Holocene humidity changes in northern Finnish Lapland inferred from lake sediments and submerged Scots pines dated by tree-rings

Abstract

Conclusive evidence for a rise in water levels has been found in connection with lake-sediment studies undertaken partly in collaboration with the pine megafossil sampling and dendrochronological work in northern Finnish Lapland. The change in lake-level stands is shown by slow sedimentation rate in the early to mid-Holocene and an increase thereafter. These data indicate a regional rise in water levels during the latter part of the Holocene following a relatively dry period between 8000 and 4000 BP. Synchronous changes, also indicating rising water levels, have been observed in the diatom and cladoceran assemblages of the sediment cores. Subfossil Scots pines (Pinus sylvestris, L.) have been preserved in large quantities in small lakes in Lapland, because in many cases the rising water level has inundated the trunks after death. The position of the subfossil trunks and stumps often indicates that the pines have been growing on dry land at the sides of the lakes in which they are now submerged. Traces of a bark beetle (Tomicus minor, Hart.) have been detected in a few old pine logs found far outside the present distribution area of this insect. A total of 1722 samples of pine subfossils have been collected in the forest-tundra ecotone region of Lapland to build a continuous pine ring-width chronology over 7000 years long. The long chronology is almost finished, but its two parts are still separated by a short discontinuity around 250 bc. An absolutely dated, year-by-year chronology over 2000 years long extends from the present close to that time. The older, over 5000 years long continuous floating chronology is fixed to the timescale by several radiocarbon dates. A total of 1212 samples of pine wood have been dated and assembled within the chronologies by tree-ring cross-matching. These substantial data indicate a gradual retreat of pine tree and forest limits with some marked regional differences during the past 5000 years. According to the preliminary interpretations of the tree-ring data the variability of Holocene summer temperatures has increased towards the present time. Shifts in climatic development leading to cooler and more unstable conditions seem to have occurred in mid-Holocene time, and between 2500 and 2000 BP. The increase in humidity is probably in association with these changes in the high-frequency variability of temperature.