EFFECTS OF GLACIAL MELTWATER INFLOWS AND MOAT FREEZING ON MIXING IN AN ICE-COVERED ANTARCTIC LAKE AS INTERPRETED FROM STABLE-ISOTOPE AND TRITIUM DISTRIBUTIONS

Perennially ice-covered lakes in the McMurdo Dry Valleys have risen se veral meters over the past two decades due to climatic warming and inc reased glacial meltwater inflow. To elucidate the hydrologic responses to changing climate and the effects on lake mixing processes we measu red the stable isotope (delta(18)O and delta D) and tritium concentrat ions of water and ice samples collected in the Lake Fryxell watershed from 1987 through 1990. Stable isotope enrichment resulted from evapor ation in stream and moat samples and from sublimation in surface lake- ice samples. Tritium enrichment resulted from exchange with the postnu clear atmosphere in stream and moat samples. Rapid injection of tritia ted water into the upper water column of the lake and incorporation of this water into the ice cover resulted in uniformly elevated tritium contents (>3.0 TU) in these reservoirs. Tritium was also present in de ep water, suggesting that a component of bottom water was recently at the surface. During summer, melted lake ice and stream water forms the moat. Water excluded from ice formation during fall moat freezing (en riched in solutes and tritium, and depleted in O-18 and H-2 relative t o water below 15-m depth) may sink as density currents to the bottom o f the lake. Seasonal lake circulation, in response to climate-driven s urface inflow, is therefore responsible for the distribution of both w ater isotopes and dissolved solutes in Lake Fryxell.