Rapid early-Holocene environmental changes in northern Sweden based on studies of two varved lake-sediment sequences
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Mineral magnetic, organic carbon and pollen studies of two varved Holocene lake-sediment sequences in the boreal forest of northern Sweden indicate that significant environmental changes took place between c. 6000 and 5700 BC. This interval is characterized by an increase in mineral-matter accumulation, which is a proxy for winter-snow accumulation, and a statistically significant decrease in total pollen influx (predominantly Pinus, Betula and Alnus), which may reflect lower spring and summer temperatures and increased frost frequency. Notable increases in the influx of deciduous tree species (including Quercus and Corylus) suggest a rapid change to warmer conditions between 5700 and 5600 BC. Given dating errors associated with the varve chronologies and the Greenland ice-core timescales, the cold interval can be considered to reflect a regional (possibly global) climatic cooling, which is often referred to as the '8.2 kyr BP cooling event'. However, the younger age of the cold event in northern Sweden does not support the hypothesis of forcing by the sudden drainage of Laurentide glacial lakes into the North Atlantic, unless a minimal 300-year delay in ocean-atmospheric coupling is accepted. The data contribute to a complex picture of early-Holocene environmental change, in response to deglaciation of the Northern Hemisphere.