OXYGEN-ISOTOPE DISEQUILIBRIUM PRECIPITATION OF CALCITE IN LAKE ARRESO, DENMARK

Lake carbonates are generally assumed to precipitate at or very close to oxygen isotopic equilibrium with the ambient water. We present resu lts from the eutrophic Lake Arreso, northeastern Sealand, Denmark, tha t demonstrate disequilibrium precipitation of present-day calcite. Acc ording to the temperature equation of Epstein et al., these calcites a re depleted in O-18 by 2 parts per thousand-3 parts per thousand when compared to the expected equilibrium values. Core studies indicate tha t the development of disequilibrium conditions in Lake Arreso is corre lated with the onset of an enhanced eutrophication. A high precipitati on rate is suggested as the mechanism responsible for the observed O-1 8 depletion of the calcite precipitates.