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.

Sediment Evidence of Early Eutrophication and Heavy Metal Pollution of Lake Mälaren, Central Sweden

“…The lake was isolated from the Baltic Sea by land upheaval in a gradual process between 900 and 1300 AD (3). At that time, the population of the area was in the order of 30␣ 000 people (4) or higher (5) and human impact from agriculture was already extensive in the large catchment of the lake (6)(7)(8).…”

Recovery from multi‐millennial natural coastal hypoxia in the Stockholm Archipelago, Baltic Sea, terminated by modern human activity

“…), a period of expanding agriculture and stock‐farming in which the first town of Sweden, Birka (∼ 750 c.e. ; ∼30 km west of Stockholm), became a permanent settlement (Risberg et al 2002; Myrdal and Morell 2011; Kalmring 2016). The large European population boom and expansion from 1000 to 1300 c.e.…”

“…206 Pb/ 207 Pb isotope ratios from a suite of records from central Sweden show that the Pb‐pollution is a mixture of both waterborne and atmospheric sources, partly originating from large‐scale mining in the Bergslagen area in central Sweden (Renberg et al 2002; Bindler et al 2009). Archeological and historical evidence also indicate increased human activity in the Stockholm region, which had become a prominent area for large‐scale international trade and politics, around that time (Risberg et al 2002; Kalmring 2016). The remainder of the sedimentary Pb record from the Stockholm Archipelago largely follows the trends observed in Swedish lakes and peats (Brännvall et al 1999; Renberg et al 2000; Renberg et al 2002).…”

De Vikingen: hun landbouwdieren en hun dierlijke voeding

“…De speciale moeilijkheden die het ecosysteem in koude sub-arctische gebie den met zich meebrengt zijn: het tekort aan hout voor bouw en vuur, de actieve onrustige vulkaanomgeving (IJsland), de natuurlijke erosie vooral in tefragebied (vulkanische bodem) en de post-glaciale 'uplifting 'waarbij vaar routes en havens va n kleine dorpjes (zoals Birka bijv.) onbevaarbaar worden (Risberg et al, 2002). De lange winters creëren ook een hoge behoefte aan brandbaar materiaal; dat kan dan bestaan uit hout (zelden) of turf (meer).…”