☆ 4.7 Article

Quantifying Nitrous Oxide Cycling Regimes in the Eastern Tropical North Pacific Ocean With Isotopomer Analysis

GLOBAL BIOGEOCHEMICAL CYCLES (2021)

Journal

GLOBAL BIOGEOCHEMICAL CYCLES

Volume 35, Issue 2, Pages -

Publisher

AMER GEOPHYSICAL UNION

DOI: 10.1029/2020GB006637

Keywords

ETNP; isotopes; N2O; nitrogen; ocean biogeochemistry; oxygen deficient zone

Funding

U.S.-NSF [OCE-1657868]
NSF Graduate Research Fellowship

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Abstract

By analyzing N2O concentration and isotopologues in the eastern tropical North Pacific (ETNP), it was found that high N2O levels observed at some stations may be due to unique conditions supporting high rates of N2O production. Additionally, the study revealed different sources and potential mechanisms contributing to N2O cycling in oxygen deficient zones.

Nitrous oxide (N2O), a potent greenhouse gas, is produced disproportionately in marine oxygen deficient zones (ODZs). To quantify spatiotemporal variation in N2O cycling in an ODZ, we analyzed N2O concentration and isotopologues along a transect through the eastern tropical North Pacific (ETNP). At several stations along this transect, N2O concentrations reached a near surface maximum that exceeded prior measurements in this region, of up to 226.1 +/- 20.5 nM at the coast. Above the sigma(theta) = 25.0 kg/m(3) isopycnal, Keeling plot analysis revealed two sources to the near-surface N2O maximum, with different delta(N2O alpha)-N-15 and delta(N2O beta)-N-15 values, but each with a site preference (SP) of 6 parts per thousand-8 parts per thousand. Given expected SPs for nitrification and denitrification, each of these sources could be comprised of 17%-26% nitrification (bacterial or archeal), and 74%-83% denitrification (or nitrifier-denitrification). Below the sigma(theta) = 25.0 kg/m(3) isopycnal, box model analysis indicated that the observed 46 parts per thousand-50 parts per thousand SPs in the anoxic core of the ODZ cannot be reproduced in a steady state context without an SP for N2O production by denitrification, and may indicate instead a transient net consumption of N2O. Furthermore, time-dependent model results indicated that while delta(N2O alpha)-N-15 and delta O-18-N2O reflect both N2O production and consumption in the anoxic core of the ODZ, delta(N2O beta)-N-15 predominantly reflects N2O sources. Finally, we infer that the high (N2O) observed at some stations derive from a set of conditions supporting high rates of N2O production that have not been previously encountered in this region.

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Re-assessing the influence of particle-hosted sulphide precipitation on the marine cadmium cycle

Gregory F. de Souza, Derek Vance, Matthias Sieber, Tim M. Conway, Susan H. Little

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GEOCHIMICA ET COSMOCHIMICA ACTA (2022)