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

Abstract

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

Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific oceans

“…This is in line with the results of two model studies of N 2 O in the ETSP by Zamora et al (2012) and Cornejo and Farias (2012), which suggested that the switching point between N 2 O production and N 2 O consumption occurs at higher O 2 concentration (∼ 8-10 µmol kg −1 ) than previously thought.…”

“…In general, we might expect enhanced production of N 2 O, CH 4 and DMS in OMZs because of the ongoing loss of O 2 . Deoxygenation in open-ocean and coastal environments may lead, on the one hand, to enhanced N 2 O production when approaching the N 2 O production-consumption switching point (see above), but, on the other hand, when O 2 concentrations fall below the switching point this may lead to a consumption of N 2 O (Zamora et al, 2012). Moreover, we do not know whether the frequency of coastal anoxic events will continue to increase and how this may affect the coastal net N 2 O production/consumption.…”

Technical Note: A simple method for air–sea gas exchange measurements in mesocosms and its application in carbon budgeting

“…However, even N 2 O production at relatively high surface layer nitrification rates, as found in upwelling regions (Rees et al, 2011), are orders of magnitudes too low to significantly bias the large fluxes caused by deliberate N 2 O addition. The only known pathway of biological N 2 O uptake as a reactive nitrogen species is by denitrifiers at anoxic conditions (<∼ 10 µmol O 2 kg −1 ) (Zumft, 1997;Zamora et al, 2012). Conditions favouring this process are unlikely to form in mesocosms and would result in a loss of the tracer (as N 2 ) but not into utilisation of N 2 O as a nitrogen source.…”

Coupled physical/biogeochemical modeling including O&lt;sub&gt;2&lt;/sub&gt;-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela

“…Recently, Zamora et al (2012) suggested that N 2 O production increases linearly with decreasing oxygen, rather than exponentially, in the eastern tropical Pacific, based on MEMENTO database. This fundamental difference in N 2 O production parameterization introduces a large uncertainty in biogeochemical models, especially in EBUS and associated OMZs (Zamora et al, 2012), even more in a context of expansion and intensification of the OMZs (Stramma et al, 2008;Deutsch et al, 2010).…”