Nitrous oxide measurements in the eastern tropical Pacific Ocean

In March 1976 the first N₂O measurements were made in the sea water and marine air of the eastern tropical Pacific Ocean during a cruise from San Diego, California, to San Martin, Peru. Continuous measurements of N₂O in the marine atmosphere were made throughout the duration of the cruise. The tropospheric mixing ratio was 332 ± 9 p.p.b. v/v N₂O. The concentration of N₂O in the marine air appeared to show a direct relationship to the N₂O concentration in the surface sea water, with the highest N₂O mixing ratios over regions which were highly supersaturated, off the coasts of Baja California and Peru, and in the equatorial upwelling region. The surface sea water samples were generally supersaturated with N₂O, with an average saturation of 145% relative to the marine atmosphere. The average flux of N₂O from the surface sea water into the atmosphere was 0.5 x 10-12 g N₂O cm^-2 s^-1. Only one surface sea water sample showed significant undersaturation; the other 28 surface water samples were approximately saturated or supersaturated. Water samples were also collected down to depths of 300 m during seven hydrocast stations. The stations showed two distinct distribution patterns for N₂O concentration versus depth for the region between the surface and 300 m. At five of the seven stations the N₂O concentration increased rapidly from the surface to a depth of 100 m, and then remained relatively constant down to the maximum depth of 300 m. At the other two stations the N₂O concentration showed the same rapid increase between the surface and 75–100 m, but then the N₂O concentration decreased in the next 50–75 m until the concentration of N₂O at 150 m was lower than that found at the surface. At these two stations the water from 150 m to 300 m was just saturated or undersaturated with N₂O with respect to the atmosphere. These two stations were located in the oxygen-deficient region off the coast of Mexico, which suggests that the low N₂O concentrations below the thermocline may be due to denitrification. However, stations in the oxygen-deficient region off the coast of Peru showed considerable N₂O supersaturation at all depths. Recent results indicate that the role of N₂O in the nitrogen cycle of the ocean may be more complex than has previously been suggested.