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

Abstract

The eastern tropical Pacific (ETP) is believed to be one of the largest marine sources of the greenhouse gas nitrous oxide (N₂O). Future N₂O emissions from the ETP are highly uncertain because oxygen minimum zones are expected to expand, affecting both regional production and consumption of N₂O. Here we assess three primary uncertainties in how N₂O may respond to changing O₂ levels: (1) the relationship between N₂O production and O₂ (is it linear or exponential at low O₂ concentrations?), (2) the cutoff point at which net N₂O production switches to net N₂O consumption (uncertainties in this parameterisation can lead to differences in model ETP N₂O concentrations of more than 20%), and (3) the rate of net N₂O consumption at low O₂. Based on the MEMENTO database, which is the largest N₂O dataset currently available, we find that N₂O production in the ETP increases linearly rather than exponentially with decreasing O₂. Additionally, net N₂O consumption switches to net N₂O production at ~ 10 μM O₂, a value in line with recent studies that suggest consumption occurs on a larger scale than previously thought. N₂O consumption is on the order of 0.01–1 mmol N₂O m⁻³ yr⁻¹ in the Peru-Chile Undercurrent. Based on these findings, it appears that recent studies substantially overestimated N₂O production in the ETP. In light of expected deoxygenation and the higher than previously expected point at which net N₂O production switches to consumption, there is enough uncertainty in future N₂O production that even the sign of future changes is still unclear.