Environmental changes and human impact as recorded in a sediment sequence offshore from a Viking Age town, Birka, southeastern Sweden

Abstract

The Viking Age town of Birka, which existed between AD 750 and 975, on the island of Björkö in the Mälar archipelago of the Baltic Sea, is known as the oldest centre used for large-scale international trading in Sweden. Birka had a strong defence: town rampart, hillfort and a water palisade. Studies of lithology, and microand macroremains in a sedimentary sequence accumulated offshore from Birka, outside the water palisade, revealed a stratigraphically distinct refuse stratum, referred to as the Birka Layer. We argue that the bulk material of the Birka Layer emanates from human activities in the town, e.g., latrine cleanings, cuttings of shore vegetation, cleaning of ditches and storage areas, and domestic animal dung. Organic remains from vegetation and animals, deposited during activities on the ice offshore from the town, e.g., winter markets, may also have contributed to the composition. The material constituting the Birka Layer started to accumulate after the construction of the protecting water palisade. In the Birka Layer and the underlying clay gyttja, there seems to be a succession of five activity phases recorded during the Birka period. After the initial establishment of the town, an expansion took place, followed by a retrogression. The most intense phase with culminating activities is registered prior to abandonment of the site, which approximately coincides with the onset of the extended final isolation process of Lake Mälaren (AD 1000-1300). The sailing routes to Birka from the south and east became too shallow and were finally cut off by the emerging thresholds. After the abandonment of Birka, the town area was utilized for cultivation and cattle-breeding.