Nitrous oxide in the oxygen minimum of the eastern tropical North Pacific: evidence for its consumption during denitrification and possible mechanisms for its production

Abstract

Investigation of the nitrous oxide (N2O), oxygen, and nutrient distributions in the eastern tropical North Pacific Ocean (ETNP) indicated biochemical N2O consumption around the core of the oxygen minimum where denitrification takes place. Subsurface N2O minima were found at stations where intermediate waters contained near-zero oxygen concentrations and well-developed secondary nitrite maxima, characteristic of denitrification. The N2O minima were absent at stations where there was little or no evidence for denitrification.Surface waters of the ETNP were on the average 110% saturated with N2O relative to a marine air content of 287 ppbv N2O. The area appears to be a source for atmospheric N2O with an average magnitude of 0.14 pg N2O cm−2s−1.Except for the oxygen-deficient layer where the N2O minima were found, N2O is highly supersaturated at all stations from just below the mixed layer to about 1000 m. Supersaturations varied from 224 to 600% in shallow waters above the base of the pycnocline and from 120 to 250% in deep waters below 1000 m. Possible mechanisms for biochemical N2O production in the ocean were examined and it is concluded that in the ETNP, nitrification is the most important.