Nitrous oxide dynamics in low oxygen regions of the Pacific: Insights from the MEMENTO database

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Abstract

<p><strong>Abstract.</strong> The eastern tropical Pacific (ETP) is believed to be one of the largest marine sources of the greenhouse gas nitrous oxide (N<sub>2</sub>O). Future N<sub>2</sub>O emissions from the ETP are highly uncertain because oxygen minimum zones are expected to expand, affecting both regional production and consumption of N<sub>2</sub>O. Here we assess three primary uncertainties in how N<sub>2</sub>O may respond to changing O<sub>2</sub> levels: (1) the relationship between N<sub>2</sub>O production and O<sub>2</sub> (is it linear or exponential at low O<sub>2</sub> concentrations?), (2) the cutoff point at which net N<sub>2</sub>O production switches to net N<sub>2</sub>O consumption (uncertainties in this parameterisation can lead to differences in model ETP N<sub>2</sub>O concentrations of more than 20%), and (3) the rate of net N<sub>2</sub>O consumption at low O<sub>2</sub>. Based on the MEMENTO database, which is the largest N<sub>2</sub>O dataset currently available, we find that N<sub>2</sub>O production in the ETP increases linearly rather than exponentially with decreasing O<sub>2</sub>. Additionally, net N<sub>2</sub>O consumption switches to net N<sub>2</sub>O production at ~ 10 μM O<sub>2</sub>, a value in line with recent studies that suggest consumption occurs on a larger scale than previously thought. N<sub>2</sub>O consumption is on the order of 0.01–1 mmol N<sub>2</sub>O m<sup>−3</sup> yr<sup>−1</sup> in the Peru-Chile Undercurrent. Based on these findings, it appears that recent studies substantially overestimated N<sub>2</sub>O production in the ETP. In light of expected deoxygenation and the higher than previously expected point at which net N<sub>2</sub>O production switches to consumption, there is enough uncertainty in future N<sub>2</sub>O production that even the sign of future changes is still unclear.</p>

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Zamora, L. M., Oschlies, A., Bange, H. W., Huebert, K. B., Craig, J. D., Kock, A., & Löscher, C. R. (2012). Nitrous oxide dynamics in low oxygen regions of the Pacific: Insights from the MEMENTO database. Biogeosciences, 9(12), 5007–5022. https://doi.org/10.5194/bg-9-5007-2012

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