Studies of the Sediments and the Vegetational History in the Ösbysjö Basin, North of Stockholm

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Abstract
Studies of the character, thickness, and volume of sediment in Lake Osbysjon made it possible to calculate the rate of sedimentation and annual deposition of organic matter in the lake, and to elucidate, mainly by means of pollen analysis, the post-glacial development of the aquatic and terrestrial macrovegetatlon and the changes of the surrounding landscape. The topographic development of the basin from a depression in an archipelago of the Baltic Sea to a small lake and the resultant changes of sedimentation are described. Its isolation (7.5-8 m above the sea) can be assigned to a period of perhaps 100 years, according to Granlund (1928) about 200-300 A.D. (but 200-300 years earlier according to C14 dating). It is marked, among other changes, by fibrous layers of a filamentous blue-alga of the Lyngbya type; here Vaucheria is obviously subordinate. The environmental change and vegetational development in the Osbysjo basin and its surroundings are elucidated by pollen analysis. During the 1st phase the deposition of mineral and organic particles, including pollen, occurred in a more or less open archipelago landscape where wind and currents more strongly influenced the transport of pollen and spores than later during the lake phase. The high frequency of Pinus pollen up to the level of isolation is, from that point of view, understandable. The hystrichosphaerid mlcrofossils (Hystrichosphaeridium centrocarpum and Hystrichosphaera furcata) are characteristic for the brackish water sediments below the level of isolation. During the 1st 100 years of the lake phase, starting in the beginning of the Christian era, the surroundings were characterized by rich oak wood with hazel. The spruce was of little importance in the landscape. Here the great spruce invasion, which happened during the 1st millennium B. C. according to several investigations in adjacent Central-Swedish areas, seems to have been almost lacking, probably for some local reasons. Pine, birch and juniper were common. Above the level of isolation the indications of agriculture increase, i.e. there is clear pollen-analytical evidence of agriculture starting from at least Roman Iron Age (birth of Christ to about 400 A.D.). About 1000 years ago the landscape seems to have changed considerably. The oak woods were strongly diminished, certainly as a result of increased agricultural activity. Ancient monuments in the area indicate an increased settlement during late Iron Age and Viking time. The greater frequency of Picea pollen should be regarded as a manifestation of some local changes, caused by man. The renewed relative increase of the Pinus pollen amount should be considered against the background of continued deforestation. The cultivation of hemp (Cannabis) is suggested to have occurred during the Viking time (approximately 9-10th centuries) and Medieval time. In the lacustrine sediment (gyttja) Pediastrum spp. and pollen of several water plants occur more or less abundantly, indicating the same eutrophic type of lake as today.

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