Supersaturated N2O in a perennially ice‐covered Antarctic lake: Molecular and stable isotopic evidence for a biogeochemical relict

Abstract

The east lobe of Lake Bonney, a permanently ice‐covered lake in the McMurdo Dry Valleys, Antarctica, has a mid‐depth maximum N2O concentration of 43.3 [mol N L‐1 (>700,000% saturation with respect to air), representing one of the highest concentrations reported for a natural aquatic system. δ15N and δ18O measurements indicate that this is the most isotopically depleted N2O yet observed in a natural environment (minimum δ15N‐N2O of ‐79.6‰ vs. air‐N2; minimum δ18O‐N2O of ‐4.7‰ vs. Vienna standard mean ocean water), providing new end points for these parameters in natural systems. The extremely depleted nitrogen and oxygen isotopes, together with nitrogen isotopic isomer data for N2O, imply that most of the N2O was produced via incomplete nitrification and has undergone virtually no subsequent consumption. However, molecular evidence provides little support for metabolically active nitrifying populations at depths where the maximal N2O concentrations occur and contemporary biogeochemical reactions cannot explain the extreme excesses of N2O in Lake Bonney. The gas appears to be a legacy of past biogeochemical conditions within the lake, and in the absence of a significant sink and the presence of a highly stable water column, gradients in N2O produced by past microbial activity could persist in the cold saline waters of Lake Bonney for >104 years.