Résumé
During January–March 1993, the vertical distributions of dissolved nitrous oxide (N
2O) and methane (CH
4), 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 N
2O and CH
4 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 ΔN
2O=8% and ΔCH
4=32%), with significantly higher values close to the continents. The N
2O vertical distributions on both sections display a broad maximum centred near 400
m depth, at the level of the oxygen minimum. The CH
4 vertical distributions also show a sharp subsurface maximum at the base of the mixed layer, associated with a chlorophyll
a maximum. Computed N
2O and CH
4 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 N
2O, 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 N
2O budget is balanced within the experimental uncertainties. This is not the case for CH
4, whose vertical transport from the maximum concentration layer accounts for <10% of the sea–air flux. It follows that the CH
4 budget requires a sustained production within the mixed layer itself to maintain CH
4 supersaturation and balance its escape to the atmosphere.