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 N2O concentration of 43.3 mmol N L21 (.700,000% saturation with respect to air), representing one of the highest concentrations reported for a natural aquatic system. d15N and d18O measurements indicate that this is the most isotopically depleted N2O yet observed in a natural environment (minimum dN-N2O of 279.6% vs. air-N2; minimum dO-N2O of 24.7% 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. Lake Bonney, located within the McMurdo Dry Valleys, Antarctica, provides a novel aquatic system to study biogeochemical transformations of water column constituents. The ice cover eliminates wind-induced turbulence, and together with low advective stream input and strong chemical stratification, vertical mixing within the liquid water column is restricted to molecular diffusion (Spigel and Priscu 1998). Under these conditions, geochemical gradients can persist for .50,000 yr (Priscu et al. 1996) and the deep saline waters are estimated to be .105 yr old (Poreda et al. 2004). The lake has two permanently icecovered lobes (east lobe, ELB; west lobe, WLB) and extensive data sets on the physical, chemical, and biological properties of ELB and WLB from 1990–present have been 1 Corresponding author ([email protected]).