AUTHOR=Bourbonnais Annie , Chang Bonnie X. , Sonnerup Rolf E. , Doney Scott C. , Altabet Mark A. 

TITLE=Marine N2O cycling from high spatial resolution concentration, stable isotopic and isotopomer measurements along a meridional transect in the eastern Pacific Ocean

JOURNAL=Frontiers in Marine Science

VOLUME=10

YEAR=2023

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2023.1137064

DOI=10.3389/fmars.2023.1137064

ISSN=2296-7745

ABSTRACT=<p>Nitrous oxide (N<sub>2</sub>O) is a potent greenhouse gas and ozone depleting substance, with the ocean accounting for about one third of global emissions. In marine environments, a significant amount of N<sub>2</sub>O is produced by biological processes in Oxygen Deficient Zones (ODZs). While recent technological advances are making surface N<sub>2</sub>O concentration more available, high temporal and spatial resolution water-column N<sub>2</sub>O concentration data are relatively scarce, limiting global N<sub>2</sub>O ocean models’ predictive capability. We present a N<sub>2</sub>O concentration, stable isotopic composition and isotopomer dataset of unprecedently large spatial coverage and depth resolution in the broader Pacific, crossing both the eastern tropical South and North Pacific Ocean ODZs collected as part of the GO-SHIP P18 repeat hydrography program in 2016/2017. We complement these data with dissolved gases (nitrogen, oxygen, argon) and nitrate isotope data to investigate the pathways controlling N<sub>2</sub>O production in relation to apparent oxygen utilization and fixed nitrogen loss. N<sub>2</sub>O yield significantly increased under low oxygen conditions near the ODZs. Keeling plot analysis revealed different N<sub>2</sub>O sources above the ODZs under different oxygen regimes. Our stable isotopic data and relationships between the N<sub>2</sub>O added by microbial processes (ΔN<sub>2</sub>O) and dissolved inorganic nitrogen (DIN) deficit confirm increased N<sub>2</sub>O production by denitrification under low oxygen conditions near the oxycline where the largest N<sub>2</sub>O accumulations were observed. The slope for δ<sup>18</sup>O-N<sub>2</sub>O versus site preference (SP, the difference between the central (α) and outer (β) N atoms in the linear N<sub>2</sub>O molecule) in the eastern tropical North Pacific ODZ was lower than expected for pure N<sub>2</sub>O reduction, likely because of the observed decrease in δ<sup>15</sup>N<sup>β</sup>. This trend is consistent with prior ODZ studies and attributed to concurrent production of N<sub>2</sub>O from nitrite with a low δ<sup>15</sup>N or denitrification with a SP &gt;0‰. We estimated apparent isotope effects for N<sub>2</sub>O consumption in the ETNP ODZ of 3.6‰ for <sup>15</sup>N<sup>bulk</sup>, 9.4‰ for <sup>15</sup>N<sup>α</sup>, -2.3‰ for <sup>15</sup>N<sup>β</sup>, 12.0‰ for <sup>18</sup>O, and 11.7‰ for SP. These values were generally within ranges previously reported for previous laboratory and field experiments.</p>