Development of the late glacial Baltic basin and the succession of vegetation cover as revealed at palaeolake Haljala, northern Estonia/ Balti basseini ja taimestiku hilisglatsiaalne areng Haljala paleojarve (Pohja-Eestis) uuringute andmetel.
The Pandivere ice-marginal formations were shaped by the ice streams of the retreating Scandinavian ice sheet. The proglacial lake systems that developed in front of the decaying ice margin, their formation and configuration were controlled by geology, local topography, and dynamics of the ice sheet as well as climatic fluctuations. The late glacial history of Estonia has been investigated for a long time, yet any palaeogeographical and palaeo-environmental reconstructions of the area at the closing stage of the last glaciation have been hampered due to lack of chronology. To fill this gap, we revisited the Haljala site and performed multi-proxy studies of sediment cores.
The Haljala overgrown lake was first examined by R. Mannil and R. Pirrus, who studied the Holocene lacustrine lime distribution and palynostratigraphy (Mannil 1961; Mannil & Pirrus 1963). The pollen diagrams by Pirrus (Mannil & Pirrus 1963; Pirrus 1965; Pirrus & Sarv 1968) provided a general picture on the vegetation succession and climatic history during the late glacial. On the basis of pollen composition she differentiated the Allerad and the Younger Dryas sediments. However, the palynological data suffered from simplified taxonomic composition (mostly tree pollen accounts were included), sparse resolution between pollen samples, limited pollen sums, absence of pollen concentration data, and, most importantly, lack of radiocarbon dates, which hindered detailed correlation of the environmental changes.
Late glacial deposits of North Estonia have only been sporadically radiocarbon dated at Kunda, Loobu, Viitna, Antu, and Raatsma (Table 1). However, according to Pirrus (1976; Karukapp et al. 1996), dates from Kunda, Viitna, and Loobu were problematic, seemed to be too old and did not match with pollen stratigraphy (Pirrus 1976; Pirrus & Raukas 1996). The AMS [sup.14]C date 10 170 [+ or -] 95 (11 885 [+ or -] 260 cal yr BP; Ukkonen et al. 2005) from a reindeer antler at Kunda, buried in lacustrine lime, seems to be correct and confirms that lacustrine lime started to deposit at the late glacial/Holocene boundary or even earlier (Sohar & Kahn 2008).
The palaeolake at Haljala was an ancient lagoon of the Baltic Ice Lake (BIL), which isolated from the BIL during the proglacial Lake Kemba ([A.sub.2]) phase (Saarse et al. 2007). The former palaeolake, today a bog, which has been drained by numerous ditches and mainly reclaimed and transformed into pasture, is located in the depression above a 20 m deep buried valley. The buried valley is filled in with till, clayey silt, lacustrine lime, and peat. The distribution of lacustrine lime roughly outlines the ancient lake about 4.6 km long and 200 m wide, in the north dammed up by a spit, which was formed during the proglacial lake [A.sub.2] phase (Saarse et al. 2007).
The key objective of the present study was to determine the chronology of the late glacial vegetation succession, as well as environmental and climatic changes, and to adjust the timing of ice recession in North Estonia. For this purpose we used the analyses of AMS [sup.14]C dating, lithological composition, pollen stratigraphy, ostracods, and macrofossil remains of plants, mosses, and algae.
MATERIAL AND METHODS
The coring site in Palaeolake Haljala (59[degrees]25'27"N, 26[degrees]17'42"E) at an elevation of 67.4 m a.s.l is located about 90 km east of Tallinn, near the crossing of Rakvere-Vosu and Tallinn Narva highways (Fig. 1), just where the road crosses the main draining trench. Multiple 1 m long sediment cores were taken from the central part of the ancient lake with a Belarus peat sampler in 2006 and 2007. One centimetre thick sub-samples for loss-on-ignition (LOI) analyses were taken continuously. Bulk samples were dried at 105[degrees]C overnight, and burnt at 525 and 900[degrees]C to calculate moisture, organic matter (OM), carbonate, and minerogenic compounds. Loss-on-ignition analyses were performed from all cores and served as a basis for the correlation of cores. Grain size distribution of clayey deposits untreated by chemicals was determined in the Institute of Ecoloav at Tallinn University with a Fritsch Analysette 22 laser particle size analyser.
Sub-samples for pollen analyses (2 [cm.sup.3]) were taken at 5 cm intervals and prepared according to Berglund & Ralska-Jasiewiczowa (1986). In addition, minerogenic samples were treated with concentrated hydrofluoric acid. Lycopodium tablets were added to calculate pollen concentration (Stockmarr 1971). Up to 350 terrestrial pollen grains were counted. Pollen identification followed Moor et al. (1991). Both the percentage and pollen accumulation rate (PAR) diagrams were constructed using TILIA and TGView programs (Grimm 2000). The zonation of pollen data is based on the constrained cluster analysis by the sum of squares (CONISS) method. Palynological richness was estimated by rarefaction analysis (Birks & Line 1992) using the PSIMPOLL 4.10 program (Bennett 1994, 1998). All identified terrestrial pollen taxa were included and standardized to the lowest pollen sum.
Macrofossils were extracted by soaking 5 cm thick silt samples in water and sieving through a 0.25 mm mesh. Thirteen samples were prepared for fossil seed and moss fragment analysis. The samples were treated according to the method proposed by Birks (2001). Ostracod subfossils were picked out of five sediment samples (sample size ~25 [cm.sup.-3]) under the binocular microscope using a fine wet brush. Species identification and ecological preferences are based on the monograph by Meisch (2000). Ostracod shells and valves were photographed by the scanning electron microscope at the Natural History Museum, London, UK.
Accelerator mass spectrometry [sup.14]C radiocarbon dating was performed in Poznan and Uppsala Radiocarbon Laboratory and dates were calibrated on the basis of the IntCa104 calibration curve (Reimer et al. 2004) and the Calib Rev 5.0.1. program (Stuiver et al. 2005). All ages mentioned in text are calibrated years before AD 1950 (cal yr BP).
[FIGURE 1 OMITTED]
Sediment lithostratigraphy and grain size distribution
On the basis of grain size composition the sediment sequence was subdivided into five lithological units (Fig. 2; Table 1). The base of the studied sequence consists of fine-grained grey silt overlain by alternating grey clayey and fine silt, and ends with greenish-grey calcareous sandy silt (Fig. 2). The grain size and LOI results revealed the homogeneous nature of silt, with clay fraction fluctuating within 7-22%, silt within 72-85%, and sand within 5-8%. The mineral matter content was between 90% and 95%. The content of OM in bottom clayey silt was low, around 1-2%, except between core depths of 340-460 cm and 210-225 cm, where it reached 4% and 7.3%, respectively (Fig. 2). The increased moisture, OM, and carbonate content and decreased sedimentation rate at 210-225 cm possibly suggest the isolation of the basin. Transition from silt to calcareous silt was sharp, with carbonates reaching 45% and mineral matter decreasing to 50%. The sediment accumulation rate, ca 3 mm [yr.sup.-1] between 13 600 and 12 940 cal BP, indicates rapid influx of minerogenic sediments (Fig. 2). After that the sedimentation rate decreased, being roughly 0.9 mm [yr.sup.-1].
[FIGURE 2 OMITTED]
Altogether eight levels in the Haljala sediment sequence were dated by the AMS [sup.14]C technique (Table 2). Three datings of terrestrial macrofossils (Haljala 2, 4, and 7) gave ages consistent with sediment depth and pollen stratigraphy. However, at an earlier stage of investigations, when only small sediment samples were available, finds of terrestrial plant remains were scarce. Therefore unidentified pieces of organic debris were sent in for AMS dating. Three [sup.14]C dates (Haljala 1, 3, and 5) provided too young ages, perhaps due to contamination by root penetration from the surface or some other reason. Two dates (Haljala 6 and 8) were apparently too old in comparison with pollen stratigraphy and deglaciation chronology (Kahn 2006). Such discrepancy is not fully understandable, but it seems that selection of the dating material is crucial. Contamination during coring seems not to be an issue, as the obtained cores displayed well-preserved lamination (Fig. 3). Controversial dates were excluded from the age--depth model, which currently is based on three dates. The AMS date Haljala 2 (10 970 [+ or -] 150 [sup.14]C yr BP; Poz-22529; 13 050-12 840 cal yr BP) marks the end of the warm episode before the Younger Dryas cooling and increase in arboreal pollen (AP) accumulation and concurrent decrease in non-arboreal pollen (NAP), first of all of Artemisia. The AMS dates Haljala4 (11 780 [+ or -] [+ or -] 60 [sup.14]C yr BP; Poz-22530; 13 740-13 570 cal yr BP) and Haljala 7 (11 750 [+ or -] 80 [sup.14]C yr BP; Poz-22531; 13 705-13 495 cal yr BP) correspond to the Allerod. By extrapolation of radiocarbon dates the studied 4.5 m core represents a time span of ca 13 800-11 300 cal yr BP. Due to the scarcity of radiocarbon dates the sedimentation rate estimations should be considered as a first approximation and the pollen accumulation diagram should be interpreted accordingly. In addition, we used in the current study the event stratigraphic units of Greenland Ice Core 2005 (GI0005) and their ages based on dates as signed to the corresponding boundaries suggested by Lowe et al. (2008), originally put forward by Bjorck et al. (1998). All these dates are calibrated ages that correspond to AD 2000:
GI-1d--Older Dryas (14 100-13 950 cal yr BP); GI-1c--Allerod warmer period (13 950-13 300 cal yr BP); GI-1b--Allerod colder period (13 300-13 100 cal yr BP); GI-1a--Allerod warmer period (13 100-12 900 cal yr BP); GS-1--Younger Dryas (12 900-11700 cal yr BP).
[FIGURE 3 OMITTED]
A total of 67 samples including 59 terrestrial taxa were analysed. Seven local pollen assemblage zones (LPAZ) were distinguished based on statistical evaluations of pollen spectra.
LPAZ Ha-1 (498-458 cm; ca 13 800-13 750 cal yr BP,fine-grained silt)
This LPAZ was characterized by upward increasing Pinus and rather stable Betula pollen around 20% (Fig. 4). Juniperus was present constantly, Salix sporadically. The percentage of Alnus (4-12%), Picea (8-17%), Ulmus, Tilia, Populus, Fraxinus, and Corylus pollen (QM--up to 7%) was rather high, indicating the redeposition phenomena, obviously from the Eemian deposits, whose outcrops are located on the islands of Prangli and Uhtju in the Gulf of Finland. The NAP content was around 25%, with Cyperaceae, Poaceae, Artemisia, and Chenopodiaceae dominating. Betula nana was present in low values (1-4%). The concentration of tree and shrub pollen fluctuated between 6000 and 15 000 grains [cm.sup.-3], whereas herb values were over three times smaller (1900-5300 grains [cm.sup.-3]). Increase in Pinus pollen, together with decrease in Betula, Picea, Juniperus, and Salix, marked the upper boundary of the LPAZ. Such a pollen assemblage is characteristic of the late glacial, obviously of the beginning of the Aller,ad chronozone (Mangerud et al. 1974) and of GI-1c (13 950-13 300 cal yr BP; Lowe et al. 2008).
LPAZ Ha-2 (458-408 cm; ca 13 750-13 600 cal yr BP, clayey silt)
The LPAZ was marked by high values of AP (57-74%) and rather low values of herbs (13-21%; Fig. 4). Tree pollen maintained dominance but was fluctuating: Pinus within 19-47% and Betula within 9-29%. The curve of Picea (2-7%), Alnus, and Quercus pollen was continuous, whereas the other QM taxa occurred sporadically. Nonarboreal pollen fluctuated between 20% and 40%. Poaceae, Cyperaceae, Chenopodiaceae, and Artemisia were present in almost equal amounts. At the Pinus peak around 430 cm Betula nana pollen disappeared and Betula, Juniperus, Artemisia, and Cyperaceae pollen decreased. Salix and Juniperus made a sporadic appearance throughout the zone. The concentration of tree pollen had decreased (3000-9700 grains [cm.sup.-3]), and that of shrubs fluctuated between 0 and 800 and of herbs between 700 and 3900 grains [cm.sup.-3]. This LPAZ corresponds to GI-lc, the Allerod warmer period (13 950-13 300 cal yr BP; Lowe et al. 2008), but supports the idea of a slight cooling inside GI-1c.
LPAZ Ha-3 (408-333 cm, ca 13 600-13 300 cal yr BP, clayey silt)
This LPAZ was characterized by irregular Betula and Pinus pollen curves; Picea and Alnus were present in almost equal values (Fig. 4). Total AP frequency was about 70%. Salix and Juniperus were more frequent than in the previous LPAZ. At the peak of Pinus pollen at 355 cm Betula nana and Salix pollen was missing and Betula had decreased. The proportion of Artemisia was decreased and that of Chenopodiaceae increased. The concentration of tree pollen increased and fluctuated between 6000 and 19 800 grains [cm.sup.-3], that of shrubs between 150 and 1000 grains [cm.sup.-3], and herbs between 2900 and 7100 grains [cm.sup.-3].
LPAZ-4 (333-258 cm, ca 13 300-13100 cal yr BP, fine-grained silt)
In LPAZ-4 pollen of Picea and thermophilous taxa, especially Corylus (Fig. 5), was abundant. Pinus was still dominant and Betula was subdominant. The percentages of trees and shrubs were high, 70-80. Herb pollen increased by the end of this zone, while tree pollen decreased. Pollen of aquatics was rare, but spores were represented by relatively high values (up to 21%). The concentration of tree pollen increased to 25 000 grains [cm.sup.-3] and that of herbs to 12 700 grains [cm.sup.-3]. The PAR of trees remained almost at the level of the previous LPAZ (2600-7300 grains [cm.sup.-2] [yr.sup.-1]), while that of shrubs (50-250 grains [cm.sup.-2] [yr.sup.-1]) and herbs (700-3800 grains [cm.sup.-2] [yr.sup.-1]) increased moderately (Fig. 5). This LPAZ correlates well with the Allerod cold event GI-lb (Lowe et al. 2008).
LPAZ Ha-5 (258-200 cm, ca 13 100-12 800 cal yr BP, upper part of fine-grained silt and lower part of clayey silt)
The LPAZ was determined by changes in the AP/NAP ratio. Herb pollen percentages had increased, especially of Artemisia, Poaceae, Cyperaceae, and Chenopodiaceae, while tree pollen percentages had decreased, first of all of Pinus and Picea (Fig. 4). Juniperus and Artemisia were the main taxa to profit from the decline in AP. They colonized fresh areas around Haljala, which emerged after the isolation of the basin from the large proglacial lake that occurred between 13 000 and 12 900 cal yr BP. The concentration of tree pollen was high but irregular, reaching up to 22 800 grains [cm.sup.-3], but declined to 11 800 grains [cm.sup.-3] at 230 cm. The PARs of trees (3700-6900 grains [cm.sup.-2] [yr.sup.-1]), shrubs (350-450 grains [cm.sup.-2] [yr.sup.-1]), and herbs (3500-4800 grains [cm.sup.-2] [yr.sup.-1]) reached their maxima at the LPAZ lower limit around 13 100 cal yr BP and decreased sharply at the LPAZ upper limit (Fig. 5). Pollen concentration remained high up to 12 500 cal yr BP. This LPAZ roughly coincides with GI-1 a (13 100-12 900 cal yr BP).
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
LPAZ Ha-6 (200-103 cm, 12 800-11750 cal yr BP, clayey silt)
This LPAZ was marked by the abundance of herb pollen, reaching 65% (Fig. 4). Cyperaceae and Artemisia had their maxima, 25% and 23%, respectively. Pinus pollen percentages decreased, but the frequency of Picea remained on the previous level. Shrubs, especially Juniperus, were abundantly present. The value of redeposited QM species had also decreased, particularly that of Corylus. Apart from NAP increase up to 22 300 grains [cm.sup.-3], AP concentration decreased (7300 and 17 000 grains [cm.sup.-3]), referring to cooling in the Younger Dryas. The amounts of Sphagnum and Pediastrum boryanum spores increased. The concentration of shrubs also reached its maximum (600-4400 grains [cm.sup.-3]). The PAR of trees remained between 800 and 1800 grains [cm.sup.-2] [yr.sup.-1], of shrubs between 50 and 400 grains [cm.sup.-2] [yr.sup.-1], and of herbs between 440 and 2100 grains [cm.sup.-2] [yr.sup.-1] (Fig. 5). The LPAZ covers the GS-1 cold event of the Younger Dryas (12 900-11700 cal yr BP; Lowe et al. 2008).
LPAZ Ha-7 (103-60 cm, 11 750-11300 cal yr BP, calcareous sandy silt)
Near the lower limit of the LPAZ tree pollen percentages were the lowest throughout the studied sequence, but shrubs (Betula nana, Juniperus, and Salix) reached their maxima (Fig. 4). The share of Artemisia diminished, but Poaceae and Urtica, on the other hand, met their maxima. At the top of the LPAZ Betula pollen increased sharply to 80% and herb pollen decreased from 50% to 20%. Especially suffered Poaceae, Cyperaceae, Artemisia, and Chenopodiaceae; constituents of thermophilous trees Quercetum mixtum and Picea disappeared from the pollen spectra. The concentration of tree pollen fluctuated between 4000 and 11 500 grains [cm.sup.-3], amounting to 61 500 grains [cm.sup.-3] in the topmost sample. The PARs of all taxa increased upwards: of tree pollen from 1000 to 2300 grains [cm.sup.-2] [yr.sup.-1], shrubs from 200 to 1200 grains [cm.sup.-2] [yr.sup.-1], and herbs from 1100 to 3300 grains [cm.sup.-2] [yr.sup.-1] (Fig. 5). This LPAZ represents the transition between the Younger Dryas and the Preboreal (Mangerud et al. 1974).
As silty sediments of Haljala were very poor in macroremains, almost the entire sequence was sieve-washed and macroscopic plant remains were picked out and identified under the microscope to find suitable material for radiocarbon dating. The content of thirteen samples, prepared specifically for plant macrofossil analysis, is presented in Table 3. The small number of samples and their non-contiguous placement within the core does not allow drawing firm conclusions about the vegetation succession in and around the palaeolake of Haljala based on plant macrofossil analysis only. As the results of plant macrofossil analysis mainly reflect species from local vegetation, it is not surprising that most of the seeds and vegetative parts belonged to the aquatic species: Ranunculus sect Batrachium, Characeae, Equisetum sp., Potamogeton sp., and different aquatic mosses (Drepanocladus sp., Scorpidium sp.). Arctic species, which are characteristic of late glacial vegetation, were represented by Dryas octopetala, Salix polaris, and Betula nana. Dryas octopetala, a dwarf shrub indicating Arctic climate conditions, is evenly distributed over the analysed sequence, being present in six samples of thirteen. Salix polaris, which nowadays is growing in northern areas, occurs in one sample in the lower part of the sediment core. One Salix sp. leaf found was left undetermined to species level because of poor preservation. The cold-tolerate shrub, Betula nana, is found rarely in the Haljala sequence. At the same time B. nana remains (seeds, leaves, buds, catkin scales) were present in different late glacial sequences in Europe (e.g. Wohlfarth et al. 2002) and in Estonia (L. Amon, unpublished data). It may be due to the small sample size in case of Haljala, but may also be limited by the lack of ecological conditions B. nana needed for spread, growth, and fructification.
While browsing plant material for radiocarbon dating or identification for plant macrofossil analysis, pyritized and poorly preserved material was observed in several samples. Pyritization is a complex process in palaeobotany (Grimes et al. 2001), linked to decomposition of organic matter in the anoxic and reducing (water) environment (Yansa 1998). Organic material decaying in suitable environment is affected by sulphate-reducing bacteria, which mediates the formation of pyrite aggregates (Tovey & Yim 2002).
Nine freshwater ostracod species were identified from the Haljala late glacial sediments: Candona candida, Cyclocypris cf. laevis, Cyclocypris ovum, Cypridopsis vidua, Cytherissa lacustris, Eucypris cf. virens, Limnocythere inopinata, Limnocytherina sanctipatricii, and Pseudocandona compressa. The most common species was C. candida (Fig. 6J), which tolerates a wide range of environmental conditions. Ostracod shells and valves were well preserved, but the number of specimens was low (Table 4). Both adult and juvenile specimens were found (the ratio 80:20).
[FIGURE 6 OMITTED]
DISCUSSION AND CONCLUSIONS
Deglaciation and palaeogeography
The new evidence from Haljala suggests that the area started to deglaciate about 13 800 cal yr BP, thus about 500 years earlier than previously thought (Vassiljev et al. 2005; Saarse et al. 2007). According to Demidov et al. (2006), decay of the ice margin since the Allerod turned from aerial downwasting to frontal-type deglaciation, the ice sheet melted quickly due to the ameliorated climate, and large proglacial lakes appeared at the front of the ice margin. This seems to be valid for Haljala as well, because the sequence reveals deposits that accumulated both in a large proglacial lake and in an isolated lake. Simulation of water level surfaces showed that during the Baltic Ice Lake stage [A.sub.1], the water level near Haljala was about 86 m, during [A.sub.2]--69 m, Baltic Ice Lake I (BIL I)--60 m, BIL II--57 m, and BIL III--54 m a.s.l. (Saarse et al. 2003, 2007; Vassiljev et al. 2005). During the AZ stage, about 13 000-12 900 cal yr BP, a spit formed between Tatruse and Vanamoisa bedrock hillocks at an elevation of 69-70 m a.s.l., which isolated the elongated narrow lagoon in the Haljala depression (Fig. 1). The isolation contact at 210-220 cm is marked by increased moisture, OM, and carbonate contents and decreased sediment accumulation rate (Fig. 2). Haljala lagoon separated finally between 13 000 and 12 900 cal yr BP, forming a coastal lake where clayey silt deposited. At the Younger Dryas/Holocene transition silt deposition was gradually replaced by calcareous sandy silt and lacustrine lime.
Based on palaeobotanical, lithological, and chronological data, five main environmental stages have been recognized. These stages coincide rather well with the event stratigraphy proposed by Lowe et al. (2008).
13 800-13 300 cal yr BP
The samples from the basal part of the Haljala sequence between 500 and 330 cm consist of fine-grained and clayey silt, incorporate pollen zones Ha 1-3, and delineate a time span of 13 800-13 300 cal yr BP. These sediments were deposited in a large proglacial lake in the middle of the Allerod-Balling warming event (GI-1c). The relatively warm climate is supported by rather high OM accumulation, peaking around 13 400 cal yr BP (Fig. 2). The stable representation of the Pediastrum algae up to 13 100 cal yr BP suggests that the water/climatic conditions were rather favourable during this late glacial period. Pollen spectra display high and uniform tree pollen percentages (around 60-70%), with a noticeable admixture of Picea, Alnus, and Corylus (Fig. 4). These taxa were not constituents of the flora at that time. Sediment deposition in a proglacial lake with a vast pollen source area and low local pollen production was the main reason why pollen spectra include abundantly redeposited pollen grains (Picea, Alnus, Corylus, QM, etc.), typical of interglacial deposits. Around 435 cm depth (13 700-13 600 cal yr BP) corresponding to Ha-2, a considerable decline in the concentration and palynological richness of pollen is recorded (Fig. 5), which might represent a short-term cooling inside GI-1c. The mentioned decline/cooling could be a result of the ice margin standstill not far from Haljala. The [sup.10]Be exposure ages in the area are some-what contradictory, but still show ice-free northern Estonia since 13 600 [sup.10]Be yr BP (Rinterknecht et al. 2006). An OSL date of sand from the Pikassaare kame field (21 km west of Haljala) gave a similar age (13 700 yr BP; Raukas & Stankowski 2005), however, the authors considered kame deposits as unpromising material to study the ice recession phenomena.
Within this time interval seven sediment samples were analysed for plant macrofossils (Table 3). Characeae oospores are present in all samples. Other aquatic species found are Ranunculus sect. Batrachium and Potamogeton sp. Ranunculus sect. Batrachium is a pioneering species often found in late glacial sediments (Birks 2000). The lowermost samples contained seeds indicating mild climate: Betula humilis, Urtica dioica, and Scirpus sylvaticus, suggesting amelioration of climate in GI-lc. However, also two common arctic species are present (Dryas octopetala and Salix polaris).
Three samples were studied for ostracods. Benthic freshwater ostracods migrated through passive transport, e.g. by wind, drifting vegetation, flowing waters, birds, mammals, fishes, amphibians, and invertebrate animals (Meisch 2000). Ostracod assemblages are typical of shallow lakes at depth intervals 450-460 cm and 440-450 cm and refer to the vegetation of the littoral zone, e.g. Limnocythere inopinata (Fig. 6A) and Cyclocypris ovum (Fig. 6E, F). Limnocythere inopinata preferred a sandy substrate (Scharf 1998). The occurrence of Eucypris cf. virens (Fig. 61) may point to a temporary water body, because the species prefers grassy pools that dry up in summer (Meisch 2000). Climate amelioration in the Allerod was favourable for littoral species. There is no record of mixed ostracod assemblages of species populating deep lake and shore areas; the subfossil material shows littoral derivation. In Scharf (1998) the existence of littoral species in deep lake is explained by limnetic sediment transportation into deeper parts of the lake during storms in the circulation periods. The high sedimentation rate caused the preservation of carapaces of both adult and juvenile specimens. Ostracods became trapped among sediment particles rapidly, which diminishes the opportunity of separation of valves. Anoxic conditions prevented microbial organisms from disarticulating ostracod valves after death (De Deckker 2002). The composition of the ostracod assemblage changed sharply at a depth of 390-400 cm. Cytherissa lacustris (Fig. 6C, D), which preferred cool and deep oxygenated lakes, appeared (Meisch 2000).
[FIGURE 7 OMITTED]
Decline in the concentration, accumulation rate, and palynological richness of pollen also shows an environmental change around 13 700-13 600 cal yr BP (Ha-2; Figs 5, 7). This change seems to be caused by the emergence of Tatruse Island, which isolated a narrow sound in the Haljala ancient valley (Fig. 1) and protected sediment influx from the large proglacial lake.
Mineral sediments, coupled with low tree pollen concentration (less than 20 000, commonly 6000-8000 grains [cm.sup.-3]) and palynological richness, indicate sparse vegetation (Fig. 4). Xerophilous steppe and tundra assemblages were dominated by Betula. Pinus was distributed only in favourable habitats characteristic of the open woodland tundra. Still, Pinus macrofossils were not found; its pollen may derive from older sediments or may be long-distance transported by winds. The pollen assemblage zones (Ha 1-3) at Haljala are quite similar to that of Visusti (central Estonia): total content of AP about 70%, NAP about 30%, pine pollen dominating over birch, and a notable percentage of thermophilous taxa (Pirrus & Raukas 1996). At Visusti such a pollen composition was correlated with the Older Dryas, in Haljala with the Aller,ad chronozone GI-1c. In the westernmost part of European Russia cold arid conditions and treeless vegetation have been reconstructed for the time period of 14 000-12 000 cal yr BP (Subetto et al. 2002; Wohlfarth et al. 2002, 2006, 2007). In southern Lithuania macrofossils of Betula, Pinus, and Picea were found before 13 700 cal yr BP (Stancikaite et al. 2008). In northern Estonia birch has been present since 13 800 cal yr BP, as indicated by Betula macrofossils and PAR around 1000 grains [cm.sup.-2] [yr.sup.-1] (Fig. 7), which according to Hicks (2001), is the threshold value for the presence of birch forest. Yet, we must bear in mind that in the late glacial environment where the vegetation is sparse, and soils and melting ice are the source for inwash of older sediments and hence pollen to the lakes, sediment focusing and artificially high PARs can be observed (Seppa & Hicks 2006), which may distort our estimations about the existence of Betula woods in the early Allerod in northern Estonia.
13 300-13 100 cal yr BP
During this short time span fine-grained silt deposited with lower OM values than during the previous years (Figs 2, 7). In pollen composition (LPAZ Ha-4) Pinus and Picea percentages reached close to their maxima, up to 41% and 11%, respectively. High percentages of secondary pollen, such as Picea, Alnus, Corylus, and Ulmus, together with a low primary pollen accumulation rate and OM content, refer to cold climate and can be correlated with the GI-1b cooling. The interpretation of the elevated Picea curve is complicated, as the high pollen percentage (11 %) and accumulation rate of Picea (300-400 pollen grains [cm.sup-2] [yr.sup.-1]) between 13 300 and 13 100 cal yr BP could suggest the presence of Picea. Finds of Picea wood and high pollen percentages (28%) in Aller,ad clayey deposits of Kunda (Thomson 1934, p. 103) support the presence of spruce in North Estonia already during the Allerod. The average pollen composition (Betula 18.5%, Pinus 56%, Picea 20%, Alnus 5.5%) of Kunda clayey deposits is also similar to that of Haljala clayey deposits at a depth of 280-320 cm, where Betula fluctuates within 16-24%, Pinus within 25-41%, Picea within 6-11%, and Alnus within 4-9%. However, considering the size of the sedimentation basin at Haljala and the large pollen source area, redeposition of Picea pollen from older sediments, first of all from Eemian deposits, could not be ruled out. As a whole, pollen concentration and accumulation rate values are low for all taxa before 13 100 cal yr BP (Fig. 5), which could be explained by a high sedimentation rate of mineral matter (3 mm [yr.sup.-1]). On the basis of pollen composition, open woodland tundra, mostly with birch, pine (?), spruce (?), juniper, and willow, spread in the Haljala basin.
The ostracod assemblage with Limnocytherina sanctipatricii (Fig. 613) and Cytherissa lacustris (Fig. 6C) at a depth interval 270-260 cm indicate a cold and deep oligotrophic freshwater lake. The highest densities of C. lacustris occur in oligotrophic lakes at depths between 12 and 40 m (Meisch 2000) and the favourable temperature of the species is below 18[degrees]C (Geiger 1993). At that time (about 13 100-13 000 cal yr BP) the Haljala basin was a narrow sound in the regressive proglacial lake [A.sub.2] (Fig. 1).
13 100-12 850 cal yr BP
Sediments of this interval are represented by fine-grained silt in which OM reached its late glacial maximum (7.3%) at 12 950 cal yr BP (215 cm). At about 13 000-12 900 cal yr BP the Haljala sedimentation basin isolated from the large proglacial water body ([A.sub.2], Kemba). After that sediment transport, mineral matter influx, and consequently the sedimentation rate decreased considerably (0.9 mm [yr.sup.-1]). These phenomena could be one reason why pollen accumulation values sharply increased (Figs 5, 7).
A decrease in Pediastrum algae might be connected with water level lowering at about 13 100-13 000 cal yr BP. The isolation age of Palaeolake Haljala dates proglacial Lake Kemba to around 13 100 cal yr BP, which agrees with an earlier estimation of 13 150 cal yr BP by Vassiljev et al. (2005), but is somewhat older than the date (12 800 cal yr BP) proposed by Rosentau et al. (2007). Two [sup.10]Be dates from boulders 20 and 30 km west and northwest from Haljala correspond to the approximate level of proglacial Lake Kemba, 12 480 [+ or -] 920 (EST-8) and 12 520 [+ or -] 890 (EST-11; Rinterknecht et al. 2006), however, these boulders are of somewhat lower level and accordingly have younger ages. This suggests 'not later than' ages for the Kemba ice lake rather than for the Palivere end moraine these dates were meant for.
Three samples from the period of 13 100-12 850 cal yr BP were analysed for plant and moss macrofossils. Aquatic and mire plant remains were prevalent: abundant Equisetum sp. stems and aquatic mosses (Drepanocladus sp., Calliergon giganteum) were common together with several Carex sp. leaves and rootlets. These finds refer to increased growth of moss and mire plants within and in the vicinity of the water body, indirectly suggesting its isolation from a larger proglacial lake. Other features indicative of water environment are abundant Characeae oospores, Ranunculus sect. Batrachium seeds, and remains of two limnic animals (Daphnia and Plumatella). Daphnia and its ephippia represent the open water component of zooplankton and, together with other cladoceras, have been used in several cases as a palaeoecological (Hoffmann 2003; Feurdean & Bennike 2004) and palaeo-climatological tool (Duigan & Birks 2000). Trichoptera remains were recorded but not identified. Moss flora contains besides aquatic species (Drepanocladus sp., Scorpidium sp., Calliergon giganteum) also terrestrial species (Tomenthypnum nitens, Rhizomnium punctatum). Tomenthypnum is presently growing in fens; Rhizomnium may occur in shoreline areas but also in forest floors or trees (Ingerpuu & Vellak 1998). In the lower part of the interval the number of Dryas octopetala leaves reaches a maximum, suggesting a more severe local climate.
At a depth of 235-225 cm the ostracod assemblage shows a still high water level and cool conditions with sparse aquatic vegetation, because Cyclocypris cf. laevis (Fig. 6G, H) does not tolerate much vegetation (Meisch 2000). This is in good accord with the pollen record where aquatics are absent (Fig. 4).
The PAR values of Betula, Pinus, and Picea are high, respectively, 1100-2700, 1600-3900, and 160-370 grains [cm.sup.-2] [yr.sup.-1] (Figs 5, 7). A rather dense forest could be suggested, referring to the threshold values proposed by Hicks (2001). This is contradicted by high PAR values of light-demanding Juniperus, Artemisia, Chenopodiaceae, and Cyperaceae. On the other hand, these species obviously colonized the new area that emerged from the proglacial lake waters. Taking into account the possible sediment focusing mentioned earlier, we may rephrase the vegetation assemblage to an open pine--birch woodland with sparse spruce (?) stands with shrubs and herbs. An early immigration of Picea seems rather likely in the light of Picea presence in the Scandinavian Mountains already at 13 000-12 900 cal yr BP (Kullman 2008) and macrofossil finds from Kunda (Thomson 1934). As Picea macroremains have not been found in the Haljala sequence, the presence of Picea at the end of the Allerod remains still open.
12 850-11 500 cal yr BP
The Allerod was succeeded by the Younger Dryas cold event, caused by a large reduction in the Atlantic thermohaline circulation (Alley 2000; McManus et al. 2004) and/or by Arctic freshwater forcing (Teller et al. 2002; Tarasov & Peltier 2005). The boundary between the Allerod and Younger Dryas stadials at 12 850 cal yr BP (present study) is defined by the increasing content of herb pollen (Artemisia, Poaceae, Cyperaceae, and Chenopodiaceae) and decreasing frequency of tree pollen (Fig. 4, Ha-5-7). Deposits of the Younger Dryas stadial (GS-1) are represented by grey and greenish-grey silt with fine-grained sand interlayers and remains of Bryales moss corresponding to the Artemisia-Betula nana pollen zone defined by Pin-as & Raukas (1996). The accumulation rate of tree and shrub pollen was very low, even lower than in the early Allerod (Figs 5, 7). The percentage of NAP pollen surpassed that of AP as herbs spread in the newly emerged area. Climate cooling resulted in a significant reduction of Sub-Arctic woodlands and their replacement by grass-shrub tundra without birch and other trees, as the PAR values of the latter stay below 500 grains [cm.sup.-2] [yr.sup.-1], thus below the level of the forest (Hicks 2001). The closest macrofossil evidence of Picea at that time is from the Valdai Highlands and close to the Ural Mountains around 12 000 cal yr BP (Valiranta et al. 2006; Wohlfarth et al. 2007).
11 500-11 300 cal yr BP
A great change in the vegetation community occurred with the start of the Holocene warming. The increased representation of all forest taxa, first of all the percentage of Betula, suggests new expansion of the forest, characteristic of the beginning of the Holocene. The PAR of birch still remained rather low (Fig. 7) and shrubs seemed to flourish, with Salix, Betula nana, and Juniperus having their maximum PARs (Fig. 5). Some delay to ameliorated climate occurred in vegetation response, especially reforestation, as was also the case on the Karelian Isthmus (Subetto et al. 2002; Wohlfarth et al. 2002). The delay was caused by the cold water of the Baltic Sea and possible increase in anticyclonic circulation due to the presence of remnants of the Scandinavian ice sheet.
Acknowledgements. We thank Tiit Vaasma from Tallinn University for grain size analyses and Anneli Poska for rarefaction analysis. We acknowledge Dr A. Cerina from the Latvian University and Prof. H. H. Birks from Bergen University for consultations on macrofossil analysis and Dr M. Valiranta from Helsinki University on fossil moss analysis. This research was supported by the Estonian Science Foundation (grants 6736 and 7029) and by the SYNTHESYS Project (http://www.synthesys.info/), which is financed by the European Community Research Infrastructure Action under the FP6 'Structure the European Research Area' Programme. The manuscript benefited from critical reviews by S. Hicks, D. Subetto, and an anonymous referee.
Received 11 March 2009, accepted 28 April 2009
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Leili Saarse (a), Eve Niinemets (b), Leeli Amon (a), Atko Heinsalu (a), Siim Veski (a), and Kadri Sohar (c)
(a) Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
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Table 1. Grain size distribution of sediments in the Haljala sequence Depth, Clay, Very fine Fine silt, Medium silt, cm >0.002 mm silt, 0.002- 0.004- 0.006- 0.004 mm 0.006 mm 0.02 mm 100 3.59 3.73 3.83 10.88 140 22.42 22.87 26.23 14.06 180 10.66 24.18 30.37 16.76 220 17.08 22.94 27.30 15.51 260 6.12 19.09 27.73 19.95 320 6.98 23.19 28.96 17.73 360 11.19 25.59 28.57 16.08 400 20.11 28.42 24.89 12.18 440 17.77 28.61 27.57 13.35 460 18.19 24.97 26.70 14.26 480 11.27 26.47 29.52 15.54 Depth, Coarse silt, Very coarse Sand, Sediment description cm 0.02- silt, 0.30- 0.63-2 mm 0.30 mm 0.63 mm 100 24.25 29.71 24.01 Calcareous sandy silt 140 4.76 4.67 5.00 Clayey silt 180 6.14 6.31 5.58 Clayey silt 220 5.74 5.96 5.47 Clayey silt 260 8.95 9.84 8.32 Fine-grained silt 320 7.33 8.06 7.75 Fine-grained silt 360 5.95 6.24 6.38 Clayey silt 400 4.41 4.75 5.24 Clayey silt 440 4.13 4.00 4.57 Clayey silt 460 5.43 5.05 5.40 Clayey silt 480 5.62 5.70 5.88 Fine-grained silt Table 2. AMS radiocarbon dates calibrated according to Stuiver et al. (2005). Dates used for the age-depth curve reconstruction of the Haljala sediment record are marked with the asterisk. Radiocarbon laboratory codes: Poz, Poznan; Ua, Uppsala; TA, Tartu; Hela, Helsinki; Tln, Tallinn Site/Sample Depth, [sup.14]C date, Calibrated age, number cm BP cal yr BP at 1 sigma Haljala 1 72-73 3 995 [+ or -] 35 4 465 [+ or -] 45 Haljala 2 210-215 10 970 [+ or -] 150 12 945 [+ or -] 105 * Haljala 3 220-225 9 205 [+ or -] 90 10 370 [+ or -] 115 Haljala 4 310-315 11 780 [+ or -] 60 13 655 [+ or -] 85 * Haljala 5 350-355 5 090 [+ or -] 35 5 830 [+ or -] 75 Haljala 6 390-400 17 860 [+ or -] 65 21 100 [+ or -] 200 Haljala 7 410-415 11 750 [+ or -] 80 13 600 [+ or -] 105 * Haljala 8 440-450 23 650 [+ or -] 70 -- Kunda 120-125 11 690 [+ or -] 150 13 550 [+ or -] 150 Kunda 100 10 170 [+ or -] 95 11 830 [+ or -] 210 Viitna 812-822 10 515 12 470 [+ or -] 220 Viitna 840-850 10 690 [+ or -] 100 12 745 [+ or -] 95 Antu 740-750 10 930 [+ or -] 200 12 930 [+ or -] 160 Antu 816 10 840 [+ or -] 60 12 845 [+ or -] 30 Loobu 180 7 450 [+ or -] 40 8 265 [+ or -] 65 Loobu n.a. 13 070 [+ or -] 115 15 440 [+ or -] 200 Loobu n.a. 14 725 [+ or -] 260 17 940 [+ or -] 540 Raatsma 1 540 12 040 [+ or -] 100 13 890 [+ or -] 100 Raatsma 2 360-370 12 050 [+ or -] 120 13 910 [+ or -] 120 Site/Sample [DELTA][sup.13]C, Laboratory number 0% [per thousand] code PDB Haljala 1 - Poz-27420 Haljala 2 - Poz-22529 Haljala 3 -24.9 Ua-33185 Haljala 4 - Poz-22530 Haljala 5 - Poz-19615 Haljala 6 -30.8 Ua-33186 Haljala 7 - Poz-22531 Haljala 8 -31.1 Ua-33187 Kunda - TA-194 Kunda - Hela-597 Viitna - Tln-2147 Viitna - TA-443 Antu - TA-2119 Antu -19.6 Poz-19684 Loobu - Poz-19616 Loobu - TA-137 Loobu - TA-138 Raatsma 1 - TA-688 Raatsma 2 - TA-687 Site/Sample Dated material References number Haljala 1 Wood Current study Haljala 2 Terrestrial macrofossil Current study Haljala 3 Moss, leaf fragments Current study Haljala 4 Terrestrial macrofossil Current study Haljala 5 Aquatic moss Current study Haljala 6 Moss, leaf fragments Current study Haljala 7 Terrestrial macrofossil Current study Haljala 8 Seeds Current study Kunda Bryales moss from Pirrus 1976; Karukapp lacustrine lime et al. 1996 Kunda Reindeer bone Ukkonen et al. 2005 Viitna Bulk gyttja Saarse et al. 1998 Viitna Bulk gyttja Saarse et al. 1998 Antu Plant remains Saarse & Liiva 1995 Antu Gyttja Sohar & Kahn 2008 Loobu Plant remains Saarse unpubl. Loobu Plant remains Pirrus 1976 Loobu Plant remains Pirrus 1976 Raatsma 1 Bryales moss from sand Ilves 1980 Raatsma 2 Bryales moss from Ilves 1980 n.a., not available; - calibration is not possible. Table 3. Macroremains from the Haljala core. Analyses by L. Amon Depth, Plant macrofossils and mosses identified in sediment samples cm 60-62 Carex sp. seed 72-73 Wood of a deciduous tree (Alnus?) 205-210 Equisetum sp. remains, Ranunculus sect. Batrachium seeds (six), Trichoptera remains. Mosses: Drepanocladus sp., Scorpidium sp., Tomenthypnum nitens, Rhizomnium punctatum 225-235 Dryas octopetala leaf fragments (six), Betula nana leaf, several Equisetum sp. remains, abundant Characeae oospores (120), remains of limnic animals (Daphnia ephippia, Plumatella statoplasts). Mosses: Drepanocladus sp., Calliergon giganteum 240-245 Abundant Equisetum sp. fragments, many Dryas octopetala leaves (14 fragments), Carex sp. remains (one leaf, two rootlets). Aquatic mosses (Drepanocladus sp.) 260-270 Many Characeae oospores(22) 340-350 One Ranunculus sect. Batrachium seed, Dryas octopetala leaves (four), few Characeae oospores 350-360 Pyritized herbs and stems, one Ranunculus sect. Batrachium seed, 20 Characeae oospores 380-390 Pyritized herbs and stems, one Salix sp. leaf, three Characeae oospores 390-400 One Sahx polaris leaf fragment, one Potamogeton sp. seed, three Characeae oospores 405-410 Dryas octopetala leaf 440-450 Pyritized macroremains, one Ranunculus sect. Batrachium sp. seed, Urtica dioica seed, Scirpus sylvaticus seed, Dryas octopetala leaves (three), abundant Characeae oospores 450-460 Pyritized macroremains, Betula humilis seed, three Dryas octopetala leaf fragments, 21 Characeae oospores Table 4. Distribution of ostracod species in the Haljala core. Analyses by K. Sohar Depth, Species Number cm of specimens 225-235 Candona candida 6 Cyclocypris cf. laevis 8 Limnocytherina sanctipatricii 3 260-270 Candona candida 1 Cytherissa lacustris 1 Limnocytherina sanctipatricii 4 390-400 Cytherissa lacustris 5 Pseudocandona cf. compressa 1 440-450 Candona candida 14 Cyclocypris ovum 3 Cypridopsis vidua 1 Eucypris cf. virens 4 Limnocythere inopinata 6 Pseudocandona cf. compressa 6 450-460 Candona candida 3 Cyclocypris ovum 7 Eucypris cf. virens 2 Pseudocandona cf. compressa 1