Geochemical study of a crater lake: Lake Pavin, Puy de Dôme, France. Constraints afforded by the particulate matter distribution in the element cycling within the lake

Abstract

Suspended particles concentrations and settling particle fluxes collected by sediment traps were analyzed along a vertical profile in Lake Pavin, France at the same time as dissolved element concentrations, which are reported elsewhere (Viollier et al., 1995). Results for iron, manganese, cobalt, vanadium, molybdenum, uranium, barium, rubidium and cesium are presented here. Particulate concentrations of all elements increase significantly just above the sediment-water interface. Manganese, iron and barium particulate concentrations exhibit a peak just above the oxic-anoxic boundary. Molybdenum and uranium particulate concentrations increase drastically below the oxic-anoxic boundary.Manganese and cobalt fluxes are at a maximum at 55 m depth, above the oxic-anoxic boundary. For all other elements, fluxes are at a maximum at 85 m depth, just above the bottom of the lake. In all cases settling particles fluxes are greater than buried fluxes, indicating that a dissolution reaction occurs at the bottom of the lake. Particulate iron is essentially present as Fe(III) oxides in the mixolimnion, as Fe(II) phosphates in the monimolimnion and as pyrite in the sediment. The change in the Fe-bearing mineral leads to either important remobilisation or immobilisation of trace element into or from solution.Comparison of fluxes in the sediment trap at 85 m depth with dissolved concentrations gradients above the bottom leads to the rejection of the previous hydrodynamic model of the lake. The fluxes and gradient data are consistent with the tritium data only if the arrival of sublacustrine waters occurs in the mixolimnion. Residence times of waters in monimolimnion in the model derived from the present study are about 20 times greater than the previous values. The dissolved element concentrations profiles are explained only by eddy diffusion with a coefficient varying from about 20 m2 a− at the bottom of the lake to 0.2 m2 a−1 at the mixolimnion-monimolimnion boundary.Transport parameters obtained in this way allow a quantitative description of the element cycles within the lake and an estimation of the elemental ratio sedimentation versus exportation by the outlets.