Bulletin of the Geological Society of Finland (Dec 1994)
Evidence on climatic variability and prehistoric human activities between 165 B.C. and A.D. 1400 derived from subfossil Scots pines (Pinus sylvestris L.) found in a lake in Utsjoki, northernmost Finland
Abstract
Samples from 1265 subfossil pines have been collected from small lakes and peat deposits in the forest-limit zone of northern Fennoscandia in order to study past variations of climate. Many of the subfossils have been dated by dendrochronology and the chronology constructed from the measured ring-width data extends as a continuous master curve from the present back until 165 B.C. and after a short gap until about 7000 years before the present time. This material has greatly increased the number of dated pine megafossils in northern Finland which had previously been restricted only to radiocarbon-dated samples. In addition to the year-by-year information provided by tree-ring width data, the temporal distribution of pine megafossils found in the vicinity of the forest-limit zone also provides information on past climatic changes. The 102 pine subfossils collected from Lake Ailigas, in Utsjoki, form part of the above material. They provide information about past variations in pine growth caused, to a large degree, by changing climate at this one site, but they also give glimpses of the local activities of Prehistoric Man. The data from 90 of these trees have been successfully dated using dendrochronological techniques and the results show that all of them grew during the time period beginning 3000 years before present, and that 79 pines lived during the time span 165 B.C. to A.D. 1952. In several lakes in the forest-limit zone, some subfossil trees are much older than those in Lake Ailigas. The relatively young ages of the subfossils at this site indicates that the lake has been in existence probably only during the past 3000 years, forming when climate turned more humid than in earlier times. The present continuous master curve is about 600 years longer than the earlier published pine chronology for northern Sweden, though this has recently been extended to A.D. 1. In the present study, the life spans of individual dated pines are considered in conjunction with the variability in the measured tree-growth curve. These provide detailed information on growth conditions and variations in temperature during the period 165 B.C. - A.D. 1400. The correlation of the present data with the previously published results from northern Sweden display great general similarity, but also some significant differences. We propose that the combined data give a more reliable indication of regional climatic variations and that the differences are due to local factors at the two locations. The similarity between these two independent series supports general conclusions made about changing summer climate over Fennoscandia after A.D. 500 and strongly suggests that our conclusions drawn here for the period 165 B.C. - A.D. 500 are on a firm basis. The present data thus enable us to infer the record of summer climate variability in northern Fennoscandia about 600 years back in time. Most of the subfossil pines preserved in the lake grew very close to the place where they were found. Reaction wood indicating tilting of the trunk, probably caused by rising water levels, was found in several trees. These trees grew on the lake shore where the soil was soft or eroding. The large number of relatively young trees indicates that storms may have occasionally felled pines. This cannot be proven (by demonstrating a common death year) however, because the outermost tree rings in the subfossil material are frequently decayed. A small number of the pines found in Lake Ailigas were cut by Man. The pines, identified by axe marks may be remnants of barriers built by the Prehistoric Saami for hunting of wild reindeer. The dates of these indicate that traps were constructed in the mid 9th century and at the end of the 11th and start of the 12th centuries, i.e. late Iron Age times.
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