Late Quaternary palaeoenvironmental reconstructions from a permafrost sequence (North Siberian Lowland, SE Taymyr Peninsula) – a multidisciplinary case study ¨ RGEN HAHNE ULRIKE KIENEL, CHRISTINE SIEGERT AND JU
Kienel, U., Siegert, C. & Hahne, J. 1999 (March): Late Quaternary palaeoenvironmental reconstructions from a permafrost sequence (North Siberian Lowland, SE Taymyr Peninsula) – a multidisciplinary case study. Boreas, Vol. 28, pp. 181–193. Oslo. ISSN 0300-9483. A Late Quaternary permafrost profile from the Labaz Lake area (Taymyr Peninsula, Central Siberia) has been investigated using analyses of diatoms, pollen, sedimentology and geochemistry. A sedimentation gap for the termination of the Pleistocene and the start of lacustrine sedimentation in the Boreal can be inferred. At that time shallow-lake conditions prevailed and a connection to the Labaz Lake is probable. The diatom data provide indications for the assessment of the Holocene environmental development of the study site in more detail. The most favourable climatic conditions and comparatively higher water level stands are inferred for the early Atlantic. Towards the end of this period the gradual transition to a wetland with poor-fen character took place. Within the Subboreal there was a short episode with mild climate conditions. At that time the last trees occurred in the Labaz Lake area and somewhat higher water level stands were likely. The Subatlantic climate deterioration led to further drying up of the wetland along with a lowering of water pH and permafrost aggradation. The transition from a locality with low-centre to one with high-centre polygons is still underway. Ulrike Kienel, Christine Siegert & Ju¨rgen Hahne, Alfred Wegener Institute for Polar and Marine Research, Research Dept. Potsdam, Telegrafenberg A 43, D-14473 Potsdam, Germany. E-mail:
[email protected]; received 22nd December 1997, accepted 14th January 1999
High-latitude aquatic ecosystems, which have adapted to low natural energy flows, are expected to be particularly sensitive to changes in the magnitude and timing of available energy and in physical and geochemical conditions (e.g. Maxwell 1992). The potential advantage of using peat profiles in this context as palaeoecological archives is that peat, even at such high latitudes, accumulates much faster than lake sediments. Peat profiles may therefore provide stratigraphic information at a much finer temporal resolution. Moreover, peat is formed in situ and is not subject to redeposition and transportation. Diatoms are the most frequently used fossil group in freshwater palaeoecology because they tend to be well preserved, diverse, ubiquitous and identifiable to a low taxonomic level. They respond quickly to environmental change because of their short life cycle (Dixit et al. 1992) and, since a considerable volume of data on their ecological characteristics exists, they are valuable and sensitive tools in environmental reconstructions. In contrast to palynology, diatom analysis provides a largely autochthonous signal. In addition to diatoms, the investigations include cryolithological field studies, radiocarbon dating, palynological, lithological and geochemical analyses. The results obtained until now (Siegert et al. in press) confirm the assumption that the eastern part of the North Siberian Lowland was non-glaciated during the Sartan (Late Weichselian) stadial (Astakhov 1997). However,
only limited information exists about the environmental development of this area following the decay of the Zyryan (Early Weichselian) ice cover, especially for the period since 18–20 000 years BP. The scope of the present study is to characterize the Holocene development of the large ‘extinct’ lake depressions (Fig. 1) which are regarded as parts of a vast Late Pleistocene ‘Pre-Labaz Lake’ (Andreeva et al. 1982). The chosen permafrost sequence, LAB2, represents syncryogenic lacustrine deposits overlain by a Holocene peat bed.
Material and methods Site Palaeogeographical studies have been carried out in the Labaz Lake area (72°N, 99°E) as part of the GermanRussian research project ‘Taymyr’ during two expeditions (Siegert et al. 1995, 1996) (Fig. 1). The study area represents a typical part of the lowland surrounding the Laptev Sea to the SW. The morphology of this landscape was formed by glacial, lacustrine and cryogenic processes. Wide and flat lacustrine depressions are situated within the glacial relief elements. Numerous small lakes, primarily of thermokarst genesis, occur at different altitudes (Fig. 2). Holocene peat beds are widespread. The recent soil cover is deter-
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Fig. 1. Geographical setting of the Labaz Lake with vegetation zones and northern tree line (Matveyeva 1994)
mined by weakly drained cryosols underlain by ice-rich permafrost. The following data characterize the modern climatic and geocryological conditions: mean annual air temperature ÿ13.4°C; mean January air temperature ÿ33.8°C; mean July air temperature 12.3°C; annual precipitation 237 mm; mean thickness of the active layer 0.2–0.5 m; permafrost temperature at the zero annual amplitude ÿ7 to ÿ9°C and permafrost thickness 300–600 m. Typical subarctic treeless tundra vegetation covers the area. The permafrost sequence LAB2 was recovered in a
plain ‘extinct’ lake depression at the northern shore zone of the Labaz Lake. As shown in Figs 2 and 3, the study location is situated near a remnant terrace. According to the results presented by Siegert et al. (in press), the lacustrine sediments which formed this terrace originated from the end of the Kargin interstadial and the Sartan (Late Weichselian). Today the study location is characterized by slightly high-centre polygons with vegetation corresponding to a moderately wet type of subarctic treeless tundra, including Eriophorum and dwarf-shrubs like Vaccinium, Rubus
Fig. 2. Topographic sketch of the study area at the northern shore of Labaz Lake with location of the study site.
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Fig. 3. Photograph of the study area (line of sight ESE) showing the flat relief of the extinct lake, subdivided only by the Tolton-PastakhYuryakh river and some channels. The study site is marked by an arrow.
and Betula nana. The soils are classified as Histic Pergelic Cryoaquept (Pfeiffer et al. 1996). To the north of the study locality younger low-centre polygons and small residual lakes have developed. Fieldwork The top of a slightly high-centre polygon was chosen for coring of the LAB2/2 sequence. Coring was carried out using portable Russian drilling equipment. Soil samples from the active layer were collected from additional digging wells (LAB 2/1). The permafrost core was taken in steps of 0.20–0.30 m. Immediately after extraction the frozen core-samples were cleaned, photographed and their lithological and cryogenic structure was described in detail. Subsequently, samples of frozen ground were taken in order to estimate the ice/ water content using the gravimetric method. The remaining core samples were first stored in double plastic bags and thawed in the camp. After complete thawing, pH and conductivity of the extracted water were estimated using a portable WTW microprocessor. The remaining sediment or peat samples were air-dried and stored in sealed plastic bags for different laboratory analyses. Laboratory investigations Granulometric composition. Bulk samples of the sediments were oxidized and disaggregated by means of 3– 10% H2O2 solution. After sieving the samples (mesh size 63 mm), the clay fraction was separated from the silt fraction by the Atterberg method. The silt fraction (2– 63 mm) was further subdivided by means of a Sedigraph. Chemical composition. The total carbon and nitrogen contents of the freeze-dried and raw bulk samples were measured using a CHNS-932 MIKRO (Leco Corporation). The total organic carbon content (TOC) was estimated in corresponding samples using a Metalyt-CS 100/1000 S (Eltra). Treatment with hydrochloric acid to
remove possible carbonates was a routine step in the sample preparation. Radiocarbon dating. Fine plant remains selected under a stereomicroscope were used for dating. Measurements were performed by Accelerator Mass Spectroscopy (AMS) at the Leibnitz Laboratory of the University of Kiel. Two peat samples from the permafrost table were dated by Liquid Scintillation Counting (LSC), using Packard Tri-Carb 2560 TR/XL spectrometers at the Alfred Wegener Institute, Potsdam and at Leipzig University. The 14C ages are given as ‘conventional ages’ (Geyh & Schleicher 1990), i.e. the ages given in ‘yr BP’ were calculated on the basis of the measured activity using a 14C half-life of 5568 years. The activity was corrected for isotopic fractionation by normalizing it to a 13C value of ÿ25%. Pollen analysis. All samples except the three lowest in the sequence were more or less rich in organic matter. Therefore they were first treated with KOH (10%). Silicates were removed using 70% HF, followed by ultrasonic sieving (mesh size 6 8 mm) and acetolysis treatment. The pollen sum in all samples, except the above-mentioned lowest samples, amounted to at least 500. The palynological results are represented as a total pollen diagram. The pollen sum (100%) includes all pollen except those of aquatic plants and spores. The Betula curve includes three Betula tree species and Betula shrubs (Betula nana and other shrub birches without a clearly visible vestibulum, summarized as ‘Betula exilis type’). The sequence was subdivided into four pollen assemblage zones (PAZ) labelled LB 1 to 4. Diatom analysis. Sample processing of 1 g freezedried sediment followed in general the method outlined by Schrader & Gersonde (1978). Briefly, the organic ¨ SOL compounds were removed by digestion in LO (washing benzene) followed by boiling with H2O2 (30%) at 60°C. The samples were allowed to settle overnight. Repeated flushing with distilled water (up to 10 times) removed finer particles. The use of HCl was not necessary since the material yielded no calcareous compounds. The remaining suspension was filled to 50
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Table 1. Terminology used for the relative abundance of the diatom species. Percentage proportion
Terminology
1 individual 10%
Sparse Rare Few Frequent Abundant
ml and stored in plastic bottles. Preparation of slides began with 0.8 ml aliquots taken from the stored suspension according to an evaporation method developed in the laboratory of the Alfred Wegener Institute, Bremerhaven and outlined by Zielinski (1993). The mounting medium MOUNTEX (refraction index Ri = 1.67) was used for permanent slides. Diatoms were identified using a ZEISS Axioplan microscope (equipped with differential interference contrast) at 1000 magnification. The diatom valves were counted along transects measured by stage micrometer until 40 mm of transect were examined or until 500 valves per sample were counted. At the same time, the chrysophycean cysts were included, but not further identified. At least 100 valves were counted in
samples extremely poor in diatoms. Since aliquots from the sample material and suspension were used for all slides, the number of diatom valves and chrysophycean cysts encountered per millimetre of microscope transect in each sample gives a semi-quantitative estimate of their concentration (Bradbury 1993). Taxa with less than five occurrences and with a relative maximum abundance below a limiting value of 5% were excluded from graphical illustration. The following literature was used as reference for determination of diatom taxa: Krammer & Lange-Bertalot (1986, 1988, 1991a, b), Lange-Bertalot (1993) and Camburn & Kingston (1986). According to the percentage proportions of species in diatom assemblages, the terminology used for relative abundance is listed in (Table 1.) In order to subdivide the set of samples, we applied a stratigraphically constrained cluster analysis. Prior to the procedure, we culled the data by eliminating all taxa with maximum percentages below a limiting value of 2%. The analysis started with the individual samples as initial ‘clusters’. We repeatedly joined two spatially neighbouring clusters. They were selected in such a way that the change of sample properties at the boundary between the two clusters was as ‘smooth’ as possible. For this, we chose the two clusters with the most
Fig. 4. Relative abundances of diatoms and local diatom assemblage zonation (DAZ) in the profile LAB2. For explanation of symbols, see Fig. 8.
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Fig. 5. Organic carbon content, concentration of diatom valves and chrysophycean crysts of the sediments in the profile LAB2. (For explanation of the symbols, see Fig. 8.)
‘similar’ adjacent boundary samples (i.e. the lowermost sample of the upper cluster and the uppermost sample of the lower cluster). As a measure of the dissimilarity of two samples we applied the squared Euclidean distance (sum of squares of the differences of the diatom percentages in the samples). Finally, a dendrogram was plotted in order to check the subdivisions (DAZ) of the diatom stratigraphy.
Diatom data Diatom stratigraphy Diatoms in the sequence were first recorded from a sample taken at 3.45 m sediment depth (samples 27– 30). The observed diatoms in the section from 3.45 to 3.05 m sediment depth were poorly preserved. Chains of Fragilaria pinnata and F. construens var. venter were
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the main constituents. From larger forms such as Amphora libyca, Navicula cf. rhynchocephala, Gyrosigma attenuatum, Stauroneis anceps, Sellaphora pupula, Cymbella sp. and Encyonema sp. only broken pieces or the more heavily silicified central areas occur. In total over 70 diatom taxa were identified. The abundances of the frequent and important species are presented in Figure 4. The Labaz sequence was partitioned into six local diatom assemblage zones (DAZ) applying a stratigraphically constrained cluster analysis (Fig. 4, Table 2). In the lower part of the sequence (DAZ-1 to 4) Fragilaria construens var. venter and F. pinnata dominated the assemblages. In DAZ-2 Tabellaria flocculosa was first recorded. Achnanthes minutissima var. minutissima co-occurring with Achnanthes nodosa, Eunotia faba and Nitzschia spp. showed maximum abundance in the sequence and decreased sharply at the end of the zone. DAZ-3 is marked by increasing percentages of Eunotia spp. (E. incisa, E. faba and later E. arcus). Gomphonema cf. procerum, Navicula cf. rhynchocephala and Pinnularia subcapitata became frequent. A strong increase in these taxa was recorded towards the end of the zone. Finally, the assemblages were dominated by E. arcus and E. incisa in association with G. cf. procerum. In DAZ-4 the E. arcus/incisa association was substituted by E. implicata and E. faba. Navicula cf. rhynchocephala co-occurred. In the topmost sample of DAZ-4 only the E. arcus/incisaassociation reappeared together with G. cf. procerum and T. flocculosa. A generally changed diatom assemblage was observed in DAZ-5. Until this point the dominant assemblage of Fragilaria and Eunotia species declined. Only T. flocculosa remained present. The DAZ-5 diatom assemblage is characterized by the dominating Aulacoseira nygaardii associated with Pinnularia brevicostata, Eunotia bilunaris, P. subcapitata, Cymbella tynnii and Neidium ampliatum. The diatom assemblage observed in the uppermost sample (DAZ-6) of the sequence differs significantly. Caloneis aerophila and Diadesmis contenta clearly dominated. The species characteristic of DAZ-5 contribute with low percentages.
Diatom concentration Table 2. Range of local diatom assemblage zones in the profiles LAB2/1 and 2. Diatom assemblage zone
LAB sample numbers
Depth range
DAZ-6 DAZ-5 DAZ-4 DAZ-3 DAZ-2 DAZ-1
LAB2/1: 1a LAB2/1: 1b, 2, 3 LAB2/2: 1, 3 LAB2/2: 3, 5–7 LAB2/2 8–12, 14 LAB2/2: 15–21, 23 LAB2/2: 25, 26
(0.00–0.05 m) (0.05–0.40 m) (0.40–0.85 m) (0.85–1.55 m) (1.55–1.75 m) (2.75–3.00 m)
Semi-quantitative approximations of diatom valve and chrysophycean cyst concentrations, calculated per millimetre of microscope transect, are given in Figure 5. Diatom concentration increased strongly from the first record until it reached its highest values in DAZ-2 (330 valves). During DAZ-3 to 4 it remained low. A slight increase was noticeable in DAZ-5. The concentration of chrysophycean cysts increased as well from the first record to the lowest sample in DAZ-3 and remained stable during the upper part of the sequence (10–15 cysts).
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Fig. 6. Relative abundances of diatom taxa in life form groups in the profile LAB2. (For explanation of the symbols, see Fig. 8.)
Life form groups Generally, it is considered difficult to subdivide periphytic taxa in sediment assemblages into detailed life form groups, as few taxa show exclusive preferences. A general distinction between planktonic and periphytic diatom taxa, however, can usually be made (Battarbee 1986). Some taxa often found in the plankton can also occur in the periphyton (e.g. Tabellaria flocculosa, Aulacoseira sp.). The small chain-forming Fragilaria species such as F. pinnata and F. construens are found to be attached only passively to the substrate and can thus easily drift and occur in the plankton. The life form of such species is often described as ‘tychoplanktonic’ (cf. Voigt 1997). In diatom analysis the changes in the euplanktonic/ periphytic ratio were conventionally interpreted in two ways: as a result of a change in the morphology of the lake basin as lake water level changes, and/or in productivity (Battarbee 1986). A lake level lowering is considered to be indicated by high percentages of periphytic diatom taxa or an increase in the area of shallower marginal regions, since this creates large areas suitable for macrophyte growth. In turn, macrophyte growth forms the substrate for periphytic diatom colonization (e.g. Bennion 1995). A strong influence of the lakes’ summer surface temperature on the composition of diatom assemblages is recorded from arctic and alpine regions (Pienitz 1993; Pienitz et al. 1995; Wunsam & Schmidt 1995; Lotter et al. 1997). In high- and mid-arctic sites the extent of snow and ice cover and especially the timing of ice
break-up and stratification can play an important role (e.g. Lemmen et al. 1988; Wolfe 1994; Lotter et al. 1997). During cold periods with a long ice cover season, when lakes have only a small ‘moat’ of open water (Smol 1988), shallow water or periphytic taxa are favoured. A distinctive feature in the study sequence is the occurrence of aerophilous taxa, which are related to mosses and soils, especially to the air/water interface and are able to colonize under reduced-light conditions. The proportions of the diatom life form groups (aerophilous, tychoplanktonic and periphytic, based on relative species abundances) showed distinctive trends in the sequence (Fig. 6). The proportion of tychoplanktonic taxa decreased continuously, while those of periphytic taxa increased gradually. Aerophilous taxa became important while both groups finally retreated. Considering the ratio of tychoplanktonic vs. periphytic plus aerophilous taxa, these trends are even more evident.
pH-indication Diatoms are sensitive indicators of lake water pH (Battarbee et al. 1986). Various numerical procedures have been developed for quantitative inference of pH from fossil diatom assemblages. Indices by Merila¨inen (1967) and Nygaard’s index (1956), improved by Renberg & Hellberg (1982), were based on Hustedt’s pH spectra (Hustedt 1937–39) and have been widely used. The application of several statistical techniques
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Fig. 7. Relative abundances of diatoms in pH preference groups in the profile LAB2. (For explanation of the symbols, see Fig. 8.)
(e.g. Birks et al. 1990) eliminated the subjectivity of preference grouping. The conventional preference grouping using the Hustedt classification (Hustedt 1937–39) has been applied to the diatom data in this study in order to assess trends in water pH. Additional data (e.g. Stevenson et al. 1991; Korsman & Birks 1996; Voigt 1997) were also included. Fragilaria pinnata, F. construens var. venter and Achnanthes minutissima var. minutissima are generally considered alkaliphilous. In recent calibration or training sets (Stevenson et al. 1991; Hall & Smol 1995; Korsman & Birks 1996), however, the pH optima are shifted towards the circumneutral point depending on the aim and purpose of the studies, which determined the choice of sites incorporated in the training sets. Hence, these mentioned species are labelled ‘circum-
neutral/alkaliphilous’ for illustrative purposes in this study. The trends observed in all the preference groups were distinctive (Fig. 7). In the lower part (DAZ-1 to 2) of the sequence, ‘circumneutral/alkaliphilous’ taxa dominated but tended to decrease. A clear parallel increase in acidophilous taxa was recorded, while circumneutral taxa showed stable proportions around 10% in this subunit. In the lower DAZ-3 a pulse of the ‘circumneutral/alkaliphilous’ taxa interrupted their gradual decrease. In contrast, the proportions of acidophilous taxa comprise about 25% of the assemblages, while the circumneutral taxa increased. In the upper DAZ-3 they began to dominate over the ‘circumneutral/alkaliphilous’ taxa. Only in DAZ-4 did the ‘circumneutral/ alkaliphilous’ taxa increase once more before they disappeared. Subsequently, in DAZ-5 and -6 acidophi-
Table 3. Radiocarbon ages from the permafrost section LAB2. Radiocarbon ages–permafrost profile LAB2 Sample ID
Sample
Description
Conventional age [yr BP]
Method
AWI-105 LZ-1278 KIA1406 KIA1407 KIA1408 KIA1409 KIA1410 KIA1411 KIA1412
LAB2/2; 0.40 m LAB2/3; 0.36–0.40 m LAB2; 0.5–0.57 m LAB2; 1.4–1.5 m LAB2; 2.25–2.4 m LAB2; 2.95–3.05 m LAB2; 3.24–3.32 m LAB2; 3.65–3.75 m LAB2; 3.79–3.85 m
Peaty soil Peaty soil Sedges, woody plant remains, seed Different plant remains Different plant remains Different plant remains Different plant remains, charcoal? Different plant remains Sand with rare plant remains, seed
2900 50 4200 60 5770 70 6580 70 7360 60 8960 90 8710 100 20400 300/ÿ290 24990 520/ÿ480
LSC LSC AMS AMS AMS AMS AMS AMS AMS
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lous and tolerant taxa increased strongly. For many of the diatoms occurring in this section no data on pH preferences were available.
Discussion Reconstructions of the Late Quaternary environmental history in the Labaz Lake area, as well as in other lowlands of Northern Siberia, are complicated by the following circumstances: *
*
*
Exceptionally cold climatic conditions favoured permafrost aggradation and persisted during the entire Late Quaternary. Geomorphologic processes and soil formation were permanently affected by the comparatively cold ice-rich permafrost (c. ÿ10°C). A change in vegetation occurred at the Pleistocene/Holocene transition.
During the last glacial – the Sartan – the high continentality of the climate in the north of Central and East Siberia permitted relatively high biological activity during the short but comparatively warm summers and enabled the spread of a specific ‘tundrasteppe vegetation’ (Ukraintseva 1993; Sher 1997). Already at the onset of the Holocene, the tree line shifted far to the north (Velichko et al. 1994; Hahne & Melles 1997). Only at the end of the Atlantic can climate deterioration be inferred. The tree line returned to similar southern latitudes to those it occupied during the Allerød (Fig. 1). It should be emphasized that in the Labaz Lake area, characterized by exceptionally low relief energy, even the Holocene climate warming did not change the geocryological conditions radically. Cold ice-rich permafrost and low active layer thicknesses persisted. As calculated by Derevyagin et al. (1996), the increase in the active layer thickness during the Holocene climatic optimum was in the order of 0.5–1.0 m on the watersheds and 0.2–0.4 m in depressions. The formation of thin taliks below lakes is assumed. Despite this, the results obtained from the diatom analysis of section LAB2 give a clear indication of changes in the environmental conditions. In the studied section a sedimentation gap at about 3.5 m sediment depth seems evident. The underlying sandy sediments with low organic carbon contents were radiocarbon dated to 24 990 520 and 20 400 300 yr BP. In contrast, the overlying silty deposits, comparatively rich in organic material, belong to the Holocene (Fig. 8, Table 3). The pollen assemblages observed in the sandy sediment horizon (PAZ LB 1 in Fig. 9) showed spectra typical of tundra-steppe vegetation characterized by high pollen proportions of Artemisia, Poaceae and Cyperaceae. One striking argument for an interruption
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of the sedimentation is the lack of gradual transition via shrub-tundra vegetation, as recorded, for instance, from the Lama Lake and Levinson-Lessing Lake regions (Hahne & Melles 1997; Hahne & Melles in press) and from other sections recovered in the Labaz area (Shilova et al. 1996; Siegert et al. in press). The large-scale shallow lakes probably formed during the Kargin Interstadial, in connection with ice cover decay (Siegert et al. in press), dried up during the termination of the last glacial period and thus provided for the further spread of tundra-steppe vegetation. Dune fields of Sartan age recorded in the surroundings of the Labaz depression indicate aeolian processes during this period (Isaeva 1982). Under the inferred cold climate conditions with high continentality, the syngenetic transition of these sediments to permafrost is highly probable. The relatively high pH and electrical conductivity values measured in water samples from segregated ground ice (Fig. 8) could be caused by repeated freezing of the sediments in a subaerial or subaquatic active layer. During winter freezing the mineral compounds became relatively enriched in the low volume of remaining water. However, evaporation effects are also of importance. We provide the following interpretations concerning the supposed sedimentation gap. (1) The study locality dried up and the sedimentation was interrupted. (2) During the transition from the Late Pleistocene to the Holocene water currents caused erosion and redeposition of sediments. The lowest altitude (70 m) of the depression remains right underneath the remnant terrace within the lake plane, providing the necessary relief energy. Even remnant lakes are existent. The morphology and lithology indicates a delta-like relief of runoffrills (characterized by peat deposits) and remnant embankment dams where sandy-silty sediments underlie the modern soil. These soils are classified as wet Pergelic Cryoaquept (Pfeiffer et al. 1996). Since it explains the observed inversion in the 14C ages as well, the second argument is probable. No diatoms have been found in samples below a sediment depth of 3.45 m. Proceeding from the basic ecological requirements of diatoms and the sediment properties, the following factors likely inhibited diatom establishment. (1) The sediments are of subaerial origin. (2) The existing lake was perennially ice covered and/or frozen down to the bottom during winter. (3) The diatom preservation is affected by dissolution and or turbation processes. In particular, an increasing pH accelerates the rate of diatom silica solubility (Lewin 1961) and affects diatom assemblages in such a way that small-sized forms with a low surface area/volume ratio and strongly silicified parts of larger forms are left (Barker 1992). Post-depositional dissolution has been related, for example, to turbulence-induced intensive mixing of the water body accompanied by oxygenation of the bottom waters (McMinn 1995). It appears that a combination of these processes may
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Fig. 8. Lithology, cryostructure and chemical characteristics of the sediments and ground ice parameters in the profile LAB2.
explain the absence of diatoms, but they are difficult to disentangle. The clayey-silty sediments, which overlay the Late Pleistocene sand horizon, are rich in allochthonous plant remains. The radiocarbon dates (8710 100, 8960 90 yr BP) puts them in the Boreal chronozone (Mangerud et al. 1974). The observed age inversion (Fig. 8) may indicate a redeposition of sediments, probably in a newly established shallow lake environment. This is confirmed by the high content in soluble compounds (Ca, Mg, Na, Cl', SO4'') and high pH values of the segregation ice. The pollen assemblages indicate the development of open woodland during the Boreal chronozone (PAZ LB 2, Fig. 9). Larix, Betula and Alnus trees dominated. In the scrub wood shrubs of Alnus and Betula were abundant. Although the pollen diagram of profile LB 2 does not depict the Holocene climate optimum as clearly as pollen sequences from deeper lakes on the Taymyr Peninsula (Hahne & Melles 1997; Hahne & Melles in press), a strong first warming is noticeable. Contemporarily, the TOC content of the sediments increases (Fig. 8). From the decreasing conductivity
values of segregated ice and the finer grain composition of the sediments, a gradual development of more stable hydrological conditions is inferred. Only at the end of PAZ LB2 were the first poorly preserved diatom assemblages recorded. Dissolution processes are likely to have affected the first diatom assemblages in the Labaz sequence recorded from 3.45 m sediment depth, as suggested by their poor preservational state. The assemblages observed from 3.45–3.0 m depth consist of the small Fragilaria construens, F. pinnata and broken pieces or the more heavily silicified central areas of larger forms such as Amphora libyca, Navicula cf. rhynchocephala, S. pupula, Gyrosigma attenatum, Stauroneis anceps, and Cymbella sp., and thus are considered to indicate an assemblage affected by dissolution. At a depth of about 2.7 m, a transition to peat deposits was observed. The mineral content decreased strongly, while TOC and TN percentages increased. Only sporadically did very thin silt layers occur in the peat bed (Fig. 8). The prominent silt layer at 2.15 m depth is mirrored by decreasing TOC and TN values. The accumulation of organic matter was accompanied by a
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Fig. 9. Pollen diagram of profile LAB2.
continuous decrease in the pH values from segregated ice. According to the available radiocarbon ages, the PAZ LB3 was placed in the Atlantic. The Larix pollen proportions achieved a maximum followed by the maximum proportion of Alnus pollen in the sequence. A second Larix peak was observed towards the end of PAZ LB3. Within this PAZ, aquatics (Menyanthes, Hydrocharis) as well as peat-forming plants (Equisetum) gained importance (Fig. 9). During the Atlantic (radiocarbon ages 6580 70 and
5770 70 yr BP), the values of arboreal pollen, especially those of Larix, decreased while Betula shrubs became more frequent. The pollen proportions of the aquatics, as well as Equisetum, diminished gradually. In parallel Ericaceae, represented by Vaccinium species, became more important. Subsequently the percentages of Cyperaceae reached maximum values. These data indicate the gradual transition from a wet site inhabited by Menyanthes, Hydrocharis and Equisetum with Larix and Alnus growing in the surroundings, to lower water
Fig. 10. Compilation of local pollen and diatom zonations and the important analytical results used for assessing the environmental development of the study site at Labaz Lake.
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levels supporting peat formation (Equisetum). Eventually dryer conditions allowed Ericaceae and Cyperaceae to grow. Given the regional peculiarities, the development from a wetland with poor-fen character to a locality with low-centre polygons in transition to highcentre polygons is inferred. Aquatic plants were still likely to have persisted in the trenches, which developed above the ice wedges. These features are similar to the present-day landscape of the study area. The upper two radiocarbon-dated samples (4200 65 and 2900 yr BP) belong to the Subboreal (PAZ LB 2). Larix pollen values amount to 12%, probably indicating a mild Subboreal event (Khotinsky 1984). Radiocarbon ages from Larix stumps found in situ in the surroundings of Labaz Lake (3680 70 and 2880 60 yr BP; Siegert et al. in press) confirm this assumption. Subsequently, pollen of Betula shrubs reach maximum percentages. The TOC and TN contents decrease. According to the 14C dates, peat accumulation decreased remarkably. The pollen assemblages observed in two soil samples from the recent active layer point towards vegetation dominated by Betula shrubs, Ericaceae and Cyperaceae in the Subatlantic. The re-increasing TOC value, along with the further decline in TN percentage, leads to an increasing C/N ratio, which reflects emergent and terrestrial types of vegetation as well. From the diatom-based ecological implications the inferred trends in the environment of the study location, starting from the establishment of stable hydrological conditions in the late Boreal, can be described in more detail. From the absence of ‘true’ planktonic diatom taxa in the sequence generally relatively low water level stands are inferred. However, from high-latitude sites higher water transparency is recorded as an alternative explanation (Lotter et al. 1997; Pienitz et al. 1997). The parallel trends of decreasing proportions of tychoplanktonic taxa and increasing proportions of periphytic taxa appear to indicate a further lowering of the water level and a transition to an open-water wetland (Fig. 6). The ratio of the relative percentages of tychoplanktonic vs. the sum of periphytic and aerophilous taxa clearly depicts this trend. Values below ‘1’ are calculated first for upper DAZ 3. At the same time the first aerophilous species and species associated with mosses (e.g. Eunotia arcus) are recorded and considered to indicate a temporary, partial drying of the wetland. Probably at that time the formation of the low-centred polygons started. From the inversion of the ratio in DAZ-4 and in lower DAZ-5 an episode of higher water level may be inferred. In DAZ-5, contemporary with a sharp increase in periphytic taxa, aerophilous taxa became important. Many of the observed diatom species were recorded to be associated with mosses (Eunotia bilunaris, E. flexuosa, E. paludosa, E. parallela; Krammer & Lange-Bertalot 1991a). Finally, aerophilous taxa and taxa associated with soils and reduced light conditions (Caloneis aerophila and Diadesmis contenta; Krammer
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& Lange-Bertalot 1986) dominated in the assemblages in DAZ-6, indicating the drying up of the wetland along with soil formation. Most of the recorded diatom taxa are known to be trophic-tolerant. However, the diatom concentration increased along with increasing TOC content and achieved highest values in the sequence in DAZ-2 (Fig. 5). Considering the diatom/crysophycean cyst ratio as a productivity indicator, as proposed by Smol (1985) for temperate lakes, interesting parallels emerge because this ratio showed its highest values at the same time. The proportions of the pH preference groupings display a clear development in the sequence (Fig. 7) in good coincidence with the pH values estimated from ground-ice water. The decreasing percentage of circumneutral/alkaliphilous taxa and the opposite trend of circumneutral and acidophilous taxa could indicate initially circumneutral to slightly alkaline conditions developing in a first stage towards the circumneutral point. This trend was probably interrupted in DAZ-4 where the proportions of circumneutral/alkaliphilous taxa re-increased. In DAZ-5 the transition towards acidic conditions can be inferred from a sharp increase in acidophilous taxa. For most of the aerophilous taxa occurring in DAZ-6 the pH preferences are unknown.
Conclusions The following conclusions may be drawn from the present study (compiled in Fig. 10): 1. The Pleistocene sediments are deposited under a continental cold climate (tundra steppe-character of the vegetation). 2. A sedimentation gap for the termination of the Pleistocene and the start of lacustrine sedimentation in the Boreal can be inferred from the gap in the 14C ages, differences in grain size and organic matter content of the sediments, and incomplete pollen record lacking the transitional shrub-tundra vegetation. 3. The early Holocene climatic amelioration reached the optimum in the Boreal and led to a new onset of lacustrine sedimentation in a shallow lake, which was probably connected to the Labaz Lake. 4. The early Atlantic sediments were deposited in a shallow lake with beginning peat formation, as evidenced by pollen of aquatics and peat forming plants, highest diatom concentration and dominance of tychoplanktonics. 5. Towards the late Atlantic the water level and water pH value lowered. Finally, the gradual transition to a wetland with poor-fen character and a no-longerclosed water surface took place. This is evidenced by increasing percentages of periphytic diatoms with circumneutral/acidophilous preferences, diminishing
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6.
7.
8.
9.
Ulrike Kienel et al.
aquatic plant pollen, and increasing proportions of Cyperaceae and domed-mire communities. A short episode with milder conditions and somewhat higher water level stands is inferred for Subboreal times from maximum TOC values, higher Larix pollen percentages and the dominance of tychoplanktonic diatom taxa. The Subatlantic climatic deterioration led to a further drying up of the wetland along with acidification, permafrost aggradation and decreasing peat accumulation. This is evidenced by high proportions of Cyperaceae pollen, re-increasing percentages of mainly acidophilous periphytic diatoms and decreasing TOC and TN values. Subsequently, the (still ongoing) formation of highcentre polygons started, as evidenced by a dominance of aerophilous and soil-related diatom taxa in the uppermost horizon. The peat formation in the study site is determined by the overlapping of two processes: first, the process of shallowing and final drying up of the ‘Boreal lake’, and second, the contemporary permafrost aggradation, with the permafrost table in near-surface position acting as aquiclude.
Acknowledgements. – We thank A. Yu. Dervevyagin and S. F. Khrutsky (both Moscow State University) for their help during the fieldwork, R. M. Crawford (Alfred Wegener Institute, Bremerhaven) for help with taxonomical problems, G. Vannahme for providing us with the topographic sketch, R. Lehmann for help with the application of cluster analysis, M. Melles and R. Raab for helpful discussions (all Alfred Wegener Institute, Potsdam) and S. A. Arcone (CRREL, Hanover, USA) for reviewing the English.
References Andreeva, S. M., Isaeva, L. L., Kind, N. V. & Nikolskaya, M. V. 1982: Glaciations, sea transgressions and climate in the Late Pleistocene and Holocene. In Kind, N. V. & Leonov, B. N. (eds.): The Antropogen of the Taymyr Peninsula, 157–165. Nauka, Moscow. In Russian. Astakhov, V. 1997: Late Glacial events in the Central Russian Arctic. Quaternary International 41/42, 17–26. Barker, P. 1992: Differential diatom dissolution in Late Quaternary sediments from Lake Manyara, Tanzania: An experimental approach. Journal of Paleolimnology 7, 235–251. Battarbee, R. 1986: Diatom analysis. In Berglund, B. E. (ed.): Handbook of Holocene Palaeoecology and Palaeohydrology, 530–570. John Wiley & Sons, London. Battarbee, R. W., Smol, J. P. & Merila¨inen, J. 1986: Diatoms as indicators of pH: An historical review. In Smol, J. P., Battarbee, R. W., Davis, R. B. & Merila¨inen, J. (eds.): Diatoms and Lake Acidity, 5–14. Dr. W. Junk, Dordrecht. Bennion, H. 1995: Surface-sediment diatom assemblages in shallow, artificial, enriched ponds, and implications for reconstructing trophic status. Diatom Research 10, 1–19. Birks, H. J., Line, J. M., Juggins, S., Stevenson, A. C. & ter Braak, C. J. F. 1990: Diatoms and pH reconstruction. Philosophical Transactions Royal Society of London B 327, 263–278. Bradbury, J. P. 1993: A diatom record of climate and hydrology for the past 200 ka from Owens Lake, California with compari-
BOREAS 28 (1999)
son to other Great Basin records. Quaternary Science Review 16, 203–219. Camburn, K. E. & Kingston, J. C. 1986: The genus Melosira from soft-water lakes with special reference to northern Michigan, Wisconsin and Minnesota. In Smol, J. P., Battarbee, R. W., Davis, R. B. & Merila¨inen, J. (eds.): Diatoms and Lake Acidity, 17–34. Dr. W. Junk, Dordrecht. Derevyagin, A. Y., Siegert, C., Troshin, E. V. & Shilova, G. N. 1996: New data on Quaternary geology and permafrost of Taymyr (Labaz Lake region). First Conference of Russian Geocryologists, June 3–5, 1996, 204–212. In Russian. Dixit, S. S., Smol, J. P., Kingston, J. C. & Charles, D. F. 1992: Diatoms: Powerful indicators of environmental change. Environmental Sciences and Technology 26, 22–33. Geyh, M. A. & Schleicher, H. 1990: Radiocarbon (14C) method. In: Absolute Age Determination, 162–180. Springer-Verlag, Berlin. Hahne, J. & Melles, M. 1997: Late- and post-glacial vegetation and climate history of the south-west Taymyr Peninsula, central Siberia, as revealed by pollen analysis of a core from Lake Lama. Vegetation History and Archaeobotany 6, 1–8. Hahne, J. & Melles, M.: Climate and vegetation history on the Taymyr Peninsula since Middle Weichselian time – Palynological evidence from lake sediments. In Kassens, H., Bauch, H. A., Dimitrenko, I., Eicken, H., Hubberten, H.-W., Melles, M., Thiede, J. & Timokhov, L. (eds.): Land-Ocean Systems in the Siberian Arctic: Dynamics and History. Lecture Notes in Earth Sciences, Springer, Berlin, Heidelberg, New York (In press). Hall, R. I. & Smol, J. P. 1995: Paleolimnological assessment of long-term water-quality changes in south-central Ontario lakes affected by cottage development and acidification. Canadian Journal of Fisheries and Aquatic Sciences 53, 1–17. Hustedt, F. 1937: Systematische und o¨kologische Untersuchungen der Diatomeen floren von Java, Bali, Sumatra. Archiv Hydrobiologie Supplement 15, 131–506. Hustedt, F. 1939: Systematische und o¨kologische Untersuchungen der Diatomeen floren von Java, Bali, Sumatra. Archiv Hydrobiologie Supplement 16, 1–394. Isaeva, L. L. 1982: Sartan deposits of the North-Siberian Lowland. In Kind, N. V. & Leonov, B. N. (eds.): The Antropogen of the Taymyr Peninsula, 114–136. Nauka, Moscow. In Russian. Khotinsky, N. A. 1984: Holocene climatic change. In Velichko, A. A. (ed.): Late Quaternary Environments of the Soviet Union, 305–309. Longman, London. Korsman, T. & Birks, H. J. B. 1996: Diatom-based water chemistry reconstructions from northern Sweden: A comparison of reconstruction techniques. Journal of Paleolimnology 15, 65–77. Krammer, K. & Lange-Bertalot, H. 1986: Die Su¨ßwasserflora von Mitteleuropa, Bacillariophyceae, v. 1, Naviculaceae. 876 pp. Fischer, Stuttgart. Krammer, K. & Lange-Bertalot, H. 1988: Die Su¨ßwasserflora von Mitteleuropa, Bacillariophyceae, v. 2, Bacillariaceae, Epithemiaceae, Surirellaceae. 596 pp. Fischer, Stuttgart. Krammer, K. & Lange-Bertalot, H. 1991a Die Su¨ßwasserflora von Mitteleuropa, Bacillariophyceae, v. 3, Centrales, Fragilariaceae, Eunotiaceae. 576 pp. Fischer, Stuttgart. Krammer, K. & Lange-Bertalot, H. 1991b: Die Su¨ßwasserflora von Mitteleuropa, Bacillariophyceae, v. 4, Achnanthaceae, kritische Bemerkungen zu Navicula. 437 pp. Fischer, Stuttgart. Lange-Bertalot, H. 1993: 85 neue Taxa. Bibliotheca Diatomologica v. 27. 454 pp. Cramer, Berlin, Stuttgart. Lemmen, D. S., Gilbert, R., Smol, J. P. & Hall, R. I. 1988: Holocene sedimentation in glacial Tasikutaaq Lake, Baffin Island. Canadian Journal of Earth Sciences 25, 810–823. Lewin, J. 1961: The dissolution of silica from diatom walls. Geochimica Cosmochimica Acta 21, 182–198. Lotter, A. F., Birks, H. J. B., Hofmann, W. & Marchetto, A. 1997: Modern diatom, cladocera, chironomid, and chrysophyte cyst assemblages as quantitative indicators for the reconstruction of
BOREAS 28 (1999)
Late Quaternary palaeoenvironmental reconstructions, Taymyr
past environmental conditions in the Alps. I. Climate. Journal of Paleolimnology 18, 395–420. Mangerud, J., Andersen, S. T., Berglund, B. E. & Donner, J. J. 1974: Quaternary stratigraphy of Norden, a proposal for terminology and classification. Boreas 3, 109–128. Matveyeva, N. V. 1994: Floristic classification and ecology of tundra vegetation of the Taymyr Peninsula, northern Siberia. Journal of Vegetation Sciences 5 (6), 813–838. Maxwell, B. 1992: Arctic climate potential for change under global warming. In Chapin, F. S., Reynolds, J. F., Jeffries, R. L., Shaver, G. R., Svoboda, J. & Chu, E. W. (eds.): Arctic Ecosystems in a Changing Climate, 11–34. Academic Press, New York. McMinn, A. 1995: Comparison of diatom preservation between oxic and anoxic basins in Ellis Fjord, Antarctica. Diatom Research 10, 145–151. Merila¨inen, J. 1967: The diatom flora and the hydrogen-ion concentration of the water. Annales Botanicae Fennici 4, 51–58. Nygaard, G. 1956: Ancient and recent flora of diatoms and chrysophyceae in Lake Gribsø: Studies on the humic and acid Lake Gribsø. Folia Limnologica Scandinavica 8, 32–94. Pfeiffer, E. -M., Gundelwein, A., No¨then, T., Becker, H. & Guggenberger, G. 1996: Characterization of the organic matter in Permafrost soils and sediments of the Taymyr Peninsula/Siberia and Severnaya Zemlya/Arctic region. In Bol’shiyanov, D. & Hubberten, H. W. (eds.): Russian German Cooperation: The Expedition TAYMYR 1995 and the Expedition KOLYMa 1995 of the ISSP Pushchino Group, Reports on Polar Research, 211, 46–63. Pienitz, R. 1993: Paleoclimate proxy data inferred from freshwater diatoms from the Yukon and the Northwest Territories, Canada. Ph.D. thesis, Queen’s University, 220 pp. Kingston, Ontario. Pienitz, R., Smol, J. P. & Birks, H. J. B. 1995: Assessment of freshwater diatoms as quantitative indicators of past climatic change in the Yukon and Northwest Territories, Canada. Journal of Paleolimnology 13, 21–49. Pienitz, R., Smol, J. P. & Lean, D. R. S. 1997: Physical and chemical limnology of 59 lakes located between the southern Yukon and the Tuktoyaktuk Peninsula, Northwest Territories (Canada). Canadian Journal of Fisheries and Auquatic Sciences 54, 330– 346. Renberg, I. & Hellberg, T. 1982: The pH history of lakes in southwestern Sweden, as calculated from the subfossil diatom flora of the sediment. Ambio 11, 30–33. Schrader, H. J. & Gersonde, R. 1978: Diatoms and silicoflagellates. In Zachariasse, W. J. et al. (eds.): Micropaleontological Counting Methods and Techniques – An Exercise on an Eight-metre Section of the Lower Pliocene of Capo Rossello, Sicily. Micropal. Bull., 129–176. Utrecht. Sher, A. V. 1997: Environmental restructuring at the Pleistocene/ Holocene boundary in the East-Siberian Arctic and its role in mammalian extinction and establishment of modern ecosystems (Communication 1). Earth Cryosphere 1, 21–29 (in Russian). Shilova, G. N., Derevyagin, A. & Siegert, C. 1996: Paleogeographical conditions during the lake sediment formation in the Labaz Lake basin (East Taymyr). All-Russian Conference on
193
Palynology, ‘Palynology in Biostratigraphy, Palaeoecology and Paleogeography’, August 15–18, 171–172, Moscow. In Russian. Siegert, C., Khrutsky, S. F. & Derevyagin, A. Y. 1995: Investigations in the Labaz Lake area: Palaeogeographical permafrost studies. In Siegert, C. & Bol’shiyanov, D. (eds.): Russian German Cooperation: The Expedition TAYMYR 1994, Reports on Polar Research, 175, 27–36. Siegert, C., Derevyagin, A. Y. & Vannahme, G. 1996: Geocryological and palaeogeographical studies in the Labaz Lake area. In Bol’shiyanov, D. & Hubberten, H. W. (eds.): Russian German Cooperation: The Expedition TAYMYR 1995 and the Expedition KOLYMa 1995 of the ISSP Pushchino Group, Reports on Polar Research, 211, 28–45. Siegert, C., Derevyagin, A. Y., Shilova, G. N., Hermichen, W. -D. & Hiller, A. In press: Palaeoclimatic evidences from permafrost sequences in the Eastern Taymyr Lowland. In Kassens, H., Bauch, H. A., Dimitrenko, I., Eicken, H., Hubberten, H.-W., Melles, M., Thiede, J. & Timokhov, L. (eds.): Land-Ocean Systems in the Siberian Arctic: Dynamics and History, Lecture Notes in Earth Sciences, Springer, Berlin, Heidelberg, New York. Smol, J. P. 1985: The ratio of diatom frustules to chrysophyte stratospores. A useful palaeolimnological index. Hydrobiologia 123, 199–208. Smol, J. P. 1988: Palaeoclimate proxy data from freshwater arctic diatoms. Verhandlungen der Internationalen Vereinigung von Limnologen 23, 837–844. Stevenson, A. C., Juggins, S., Birks, H. J. B., Anderson, D. S., Anderson, N. J., Battarbee, R. W., Berge, F., Davis, R. B., Flower, R. J., Haworth, E. Y., Jones, V. J., Kingston, J. C., Kreiser, A. M., Line, J. M., Munro, M. A. R. & Renberg, I. 1991: The Surface Waters Acidification Project Palaeolimnology Programme: Modern Diatom/Lake-water Chemistry Data Set. 86 pp. ENSIS Publishing, London. Ukraintseva, V. U. 1993: Vegetation cover and environment of the ‘Mammoth Epoch’ in Siberia. In Agenbroad, L. D., Mead, J. I. & Hevly, R. H. (eds.): 309 pp. Hot Springs, South Dakota. Velichko, A. A. Andreev, A. A. & Klimanov, V. A. 1994: Vegetation and climate dynamics during Late Glacial and Holocene in tundra and forest zones of Northern Eurasia. In Velichko, A. A. (ed.): Short-term and Sharp Landscape and Climate Changes during the Last 15 000 yrs, 4–60. Nauka, Moscow. In Russian. Voigt, R. 1997: Untersuchungen zur Pala¨olimnologie und Vegetationsgeschichte an Sedimenten von Fuschlsee und Chiemsee (Salzburg und Bayern). Dissertationes Botanicae 270, 303. Wolfe, A. P. 1994: Late Wisconsinian and Holocene diatom stratigraphy from Amarok Lake, Baffin Island, N. W. T., Canada. Journal of Paleolimnology 10, 129–139. Wunsam, S. & Schmidt, R. 1995: A diatom-phosphorus transfer function for Alpine and pre-alpine lakes. Memorie dell’Istituto Italiano di Idrobiologia 53, 85–99. Zielinski, U. 1993: Quantitative Bestimmung von Palaeoumweltparametern des Antarktischen Oberfla¨chenwassers im Spa¨tquarta¨r anhand von Transferfunktionen mit Diatomeen. Berichte zur Polarforschung 126, 148.
