210Pb, 226Ra and 32Si in Pavin lake (Massif Central, France)

Abstract

Measurements of cosmogenic 32Si and the U-decay series' nuclides 210Ph and 226Ra in waters and sediments of lake Pavin are reported. Both 210Pb and 226Ra are enriched in the anoxic deep waters compared to the oxic surface waters, respectively by a factor of 4 and 10, whereas 32Si is depleted by a factor of ∼ 2. Redox conditions in the lake appear to have no marked effect on the 32Si. Using a steady-state box model it is shown that the deep-water 32Si concentration is controlled by the underground lacustrine springs. The residence times of 210Pb, 32Si and 226Ra are ∼ 1, ∼ 10 and ∼ 80 a, respectively. In the case of 32Si where more data are available, the assessed inventory data from the overhead atmospheric fallout and that measured in the sediments agree very well as expected. The 210Pb, and 32Si-based deposition rates during the past ∼ 100 a ranged from 0.8 to 1.9 mm a −1, earlier these were a factor of ∼ 3–5 faster. The geochemistry of 32Si and 210Pb, in lake Pavin in many ways resembles that in the ocean, only the time scales of the processes involved are faster.