The AAIW is formed along the front of the Antarctic Circumpolar Current ( Sverdrup et al., 1942 McCartney, 1977 Talley, 1999) and spreads equatorward along the lower boundary of the thermocline, in a core layer with a potential density (σ θ) of about 27.0–27.1 kg m −3 (AAIWσ θ). The intermediate depths of the eastern South Pacific (ESP) are occupied by two water masses that move in opposite directions and mix as they converge, i.e., Equatorial Subsurface Water (ESSW) and Antarctic Intermediate Water (AAIW). Very limited information is available regarding N 2O production in intermediate waters, approximately between 5 m depth, ( Suntharalingam and Sarmiento, 2000 Nevison et al., 2003 Freing et al., 2012), with several of these studies carried out in the eastern tropical North Pacific and the Arabian Sea ( Naqvi and Noronha, 1991 Bange et al., 2005 Yamagishi et al., 2007 Fujii et al., 2013). In contrast, denitrification, the process by which NO 3 − is reduced to produce N 2 gas as final end product and N 2O as an intermediate, only occurs at O 2levels close to anoxia ( Dalsgaard et al., 2014). In addition, in a process known as nitrifier denitrification, autotrophic NH 4 + oxidizing bacteria are able to produce N 2O as a final product of NO 2 − reduction ( Poth and Focht, 1985). An increased N 2O yield is triggered by lower concentrations of O 2 ( Goreau et al., 1980) and previous studies have shown that significant quantities of N 2O are produced by Bacteria and Archaea at O 2 concentrations below 5 μmol L −1 ( Frame and Casciotti, 2010 Santoro et al., 2011 Löscher et al., 2012). N 2O production by nitrification i.e., the aerobic oxidation of ammonium ( NH 4 +) to nitrite ( NO 2 −), and then to nitrate ( NO 3 −), is controlled by O 2 concentration. The formation/maintenance of OMZs is predominantly determined due to slow ventilation while high aerobic respiration of particulate organic matter (POM) is probably of second importance ( Karstensen et al., 2008). Coastal upwelling systems with high primary production (PP) rates meet both these conditions for enhanced N 2O production ( Nevison et al., 2004) and are closely linked to Oxygen Minimum Zones (OMZs). Both processes depend on organic matter availability and oxygen (O 2) levels, but whereas the former occurs under a wide range of oxygen conditions, the latter takes place only at suboxic and anoxic levels ( Codispoti et al., 2001 Bange, 2008). Nitrification and denitrification are the principal processes involved in N 2O production. We conclude that the non-linearity in the relationship between N 2O 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.Īpproximately 90% of the total marine nitrous oxide (N 2O) inventory occurs between depths just below the pycnocline down to ~1000 m ( Nevison et al., 1995, 2003 Suntharalingam and Sarmiento, 2000). By making use of water mass mixing analyses, we show that an increase in N 2O occurs in the region where high oxygen from AAIW merges with low oxygen from Equatorial Subsurface Water (ESSW), creating favorable conditions for local N 2O production. N* (a Nitrogen deficit index) and because ΔN 2O and AOU do not follow a linear trend, we suspect that, in addition to nitrification, denitrification also takes place in N 2O cycling. Because negative correlations were found between ΔN 2O vs. Closer to the equator, between 20° and 10°S at AAIW core, a strong N 2O increase up to 75 nmol L −1 was observed. Positive correlations were found between apparent N 2O production (ΔN 2O) and O 2 utilization (AOU), and between ΔN 2O and NO 3 −, which suggested that local N 2O production is predominantly produced by nitrification. Between 60° and 20°S, a gradual equatorward increase of N 2O from 8 to 26 nmol L −1 was observed at density 27.0–27.1 kg m −3 where AAIW penetrates. 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. Nitrous oxide (N 2O) is a powerful greenhouse gas principally produced by nitrification and denitrification in the marine environment. 3Department of Oceanography, University of Concepcion and Center for Climate Resilience and Research, Concepción, Chile.2GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany.1Programa de Postgrados en Oceanografía, Departamento de Oceanografía, Universidad de Concepción, Concepción, Chile.Cristina Carrasco 1, Johannes Karstensen 2 and Laura Farias 1,2 *
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