International Geologiical Congress - Oslo 2008

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CGC-08 Reconstruction of past climates based on combinations of microfossil records

 

A temperature and mire hydrology record for the last 500 years based on pollen, testate amoebae and pine needle production

 

Sheila Hicks, University of Oulu (Finland)
Kristian Schoning, Stockholm University (Sweden)
Heidi Hyyppä, University of Oulu (Finland)
Antti Huusko, University of Oulu (Finland)
Risto Jalkanen, METLA (Finland)
Tomasz Goslar, Adam Mickiewicz University (Poland)
 

 

A peat profile from Palomaa, northern Lapland, located close to the forest limit of Pinus sylvestris has been analysed for pollen and testate amoebae content. A robust age-depth chronology was achieved through 32 14C dated samples. On the basis of this, the frozen peat was cut into contiguous slices, each with a temporal resolution close to 1 year in the top 32 cm but falling to 11 years per cm with depth. The upper 47 cm cover the last 500 years. The pollen content of each slice is expressed as PAR (Pollen Accumulation Rates: grains cm-2 year-1) while the testate amoebae content is expressed as the percentage presence of each taxon in the whole assemblage.
A pollen deposition series measured with a pollen trap at the same mire since 1982, shows good agreement with the occurrence of 'highs' and 'lows' in Pinus pollen in the peat profile for this 23 year period. However, the average Pinus, Betula and Picea PARs for the same period are much lower in the peat profile than in the monitored series. On the basis of the monitored pollen record we hypothesize that the highs and lows in Pinus PARs in the peat record are a temperature proxy while the quantity of pollen is a biomass proxy.
To test whether the fossil pollen retains the temperature signal a Kalman filter was applied to the peat Pinus PAR record, the temperature record from Sodankylä and the monitored Pinus record. Using this approach it is possible to demonstrate to what extent the pollen record can be used to reconstruct temperature by linear models providing there are no big changes in biomass. A further check was made by comparing the peat Pinus pollen record with the Pinus needle production record from nearby Laanila, since both pollen and needles are initiated in the same bud and since it has been demonstrated that needle production is a strong proxy for summer temperature.
The most visible change in the pollen diagram is around AD1959. Before this date the peat is more humified, the rate of accumulation is much slower and the PARs of all taxa are distinctly lower. The road running north to Utsjoki was built in 1959, almost certainly changing the hydrology of the Palomaa mire. Shortly after this, in the early 60s, the testate amoebae assemblage changes from wet associated species to dry associated species and a distinct drop in the testate amoebae derived water table is registered. Prior to this change the testate amoebae assemblages are related to annual precipitation but after the altered hydrology no correlation with climate parameters are achieved.
In the peat pollen record, when the temporal resolution is low, the PARs are most indicative of changing biomass. The diagram would then have to be interpreted such that between AD1630 and 1870 pine of the area was virtually unable to produce pollen due to low temperatures.

 

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