Sediment profile dating and reconstructing nuclear events from annually laminated lake sediments in northern Finland

The sediments deposited in Lake Kevojärvi (69°45N, 27°00'E) in northernmost Finland were investigated for radioactivity. Freeze cores recovered from the 35-m deep basin has preserved a distinct succession of annual laminations deposited between 1909 and 2015. The basic varve structure is composed of a detrital snowmelt layer and an organic-rich post-snowmelt layer lying on top of the snowmelt layer. The past nuclear events have been preserved in the lake sediment. In order to study these, sediment freeze with annual sediment laminations were recovered from the lake bottom and measured for gamma emitting radionuclides. A total of 53 sediment subsamples, each incorporating sediment from one to two varves i.e. sediment deposited during one to two years, were taken for gamma spectroscopy measurements using low-background HPGe detector system. The measurements showed large variations in ¹³⁷Cs and ²⁴¹Am concentrations between different laminae marking different nuclear events in history. This highly resolved profile revealed a detailed record of anthropogenic radioactive fallout from atmospheric nuclear weapons testing during 1950s and 1960s and the Chernobyl accident in 1986. The measured ¹³⁷Cs concentrations varied between 0.6 and 229 Bq/kg dry weight (dw) while the measured ²⁴¹Am concentrations varied between 0.6 and 6.4 Bq/kg dw. The ¹³⁷Cs showed downward migration in the sediment column since ¹³⁷Cs was also found in varves dating before 1945. The first clear increase in ¹³⁷Cs concentration was observed in 1956 varve and the ¹³⁷Cs concentrations were found to peak in the 1964, 1970 and 1986 varves. The ²⁴¹Am was first observed in 1957 varve and the concentrations peaked in 1960-1962, 1964 and 1970 varves. This demonstrated that ²⁴¹Am can be a complementary chronostratigraphic marker to ¹³⁷Cs in sediment. A time delay of one to two years was observed between the years of intensive atmospheric nuclear weapons testing and the increased ¹³⁷Cs and ²⁴¹Am concentrations in the sediment varves. In the ²¹⁰Pb concentrations long-time periods of low (~1930-1950 and ~1990-2000) and high concentrations (~1950-1980) were observed. The reason for large variation was the amount of sediment input. The Constant rate of supply (CRS) model results showed good agreement with the varve counting. The model results showed that the ²¹⁰Pb deposition has been constant over the last 80 years and no effect of climate change to the ²¹⁰Pb deposition was observed.