ABRUPT CLIMATIC CHANGE IN SWEDEN CENTRED ON 3,750 CAL. YEARS BP: EXPLORING ENVIRONMENTAL RESPONSES AND THEIR RELATIONSHIP TO ATMOSPHERIC CO 2

Many proxy records from the northeast Atlantic region register an abrupt response to climatic change shortly after 4000 years ago, generally marking the beginning of Late Holocene increased instability. Among these is a stable carbon and oxygen isotope record from Lake Igelsjön, south central Sweden showing a distinct change to a ’wetter/cooler’ climate.

New cores have been extracted from Lake Igelsjön and a multi-proxy approach applied at high-resolution (ca. 20-25 years) to examine the character and timing of the different proxy responses. Lake sediment chemistry data, magnetic parameters and pollen assemblages have been investigated and compared using a chronology based on high-resolution AMS radiocarbon dates. In addition, by applying the inverse relationship of atmospheric CO₂ concentration and the stomatal index of leaf fragments at the same high-resolution, a quantified record of global CO₂ has been obtained.

The results show a distinct reduction in lake productivity beginning at 3,880 cal. years BP and centred on 3,750 cal BP. Lake productivity was generally low for a period of 160 years with only a short-term recovery (ca. 140 years) before further fluctuations occur. Beginning at precisely the same time, the local/regional vegetation reconstruction shows a clear response for 360 years. The decline occurred over a period of only 60 years and possibly due to climatically induced alterations in human land-use. The preliminary stomatal index (SI) reconstruction of atmospheric CO₂ concentrations show a possible reduction for a period of 280 years beginning at 3,740 cal years BP, i.e. 140 years after the start of the vegetation and lake production responses. This could not therefore be considered the forcing behind this climate change.

The concurrent, independent response of different environmental proxies beginning at 3,880 cal. years BP from Lake Igelsjön, along with evidence from other Scandinavian sites, suggests that the climatic change affecting this region occurred rapidly and impacted sensitive environments for at least 150 years.