HOLOCENE ANNUAL MEAN TEMPERATURE CHANGES IN THE BOREAL ZONE OF EUROPE: POLLEN-BASED RECONSTRUCTIONS

Holocene annual mean temperature (Tann) trends were reconstructed from radiocarbon-dated pollen stratigraphies from lake sediments in Estonia, Finland, and Sweden. The reconstructions were carried out using two North-European pollen-climate calibration models based on weighted averaging partial least squares regression. Palaeotemperatures from Estonia and Finland were reconstructed with a model where the cross-validated prediction error is 0.89^oC and the coefficient of determination between observed Tann values and those predicted by the model is 0.88. Reconstructions from Sweden were carried out with a model with the prediction error 0.95^oC and coefficient of determination 0.88. In the reconstructions, Holocene thermal maximum (HTM) is distinguishable at 8000 - 4500 cal yr BP, with Tann on average 2.5^oC higher than at present. The pollen-stratigraphical data reflects progressively warmer and drier summers during the HTM. Analogously with the modern decadal-scale climatic variability in North Europe, we interpret this as an indication of increasing climatic continentality due to the intensification of anticyclonic circulation. Post-HTM cooling started at around 4500 cal yr BP. All reconstructions show a transient cooling of 1.5 - 2.0^oC at 8500 - 8100 cal yr BP, caused predominantly by the declines of Corylus, Ulmus, and Alnus. We correlate this cold event with the North-Atlantic 8.2 ka event. Provided that the 8.2 ka event was caused by freshening of the North-Atlantic surface water, our data provides evidence of the climatic responsiveness of the boundary of the boreal and temperate zones to the weakening of the North-Atlantic thermohaline circulation and the zonal energy transport over Europe. No other cold events of comparable magnitude are indicated during the last 8000 years.