An N isotopic mass balance of the Eastern Tropical North Pacific oxygen deficient zone

Abstract

Oxygen deficient zones host up to 50% of marine N2 production and the Eastern Tropical North Pacific (ETNP) is the largest marine oxygen deficient zone. We measured δ15N-NO3-, δ15N-NO2-, and δ15N-N2 at 7 stations along a transect normal to the coast in the heart of the ETNP oxygen deficient zone in 2012. The δ15N-N2 minimum was 0.34‰ at 300 m, which corresponded with the N2:Ar maximum. When the atmospheric N2 background was removed, the biological δ15N-N2 for the ODZ ranged from −7‰ to −22‰. In the ODZ, δ15N-NO3- ranged from 15 to 24‰ while δ15N-NO2- was generally between − 11 and −18‰, generating differences up to 40‰ between δ15N-NO3- and δ15N-NO2-. The isotopic separation between nitrite and nitrogen gas (Δ15N NO2-N2) changed sign from ~5‰ at the top of the oxygen deficient zone to ~−10‰ at 300 m, indicating an important shift in nitrogen cycling with depth. We calculated the closed system Rayleigh isotope effect (ε) for N2 production from both the δ15N-DIN (εDIN = 26 ± 11‰) and δ15N-N2 (εN2 = 27 ± 6‰) data. When examined individually by depth, both εDIN and εN2 matched closely and both ε depth profiles showed maximal fractionation at 300 m. Additionally, closed system isotope effects were calculated for one offshore station from the Arabian Sea in 2007 using δ15N-N2 ( εN2 = 24 ± 4‰) and δ15N-DIN (εDIN = 26 ± 3‰). The relatively large isotope effects for N2 production appear to be found in both major offshore oxygen deficient zones, which implies a large denitrification term in the marine N budget.