CLIMATE AND ENVIRONMENTAL RECONSTRUCTIONS FROM NAUTAJÄRVI CLASTIC-BIOGENIC VARVES IN CENTRAL SOUTHERN FINLAND

Annually resolved proxy records of varved lake sediments are important archives of past climate variability and environmental change. They provide a continuous and inherent calendar year timescale and a possibility to express many physical, chemical and biological parameters as accurate fluxes. The structure and composition of varves are governed by a multitude of interacting factors in different sedimentary environments that, in the best cases, can be related to external forcing factors and subsequently interpreted as records of regional climate change. In Fennoscandia, lacustrine varves are typically composed of an allochthonous fine-grained minerogenic layer deposited as a result of the spring snowmelt, grading into an organic summer layer formed by primary productivity and a thin dark-coloured layer composed of fine-grained organic layer that settles during the winter when the lake is ice-covered. Such clastic-biogenic varves have been deposited in Lake Nautajärvi, central southern Finland, since the development of the basin at ca. 10,000 years ago. Physical properties of varves are indicative of the annual influx of mineral matter into the basins, which depends principally on the rate of catchment erosion and the transportation of detritus during the intense spring snowmelt flood. Hence, clastic-biogenic varves provide a proxy for winter and spring precipitation and temperature. One of the phenomena known to regulate winter climate in the North Atlantic ? European region is the atmospheric circulation pattern called the North Atlantic Oscillation (NAO). During years of positive NAO winters are mild and wet, whereas years of negative NAO are characterized by colder and dryer winters. Lake Nautajärvi is located at a sensitive climatic boundary where just a few degrees warming during the winter (+NAO) results in a considerably shortened period of snow cover; even to the point where no persistent snow cover forms. As a consequence, the peak discharge in spring is severely reduced, catchment erosion is significantly lower and less allochthonous mineral matter is transported and deposited on the lake bottom. However, due to local system dynamics the relationships between external forcing factors and the varve formation is neither linear nor stable over time.