Hannu Hyvärinen

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

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.

Holocene lake-level changes in the Fennoscandian tree-line region, western Finnish Lapland: diatom and cladoceran evidence

Lithostratigraphical, diatom and cladoceran evidence were used to demonstrate Holocene lake-level changes in two closed-basin lakes in western Finnish Lapland, north of the coniferous tree- line. The lake levels were significantly lower than present in early to mid-Holocene times, from about 8000 until about 4000BP, after which they rose to the present level. The changes are believed to be climatically controlled and regionally representative. The proposed mid-Holocene dryness in Lapland contrasts with the evidence from southern Sweden and Finland, where past lake levels indicate humid conditions during much of the mid-Holocene (Atlantic) time.

Subfossil pine dates and pollen diagrams from northern Fennoscandia

Abstract As a continuation of earlier work on the Holocene history of the pine forests of northern Fennoscandia, new finds of subfossil pine (Pinus silvestris) are reported from sites in Enontekio, northwestern Finnish Lapland, and at one location in the Lyngenfjorden area in North Norway. Two pollen diagrams from lake sediments are also presented, one from near Kilpisjarvi in Finland, the other from close to Skibotn in Norway. The megafossil, pollen, and radiocarbon data provide a more or less consistent picture of the history of the pine forests of northern Fennoscandia. Pine spread to the Kilpisjarvi and Lyngen area around 7500–7000 B.P., and the pine forests were at their maximum there from about 7000 to 4000 B.P. Their subsequent gradual decline meant a retreat of the pine limit by some 70 km or so. Discussions are provided on climatic variations in the area, and on possible further research into these and other palaeo-ecological changes.

Humic- and fulvic-acid stratigraphy of the Holocene sediments from a small lake in Finnish Lapland

Holocene sediments from a small lake (Hirvaslompolo) in eastern Finnish Lapland have been ana lysed for their content of humic substances. The amount and proportion of humic and fulvic acids were found to be subject to significant changes with time through the Holocene. The humus stratigraphy shows a good correlation with litho- and biostratigraphical indicators of environmental change at and around the site. The main vegetational changes were the replacement of the early-Holocene birch woods by pine about 7500 yr BP and the retreat southwards of the pine limit since 4000-5000 yr BP. The latter change is associated with climatic cooling and an increase in effective humidity, resulting in the spread of peatlands and a lake-level rise. The present results are preliminary but nevertheless demonstrate the potential of humus analysis for providing palaeoenvironmental information.

Holocene vegetation history at Vätsäri, Inari Lapland, northeastern Finland, with special reference to Betula

A Holocene sediment series from a lake in the northern birch forest region of eastern Finnish Lapland was studied pollen-analytically. In addition to conventional pollen analysis, birch pollen measurements were carried out. Betula pollen diameters were measured systematically. The resulting size-frequency distributions were analysed statistically to infer their species composition and to reconstruct the local history of birch. The results indicate pine arrival about 7500 14C years BP. Even during its optimum period of 7000–6000 BP, pine cover seems to have been thin and discontinuous. Pine and birch started to retreat soon after the pine optimum. In the interpretation of the birch pollen-size frequency distributions, no continuous record of Betula tortuosa was found. B. pendula seems to have had a more northerly distribution in the past than it has today.

The sediment history of a 100 year old pond near Lake Höytiäinen, Eastern Finland

Hyvarinen, H., and Alhonen, P.: The sediment history of a 100 year old pond near Lake Hoytiainen, Eastern Finland. Geologiska Foreningens i Stockholm For‐handlingar, Vol. 92, pp. 410–414, Stockholm, September 30, 1970. The pond investigated was formed in connexion with the drainage of Lake Hoytiainen in 1859. In the sediment record the drainage is represented by a 5 cm layer of sand. This is overlain by about 6 cm of sandy mud deposited in the pond during the 100 years since its isolation. A thin sulphide‐rich horizon occurs at the isolation contact.