STABLE CARBON AND OXYGEN ISOTOPES AND SPORE-POLLEN SPECTRA IN LATE GLACIAL-HOLOCENE CARBONATE SEDIMENTS FROM THE LAKE SERGEYEVSKOYE (BELARUS)
A pair of isotope curves (δ13C and δ18O in calcite) covering almost the entire Late Glacial- Holocene stage of the geological history was derived from five meters thick carbonate sediments of the lake Sergeyevskoye (53°30'40" N, 27°45'40" E). Simultaneous consideration of lithological, isotopic, palynological and chronometric peculiarities allowed the authors to recognize several palaeogeographical events and to suggest ideas of their precise chronology. The Late Glacial is characterized by abundant herbs and willow and by the presence of dwarf varieties of pine, alder and birch in the lake vicinity. In the Oldest Dryas, the arboreal vegetation was dominated by birch with the less important role of pine. Fir was present in small amounts. Herbs were especially abundant, the most important role belonging to sedge. In the time interval between the beginning of the Bolling and the end of the Allerod (local pollen zones Pinus-NAP and Pinus-Betula-NAP) the heterogeneity of environments was revealed on the basis of stable isotope data. Carbonates with very light oxygen isotope composition were deposited during a short-term phase recognized within the Bolling-Allerod stage. This phase may be attributed either to cold and wet Older Dryas stadial, or to an episode of active thawing of permafrost grounds. In the latter case, the Older Dryas stadial appears to be undetectable within the single warm Bolling-Allerod period. The Allerod (local pollen zone Pinus-Betula-NAP) is interpreted as a dry warm phase, which is evidently suggested from strong positive δ18O and δ13C spikes, a low organic matter content of sediments, and relatively abundant herbs in the pollen spectra. The cold Younger Dryas is well expressed in both isotope curves (wide negative δ18O and δ13C spikes) and the pollen spectra. Pine dominated over birch significantly; a notable expansion of fir occurred. Grasses and shrub birch were more abundant than in the other periods of the Late Glacial-Holocene (with the exception of the Oldest Dryas). A complex of data available did not provide evidence of especial aridity of the stadial climate. A sharp shift toward the higher δ13C values in the sediments marking the Younger Dryas - Preboreal transition indicates an episode of active permafrost degradation. The Preboreal is characterized by the arboreal vegetation strongly dominated by birch and is interpreted as a relatively dry phase. During the time interval of a widespread occurrence of heat-loving trees (Quercetum mixtum) the following climatic stages were revealed: wet final Boreal with moderate and gradually increasing temperatures; a short-term cold event at ~ 8200 cal. yrs BP; a trend of rapid warming up to 7800-7900 cal. yrs BP (till the beginning of the Atlantic period, the Holocene climatic optimum). Within the Atlantic (till the elm decline), a warmer and/or dryer phase was followed by a colder and/or wetter phase, and then again by a warmer and/or dryer phase. Palynological evidences of the human impact on the vegetation recorded in the upper 50- 60 cm layer of sediments appear to be synchronous with lithological and isotopic indications of rapid eutrophication of the lake (significant increase of the organic matter content, trends of decreasing δ18O and δ13C toward the top of the sequence). Some trends, peaks and shifts of the oxygen isotope curve from the Sergeyevskoye profile have counterparts in the other Belarusian δ18O curves, as well as in some West European δ18O curves. These are the shifts reflecting the initiation and termination of the Younger Dryas (12 650-12 750 and 11 450-11 500 cal. yrs BP), the peaks of the 8200 cal. yrs BP cold event and of the subsequent warming. These features of the oxygen isotope records are of chronometric significance and may be used to improve dating of sediment layers in Belarusian deposits by matching δ18O trends with their analogues revealed in reliably dated European curves.