Abiotic nitrous oxide emission from the hypersaline Don Juan Pond in Antarctica

Nitrous oxide is a potent atmospheric greenhouse gas1 that contributes to ozone destruction2. Biological processes such as nitrification and denitrification are thought to drive nitrous oxide production in soils, which comprise the largest source of nitrous oxide to the atmosphere1. Here we present measurements of the concentration and isotopic composition of nitrous oxide in soil pore spaces in samples taken near Don Juan Pond, a metabolically dormant hypersaline pond in Southern Victoria Land, Antarctica in 2006, 2007 and 2008, together with in situ fluxes of nitrous oxide from the soil to the atmosphere. We find fluxes of nitrous oxide that rival those measured in fertilized tropical soils3. Laboratory experiments —in which nitrite-rich brine was reacted with a variety of minerals containing Fe(II)—reveal a new mechanism of abiotic water–rock reaction that could support nitrous oxide fluxes at Don Juan Pond. Our findings illustrate a dynamic and unexpected link between the geosphere and atmosphere. Don Juan Pond (DJP), a shallow hypersaline playa lake located in the south fork of the Wright Valley (Southern Victoria Land) Antarctica (Supplementary Fig. S1), contains a groundwaterderived calcium-chloride-rich eutectic brine4 (413 g CaCl2 and 29 gNaCl kg−1; up to 671h or 40.2% salt) and is the most saline water body on Earth. Despite extremely low temperatures (Supplementary Fig. S2), the brine has no significant ice cover, even during winter, and is the only ice-free body of water in the Antarctic McMurdo Dry Valleys. DJP is bounded to the north and south by steep mountains of Beacon sandstone through which cut numerous intrusive dykes of igneous Ferrar dolerite. Ferrar dolerite contains pyroxenes (augite, pigeonite), phyllosilicates (biotite) and feldspars (plagioclase, labradorite, alkaline feldspars), is up to nine per cent weight iron(ii) (ref. 5) and has a similar composition to basaltic rocks onMars6. The geological setting of theWright Valley, subzero temperatures and brine geochemistry make DJP an ideal Mars analogue environment4. In November of 2006, 2007 and 2008, we measured the isotopic composition and concentration of N2O in the soil pore space (aqueous and gas phases) near the DJP shore (see the Methods section) and determined in situ soil–atmosphere N2O fluxes. Soil pore space N2O concentrations varied from 1.16 to 2.03 μmol l−1 of bulk soil (approximately 3.9 to 6.8 μmol l−1 of pore fluid assuming 30% porosity), and were up to 500 times higher than expected for soils in equilibrium with atmospheric N2O. Soil– atmosphere N2O fluxes (0.7 to 55 μmolm−2 h−1) varied inversely with water table height (Supplementary Table S1). DJP N2O fluxes were comparable to those in urea-fertilized tropical soils