THE BIOGEOCHEMISTRY OF NITROUS-OXIDE IN PERMANENTLY ICE-COVERED LAKESOF THE MCMURDO-DRY-VALLEYS, ANTARCTICA

This manuscript presents an overview of published work on nitrous oxid e in the permanently ice-covered lakes of the McMurdo Dry Valleys, Ant arctica One of these lakes contains the highest concentration of nitro us oxide reported for natural aquatic systems (> 500 000% with respect to the global average mixing ratio in air). Recent data on nitrous ox ide from the major lakes in this region of Antarctica are used to draw general conclusions regarding sources and sinks for this gas within t he liquid water column, and to estimate exchanges with the atmosphere. Nitrous oxide maxima are usually found in regions where oxygen concen trations and redox potentials are decreasing (i.e. where high gradient s exist); nitrous oxide is virtually absent in anoxic, and very low re dox zones. These trends, together with positive relationships between apparent oxygen utilization (AOU) and apparent nitrous oxide productio n (ANP) indicate that nitrous oxide is primarily a product of nitrific ation; experiments showed that denitrification is a sink for this gas in anoxic water. ANP/AOU ratios are several orders of magnitude higher than that for the ocean. Yield ratios for nitrous oxide [ANP/(NO2- NO3-)] averaged 4.2% (i.e. 1 atom of N appears in nitrous oxide for ev ery 24 atoms appearing in oxidized N), greatly exceeding existing repo rts for pelagic systems, being similar to that from reduced sediments. Production and consumption rates, computed with a one-dimensional dif fusion model, ranged from 0 to 5.3 nM-N d(-1) and 0-2.7 nM-N d(-1), re spectively. Rates were usually greatest in the region of largest oxyge n and inorganic nitrogen gradients. Turnover times averaged 2917 and 1 277 years for production and consumption which is in the range of the mixing times for the lakes. Areal flux from the lakes to the atmospher e (6.17 gN m(-2) y(-1)) is several hundred times greater than areal fl uxes reported for oceanic systems. Owing to the relatively small combi ned surface area of these lakes, absolute atmospheric transfer (1.2 x 10(5) gN y(-1)) is only a small fraction of annual global emission.