Holocene climatic and environmental changes inferred from midge records (Diptera : Chironomidae, Chaoboridae, Ceratopogonidae) at Lake Berkut, southern Kola Peninsula, Russia

A radiocarbon-dated sediment sequence from Lake Berkut in the southern part of the Kola Peninsula, northwest Russia, was investigated by means of midge analysis in order to reconstruct the Holocene climatic and environmental history of the region. Past mean July air temperatures at the study site and hypolimnetic oxygen contents of the lake water were inferred from chironomid-based transfer functions. The early Holocene (c. 10 100 - 8400 cal. yr BP) is characterized by summer temperatures and hypolimnetic oxygen concentrations broadly similar to present-day conditions. The midge records give evidence of a lake- level fall at c. 7000 cal. yr BP, resulting in weakened thermal stratification of the lake water, and improvement of the hypolimnetic oxygen conditions. After c. 4000 cal. yr BP midge assemblages suggest a lake- level rise related to increased effective moisture. A secondary mid-Holocene climatic optimum with inferred mean July air temperatures of c. 12.1 degrees C was recorded between 6000 and 4400 cal. yr BP, followed by minimum values (c. 11.3 degrees C) between 3500 and 1500 cal. yr BP, accompanied by the onset of natural acidification of the lake. The late-Holocene midge assemblages suggest weakly acidic conditions, low hypolimnetic oxygen contents and a general warming trend during the last c. 1500 years, although paludification and natural acidification may have influenced the temperature reconstruction at this stage. However, the modern chironomid-inferred mean July air temperature (12.5 degrees C) is consistent with meteorological data from a nearby station. In general, the midge records reflect relatively warm and moist conditions at Lake Berkut before c. 7000 cal. yr BP, a relatively warm and dry climate at c. 7000 - 4000 cal. yr BP, and a shift to cooler and wetter climatic conditions in the region after c. 4000 cal. yr BP