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

The east lobe of Lake Bonney, a permanently ice‐covered lake in the McMurdo Dry Valleys, Antarctica, has a mid‐depth maximum N 2 O 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. δ 15 N and δ 18 O measurements indicate that this is the most isotopically depleted N 2 O yet observed in a natural environment (minimum δ 15 N‐N 2 O of ‐79.6‰ vs. air‐N 2 ; minimum δ 18 O‐N 2 O 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 N 2 O, imply that most of the N 2 O 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 N 2 O concentrations occur and contemporary biogeochemical reactions cannot explain the extreme excesses of N 2 O 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 N 2 O produced by past microbial activity could persist in the cold saline waters of Lake Bonney for >10 4 years.