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HOLOCENE PALEOCLIMATES IN THE NORTH-EUROPEAN TREE-LINE REGION – QUANTITATIVE RECONSTRUCTIONS OF TRENDS AND ABRUPT CLIMATE EVENTS
1 Department of Geology, P.O. Box 64, Fi-00014, University of Helsiinki, Finland
2 Department of Biological and Environmental Sciences, University of Helsinki, Finland
The last 10 years have seen a major progress of quantitative Holocene climate reconstructions in the North-European and North-Atlantic regions. New quantitative calibration models have been developed for chironomids, cladocera, diatoms, and pollen, and accurately-dated fossil records have been produced from various parts of the Fennoscandian and North-West Russian tree-line region with higher time resolution than earlier. The results of these reconstructions can be compared with the new climatic proxy records obtained from the Greenland ice cores, from the marine sediment cores of the North Atlantic and the Barents Sea, and with the output of the palaeoclimate simulations. Many proxy records reflect short-term cooling events during the early Holocene, dating to 11,300 cal yr BP, 10,300 cal yr BP, 9400 cal yr BP and 8200 cal yr BP. The event at 8200 cal yr BP is the only unambiguous cold event that can be observed in many records. It is reflected by a decline of 1.5°C in the records in the boreal-temperate ecotone, but, intriguingly, is missing from the records from the tree-line area. These early-Holocene cooling events may be related by the perturbations of the Atlantic Thermohaline Circulation, ultimately caused by the fluxes of freshwater from the Laurentide ice sheet.
Most of the continental records and Greenland ice core data indicate a steady rise of summer from the onset of the Holocene at 11,700 cal yr BP towards a clear Holocene Thermal Maximum (HTM) at about 8000 to 4000 cal yr BP (Fig. 1). The beginning of the HTM in the proxy records is later than suggested by Holocene climate simulations, probably because the simulations do not account for the existence of the Laurentide ice sheet until about 7000 cal yr BP. In addition to the 1.5-2.0°C higher-than-present summer temperatures, the HTM was characterized by low lake-levels and markedly dry conditions, suggesting a major change of the atmospheric circulation pattern from the early Holocene. The neo-glacial cooling starts abruptly at about 4200 cal yr BP, followed by gradual cooling towards the present. The historically documented Little Ice Age at 400– 200 cal yr BP is reflected in some proxy records, and possible evidence exist for other cooling events during the late Holocene.
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Fig 1. Records reflecting summer temperature trends in the North-European tree-line region during the last 10,000 years. A) A chironomid-based Tjul record from Lake Toskaljavri, an Arctic-Alpine lake in northern Finland (Seppä et al., 2002b), B) A pollen-based Tjul record from Lake KP-2 on the Kola Peninsula (Seppä et al., 2008), C) A diatom-based Tjul reconstruction from the Scandes mountains in northern Sweden (Bigler et al., 2006), D) A lake-level record generated from a cladoceran-based transfer function model from Lake Isohattu, a closed basin in northern Finland, showing lake-level in relation to modern lake-level (Korhola et al., 2005). The record has been inverted to show low lake-levels at 8000 to 4000 cal yr BP, E) A glacier equilibrium altitude record from the Lyngen peninsula, northern Norway (Bakke et al., 2005), F) A pine megafossil record indicating the altitude of the highest pine megafossil in 200-year periods in Finnish Lapland (Eronen et al., 1999; Helama et al., 2004), F) A pine stomata record from Lake KP-2 is shown above the megafossil record. Each dot represents a pollen sample containing one or more stomata (Gervais et al., 2002). Curves in Figures A, B, and C are shown with a LOESS smoother fitted with a span of 0.10. The gray column indicates the period of highest summer temperature (Holocene thermal maximum, HTM) in the region