Dissolved gases in perennially ice-covered lakes of the McMurdo dry valleys, Antarctica

Measurements of dissolved N-2, O-2, Ar, CO2, and CH4 were made in perennially ice-covered Lake Hoare. Results confirm previous reports that O-2 concentrations in the upper water column exceed atmospheric equilibrium and that N-2 and Ar are supersaturated throughout the water column. The mean supersaturation of N-2 was found to be 2.0 (+/-0.37) and Ar was 3.8 (+/-1.1). The ratios of N-2/Ar (20.3 +/- 3.8), and O-2/Ar (22.5 +/- 4.0) at the ice-water interface are consistent with those previously measured, suggesting that bubble formation is the main process for removing gas from the lake. However, the saturations of N-2 and Ar greatly exceed those previously predicted for degassing by bubble formation only at the ice-water interface. The data support the hypothesis that removal of gas by bubbles occurs in the water column to a depth of 11 m in Lake Hoare. CO2 concentration increases from near zero at the ice-water interface to 80-100 times saturation at and below the chemocline at c. 28 m. There is considerable variability in the gas concentrations throughout the water column; samples separated in depth by one metre may vary by more than 50% in gas content. It is likely that this phenomenon results from the lack of turbulent mixing in the water column. Methane (c. 2 mu g 1(-1)) was detected below the chemocline and immediately above the sediment/water interface at a depth of 30 m. Samples from lakes Vanda, Joyce, and Miers, also show supersaturations of O-2, N-2, and Ar at levels similar to levels found in Lake Hoare.