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.