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The isolation age and history of Lake Sågsjön, Stockholm, based on different dating techniques

Abstract Laminated sediments from Lake Sågsjön, situated east of Stockholm, were used to study the last 1000 years history of the lake. A combination of 14C dated (AMS) terrestrial macrofossils and annually laminated sediments were used, together with diatom stratigraphy, to determine the age of the isolation from the Baltic Sea. The laminae were counted using greyscale analysis and studied in detail by thin sections. Three new pair of laminae were formed between 1993 and 1996, indicating that the laminae are annually formed as varves. This is further supported by studies of thin sections, which reveal organic and minerogenic layers reflecting summer algae bloom and spring flood, respectively. Two of the 14C dated macrofossils are interpreted as redeposited wheras the calibrated ages received from two leaf fragments are believed to represent the age of sediment deposition. According to the varve counting, Lake Sågsjön (2.0 m a.s.1.) was isolated from the Baltic Sea between AD 1500 and 1600. The two radiocarbon dated leafs give an isolation age between AD 1150 and 1400. Compared to the 14C dates, the laminae based chronology yields too young ages, which indicates that the deposition of the laminae was not continuous. Shortly after the isolation the rate of sediment accumulation increases, which most probably is an effect of increased erosion when the catchment area was partly brought under cultivation.