Flax use, weeds and manuring in Viking Age Åland: archaeobotanical and stable isotope analysis

Charred masses of nearly 100,000 Linum usitatissimum (flax) seeds were found in house remains from Tjudnäs on the Åland Islands, Finland, and radiocarbon dated to ad 880–1020. The flax seeds were large and discovered in lumps, which indicates that they had been pressed for their oil. The flax was accompanied by seeds of Cuscuta epilinum, Lolium remotum, Galium spurium and Camelina alyssum/sativa, which are specific weeds of flax fields that mimic it in the flax fields and during crop processing. Several of these species have now become extinct or rare after the introduction of more effective methods of weed control and less flax growing. This find demonstrates the antiquity of this obligate flax weed flora, which appears to have been introduced to Åland together with flax seeds intended for cultivation. Thousands of cereal grains discovered in Viking Age/early medieval (ad 750–1300) Kohagen show that a broad range of crops was grown, with high proportions of Triticum aestivum (naked wheat). Stable isotope analyses of carbon (δ¹³C) and nitrogen (δ¹⁵N) ratios were done on the flax from Tjudnäs and other crops from Kohagen, all from samples dated ad 880–1020. The analyses showed high values of δ¹⁵N, which can be explained by considerable soil enrichment with manure, possibly also with maritime resources such as seaweed and fish. The δ¹³C values varied between crops, with those for flax being lower than for cereals and Pisum sativum (peas), which could reflect different environmental conditions or differences in plant physiology. The factors affecting δ¹⁵N and δ¹³C in flax are, however, poorly understood due to the lack of experimental studies.