Dissolved gases in perennially ice-covered lakes of the McMurdo Dry Valleys, Antarctica

Abstract

Measurements of dissolved N₂, O₂, Ar, C_O2, and CH₄ were made in perennially ice-covered Lake Hoare. Results confirm previous reports that O₂ concentrations in the upper water column exceed atmospheric equilibrium and that N₂ and Ar are supersaturated throughout the water column. The mean supersaturation of N₂ was found to be 2.0 (±0.37) and Ar was 3.8 (±1.1). The ratios of N₂/Ar (20.3 ±3.8), and O₂/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 N₂ 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 μg 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 O₂, N₂, and Ar at levels similar to levels found in Lake Hoare.