On the Nitrous Oxide Accumulation in Intermediate Waters of the Eastern South Pacific Ocean
Frontiers in Marine Science
Nitrous oxide (N 2 O) is a powerful greenhouse gas principally produced by nitrification and denitrification in the marine environment. Observations were made in the eastern South Pacific (ESP), between 10 • and 60 • S, and ∼75 • -88 • W, from intermediate waters targeting Antarctic Intermediate Water (AAIW) at potential density of 27.0-27.1 kg m −3 . Between 60 • and 20 • S, a gradual equatorward increase of N 2 O from 8 to 26 nmol L −1 was observed at density 27.0-27.1 kg m −3 where AAIW
... −3 where AAIW penetrates. Positive correlations were found between apparent N 2 O production ( N 2 O) and O 2 utilization (AOU), and between N 2 O and NO − , which suggested that local N 2 O production is predominantly 3 produced by nitrification. Closer to the equator, between 20 • and 10 • S at AAIW core, a strong N O increase up to 75 nmol L −1 2 was observed. Because negative correlations were found between N 2 O vs. NO − and N 2 O vs. N * (a Nitrogen deficit index) and 3 because N 2 O and AOU do not follow a linear trend, we suspect that, in addition to nitrification, denitrification also takes place in N 2 O cycling. By making use of water mass mixing analyses, we show that an increase in N 2 O occurs in the region where high oxygen from AAIW merges with low oxygen from Equatorial Subsurface Water (ESSW), creating favorable conditions for local N 2 O production. We conclude that the non-linearity in the relationship between N 2 O and O 2 is a result of mixing between two water masses with very different source characteristics, paired with the different time frames of nitrification and denitrification processes that impact water masses en route before they finally meet and mix in the ESP region.