Nitrous oxide flux and response to increased iron availability in the Antarctic Circumpolar Current

Nitrous oxide (N2O) profiles were obtained at stations inside and outside a n area of iron-fertilised surface water at 61 degreesS 140 degreesE during the Southern Ocean Iron Enrichment Experiment (SOIREE). Surface N-2(O) satu ration and air-sea flux during SOIREE (98-103%; - 1.18-1.75 mu mol/m(2)/d) were consistent with that obtained between 58 degreesS 158 degreesE and 49 degreesS 162 degreesE (99-104%; - 0.3-4.7 mu mol/m(2)/d), and confirmed pre dicted flux estimates for this region. Turbulent eddy diffusion across the pycnocline supplied an average 38% of the air-sea N2O flux, indicating a pr oduction mechanism in the upper 80 m. There was no significant difference i n N2O saturation and flux between stations inside and outside the patch, al though a N2O saturation maximum in the pycnocline at most stations inside t he iron-fertilised patch was not present at stations outside. The mean N2O profile for the stations outside the patch was used as a control to identif y pycnocline N2O production, which increased during SOIREE and co-varied wi th iron-mediated increases in phytoplankton biomass. The mechanisms for iro n-mediated N2O production in the pycnocline are considered. On longer times cales, the decrease in radiative forcing resulting from carbon fixation and CO, uptake during SOIREE may be subsequently offset by 6-12% by N2O produc tion. Furthermore, analysis of scenarios of large-scale Southern Ocean fert ilisation supports previous observations that any decrease in radiative for cing due to CO2 drawdown may be partially or totally negated by an increase in N2O production. (C) 2001 Elsevier Science Ltd. All rights reserved.