Abstract
Abstract. Oxygen minimum zones (OMZs), such as those found in the eastern South Pacific (ESP), are the most important N2O sources in the global ocean relative to their volume. N2O production is related to low O2 concentrations and high primary productivity. However, when O2 is sufficiently low, canonical denitrification takes place and N2O consumption can be expected. N2O distribution in the ESP was analyzed over a wide latitudinal and longitudinal range (from 5° to 30° S and from 71–76° to ~ 84° W) based on ~ 890 N2O measurements. Intense N2O consumption, driving undersaturations as low as 40%, was always associated with secondary NO2– accumulation (SNM), a good indicator of suboxic/anoxic O2 levels. First, we explore relationships between ΔN2O and O2 based on existing data of denitrifying bacteria cultures and field observations. Given the uncertainties in the O2 measurements, a second relationship between ΔN2O and NO2– (> 0.75 μM) was established for suboxic waters (O2 < 8 μM). We reproduced the apparent N2O production (ΔN2O) along the OMZ in ESP with high reliability (r2 = 0.73 p = 0.01). Our results will contribute to the quantification of the N2O that is recycled in O2 deficient waters, and improve the prediction of N2O behavior under future scenarios of OMZ expansion and intensification.
Highlights
Nitrous oxide (N2O), a strong greenhouse gas and contributor to stratospheric ozone depletion, is produced in the oceans by archaeal and bacterial nitrification (Santoro et al, 2011)under a wide range of oxygen concentrations, including hypoxic and suboxic levels (Goreau et al, 1980; Frame and Casciotti, 2010)
When O2 is near zero or anoxia is found, N2O is consumed by canonical denitrification, producing N2
In Pseudomonas nautica cultures, NO−3 begins to be consumed at O2 < 125 μM, whereas N2O is consumed at O2
Summary
Nitrous oxide (N2O), a strong greenhouse gas and contributor to stratospheric ozone depletion, is produced in the oceans by archaeal and bacterial nitrification (Santoro et al, 2011). Consumption in the OMZ’s core is offset by high N2O production at the oxycline Many of these areas are associated with eastern boundary upwelling ecosystems, which are major sources of oceanic atmospheric N2O (Naqvi et al, 2010). Recent results suggest that the accumulation of more than 0.5 mmol kg−1 nitrite is a robust indicator of oxygen depletion (at least in nanomolar range, Thamdrup et al, 2012) This layer is subject to intense denitrification with N2O consumption exceeding its production (Codispoti et al, 1986). The models do not include consumption by denitrification at low O2 concentrations (< 8 μM) (Nevison et al, 2003) For this reason, the results of these model outputs are poorly fitted in areas such as OMZ’s cores of the Arabian Sea and eastern tropical North Pacific. We combine our results with previously reported equations for N2O production in the OMZ, when O2 concentrations are higher than 8 μM
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