Reuieu'of Palaeobotany and Palynology, 53 (1988): 185 231
Elsevrer Science Publishers B.V., Amsterdam - Printed in The Netherlands
LATE WEICHSELIAN/EARLY HOLOCENE POLLEN- AND LITHOSTRATIGRAPHY IN LAKES IN THE AITSUruD AREA,
WESTERN NORWAY
INGER LISE KRISTIANSEN1, JAN MANGERUD and LEIF LOMOl
Department of Geology, Sect. B, Uniuersity of Bergen, AIIegt. 41, N-5000 Bergen (Norway) (Received and accepted May 5, 1987)
Abstract
Kristiansen, I.T,., Mangerud, J. and Lamo,L.,l988. Late Weichselian/Early Holocene pollen- and lithostratigraphy in Iakes in the Alesund area, western Norway. Rev. Palaeobot. Palynol.,53: l8b-281.
Three palaeolakes at elevations from 44 m a.s.l. to 30 m a.s.l. 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 Rumer and Oryria. Later in the Allersd the flora was more diversified, but a tree.Iess 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 Solir. An Older Dryas climatic deterioration was not detected. In the Preboreal a Juniperus maximum occurs after the Betula 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 lithostratigra- phy and palynomorph stratigraphy for three palaeolakes in the Alesund area, Western Norway (Fig.1). Two of the palaeolakes, Torvlo- myra and Saudedalsmyra (see Fig.9), are com- pletely filled in with Holocene organic sedi- ments, 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).
rPresent address: Norsk Hydro, Research Centre, P. Box 43f3, N-5028 Bergen (Norway).
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 45-30 m a.s.l.) that emerged from the sea during the Late Weichselian. Actually there were hardly any alternative.basins. The result- ing sea-level curve is presented by Lie et al.
(1983). The main purpose of the palynological investigation was to provide a biostratigraphi- cal tool for inter-correlation of the strata in the three paleolakes.
In this paper we also use the pollen-stratigra- phy to deduce the vegetational history of the area. As stated above, the sites were not chosen for that purpose. However, Lerstadvatn
0034-6667/88/$03.50 O 1988 Elsevier Science Publishers B.V
Fig.l. Southern Norway with Younger Dryas end moraines (after Mangerud etal., 1979). Discussed locautles are marked with crosses.
meets nearly all the requirements for such a study. In Torvlsmyra 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 infor- mation, 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 Krikenes (Fig.1) to north of Alesund.
Topography
The most characteristic topographic fea- ture of Sunnmsre is the sharp relief. The investigated area lies at the coast, but there are mountains up to 900 m a.s.l. 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
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(Holtedahl, 1960). The three investigated local- ities are situated on the upper, undulating part of the strandflat (Figs.2 and 3).
Deglaciation
From the area around the investigated sites, we have six radiocarbon datings on shells giving a minimum age for the deglaciation r a n g i n g f r o m 1 2 , 3 1 0 * 1 3 0 to 1 2 , 6 3 0 + 6 0 y r 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 ( F i g . 1 ) ( S o l l i d and Sorbel, 1979). Cirque gla- ciers were frequent in the area outside the ice sheet during Younger Dryas (Reite, 1967;
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Larsen et al., 1984) and cirque moraines are present only 3 km west of the sites Torvlomyra and Saudedalsmyra (see Fig.21).
Present uegetation
Outside cultivated areas, the coastal low- land 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 auella,na and Ulmus glabra are occa- sionally found in favourable localities near the outer coast, but they occur more frequently
B r u s d o l s v q t n e t
' . A .
Fig.3. Photo towards east, showing the general topography of the area, and location of the site Lerstadvatn, compare Fig.2.
Photo: Svein Skare, Historical Museum, University of Bergen.
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. Taxus 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 Dresent cultivated.
Methods Field work
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.
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 HrOr.
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
189 all the analyzed cores. In addition the content of organic carbon has been determined with a Leco carbon determinator EC 12 in two cores from Lerstadvatn (502-30-0ll2a 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, 1.977, 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) Torvlsmyra and Saude- dalsmyra: plastic bottles standing for 3-4 days in warm sand with shaking once each dav, (b) Lerstadvatn: hot HF for 10 min, then cold HF for two days, (2) disaggregation with NaoPrO, (Bates et al., 1978), (3) acetolysis (Fegri and Iversen, 1975) and ( ) 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).
PoIIen diagrams
Each pollen sample is given a number corresponding to its depth in cm. Apart from the lowermost samples in each diagram, the pollen sum (IP) 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 Torvlsmyra (see Fig.19) and Saudedalsmyra (see Fig.20) the palynomorphs which are not included in fP (e.g. Equisetum) are calculated as percent of fP+the actual palynomorph (Equisetun). In Lerstadvatn (see Fig.17) these percentages are
190
based on tP + t 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 observa- tions 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 0.2o/".
The pollen diagram from Lerstadvatn (see Fig.17) is composed from two cores, b0Z-80- 01/2a (lower part) and b02-80-08 (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 (10,600+60 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 {ue 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.fg) 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 820-1000 times mag- nification.
The pollen identifications are based mainly on Fegri and Iversen (lgZS) and the pollen herbarium at the Department of Botany, Uni- versity of Bergen. Moe (1924) and Eide (lgg1) have been used to identify the trilete pterido- phyte 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. niualis among others.
Initially, Lycopodium inundqtum spores were identified (Lie and Lsmo, 19g1). 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. inundaturn like types. The latter lacks the trilete mark and is probably not a fully developed spore. The "2. inundqtum,'spores in the fossil material also lack the trilete mark, and they may be not fully developed L. selago spores.
According to Fegri and Iversen (1925) it is possible to distinguish Solir herbacea from the other Sa/ir species (see also Fegri, 1gb3 and Fredskild, 1973). The most important criterion is "knot-like
thickenings" of the muri (colu- mellae) in S. herbacea. Our experience is, however, that corrosion of pollen grains from other So/ir species in some cases may result in a similar appearance. Because the So/ir pollen in our material were usually corroded and crumpled, we have not distinguished S. herba_
cea.
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 prepa- ration (Berglund and Digerfeldt, 1920; Kristi_
ansen, 1979; Prentice, lg81). We have at.
tempted to solve this problem by regarding the total populations as consisting of sub-popula_
tions with normal distributions (Usinger, lgZS;
Andersen, 1980; Prentice, 198i) instead ofusing absolute size (Eneroth, lgbl).
Birks (1968) found that B. pubescens and B- nana could be distinguished by the rario grain diameter/pore depth (D/p, see Fig.4).
Theoretically, this parameter should not be affected by changes in the absolute dimensrons of the pollen gtain. In Fig.5 we have plotted the distribution curyes for the D/P ratios and for
F i g . 4 . P o l l e n g r a i n o f B e t u l o . D = d i a m e t e l , p : p e r e d e p t h .
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 differ- ent. 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. Freds- kild (1975) found that cold 40o/o 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
i 9 1
(Fig.s), and all four distribution curves would be assigned to almost only B. pubescens accord- ing to Birks (1968) and Van Leeuwaarden (1982). It thus seems that the D/P ratio is a better method for distinguishing ttre Betula species if the preparation methods have varied.
The grain diameter and the pore depth (Fig. ) were measured in eleven fossil samples from Lerstadvatn. The size units of the meas- urements 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/o were measured in each sample. Unfortunately processing with differ- ent HF-treatments was used, and therefore only the distribution curves for the D/P ratios are used to distinguish the Betula species.
The D/P 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 D/P ratio larger than 9. The DIP ratios for the different Betula species are not well established, but the majority of those with ratios above 9 are probably B. nono (Birks, 1968; Van Leeuwarden, 1982), even though
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Birks (1968) found that B. pubescens subsp.
tortuosa Nyman has D/Pratios between B. nana and B. pubescens. If this interpretation is cor- rect, B. nonq was most frequent in the oldest Allersd sample and during the Younger Dryas.
B. nana has hardly contributed to the Holocene curyes. 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 D/P ratio slightly greater than B. pubescens (c.8.5 according to Van Leeuwaarden, 1982).
Diatoms
Diatom analysis has been carried out by Lie et al. (1983) to determine the isolation of the basins from the sea. The distribution of the salinity groups of diatoms are shown in the relative pollen diagrams.
Radiocarbon dates
The radiocarbon datings (Table I) were car- ried 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 solu- ble and insoluble fractions. The insoluble fraction yielded the youngest age in Lerstad- vatn, 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 KrAkenes (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 lacus- trine gyttja dates.
In addition, three samples of marine gyttja were dated from Saudedalsmyra (Table I, T- 3956 A/B and T-3951A). The composition of the
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marine gyttja has not been examined in detail, but it is a mixture of marine and terrestrial material, as indicated by the dl3C 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 (T- 3956 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 conclu- sion 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 (T- 3959, Torvlsmyra, and T-4161 A and B, Lerstad- vatn) 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 alterna- tive 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 lower- most date (T-3957A) should be consistent with other dates of that event. As mentioned before, we have obtained many dates suggesting that the deglaciation took place between 12,500-12,300, probably 12,400-72,300 yr 8.P., or approximately one standard deviation youn- ger than the Lerstadvatn date 12,650*230 yr B.P. (T-3957A). For the Ase member in Lerstad- vatn, 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+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
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Fig.7. The radiocarbon dates of the Ase Member in Lergtadvatn against depth. Dates with one standard deviation.
constant sedimentation rate curve is shown.
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 Torvlamyra by the brook, and the level can be identified as a peak of T. minirnum at 600 cm depth in the marine sediment in Saudedals- myra. 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 Torvlomyra 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-39b8A, 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 sarnple from Torvlsmyra gave 11,840*120 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,g00, for KrA- kenes. 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, 10,600+50 yr B.P., is from Mangerud etal.
(1984).
There are several dates available for the base of the Hatlen Formation. Taking sample thickness into account, the dates for Torvlo- myra and Lerstadvatn suggest an age of 10,400-10,500 yr B.P., while the dates from
195
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, how- ever, more likely that the discrepancy is 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 10,200-10,800 yr B.P. for the Leirstad/Hatlen boundary, stressing that a more precise age is desirable for this important boundary. For the calcula- tions of sedimentation rates and pollen influx, 10,200 is used in Fig.18.
Lithostratigraphy, macrofossils, and descriptions of the basins
Lithostratigraphical units ; general considerations
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 for- mations 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 com-
position nearly correspond with the lithologi-
cal changes, although the boundaries do not
always coincide. Mainly based on this litho-
and pollen stratigraphy, in combination with
radiocarbon dating, chronostratigraphical
subdivisions are defined, e.g. the Bolling,
Older Dryas, Allerod, Younger Dryas chrono-
Chronostratigraphy
( M a n q e . u d e t a l . t 9 7 4 )
B o r e a l
P r e b o r e a l 1 0 0 0 0
1 1 0 0 0
l 2000
1 3 0 0 0
zones in Norden (Mangerud et al., 1974) and climatostratigraphical subdivisions, e.g. the Windermere Interstadial, Loch Lomond Sta- dial in Britain (Coope and Pennington, 1977) and, proposed for entire NW-Europe: the La- teglacial Interstadial, Younger Dryas Stadial (Lowe and Gray, 1980). However, useful as chronostratigraphical and climatostratigraph- ical 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 stratigra- phy 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
Formal stratigraphical units, lacustrine sediments
Glacial sediments or bedrocK
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 forma- tions 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.
Forrnation A
This informal formation comprises the low- ermost marine sediments, the lower boundary normally being to bedrock or glacial sedi- ments. The formation consists mainly of grey silt, with beds of silty sand and clayey silt. The loss on ignition is <3/o, the content ofcarbon
< 1/o. 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).
O
Eo .9
E uo
I
. G
o)
c
=
0) o.:
B
q)
)
(!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. l,erstadvatn) have more lacustrine sediments than lakes at lower elevations (e.g. Gjslvatn). 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).
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 4-l2o/o. The formation is generally homoge- nous, 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 time- transgressive due to the deglaciation, gener- ally 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. (198a, fig.5) combined formation A and B into "marine sediments, mainly silt".
The Langeu&g Forntation
Langevig is the name of the small town west of Torvlomyra and Saudedalsmyra. The Lan- gevig Formation consists mainly of clay/silt with a varying but low content of organic matter. The loss on ignition rarely exceeds 40o/", and is generally much lower. The Forma- tion may include more sandy facies. All sediments are of lacustrine origin. The lower boundary stratotype is the lake Kr6kenesvatn (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.l0, 11, 14 and 15).
The stratotype for the Langevig 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-
197
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 igni- tion 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 frus-
tules 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 Krrikenes
(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 f0lo. The boundaries are easily
recognized through changing colours, even
though there are frequently transitions cover-
ing some few cm. The Vedde Ash Bed with an
age of 10,600+60 yr B.P. (Mangerud etal.,
19&1) 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 lacus- trine, and of Younger Dryas age. The bounda- ries 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 Forma- tion 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 inter- face. 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 defini- tion 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 Torulomyra basin
The two bogs Torvlomyra and Saudedalsmyra are situated on the undulating strand-flat (10-50 m a.s.l.) on the NE side of the island Sula (Fig.2). South of the bogs is a mountain reaching 776 m a.s.l. The area around the two bogs is characterized by small hills and ridges up to 10-15 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.
tig.9. A. Detailed map of Saudedalsmyra and Torvlomyra.
Contour interval 5 m. For location see Fig.2. B. Recon- struction on a larger scale of the lakes that during the Younger Dryas occupied parts of the present day bogs Torvlomyra and Saudedalsmyra. Crosses mark the coring points shown in Fig.10 and 11. Dots mark other cored points. (In Saudedalsmyra the coring points are ehown rn Fig.l1 connected with a line.)
Both Torvlstmyra and Saudedalsmyra are
small bogs, just greater than 100 m across, and they are only 50 m apart (Fig.g). 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 Saudedals- myra. The most important species in the poor oligotrophic vegetation on the two bogs are Calluna uulgaris, Erica tetralix, Eriophorum spp., Scirpus caespitosus and Narthecium ossi- fragum. Some small pines are also found.
The southern part of Torvlsmyra 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.
€ g o g E L a k e , S O o m
T-5 T-4 T-3 T-2 T-1
+\fi/
Fig.10. Cross section from Torvlsmyra. Depth below bog surface, distance in meters from brook outlet in west.
Legend as for Fig.1l. The analyzed core shown in Fig.lg 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 Saudedalsmyr.a, and occur throughout the sequence. The upper 5 cm (brackish) consists of well defined red- dish, 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 difrerent thick- ness variation. The diatomite-silt (subtracted the ash) is 16 cm thick in the deepest part ofthe basin and increases to more than 25 cm towards east and west. This demonstrates that different processes governed the deposition of
r99
s-3
ffi xutt"n Formation r---l Leirstad Member F with Vedde Ash Bed E t "
lEl Ase Member E€ Formation B [] sano
4 0 m
Fig.ll. 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 (Man- gerud et al., 1984), while a considerable part of the diatomite-silt was produced in the water- column, with diatom frustules "raining"
to the bottom. The thinning of the diatomite-silt in the deeper part is, however, difficult to under- stand, as depth differences are too small to encounter for a considerably differentiated dissolution
The Hatlen Formation (617-560 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
' 1 0 40
200
direction (Figs.9 and 11). The analyzed core (see Fig.20) was taken from the deepest part near the centre ofthe basin. The lithostratigra- phy 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 (780-764 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 (764-519 cm) consists of a rela- tively homogenous brownish grey gyttja silt. A lighter grey zone in the lower part is found in all cores. At 7I2 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 (629-561cm).
Shell fragments are abundant up to 541 cm, with enrichment at 7M-740 cm, 7I2 cm, and 661-629 cm. Mytilus edulis, Littorina littorea and. Balanus balanoides are identified. Two bones from 729 and 708 cm are identified as cod (Gadus sp.) (Rolf Lie, det.) A 1cm thick reddish- brown 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 (519-511cm) is only a thin bed of silty gyttja, due to the late isolation from the sea. Equisetum sp. is most important
€rmong the plant macrofossils.
The Leirstad Member (511-500 cm) consists of a diatomite silt with relatively high loss on ignition, 10*l2oA.
Lerstaduatn basin
The lake Lerstadvatn is situated 44 m a.s.l.
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 undulat- ing (Fig.3) with heights up to 130 m a.s.l. 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 com- pared 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 Potarnogeton 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 uulgaris, Erica tetralix and Myrica gale. Molinia caerulea, Eriophorum angustifolium, E. uaginatum, Scir- pus caespitosus, Carex rostrata, Potentilla erecta, and, Narthpcium ossifragu^ ir"the main grasses and herbs. Sphagnum epp., Hylocomium splen- dens and Pleurozium schreberi, are important in the ground layer (. Rossberg, pers. courm., 1982).
Along the northwestern and western margins of the bog there are also more base-demanding vegetation types with Carex'hostiana, Eriopho rum latifolium, Drepanocladus reuolvens, and Scorpidium scorpioides as important species.
201
L---J Area wilh garage bu{dangs
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 iake. On the bog in the western end the coring points are numbered, and the two profiles are named A and B.
Hatlen Formation
Fig.l3. Composite E-W profle ofthe bog (profiIe A) and the lake basins at Lerstadvatn. Depth below bog and water surface are given on the vertical scale,, and distance in meters from eastern shore on the horizontal. The figures in parentheses at the top of the lake profile show the water depth in meters. Note the break in the horizontal scale; coring point 5 in the bog basin is situated 50 m south of point E in the lake basin (see Fig.12). The analyzed cores shown in Figs.l? and 18 were taken near pbint C in the lake basin and between point I and 6 in the bog basin.
H a t l € n F o r m a l r o n L a r s t a d u a m b r
B r 3 c o t c o r a
a k e b a s i n
5 4 0
4 5 0 m
Fig'ra' E-W profile lprofile A in Fig.r2) 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 ofthe profile (see a l s o F i g . l 6 ) .
202
z u 3 u 4 0 5 0 6 0
Fig.l5. N-S profile (profile B in Fig.12) from the bog basin southern border of the present bog.
f'',,i
l i : : : l
l::,:j
l,t,l:,1
,it
f +'/
I l i l n r t r " n F o r m a t , o n- L e r r s t a d M e m b e r L - - l w i t h V e d d e A s h 8 e d i - : A s e Membef l - l w i i h 4 beds
F t " , A z
I r o r m a r i o n F : - : 1 g e o e ' I A I o o craver B e o
]
t:77)
EAI Hatlen Formatron
E Leirstad Member
L--J wilh Vedde Ash Bed
=l Ase Formation
€ with 4 beds
B e d A 2 l - I r o r m a l t o n B e d A r
J A
o o G r a v e l B e d
7 0 8 0 9 0 i o o 1 1 0 m
at Lerstadvatn. Depth below bog surface, distance from the
' 8
B
t , ,
o / ,
)un
\0
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.1B).
The formation A is subdivided into two beds, A, and A, (Figs.14 and l5) mainly based on the difference in organic carbon (Fig.1?), resulting in a slight difference in colour.
The lower bed Ar (718-660 cm, Fig.lZ) is light grey. Loss on ignition is c.0.S/o and organic carbon content 0.1/o. 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 A2 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 B%
and organic carbon content c.l"/". The bed is from 7.5 to 42.5cm thick, with the greatest thicknesses in the deepest part of the basins. A leaf of Salix herbocea and a bone of cod (Gadus rnorrhua L.) (Rolf Lie, det.) were found in bed Ar. The upper boundary is smooth but distinct.
Lamination at this boundary is only recorded in the 110 mm core from the lake basin (502-30-08).
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).
l s o p a c h m a p l o r t h e L e i r s t a d M e m b e r T h t c k n e s s in c m .
The Ase Member (635-565 cm) consists of a silty gyttja with loss on ignition lb-B7lo, and organic carbon content 5-l5o/o. The thickness varies from 5-79 cm in the bog basin with the greatest thicknesses in the deepest parts (Figs.f5 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 Ase-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 7-9o/". It is more silty than the rest of the Member. The thickness varies from 10-20 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 Ase.
I 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. Ase- 2 is found up to 50 cm thick, but there is great variation in thickness.
Ase-3 (585-573.5 cm) is light, brownish green.
l s o p a c h m a p l o r t h e A s e M e m b e r T h i c k n e s s in c m .
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 l,eirstad members. Profile A and B (Fig.12) are marked with lines with crosses for the coring points. The maps are based on 20 cores.
)f',./,
D e p t h s I n c m l o r t h e b a s e o f t h e A s e M e m b e r
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/o. It is up to 10 cm thick, but usually thinner.
The Leirstad Member (565-553.5 cm) con- sists of diatomite-silt with 2-3o/o organic car- bon. 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): Drepanocla- dus sp., D. exannulalus (Bruch, Schimper, Gu- embel) Warnstorf, Bryum sp., Hygrohypnum ochraceum (Wils) Loeske, and Calliergon trifar- ium (Web and Mohr) Kindberg (Hans H. Blom, det.). The diatomite-silt is usually more yellow- ish 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.16), partly because the posi- tion 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.l. in Saudedalsmyra. The maximum water depth during deposition therefore was 22m. 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 trans- ported the fines into local basins which acted as sediment-traps on the shallow sea-floor. This explains the high sedimentation rates (1-2 mm per year) found in formations A and B in Torvlomyra and Saudedalsmyra. The main dif- ference between the two marine formations is the content of organic carbon, resulting from the balance between production and destruc- tion 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 sedi- ment 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 forma- tion 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 sedimenrs 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 shaUow 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 se- quence also indicates that burrowing organ- isms 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
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 sug- gests 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.8. 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 sedi- ments we assume two factors to be of special importance. First, the autochthonous organic matter, produced within the basin, changed from marine organisms to freshwater orga- nisms. Secondly, the intertidal zone, which we assume was the major source of the mlnero- genic 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 ofeach basin, wedging out towards the shores. This pattern is a result ofdeposition 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.14, 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
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