Ventilation versus biology: What is the controlling mechanism of nitrous oxide distribution in the North Atlantic?

Abstract

The extent to which water mass mixing and ocean ventilation contribute to nitrous oxide (N2O) distribution at the scale of oceanic basins is poorly constrained. We used novel N2O and chlorofluorocarbon measurements along with multiparameter water mass analysis to evaluate the impact of water mass mixing and Atlantic Meridional Overturning Circulation (AMOC) on N2O distribution along the Observatoire de la variabilité interannuelle et décennale en Atlantique Nord (OVIDE) section, extending from Portugal to Greenland. The biological N2O production has a stronger impact on the observed N2O concentrations in the water masses traveling northward in the upper limb of the AMOC than those in recently ventilated cold water masses in the lower limb, where N2O concentrations reflect the colder temperatures. The high N2O tongue, with concentrations as high as 16 nmol kg−1, propagates above the isopycnal surface delimiting the upper and lower AMOC limbs, which extends from the eastern North Atlantic Basin to the Iceland Basin and coincides with the maximum N2O production rates. Water mixing and basin‐scale remineralization account for 72% of variation in the observed distribution of N2O. The mixing‐corrected stoichiometric ratio N2O:O2 for the North Atlantic Basin of 0.06 nmol/μmol is in agreement with ratios of N2O:O2 for local N2O anomalies, suggesting than up to 28% of N2O production occurs in the temperate and subpolar Atlantic, an overlooked region for N2O cycling. Overall, our results highlight the importance of taking into account mixing, O2 undersaturation when water masses are formed and the increasing atmospheric N2O concentrations when parameterizing N2O:O2 and biological N2O production in the global oceans.

An intercomparison of oceanic methane and nitrous oxide measurements

“…Such intercomparison exercises are critical to determining the spatial and temporal variability of methane and nitrous oxide across the world oceans with confidence, since no single laboratory can single-handedly provide all the required measurements at sufficient resolution. Previous comparative exercises have been conducted for other trace gases, e.g., carbon dioxide, dimethylsulfide, and sulfur hexafluoride (Dickson et al, 2007;Bullister and Tanhua, 2010;Swan et al, 2014), and for trace elements (Cutter, 2013). These exercises confirm the value of the intercomparison concept.…”

“…In addition to the self-assessments provided by the analysis of air-equilibrated seawater, this study revealed the need for reference seawater to help assess the accuracy of highconcentration methane and nitrous oxide measurements. Reference seawater in this instance refers to batches of dissolved methane and nitrous oxide samples prepared in the laboratory using an equilibrator setup, as used for dissolved inorganic carbon (Dickson et al, 2007). In the absence of plans for additional intercomparison exercises, the provision of reference seawater will allow laboratories to continue evaluating their own measurements.…”

“…Consequently, there is generally a lower inherent uncertainty in its surface ocean saturation state, as previously proposed (Law and Ling, 2001;Forster et al, 2009). The interlaboratory differences highlighted by this study should be viewed in the context of numerous individual efforts to assess temporal and/or spatial trends in methane and nitrous oxide by way of time series observations Farías et al, 2015;Wilson et al, 2017;Fenwick and Tortell, 2018), repeat hydrographic survey lines (de la Paz et al, 2017), and single expeditions. While the value of these in integrating the behavior of methane and nitrous oxide into the hydrography and biogeochemistry of localregional ecosystems is beyond question, their value would be enhanced by the rigorous cross-validation of analytical protocols.…”

“…Some laboratories employ purge-and-trap methods for extracting and concentrating the gases prior to their analysis (e.g., Zhang et al, 2004;Bullister and Wisegarver, 2008;Capelle et al, 2015;Wilson et al, 2017). Others equilibrate a seawater sample with an overlying headspace gas and inject a fixed volume of the gaseous phase into a gas analyzer (e.g., Upstill-Goddard et al, 1996;Walter et al, 2005;Farías et al, 2009). The purge-and-trap technique is typically more sensitive by 1-2 orders of magnitude over headspace equilibrium (Magen et al, 2014;Wilson et al, 2017).…”

Water mass distributions and transports for the 2014 GEOVIDE cruise in the North Atlantic

“…The separation between biological and mixing components allows a better quantification of the oxygen consumption (de la Paz et al, 2017), and can be a valuable tool when interpreting tracer distributions such as the TEIs. 25…”