Mid-Holocene reinforcement of North Atlantic atmospheric circulation variability from a western Baltic lake sediment record

. Knowledge about timing, amplitude and spatial gradients of Holocene environmental variability in the Circum-Baltic region is key to understand its responses to ongoing climate change. Based on a multi-dating and proxy approach, we reconstruct changes in productivity from TOC contents in sediments of Lake Kälksjön (KKJ) in west-central Sweden spanning the last 9612 (+255/-144) years. An exception is the period from AD 1878 until today, in which sedimentation was dominated by anthropogenic lake level lowering and land use. In-lake productivity was higher during periods of warmer winters with shortened ice cover and prolonged growing seasons. A multi-millennial increase in productivity throughout the last ~9600 years is associated with progressively warmer winters in north-western Europe, likely triggered by the coinciding increase in Northern Hemisphere winter insolation. Decadal to centennial periods of higher productivity in KKJ correspond to warmer winters during a more positive North Atlantic Oscillation (NAO) polarity, as reconstructed for the last 5200 years. In consequence, we assume our decadal to centennial productivity record from KKJ sediments for the complete ~9600 years to provide a qualitative record of NAO polarity. A shift towards higher productivity variability at ~5450 cal. a BP is hypothesized to reflect a reinforcement of NAO-like atmospheric circulation variability, possibly driven by more vigorous changes in North Atlantic deep water formation.