Sediment as an archive of a lake’s history

The Geological Survey of Finland (GTK), together with the Finnish Environment Institute and the Universities of Turku and Jyväskylä, is currently studying how peat production and forest drainage affect lakes. The research is being conducted at two lakes in Keuruu: Martinjärvi and Iso Kivijärvi. The research started in March 2012 with samples of lake sediment being taken. The findings are expected to be available in 2013.

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Sediments of lakes began to be formed about 10,000 years ago, i.e. after the last ice age. Sediment is matter deposited at the bottom of the basin after the lake was formed. This matter may have been transported from a catchment area, been re-deposited in the lake basin or have formed in the lake.

Sediment varies in thickness, both between lakes and within a lake, and the thickness generally varies from 2 to 10 metres. Human activity generally accounts for some tens of centimetres of the sediment.

In small lakes the sediment layer is on average thicker than in large lakes, which is largely due to matter being transported from catchment areas and being deposited in the small basins of headwaters. So far around one third of the original volume of small lake basins has been filled, but only less than one tenth of the volume of large basins.

The age of lake sediment is determined using methods based on radioactive decay and, among other things, by searching sediment for the fallout of caesium from the 1986 Chernobyl nuclear accident.

In addition to caesium fallout, for example changes in the lead concentration in sediment and its isotopic composition indicate man-made deposition of lead, whereas the content of sulphur and vanadium (a naturally occurring metal) reflect the airborne load from the combustion of fossil fuels. In addition to the physical and chemical properties of sediment, the remains of various organisms in the samples are studied, because they indicate changes in the water quality and conditions at the bottom of the lake as well as e.g. in the oxygen content of the hypolimnion (bottom layer of water).