Density-driven exchange between the basins of Lake Lucerne (Switzerland) traced with the 3H-3He method

Abstract

Lake Lucerne is divided by sills into four major and two minor basins. During winter, differential vertical mixing due to non-uniform wind exposure and different total dissolved solid concentrations in the main tributaries of the lake cause considerable interbasin density gradients. These gradients induce density-driven currents across the sills that contribute significantly to the deep water exchange in the basins and gradually reduce the density gradients during summer. Over a period of 2 yr (1990-1992), the spatial and temporal evolution of water density and water age (measured by the 3H-3He 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 exchange flows the density gradients cause. Water age provides an integral measure of the magnitude of deep-water renewal, which in some basins can be roughly separated into contributions from density-driven currents and wind-induced vertical mixing. By correlating water age with dissolved 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 2 x 1010 atoms m-2 s-1, indicating dynamic equilibrium with helium production in the underlying crust. Oxygen depletion rates per unit sediment area were roughly equal in all basins (between 220 and 290 g m-2 yr-1), whereas volumetric 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.