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