HOLIVAR2006 Abstracts
Changes in temperature and effective humidity during the Holocene in central Sweden: Implications for atmospheric and oceanic forcings of climate.
H. Seppä1, D. Hammarlund2 and K. Antonsson3
1Department of Geology, P.O. Box 64, FIN-00014, University of Helsinki, Finland
2GeoBiosphere Science Centre, Quaternary Sciences, Lund University, Sölvegatan 12, 22362, Lund, Sweden
3Department of Earth Sciences, Uppsala University, Villavägen 16, 75236 Uppsala, Sweden
Contact: H. Seppä (heikki.seppa@helsinki.fi)
The combined use of palaeoclimatic proxies reflecting different components of the climate system can help to generate more comprehensive climate reconstructions. Here we report a 10,000 yr quantitative record of annual mean temperature, based on pollen-climate transfer functions and pollen-stratigraphical data from Lake Flarken, South central Sweden, and compare this record with a reconstruction of effective humidity, as reflected by δ18O stratigraphy of lacustrine carbonates from Lake Igelsjön, c. 10 km from Lake Flarken (Hammarlund et al. 2003, Seppä et al. 2005). In general, the independent records of annual mean temperature and effective humidity in South central Sweden show coherent long-term trends. Consistent evidence exists of enhanced zonal circulation and predominantly humid conditions during the Early Holocene, replaced by a stable period of warm and dry conditions during the Holocene Thermal Maximum (HTM), at about 8000 to 4300 cal. yr BP, probably due to the increasing influence of blocking anticyclonic circulation during the summer. The abrupt end of the HTM at 4300 cal. yr BP in South central Sweden agrees in general with other climate reconstructions from Boreal regions in Europe. Thus, a growing body of evidence suggests that the HTM came to a sudden end adjacent to the North Atlantic, probably as a result of a threshold response of the dominant atmospheric circulation mode to changes in oceanic and astronomical forcings and a subsequent cessation of the stable anticyclonic climatic conditions.
Another picture emerging from the present data in conjunction with other recent palaeoclimate records from northern Europe concerns the cause and nature of the 8200 cal. yr BP cold event. The increasing evidence of the amplitude, the geographical extent in northern Europe, and in particular the abrupt start and end of the event suggest a rapid threshold response of the zonal flow to a sudden and strong change in a major forcing factor, and seem inconsistent with a suggested weak-forcing scenario or more regional atmospheric re-organisations as causes of the event. Although there is currently no deep-sea evidence for a shutdown of Atlantic Meridional Overturning Circulation during the 8200 cal. yr BP event, we conclude that the evidence from Scandinavia supports the argument that the cause of this prominent climatic perturbation must be searched for in high-resolution marine records.
Hammarlund, D., Björck, S., Buchardt, B., Israelson, C., Thomsen, C.T., 2003. Rapid hydrological changes during the Holocene revealed by stable isotope records of lacustrine carbonates from Lake Igelsjön, southern Sweden. Quaternary Science Reviews 22, 353-370.
Seppä, H., Hammarlund, D., Antonsson, K., 2005. Low-frequency and high-frequency changes of temperature and effective humidity during the Holocene in South central Sweden: Implications for atmospheric and oceanic forcings of climate. Climate Dynamics 25, 285-297.
Heikki Seppä is a quaternary palaeoclimatologist and palaeoecologist.