northern Pinus border in northwest Germany and south Sweden (Berglund, 1966a; Huntley and Birks, 1983). The few grains of Corylus, Alnus, Ulmus and Quercus are also assigned to long distance transport, together with two pollen grains from the steppe shrub Ephedra distachya. In the marine sediment in the lower part of Saudedalsmyra is found up to 4% Alnus and one pollen grain of Tilia, which indicate redeposition of older sediments. However, long distance transport by sea surface currents can not be excluded. Definite pre-Quaternary pollen or spores have not been found. Redeposition appears to be a negligable problem in the analyzed cores. The lowermost sediments in the three basins were not analyzed, due to the extremely low pollen content.

Al 1 Rumex-Oxyria

assemblage

zone

The zone is characterized by high (above 20%) percentages of Rumex and Oxyria and a low Betula curve (less than 10%). Rumer and Oxyria generally decline upwards. The zone includes the lowermost sample in all the studied cores. The base of the zone is younger than the deglaciation of the area, which we assume took place 12,300-12,400 yr B.P. (p.194), and the top of the zone has an age of 12,000 yr B.P. (Table I), indicating that the zone represents the Upper Boiling Chronozone (in the sense of Mangerud et al., 1974). Zone Al 1 in Saudedalsmyra and Torvlomyra is deposited during marine conditions, whereas in Lerstadvatn the upper part (subzone lb) shows a lacustrine environment. The basal part of zone Al 1 in Lerstadvatn and Saudedalsmyra (Figs.17 and 20), are interpreted as older than the base of Torvlomyra (Fig.19) because of the rising RumexlOxyria curve at the base of the two former diagrams and the lack of formation A in Torvlomyra. A rise in both Rumex and Oxyria is found after a decrease of Artemisia below Balling Chronozone at Karsto (Eide and Paus, 1982). Oxyria digyna, constituting 30940% in the lowermost spectra in Lerstadvatn, (subzone la) is common in present-day mesotrophic and eutrophic snowbed communities (Nordhagen, 1943; Iversen, 1954; Dahl et al., 1971) and along brooks (Gjaerevoll, 1956) and may also occur in oligotrophic snowbed communities (Gjaere~011, 1956; Sjors, 1967). The high quantity of Oxyria, which could hardly be as dominant in the total vegetation cover as in the pollen composition, indicates the importance of transport of sporomorphs by (melt-) water. As mentioned earlier, we have not distinguished Salk herbacealpolaris from the other Salix species. But as snowbed communities were apparently an important vegetation type in the area at this time, S. herbacea-snowbed communities may have existed. A leaf of S. herbacea was found in the upper part of the zone in Lerstadvatn. Willow shrubs are also

207

characteristic in the present day low alpine region, and a considerable part of the Salixpollen in zone Al 1 may have been derived from a similar vegetation occurring at favourable localities. Bet&a pollen has not been measured in this zone because of the few and often damaged grains. Some of the Betula pollen is possibly long distance transported B. pubescens pollen (cf. sample 605, Fig.6) but B. nana probably occurred locally at protected localities. B. nana is wind- and frost sensitive but it also does not tolerate too long-lasting snowcover (Nordhagen, 1943, p.128). Poaceae, together with herbs of Caryophyllaceae, Chenopodiaceae, Compositae, Ranunculaceae,, Apiaceae, Rumex spp. and Rubiaceae show the existence of grass heath and grass meadows probably with both chionophilous (demanding long snow cover) and chionophobous (not snow tolerant) communities. Artemisia, Hippophae and Helianthemum indicate the existence of unstable soils. Hippophae is found only in Lerstadvatn (lacustrine sediment); three grains in sample 630 and one grain in each of the samples 615,612.5 and 610. Single grains of Hippophae have often been found in Late Weichselian sequences from western Norway (Faegri, 1940; Chanda, 1965; Hafsten, 1966; Eide and Paus, 1982; Krzywinski and Stabell, 1984), but as shieldhairs or other macrofossils have not been found, it has been uncertain if Hippophae grew in western Norway during the Late Weichselian (Hafsten, 1966). However, Hafsten (op. cit.) concluded that it probably occurred at Lista. Taking into account the low pollen production of Hippophab and that Lerstadvatn received few long distance transported pollen at this time (only 0.2-2.2% Pinus), we conclude that Hippophab grew in the area. There were many habitats well suited to Hippophad along the newly formed seashores, due to the rapid emergence. The Polypodiaceae spores are most probably attributed to the Athyrium distentifolium communities typical of stony snowbeds, along brooks and other stony areas with excess of melt water (Gjaerevoll, 1956; Sjors, 1967). Lycopo-

dium selago is abundant in zone Al 1 in Torvlomyra, but less in Lerstadvatn. L. selago grows on stony substrata and screes, and the spore frequencies depend on the presence of such habitats in the immediate catchment of the lake (e.g. Pennington et al., 1972). L. selago seems to have been frequent at some sites in western Norway in the Late Weichselian (e.g. Krzywinski and Stabell, 1984). Subzone lb in Lerstadvatn is characterized by high values of Anemone type (17%), probably Ranunculus glacialis and/or R. nivalis, which are both snowbed plants (Nordhagen, 1943). The pronounced increase in species diversity at the transition to subzone lb is probably partly an effect of the increase in P at this level and partly real. The occurrence of Empetrum and Dryas octopetala in subzone lb in Lerstadvatn demonstrates that a chionophobous heath vegetation also existed at this time (Late Belling). Calciphilous herbs such as Thalictrum cf. alpinum, Saxifraga cf. oppositifolia and Dryas octopetala indicate, together with Oxyria digyna and Artemisia, a soil rich in mineral nutrients. The vegetation in zone Al 1 suggests an arctic climate, which is also expected in an area near the ice margin. But the climate may have been more favourable than the vegetation indicates, due to delayed immigration. The vegetation may be characterized as an arctic/ alpine pioneer vegetation consisting of a mosaic of different communities. Snowbeds and other communities influenced by melt water were .abundant, but also dry grasssteppes and true pioneer vegetation on more or less bare soil were important. On a larger geographical scale, the vegetation both in zone Al 1 and the following zones fits into the general pattern of NW-Europe, recently analyzed by Birks (1986). Al 2 Betula-Empetrum

assemblage

zone

The lower boundary of this zone in Lerstadvatn is defined by the rising Betula curve from approximately 10% to 15-2Oo/o and the Empe-

208

trum curve exceeding 1%. Juniperus also shows an almost continuous curve from this level. In Saudedalsmyra (Fig.20) the boundary is identified by an increase of Bet&a contemporary with the start of the Empetrum curve. In Torvlomyra (Fig.19) there is no distinct Bet&a rise. The boundary Al l/2 is here identified by a small increase in Bet&a just after the start of the continuous Juniperus curve. In Lerstadvatn the zone is subdivided into three subzones, (Al 2a, 2b and 2c) mainly based on the variations of the Bet&a curve. Lower part of Al 2 In Lerstadvatn (Fig.17) Betula rises throughout the subzone 2a and Empetrum reaches its peak (nearly 4%) during the Late Weichselian within this interval. The Poaceae and Cyperaceae curves are relatively high and the subzone contains many different herbs. HippophaE is found in the lowermost part of the subzone. The Betula measurements (Fig.6) from the lowermost part (605 cm) of the subzone indicate a dominance of B.pubescens pollen, but also a considerable amount of B. nana. Pollen of B. nana spreads very short distances (Salmi, 1962; Andrews et al., 1980). Nevertheless, it is over-represented in pollen samples from treeless tundra (Pennington, 1980; Van Leeuwaarden, 1982, p.39) and seems to be under- (or fairly) represented near B. pubescens vegetation (Van Leeuwaarden, op. cit.). Our interpretation is that B.pubescens probably grew in the area as single stands from the start of the zone. B. nana was, however, the most abundant of the two. At 592.5 cm the Bet&a curve increases to c. 30% and in the next spectrum the pollen influx of Bet&a shows an increase to c. 150 pollen cm-* yr-i (Fig.18). In the D/P curve from 585 cm (Fig.6) the amount of B. nana has decreased compared to 605 cm. This may indicate that open birch forests became established at favourable localities approximately 11,600 yr B.P. In the lower part of zone Al 2 in Torvlomyra and Saudedalsmyra the Betula frequencies are lower and Poaceae higher than in Lerstadvatn.

B. pubescens probably did not grow in the area around Torvlomyra and Saudedalsmyra, and birch forests were definitely not established. During this zone, with a sea level approximately 30 m higher than today, there was very little lowland around Torvlomyra and Saudedalsmyra compared to Lerstadvatn (Fig.21). The steep hillside just south of Torvlomyra is north-facing, and we assume it was poorly vegetated. The area around Lerstadvatn has more quaternary sediments, favouring vegetation development, especially species demanding soil of some depth, e.g. B.pubescens. The three diagrams show that the taxa in the field vegetation were generally the same in the lower part of zone Al 2 as in zone Al 1, but the relative importance of the different communities was changed. The chionophobous Epetrum heaths expanded, while the wet snowbeds and the pioneer vegetation decreased, indicating that the snow cover was less heavy and lasted shorter. Grass heaths also expanded in the area around Torvlomyra and Saudedalsmyra. Empetrum heaths form an acid humus promoting podsolization (Berglund, 1966a). On the other hand, the abundance of lime-demanding plants, especially Dryas octopetala, demonstrate that there also existed unleached soils. D. octopetala is insect-pollinated, and was probably more important in the vegetation than the wind-pollinated Empetrum sp., even though the pollen frequencies are lower.

Upper part of Al 2 In Torvlomyra (Fig.lS), the upper part of Al 2 (sample 681 and above), is characterized by high values of Cyperaceae, Caltha type, Apiaceae, and Equisetum, which all include species typical of wet areas. We assume they reflect the vegetation in and around the basin which had recently been isolated from the sea. This swamp-vegetation probably consisted of different Carex spp., Caltha palustris, Angelica syluestris, Equisetum fluviatile, and others. The development of this vegetation type had already started in the previous subzone, but the main expansion was delayed several hundred

pp.209-211

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pp. 215-217

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years after to the isolation of the basin, probably because earlier it was too cold. In Saudedalsmyra (Fig.20) Caltha, Apiaceae and Cyperaceae are frequent from 535 cm, that is 16 cm below the isolation contact. Saudalsmyra is downstream from Torvlramyra and received palynomorphs from Torvlramyra. This is clearly demonstrated by the content of the freshwater algae Tetraedron minimum, Pediastrum and Scenedesmus in Saudedalsmyra at 600 cm, while Saudedalsmyra was still connected with the sea. This level is correlated with the flourishing of T. minimum in Torvlomyra immediately after the isolation there. Similarly, the abundance of Cyperaceae, Caltha type, Apiaceae and Equisetum in the marine sediments in Saudedalsmyra probably reflects the swamp-vegetation in and around Torvlomyra. In Saudedalsmyra there is a general increase in Cyperaceae in the uppermost part of Al 2 to a maximum of 37%. This, and the declining Pinus curve are interpreted as caused by the transition to lacustrine conditions. The dominance of the local swamp vegeta-

tion in these diagrams (Figs.19 and 20) hampers an interpretation of the general vegetation. The upper part of zone Al 2 in Lerstadvatn is separated into subzones 2b and 2c. The lower boundary of subzone 2b is defined by Bet&a declining from about 30% to 20%. Other characteristics of the subzone are somewhat higher values for Salix, Oxyria, Artemisia and Lychnis type, and slightly less Empetrum, compared to the preceding subzone. The Betula analysis from the top of the subzone (Fig.6, 572.5 cm) shows mainly B. pubescens, but also some B. nana. The Betula decline occurs simultaneously with a decrease in organic carbon in the sediments. The more minerogenic bed (Ase 3) has been described in almost all cores in the bog basin at Lerstadvatn (Figs.14 and 15). Both the decrease of Bet&a and of organic content suggest a small climatic deterioration, to colder and/or more humid conditions. The radiocarbon age of the change is c. 11,400 yr B.P. Less Betula pubescens and Empetrum, and more Oxyria and Artemisia may indicate that snowbeds and other areas with unstable soil expanded. Poaceae increases from the middle of the subzone 2b and has high values also in subzone 2c. We find this difficult to interpret, but it suggests a change in the local vegetation around the basin. The abundance and diversity of herbs indicate that the grass heaths and meadows were still important. The boundary between the subzones 2b and 2c in Lerstadvatn is defined by a sharp increase in Betula from about 23 to 43%. The influx of Betula pollen was more than doubled. This Betula peak together with the high Poaceae values depresses all the other percentage curves. The D/P ratio curve for Betula in this subzone (Fig.6, 567.5 cm) shows mainly B.pubescens. The NAP curve in the influx diagram from Lerstadvatn (Fig.18) shows that the Alesund area was not covered by extensive birch forests at this time. We interpret the general vegetation in the lowland around Alesund in Al 2 as park tundra with Betula pubescens and Empetrum heaths, grass heaths and meadows, but probably also

some snowbeds. B. pubescens became more important throughout the zone and limited birch forests were established at favourable localities. In general, there was a large difference between localities with different exposure. The diagrams from Torvlomyra and Saudedalsmyra in the upper part of the zone are strongly influenced by the strictly local vegetation. According to the radiocarbon dates, the Bet&a-Empetrum assemblage zone corresponds approximately to Older Dryas and Allerod chronozones (Mangerud et al., 1974). Al 3 Salix-Artemisia-assemblage

zone

The lower zone boundary is defined by the very sharp increase in Salk (from 10 to 30-40%), and a less pronounced increase in Artemisia. The zone is further characterized by the increased importance of many other herbs such as Caryophyllaceae, Chenopodium, Anemone type, Sedum sp. and the pteridophyte Lycopodium selago, and a decrease of especially Bet&a, Poaceae and Cyperaceae. The influx diagram from Lerstadvatn (Fig.18) shows a pronounced decrease in the total pollen influx at the zone boundary. The influx decreases for almost all pollen types, except Artemisia which shows a small increase. There are several radiocarbon dates (Table I) for the zone boundaries yielding an average age of approximately 11,000 yr B.P. for the lower and 10,200-10,300 for the upper boundary. Accordingly, Al 3 nearly coincides with the Younger Dryas Chronozone. In Lerstadvatn both the relative and the influx diagram show a marked decrease for Betula at the zone boundary (from 40 to 15%). The D/P ratio curve (Fig.6) shows mainly B.pubescens but also a considerable amount B. nana in zone Al 3. Since B. pubescens dominated in the preceding zone, this means an even stronger decrease for B. pubescens than for total Betula. The B.pubescens pollen are probably mainly long distance transported or redeposited, but it is likely that some indi-

viduals survived at favourable localities. The smaller pollen production and the less efficient dispersal of the pollen of B. nana means that dwarf birch was more important in the vegetation than the D/P ratio curve indicates. The small or almost imperceptable decrease for Betula in Torvlplmyra and Saudedalsmyra indicates that Betula here was mainly B. nana also in the underlying zone. Salk spp., which shows a marked increase in the relative diagrams, shows no increased influx in zone Al 3 (Fig.18). Some of the Salices are probably from the snowbed plants S. herbatea and S. polaris, but some may also derive from willow shrubs, which often grow along brooks and wet depressions in the mountains today. A marked increase in Salk at the Allerod/Younger Dryas transition is also found in the diagram from Krakenes (Larsen et al., 1984), and there is caused by an increase of willows. Krakenes is similar to the present diagrams in being situated in the part of Norway that had an extensive local glaciation during the Younger Dryas (Figs.1 and 21) (Reite, 1967; Mangerud et al., 1979). Rivers from glaciers and snowfields offered habitats favourable for willow vegetation and provided efficient transportation of pollen into the basins. A relative increase of Salk pollen may therefore be a general feature of the Younger Dryas climatic deterioration in these areas. The increase of Oxyria (relative diagrams), indicates that snowbeds became more important around Alesund during the Younger Dryas Chronozone. The heliophilous herb Helianthemum is found in zone Al 3 in all three diagrams, which probably indicates that it was growing in the area. During the Younger Dryas there were many cirque glaciers in this area (Reite, 1967; Mangerud et al., 1979; Sollid and Ssrbel, 1979; Larsen et al., 1984), suggesting an environment with active solifluction and other slope processes. Indeed, large Younger Dryas solifluction tongues are found at Godoy, 15 km west of Lerstadvatn (J. Landvik and J. Mangerud, unpublished). The vegetational changes at the

223

boundary between Al 2 and Al 3 indicate a climatic deterioration, and the increase of Artemisia and other herbs may indicate areas with unstable soil (see discussion about Artemisia below). Close to this boundary there is also a distinct lithological change with a decrease in organic matter. The sedimentation rate in zone Al 3 was, however, extremely low and a great part of the sediment consists of diatom frustules. The deposition of minerogenie particles was thus probably only slightly higher than in zone Al 2. Our interpretation is that the well established vegetation cover in the area at the end of Allerod was destroyed only to a small degree during the Younger Dryas. Thus the vegetation was relatively continuous, also covering active solifluction tongues, and therefore restricted erosion and transportation of minerogenic particles into the basins. The strong decrease in the total pollen influx reflects more a decrease in pollen production of the existing vegetation, than a destruction of the vegetational cover. We thus interpret the lithological change mainly as a response to lower organic production on land around the lake, while there was no pronounced increase of erosional processes as e.g. at Krakenes (Larsen et al., 1984). Larsen et al. (op. cit.) examined the relative response time of pollen and sediments to climatic change at the Allerod/Younger Dryas boundary in a lake that received a glacial river. They found a change in pollen composition distinctly before the first inorganic lamina. In Lerstadvatn a decrease of the Bet&a curve starts simultaneously at 567.5 cm in the influx and relative diagrams, but the decrease is faster in the influx diagram, where the minimum value is reached already in the next sample, 565.5 cm, which is very near to the lithological boundary at 565 cm. The minimum in the relative diagram is reached in the sample above at 563.5 cm. This shows that the pollen production decreased faster as a response to the climatic deterioration than the composition of the vegetation. The difference in response time is difficult to evaluate because the sedimentation rate changed drastically

around this level, and because the sample interval only yields a maximum difference, which, however, is estimated to c. 100 years. The curve for organic carbon in Lerstadvatn is nearly parallel to the Bet&a curve (and the AP curve) in the relative diagram (Fig.l7), indicating that the organic matter in this lake mainly reflects the organic production of the vegetation on land. Thus the main lithological change is probably a result of the decreased organic production around the lake, that is mainly the reduction of the birch forests. Artemisia was an important species in the vegetation during the Younger Dryas Chronozone in the Alesund area and at most localities in south and west Scandinavia (Iversen, 1954; Hafsten, 1963; Berglund, 1966a, 1971; Mangerud, 1970, 1977; Fredskild, 1975: Anundsen, 1978; Eide and Paus, 1982; Kjemperud, 1982; Selnes, 1982; Larsen et al., 1984; Johansen et al., 1985). There are, however, also some exceptions. Several diagrams from Sotra (Krzywinski and Stabell, 1984) showed Artemisia dominance in different Late Weichselian zones, which we, however, suggest all represent the Younger Dryas (see p.226). In Trondelag Selnes (op. cit.) found higher values for Artemisia during Allerod than during Younger Dryas in one basin. Chanda’s (1965) diagram from Jmren showing maximum Artemisia in Allerod, is probably a mis-interpretation as the Artemisia phase corresponds to a gyttja with sand (Watts, 1980). Artemisia is heliophile and usually associated with unstable soils and/or a continental climate. In Scotland the Artemisia frequencies were found to be highest in areas with low precipitation during Loch Lomond Stadial (Macpherson, 1980). The fossil Artemisia material seems to embrace several species (Iversen, 1954; Danielsen, 1970; Paus, 1982), but at present they can not be satisfactorily differentiated. Most of the Scandinavian species require a dry climate, and some are thermophilous (Iversen, 1954). A. WEgaris, however, which is the most widespread Artemisia species in Norway today, is indifferent concerning climate (Iversen, op. cit.).

224

In western Norway the greatest frequencies, 20&25:/,, of Artemisia in Younger Dryas are recorded at Sotra (Mangerud, 1977; Krzywinski and Stabell, 1984) and Karsto (Eide and Paus, 1982). North and south of this area the Artemisia frequencies are usually 12% or lower. Based on firn-line calculations, Larsen et al. (1984) deduced a cold and oceanic climate for the Bergen-More region, with more continental conditions to the north and south during Younger Dryas. This indicates that other factors than dryness favoured Artemisia in western Norway during Late Weichselian, and we suggest that unstable minerogenic soils and wind, creating habitats with short periods of snow cover, were most important (cf. Pennington, 1980). One exception from this general picture is the mountain site Grodalen 778 m a.s.l., where Artemisia reached 50% in one Younger Dryas sample (Johansen et al., 1985). Al 4 Betula-JuniperussEmpetrum zone

assemblage

The lower zone boundary is defined by the start of the Bet&a rise (relative and influx diagrams). In Lerstadvatn zone Al 4 is very clearly separated into two parts, the lower with a relatively high amount of Poaceae and Empetrum (4a) and the upper with a high Juniperus content (4b). This is recognizable but less pronounced in the other two diagrams. The start of the Bet&a rise coincides with the boundary between the Leirstad Member and the Hatlen Formation, dated to 10,200~10,300 yr B.P. (Table I). The Bet&a D/P ratios in the samples 945,935 and 925 cm (Fig.6) indicate a total dominance of B.pubescens in zone Al 4, which is easier to understand if some B.pubescens survived the Younger Dryas in the area, than if it immigrated from Denmark and Germany. In the two or three lowermost samples in zone Al 4, a considerable amount of the Betula pollen may be transported over a long distance. Concerning the rest of zone Al 4 (above 945 cm), however, tree birch must have been a very important constituent within the local vegetation.

The maximum influx of Bet&a (1900 pollen cm-’ yr~ ‘) in Lerstadvatn is reached in spectrum 930. Further upwards the influx varies between 1350 and 1750. We suggest that closed birch forest was established in the area at this level, or between 9750 and 9600 yr B.P. There was also a pronounced increase in the influx of NAP at the lower boundary of Al 4 and throughout the zone. The influx of NAP was higher than during the entire Late Weichselian. The reason for this is probably mainly a better climate, but partly also that the NAP constituents were different from those in the Late Weichselian. The main NAP constituents in the lowermost part of subzone Al 4a were Poaceae and Cyperaceae. The high amount of Empetrum in the upper part of the subzone, where Betula shows a high influx, indicates an open and discontinuous birch forest; a heath type with Empetrum was probably well developed. Vaccinium myrtillus and Filipendula &maria are today characteristic species in the field layer of different types of open birch forests, whereas Polypodiaceae represents a more shade tolerant understorey. Populus has a continuous curve from the base of the zone in Lerstadvatn, but probably did not belong to the local vegetation before the upper part of the zone. NAP constituents favoured by unstable soil and/or snowbeds disappeared at the zone boundary. The change from Al 3 is striking and reflects the marked climatic change around the transition Late Weichselian/Preboreal. Subzone Al 4a, which could be called a Poaceae-Empetrum phase, is followed by a Juniperus phase (subzone Al 4b); Juniperus constitutes more than 20% in sample 930 and 925. Vorren (1978, pp.31-32) considers the Empetrum phase as a northern type of the Juniperus maximum at the Younger Dryas/ Preboreal transition in Denmark (Iversen, 1954, 1973; Fredskild, 1975) and southern Sweden (Berglund, 1966a). The decrease after this Juniperus maximum has been interpreted as a shading out of the light demanding juniper when dense birch forest was established (Iversen, 1954). In Denmark and Sweden there is

225

also an

Pinus-Betulaaassemblage

zone

The lower boundary is the Pinus rise, the upper boundary the Corylus rise. The influx diagram from Lerstadvatn (Fig.18) indicates that pine did not expand at the expense of the birch forest. Pine probably occupied the more well-drained soils (Berglund, 196613) and areas with only a thin soil cover on the bedrock while Bet&a pubescens was growing at the wetter localities. Pollen from Corylus avellana and the other wind-pollinated tree species are probably long distance transported. However, the insect-pollinated Sorbus sp. was probably growing in the neighbourhood. Calluna was established in the field layer during the last part of the underlying zone (Juniperus-phase) and was important also in this zone. From south to north along the western coast of Norway the Pinus rise is radiocarbon dated to c. 9000 yr B.P. at Karstpl near Haugesund (Fig.1) (Eide and Paus, 1982), 880068900 near Bergen (Ssnstegaard and Mangerud, 1977), c. 8400 at the coast northwest of Bergen (Kaland, 1984) 8900 in the Alesund area (Table I), and c. 9000 yr B.P. over a large area in Trrandelag (just north of the map, Fig.1) (Paus, 1982). These dates are not compatible with a gradual delay of the Pinus maximum from Denmark along the Norwegian west coast as proposed by F;Egri (1944, pp.28829, 1954, p.237). The most likely explanation for the early Pinus rise at Sunnmore and in Trondelag seems to be a migration from the east, through Sweden, as Paus (1982) has also concluded. This route is also indicated on the Pinus isopoll map for 9000 yr B.P. (Huntley and Birks, 1983). Pine was an early immigrant in the mountain areas of Jamtland (Lundquist, 1969). In the area from Jaeren to Bergen (Fig.1) the Pinus rise is usually younger than the Corylus rise (Fmgri, 1940, 1944, 1954; Hafsten, 1965; Mangerud, 1970; Kristiansen, 1979; Kryzwinski and Stabell, 1984; Kaland, 1984). An exception is Banketjorn in OS, south of Bergen (Sonstegaard and Mangerud, 1977) where Pinus expanded simultaneously with Corylus. The radiocarbon dates of the Corylus expansion in

226

the Bergen area vary between 9400 and 8800 yr B.P. (Kaland and Kryzwinski, 1978). In the Alesund area, however, the Pinus rise, and even the Pinus maximum, occur before the Corylus rise as in Denmark (Iversen, 1960) and southernmost Norway (Hafsten, 1965). The Corylus rise is radiocarbon-dated to 8340 f 130 yr B.P. (T-3988 A) in Lerstadvatn. Discussion Older Dryas The Older Dryas, which traditionally was thought to represent a climatic deterioration is much discussed lately. A recent summary is given by Bjorck (1984). We will shortly analyze some evidence from Western Norway. In the diagrams in this paper, the Older Dryas Chronozone sensu Mangerud et al. (1974), should correlate to the lowermost part of pollenzone Al 2. However, at the lower zone boundary of Al 2, the vegetation changed from an arctic-alpine vegetation to a drier and more humus-demanding vegetation with some single birch trees. This may suggest a climatic amelioration, or could be the result of successions due to immigration and soil development. A climatic deterioration of Older Dryas Chronozone age (12,000-11,800 yr B.P.) is not found in any of the diagrams from the Alesund area, and is clear in very few other diagrams from Norway covering this period. Faegri (1940) based the Older Dryas (zone IV) in Jeeren entirely on Bet&a analysis, and Chanda’s (1965) diagram from one of the same basins shows a small, one-spectrum decrease in Bet&a in the Older Dryas Chronozone. Thomsen (1982) found very weak indications of a colder period between Allerod and Balling, and she concluded that either a less favourable Older Dryas period did not exist, or it had no influence on the vegetation at JEeren. Paus (in Eide and Paus, 1982), also found very weak indications of an Older Dryas climatic deterioration at KBrsto (Fig.1). Krzywinski and Stabell (1984) described a

climatic deterioration at Sotra (Fig.1) interpreted as the Older Dryas. We interpret their diagrams differently, as Anundsen (1985) has also partly done, and suggest that their “Older Dryas” Artemisia zone is of lower Younger Dryas age, and thus also the Younger Dryas vegetation is more similar to adjacent areas. Furthermore, we suggest the Younger Dryas/Preboreal boundary to be near to the first distinct increase in Betula above the “Artemisia zone” in all diagrams. A Betula decrease above this level in two of the diagrams (Klsesvatn and Hamravatn), interpreted by Krzywinski and Stabell(l984, p.195), as the Younger Dryas may be a statistical artifact of the increase of Poaceae and Pinus. The sequences at Sotra then, according to our interpretation, start with, or post-date, the Older Dryas. This is also more compatible with the radiocarbon dates, and the glacial history of that area as suggested by Mangerud (1977). Larsen et al. (1984) described a colder period (pollen zone 2) at Krakenes (Fig.1) based on a decrease in humus-demanding plants and low pollen influx. However, the zone is younger than the Older Dryas Chronozone and the sediment is in fact more organic than below. At Andraya, Vorren (1978) registered cryocratic conditions with an increase of unstable soil plants in Older Dryas Chronozone. Climatic changes can be detected pollen analytically only if the vegetation at the site and its surroundings was sensitive to the amplitude or duration of the actual climatic change. The Older Dryas may have been a short climatic deterioration that can only be registered in sensitive areas, It is also important to realise that a period as short as the proposed Older Dryas, will usually cover only one or two pollen spectra, and the response of the pollen constituents may be very vague and therefore easily overlooked, or doubted (e.g. Pennington, 1975). Nevertheless, for Western Norway, as for southern Sweden (Bjrarck, 1984) the conclusion seems clear; no climatic deterioration can be seen in the pollendiagrams for the period

221

(12,OOOG11,800 yr B.P.) that Mangerud et al. (1974) proposed for the Older Dryas Chronozone. The next logical question would be if the Older Dryas climatic event is in fact real, but that the response was erroneously dated by Mangerud et al. (1974). A glacial re-advance in western Norway is dated to around 12,000~12,400 yr B.P. (Mangerud, 1977; Sindre, 1980; Anundsen and Fjeldskaar, 1983; Vorren et al., 1983). Bjplrck (1984) found a response at the same time in pollenstratigraphy in southern Sweden. If there was a regional (Older Dryas) climatic deterioration, these events are the most probable candidates. A major obstacle to solving these problems is in fact the precision of correlations, which basically means the precision of radiocarbon dates. Betula

62

60

in Allemd

22 shows the maximum (%) during Allererd at in southwest Scandinavia where the percent values of is slightly different and Birks (1983) as map shows the maximum, and the local optimum during Allerod at each site. The time of optimum might be at each site, generally in younger of Allered. In and Jzeren, 50% Betula in it generally assumed of pubescens For concluded

on north-facing slopes and Saudedalsmyra and other unfavourable localities. et al., 1984), the lowest percentages of in Allered in and Saude-

localities most authors or probable occurrence of in the Allered Chronozone. main picture is similar to one given by and Birks (1983), but the additional sites show locally The Bet&a frequencies (43%) in subzone 2c in Lerstadvatn are to most localities in southern Norway. In and Saudedalsmyra it is well below during the entire We assume to representative for Western at favourable localities, the surroundings of

58c

56’

the other

54

Fig.22. Maximum percentages of Betula in Allered in southwestern Scandinavia north of the pine-forests, from Iversen (1954), Denmark and Hafsten (1963) and Chanda (1965) in Mangerud (1970, 1977), Fredskild (1975). Anundsen (1978), Anderson (1980), Eide and Paus (1982), Kjemperud (1982), Thomsen (1982), Krzywinski and Stabell (1984), Larsen et al. (1984) and Johansen et al. (1985) in Norway. Sites in south Sweden (e.g. Berglund, 1966a) are not plotted, since the relative content of Betula there is depressed by Pinus.

dalsmyra) is exposed to the open sea. Johansen et al. (1985) found an increase of Betula from the open coast into the inner fiord areas, which would parallel present day climatic and vegetational gradients. Conclusions (1) In all the studied lake basins there are shallow marine sediments in the lower parts of the sequences. When the basins were isolated from the sea, the influx of minerogenic particles decreased strongly, due to the disappearance of the intertidal zone. (2) The brackish sediments are frequently laminated. (3) The lacustrine sedimentary sequence is very similar in many lake basins and formal lithostratigraphical units are defined. (4) The composition of the lacustrine sediments is partly determined by the production in the lake, but even more by the vegetation around the lake. Climate was the main forcing agent for changes in the lacustrine sediments. (5) The lacustrine sediments are generally thickest in the deepest part of the basin. In one small basin the focusing of the Allerpld sediments continued to the earlier deepest part, so that the maximum depth moved laterally, and the maximum thickness of the Younger Dryas sediments is displaced compared to the Allerrad sediments. (6) The strong decrease of organic matter from Allererd to Younger Dryas sediments is due to the decrease of the supply of organic matter and not to increased influx of minerogenic particles, as commonly concluded in other areas. (7) The very low sedimentation rates during the Younger Dryas Chronozone suggest a limited erosion on land and an extensive vegetation cover. (8) The Younger Dryas is characterized by high percentages of Salk and Artemisia. Artemisia is in western Norway believed to be favoured by unstable minerogenic soil and areas with short periods of snow cover. (9) Soon after the deglaciation 12,300-12,500

yr B.P. there was an arctic-alpine vegetation around Alesund consisting of a mosaic of different plant communities. The dominating pollen types were Oxyria, Rumex and Salk, indicating extensive snowbeds and meltwaterinfluenced communities. Drier grass steppes and bare soils with pioneer plants such as Artemisia and Hippophae were also present. (10) Throughout the Berlling-Older DryasAllercad Chronozones there was a gradual development of a more humus-demanding vegetation. This indicates that a climatically less favourable Older Dryas period did not exist, or that the vegetation was not sensitive enough to respond to an “Older Dryas climatic deterioration”. (11) The vegetation during Allerclld Chronozone was a park tundra with B.pubescens, Empetrum heaths and grass heaths. Open birch forest seemed to be established at favourable localities approximately 11,600 yr B.P. At that time there were large differences in the vegetation between lowland localities with different exposure. (12) At the Allerad/Younger Dryas boundary the pollen production decreased faster than the composition of the vegetation changed. All detected time-lags are, however, in the order of decades, suggesting an abrupt climatic deterioration of considerable amplitude. (13) Tree birch probably survived in the area during the Younger Dryas. (14) In the Preboreal, a Juniperus maximum occurs after both the Bet&a rise and the maximum pollen influx of Bet&a, in contrast to southern Scandinavia. There is a gradual delay of the Juniperus phase as one goes north along western Norway probably due to the slow migration of Juniperus. (15) The early Pinus rise in the Alesund area (c. 9000 yr B.P.) compared to southwestern Norway suggests an immigration from the east, through Sweden. Acknowledgements Peter Emil Kaland, Knut Krzywinski and Alfred Kjemperud have critically read an

earlier draft of the manuscript. Hilary H. Birks has corrected the English language and also critically read the manuscript. Kjell Sognen has constructed the coring equipment. Svein Erik Lie and Jon Landvik assisted during the field work. Eva Krzywinski prepared the pollen samples from Lerstadvatn. Ingvald Rersberg gave information on the recent vegetation in the area and Hans H. Blom identified mosses from the cores. Ellen Irgens and Jane Ellingsen made the drawings. All the radiocarbon analyses were carried out at the Trondheim laboratory under supervision of Reidar Nydal and Steinar Gulliksen. The work was supported by the Norwegian Research Council for Science and the Humanities (NAVF) through a grant (to Mangerud) under IGCP-project 24. The typing of the manuscript was done at Norsk Hydro, Research Centre Bergen. To all these persons and institutions we proffer our sincere thanks. References Andersen, ST., 1980. Early and Late Weichselian chronology and birch assemblages in Denmark. Boreas, 9: 53369. Andrews, J.T., Mode, W.N., Webber, P.J., Miller, G.H. and Jacobs, J.D., 1980. Report on the distribution of dwarf birches and present pollen rain, Baffin Island, N.W.T. Arctic, 33: 50-58. Anundsen, K., 1978. Marine transgression in Younger Dryas in Norway. Boreas, 7: 49960. Anundsen, K., 1985. Changes in shore-level and ice-front position in Late Weichsel and Holocene, southern Norway. Nor. Geogr. Tidsskr., 39: 205-225. Anundsen, K. and Fjeldskaar, W., 1983. Observed and theoretical Late Weichselian shore-level changes related to glacier oscillations at Yrkje, southwest Norway. In: H. Schroeder-Lanz (Editor), Late- and Postglacial Oscillations of Glaciers: Glacial and Periglacial Forms. Balkema, Rotterdam, pp.1333170. Bates, C.D., Coxon, P. and Gibbard, P.L., 1978. A new method for the preparation of clay-rich sediment samples for palynological investigation. New Phytol., 81: 4599463. Berglund, B.E., 1966a. Late-Quaternary vegetation in eastern Blekinge, southeastern Sweden. A pollen-analytical study. I. Late-Glacial time. Oper. Bot., 12: 180 pp. Berglund, B.E., 1966b. Late-Quaternary vegetation in eastern Blekinge, southeastern Sweden. A pollen-analytical study II. Post-Glacial time. Oper. Bot., 12, 190 pp. Berglund, B.E., 1971. Late-Glacial stratigraphy and chronology in South Sweden in the light of biostratigraphic

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