Transatlantic equatorial distribution of nitrous oxide and methane

During January–March 1993, the vertical distributions of dissolved nitrous oxide (N2O) and methane (CH4), and their atmospheric mixing ratios were measured in the equatorial Atlantic on the WOCE lines A6 (7°30′N) and A7 (4°30′S). Measured mean N2O and CH4 atmospheric mixing ratios were 0.316 and 1.786 ppm respectively, with an analytical precision of 1%. Surface waters were everywhere supersaturated with both gases (mean excess ΔN2O=8% and ΔCH4=32%), with significantly higher values close to the continents. The N2O vertical distributions on both sections display a broad maximum centred near 400 m depth, at the level of the oxygen minimum. The CH4 vertical distributions also show a sharp subsurface maximum at the base of the mixed layer, associated with a chlorophyll a maximum. Computed N2O and CH4 sea–air fluxes are in the range 1.1–1.8 μmol m−2 d−1 and 1.2–2.0 μmol m−2 d−1 respectively. For N2O, we calculate that the upward transport through the thermocline matches the horizontal transport in the mixed layer and the emission to the atmosphere. Hence, the N2O budget is balanced within the experimental uncertainties. This is not the case for CH4, whose vertical transport from the maximum concentration layer accounts for <10% of the sea–air flux. It follows that the CH4 budget requires a sustained production within the mixed layer itself to maintain CH4 supersaturation and balance its escape to the atmosphere.