Characterizing changes in the sedimentary environment of a varved lake sediment record in southern central Finland around 8000 cal. yr BP

摘要

The varved lake sediment record of Lake Nautajarvi in southern central Finland indicates fluctuations in the annual sedimentation pattern and lake environment around 8000 cal. yr BP. Annual accumulation of catchment-derived mineral matter peaks a number of times and there are changes in the deposition rate of organic matter. Based on evidence from the Lake Nautajarvi sediment record, the climate was characterised by a millennial-scale colder period that terminated at one or several cooling events around 8000 cal. yr BP. During one of these events, diatom composition undergoes a change from planktonic species dominance to more littoral species dominance at 8160 cal. yr BP, and a recovery around 8010 cal. yr BP. The timing of this 150-year-long diatom anomaly is convergent with increased catchment erosion and decreased organic matter accumulation. We interpret that during such a period, the environment was characterised by colder winter temperatures, prolonged duration of ice cover, higher than normal spring discharge and rapid development of thermal stratification in the spring, coupled with reduced water circulation and shorter growing seasons. Copyright (C) 2008 John Wiley & Sons, Ltd.