Understanding the fundamentals of the Earth’s climate system during the Quaternary Period (the past ca 2.6 million years) is of imminent interest in light of the reported recent climatic changes. The Eemian Interglacial (ca 129,000-116,000 years ago) is the last period before the present interglacial (the Holocene, the last ca 11,700 years) when temperatures were as warm as or warmer than today and may serve as an analogue to future climatic warming. The Eemian is a relatively well-studied period, but many aspects of the interglacial climatic dynamics still remain unresolved. One reason for this is the chronological uncertainties of many records from this time period; another is the relatively uneven distribution of palaeoarchives. Especially from high latitude areas, terrestrial records from the Eemian are few due to the impact of glacial erosion during the Weichselian Glacial (ca 116,000-11,700 years ago).

This thesis is based on data from Sokli, N Finland, where cold-based conditions of the Weichselian ice sheets and non-typical bedrock conditions have allowed preservation of an unusually thick Eemian lacustrine deposit in stratigraphic sequence with overlying Weichselian and Holocene deposits. The main purpose of this thesis is to contribute to the knowledge of climate variability in Fennoscandia and the North Atlantic region during the Eemian, and it involves a qualitative multi-proxy reconstruction of the Eemian lake and its development as well as a quantitative reconstruction of the regional Eemian July temperature evolution using subfossil chironomids. Attention is paid also to the identification and characterization of short-term climatic events in the Sokli record.

The main contribution of this study is to increase the spatial coverage of Eemian palaeoclimatological data. Specific results include the description of the lake development, identifying phases where non-climatic variables likely affect the quantitative chironomid-based temperature reconstruction, as well as qualitative and quantitative reconstructions of climatic features such as changes in seasonality and temperature. Five major lake phases are distinguished. A glaciolacustrine phase is followed by an early Eemian lake phase characterized by short mixing periods and rising nutrient levels. The following early mid-Eemian lake phase is characterized by a prolonged open water season and high productivity and possibly corresponds to the Eemian climatic optimum. The late mid-Eemian lake phase is characterized by infilling and eutrophic conditions while the late Eemian lake phase reflects shallow, dynamic and cooling conditions.

The qualitative study suggests a shift from a higher seasonality in the early Eemian towards more oceanic conditions during the mid-Eemian, while chironomid-inferred July temperatures indicate that near present-day temperatures prevailed at Sokli already from the beginning of the interglacial and that optimum temperatures were only moderately (ca + 1°C) higher  than at present. These results suggest that the climatic development at Sokli followed that of central Europe during the Eemian. Two events characterized by colder and more arid conditions are identified, and can be tentatively correlated to meltwater events registered in marine records from the Nordic seas and subpolar north Atlantic, possibly reflecting the effect that meltwater pulses and related disruptions in the thermohaline circulation had on adjacent continental areas.