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 (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 O-2 levels: (1) the relationship between N2O production and O-2 (is it linear or exponential at low O-2 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 O-2. 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 O-2. Additionally, net N2O consumption switches to net N2O production at similar to 10 mu MO2, 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 Under-current. 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.