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.