DENSITY-DRIVEN EXCHANGE BETWEEN THE BASINS OF LAKE-LUCERNE (SWITZERLAND) TRACED WITH THE H-3 HE-3 METHOD

Lake Lucerne is divided by sills into four major and two minor basins. During winter, differential vertical mixing due to non-uniform wind e xposure and different total dissolved solid concentrations in the main tributaries of the lake cause considerable interbasin density gradien ts. These gradients induce density-driven currents across the sills th at contribute significantly to the deep water exchange in the basins a nd gradually reduce the density gradients during summer. Over a period of 2 yr (1990-1992), the spatial and temporal evolution of water dens ity and water age (measured by the H-3-He-3 method) was investigated. Variation in water age between the different basins and the occasional occurrence of age inversions (older water overlying younger one) can be explained in terms of the observed density distribution and the exc hange flows the density gradients cause. Water age provides an integra l measure of the magnitude of deep-water renewal, which in some basins can be roughly separated into contributions from density-driven curre nts and wind-induced vertical mixing. By correlating water age with di ssolved helium and oxygen concentrations, radiogenic helium fluxes and average oxygen depletion rates were determined for the main basins of the lake. The helium flux was between 1 and 2x10(10) atoms m(-2) s(-1 ), indicating dynamic equilibrium with helium production in the underl ying crust. Oxygen depletion rates per unit sediment area were roughly equal in all basins (between 220 and 290 g m(-2) yr(-1)), whereas vol umetric depletion rates varied from 1.8 g m(-3) yr(-1) in the deepest basin to 10 g m(-3) yr(-1) in the shallowest.