Abrupt climatic changes and an unstable transition into a late Holocene Thermal Decline: a multiproxy lacustrine record from southern Sweden

The transition from a middle Holocene relatively warm and stable climate to a cooler and unstable late Holocene climate is reconstructed using sediments from Lake Igelsjon, south-central Sweden. This multiproxy study illustrates local, regional and global scale responses to climatic change by focusing on a previously identified abrupt hydrological shift to cooler and/or wetter conditions around 4000 cal. yr BP. The results suggest that between ca. 4600 and ca. 3400 cal. yr BP, the environment around and within the lake responded in two major, well-defined steps: the first between 4450 and 4350 cal. yr BP and the second between 4000 and 3800 cal. yr BP. A series of rapid fluctuations of short duration were superimposed on the general cooling trend, with the most severe aquatic response peaking at ca. 3800 cal. yr BP. Pollen percentage and influx values show forest composition and pollen productivity changes and a distinct decline in total and Corylus pollen influx in the period of 4000-3500 cal. yr BP. Stomatal-based reconstruction of atmospheric CO2 concentration produced a tenuous decrease with a minimum between 3650 and 3500 cal. yr BP.