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Late Weichselian vegetation, climate and floral migration at Eigebakken, South Rogaland, Southwestern Norway
Review of Palaeobotany and Palynology, 61 (1989): 177-203 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
177
LATE WEICHSELIAN VEGETATION, CLIMATE AND FLORAL MIGRATION AT EIGEBAKKEN, SOUTH ROGALAND, SOUTHWESTERN NORWAY AAGE PAUS
Botanical Institute, University of Bergen, Alldgt. 41, N-5007 Bergen (Norway) (Received November 17, 1988; revised and accepted April 25, 1989)
Abstract Paus, Aa., 1989. Late Weichselian vegetation, climate and floral migration at Eigebakken, South Rogaland, southwestern Norway. Rev. Palaeobot. Palynol., 61:177 203. The Eigebakken, Jseren, pollen diagram shows a tripartite division of the Late Weichselian into three main climatic periods. The pleniglacial, from local deglaciation c. 14,000 B.P. to c. 13,000 B.P., reflects an Artemisia-dominated pioneer vegetation on disturbed mineral-soil, prevented from further development by cold winters and katabatic winds. The Bolling amelioration opens the Late Weichselian Interstadial (13,000-11,000 B.P.) and initiates soil development and vegetational closure into a Salix.shrub consolidation phase (to c. 12,650 B.P.). Thereafter an open birch vegetation phase (to c. 12,200 B.P.) follows. The subsequent birch-forest phase (c. 12,200 11,000B.P.) reflects the interstadial vegetational and edaphical optimum. In this phase July mean temperature reached at least 14°C. In contrast to late-glacial studies from N Rogaland, the Eigebakken diagram gives no biostratigraphical indications of climatic deteriorations such as "Older Dryas" within the interstadial. This is probably explained by denser local birch forests with higher ecological inertia on Jmren. Furthermore, no traces of Fsegri's "Brondmyra interstadial" are recorded. The Younger Dryas Stadial (11,000-c. 10,500 B.P.) shows a two-step regressional succession. In the first step (to 10,600 B.P.) the birch forests degraded into open birch woodland. The second phase (10,600-c. 10,500 B.P.) involved the maximum extent of open-ground vegetation and possibly temporary local deforestation. Critical climatic factors included cold winters and strong winds. The first vegetational responses of the Holocene climatic amelioration are recorded locally as early as c. 10,500 B.P. Tree-birches re-established, finally developing into dense forests c. 10,000 B.P. Boreal-circumpolar, eurasiatic and arctic-alpine plants dominated the late-glacial flora. For the majority of the late-glacial taxa a northward migration into SW Norway is suggested.
Introduction T h i s p a p e r is t h e t h i r d c o n t r i b u t i o n f r o m a project aimed at reconstructing the vegetational and climatic development during the L a t e W e i c h s e l i a n i n S W N o r w a y (see P a u s , 1988, 1989). T h e E i g e b a k k e n b o g is o n e o f t h e t h r e e l a t e g l a c i a l l o c a l i t i e s w h e r e Fsegri (1935, 1940) demonstrated the ~'Brondmyra interstadial" followed by a climatic deterioration. This s e q u e n c e h a s l a t e r b e e n c o r r e l a t e d w i t h Boll-
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i n g a n d O l d e r D r y a s , r e s p e c t i v e l y (cf. C h a n d a , 1965). H o w e v e r , T h o m s e n (1982) q u e s t i o n s t h i s i n h e r r e i n t e r p r e t a t i o n of t h e B r o n d m y r a d i a g r a m s o f F~egri (1940) a n d C h a n d a (1965). She claims that no biostratigraphical zones r e p r e s e n t i n g the i n t e r s t a d i a l or the s u b s e q u e n t d e t e r i o r a t i o n , c a n be d i s t i n g u i s h e d . I n 1975, b e f o r e i n d u s t r i a l e x p a n s i o n t o t a l l y destroyed the Eigebakken locality, a new sedimentary sequence was obtained by coring at the locality. Its well-developed late-glacial deposits were considered too scientifically
© 1989 Elsevier Science Publishers B.V.
178
important to be lost. Later, when I initiated my late-glacial project, Professor Knut Faegri kindly placed the new Eigebakken material at my disposal. In addition to elucidating the vegetational and climatic late-glacial development on Jseren, this re-analysis also attempts to resolve the interpretative problems of the “Brondmyra interstadial” and the “Older Dryas deterioration”. Site description Eigebakken bog (5”42’E, 58”47’N, 27 m a.s.1.) is located in the central part of Jzren, close to Froylandsvatn (Fig.1). The sizes of the catchment and the bog surface (before 1960) are estimated to about 1.8 km2 and 320,000 m2, respectively. The 1975material for pollen analysis was taken in a bog area between the Block Berge and Kvernaland factories, west of the railway. Today, due to further industrial expansion, there are only remnants of the bog. Moreover, the whole Froylandsvatn area is
Fig.1. Map of the Boknfjord
area and the site Investigated,
influenced by human impact, mainly agriculture and industry, and there are few traces to indicate that the landscape once belonged to the treeless coastal heath zone. For further description of the natural vegetation and flora of the area and Eigebakken, see Faegri (1940). The area has an oceanic climate with July and January mean temperatures of 14°C and 1 C, respectively (Bruun, 1967). Annual precipitation is about 1050 mm (Bruun and Haland, 1970). The bedrock in the eastern part of Jaeren is mostly Precambrian granites and gneisses: in the west Precambrian mica-gneisses dominates (Birkeland, 1970 in Andersen et al., 1987; Sigmond et al., 1984). Between these areas and across the investigation area there is a narrow, north- south running band of phyllite and mica shists (Sigmond et al., 1984). The Froylandsvatn area is covered by till and fluvial/glaciofluvial deposits (Andersen et al., 1987). Close to and west and north of the coring point, a welldeveloped esker system delimits the catchment and shelters the locality. The esker system
showing areas mentioned
m the text
179 runs in N E - S W direction, demonstrating a southwest transport of meltwater during deglaciation (F~egri, 1939; Andersen et al., 1987). Movement of the retreating ice was in the same direction. The marine limit in the area is estimated to about 15 m a.s.1, and the nearest Younger Dryas moraine is in the outermost part of Lysefjorden, c. 25 km NE of Eigebakken (Anundsen, 1985; Andersen et al., 1987). Thus, in the absence of any influence of the sea or Younger Dryas ice, Eigebakken has undisturbed sediments extending back to preAllerod deglaciation times. Methods
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Field work. K n u t Krzywinski (Botanical Institute, University of Bergen) and Lotte Selsing (Archeological Museum, Stavanger) carried out the field work. Material for pollen analysis was taken using a l l 0 m m - d i a m e t e r Livingstone sampler (see Table I for sediment description). Unfortunately, destructions in the bog area caused by industrial development, prevented thorough lithostratigraphic-transects of the bog. Laboratory work and analytical methods follow Paus (1988). Betula-pollen analysis was carried out using the approach of Paus (1988). Originally, in 19 selected samples (preparation: a c e t o l y s i s + H F ) from Bolling to Preboreal Chronozones, grain-diameters were measured a t 1280 × magnification to the nearest ocular line (one line: 0.79 I~m). This resulted in curves hard to interpret. An extended Betula analysis, including D/P-ratio statistics (Birks, 1968) and morphological studies (Terasma~, 1951), was then carried out in 13 samples. Six of these samples were not included in the previous Betula analysis. Pore depth was measured to the nearest ocular half-line. 100 grains or more per sample were measured. The pollen diagrams. Pollen data were processed, and the percentage- and absolutediagrams (Figs.3, 4, 5) drawn by the program CORE-SYSTEM (Michelsen, 1985). The per-
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-o- NaOH soluble (A) fraction ........... ble ( B ) ' . . . . . . .
// / // 14,000
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Fig.2. Sedimentaccumulationcurve based on the assumed ages of dating levels. 14C-dates are shown with one standard error.
centage calculation basis, ZP, comprises the terrestrial (including assumed secondary) and unidentified pollen. For a constituent X within spores, aquatic pollen and pre-Quaternary microfossils the calculation basis is ZP+X. Sediment accumulation rates, used in estimating pollen influx, were calculated by linear interpolation between 14C-dated levels. The pollen diagrams contain 69 spectra analysed at 1.25-10 cm intervals. Z P per spectrum ranges from 793 to 2830 (mean 1997) comprising 30-59 (mean 38) taxa. The pollen diagrams include 131 terrestrial taxa. Pollen and spore types are grouped following Paus (1988). Plant nomenclature follows Lid (1985). Biostratigraphic zones, based on Figs.3 and 4, are defined as local pollen-assemblage zones (paz). Chronostratigraphy younger t h a n 13,000 B.P. follows
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182 TABLE I
Sediment lithology for the core from Eigebakken studied pollen analytically. The description is based on Troels-Smith's (1955) system. DEPTH cm
TYPE
COLOUR
CONSTITUENTS
Dgl, Ld33 Th + , Ag+
PHYSICAL CHARACTERISTICS
640-698
DY
DARK BROWN
698-719
SILTY GYTTJA/DY
GREENISH BROWN
Dgl, Ld22, Agl Th +, D1 +, As +
nig.2 sicc.2 strat.1 elas.2
719-731
SILTY GYTTJA
YELLOWBROWN
Dgl, Ld12, Agl Th + , As+
nlg. 2- sicc.2 strat.1 elas.3
731-744
SILTY DY
DARK BROWN
Dgl, Ld32, Agl Th+, Dh+, As+
nig.3 sicc.2 strat.! elas.2
744-747
MINEROGENIC GYTTJA
BROWN
Dg1,Ld21,As1,Agl Th+,Dg+,Gs+,Gg+
nig. 2 sicc.1 strat.1 elas.2
747-749
ORGANIC SAND
LIGHT BROWN
Ld21,Ga1,Gs1,Gg1 Th+, Dg +
nig. 2- sicc.1 strat.0 elas.1
749-758
CLAYEY GYTTJA
YELLOWBROWN
Dgl, Ld12, Asl Tb+,Th+,Dh+,Gg+
nig. 1+ slcc.2 strat.2 elas.3
758-759
MOSSLAYER
DARK BROWN
Tb14
nig.3 site.2 strat.0 elas.1
759-818
CLAYEY GYTTJA
LIGHT YELLOW
Dgl, Ld12, Asl Tb +, Dh +
nig.1 sicc.2 strat.1 elas.3
818-819
GRAVEL/ SILT
BROWNISH GREY
Ld11, Agl, Ga2
nig.1 sicc.1 strat.0 elas.1
819-832
CLAYEY GYTTJA
LIGHT YELLOW
Dgl, Ld12, Asl Dh+, DI+, Ag+
nig. 1 sicc.2 strat.1 elas.3
832-836
CLAYEY GY2~rJA
REDDISH BROWN
Dgl, Ld21, Thl, Asl
nig.2 sicc. 2 strat.2 elas.3
836-839
ORGANIC GRAVEL/SAND
YELLOWBROWN
Dg1,Ld11,Ga1,Gsl Ag+
nig. 1 sicc.2 strat.0 elas.1
839-854
CLAYEY GYTTJA
GREENISH BROWN
Dgl, Ld22, Asl T1 + , DI+
nig.2 sicc.2 strat.2 elas.2
854-858
ORGANIC GRAVEL/SAND
GREYISH BROWN
Ld11, Gal, Gs2 Dh+, D1 +
nig.1+ sicc.1 strat.1 elas.0
858-923
SILT
BROWNISH BLUE-GREY
Asl,oAg2, Gal Dh+,Ld +,Gs+,Gg+
nig.1+ sicc.1 strat.2 elas.0
Mangerud et al. (1974). Between local deglaciation and Bolling the term Oldest Dryas (DR 1) is used following Welten (1982). Independently of this chronostratigraphy the zonation of Firbas (1949) is added. Chrono-/bio-zone borders not dated are based on interpolated ages.
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R a d i o c a r b o n dates Ten gyttja samples (thickness 2.5-3.0cm) have been dated conventionally. In all levels the NaOH-soluble (A) fractions are dated; with one exception also the NaOH-insoluble (B)
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185
TABLE II Results and characteristics of the Eigebakken 14C-dates. The letter behind the lab reference indicates NaOH soluble (A) and NaOH insoluble (B) fractions of the samples. The datings were carried out by the Laboratory of Radiological Dating in Trondheim, Norway.
DATING LEVEL
0EPTH cm
LOSS ON MATERIAL ~ IGNIT- SECONOARY LAB.REF. ~ 10N(Z) POLLEN(Z)
14C_YEARS B.P.
~ 13C
PRONOUNCED DATING ERRORS
ASSUMED AGE B.P
10
700 - 702.5
SILTY GYTT3A 49-54
-
T-5895A
-22.9
9,900
9
746 - 748.5
SANDYGYTT3A ca.10
0.t
T-G1ESA T-61698
10 ,2402170 - 2 0 . 0 7 ,000_+100 -19.2
10,600
9
,9202100
DATING
FEATURE
BETULA INCREASE YD ASH BED
AOUAT. ROOTS
8
757.5 - 760
CLAYEY GYTT3A 26-35
-
T-5694A T-5894B
11 ,280+150 10 ,780~120
-23.9 -21.9
11,000
AL/YD TRANSITION
7
775 - 777.5
CLAYEY GYTT3A 32-35
0.1
T-TS86A !T-75068
11 ,0202110 11 ,7802150
-20.5 -20.7
11,400
JUNIPER MAXIMUM
G
807.5 - 810
CLAYEY GYTT3A 30-33
0.1
T-7585A ;T-7585B
12 ,120~120 12 ,110280
-20.0 -20.8
12,000
LOSS-ON-IGNITION MAXIMUM
5
817.5 - 820
MIN. GYTTJA
7-12
0.1
IT-5893A T-56938
12 ,670÷190 10 ,800~140
-20.8 -22.1
HARDWATER AOUAT. ROOTS
12,200
EPIPETRUMINCREASE
4
825
827.5
CLAYEY 8YTT3A
29-30
-
:T-7504A iT-7584B
12 ,800+210 13 ,4802150
-19.4 -17.7
HAROWATER
12,400
LOSS-0N-IGNITION
3
833.5
836
CLAYEY GYTT3A ca.lO
0.1
T-5692A :T-56928
13 ,180~170 9 ,580±120
-22.0 -22.1
HAROWATER AOUAT. ROOTS
12,600
8ETULA INCREASE
2
843.3-848.3
CLAYEY GYTT3A ca.14
0.6
T-TS83A T-TS838
13 ,790~150 14 ,370±190
-20.2 -21.0
HAROWATER REWORKE8 MAT.
12,800
SALIX MAXIMUM
1
851.5
854
CLAYEY GYTT3A ca.10
0.6
14 ,380._.210 - 2 2 . 2 12 ,390_+200 - 2 1 . 2
HAROWATER REWORKEDMAT. AOUAT. ROOTS
13,000
BOLLIN6 AMELIORATION
-
-
-
T-SGS1A T-58918
fractions are dated. Results are given in Table II and Fig.2. Discrepancies, in part considerable, between dates of different fractions from the same level indicate serious dating errors. In light of the well-developed aquatic macrophyte flora throughout the late-glacial, the calcareous local bedrock, and redeposition in the basal sediments (see below), both fractions may have been influenced by the hard-water effect (Donner et al., 1971) and/or reworked material (Olsson, 1974), whereas downwardpenetrating roots (Kaland et al., 1984) and/or inorganic carbon (Olsson, 1979) may have contaminated the NaOH-insoluble fractions. Due to the antagonistic effects of these sources of error, it is difficult to estimate quantitatively their influence. However, it is important to note how the A and B fractions from the same sample vary throughout the late-glacial deposits. Levels with the B frac-
MAXIMUM
tion more than 2000 z4C-years younger than the A fraction are situated in or just above sandy/silty horizons. These minerogenic layers may have been less penetrable for roots thereby representing root concentration levels in the sediments. The serious dating errors involved imply that an Eigebakken chronology must be defined by assumed ages of levels. These include the age of deglaciation (c. 14,000 B.P., see Anundsen, 1985; Paus, 1989), the transition minerogenic/organic sediments (c. 13,000 B.P., see Chanda, 1965; Thomsen, 1982; Paus, 1989), the AL/YD-transition (c. 11,000 B.P.) and the YD ash layer (c. 10,600 B.P., Mangerud et al., 1984). In addition, levels 6 and 10 (Fig.2, Table II) are considered reliable. These levels form the chronological basis for further discussion and provide the basis for estimating the sediment accumulation rates shown in Fig.2.
186
Late Weichselian vegetation history Introduction The Eigebakken results are, in many respects, very similar to those obtained from the other two sites included in my late-glacial project (Paus, 1988, 1989). In the following interpretation of the Eigebakken data, I will, when appropriate, refer to more detailed discussions and arguments in Paus (1988, 1989) to avoid frequent repetitions. Prior to the reconstructions and discussion of the late-glacial vegetation, some limitations and problems preventing a precise reconstruction of past vegetation (cf. Fsegri and Iversen, 1975; Birks, 1981), are stressed. First, the presence of reworked and deteriorated pollen in the minerogenic basal sediments causes interpretative problems (Iversen, 1936; see Paus, 1988, 1989). Second, the diameter statistics of Betula pollen show no clear patterns and often contradict the results from D/P-ratio statistics and the morphology study (Fig.6). As grain size is more sensible than D/P-ratio to abiotic factors such as preparation techniques (Kristiansen et al., 1988; Paus, 1988) and sediment types (Praglowski, 1966; Paus, 1989), the size statistics are omitted in the interpretation of the Betula analysis. Third, in the absence of reliable dates, the Eigebakken influx estimates are largely based on assumed ages. In particular, the assumed age of the basal sediments, representing the minimal age of local deglaciation, may deviate from the real age (cf. Andersen, 1979; Anundsen, 1985; Andersen et al., 1987). Furthermore, without any dates in the diagram top, the influx estimates are here based on extrapolated sediment-accumulation rates (Fig.2). In fact, influx estimates and hence, the establishment of a reliable chronology, represent one of the most serious problems in late-glacial studies. The necessary chronological precision will not be obtained until chronologies based on accelerator datings of terrestrial material, are
constructed (Andree et al., 1986; MacDonald et al., 1987; Lowe et al., 1988). Fourth, pollen-stratigraphical, -morphological and -dispersal limitations permit only a coarse recognition of late-glacial plant communities (cf. Firbas, 1949; Berglund, 1966; Webb and Moore, 1982). In the following discussion of vegetation history I will refer to the simplified classification into late-glacial plant communities used by Paus (1988, 1989). In brief, these communities are: (la) Dry, open-ground communities on rocks or unstable mineral soil. Vegetation discontinuous. (lb) Dry grassland on well-drained humus and shallow soils. Vegetation mostly continuous. (2a) Wet, open-ground communities, including extreme snow-beds. On rocks, in hollows and by streams, springs and flushes. Vegetation discontinuous. (2b) Moist grassland, including early-melting snow-beds. In more sheltered areas, on humus soil varying in moisture. Vegetation mostly continuous. (3) Juniper-dwarf-shrub heaths. Heterogeneous in ecology and composition. On humus soil varying in dryness, thickness and fertility. Vegetation mostly continuous. (4) Tall-herb grassland in sheltered areas on fertile, moist humus soil. Includes vegetation in wet lake margins. (5) Willow shrubs/copses. In favourable, moist sites with a tall-herb field-layer. (6a) Tall-herb birch-forests on fresh to moist humus soil. (6b) Empetrum birch-forest on well-drained, humus-rich soils. (6c) Open birch vegetation. Scattered trees/ copses on moist, deep soils.
Vegetation development c. 14,000-c. 13,000 B.P.: Artemisia-Pinus local pollen-assemblage zone (El) The E1 lithostratigraphy is clayey silt (losson-ignition 2%) containing 30-35% reworked
187
% BETULA OF Z P
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--x 752.5 ,_~ 755
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Fig.6. R e s u l t of t h e Betula a n a l y s i s from E i g e b a k k e n . T h e v a l u e s of t h e m o r p h o l o g i c a l Betula nana-type a r e i n b l a c k .
188 pollen (Fig.3). This may be an underestimate as unidentified pollen is not included. Parts of the pine pollen is assumed to be long-distance transported (see de Beaulieu and Reille, 1984). The presence of pre-Weichselian material in Oldest Dryas sediments is a common feature in Rogaland (Paus, 1988, 1989) and is most likely explained by erosion of glacial deposits within the catchments or by winds carrying eroded material from the North Sea Continent (cf. finds of marine dinoflagellate cysts). Influx estimates (220-340 grains cm 2 a-1) correspond to influx in newly deglaciated areas elsewhere (Ammann and Tobolski, 1983; Gaillard, 1985; Paus, 1988, 1989) and modern tundra (Ritchie and Lichti-Federovich, 1967; Fredskild, 1973). The taxonomically rich pollenassemblage includes arctic-alpines such as
Dryas octopetala, Papaver radicatum-type, Saxifraga spp. and presumably also Artemisia norvegica (cf. relict-localities in Ryfylke, c. 50 km NE of Eigebakken; Ryvarden and Kaland, 1968). Furthermore there are steppeelements such as taller Artemisia spp., Helianthemum and Hippophag rhamnoides and plants that today are weeds and/or coastal plants (Chenopodiaceae, Euphorbia, Plantago maritima, Spergularia spp., Polygonum avicularetype). The presence of plants with N-fixing root nodules (Dryas, Hippopha& Fabaceae, Ononistype, Lotus-type, Onobrychis) suggests immature soils. This, together with low influx and the present ecological demands of the El-taxa, suggest a discontinuous vegetation on mineral soil, possibly as mosaics of unstable and temporary communities such as la in exposed areas and 2a in sheltered areas. The better representation of Salix (S. herbacea/polaris?), Saxifraga oppositifolia-type and Cerastiumtype than in the pioneer phase of N Rogaland (Paus, 1988, 1989) is interpreted as more widespread wet, open-ground communities (2a) at Eigebakken. This may be ascribed to the eskers within the catchment creating a hummocky landscape with numerous lee-sides. Here, in the last part of El, some denser vegetation could have developed, as suggested
by the occurrences (if not secondary) of PimpineUa-type, Plantago major (dry grassland), Parnassia palustris, Polygonum bistorta-type (wet grassland), Filipendula, Urtica (tall-herb grassland), and Juniperus, Gymnocarpium dryopteris (juniper-dwarf-shrub heaths). The E1 aquatic macrophyte flora was poorly developed (Fig.5). Climate was probably the limiting factor, either directly (e.g., low temperatures) or indirectly (cryoturbation/soil erosion causing reduced light transparency in water). As in N R o g a l a n d (Paus, 1988, 1989) Sparganium-type is the pioneer type among the pollenproducing aquatics. E1 is directly related to the Oldest Dryas zones S1 and L1 in N Rogaland (Paus, 1988, 1989). Ecologically these are younger parallels to the Artemisia-dominated, pioneer Oldest Dryas zone on the Continent (de Beaulieu and Reille, 1984; Gaillard, 1985).
c. 13,000-c. 12,650 B.P.: Salix-RumexPoaceae local pollen-assemblage zone (E2) The E1/E2-transition occurs in a sandy layer. Otherwise E2 is represented by clayey gyttja (loss-on-ignition 12-14%). Biostratigraphic changes are increases in Salix, Rumex, Cyperaceae and aquatic pollen and declining Artemisia, Chenopodiaceae, Helianthemum, and Saxifraga oppositifolia-type. Simultaneously secondary pollen drops strongly. Thus, the actual increase in influx of locally produced pollen is more pronounced at the E1/E2transition than shown (Fig.4). The E2-influx reaches 400-500 grains cm 2 a - l , comparable to low-producing forest-tundra today (Ritchie and Lichti-Federovich, 1967). This vegetational closure reflects some humus-soil development. Open-ground communities declined (cf. disappearance of e.g. Papaver), whereas dry grassland (Myricaria, Sanguisorba minor, Rumex spp.) and wet grassland (Pinguicula vulgaris, Botrychium, Selaginella, Rhinanthus-type) expanded. The development of juniper-dwarf-shrub and tallherb communities is reflected by Juniperus, Betula nana-type (Fig.6), Urtica and Sanguisorba officinalis. High percentage- and abso-
189
lute-values of Salix most likely originate from willow shrubs/copses (5), representing the shrub-stage in this progressive succession (see Paus, 1988). The next step in the aquatic succession is dominant Myriophyllum spicatum, Potamogeton sect. Eupotamogeton and Ranunculus sect. Batrachium (Fig.5). Macrofossils of Myriophyllure alterniflorum and Potamogeton filiformis are present (det. Hilary Birks). Similar macrofossils are reported from late-glacial pioneer phases elsewhere on Jseren (Holmboe, 1903). Ecologically and chronologically E2 directly relates to the Salix-shrub-Rumex phase in N Rogaland (Paus, 1988, 1989). Similar pollenassemblages are described from other sites in SW Norway (Fsegri, 1940; Hafsten, 1963; Chanda, 1965; Thomsen, 1982) and are interpreted as representing snow-bed, pioneer stages. Early Salix-phases in the British lateglacial (Pennington, 1977a; Webb and Moore, 1982) probably represent the same successional stage as E2.
c. 12,650-c. 12,200 B.P.: Betula-Salix local pollen-assemblage zone (E3) Lithostratigraphically the E2/E3-transition is represented by a sandy layer (loss-onignition 10%). Otherwise the E3 sediment is minerogenic gyttja strongly varying in organic content (loss-on-ignition 12-30%). Biostratigraphic features are rises in Betula (both treebirch- and B. nana-type; Fig.6) and decreases in the Salix-, Poaceae- and Rumex sect. acetosapercentages and Polypodiaceae (both absolute and percentage values). Total pollen influx increases to about 1000 grains cm -2 a-1, comparable to present high productive foresttundra (Ritchie and Lichti-Federovich, 1967). E3 reflects further vegetational succession and soil development. Tree-birches rapidly invaded locally (cf. D/P-ratio decrease, 35% tree-birch pollen ZP; Fig.6) and established open birch vegetation (community 6c). Additional trees were Populus and Sorbus, both sparsely recorded earlier. Salix-shrubs and Betula nana existed within the tree-birch vegetation or in their own shrub (5) and dwarf-
shrub (3) communities. New or better represented tall-herbs (Geum, Filipendula, Lychnis/ Dianthus-type; cf. 4) are recorded. In addition, new taxa (Anthyllis vulneraria, Plantago lanceolata) appear in dry grassland (lb), in which Dryas expanded. These mineral-soil communities may have occurred in less protected areas such as on the esker ridges within the catchment. Although pronouncely changing, the E3 aquatic macrophyte flora was well-developed. After a short-lived maximum at the lower zone border, Potamogeton sect. Eupotamogeton drops to low values whereas Ranunculus sect. Batrachium expands. The shift from dominant Myriophyllum spicatum to frequent M. alterniflorurn could reflect leaching processes leading to lower trophic conditions in the lake (cf. Iversen, 1954; Berglund, 1966). Chronologically and partly vegetationally, E3 parallels paz $3 in Sandvikvatn (Paus, 1988) and paz L 3 + L 4 in Liastemmen (Paus, 1989). Similarly, E3 also compares to zones Ib + c of Iversen (1954) and to the main part of zone II of Fsegri (1940).
c. 12,200-c. 10,600 B.P.: Betula-Empetrum local pollen-assemblage zone (E4) At the E3/E4-transition, a narrow horizon of sand/silt occurs. Otherwise the lithology is gyttja (loss-on-ignition 15-35%) except for the moss layer at the E4b/E4c-transition. Biostratigraphic features are rises in Empetrum, Pinus, and Filipendula, and declining Salix (%), Artemisia, Caltha, Dryas, and Rumex sect. longifolius. Total pollen influx increases and varies between 1200 and 2800 cm-2 a-1 throughout the zone. This compares to modern high productive forest-tundra or birch woodland/conifer forests (Ritchie and Lichti-Federivich, 1967; Hyv/irinen, 1976). The parallel Betula-influx rise is due mainly to increased proportions of tree-birch pollen (Fig.6). The strong Empetrum rise at the lower zone boundary most likely reflects the expansion of the present lowland taxon E. nigrum. No size statistics which could distinguish the two Empetrum taxa (Andersen, 1961; Berglund,
190 1966), have been carried out. However, the bigger E. hermaphroditum tetrads differ by being more rounded/sub-sphaerical, which places E. hermaphroditum within the pollentaxon Vaccinium-type (cf. revised pollen-keys in Fsegri and Iversen, 1989). These morphological criteria indicate the total dominance of E. nigrum in the Eigebakken deposits. Similar conclusions are reached in other NW European late-glacial studies differentiating between the two Empetrum taxa (Huntley and Birks, 1983). Correspondingly strong late-Boiling Empetrum expansions are demonstrated throughout Rogaland (Thomsen, 1982, Braathen and Hermansen, 1985; Austad and Erichsen, 1987; Paus, 1988, 1989). Furthermore, smaller Empetrum increases at about the same time are recorded further north in W Norway (Krzywinski and Stabell, 1984; Larsen et al., 1984; Kristiansen et al., 1988). This apparent synchroneity could indicate a rapid, late-Boiling Empetrum migration into W Norway. However, the find of one Empetrum seed in early Bolling and a short-lived, middle Bolling Empetrum-pollen maximum in the Liastemmen deposits (Paus, 1989) demonstrate local Empetrum occurrences in Rogaland before the late Bolling expansion. The success of Empetrum at Liastemmen is probably caused by local soil factors. Sparse remnants of glacial deposits (Ringen, 1962) and large areas of exposed, granitic bedrock within the catchment suggest a limited late-glacial reservoir of mineral nutrients. Thus, leaching processes could rapidly acidify the soil to levels at which Empetrum could compete and expand. An early acidification of the Liastemmen soil is indicated by the strong decrease in the eutrophic Myriophyllum spicatum as early as 12,800 B.P. (Paus, 1989), whereas in Eigebakken, the final drop in M. spicatum occurs abut 500 years later (Fig.5). N W E u r o p e a n studies demonstrate metachroneous late-glacial Empetrum rises which may be both climatically and edaphically conditioned (Berglund, 1966). At a broad-scale, the earliest Empetrum expansions occurred
about 13,000-12,500 B.P. in Scotland (Lowe and Walker, 1977; Pennington, 1977b) and Ireland (Watts, 1977), about 12,500-12,000 B.P. in W Norway and about ll,700 11,500 B.P. in S Sweden (Berglund, 1966; Berglund and Ralska-Jasiewiczowa, 1986; BjSrck and MSller, 1987). Empetrum rises in the interval 11,50011,000 B.P. are recorded from Denmark (Iversen, 1954; Fredskild, 1975; Kolstrup, 1982), the Netherlands (Van der Hammen, 1951; Cleveringa et al., 1977; Bohncke et al., 1988), N Germany (Usinger, 1985), and N Poland (Zachowics et al., 1982). Taking into account the existence of the North Sea Continent (cf. Nesje and Sejrup, 1988), this pattern suggests a time lag in the Empetrum expansion from the coast towards continental areas. In line with presentday conditions, this probably reflects decreasing precipitation and, thus, delayed leaching of soil nutrients from the coast inwards. In broad outline, climate may then appear as the most decisive factor in determining the time of the late-glacial Empetrum expansion. On the other hand, edaphical differences may be important on a local scale, as at Liastemmen and Eigebakken. Similarly, the acid rocks and tills of S Sweden compared with the Danish calcareous rocks and tills (Berglund, 1966) may be responsible for the earlier Empetrum rise in S Sweden than in Denmark. An additional factor regulating the time of the NW European Empetrum expansion may be vegetational density. Using the Betula percentages and/or total pollen influx as an index of vegetation density, there seem to be positive correlations between a dense, early late-glacial vegetation and a late Empetrum rise. Both are indicative for continental localities. Several factors may have been responsible for a generally denser early late-glacial vegetation in continental than in marginal, coastal areas, e.g. more favourable climate, earlier deglaciation (if ever glaciated) and a shorter migrational delay. A closed vegetation and soil cover may have hindered wide-spread Empetrum establishment by (1) the low availability of light, which would be unfavourable for the light-demanding Empetrum and (2) slower acidi-
191 fication than in bare soils covered by pioneer, discontinuous vegetation (Jacobsen and Birks, 1980; Messer, 1988). As a whole, E4 represents the late-glacial vegetational optimum with dominant birch vegetation (communities 6) and dwarf-shrub heaths (3). Changes in representation of these humus-soil communities are the basis for dividing E4 into three subzones. E4a (Rumex-Filipendula subzone, c. 12,200c. 11,600 B.P.) reflects the closure of the local interstadial birch-forests. Tree-birch pollen reaches values of about 50% Z P and 600-1000 grains cm -2 a-1. According to th~ D/P-ratio concentrating around 8, Betula pubescens s.s. was the main forest component (cf. Birks, 1968; Van Leeuwaarden, 1982). The forests may have been of the drier Empetrum-birch type (6b) including Populus, Sorbus, Juniperus, Empetrum, Calluna and Melampyrum and of the fresher tall-herb birch-forests (6a) including
Salix-shrubs, Filipendula, Sanguisorba officinalis, Urtica, Geum, Trollius, Valeriana, Lychnis/Dianthus-type and other tall-herbs. Empetrum together with decreasing Betula nana during the subzone (Fig.6), also formed their own dwarf-shrub communities (3). Similarly, Salix and tall-herbs may have existed within communities 5 and 4, respectively. These humus-soil communities formed a more or less continuous vegetation cover. Nevertheless, occurrences of, e.g., Myricaria, Artemisia, Dryas, Helianthemum, Plantago spp., Cerastiumtype, Montia and Saxifraga spp. indicate areas supporting herb communities such as 1 and 2. A diverse vegetation thus characterized E4a. E4b (Populus-Filipendula subzone, c. 11,600c. 11,000 B.P.) reflects minor changes in local vegetation by decreases in total pollen/Betula influx (Fig.4), Betula nana-type (Fig.6), Salix, Rumex spp., Caltha-type, Montia and Lychnis/ Dianthus-type, whereas Populus, Juniperus and Empetrum were better represented. These fluctuations are interpreted as reflecting drier conditions which destabilized the birch-forests (Coope and Joachim, 1980; Pennington, 1986),
in particular on sandy, well-drained eskerslopes within the catchment. The resulting more open conditions (cf. influx decrease) obviously favoured Empetrum, followed by maxima of Juniperus and Populus. The resemblance to a progressive succession is striking, ending with closure of the Betula forests (cf. rising influx) with increased Populus proportions. Thus, the early E4b-changes indicate a strengthening of the drier dwarf-shrub communities (3) and Empetrum-birch forests (6b) at the expense of the moister birch-, willow- and herb-communities. However, some communities in more protected areas on soils with sufficient water supply, e.g. in lake margins, seem unaffected by the drier conditions (cf. Filipendula, community 4). New species (Polemonium caeruleum, Solanum dulcamara) are also recorded in these communities (4, 6a). In the drier herb-communities (1) Jasione montana, Plantago coronopus, Cynoglossum and Scleranthus cf. perennis show their first local late-glacial appearance. In the last part of E4b, Populus, Juniperus and Empetrum slowly decrease, whereas Salix, Rumex spp., Betula nana and Caltha rise. Montia reappears and the pollen-assemblage indicate the turn to conditions as in late E4a. E4c (Betula nana-Koenigia-Lychnis/Dianthus subzone, c. 11,000-c. 10,600 B.P.) is interpreted as representing more open vegetation although a small increase in total pollen influx is recorded (Fig.4). However, tree-pollen decreases both in percentages (to 20-30~/o ZP) and absolute values (Figs.4, 6), predominantly originating from Betula pubescens s.s. according to a D/P-ratio maximum about 8 (Birks, 1968; Van Leeuwaarden, 1982). The remaining Betula pollen reflects increased proportions of Betula nana-type, reaching c. 15% ZP. Otherwise E4c shows rises in Salix (influx), Empetrum, Caltha, Chenopodiaceae and Rumex sect. longifolius (% and influx), decreases in loss-onignition, Populus, Juniperus, Filipendula and Urtica and appearance and/or better representation of Arctostaphylos alpina, Echium, Lych-
192
nis/Dianthus-type, Koenigia, Cerastium-type, Saxifraga spp., Sagina and Alchemilla. Together, these features suggest degradation of the local forests to open birch vegetation (6c), an increased vigour of the shrub-and dwarfshrub-vegetation (5, 3) due to improved light and more widespread herb-vegetation (1, 2). Especially, Koenigia indicates expanding wet, open-ground communities including extreme snow-beds (2a). The E4b/E4c changes were probably initiated by the Younger Dryas climatic deterioration. A moss layer just above the E4b/E4c-transition (758-759 cm) also containing leaves of Betula nana, confirms this interpretation. The layer consists of Drepanocladus uncinatus (main constituent), Rhizomnium pseudopunctaturn (several leaves and stems) and a few stems of Polytrichum juniperinum and P. norvegicum (det. D.O. Ovstedal). Their good preservation suggests deposition when fresh and the moss fragments may thus be contemporaneous with their stratigraphical position. Though Rhizomnium pseudopunctatum can reflect wet, closed vegetation, the moss assemblage suggests snow-beds or snow-bed influence (D.O. Ovstedal, pers. commun.). Moreover, Polytrichum norvegicum rarely occurs below the tree-limit (Nyholm, 1954-1969) and is almost exclusive to late snow areas. The moss layer in combination with the decreasing loss-on-ignition values suggest the onset of erosion of vegetation and soil. In conclusion, the E4 local pollen-assemblage includes two episodes that are usually biostratigraphically distinct in NW European late-glacial studies, namely the Allerod and Younger Dryas periods. Regionally, E4a + E4b correlate with the Allerod-zones $4 (Paus, 1988) and L5 (Paus, 1989), whereas E4c is related to S5a and the first half of L6, i.e., the early YD.
c. 10,600-c. 10,000 B.P.: Betula-SedumCyperaceae local pollen-assemblage zone (E5) E5 is represented by heterogenous lithology; silty sand (loss-on-ignition 8%) in the lower part and silty gyttja (loss-on-ignition 35-50%)
in the upper part. The E4/E5-transition is characterized by the sudden appearance of a volcanic ash layer, only detectable by polarizing microscopy (Persson, 1966) and referred to the Vedde Ash Bed (Mangerud et al., 1984). The same ash layer was demonstrated by Fsegri (1940), but wrongly correlated to the late Allerod Laacher See Tephra (cf. Mangerud et al., 1984, see below). Biostratigraphical features at the transition are rising Cyperaceae, Juniperus, Filipendula, Urtica, Sedum and aquatic pollen, all well-represented throughout the zone, and decreasing Betula, Salix, Empetrum and Lychnis/ Dianthus-type. Betula (varying between 20-30% ZP) includes dominant B. pubescens s.s. according to a D/P-ratio of about 8, and decreasing and low representation of B. nana-type (Fig.6). E5 reflects the last half of the Younger Dryas Chronozone (Mangerud et al., 1974) and is divided into two subzones. E5a (Artemisia-Dryas-Ononis peakzone, c. 10,600-c. 10,500 B.P.) includes maxima in the taxa naming the zone, and distinct minima in total pollen influx (1000-1300 grains cm-2 a-1) and Empetrum (5%). The Betula analysis (Fig.6) and low Betula influx (250-300 grains cm -2 a - l ) indicate very scattered or even a temporary absence of local tree-birch vegetation (6c). Improved light, unstable soils and/or soil erosion (loss-on-ignition 8%) favoured open-ground communities, especially la (cf. Artemisia, Dryas). Soil break-up probably caused the Empetrum decrease and also the flourishing of aquatics due to the inwash of nutrients to the basin. However, the stable influx/increasing percentages of Filipendula and Urtica indicate that more protected areas retained fertile, humus soils. Most of the Ononis-type in E5a is small (< 25 Ilm) and referred to Astragalus alpinus. Some bigger and more coarsely sculptured grains belong to Oxytropis/Astragalus-type, which includes species such as Oxytropis campestris, Astragalus glycophyllus and A. norvegicus. Their close pollen-morphological similarity excludes any species determination.
193
Oxytropis campestris may, however, be a possible contributor to the Ononis-type curve. In S Norway today, O. campestris occurs only in landslide slopes in Ryfylke (Lid, 1985), c. 50 km northeast of Eigebakken, where it gives the impression of being a late-glacial relict. Parallels can be drawn to Artemisia norvegica, also with a disjunct distribution and occurring in Ryfylke (Ryvarden and Kaland, 1968). E5a is directly related to S5b at Sandvikvatn (Paus, 1988). Both reflect the sparsest local vegetation in YD and represent the time when YD ice reached its maximum extent in SW Norway (10,500-10,700 B.P.; Anundsen, 1985). E5b (Poaceae subzone, c. 10,500-c. 10,000 B.P.) reflects pronounced humus-soil development and vegetational development. Total pollen influx rises to 2500-3600 grains cm- 2 a - 1, comparable to modern birch-/coniferforests (Ritchie and Lichti-Federovich, 1967; Hyvfirinen, 1976). However, Cyperaceae and Poaceae constitute 40-50% ZP, possibly reflecting local over-representation of, for example, a well-developed telmatic flora. Otherwise, loss-on-ignition rises to 40-50% and Betula influx, almost exclusively including tree-birch pollen (Fig.6), reaches 700-1100 grains cm -2 a -~. Populus, Empetrum, Juniperus, Filipendula and Polypodiaceae gain better representation, whereas mineral-soil taxa such as Artemisia, Koenigia, Dryas, Ononis-type and Rumex sect. longifolius decrease. This all suggests the expansion of humus-soil communities such as tall-herb birch-forest (cf. Prunus padus, Solanum dulcamara, Trollius), Empetrum-birch-forests (cf.
Populus, Rhamnus catharticus,
c. 10,000-c. 9700 B.P.: Betula local pollenassemblage zone (E6) In E6 the sediments become highly organic (loss-on-ignition>55%), total pollen influx rises to more than 5000 grains cm -2 a -1 (comparable to modern forests; Ritchie and Lichti-Federovich, 1967; Hyvfirinen, 1976) and Betula increases to 55% Z P and 2800 grains cm -2 a -t, respectively. Betula analysis shows no representation of B. nana-type and the D/Pratio, displaced towards 8.5 (Fig.6), indicates increased proportions of B.pendula (Van Leeuwaarden, 1982). This further closing of local vegetation into dense birch forests caused the extinction or reduced representation of light-demanding, low competitive herbs (e.g., Dryas, Helianthemum, Plantago major, Sedum, Apiaceae) during the Potentilla-Campanula subzone (E6a). The local Betula pubescens/B.pendula-forests reached their greatest density in the subsequent Juniperus-Salix subzone (E6b), when a shrub-layer fully developed.
Climatic history The following climatic interpretation of the Eigebakken late-glacial pollen diagram is based on the qualitative indicator-species method and the quantitative pollen-assemblage method described in Paus (1989). The latter uses the ratio between the representation of mineral-soil taxa versus humus-soil taxa, to measure the degree of soil erosion/cryoturbation within the local area. High ratios are interpreted as reflecting widespread soil erosion initiated by harsh winter conditions such as low temperatures and strong winds (Paus, 1988, 1989).
Vaccinium-
type), juniper-dwarf-shrub heaths (3) and tallherb grasslands (4) at the expense of openground communities (1, 2). In E5b the aquatics reach their late-glacial influx maxima. Especially, Myripohyllum alterniflorum is well represented (Fig.5). Simultaneously, there are the first occurrences of the telmatic Cladium mariscus and Typha latifolia.
Paz E1 (Oldest Dryas). Single occurrences of Echium and Hippophag indicate a July mean temperature of about 12°C (Skre, 1979; Kolstrup, 1980), whereas the slightly better represented Filipendula, Urtica, and Myriophyllum spicatum suggest a July mean not less than 8-10°C (Kolstrup, 1980). As pronounced pollen redeposition is recognized in E1 (see
194 above), these indications may, however, be unreliable. High frequencies of reworked pollen and mineral-soil taxa suggest strong soil erosion within the newly deglaciated terrain. Cold winters (Atkinson et al., 1987) and strong winds, probably katabatic (Nickling and Brazel, 1985), may have been important influencing factors. As E1 lasts about 1000 years, i.e., considerably longer than could be explained by migrational delay (Fridriksson, 1975) or natural succession (Fsegri, 1934; Lindroth, 1965), this earliest paz reflects a pioneer vegetation prevented from further development by climatic extremes.
Paz E2 (early BoUing). At the E1/E2-transition, humus-soil taxa increase at the expense of mineral-soil taxa. Reworked pollen decline suggesting reduced erosion and stabilized, developing soils. The causal factors probably included milder winters (Atkinson et al., 1987) and a weaker wind-influence from the retreating ice. Local indications of a July mean reaching at least 10-12°C are given by Hippopha6, Sanguisorba minor, S. officinalis and frequent Myriophyllum spicatum (Kolstrup, 1980). The El/E2 biostratigraphical changes reflect the effects of the B~lling climatic amelioration. These are assumed approximately synchronous with almost identical Bolling changes elsewhere in Rogaland (c. 13,000 B.P.; Chanda, 1965; Thomsen, 1982; Paus, 1988, 1989). Some metachroneity within Rogaland is, however, likely due to differences in the geographical and ecological setting of the localities (Fsegri and Iversen, 1975; fig.23). Throughout E2 climate improved, probably reaching the late-glacial thermal maximum in middle Bolling (Atkinson et al., 1987; Vorren et al., 1988). At the E2/E3-transition, about 12,650 B.P., the local climate had passed the ecotone of tree-birches. However, due to delays in migration and soil maturation (cf. Wright, 1984; Birks, 1986), this critical threshold may have been reached earlier.
Paz E3+ E4a, b (late Bolling-Allerod). July means not less than 10°C for the whole period are indicated by Viburnum, Sanguisorba minor, S. officinalis, Myriophyllum alterniflorum and Nymphaea (Kolstrup, 1980). Furthermore, in E3 Hippopha6 suggests a July mean of at least 11-12°C, whereas the July mean reached 13-14°C or more in E4b, according to the occurrences of Solanum dulcamara and Cynoglossum (Iversen, 1954; Skre, 1979). One single grain of Pleurospermum austriacum suggest the E3 J a n u a r y mean below -2~'C (Iversen, 1954). Throughout this interstadial tree-birch period, the pollen assemblage shows a dominant representation of humus-soil taxa which, in turn, suggests stable soil conditions. Although rather fluctuating, the pollen curves show no clearly concurrent patterns and no distinct climatic oscillations such as the Older Dryas deterioration can be demonstrated. Similar conclusions are reached in the Jseren studies of Thomsen (1982). In contrast, F~egri (1940) demonstrated the "Br~ndmyra Interstadial" followed by a climatic deterioration. However, his interpretation is questioned. On the other hand, in N Rogaland three short-lasting deteriorations are detectable within the Bolling-Aller~d complex (Paus, 1988, 1989). These appear as periods unfavourable to local woody vegetation and with increased proportions of mineral-soil taxa. The diverging biostratigraphial records between N and S Rogaland are probably due to more mature and denser vegetation (Betula>50% EP) with higher inertia to change (Smith, 1965) in the southern areas. In the north the interstadial vegetation was closer to the ecotone of birch-forest (Betula 25-40% EP) and was, thus, more sensitive to climatic fluctuations (see fig.9, cf. Feegri and Iversen, 1975; fig.23, Watts, 1980). Although distinct biostratigraphical traces of climatic deteriorations are lacking in the Eigebakken diagram, such events may weakly be indicated in the lithological sequence. At about 12,200 B.P., 11,750 B.P. and 11,200 B.P.
195 (interpolated ages), loss-on-ignition values have distinct minima; the oldest also being accompanied by a 1 cm thick sandy/silty horizon. These could reflect periods of unstable soil and soil erosion initiated by climatic oscillations. There seems to be good chronological conformity between the Eigebakken loss-on-ignition minima and the N Rogaland interstadial deteriorations (Paus, 1988, 1989). From c. 11,600 B.P., i.e. early E4b, drier conditions are indicated by slightly more lightopen local birch-forests (see above). Most likely the causal factors involve decreasing precipitation and/or increased summer temperatures (cf. Coope and Joachim, 1980; Pennington, 1986). At the same time (if the chronological correlations are reliable), climatic change led to a closing of the N Rogaland birch-forests (Paus, 1988, 1989). This illustrates how the same regional climatic fluctuation can result in different local vegetational changes, depending on the geographical and altitudal position, exposition, soil type, etc. of the site.
Paz E4c ÷ E5a reflect the Younger Dryas climatic deterioration that caused local treebirch decrease and disappearance, vegetational disruption and soil erosion (see above). The climatic shift involved drops in summer and winter temperatures (Iversen, 1954; Rind et al., 1986; Atkinson et al., 1987) and the Scandinavian ice-sheet halted or readvanced. At about 10,500-10,700 B.P. the ice-front reached the outer part of Lysefjorden, c. 25 km northeast of Eigebakken (Fig.l), and simultaneously its maximal extension in SW Norway (Anundsen, 1985). This coincides chronologically with E5a, representing the YD period of strongest cryoturbation and soil erosion (cf. loss-on-ignition minimum, maximum in mineral-soil taxa). Thus, strong katabatic winds (Nickling and Brazel, 1985) may have been one of the factors that made E5a the period of maximal climatic stress during YD. On the other hand, local summer temperatures were not critical. According to the occurrences of
MyriophyUum alterniflorum, Sanguisorba officinalis and Filipendula, July mean temperature never reached below 9-10°C (Kolstrup, 1980). If not redeposited, the single-grain occurrences in E4c of Mercurialis perennis and Echium indicate a July mean of 11-13°C or more (Skre, 1979; Kolstrup, 1980). Paz E5b ÷ E6 reflect the effects of the climatic amelioration associated with the onset of the Holocene, initiated already in YD Chronozone about 10,500 B.P. (interpolated age). However, due to delays in soil maturation and plant migration (Iversen, 1954; Wright, 1984), the improvement is not fully biostratigraphically manifested until E6 (PB), when dense local birch forests establish. Nevertheless, occurrences of Typha latifolia and Solanum dulcamara in early E5b indicate a rapid increase in summer temperatures with a July mean above 13-14°C (Iversen, 1954; Kolstrup, 1980). Later, July means of at least 15-16°C are suggested by the sparse occurrences of Cladium mariscus, Rhamnus catharticus and Prunus spinosa (Von Post, 1925; Skre, 1979). In addition, Cladium suggests a J a n u a r y mean not less than - 5 ° C (Von Post, 1925; Conway, 1942). Usually the date of this final "Holocene" amelioration falls within the interval 10,000-10,200 B.P. in W Norway (e.g., Krzywinski and Stabell, 1984; Braathen and Hermansen, 1985; Austad and Erichsen, 1987; Kristiansen et al., 1988; Paus, 1988). At Eigebakken, however, this event occurs at about 10,500 B.P. (if reliably interpolated). Similar early dates are recorded elsewhere in NW Europe (e.g., Pennington, 1977b; Lowe and Walker, 1980; BjSrck and MSller, 1987) indicating that the finiglacial climatic improvement is a metachronous, biostratigraphical event. Early biostratigraphical records of this amelioration may be from sites that supported vegetation with a close ecotonal position. In such sites local vegetation could rapidly respond and change to a different vegetational type following climatic amelioration.
196
Hippopha# rhamnoides, Polemonium caeruleum, Gypsophila) a n d t h e o c e a n i c e l e m e n t s (e.g., Sanguisorba minor, Montia fontana, Cladium mariscus). T h e a r c t i c e l e m e n t is r e p r e s e n t e d by only one t a x o n , Papaver radicatum-type. All
Plant geography T h e s a m e p h y t o g e o g r a p h i c a l division of the p r e s e n t N o r d i c flora used by P a u s (1988, 1989) and b a s e d on B e r g l u n d (1966), g r o u p 73 of the 141 l a t e - g l a c i a l pollen a n d spore t a x a as s h o w n in Fig.7. F o r t h e t o t a l late-glacial, the m a j o r p l a n t - g e o g r a p h i c a l p a t t e r n s a r e d o m i n a n t bor e a l - c i r c u m p o l a r (e.g., Populus tremula, Betula nana, Parnassia palustris) a n d e u r a s i a t i c species (e.g., Rhamnus catharticus, Pleurospermum austriacum, Anthyllis vulneraria) a n d f r e q u e n t a r c t i c - a l p i n e e l e m e n t s (e.g., Koenigia
e l e m e n t s w e r e p r e s e n t d u r i n g the pre-birch period (El + E2). In t h e following b i r c h period ( E 3 + E4a, b), b o r e a l - c i r c u m p o l a r a n d eurasiatic e l e m e n t s w e r e the m o s t i m p o r t a n t , also a m o n g the new a r r i v a l s , w h e r e a s the arctica l p i n e species g a i n e d i n c r e a s e d i m p o r t a n c e during the Younger Dryas climatic deterioration ( E 4 c + E5a). C o r r e s p o n d i n g p h y t o g e o g r a p h i c a l p a t t e r n s are r e c o r d e d in late-glacial studies from N R o g a l a n d (Paus, 1988, 1989), S S c a n d i n a v i a (Berglund, 1966) and I r e l a n d (Mitchell, 1954). This s u g g e s t s t h a t N W E u r o p e
islandica, Dryas octopetala, Selaginella selaginoides). Less r e p r e s e n t e d a r e the b o r e a l - a l p i n e (e.g., Betula pubescens, Trollius europaeus, Myricaria germanica), the c o n t i n e n t a l (e.g.,
PHYTOGEOGRAPHICAL GROUPS HULTEN 1950
LATE WEICHSELIAN,TOTAL SANDVIKVATN (PAUS 1988) EIGEBAKKEN LIASTEMMEN ( PAUS 1989) lo ,,,
20:
j
....................................................
t
LOCAL ]~-~PERIODS
]
P A Z E4c+ESo( 45 T A X A ]
PAZ E4c+ESb{ 8 N E W
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CONTINENTAL
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Fig.7. The relative frequencies of phytogeographical groups for the total Late Weichselian, for the three main periods (El + E2, E3 + E4a, b, E4c + E5), and for the new taxa within each period. Because of the low number of new taxa in E4c + E5 their relative frequencies are stippled. Comparable data from N Rogaland (Paus, 1988, 1989) are added.
197 was plant-geographically uniform during the late-glacial. According to Hult~n's (1950) division into migration groups, about 60~/o of the late-glacial Eigebakken taxa included in Fig.7, are classified as so-called southern species. All are found in the early late-glacial on the Continent. A similar pattern exists among the eastern/south-eastern taxa (e.g., Parnassia, Pleurospermum, TroUius) and the arctic-alpine (e.g., Koenigia, Dryas), which together suggest that the predominant migration route was from the Continent into SW Norway (Danielsen, 1971; Paus, 1988). One single grain in E5b of the entomophilous Polemonium caeruleum, not previously found in the Norwegian late-glacial, suggests local occurrence. In the middle Weichselian, this south-eastern species was better represented in western Europe than today (Godwin, 1975; Kolstrup, 1980). Danish late-glacial finds of Polemonium-pollen (Iversen, 1954; Kolstrup, 1982) obviously represent its northwards spread into SW Norway. Thus, Polernonium caeruleum seems to parallel other eastern/ south-eastern species, e.g. Pleurospermurn austriacum and Gypsophila fastigiata/repens, that were more westerly distributed in Europe in the Late Weichselian than today (Iversen, 1954; Berglund, 1966; Webb and Moore, 1982).
The ~Brondmyra interstadial" of F~gri (1940) The two main pollen diagrams in which F~egri (1940) demonstrated the Brondmyra interstadial (biozone III), were from the Jseren localities Brondmyra and Eigebakken. Later, it has become widely accepted that the Brondmyra interstadial correlates with the Bolling interstadial (cf. Chanda, 1965). However, Thomsen (1982) re-interpreted the Brondmyra diagrams of F~egri (1940) and Chanda (1965) and concluded that the Brondmyra interstadial belongs to the earliest part of Younger Dryas Chronozone. A correlation between the two Eigebakken diagrams (Feegri, 1940 and this paper, Fig.3) is therefore essential in resolving the chronological problem of the Brondmyra interstadial.
Figure 8 shows the Eigebakken diagram of F~egri (1940) converted into a total pollen diagram. My re-interpretation resulting from correlation with the re-analysed diagram (Fig.3) is included.The lithostratigraphical correlation between the two diagrams is not straightforward. However, the sandy layer (Fsegri: 600-605 cm, this paper: 744-749cm) containing the first part of the Vedde Ash Bed (Mangerud et al., 1984), is easily recognized in both diagrams. Otherwise, there may be parallels between F~egri's bottom "Ton Gyttj a" layer (767 cm --*) and the clayey gyttja below 839 cm in Fig.3. Similarly, the "Diatomeen-erde" of Fsegri (605-767 cm) may correlate with the clayey gyttja layers between 757 and 839 cm in Fig.3, and lastly, Fsegri's gyttja layer (560-600 cm) may equate with the three upper layers of minerogenic gyttja/dy (698-744 cm) in Fig.3. Biostratigraphically the two diagrams are highly conformable. Biozone I of Feegri, in the upper part delimited by a Betula increase and Salix/Poaceae/Cyperaceae decreases, correlates with paz E2. The Ericales rise represents the upper limit of F~egri's zone II, thus paralleling E3. Zone III (Brondmyra interstadial) constitutes the first quarter of E4, zone IV (previously correlated with Older Dryas) the second quarter of E4, whereas zone V of F~egri comprises the last half of E4 plus E5. Diverging characteristics are the absence of zones comparable to E1 and E6 in Fsegri's diagram (Fig.8). Furthermore, due to fewer pollen constituents in Fig.8, the percentages are here generally higher than in Fig.3. In conclusion, the chronostratigraphical reinterpretation of the Eigebakken diagram of Fsegri (1940) is as follows: Zones I and II constitute the Bolling Chronozone; zone III (Brondmyra interstadial) has the approximate age of the Older Dryas Chronozone; zone IV comprises the first part of the Allerod Chronozone, whereas zone V, by Fsegri called the Allerod interstadial, constitutes the last half of Allerod plus Younger Dryas Chronozones. Consequently, in the two main diagrams in which the Brondmyra interstadial has been
198
o,
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IV 313
III
BC E 3
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I
GYTTJA
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m
CLAYGYTTJA
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DIATOME GYTTJA
Fig.8. The Eigebakken diagram of F~egri (1940) converted into a total pollen diagram. A re-interpretation, based on correlation with the re-analysed Eigebakken diagram (Fig.3) and including chronozones, is added. demonstrated, it appears as two metachronous biozones (OD/early YD Chronozones in Eigebakken/Brondmyra, respectively). In neither of the two diagrams, can the interstadial be correlated with the Bolling interstadial. Moreover, zone III is not refound as a distinct biostratigraphical unit either in the re-interpretated Brondmyra diagrams (Thomsen, 1982) or in the re-analysed Eigebakken diagram (this paper). The only criterion of Fsegri (1935, 1940) for distinguishing the Brondmyra interstadial was Betula-pollen diameter statistics showing the presence of B. pendula pollen. His results are, however, not confirmed by the new Betula analyses from Eigebakken (Fig.6). Several abiotic factors influencing grain-size (Ander-
sen, 1960, 1980; Cushing, 1961; Praglowski, 1966; Kristiansen et al., 1988; Paus, 1988), could explain this divergency. Furthermore, these abiotic factors represent a warning for drawing firm conclusions solely based on Betulapollen size statistics. Conclusions Due to the several sources of dating errors, the Eigebakken chronology, and hence the influx estimates, are to great extent based on assumed ages. The assumptions refer to dates in other late-glacial studies from Rogaland (Chanda, 1965; Thomsen, 1982; Paus, 1988, 1989).
199 BIRCH FOREST OPEN BIRCH VEGETATION SHRUB VEGETATION OPEN-GROUND VEGETATION
_~cc~E. STAG ~___SS LIASTEMMEN
L1
~
L2 11 L3~ blL41~ LS_ .__bb
SANDVlKVATN
$1
I
$2
EIGEBAKKEN YEARS
E1 r 141',000
B.P.
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E2
E3
13,000
S4a E4a 12,000
c
b
d
e l
c
d
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~
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b
c 11,000
10,000
L
P E R I O DS
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PLE N I G L A C I AL
~
"M-~ I'-MU M JULY MEAN (°C) ~ E " ~ + STRONG Wl NDS
YD STADIAL
POST-GL ACIAL
15 - 1 6
Fig.9. Survey of the Late Weichselianvegetational and climatic history in Rogaland based on results from J~eren (Chanda, 1965; Thomsen, 1982; this paper) and N Rogaland (Paus, 1988, 1989). Chronological correlation of the biozones from Eigebakken, Sandvikvatn (Paus, 1988), and Liastemmen(Paus, 1989) is added.
From local deglaciation, about 14,000 B.P., to 11,000 B.P. the Eigebakken diagram demonstrates a local vegetational succession including four progressive stages (Fig.9). The earliest, the Artemisia-dominated pioneer stage, lasted to c. 13,000 B.P. and reflects discontinuous vegetation cover of open-ground communities on disturbed, mineral soils. The subsequent Salix-shrub consolidation stage ending c. 12,650 B.P., represents stable soils, humus-soil development and vegetational closure. From c. 12,650 t O c. 12,200 B.P. open tree-birch vegetation established locally. Thereafter a birch-forest stage commenced. At about 11,000 B.P. retrogressive succession was initiated by a change in the birch forests to open birch vegetation. Simultaneously, dwarfshrub heaths and open-ground communities expanded. In a short-lived and probably temporary treeless period, from c. 10,600 to c. 10,500 B.P., open-ground vegetation reached its maximum extention. Finally, rapid vegetational closure is reflected, ending with the establishment of dense, local Holocene birchforests.
This vegetational development indicates a tripartite division of the local, late-glacial climate into a pleniglacial period (older than 13,000 B.P.), an interstadial (13,000-11,000 B.P.) and the Younger Dryas Stadial (Fig.9). Cold winters and strong, probably katabatic winds, exposing vegetation and soil to drought and erosion, are proposed as critical factors in Younger Dryas and the pleniglacial period. Throughout the late-glacial the July mean did not fall below 8-10°C and reached at least 11-12°C in the first half and 13-14°C in the last half of the interstadial. In early parts of Younger Dryas J u l y mean was probably about 11-13°C. Decreasing precipitation and/or warmer summers in the mid-interstadial (early paz E4b; from about 11,600 B.P.) may explain the slight and temporary thinning of the local birch forests and the increase of vegetation t h a t prefers drier habitats. Otherwise there is no biostratigraphical evidence of climatic oscillations such as the Older Dryas deterioration within the interstadial. By contrast, late-glacial studies from N Rogaland (Paus, 1988, 1989) show three
200 short-lived, interstadial climatic deteriorat i o n s . T h i s d i v e r g e n c y m a y be e x p l a i n e d by dense and mature birch-forests with high i n e r t i a ( S m i t h , 1965) i n t h e J~eren a r e a , whereas the N Rogaland interstadial vegetation lay close to the ecotone of birch-forests and was thereby more easily influenced by c l i m a t i c o s c i l l a t i o n s (see Fig.9). T h e f a s t e r d e v e l o p m e n t i n t o s t a b l e b i r c h - f o r e s t s o n J~eren is p r o b a b l y a s s o c i a t e d w i t h t h e m o r e s o u t h e r n p o s i t i o n w h i c h , in t u r n , s u g g e s t s a g e n e r a l l y milder climate and a shorter migrational delay. I n a d d i t i o n , E i g e b a k k e n is s h e l t e r e d b y w e l l developed eskers, thereby causing a favourable local climate. My re-analysis of the Eigebakken material t o g e t h e r w i t h T h o m s e n s ' s (1982) r e - i n t e r p r e t a t i o n o f t h e B r o n d m y r a d i a g r a m s (F~egri, 1935, 1940; C h a n d a , 1965) s t r o n g l y i n d i c a t e t h a t t h e e x i s t e n c e o f ' ~ B r o n d m y r a i n t e r s t a d i a l " o f F~egri (1940) m u s t be r e j e c t e d . I n t h e s e d i a g r a m s , B r o n d m y r a b i o z o n e (III) a p p e a r s to be m e t a chroneous and includes the Older Dryas Chron o z o n e in t h e E i g e b a k k e n m a t e r i a l a n d t h e early part of Younger Dryas Chronozone in the Brondmyra diagrams. The only biostratig r a p h i c c r i t e r i o n o f F m g r i (1935, 1940) f o r distinguishing the interstadial was his results from Betula-pollen statistics. Otherwise there is n o b a s i s f o r t h e b i o s t r a t i g r a p h i c a l u n i t s corresponding with "Brondmyra interstadial" in a n y o f t h e E i g e b a k k e n / B r o n d m y r a diagrams.
Acknowledgements Knut Krzywinski and Lotte Selsing did the field w o r k . P r o f . K n u t F~egri p l a c e d t h e m a t e rial to my disposal. Eva Krzywinski prepared the pollen samples. Dating problems were discussed with Steinar Gulliksen, Siri Herland drew some of the figures and Hilary Birks and Dag Olav Ovstedal determined the macrofossils. I t h a n k all. I a l s o s i n c e r e l y t h a n k J o h n Birks for reading and correcting the manuscript. This work was financed by the Norweg i a n C o u n c i l for S c i e n c e a n d H u m a n i t i e s ( G r a n t D.71.49:038).
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177
LATE WEICHSELIAN VEGETATION, CLIMATE AND FLORAL MIGRATION AT EIGEBAKKEN, SOUTH ROGALAND, SOUTHWESTERN NORWAY AAGE PAUS
Botanical Institute, University of Bergen, Alldgt. 41, N-5007 Bergen (Norway) (Received November 17, 1988; revised and accepted April 25, 1989)
Abstract Paus, Aa., 1989. Late Weichselian vegetation, climate and floral migration at Eigebakken, South Rogaland, southwestern Norway. Rev. Palaeobot. Palynol., 61:177 203. The Eigebakken, Jseren, pollen diagram shows a tripartite division of the Late Weichselian into three main climatic periods. The pleniglacial, from local deglaciation c. 14,000 B.P. to c. 13,000 B.P., reflects an Artemisia-dominated pioneer vegetation on disturbed mineral-soil, prevented from further development by cold winters and katabatic winds. The Bolling amelioration opens the Late Weichselian Interstadial (13,000-11,000 B.P.) and initiates soil development and vegetational closure into a Salix.shrub consolidation phase (to c. 12,650 B.P.). Thereafter an open birch vegetation phase (to c. 12,200 B.P.) follows. The subsequent birch-forest phase (c. 12,200 11,000B.P.) reflects the interstadial vegetational and edaphical optimum. In this phase July mean temperature reached at least 14°C. In contrast to late-glacial studies from N Rogaland, the Eigebakken diagram gives no biostratigraphical indications of climatic deteriorations such as "Older Dryas" within the interstadial. This is probably explained by denser local birch forests with higher ecological inertia on Jmren. Furthermore, no traces of Fsegri's "Brondmyra interstadial" are recorded. The Younger Dryas Stadial (11,000-c. 10,500 B.P.) shows a two-step regressional succession. In the first step (to 10,600 B.P.) the birch forests degraded into open birch woodland. The second phase (10,600-c. 10,500 B.P.) involved the maximum extent of open-ground vegetation and possibly temporary local deforestation. Critical climatic factors included cold winters and strong winds. The first vegetational responses of the Holocene climatic amelioration are recorded locally as early as c. 10,500 B.P. Tree-birches re-established, finally developing into dense forests c. 10,000 B.P. Boreal-circumpolar, eurasiatic and arctic-alpine plants dominated the late-glacial flora. For the majority of the late-glacial taxa a northward migration into SW Norway is suggested.
Introduction T h i s p a p e r is t h e t h i r d c o n t r i b u t i o n f r o m a project aimed at reconstructing the vegetational and climatic development during the L a t e W e i c h s e l i a n i n S W N o r w a y (see P a u s , 1988, 1989). T h e E i g e b a k k e n b o g is o n e o f t h e t h r e e l a t e g l a c i a l l o c a l i t i e s w h e r e Fsegri (1935, 1940) demonstrated the ~'Brondmyra interstadial" followed by a climatic deterioration. This s e q u e n c e h a s l a t e r b e e n c o r r e l a t e d w i t h Boll-
0034/6667/89]$03.50
i n g a n d O l d e r D r y a s , r e s p e c t i v e l y (cf. C h a n d a , 1965). H o w e v e r , T h o m s e n (1982) q u e s t i o n s t h i s i n h e r r e i n t e r p r e t a t i o n of t h e B r o n d m y r a d i a g r a m s o f F~egri (1940) a n d C h a n d a (1965). She claims that no biostratigraphical zones r e p r e s e n t i n g the i n t e r s t a d i a l or the s u b s e q u e n t d e t e r i o r a t i o n , c a n be d i s t i n g u i s h e d . I n 1975, b e f o r e i n d u s t r i a l e x p a n s i o n t o t a l l y destroyed the Eigebakken locality, a new sedimentary sequence was obtained by coring at the locality. Its well-developed late-glacial deposits were considered too scientifically
© 1989 Elsevier Science Publishers B.V.
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important to be lost. Later, when I initiated my late-glacial project, Professor Knut Faegri kindly placed the new Eigebakken material at my disposal. In addition to elucidating the vegetational and climatic late-glacial development on Jseren, this re-analysis also attempts to resolve the interpretative problems of the “Brondmyra interstadial” and the “Older Dryas deterioration”. Site description Eigebakken bog (5”42’E, 58”47’N, 27 m a.s.1.) is located in the central part of Jzren, close to Froylandsvatn (Fig.1). The sizes of the catchment and the bog surface (before 1960) are estimated to about 1.8 km2 and 320,000 m2, respectively. The 1975material for pollen analysis was taken in a bog area between the Block Berge and Kvernaland factories, west of the railway. Today, due to further industrial expansion, there are only remnants of the bog. Moreover, the whole Froylandsvatn area is
Fig.1. Map of the Boknfjord
area and the site Investigated,
influenced by human impact, mainly agriculture and industry, and there are few traces to indicate that the landscape once belonged to the treeless coastal heath zone. For further description of the natural vegetation and flora of the area and Eigebakken, see Faegri (1940). The area has an oceanic climate with July and January mean temperatures of 14°C and 1 C, respectively (Bruun, 1967). Annual precipitation is about 1050 mm (Bruun and Haland, 1970). The bedrock in the eastern part of Jaeren is mostly Precambrian granites and gneisses: in the west Precambrian mica-gneisses dominates (Birkeland, 1970 in Andersen et al., 1987; Sigmond et al., 1984). Between these areas and across the investigation area there is a narrow, north- south running band of phyllite and mica shists (Sigmond et al., 1984). The Froylandsvatn area is covered by till and fluvial/glaciofluvial deposits (Andersen et al., 1987). Close to and west and north of the coring point, a welldeveloped esker system delimits the catchment and shelters the locality. The esker system
showing areas mentioned
m the text
179 runs in N E - S W direction, demonstrating a southwest transport of meltwater during deglaciation (F~egri, 1939; Andersen et al., 1987). Movement of the retreating ice was in the same direction. The marine limit in the area is estimated to about 15 m a.s.1, and the nearest Younger Dryas moraine is in the outermost part of Lysefjorden, c. 25 km NE of Eigebakken (Anundsen, 1985; Andersen et al., 1987). Thus, in the absence of any influence of the sea or Younger Dryas ice, Eigebakken has undisturbed sediments extending back to preAllerod deglaciation times. Methods
d
o00
/
7
000 le
Field work. K n u t Krzywinski (Botanical Institute, University of Bergen) and Lotte Selsing (Archeological Museum, Stavanger) carried out the field work. Material for pollen analysis was taken using a l l 0 m m - d i a m e t e r Livingstone sampler (see Table I for sediment description). Unfortunately, destructions in the bog area caused by industrial development, prevented thorough lithostratigraphic-transects of the bog. Laboratory work and analytical methods follow Paus (1988). Betula-pollen analysis was carried out using the approach of Paus (1988). Originally, in 19 selected samples (preparation: a c e t o l y s i s + H F ) from Bolling to Preboreal Chronozones, grain-diameters were measured a t 1280 × magnification to the nearest ocular line (one line: 0.79 I~m). This resulted in curves hard to interpret. An extended Betula analysis, including D/P-ratio statistics (Birks, 1968) and morphological studies (Terasma~, 1951), was then carried out in 13 samples. Six of these samples were not included in the previous Betula analysis. Pore depth was measured to the nearest ocular half-line. 100 grains or more per sample were measured. The pollen diagrams. Pollen data were processed, and the percentage- and absolutediagrams (Figs.3, 4, 5) drawn by the program CORE-SYSTEM (Michelsen, 1985). The per-
8SO
N ~
14
12
~.~ ['~'~L~t 1:[[~.½~,_., I..L~:t: ~k~LL~
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//
-o- NaOH soluble (A) fraction ........... ble ( B ) ' . . . . . . .
// / // 14,000
13.000
12,000
11,000
10,000
ye.rs e~ 9,OOb
Fig.2. Sedimentaccumulationcurve based on the assumed ages of dating levels. 14C-dates are shown with one standard error.
centage calculation basis, ZP, comprises the terrestrial (including assumed secondary) and unidentified pollen. For a constituent X within spores, aquatic pollen and pre-Quaternary microfossils the calculation basis is ZP+X. Sediment accumulation rates, used in estimating pollen influx, were calculated by linear interpolation between 14C-dated levels. The pollen diagrams contain 69 spectra analysed at 1.25-10 cm intervals. Z P per spectrum ranges from 793 to 2830 (mean 1997) comprising 30-59 (mean 38) taxa. The pollen diagrams include 131 terrestrial taxa. Pollen and spore types are grouped following Paus (1988). Plant nomenclature follows Lid (1985). Biostratigraphic zones, based on Figs.3 and 4, are defined as local pollen-assemblage zones (paz). Chronostratigraphy younger t h a n 13,000 B.P. follows
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182 TABLE I
Sediment lithology for the core from Eigebakken studied pollen analytically. The description is based on Troels-Smith's (1955) system. DEPTH cm
TYPE
COLOUR
CONSTITUENTS
Dgl, Ld33 Th + , Ag+
PHYSICAL CHARACTERISTICS
640-698
DY
DARK BROWN
698-719
SILTY GYTTJA/DY
GREENISH BROWN
Dgl, Ld22, Agl Th +, D1 +, As +
nig.2 sicc.2 strat.1 elas.2
719-731
SILTY GYTTJA
YELLOWBROWN
Dgl, Ld12, Agl Th + , As+
nlg. 2- sicc.2 strat.1 elas.3
731-744
SILTY DY
DARK BROWN
Dgl, Ld32, Agl Th+, Dh+, As+
nig.3 sicc.2 strat.! elas.2
744-747
MINEROGENIC GYTTJA
BROWN
Dg1,Ld21,As1,Agl Th+,Dg+,Gs+,Gg+
nig. 2 sicc.1 strat.1 elas.2
747-749
ORGANIC SAND
LIGHT BROWN
Ld21,Ga1,Gs1,Gg1 Th+, Dg +
nig. 2- sicc.1 strat.0 elas.1
749-758
CLAYEY GYTTJA
YELLOWBROWN
Dgl, Ld12, Asl Tb+,Th+,Dh+,Gg+
nig. 1+ slcc.2 strat.2 elas.3
758-759
MOSSLAYER
DARK BROWN
Tb14
nig.3 site.2 strat.0 elas.1
759-818
CLAYEY GYTTJA
LIGHT YELLOW
Dgl, Ld12, Asl Tb +, Dh +
nig.1 sicc.2 strat.1 elas.3
818-819
GRAVEL/ SILT
BROWNISH GREY
Ld11, Agl, Ga2
nig.1 sicc.1 strat.0 elas.1
819-832
CLAYEY GYTTJA
LIGHT YELLOW
Dgl, Ld12, Asl Dh+, DI+, Ag+
nig. 1 sicc.2 strat.1 elas.3
832-836
CLAYEY GY2~rJA
REDDISH BROWN
Dgl, Ld21, Thl, Asl
nig.2 sicc. 2 strat.2 elas.3
836-839
ORGANIC GRAVEL/SAND
YELLOWBROWN
Dg1,Ld11,Ga1,Gsl Ag+
nig. 1 sicc.2 strat.0 elas.1
839-854
CLAYEY GYTTJA
GREENISH BROWN
Dgl, Ld22, Asl T1 + , DI+
nig.2 sicc.2 strat.2 elas.2
854-858
ORGANIC GRAVEL/SAND
GREYISH BROWN
Ld11, Gal, Gs2 Dh+, D1 +
nig.1+ sicc.1 strat.1 elas.0
858-923
SILT
BROWNISH BLUE-GREY
Asl,oAg2, Gal Dh+,Ld +,Gs+,Gg+
nig.1+ sicc.1 strat.2 elas.0
Mangerud et al. (1974). Between local deglaciation and Bolling the term Oldest Dryas (DR 1) is used following Welten (1982). Independently of this chronostratigraphy the zonation of Firbas (1949) is added. Chrono-/bio-zone borders not dated are based on interpolated ages.
nlg. 3 sicc.2 strat.1 elas.2
R a d i o c a r b o n dates Ten gyttja samples (thickness 2.5-3.0cm) have been dated conventionally. In all levels the NaOH-soluble (A) fractions are dated; with one exception also the NaOH-insoluble (B)
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TABLE II Results and characteristics of the Eigebakken 14C-dates. The letter behind the lab reference indicates NaOH soluble (A) and NaOH insoluble (B) fractions of the samples. The datings were carried out by the Laboratory of Radiological Dating in Trondheim, Norway.
DATING LEVEL
0EPTH cm
LOSS ON MATERIAL ~ IGNIT- SECONOARY LAB.REF. ~ 10N(Z) POLLEN(Z)
14C_YEARS B.P.
~ 13C
PRONOUNCED DATING ERRORS
ASSUMED AGE B.P
10
700 - 702.5
SILTY GYTT3A 49-54
-
T-5895A
-22.9
9,900
9
746 - 748.5
SANDYGYTT3A ca.10
0.t
T-G1ESA T-61698
10 ,2402170 - 2 0 . 0 7 ,000_+100 -19.2
10,600
9
,9202100
DATING
FEATURE
BETULA INCREASE YD ASH BED
AOUAT. ROOTS
8
757.5 - 760
CLAYEY GYTT3A 26-35
-
T-5694A T-5894B
11 ,280+150 10 ,780~120
-23.9 -21.9
11,000
AL/YD TRANSITION
7
775 - 777.5
CLAYEY GYTT3A 32-35
0.1
T-TS86A !T-75068
11 ,0202110 11 ,7802150
-20.5 -20.7
11,400
JUNIPER MAXIMUM
G
807.5 - 810
CLAYEY GYTT3A 30-33
0.1
T-7585A ;T-7585B
12 ,120~120 12 ,110280
-20.0 -20.8
12,000
LOSS-ON-IGNITION MAXIMUM
5
817.5 - 820
MIN. GYTTJA
7-12
0.1
IT-5893A T-56938
12 ,670÷190 10 ,800~140
-20.8 -22.1
HARDWATER AOUAT. ROOTS
12,200
EPIPETRUMINCREASE
4
825
827.5
CLAYEY 8YTT3A
29-30
-
:T-7504A iT-7584B
12 ,800+210 13 ,4802150
-19.4 -17.7
HAROWATER
12,400
LOSS-0N-IGNITION
3
833.5
836
CLAYEY GYTT3A ca.lO
0.1
T-5692A :T-56928
13 ,180~170 9 ,580±120
-22.0 -22.1
HAROWATER AOUAT. ROOTS
12,600
8ETULA INCREASE
2
843.3-848.3
CLAYEY GYTT3A ca.14
0.6
T-TS83A T-TS838
13 ,790~150 14 ,370±190
-20.2 -21.0
HAROWATER REWORKE8 MAT.
12,800
SALIX MAXIMUM
1
851.5
854
CLAYEY GYTT3A ca.10
0.6
14 ,380._.210 - 2 2 . 2 12 ,390_+200 - 2 1 . 2
HAROWATER REWORKEDMAT. AOUAT. ROOTS
13,000
BOLLIN6 AMELIORATION
-
-
-
T-SGS1A T-58918
fractions are dated. Results are given in Table II and Fig.2. Discrepancies, in part considerable, between dates of different fractions from the same level indicate serious dating errors. In light of the well-developed aquatic macrophyte flora throughout the late-glacial, the calcareous local bedrock, and redeposition in the basal sediments (see below), both fractions may have been influenced by the hard-water effect (Donner et al., 1971) and/or reworked material (Olsson, 1974), whereas downwardpenetrating roots (Kaland et al., 1984) and/or inorganic carbon (Olsson, 1979) may have contaminated the NaOH-insoluble fractions. Due to the antagonistic effects of these sources of error, it is difficult to estimate quantitatively their influence. However, it is important to note how the A and B fractions from the same sample vary throughout the late-glacial deposits. Levels with the B frac-
MAXIMUM
tion more than 2000 z4C-years younger than the A fraction are situated in or just above sandy/silty horizons. These minerogenic layers may have been less penetrable for roots thereby representing root concentration levels in the sediments. The serious dating errors involved imply that an Eigebakken chronology must be defined by assumed ages of levels. These include the age of deglaciation (c. 14,000 B.P., see Anundsen, 1985; Paus, 1989), the transition minerogenic/organic sediments (c. 13,000 B.P., see Chanda, 1965; Thomsen, 1982; Paus, 1989), the AL/YD-transition (c. 11,000 B.P.) and the YD ash layer (c. 10,600 B.P., Mangerud et al., 1984). In addition, levels 6 and 10 (Fig.2, Table II) are considered reliable. These levels form the chronological basis for further discussion and provide the basis for estimating the sediment accumulation rates shown in Fig.2.
186
Late Weichselian vegetation history Introduction The Eigebakken results are, in many respects, very similar to those obtained from the other two sites included in my late-glacial project (Paus, 1988, 1989). In the following interpretation of the Eigebakken data, I will, when appropriate, refer to more detailed discussions and arguments in Paus (1988, 1989) to avoid frequent repetitions. Prior to the reconstructions and discussion of the late-glacial vegetation, some limitations and problems preventing a precise reconstruction of past vegetation (cf. Fsegri and Iversen, 1975; Birks, 1981), are stressed. First, the presence of reworked and deteriorated pollen in the minerogenic basal sediments causes interpretative problems (Iversen, 1936; see Paus, 1988, 1989). Second, the diameter statistics of Betula pollen show no clear patterns and often contradict the results from D/P-ratio statistics and the morphology study (Fig.6). As grain size is more sensible than D/P-ratio to abiotic factors such as preparation techniques (Kristiansen et al., 1988; Paus, 1988) and sediment types (Praglowski, 1966; Paus, 1989), the size statistics are omitted in the interpretation of the Betula analysis. Third, in the absence of reliable dates, the Eigebakken influx estimates are largely based on assumed ages. In particular, the assumed age of the basal sediments, representing the minimal age of local deglaciation, may deviate from the real age (cf. Andersen, 1979; Anundsen, 1985; Andersen et al., 1987). Furthermore, without any dates in the diagram top, the influx estimates are here based on extrapolated sediment-accumulation rates (Fig.2). In fact, influx estimates and hence, the establishment of a reliable chronology, represent one of the most serious problems in late-glacial studies. The necessary chronological precision will not be obtained until chronologies based on accelerator datings of terrestrial material, are
constructed (Andree et al., 1986; MacDonald et al., 1987; Lowe et al., 1988). Fourth, pollen-stratigraphical, -morphological and -dispersal limitations permit only a coarse recognition of late-glacial plant communities (cf. Firbas, 1949; Berglund, 1966; Webb and Moore, 1982). In the following discussion of vegetation history I will refer to the simplified classification into late-glacial plant communities used by Paus (1988, 1989). In brief, these communities are: (la) Dry, open-ground communities on rocks or unstable mineral soil. Vegetation discontinuous. (lb) Dry grassland on well-drained humus and shallow soils. Vegetation mostly continuous. (2a) Wet, open-ground communities, including extreme snow-beds. On rocks, in hollows and by streams, springs and flushes. Vegetation discontinuous. (2b) Moist grassland, including early-melting snow-beds. In more sheltered areas, on humus soil varying in moisture. Vegetation mostly continuous. (3) Juniper-dwarf-shrub heaths. Heterogeneous in ecology and composition. On humus soil varying in dryness, thickness and fertility. Vegetation mostly continuous. (4) Tall-herb grassland in sheltered areas on fertile, moist humus soil. Includes vegetation in wet lake margins. (5) Willow shrubs/copses. In favourable, moist sites with a tall-herb field-layer. (6a) Tall-herb birch-forests on fresh to moist humus soil. (6b) Empetrum birch-forest on well-drained, humus-rich soils. (6c) Open birch vegetation. Scattered trees/ copses on moist, deep soils.
Vegetation development c. 14,000-c. 13,000 B.P.: Artemisia-Pinus local pollen-assemblage zone (El) The E1 lithostratigraphy is clayey silt (losson-ignition 2%) containing 30-35% reworked
187
% BETULA OF Z P
GRAIN
o_
io%
SIZE
D/P-RATIO
(pm)
~
~±s n
I
cm
E6b
~/~/~ ~,/ /./.
a
1°~t8.°7 1 0 0 , 0.86
~"~t ° -~, 705
1z91j 1 0 0 10.821 ,/
\
,
,A ,
E5b
--x 752.5 ,_~ 755
,
,
.
|097
100
~ _ ~
765 -
Eao'L [~-:-'= "........ : - - ~ = - ~
.
N
.
.
.
-:~-
_h
--//
t10012~'53~
f/
"~-
~ \ ~ / ~
,
7825
200 2~:~
,
'-"
,
/
,
\
•
17~Z11001 ....
~95 ,oo ~:~
..
Fig.6. R e s u l t of t h e Betula a n a l y s i s from E i g e b a k k e n . T h e v a l u e s of t h e m o r p h o l o g i c a l Betula nana-type a r e i n b l a c k .
188 pollen (Fig.3). This may be an underestimate as unidentified pollen is not included. Parts of the pine pollen is assumed to be long-distance transported (see de Beaulieu and Reille, 1984). The presence of pre-Weichselian material in Oldest Dryas sediments is a common feature in Rogaland (Paus, 1988, 1989) and is most likely explained by erosion of glacial deposits within the catchments or by winds carrying eroded material from the North Sea Continent (cf. finds of marine dinoflagellate cysts). Influx estimates (220-340 grains cm 2 a-1) correspond to influx in newly deglaciated areas elsewhere (Ammann and Tobolski, 1983; Gaillard, 1985; Paus, 1988, 1989) and modern tundra (Ritchie and Lichti-Federovich, 1967; Fredskild, 1973). The taxonomically rich pollenassemblage includes arctic-alpines such as
Dryas octopetala, Papaver radicatum-type, Saxifraga spp. and presumably also Artemisia norvegica (cf. relict-localities in Ryfylke, c. 50 km NE of Eigebakken; Ryvarden and Kaland, 1968). Furthermore there are steppeelements such as taller Artemisia spp., Helianthemum and Hippophag rhamnoides and plants that today are weeds and/or coastal plants (Chenopodiaceae, Euphorbia, Plantago maritima, Spergularia spp., Polygonum avicularetype). The presence of plants with N-fixing root nodules (Dryas, Hippopha& Fabaceae, Ononistype, Lotus-type, Onobrychis) suggests immature soils. This, together with low influx and the present ecological demands of the El-taxa, suggest a discontinuous vegetation on mineral soil, possibly as mosaics of unstable and temporary communities such as la in exposed areas and 2a in sheltered areas. The better representation of Salix (S. herbacea/polaris?), Saxifraga oppositifolia-type and Cerastiumtype than in the pioneer phase of N Rogaland (Paus, 1988, 1989) is interpreted as more widespread wet, open-ground communities (2a) at Eigebakken. This may be ascribed to the eskers within the catchment creating a hummocky landscape with numerous lee-sides. Here, in the last part of El, some denser vegetation could have developed, as suggested
by the occurrences (if not secondary) of PimpineUa-type, Plantago major (dry grassland), Parnassia palustris, Polygonum bistorta-type (wet grassland), Filipendula, Urtica (tall-herb grassland), and Juniperus, Gymnocarpium dryopteris (juniper-dwarf-shrub heaths). The E1 aquatic macrophyte flora was poorly developed (Fig.5). Climate was probably the limiting factor, either directly (e.g., low temperatures) or indirectly (cryoturbation/soil erosion causing reduced light transparency in water). As in N R o g a l a n d (Paus, 1988, 1989) Sparganium-type is the pioneer type among the pollenproducing aquatics. E1 is directly related to the Oldest Dryas zones S1 and L1 in N Rogaland (Paus, 1988, 1989). Ecologically these are younger parallels to the Artemisia-dominated, pioneer Oldest Dryas zone on the Continent (de Beaulieu and Reille, 1984; Gaillard, 1985).
c. 13,000-c. 12,650 B.P.: Salix-RumexPoaceae local pollen-assemblage zone (E2) The E1/E2-transition occurs in a sandy layer. Otherwise E2 is represented by clayey gyttja (loss-on-ignition 12-14%). Biostratigraphic changes are increases in Salix, Rumex, Cyperaceae and aquatic pollen and declining Artemisia, Chenopodiaceae, Helianthemum, and Saxifraga oppositifolia-type. Simultaneously secondary pollen drops strongly. Thus, the actual increase in influx of locally produced pollen is more pronounced at the E1/E2transition than shown (Fig.4). The E2-influx reaches 400-500 grains cm 2 a - l , comparable to low-producing forest-tundra today (Ritchie and Lichti-Federovich, 1967). This vegetational closure reflects some humus-soil development. Open-ground communities declined (cf. disappearance of e.g. Papaver), whereas dry grassland (Myricaria, Sanguisorba minor, Rumex spp.) and wet grassland (Pinguicula vulgaris, Botrychium, Selaginella, Rhinanthus-type) expanded. The development of juniper-dwarf-shrub and tallherb communities is reflected by Juniperus, Betula nana-type (Fig.6), Urtica and Sanguisorba officinalis. High percentage- and abso-
189
lute-values of Salix most likely originate from willow shrubs/copses (5), representing the shrub-stage in this progressive succession (see Paus, 1988). The next step in the aquatic succession is dominant Myriophyllum spicatum, Potamogeton sect. Eupotamogeton and Ranunculus sect. Batrachium (Fig.5). Macrofossils of Myriophyllure alterniflorum and Potamogeton filiformis are present (det. Hilary Birks). Similar macrofossils are reported from late-glacial pioneer phases elsewhere on Jseren (Holmboe, 1903). Ecologically and chronologically E2 directly relates to the Salix-shrub-Rumex phase in N Rogaland (Paus, 1988, 1989). Similar pollenassemblages are described from other sites in SW Norway (Fsegri, 1940; Hafsten, 1963; Chanda, 1965; Thomsen, 1982) and are interpreted as representing snow-bed, pioneer stages. Early Salix-phases in the British lateglacial (Pennington, 1977a; Webb and Moore, 1982) probably represent the same successional stage as E2.
c. 12,650-c. 12,200 B.P.: Betula-Salix local pollen-assemblage zone (E3) Lithostratigraphically the E2/E3-transition is represented by a sandy layer (loss-onignition 10%). Otherwise the E3 sediment is minerogenic gyttja strongly varying in organic content (loss-on-ignition 12-30%). Biostratigraphic features are rises in Betula (both treebirch- and B. nana-type; Fig.6) and decreases in the Salix-, Poaceae- and Rumex sect. acetosapercentages and Polypodiaceae (both absolute and percentage values). Total pollen influx increases to about 1000 grains cm -2 a-1, comparable to present high productive foresttundra (Ritchie and Lichti-Federovich, 1967). E3 reflects further vegetational succession and soil development. Tree-birches rapidly invaded locally (cf. D/P-ratio decrease, 35% tree-birch pollen ZP; Fig.6) and established open birch vegetation (community 6c). Additional trees were Populus and Sorbus, both sparsely recorded earlier. Salix-shrubs and Betula nana existed within the tree-birch vegetation or in their own shrub (5) and dwarf-
shrub (3) communities. New or better represented tall-herbs (Geum, Filipendula, Lychnis/ Dianthus-type; cf. 4) are recorded. In addition, new taxa (Anthyllis vulneraria, Plantago lanceolata) appear in dry grassland (lb), in which Dryas expanded. These mineral-soil communities may have occurred in less protected areas such as on the esker ridges within the catchment. Although pronouncely changing, the E3 aquatic macrophyte flora was well-developed. After a short-lived maximum at the lower zone border, Potamogeton sect. Eupotamogeton drops to low values whereas Ranunculus sect. Batrachium expands. The shift from dominant Myriophyllum spicatum to frequent M. alterniflorurn could reflect leaching processes leading to lower trophic conditions in the lake (cf. Iversen, 1954; Berglund, 1966). Chronologically and partly vegetationally, E3 parallels paz $3 in Sandvikvatn (Paus, 1988) and paz L 3 + L 4 in Liastemmen (Paus, 1989). Similarly, E3 also compares to zones Ib + c of Iversen (1954) and to the main part of zone II of Fsegri (1940).
c. 12,200-c. 10,600 B.P.: Betula-Empetrum local pollen-assemblage zone (E4) At the E3/E4-transition, a narrow horizon of sand/silt occurs. Otherwise the lithology is gyttja (loss-on-ignition 15-35%) except for the moss layer at the E4b/E4c-transition. Biostratigraphic features are rises in Empetrum, Pinus, and Filipendula, and declining Salix (%), Artemisia, Caltha, Dryas, and Rumex sect. longifolius. Total pollen influx increases and varies between 1200 and 2800 cm-2 a-1 throughout the zone. This compares to modern high productive forest-tundra or birch woodland/conifer forests (Ritchie and Lichti-Federivich, 1967; Hyv/irinen, 1976). The parallel Betula-influx rise is due mainly to increased proportions of tree-birch pollen (Fig.6). The strong Empetrum rise at the lower zone boundary most likely reflects the expansion of the present lowland taxon E. nigrum. No size statistics which could distinguish the two Empetrum taxa (Andersen, 1961; Berglund,
190 1966), have been carried out. However, the bigger E. hermaphroditum tetrads differ by being more rounded/sub-sphaerical, which places E. hermaphroditum within the pollentaxon Vaccinium-type (cf. revised pollen-keys in Fsegri and Iversen, 1989). These morphological criteria indicate the total dominance of E. nigrum in the Eigebakken deposits. Similar conclusions are reached in other NW European late-glacial studies differentiating between the two Empetrum taxa (Huntley and Birks, 1983). Correspondingly strong late-Boiling Empetrum expansions are demonstrated throughout Rogaland (Thomsen, 1982, Braathen and Hermansen, 1985; Austad and Erichsen, 1987; Paus, 1988, 1989). Furthermore, smaller Empetrum increases at about the same time are recorded further north in W Norway (Krzywinski and Stabell, 1984; Larsen et al., 1984; Kristiansen et al., 1988). This apparent synchroneity could indicate a rapid, late-Boiling Empetrum migration into W Norway. However, the find of one Empetrum seed in early Bolling and a short-lived, middle Bolling Empetrum-pollen maximum in the Liastemmen deposits (Paus, 1989) demonstrate local Empetrum occurrences in Rogaland before the late Bolling expansion. The success of Empetrum at Liastemmen is probably caused by local soil factors. Sparse remnants of glacial deposits (Ringen, 1962) and large areas of exposed, granitic bedrock within the catchment suggest a limited late-glacial reservoir of mineral nutrients. Thus, leaching processes could rapidly acidify the soil to levels at which Empetrum could compete and expand. An early acidification of the Liastemmen soil is indicated by the strong decrease in the eutrophic Myriophyllum spicatum as early as 12,800 B.P. (Paus, 1989), whereas in Eigebakken, the final drop in M. spicatum occurs abut 500 years later (Fig.5). N W E u r o p e a n studies demonstrate metachroneous late-glacial Empetrum rises which may be both climatically and edaphically conditioned (Berglund, 1966). At a broad-scale, the earliest Empetrum expansions occurred
about 13,000-12,500 B.P. in Scotland (Lowe and Walker, 1977; Pennington, 1977b) and Ireland (Watts, 1977), about 12,500-12,000 B.P. in W Norway and about ll,700 11,500 B.P. in S Sweden (Berglund, 1966; Berglund and Ralska-Jasiewiczowa, 1986; BjSrck and MSller, 1987). Empetrum rises in the interval 11,50011,000 B.P. are recorded from Denmark (Iversen, 1954; Fredskild, 1975; Kolstrup, 1982), the Netherlands (Van der Hammen, 1951; Cleveringa et al., 1977; Bohncke et al., 1988), N Germany (Usinger, 1985), and N Poland (Zachowics et al., 1982). Taking into account the existence of the North Sea Continent (cf. Nesje and Sejrup, 1988), this pattern suggests a time lag in the Empetrum expansion from the coast towards continental areas. In line with presentday conditions, this probably reflects decreasing precipitation and, thus, delayed leaching of soil nutrients from the coast inwards. In broad outline, climate may then appear as the most decisive factor in determining the time of the late-glacial Empetrum expansion. On the other hand, edaphical differences may be important on a local scale, as at Liastemmen and Eigebakken. Similarly, the acid rocks and tills of S Sweden compared with the Danish calcareous rocks and tills (Berglund, 1966) may be responsible for the earlier Empetrum rise in S Sweden than in Denmark. An additional factor regulating the time of the NW European Empetrum expansion may be vegetational density. Using the Betula percentages and/or total pollen influx as an index of vegetation density, there seem to be positive correlations between a dense, early late-glacial vegetation and a late Empetrum rise. Both are indicative for continental localities. Several factors may have been responsible for a generally denser early late-glacial vegetation in continental than in marginal, coastal areas, e.g. more favourable climate, earlier deglaciation (if ever glaciated) and a shorter migrational delay. A closed vegetation and soil cover may have hindered wide-spread Empetrum establishment by (1) the low availability of light, which would be unfavourable for the light-demanding Empetrum and (2) slower acidi-
191 fication than in bare soils covered by pioneer, discontinuous vegetation (Jacobsen and Birks, 1980; Messer, 1988). As a whole, E4 represents the late-glacial vegetational optimum with dominant birch vegetation (communities 6) and dwarf-shrub heaths (3). Changes in representation of these humus-soil communities are the basis for dividing E4 into three subzones. E4a (Rumex-Filipendula subzone, c. 12,200c. 11,600 B.P.) reflects the closure of the local interstadial birch-forests. Tree-birch pollen reaches values of about 50% Z P and 600-1000 grains cm -2 a-1. According to th~ D/P-ratio concentrating around 8, Betula pubescens s.s. was the main forest component (cf. Birks, 1968; Van Leeuwaarden, 1982). The forests may have been of the drier Empetrum-birch type (6b) including Populus, Sorbus, Juniperus, Empetrum, Calluna and Melampyrum and of the fresher tall-herb birch-forests (6a) including
Salix-shrubs, Filipendula, Sanguisorba officinalis, Urtica, Geum, Trollius, Valeriana, Lychnis/Dianthus-type and other tall-herbs. Empetrum together with decreasing Betula nana during the subzone (Fig.6), also formed their own dwarf-shrub communities (3). Similarly, Salix and tall-herbs may have existed within communities 5 and 4, respectively. These humus-soil communities formed a more or less continuous vegetation cover. Nevertheless, occurrences of, e.g., Myricaria, Artemisia, Dryas, Helianthemum, Plantago spp., Cerastiumtype, Montia and Saxifraga spp. indicate areas supporting herb communities such as 1 and 2. A diverse vegetation thus characterized E4a. E4b (Populus-Filipendula subzone, c. 11,600c. 11,000 B.P.) reflects minor changes in local vegetation by decreases in total pollen/Betula influx (Fig.4), Betula nana-type (Fig.6), Salix, Rumex spp., Caltha-type, Montia and Lychnis/ Dianthus-type, whereas Populus, Juniperus and Empetrum were better represented. These fluctuations are interpreted as reflecting drier conditions which destabilized the birch-forests (Coope and Joachim, 1980; Pennington, 1986),
in particular on sandy, well-drained eskerslopes within the catchment. The resulting more open conditions (cf. influx decrease) obviously favoured Empetrum, followed by maxima of Juniperus and Populus. The resemblance to a progressive succession is striking, ending with closure of the Betula forests (cf. rising influx) with increased Populus proportions. Thus, the early E4b-changes indicate a strengthening of the drier dwarf-shrub communities (3) and Empetrum-birch forests (6b) at the expense of the moister birch-, willow- and herb-communities. However, some communities in more protected areas on soils with sufficient water supply, e.g. in lake margins, seem unaffected by the drier conditions (cf. Filipendula, community 4). New species (Polemonium caeruleum, Solanum dulcamara) are also recorded in these communities (4, 6a). In the drier herb-communities (1) Jasione montana, Plantago coronopus, Cynoglossum and Scleranthus cf. perennis show their first local late-glacial appearance. In the last part of E4b, Populus, Juniperus and Empetrum slowly decrease, whereas Salix, Rumex spp., Betula nana and Caltha rise. Montia reappears and the pollen-assemblage indicate the turn to conditions as in late E4a. E4c (Betula nana-Koenigia-Lychnis/Dianthus subzone, c. 11,000-c. 10,600 B.P.) is interpreted as representing more open vegetation although a small increase in total pollen influx is recorded (Fig.4). However, tree-pollen decreases both in percentages (to 20-30~/o ZP) and absolute values (Figs.4, 6), predominantly originating from Betula pubescens s.s. according to a D/P-ratio maximum about 8 (Birks, 1968; Van Leeuwaarden, 1982). The remaining Betula pollen reflects increased proportions of Betula nana-type, reaching c. 15% ZP. Otherwise E4c shows rises in Salix (influx), Empetrum, Caltha, Chenopodiaceae and Rumex sect. longifolius (% and influx), decreases in loss-onignition, Populus, Juniperus, Filipendula and Urtica and appearance and/or better representation of Arctostaphylos alpina, Echium, Lych-
192
nis/Dianthus-type, Koenigia, Cerastium-type, Saxifraga spp., Sagina and Alchemilla. Together, these features suggest degradation of the local forests to open birch vegetation (6c), an increased vigour of the shrub-and dwarfshrub-vegetation (5, 3) due to improved light and more widespread herb-vegetation (1, 2). Especially, Koenigia indicates expanding wet, open-ground communities including extreme snow-beds (2a). The E4b/E4c changes were probably initiated by the Younger Dryas climatic deterioration. A moss layer just above the E4b/E4c-transition (758-759 cm) also containing leaves of Betula nana, confirms this interpretation. The layer consists of Drepanocladus uncinatus (main constituent), Rhizomnium pseudopunctaturn (several leaves and stems) and a few stems of Polytrichum juniperinum and P. norvegicum (det. D.O. Ovstedal). Their good preservation suggests deposition when fresh and the moss fragments may thus be contemporaneous with their stratigraphical position. Though Rhizomnium pseudopunctatum can reflect wet, closed vegetation, the moss assemblage suggests snow-beds or snow-bed influence (D.O. Ovstedal, pers. commun.). Moreover, Polytrichum norvegicum rarely occurs below the tree-limit (Nyholm, 1954-1969) and is almost exclusive to late snow areas. The moss layer in combination with the decreasing loss-on-ignition values suggest the onset of erosion of vegetation and soil. In conclusion, the E4 local pollen-assemblage includes two episodes that are usually biostratigraphically distinct in NW European late-glacial studies, namely the Allerod and Younger Dryas periods. Regionally, E4a + E4b correlate with the Allerod-zones $4 (Paus, 1988) and L5 (Paus, 1989), whereas E4c is related to S5a and the first half of L6, i.e., the early YD.
c. 10,600-c. 10,000 B.P.: Betula-SedumCyperaceae local pollen-assemblage zone (E5) E5 is represented by heterogenous lithology; silty sand (loss-on-ignition 8%) in the lower part and silty gyttja (loss-on-ignition 35-50%)
in the upper part. The E4/E5-transition is characterized by the sudden appearance of a volcanic ash layer, only detectable by polarizing microscopy (Persson, 1966) and referred to the Vedde Ash Bed (Mangerud et al., 1984). The same ash layer was demonstrated by Fsegri (1940), but wrongly correlated to the late Allerod Laacher See Tephra (cf. Mangerud et al., 1984, see below). Biostratigraphical features at the transition are rising Cyperaceae, Juniperus, Filipendula, Urtica, Sedum and aquatic pollen, all well-represented throughout the zone, and decreasing Betula, Salix, Empetrum and Lychnis/ Dianthus-type. Betula (varying between 20-30% ZP) includes dominant B. pubescens s.s. according to a D/P-ratio of about 8, and decreasing and low representation of B. nana-type (Fig.6). E5 reflects the last half of the Younger Dryas Chronozone (Mangerud et al., 1974) and is divided into two subzones. E5a (Artemisia-Dryas-Ononis peakzone, c. 10,600-c. 10,500 B.P.) includes maxima in the taxa naming the zone, and distinct minima in total pollen influx (1000-1300 grains cm-2 a-1) and Empetrum (5%). The Betula analysis (Fig.6) and low Betula influx (250-300 grains cm -2 a - l ) indicate very scattered or even a temporary absence of local tree-birch vegetation (6c). Improved light, unstable soils and/or soil erosion (loss-on-ignition 8%) favoured open-ground communities, especially la (cf. Artemisia, Dryas). Soil break-up probably caused the Empetrum decrease and also the flourishing of aquatics due to the inwash of nutrients to the basin. However, the stable influx/increasing percentages of Filipendula and Urtica indicate that more protected areas retained fertile, humus soils. Most of the Ononis-type in E5a is small (< 25 Ilm) and referred to Astragalus alpinus. Some bigger and more coarsely sculptured grains belong to Oxytropis/Astragalus-type, which includes species such as Oxytropis campestris, Astragalus glycophyllus and A. norvegicus. Their close pollen-morphological similarity excludes any species determination.
193
Oxytropis campestris may, however, be a possible contributor to the Ononis-type curve. In S Norway today, O. campestris occurs only in landslide slopes in Ryfylke (Lid, 1985), c. 50 km northeast of Eigebakken, where it gives the impression of being a late-glacial relict. Parallels can be drawn to Artemisia norvegica, also with a disjunct distribution and occurring in Ryfylke (Ryvarden and Kaland, 1968). E5a is directly related to S5b at Sandvikvatn (Paus, 1988). Both reflect the sparsest local vegetation in YD and represent the time when YD ice reached its maximum extent in SW Norway (10,500-10,700 B.P.; Anundsen, 1985). E5b (Poaceae subzone, c. 10,500-c. 10,000 B.P.) reflects pronounced humus-soil development and vegetational development. Total pollen influx rises to 2500-3600 grains cm- 2 a - 1, comparable to modern birch-/coniferforests (Ritchie and Lichti-Federovich, 1967; Hyvfirinen, 1976). However, Cyperaceae and Poaceae constitute 40-50% ZP, possibly reflecting local over-representation of, for example, a well-developed telmatic flora. Otherwise, loss-on-ignition rises to 40-50% and Betula influx, almost exclusively including tree-birch pollen (Fig.6), reaches 700-1100 grains cm -2 a -~. Populus, Empetrum, Juniperus, Filipendula and Polypodiaceae gain better representation, whereas mineral-soil taxa such as Artemisia, Koenigia, Dryas, Ononis-type and Rumex sect. longifolius decrease. This all suggests the expansion of humus-soil communities such as tall-herb birch-forest (cf. Prunus padus, Solanum dulcamara, Trollius), Empetrum-birch-forests (cf.
Populus, Rhamnus catharticus,
c. 10,000-c. 9700 B.P.: Betula local pollenassemblage zone (E6) In E6 the sediments become highly organic (loss-on-ignition>55%), total pollen influx rises to more than 5000 grains cm -2 a -1 (comparable to modern forests; Ritchie and Lichti-Federovich, 1967; Hyvfirinen, 1976) and Betula increases to 55% Z P and 2800 grains cm -2 a -t, respectively. Betula analysis shows no representation of B. nana-type and the D/Pratio, displaced towards 8.5 (Fig.6), indicates increased proportions of B.pendula (Van Leeuwaarden, 1982). This further closing of local vegetation into dense birch forests caused the extinction or reduced representation of light-demanding, low competitive herbs (e.g., Dryas, Helianthemum, Plantago major, Sedum, Apiaceae) during the Potentilla-Campanula subzone (E6a). The local Betula pubescens/B.pendula-forests reached their greatest density in the subsequent Juniperus-Salix subzone (E6b), when a shrub-layer fully developed.
Climatic history The following climatic interpretation of the Eigebakken late-glacial pollen diagram is based on the qualitative indicator-species method and the quantitative pollen-assemblage method described in Paus (1989). The latter uses the ratio between the representation of mineral-soil taxa versus humus-soil taxa, to measure the degree of soil erosion/cryoturbation within the local area. High ratios are interpreted as reflecting widespread soil erosion initiated by harsh winter conditions such as low temperatures and strong winds (Paus, 1988, 1989).
Vaccinium-
type), juniper-dwarf-shrub heaths (3) and tallherb grasslands (4) at the expense of openground communities (1, 2). In E5b the aquatics reach their late-glacial influx maxima. Especially, Myripohyllum alterniflorum is well represented (Fig.5). Simultaneously, there are the first occurrences of the telmatic Cladium mariscus and Typha latifolia.
Paz E1 (Oldest Dryas). Single occurrences of Echium and Hippophag indicate a July mean temperature of about 12°C (Skre, 1979; Kolstrup, 1980), whereas the slightly better represented Filipendula, Urtica, and Myriophyllum spicatum suggest a July mean not less than 8-10°C (Kolstrup, 1980). As pronounced pollen redeposition is recognized in E1 (see
194 above), these indications may, however, be unreliable. High frequencies of reworked pollen and mineral-soil taxa suggest strong soil erosion within the newly deglaciated terrain. Cold winters (Atkinson et al., 1987) and strong winds, probably katabatic (Nickling and Brazel, 1985), may have been important influencing factors. As E1 lasts about 1000 years, i.e., considerably longer than could be explained by migrational delay (Fridriksson, 1975) or natural succession (Fsegri, 1934; Lindroth, 1965), this earliest paz reflects a pioneer vegetation prevented from further development by climatic extremes.
Paz E2 (early BoUing). At the E1/E2-transition, humus-soil taxa increase at the expense of mineral-soil taxa. Reworked pollen decline suggesting reduced erosion and stabilized, developing soils. The causal factors probably included milder winters (Atkinson et al., 1987) and a weaker wind-influence from the retreating ice. Local indications of a July mean reaching at least 10-12°C are given by Hippopha6, Sanguisorba minor, S. officinalis and frequent Myriophyllum spicatum (Kolstrup, 1980). The El/E2 biostratigraphical changes reflect the effects of the B~lling climatic amelioration. These are assumed approximately synchronous with almost identical Bolling changes elsewhere in Rogaland (c. 13,000 B.P.; Chanda, 1965; Thomsen, 1982; Paus, 1988, 1989). Some metachroneity within Rogaland is, however, likely due to differences in the geographical and ecological setting of the localities (Fsegri and Iversen, 1975; fig.23). Throughout E2 climate improved, probably reaching the late-glacial thermal maximum in middle Bolling (Atkinson et al., 1987; Vorren et al., 1988). At the E2/E3-transition, about 12,650 B.P., the local climate had passed the ecotone of tree-birches. However, due to delays in migration and soil maturation (cf. Wright, 1984; Birks, 1986), this critical threshold may have been reached earlier.
Paz E3+ E4a, b (late Bolling-Allerod). July means not less than 10°C for the whole period are indicated by Viburnum, Sanguisorba minor, S. officinalis, Myriophyllum alterniflorum and Nymphaea (Kolstrup, 1980). Furthermore, in E3 Hippopha6 suggests a July mean of at least 11-12°C, whereas the July mean reached 13-14°C or more in E4b, according to the occurrences of Solanum dulcamara and Cynoglossum (Iversen, 1954; Skre, 1979). One single grain of Pleurospermum austriacum suggest the E3 J a n u a r y mean below -2~'C (Iversen, 1954). Throughout this interstadial tree-birch period, the pollen assemblage shows a dominant representation of humus-soil taxa which, in turn, suggests stable soil conditions. Although rather fluctuating, the pollen curves show no clearly concurrent patterns and no distinct climatic oscillations such as the Older Dryas deterioration can be demonstrated. Similar conclusions are reached in the Jseren studies of Thomsen (1982). In contrast, F~egri (1940) demonstrated the "Br~ndmyra Interstadial" followed by a climatic deterioration. However, his interpretation is questioned. On the other hand, in N Rogaland three short-lasting deteriorations are detectable within the Bolling-Aller~d complex (Paus, 1988, 1989). These appear as periods unfavourable to local woody vegetation and with increased proportions of mineral-soil taxa. The diverging biostratigraphial records between N and S Rogaland are probably due to more mature and denser vegetation (Betula>50% EP) with higher inertia to change (Smith, 1965) in the southern areas. In the north the interstadial vegetation was closer to the ecotone of birch-forest (Betula 25-40% EP) and was, thus, more sensitive to climatic fluctuations (see fig.9, cf. Feegri and Iversen, 1975; fig.23, Watts, 1980). Although distinct biostratigraphical traces of climatic deteriorations are lacking in the Eigebakken diagram, such events may weakly be indicated in the lithological sequence. At about 12,200 B.P., 11,750 B.P. and 11,200 B.P.
195 (interpolated ages), loss-on-ignition values have distinct minima; the oldest also being accompanied by a 1 cm thick sandy/silty horizon. These could reflect periods of unstable soil and soil erosion initiated by climatic oscillations. There seems to be good chronological conformity between the Eigebakken loss-on-ignition minima and the N Rogaland interstadial deteriorations (Paus, 1988, 1989). From c. 11,600 B.P., i.e. early E4b, drier conditions are indicated by slightly more lightopen local birch-forests (see above). Most likely the causal factors involve decreasing precipitation and/or increased summer temperatures (cf. Coope and Joachim, 1980; Pennington, 1986). At the same time (if the chronological correlations are reliable), climatic change led to a closing of the N Rogaland birch-forests (Paus, 1988, 1989). This illustrates how the same regional climatic fluctuation can result in different local vegetational changes, depending on the geographical and altitudal position, exposition, soil type, etc. of the site.
Paz E4c ÷ E5a reflect the Younger Dryas climatic deterioration that caused local treebirch decrease and disappearance, vegetational disruption and soil erosion (see above). The climatic shift involved drops in summer and winter temperatures (Iversen, 1954; Rind et al., 1986; Atkinson et al., 1987) and the Scandinavian ice-sheet halted or readvanced. At about 10,500-10,700 B.P. the ice-front reached the outer part of Lysefjorden, c. 25 km northeast of Eigebakken (Fig.l), and simultaneously its maximal extension in SW Norway (Anundsen, 1985). This coincides chronologically with E5a, representing the YD period of strongest cryoturbation and soil erosion (cf. loss-on-ignition minimum, maximum in mineral-soil taxa). Thus, strong katabatic winds (Nickling and Brazel, 1985) may have been one of the factors that made E5a the period of maximal climatic stress during YD. On the other hand, local summer temperatures were not critical. According to the occurrences of
MyriophyUum alterniflorum, Sanguisorba officinalis and Filipendula, July mean temperature never reached below 9-10°C (Kolstrup, 1980). If not redeposited, the single-grain occurrences in E4c of Mercurialis perennis and Echium indicate a July mean of 11-13°C or more (Skre, 1979; Kolstrup, 1980). Paz E5b ÷ E6 reflect the effects of the climatic amelioration associated with the onset of the Holocene, initiated already in YD Chronozone about 10,500 B.P. (interpolated age). However, due to delays in soil maturation and plant migration (Iversen, 1954; Wright, 1984), the improvement is not fully biostratigraphically manifested until E6 (PB), when dense local birch forests establish. Nevertheless, occurrences of Typha latifolia and Solanum dulcamara in early E5b indicate a rapid increase in summer temperatures with a July mean above 13-14°C (Iversen, 1954; Kolstrup, 1980). Later, July means of at least 15-16°C are suggested by the sparse occurrences of Cladium mariscus, Rhamnus catharticus and Prunus spinosa (Von Post, 1925; Skre, 1979). In addition, Cladium suggests a J a n u a r y mean not less than - 5 ° C (Von Post, 1925; Conway, 1942). Usually the date of this final "Holocene" amelioration falls within the interval 10,000-10,200 B.P. in W Norway (e.g., Krzywinski and Stabell, 1984; Braathen and Hermansen, 1985; Austad and Erichsen, 1987; Kristiansen et al., 1988; Paus, 1988). At Eigebakken, however, this event occurs at about 10,500 B.P. (if reliably interpolated). Similar early dates are recorded elsewhere in NW Europe (e.g., Pennington, 1977b; Lowe and Walker, 1980; BjSrck and MSller, 1987) indicating that the finiglacial climatic improvement is a metachronous, biostratigraphical event. Early biostratigraphical records of this amelioration may be from sites that supported vegetation with a close ecotonal position. In such sites local vegetation could rapidly respond and change to a different vegetational type following climatic amelioration.
196
Hippopha# rhamnoides, Polemonium caeruleum, Gypsophila) a n d t h e o c e a n i c e l e m e n t s (e.g., Sanguisorba minor, Montia fontana, Cladium mariscus). T h e a r c t i c e l e m e n t is r e p r e s e n t e d by only one t a x o n , Papaver radicatum-type. All
Plant geography T h e s a m e p h y t o g e o g r a p h i c a l division of the p r e s e n t N o r d i c flora used by P a u s (1988, 1989) and b a s e d on B e r g l u n d (1966), g r o u p 73 of the 141 l a t e - g l a c i a l pollen a n d spore t a x a as s h o w n in Fig.7. F o r t h e t o t a l late-glacial, the m a j o r p l a n t - g e o g r a p h i c a l p a t t e r n s a r e d o m i n a n t bor e a l - c i r c u m p o l a r (e.g., Populus tremula, Betula nana, Parnassia palustris) a n d e u r a s i a t i c species (e.g., Rhamnus catharticus, Pleurospermum austriacum, Anthyllis vulneraria) a n d f r e q u e n t a r c t i c - a l p i n e e l e m e n t s (e.g., Koenigia
e l e m e n t s w e r e p r e s e n t d u r i n g the pre-birch period (El + E2). In t h e following b i r c h period ( E 3 + E4a, b), b o r e a l - c i r c u m p o l a r a n d eurasiatic e l e m e n t s w e r e the m o s t i m p o r t a n t , also a m o n g the new a r r i v a l s , w h e r e a s the arctica l p i n e species g a i n e d i n c r e a s e d i m p o r t a n c e during the Younger Dryas climatic deterioration ( E 4 c + E5a). C o r r e s p o n d i n g p h y t o g e o g r a p h i c a l p a t t e r n s are r e c o r d e d in late-glacial studies from N R o g a l a n d (Paus, 1988, 1989), S S c a n d i n a v i a (Berglund, 1966) and I r e l a n d (Mitchell, 1954). This s u g g e s t s t h a t N W E u r o p e
islandica, Dryas octopetala, Selaginella selaginoides). Less r e p r e s e n t e d a r e the b o r e a l - a l p i n e (e.g., Betula pubescens, Trollius europaeus, Myricaria germanica), the c o n t i n e n t a l (e.g.,
PHYTOGEOGRAPHICAL GROUPS HULTEN 1950
LATE WEICHSELIAN,TOTAL SANDVIKVATN (PAUS 1988) EIGEBAKKEN LIASTEMMEN ( PAUS 1989) lo ,,,
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....................................................
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Fig.7. The relative frequencies of phytogeographical groups for the total Late Weichselian, for the three main periods (El + E2, E3 + E4a, b, E4c + E5), and for the new taxa within each period. Because of the low number of new taxa in E4c + E5 their relative frequencies are stippled. Comparable data from N Rogaland (Paus, 1988, 1989) are added.
197 was plant-geographically uniform during the late-glacial. According to Hult~n's (1950) division into migration groups, about 60~/o of the late-glacial Eigebakken taxa included in Fig.7, are classified as so-called southern species. All are found in the early late-glacial on the Continent. A similar pattern exists among the eastern/south-eastern taxa (e.g., Parnassia, Pleurospermum, TroUius) and the arctic-alpine (e.g., Koenigia, Dryas), which together suggest that the predominant migration route was from the Continent into SW Norway (Danielsen, 1971; Paus, 1988). One single grain in E5b of the entomophilous Polemonium caeruleum, not previously found in the Norwegian late-glacial, suggests local occurrence. In the middle Weichselian, this south-eastern species was better represented in western Europe than today (Godwin, 1975; Kolstrup, 1980). Danish late-glacial finds of Polemonium-pollen (Iversen, 1954; Kolstrup, 1982) obviously represent its northwards spread into SW Norway. Thus, Polernonium caeruleum seems to parallel other eastern/ south-eastern species, e.g. Pleurospermurn austriacum and Gypsophila fastigiata/repens, that were more westerly distributed in Europe in the Late Weichselian than today (Iversen, 1954; Berglund, 1966; Webb and Moore, 1982).
The ~Brondmyra interstadial" of F~gri (1940) The two main pollen diagrams in which F~egri (1940) demonstrated the Brondmyra interstadial (biozone III), were from the Jseren localities Brondmyra and Eigebakken. Later, it has become widely accepted that the Brondmyra interstadial correlates with the Bolling interstadial (cf. Chanda, 1965). However, Thomsen (1982) re-interpreted the Brondmyra diagrams of F~egri (1940) and Chanda (1965) and concluded that the Brondmyra interstadial belongs to the earliest part of Younger Dryas Chronozone. A correlation between the two Eigebakken diagrams (Feegri, 1940 and this paper, Fig.3) is therefore essential in resolving the chronological problem of the Brondmyra interstadial.
Figure 8 shows the Eigebakken diagram of F~egri (1940) converted into a total pollen diagram. My re-interpretation resulting from correlation with the re-analysed diagram (Fig.3) is included.The lithostratigraphical correlation between the two diagrams is not straightforward. However, the sandy layer (Fsegri: 600-605 cm, this paper: 744-749cm) containing the first part of the Vedde Ash Bed (Mangerud et al., 1984), is easily recognized in both diagrams. Otherwise, there may be parallels between F~egri's bottom "Ton Gyttj a" layer (767 cm --*) and the clayey gyttja below 839 cm in Fig.3. Similarly, the "Diatomeen-erde" of Fsegri (605-767 cm) may correlate with the clayey gyttja layers between 757 and 839 cm in Fig.3, and lastly, Fsegri's gyttja layer (560-600 cm) may equate with the three upper layers of minerogenic gyttja/dy (698-744 cm) in Fig.3. Biostratigraphically the two diagrams are highly conformable. Biozone I of Feegri, in the upper part delimited by a Betula increase and Salix/Poaceae/Cyperaceae decreases, correlates with paz E2. The Ericales rise represents the upper limit of F~egri's zone II, thus paralleling E3. Zone III (Brondmyra interstadial) constitutes the first quarter of E4, zone IV (previously correlated with Older Dryas) the second quarter of E4, whereas zone V of F~egri comprises the last half of E4 plus E5. Diverging characteristics are the absence of zones comparable to E1 and E6 in Fsegri's diagram (Fig.8). Furthermore, due to fewer pollen constituents in Fig.8, the percentages are here generally higher than in Fig.3. In conclusion, the chronostratigraphical reinterpretation of the Eigebakken diagram of Fsegri (1940) is as follows: Zones I and II constitute the Bolling Chronozone; zone III (Brondmyra interstadial) has the approximate age of the Older Dryas Chronozone; zone IV comprises the first part of the Allerod Chronozone, whereas zone V, by Fsegri called the Allerod interstadial, constitutes the last half of Allerod plus Younger Dryas Chronozones. Consequently, in the two main diagrams in which the Brondmyra interstadial has been
198
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Fig.8. The Eigebakken diagram of F~egri (1940) converted into a total pollen diagram. A re-interpretation, based on correlation with the re-analysed Eigebakken diagram (Fig.3) and including chronozones, is added. demonstrated, it appears as two metachronous biozones (OD/early YD Chronozones in Eigebakken/Brondmyra, respectively). In neither of the two diagrams, can the interstadial be correlated with the Bolling interstadial. Moreover, zone III is not refound as a distinct biostratigraphical unit either in the re-interpretated Brondmyra diagrams (Thomsen, 1982) or in the re-analysed Eigebakken diagram (this paper). The only criterion of Fsegri (1935, 1940) for distinguishing the Brondmyra interstadial was Betula-pollen diameter statistics showing the presence of B. pendula pollen. His results are, however, not confirmed by the new Betula analyses from Eigebakken (Fig.6). Several abiotic factors influencing grain-size (Ander-
sen, 1960, 1980; Cushing, 1961; Praglowski, 1966; Kristiansen et al., 1988; Paus, 1988), could explain this divergency. Furthermore, these abiotic factors represent a warning for drawing firm conclusions solely based on Betulapollen size statistics. Conclusions Due to the several sources of dating errors, the Eigebakken chronology, and hence the influx estimates, are to great extent based on assumed ages. The assumptions refer to dates in other late-glacial studies from Rogaland (Chanda, 1965; Thomsen, 1982; Paus, 1988, 1989).
199 BIRCH FOREST OPEN BIRCH VEGETATION SHRUB VEGETATION OPEN-GROUND VEGETATION
_~cc~E. STAG ~___SS LIASTEMMEN
L1
~
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S4a E4a 12,000
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"M-~ I'-MU M JULY MEAN (°C) ~ E " ~ + STRONG Wl NDS
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POST-GL ACIAL
15 - 1 6
Fig.9. Survey of the Late Weichselianvegetational and climatic history in Rogaland based on results from J~eren (Chanda, 1965; Thomsen, 1982; this paper) and N Rogaland (Paus, 1988, 1989). Chronological correlation of the biozones from Eigebakken, Sandvikvatn (Paus, 1988), and Liastemmen(Paus, 1989) is added.
From local deglaciation, about 14,000 B.P., to 11,000 B.P. the Eigebakken diagram demonstrates a local vegetational succession including four progressive stages (Fig.9). The earliest, the Artemisia-dominated pioneer stage, lasted to c. 13,000 B.P. and reflects discontinuous vegetation cover of open-ground communities on disturbed, mineral soils. The subsequent Salix-shrub consolidation stage ending c. 12,650 B.P., represents stable soils, humus-soil development and vegetational closure. From c. 12,650 t O c. 12,200 B.P. open tree-birch vegetation established locally. Thereafter a birch-forest stage commenced. At about 11,000 B.P. retrogressive succession was initiated by a change in the birch forests to open birch vegetation. Simultaneously, dwarfshrub heaths and open-ground communities expanded. In a short-lived and probably temporary treeless period, from c. 10,600 to c. 10,500 B.P., open-ground vegetation reached its maximum extention. Finally, rapid vegetational closure is reflected, ending with the establishment of dense, local Holocene birchforests.
This vegetational development indicates a tripartite division of the local, late-glacial climate into a pleniglacial period (older than 13,000 B.P.), an interstadial (13,000-11,000 B.P.) and the Younger Dryas Stadial (Fig.9). Cold winters and strong, probably katabatic winds, exposing vegetation and soil to drought and erosion, are proposed as critical factors in Younger Dryas and the pleniglacial period. Throughout the late-glacial the July mean did not fall below 8-10°C and reached at least 11-12°C in the first half and 13-14°C in the last half of the interstadial. In early parts of Younger Dryas J u l y mean was probably about 11-13°C. Decreasing precipitation and/or warmer summers in the mid-interstadial (early paz E4b; from about 11,600 B.P.) may explain the slight and temporary thinning of the local birch forests and the increase of vegetation t h a t prefers drier habitats. Otherwise there is no biostratigraphical evidence of climatic oscillations such as the Older Dryas deterioration within the interstadial. By contrast, late-glacial studies from N Rogaland (Paus, 1988, 1989) show three
200 short-lived, interstadial climatic deteriorat i o n s . T h i s d i v e r g e n c y m a y be e x p l a i n e d by dense and mature birch-forests with high i n e r t i a ( S m i t h , 1965) i n t h e J~eren a r e a , whereas the N Rogaland interstadial vegetation lay close to the ecotone of birch-forests and was thereby more easily influenced by c l i m a t i c o s c i l l a t i o n s (see Fig.9). T h e f a s t e r d e v e l o p m e n t i n t o s t a b l e b i r c h - f o r e s t s o n J~eren is p r o b a b l y a s s o c i a t e d w i t h t h e m o r e s o u t h e r n p o s i t i o n w h i c h , in t u r n , s u g g e s t s a g e n e r a l l y milder climate and a shorter migrational delay. I n a d d i t i o n , E i g e b a k k e n is s h e l t e r e d b y w e l l developed eskers, thereby causing a favourable local climate. My re-analysis of the Eigebakken material t o g e t h e r w i t h T h o m s e n s ' s (1982) r e - i n t e r p r e t a t i o n o f t h e B r o n d m y r a d i a g r a m s (F~egri, 1935, 1940; C h a n d a , 1965) s t r o n g l y i n d i c a t e t h a t t h e e x i s t e n c e o f ' ~ B r o n d m y r a i n t e r s t a d i a l " o f F~egri (1940) m u s t be r e j e c t e d . I n t h e s e d i a g r a m s , B r o n d m y r a b i o z o n e (III) a p p e a r s to be m e t a chroneous and includes the Older Dryas Chron o z o n e in t h e E i g e b a k k e n m a t e r i a l a n d t h e early part of Younger Dryas Chronozone in the Brondmyra diagrams. The only biostratig r a p h i c c r i t e r i o n o f F m g r i (1935, 1940) f o r distinguishing the interstadial was his results from Betula-pollen statistics. Otherwise there is n o b a s i s f o r t h e b i o s t r a t i g r a p h i c a l u n i t s corresponding with "Brondmyra interstadial" in a n y o f t h e E i g e b a k k e n / B r o n d m y r a diagrams.
Acknowledgements Knut Krzywinski and Lotte Selsing did the field w o r k . P r o f . K n u t F~egri p l a c e d t h e m a t e rial to my disposal. Eva Krzywinski prepared the pollen samples. Dating problems were discussed with Steinar Gulliksen, Siri Herland drew some of the figures and Hilary Birks and Dag Olav Ovstedal determined the macrofossils. I t h a n k all. I a l s o s i n c e r e l y t h a n k J o h n Birks for reading and correcting the manuscript. This work was financed by the Norweg i a n C o u n c i l for S c i e n c e a n d H u m a n i t i e s ( G r a n t D.71.49:038).
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