Holocene temperature changes in northern Fennoscandia reconstructed from chironomids using Bayesian modelling

The recently developed Bayesian multinomial calibration model, Bummer, was applied to the fossil chironomid assemblages of a lake situated at the present tree-line in northern Fennoscandia to reconstruct patterns in Holocene climates. The results are compared with inferences derived using non-Bayesian weighted averaging partial least-squares regression. The reconstruction exhibits both long-term trends as well as a series of shorter-term oscillations. Abrupt climatic excursions were particularly characteristic for the early Holocene period, with cooling episodes around 9200, 8600 and 8200 cal yr BP. The interval from ca 8000 to 5800 cal yr BP appeared to be warm and stable, but was followed by a pronounced climatic cooling around 5800 cal yr BP leading to a relatively long-lasting mid-Holocene temperature minimum between ca 5800 and 4000 cal yr BP. The reconstruction during the latter part of the Holocene is relatively featureless, which may partly be due to gradual shallowing of the lake by sedimentation that may have affected chironomid compositions, thereby masking the climate signal. The major climatic patterns in our reconstruction agree well with inferences from Greenland ice and marine sediments and are also in line with the previous reconstructions based on diatoms and pollen from the same core, although the finer-scale variations reflected by the three proxies do not always match. The overall results provide evidence for climatic complexity during the Holocene epoch and demonstrate that Bayesian modelling is highly suitable for reconstruction purposes. r 2002 Elsevier Science Ltd. All rights reserved.