Differences between coastal and open ocean distributions of N2O

Introduction Conclusions References


Introduction
Nitrous oxide (N 2 O) acts as a strong atmospheric greenhouse gas and contributes sub-20 stantially to the stratospheric ozone depletion (IPCC, 2013;WMO, 2011). The ocean is a major source for N 2 O as it is naturally produced in the water column (Ciais et al., 2013;Bange, 2008). While in large parts of the surface ocean N 2 O concentrations are close to saturation, high emissions of N 2 O have been observed in upwelling areas where subsurface waters enriched in N 2 O are transported to the surface (e.g. Nevison place. Recent studies furthermore indicate that they are also zones of intense nitrogen fixation (Deutsch et al., 2007;Löscher et al., 2014;Fernandez et al., 2011), and in areas where the OMZ is fuelled by high export production, high rates of other N transformation processes, such as nitrification, have been observed (Hu et al., 2015;Kalvelage et al., 2013).
Within the nitrogen cycle, N 2 O evolves during nitrification and denitrification (Bange, 2008). Both processes strongly depend on the oxygen availability in the water column, with different responses to the oxygen concentration. Under oxic conditions the first step of nitrification, ammonium-oxidation to nitrite, is known to be the main production pathway for N 2 O, with an increasing N 2 O yield at decreasing oxygen concentra- 15 tions (Goreau et al., 1980;Löscher et al., 2012;Frame and Casciotti, 2010). During bacterial ammonium-oxidation, N 2 O can either be produced as a side product during the oxidation of ammonia to nitrite or through the reduction of nitrite to N 2 O (nitrifierdenitrification) (Stein, 2011). Nitrifier-denitrification has been identified as an important production pathway of N 2 O at low oxygen concentrations and may thus be responsible 20 for the increased N 2 O production under these conditions (Ni et al., 2014).
While the N 2 O production pathways during bacterial nitrification have been studied for several decades, archaeal ammonium oxidation has only recently come into focus as a main production pathway for N 2 O. The exact mechanism and the extent to which ammonium oxidation or a nitrifier-denitrification pathway are responsible for archaeal 25 N 2 O production as well as the effect of environmental controls on archaeal N 2 O production are subject to ongoing research (Stieglmeier et al., 2014;Löscher et al., 2012;Santoro et al., 2011) During denitrification, the canonical reduction of nitrate to molecular nitrogen, N 2 O evolves as an intermediate product. Denitrifying genes are widespread among different groups of microorganisms, but active denitrification is restricted to suboxic to anoxic conditions (e.g. Firestone et al., 1980;Dalsgaard et al. 2014). Depending on the environmental conditions, N 2 O production or consumption due to denitrification can be 5 observed in environmental samples.
As nitrification is one major process accompanying the remineralization of organic matter, a positive correlation between the excess N 2 O (∆N 2 O) and the apparent oxygen utilization (AOU) is often interpreted as an indication for nitrification as the main N 2 O production pathway (e.g. Walter et al., 2006;Forster et al., 2009). An increase in the 10 ∆N 2 O/AOU ratio at low oxygen concentrations has been observed in several studies in different oceanic areas with reduced oxygen concentrations Upstill-Goddard et al., 1999;De Wilde and Helder, 1997), whereas a breakdown of this relationship due to N 2 O consumption is observed when oxygen concentrations fall below a certain, not well defined, threshold (Zamora et al., 2012). 15 There is a strong indication that at low oxygen concentrations nitrification and denitrification may take place in close proximity (Kalvelage et al., 2011), and the N 2 O production and consumption under these conditions are strongly influenced by the interaction of both processes. Stable isotope measurements of N 2 O in oxygen-deficient waters indicated that N 2 O accumulation within the oxycline as a result of the coupling between 20 nitrification and denitrification whereas N 2 O consumption in the OMZ core was associated with denitrification (Farías et al., 2007). The exact oxygen concentration where N 2 O consumption starts is not yet well determined, however (Cornejo and Farías, 2012;Zamora et al., 2012).
Measurements of denitrification and anammox rates in different oceanic OMZs have 25 raised the question whether denitrification or anammox is the main pathway for nitrogen loss in the water column (Hamersley et al., 2007;Ward et al., 2009;Voss and Montoya, 2009). In the ETSP, anammox has been found to play the major role in N loss, whereas denitrification was only rarely detectable (Kalvelage et al., 2013 (Thamdrup et al., 2006). As N 2 O is not supposed to be involved in the anammox process (Kartal et al., 2011), anammox does not influence the N 2 O distribution and only denitrification is thought to be responsible for N 2 O consumption at suboxic to anoxic conditions (Bange, 2008). The widespread N 2 O consumption in the OMZ core is thus an indicator for denitrification taking place in the ETSP (Farías et al., . One explanation for these contradicting findings is that denitrification is stimulated by the supply of organic carbon or hydrogen sulfide which could lead to only sporadically increased rates of denitrification (Chang et al., 2014;Dalsgaard et al., 2014;Galan et al., 2014).
Here we present N 2 O measurements in the water column off Peru from six measure-10 ment campaigns during the upwelling in the ETSP. This region is characterized by one of the largest and most intense OMZs in the oceans, extending from the Peruvian shelf about 1000 km offshore with a maximum thickness of more than 600 m (Fuenzalida et al., 2009). It is located in the shadow zone of large ocean current systems which leads to a sluggish ventilation and long residence times of waters within the OMZ. 15 Along the continental margin, high primary productivity due to coastal upwelling and high remineralization rates in the underlying waters lead to a further drawdown in oxygen concentrations (Karstensen et al., 2008). Active N loss can be observed in large parts of the OMZ which is reflected in a pronounced secondary nitrite maximum and a strong nitrogen deficit in the OMZ core (Codispoti et al., 1986). The OMZ extends 20 over large parts of the Peruvian shelf where sulfidic conditions within the water column are frequently observed (Schunck et al., 2013). These conditions are comparable to the West Indian shelf where exceptionally high N 2 O concentrations have been measured (Naqvi et al., 2000) which indicates favorable conditions for enhanced N 2 O production (Codispoti, 2010). Only a few measurements of N 2 O from the Peruvian OMZ are 25 available so far (Friederich et al., 1985;Nevison et al., 1995;Pierotti and Rasmussen, 1980), however. N 2 O measurements from the OMZ off Chile indicated the potential for high N 2 O production and emissions due to the proximity of the OMZ to coastal up- welling taking place in this area (Charpentier et al., 2007;Castro-Gonzalez and Farías, 2004).

Methods
In total, 146 depth profiles ( Samples were treated with 0.2 mL (M77-3 and M77-4) and 0.05 mL (M90-M93) of a saturated mercuric chloride solution directly after the sampling to inhibit microbial 20 N 2 O production or consumption. The samples were either analyzed onboard (M77-3 and M77-4, M91, partly M90 and M93) within a few days or shipped to GEOMAR by air freight for later analysis (M92, partly M90 and M93). Samples that were shipped to Germany were additionally sealed with paraffin wax and stored upside down to avoid the formation of air bubbles in the samples due to temperature and pressure changes 25 during transportation. Samples were analyzed using a static equilibration method: 10 mL helium (99.9999 % AirLiquide, Düsseldorf, Germany) was manually injected into each vial which was vigorously shaken for about 20 s and allowed to equilibrate at ambient temperature for a minimum of two hours. A subsample of the equilibrated headspace was manually injected into a GC-ECD system (Hewlett-Packard 5890 Series II, Agi-5 lent Technologies, Santa Clara, CA, USA), equipped with a 6 1/8 packed column (molsieve, 5Å, W. R. Grace and Co.-Conn., Columbia, MY). The GC was operated at 190 • C, using argon/methane (95/5 %, ECD purity, AirLiquide, Düsseldorf, Germany) as carrier gas at a flow rate of 30 mL min −1 .
The GC was calibrated on a daily basis with a minimum of 2 (M77-3 and M77-4) or (1) We used the contemporary atmospheric mixing ratio measured at Cape Grim, Tasma Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | mospheric mixing ratio since the beginning of the industrial revolution (Freing et al., 2009). The use of the contemporary N 2 O mixing ratio of 2013 would lead to a maximum ∼ 17 % overestimate of [N 2 O] eq , thus leading to only a small error compared to the maximum N 2 O concentrations measured in our study, and the use of the contemporary atmospheric mixing ratio still allows a qualitative analysis of the ∆N 2 O/AOU 5 relationship in order to investigate the formation and consumption processes of N 2 O. The potential temperature of the water parcel at a certain depth was calculated using the Gibbs Seawater Oceanographic Toolbox (McDougall and Barker, 2011).
Oxygen concentrations were measured either with a Seabird (M77-3 and M77-4: SBE-5; M90-M93: SBE 43) oxygen sensor (Sea-Bird Electronics, Bellevue, WA, USA) 10 mounted on the CTD rosette or from 100 mL discrete samples taken from the Niskin bottles and analyzed using the Winkler titration method (Grasshoff et al., 1999). The oxygen sensor was calibrated against the Winkler measurements.
Recent studies using highly sensitive STOX (Switchable Trace amount Oxygen) sensors for oxygen measurements indicate that measurements with conventional oxygen 15 sensors that are calibrated against Winkler measurements may be biased towards higher concentrations at near-zero oxygen conditions. Thamdrup et al. (2012) therefore argued that anoxic conditions are prevalent in the core of the Peruvian OMZ where oxygen concentrations of several µM have been found using the conventional Winkler-calibrated measurements. As STOX sensor measurements were not available 20 for all measurement campaigns presented here, the minimum oxygen measurements reported here from the core of the OMZ (3-5 µM) should be considered as an overestimation.
The Nutrient samples from the CTD rosette were analyzed onboard following the nutrient analysis methods according to Hansen et al. (1999). Samples taken from the pump-CTD during M77-3 were stored at −20 • C and shipped to Germany for later analysis.
N was calculated as a measure for the nitrogen deficit from the nitrate ([NO  :

Results and discussion
The oxygen profiles revealed an intense oxygen minimum zone throughout the studied area, with a vertical thickness of several hundreds of meters. In the open ocean, the oxygen concentrations in the core of the OMZ increased towards the north from be-10 low 3 µM south of 5 • S to ∼ 10 µM at the equator. South of 13 • S the mixed layer depth significantly increased from ∼ 50 to ∼ 100 m, which is reflected in the oxygen and N 2 O distributions (Fig. 2). Due to the coastal upwelling, the depth of the upper OMZ boundary significantly decreased towards the coast, with a well oxygenated mixed layer of ∼ 50 m in the open ocean and a mixed layer depth of less than 5 m on the shelf. Oxygen 15 was strongly undersaturated in the surface waters on the shelf as a result of upwelling of waters from the underlying OMZ. Elevated phosphate concentrations in the surface waters at the coast also reflected the upwelling on the shelf (Fig. 3).
The vertical profiles showed characteristic nutrient distributions that marked the zones of nitrogen depletion: accumulation of nitrite was observed in the core of the 20 OMZ where oxygen concentrations fell below ∼ 5 µM and low N values coincided with the nitrite maxima in the OMZ https://www.sfb754.de/. The maximum nitrite concentration reached ∼ 13 µM, with a more pronounced maximum at the shelf than in open ocean waters. Additionally, many profiles showed an additional, less pronounced primary nitrite maximum within the upper oxycline that is associated with nitrifica-Introduction  (Codispoti and Christensen, 1985) (Fig. 4). At the shelf, strong signals of N loss throughout the water column are reflected in low N and nitrate values (Fig. 3). The N 2 O depth distribution showed a strong sensitivity to oxygen concentrations throughout the study area. In the offshore waters, surface N 2 O concentrations were close to saturation, with a strong increase below the mixed layer (Fig. 2). Two types 5 of depth profiles could be identified: a broad N 2 O maximum at the depth of the oxygen minimum was found at the northern and southern periphery of the oxygen minimum zone where the minimum oxygen concentrations did not fall below 5 µM. In contrast, N 2 O depletion was found in the core of the OMZ, where oxygen concentrations below 5 µM were observed over a wide depth range. The N 2 O depth profiles in the 10 central OMZ thus revealed a double-peak structure with narrow N 2 O maxima in the upper and lower oxycline (Fig. 4). This depth profile structure has been frequently observed in other oceanic areas with highly depleted oxygen concentrations (e.g. . N 2 O depletion coincided with nitrite accumulation in the OMZ core and high nitrate to phosphate ratios. In all offshore profiles N 2 O concentrations did not ex-15 ceed 80 nM. A large difference between offshore stations and the stations on and in proximity to the shelf could be observed: compared to the offshore waters, the N 2 O distribution at the shelf break and on the shelf showed a much larger variability. N 2 O depletion was in fact observed at oxygen concentrations below 5 µM, too, but N 2 O accumulation 20 with N 2 O concentrations above 100 nM was frequently observed, and several profiles showed an extreme N 2 O accumulation with concentrations up to ∼ 850 nM (Fig. 4). The location and shape of the N 2 O maxima in the different profiles was highly variable, which resulted in a very patchy distribution of N 2 O in the water column over the shelf and in the adjacent waters (Fig. 3). A characteristic shape of the profiles could not 25 be identified: profiles with a subsurface N 2 O maximum in the oxycline were observed as well as profiles with multiple maxima or a surface N 2 O maximum (Fig. 4). These surface maxima are not necessarily a signal for surface production of N 2 O as has been proposed before (e.g. Zamora and Oschlies, 2014)  upper oxycline for waters with oxygen concentrations higher than 5 µM during the M77-4 cruise that took place in the offshore waters of the OMZ . We found a very similar relationship for all data from the offshore cruises M77-4 and M90 with no systematic difference between the data from the M77-4 (January/February 2009) cruise and the M90 (November 2012) cruise (Figs. 2 and 5a). This 10 indicates a comparable setting of the open ocean OMZ waters during both cruises. We furthermore found no difference in the ∆N 2 O/AOU relationship between stations with a broad N 2 O maximum and a double-peak structure.
In contrast to the open ocean waters, a correlation between ∆N 2 O and AOU was not observed for the data from cruises that took place at the shelf (M77-3, M91-M93) 15 ( Fig. 5b). The ∆N 2 O/AOU ratio from the offshore waters serves as a lower limit for the coastal stations, where numerous values with much higher ∆N 2 O/AOU ratios were observed. The ∆N 2 O values that showed the strongest deviation from the offshore ∆N 2 O/AOU ratio were associated with low N values as a signal for a large nitrogen deficit (Fig. 5b). This indicates that these waters with extreme N 2 O accumulation had 20 been subject to extensive N loss. The high oxygen concentrations within these waters excludes in-situ denitrification or anammox, however (see e.g. Babbin et al., 2014;Dalsgaard et al., 2014), and the extraordinarily high N 2 O concentrations as well as the low N values seem to be old signals that were preserved during re-ventilation.
Extreme accumulation of N 2 O with concentrations up to 765 nM in the oceanic water 25 column has also been found in the Arabian Sea where oxygen depleted waters extend to the West Indian shelf (Naqvi et al., 2010(Naqvi et al., , 2006 (Naqvi et al., 2010). Farías et al. (2015) measured N 2 O accumulation during the transition from oxic to anoxic conditions, too, but at variable oxygen concentrations whereas N 2 O depletion was dominant under suboxic conditions. In 5 contrast to the results from the Indian Ocean, they identified enhanced remineralization due to short-term variability in coastal upwelling as the main driver for N 2 O accumulation. In our study, N 2 O accumulation did not coincide with the accumulation of nitrite. We found N 2 O depletion in samples that showed marked nitrite accumulation (Fig. 4) and 10 similar to the measurements off Chile, we found strongly elevated N 2 O concentrations (> 100 nM) over the full range of oxygen concentrations (Fig. 5), whereas N 2 O accumulation on the Indian shelf was restricted to suboxic conditions.
Our results indicate that N 2 O accumulation took place during increasing oxygen concentrations: the large N deficits we measured may be the result of earlier N loss and 15 subsequent re-ventilation of these waters. This re-ventilation could have also induced an abrupt increase in N 2 O production and subsequent N 2 O accumulation in the water column. The accumulated N 2 O could have remained preserved in the water column when these waters were further ventilated as there is no known consumption process for N 2 O in oxygenated waters (Bange, 2008). This effect would disturb the 20 linear ∆N 2 O/AOU relationship over a wide range of oxygen concentrations, whereas decreasing oxygen concentrations would lead only to a temporal accumulation with subsequent depletion of N 2 O, similar to the observations over the Indian shelf.
Enhanced production of N 2 O after re-ventilation is a known process occurring in soils (e.g. Morley et al., 2008). In a recent incubation study, Dalsgaard et al. (2014) found 25 no indication for an increased N 2 O production by denitrification due to changes in the oxygen concentration at nanomolar levels, however. Instead, autotrophic denitrification and N 2 O production have been shown to be stimulated by the addition of hydrogen sulfide (H 2 S) (Galan et al., 2014;Dalsgaard et al., 2014) depleted in all samples that contained H 2 S and although we found extreme N 2 O accumulation at the boundary of H 2 S-containing waters at some stations (Löscher et al., 2015), similarly high concentrations were present at stations without any signals of H 2 S, too. We cannot exclude that the high N 2 O production we frequently observed at the shelf is stimulated by a coupling of denitrification with sulfur cycling, though: Canfield et al. (2010) found evidence for active sulfur cycling in the ETSP without H 2 S accumulation, and a potential coupling between H 2 S oxidation and denitrification has been demonstrated before (Galan et al., 2014;Jensen et al., 2009). Indeed, active denitrification was only found in proximity to H 2 S plumes in the water column during M77-3 (Kalvelage et al., 2013), where the highest N 2 O concentrations during this cruise were found, too (Löscher et al., 2015). In the ocean, increased N 2 O production was also associated with the onset of nitrification after re-ventilation of the water column in a seasonal study in the Baltic Sea, but at relatively low concentrations (Naqvi et al., 2010). Yu et al. (2010) found increased N 2 O production by nitrifying bacteria that was stimulated by the availability of ammo-15 nium during recovery from anoxic conditions in a chemostat culture experiment. Their results point towards an increased N 2 O production via the ammonium-oxidation pathway, while N 2 O production by nitrifier-denitrification seemed not to be stimulated by the shift from anoxic to oxic conditions. Studies of the isotopic and isotopomeric N 2 O composition could reveal more detailed insights whether N 2 O is produced via the am-20 monium oxidation or the nitrite reduction pathway during extreme accumulation.
Together with the high N 2 O concentrations we found low N values that were associated with oxygenated waters only at the shelf, whereas in the open ocean N depletion was restricted to the OMZ core. This could indicate that re-ventilation of the oxygen-deficient waters mainly happens at the shelf, whereas waters in the open ocean 25 OMZ are less affected by mixing processes. Strong diapycnal and isopycnal mixing has indeed been observed on the shelf has indeed been reported from the Peruvian and Mauritanian upwelling region (Schafstall et al., 2010;Thomsen et al., 2015). The upwelling-induced high primary production in the surface ocean furthermore fuels rapid BGD 12,2015  oxygen consumption in the underlying waters due to the export and remineralization of organic matter, thereby creating strong small-scale variability in oxygen concentrations. Kalvelage et al. (2013) showed that these high remineralization rates also induce strong N cycling in the subsurface layer. Turnover rates for different N species are therefore much faster on the shelf than in the open ocean OMZ (Hu et al., 2015), which is 5 also reflected in the distribution of different functional gene abundances (Löscher et al., 2014). One factor that also contributes to the N 2 O accumulation on the shelf could thus be generally higher rates of nitrification and/or denitrification on the shelf than in the open ocean. 10 We observed extreme N 2 O accumulations over the Peruvian shelf with maximum concentrations similar to the observations made by Naqvi et al. (2000) and a large variability in the oxygen concentrations together with elevated nitrification and denitrification rates on the shelf. We found strong evidence that N 2 O accumulations are preserved during the ventilation of the water column and that waters with high N 2 O concentrations are directly and frequently transported to the surface ocean. This makes this region one of the most im-20 portant oceanic regions for N 2 O emissions to the atmosphere (Arévalo-Martínez et al., 2015). This direct link between unusually high N 2 O production and emissions over the Peruvian shelf makes it necessary to understand the biogeochemical processes involved in N 2 O production and consumption to produce reliable predictions of oceanic emissions from this area. Current approaches to model the N 2 O distribution rely on 25 parameterizations based on the linear ∆N 2 O/AOU relationship (Suntharalingam and Sarmiento, 2000;Nevison et al., 2003;Freing et al., 2012). These approaches could BGD 12,2015   itrification -factors controlling its biological production, Science, 208, 749-751, doi:10.1126Science, 208, 749-751, doi:10. /science.208.4445.749, 1980. Forster, G., Upstill-Goddard, R. C., Gist, N., Robinson, C., Uher, G., and Woodward, E. M. S.: