Nitrous oxide (N2O) 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 10º and 60°S, and ~75° to 88°W, from intermediate waters targeting Antarctic Intermediate Water (AAIW) at potential density of 27.0-27.1 kg m-3. Between 60° to 20°S, a gradual equatorward increase of N2O from 8 to 26 nmol L-1 was observed at density 27.0-27.1 kg m-3 where AAIW penetrates. Positive correlations were found between apparent N2O production (∆N2O) and O2 utilization (AOU), and between ∆N2O and NO3-, which suggested that local N2O production is predominantly produced by nitrification. Closer to the equator, between 20° and 10°S at AAIW core, a strong N2O increase up to 75 nmol L-1 was observed. Because negative correlations were found between ∆N2O vs. NO3- and ∆N2O vs. N* (a Nitrogen deficit index) and because ∆N2O and AOU do not follow a linear trend, we suspect that, in addition to nitrification, denitrification also takes place in N2O cycling. By making use of water mass mixing analyses, we show that an increase in N2O occurs in the region where high oxygen from AAIW merges with low oxygen from Equatorial Subsurface Water (ESSW), creating favorable conditions for local N2O production. We conclude that the non-linearity in the relationship between N2O and O2 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.

Seven oceanographic cruises which include the vertical distribution of Nitrous Oxide, Oxygen, Silicate, Nitrate, Nitrite, Phosphate, Temperature, Salinity and estimated variable such as Density and Potential Density, are shared. The cruises cover the South Eastern Pacific Ocean between -9.5 to -60 °S and -70.7 to -91.4 °W and from surface up 2000 meters depth. Nitrous Oxide was analyzed in 20 ml vials with headspace by Gas Chromatography, while the Temperature, Salinity, Pressure and Dissolved Oxygen were determinate using a calibrated CTD-O probe. Nutrients were filtered with 0.45 micras pore size glass filter and determine by colorimetry.Water samples were collected using Niskin bottles attached to rosette sampler, in order to obtain discrete measurements of dissolver oxygen (O2) and nutrients (NO3, NO2, SiO4 and PO43). Discrete samples of DO (in triplicate) were analyzed using the AULOX measurement system, an automatic Winkler method. Samples for all nutrients (15 mL in triplicate) were filtered (using a 0.45 μm GF/F glass filter) and stored (frozen) until analysis, using standard colorimetric techniques (Grasshoff, 1983). N2O samples were taken in triplicate in 20 mL vials and carefully sealed to avoid air bubbles. They were then preserved with 50 μL of saturated HgCl2 and stored in darkness until analysis. N2O was analyzed by creating a 5 mL headspace of ultrapure Helium (He) and then equilibrated within the vial, and measured by gas chromatography (Shimadzu 17A) using an electron capture detector (ECD). The calibration curves were made previous to each measurement with five points using pure He, 0.1, 0.5, and 1 of N2O standards and dry air. The ECD detector linearly responded to this concentration range and the analytical error for N2O measurements was ~3%. The uncertainty of the measurements was calculated from the standard deviation of the triplicate measurements by depth. Samples with a variation coefficient above 10% were not considered in the N2O database.

Supplement to: Carrasco, Cristina; Karstensen, Johannes; Farias, Laura (2017): On the Nitrous Oxide Accumulation in Intermediate Waters of the Eastern South Pacific Ocean. Frontiers in Marine Science, 4