Teija Alenius

Integrated varve and pollen-based temperature reconstruction from Finland: evidence for Holocene seasonal temperature patterns at high latitudes

A detailed understanding of decadal to millennial-scale climate changes requires seasonal-scale (summer-winter) reconstructions of past precipitation and temperature fluctuations. Comparing seasonally resolved varve records with pollen-based sum of growing degree-days (GDD) reconstructions from Lake Nautajärvi, we examined the intra-annual nature of climate variability in central southern Finland during the Holocene. The organic varve record and the GDD reconstruction show roughly comparable trends supporting the interpretation that both proxies predominantly reflect summer temperatures in the study area. The records suggest low but rising early-Holocene (9500 to 8500 cal. yr BP) summer temperatures. The Holocene Thermal Maximum (HTM) in the GDD record dates to about 7500 to 4500 cal. yr BP, but the organic varve record along with reconstructed changes in vegetation composition, notably a peak of Tilia pollen percentages, indicate that during the HTM there was a trend towards a more continental climate with maximum mid-summer temperatures reached at 6500 to 4500 cal. yr BP. Both records reflect the start of the post-HTM cooling at about 4500 cal. yr BP, simultaneously with an increase of the amount of catchment erosion and mineral matter influx into the lake, suggesting gradually colder and/or longer winters with high net accumulation of snow. The organic varve record and the GDD record start to diverge at 2000 cal. yr BP, possibly owing to the human influence on catchment processes. The reconstructed mid-Holocene summer temperature peak deviates from the regional climate model outputs, which suggest highest summer temperatures during the early Holocene.

Calibrated pollen accumulation rates as a basis for quantitative tree biomass reconstructions

Recent investigations show that the pollen accumulation rate (PAR) of the common tree taxa is directly related to the biomass and, by inference, to the population size of the taxa around the study site. Fossil PAR records preserved in lakes provide therefore a potential proxy for quantitative biomass and population reconstructions. We use the high-resolution PAR records obtained from two accurately dated lake sediment cores in Finland to generate quantitative Holocene biomass records for Pinus, Picea and Betula, the most common tree taxa of the European Boreal forest. PAR values were calibrated to biomass values by comparing the modern PAR values with the modern biomass values and assuming a linear relationship between the past PAR and biomass values. The obtained PAR and biomass values and trends are remarkably coherent between the two records. Pinus has a stable Holocene biomass size and its modern biomass, about 20 t/ha, corresponds with the natural Pinus biomass in the study regions. In contrast, Picea immigrated from the East during the mid Holocene, had a maximum biomass, 50—60 t/ha, at 3500—1000 cal. yr BP, and declined strongly during the last 1500—1000 years as a result of increased human activity and related rise of fire frequency. Thus, the modern Picea biomass in the study regions, about 22 t/ha, is only 35—40% of the natural Picea biomass. The results of this pilot study demonstrate the potential of the calibrated PAR data in quantitative biomass and population reconstructions. Such reconstructions can provide fresh insights into the structure of past plant communities and, when combined with records reflecting palaeoclimates, natural disturbances, and human activity, can help to disentangle the long-term importance of different enviromental drivers to changes in plants populations and ecosystems.

Palaeoecology and Archaeology of the Village of Uukuniemi, Eastern Finland

Abstract In the present article, pollen analytical results from Lake Kirkkolampi are presented and compared with results provided by archaeological material. Pollen analysis is connected with the archaeological research project at Papinniemi in Uukuniemi. Papinniemi is one of the numerous Greek Orthodox settlements that existed in Karelia in the fourteenth to seventeenth centuries. Archaeological evidence of settlement preceding this period is very scarce, and in this respect Uukuniemi represents a typical area in eastern Finland. There is no archaeological evidence of permanent settlement in Uukuniemi from the Early Metal Period (c. 1800 bc–ad 400), the Middle Iron Age (c. ad 400–800) or from the Late Iron Age (c. ad 800–1300). Pollen analysis demonstrated the onset of cultivation c. cal ad 300. Marked intensification of agricultural activities and cultivation in permanent fields took place around cal ad 800. A shift in land-use practises, including a declining use of fire, is visible at cal ad 1520–1600. The discrepancy between archaeological and palaeoecological records raises several questions, and the problems of Early Metal Period and Iron Age populations, as well as settlement continuity, are discussed.

Event reconstruction through Bayesian chronology: Massive mid-Holocene lake-burst triggered large-scale ecological and cultural change

Precise timing of natural and cultural events provides a foundation for understanding how past natural phenomena have driven changes in population and culture. In this study, we used high-resolution Bayesian chronology to describe an event sequence of a massive and abrupt water level decline of a large lake and the contemporaneous cultural changes that occurred in eastern Fennoscandia during the mid-Holocene. The study provides the first transdisciplinary analysis of the causes and effects of the events by using a combination of archaeological, geological and ecological data. Nearly 6000 years ago, ancient Lake Saimaa, estimated to cover nearly 9000 km2at the time, was abruptly discharged through a new outlet. The event created thousands of square kilometres of new residual wetlands. The archaeological record shows a profound cultural replacement and a subsequent sharp human population maximum in the area during the decades after the decline in water level. During the population maximum, the proportion of Alces alces (moose) in the diet rapidly increased and became prominent as a dietary resource. The eventual population decline in the area coincided with ecological development towards old boreal conifer forests, along with the colonization of a new species of tree Picea abies (Norway spruce). The new ecosystem was less suitable for moose to forage in, and this attenuated the dietary role of moose and thus contributed towards the eventual population and cultural decline. The methodological approach described in this paper allowed the reconstruction of past natural and cultural events and demonstrated how they can be causally intertwined.

Pollen-Analytical Results from Lake Katajajärvi – Aspects of the History of Settlement in the Finnish Inland Regions

Abstract The Early Metal Period (1800 cal BC–AD 300) of the Finnish inland regions is characterized by a scarcity of archaeological remains. Its latest stages, in particular, have proved to be difficult to interpret. This paper discusses the continuity of settlement in the Early Metal Period as indicated by archaeological survey and pollen-analytical data. The study area is situated in Repovesi National Park in southeastern Finland. The pollen analysis was constructed from a sediment sequence taken from Lake Katajajärvi. The archaeological survey data from Repovesi National Park supports earlier hypotheses of the small number of Early Metal Period sites. In the pollen data, the first anthropogenic impact is recorded from about 2600 BC onwards, with weak signs of cultivation around 2370 BC. From around 1900 BC onwards a clear change in tree, shrub and herb pollen percentages indicates anthropogenic impact, possibly grazing, in the area. The pollen-analytical results also revealed a cultivation period of about 900 years at the end of the Early Metal Period, providing proof of continuity in settlement during a period characterized by a scarcity of archaeological material. A new beginning of cultivation from cal AD 1160 to 1250 onwards can be considered as indicating population growth, developing settlements and the colonization of new areas. The results from Repovesi are challenging from an archaeological point of view, because they represent slash-and-burn cultivation that appears to have been practised in total wilderness conditions. Comparison of the Lake Katajajärvi results with other areas raises new questions that call for a discussion of early slash-and-burn cultivation in new perspective.