Description: As a major source for atmospheric CO2, the Peruvian upwelling region exhibits strong variability in surface fCO2 on short spatial and temporal scales. Understanding the physical processes driving the strong variability is of fundamental importance for constraining the effect of marine emissions from upwelling regions on the global CO2 budget. In this study, a frontal decay on length scales of 𝒪(10 km) was observed off the Peruvian coast following a pronounced decrease in down-frontal (equatorward) wind speed with a time lag of 9 h. Simultaneously, the sea-to-air flux of CO2 on the inshore (cold) side of the front dropped from up to 80 to 10 mmol m−2 day−1, while the offshore (warm) side of the front was constantly outgassing at a rate of 10–20 mmol m−2 day−1. Based on repeated ship transects the decay of the front was observed to occur in two phases. The first phase was characterized by a development of coherent surface temperature anomalies which gained in amplitude over 6–9 h. The second phase was characterized by a disappearance of the surface temperature front within 6 h. Submesoscale mixed-layer instabilities were present but seem too slow to completely remove the temperature gradient in this short time period. Dynamics such as a pressure-driven gravity current appear to be a likely mechanism behind the evolution of the front.

Description: Large-scale climatic forcing is impacting oceanic biogeochemical cycles and is expected to influence the water-column distribution of trace gases, including methane and nitrous oxide. Our ability as a scientific community to evaluate changes in the water-column inventories of methane and nitrous oxide depends largely on our capacity to obtain robust and accurate concentration measurements that can be validated across different laboratory groups. This study represents the first formal international intercomparison of oceanic methane and nitrous oxide measurements whereby participating laboratories received batches of seawater samples from the subtropical Pacific Ocean and the Baltic Sea. Additionally, compressed gas standards from the same calibration scale were distributed to the majority of participating laboratories to improve the analytical accuracy of the gas measurements. The computations used by each laboratory to derive the dissolved gas concentrations were also evaluated for inconsistencies (e.g., pressure and temperature corrections, solubility constants). The results from the intercomparison and intercalibration provided invaluable insights into methane and nitrous oxide measurements. It was observed that analyses of seawater samples with the lowest concentrations of methane and nitrous oxide had the lowest precisions. In comparison, while the analytical precision for samples with the highest concentrations of trace gases was better, the variability between the different laboratories was higher: 36% for methane and 27% for nitrous oxide. In addition, the comparison of different batches of seawater samples with methane and nitrous oxide concentrations that ranged over an order of magnitude revealed the ramifications of different calibration procedures for each trace gas. Finally, this study builds upon the intercomparison results to develop recommendations for improving oceanic methane and nitrous oxide measurements, with the aim of precluding future analytical discrepancies between laboratories.

Description: A new system for continuous, highly-resolved oceanic and atmospheric measurements of N2O, CO and CO2 is described. The system is based upon off-axis integrated cavity output spectroscopy (OA-ICOS) and a non-dispersive infrared analyzer (NDIR) both coupled to a Weiss-type equilibrator. Performance of the combined setup was evaluated by testing its precision, accuracy, long-term stability, linearity and response time. Furthermore, the setup was tested during two oceanographic campaigns in the equatorial Atlantic Ocean in order to explore its potential for autonomous deployment onboard voluntary observing ships (VOS). Improved equilibrator response times for N2O (2.5 min) and CO (45 min) were achieved in comparison to response times from similar chamber designs used by previous studies. High stability of the OA-ICOS analyzer was demonstrated by low optimal integration times of 2 and 4 min for N2O and CO respectively, as well as detection limits of 〈 40 ppt and precision better than 0.3 ppb Hz−1/2. Results from a direct comparison of the method presented here and well-established discrete methods for oceanic N2O and CO2 measurements showed very good consistency. The favorable agreement between underway atmospheric N2O, CO and CO2 measurements and monthly means at Ascension Island (7.96°S 14.4°W) further suggests a reliable operation of the underway setup in the field. The potential of the system as an improved platform for measurements of trace gases was explored by using continuous N2O and CO2 data to characterize the development of the seasonal equatorial upwelling in the Atlantic Ocean during two RV/ Maria S. Merian cruises. A similar record of high-resolution CO measurements was simultaneously obtained offering for the first time the possibility of a comprehensive view on the distribution and emissions of these climate relevant gases on the area. The relatively simple underway N2O/CO/CO2 setup is suitable for long-term deployment on board of research and commercial vessels although potential sources of drift such as cavity temperature and further technical improvements towards automation still need to be addressed.

Description: The climate active trace-gas carbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere. A missing source in its atmospheric budget is currently suggested, resulting from an upward revision of the vegetation sink. Tropical oceanic emissions have been proposed to close the resulting gap in the atmospheric budget. We present a bottom-up approach including (i) new observations of OCS in surface waters of the tropical Atlantic, Pacific and Indian oceans and (ii) a further improved global box model to show that direct OCS emissions are unlikely to account for the missing source. The box model suggests an undersaturation of the surface water with respect to OCS integrated over the entire tropical ocean area and, further, global annual direct emissions of OCS well below that suggested by top-down estimates. In addition, we discuss the potential of indirect emission from CS2 and dimethylsulfide (DMS) to account for the gap in the atmospheric budget. This bottom-up estimate of oceanic emissions has implications for using OCS as a proxy for global terrestrial CO2 uptake, which is currently impeded by the inadequate quantification of atmospheric OCS sources and sinks.

Description: Recent observations in the eastern tropical South Pacific (ETSP) have shown the key role of meso- and submesoscale processes (e.g. eddies) in shaping its hydrographic and biogeochemical properties. Off Peru, elevated primary production from coastal upwelling in combination with sluggish ventilation of subsurface waters fuels a prominent oxygen minimum zone (OMZ). Given that nitrous oxide (N2O) production–consumption processes in the water column are sensitive to oxygen (O2) concentrations, the ETSP is a region of particular interest to investigate its source–sink dynamics. To date, no detailed surveys linking mesoscale processes and N2O distributions as well as their relevance to nitrogen (N) cycling are available. In this study, we present the first measurements of N2O across three mesoscale eddies (two mode water or anticyclonic and one cyclonic) which were identified, tracked, and sampled during two surveys carried out in the ETSP in November–December 2012. A two-peak structure was observed for N2O, wherein the two maxima coincide with the upper and lower boundaries of the OMZ, indicating active nitrification and partial denitrification. This was further supported by the abundances of the key gene for nitrification, ammonium monooxygenase (amoA), and the gene marker for N2O production during denitrification, nitrite reductase (nirS). Conversely, we found strong N2O depletion in the core of the OMZ (O2 〈 5 μmol/L) to be consistent with nitrite (NO2-) accumulation and low levels of nitrate (NO3-), thus suggesting active denitrification. N2O depletion within the OMZ’s core was substantially higher in the centre of mode water eddies, supporting the view that eddy activity enhances N-loss processes off Peru, in particular near the shelf break where nutrient-rich, productive waters from upwelling are trapped before being transported offshore. Analysis of eddies during their propagation towards the open ocean showed that, in general, “ageing” of mesoscale eddies tends to decrease N2O concentrations through the water column in response to the reduced supply of material to fuel N loss, although the hydrographic variability might also significantly impact the pace of the production–consumption pathways for N2O. Our results evidence the relevance of mode water eddies for N2O distribution, thereby improving our understanding of the N-cycling processes, which are of crucial importance in times of climate change and ocean deoxygenation.

Description: In the current era of rapid climate change, accurate characterization of climate-relevant gas dynamics-namely production, consumption, and net emissions-is required for all biomes, especially those ecosystems most susceptible to the impact of change. Marine environments include regions that act as net sources or sinks for numerous climateactive trace gases including methane (CH4) and nitrous oxide (N2O). The temporal and spatial distributions of CH4 and N2O are controlled by the interaction of complex biogeochemical and physical processes. To evaluate and quantify how these mechanisms affect marine CH4 and N2O cycling requires a combination of traditional scientific disciplines including oceanography, microbiology, and numerical modeling. Fundamental to these efforts is ensuring that the datasets produced by independent scientists are comparable and interoperable. Equally critical is transparent communication within the research community about the technical improvements required to increase our collective understanding of marine CH4 and N2O. A workshop sponsored by Ocean Carbon and Biogeochemistry (OCB) was organized to enhance dialogue and collaborations pertaining to marine CH4 and N2O. Here, we summarize the outcomes from the workshop to describe the challenges and opportunities for near-future CH4 and N2O research in the marine environment.

Description: Oxygen-deficient zones (ODZs) are major sites of net natural nitrous oxide (N2O) production and emissions. In order to understand changes in the magnitude of N2O production in response to global change, knowledge on the individual contributions of the major microbial pathways (nitrification and denitrification) to N2O production and their regulation is needed. In the ODZ in the coastal area off Peru, the sensitivity of N2O production to oxygen and organic matter was investigated using 15N tracer experiments in combination with quantitative PCR (qPCR) and microarray analysis of total and active functional genes targeting archaeal amoA and nirS as marker genes for nitrification and denitrification, respectively. Denitrification was responsible for the highest N2O production with a mean of 8.7 nmol L−1 d−1 but up to 118±27.8 nmol L−1 d−1 just below the oxic–anoxic interface. The highest N2O production from ammonium oxidation (AO) of 0.16±0.003 nmol L−1 d−1 occurred in the upper oxycline at O2 concentrations of 10–30 µmol L−1 which coincided with the highest archaeal amoA transcripts/genes. Hybrid N2O formation (i.e., N2O with one N atom from NH+4 and the other from other substrates such as NO−2) was the dominant species, comprising 70 %–85 % of total produced N2O from NH+4, regardless of the ammonium oxidation rate or O2 concentrations. Oxygen responses of N2O production varied with substrate, but production and yields were generally highest below 10 µmol L−1 O2. Particulate organic matter additions increased N2O production by denitrification up to 5-fold, suggesting increased N2O production during times of high particulate organic matter export. High N2O yields of 2.1 % from AO were measured, but the overall contribution by AO to N2O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important N2O production process in low-oxygen conditions fueled by organic carbon supply, which implies a positive feedback of the total oceanic N2O sources in response to increasing oceanic deoxygenation.

Description: Carbon monoxide (CO) influences the radiative budget and oxidative capacity of the atmosphere over the Arctic Ocean, which is a source of atmospheric CO. Yet, oceanic CO cycling is understudied in this area, particularly in view of the ongoing rapid environmental changes. We present results from incubation experiments conducted in the Fram Strait in August/September 2019 under different environmental conditions: While lower pH did not affect CO production (GPCO) or consumption (kCO) rates, enhanced GPCO and kCO were positively correlated with coloured dissolved organic matter (CDOM) and dissolved nitrate concentrations, respectively, suggesting microbial CO uptake under oligotrophic conditions to be a driving factor for variability in CO surface concentrations. Both production and consumption of CO will likely increase in the future, but it is unknown which process will dominate. Our results will help to improve models predicting future CO concentrations and emissions and their effects on the radiative budget and the oxidative capacity of the Arctic atmosphere.

Description: The coastal upwelling regime off Peru in December 2012 showed considerable vertical concentration gradients of dissolved nitrous oxide (N2O) across the top few meters of the ocean. The gradients were predominantly downward, i.e., concentrations decreased toward the surface. Ignoring these gradients causes a systematic error in regionally integrated gas exchange estimates, when using observed concentrations at several meters below the surface as input for bulk flux parameterizations – as is routinely practiced. Here we propose that multi-day near-surface stratification events are responsible for the observed near-surface N2O gradients, and that the gradients induce the strongest bias in gas exchange estimates at winds of about 3 to 6 m s−1. Glider hydrographic time series reveal that events of multi-day near-surface stratification are a common feature in the study region. In the same way as shorter events of near-surface stratification (e.g., the diurnal warm layer cycle), they preferentially exist under calm to moderate wind conditions, suppress turbulent mixing, and thus lead to isolation of the top layer from the waters below (surface trapping). Our observational data in combination with a simple gas-transfer model of the surface trapping mechanism show that multi-day near-surface stratification can produce near-surface N2O gradients comparable to observations. They further indicate that N2O gradients created by diurnal or shorter stratification cycles are weaker and do not substantially impact bulk emission estimates. Quantitatively, we estimate that the integrated bias for the entire Peruvian upwelling region in December 2012 represents an overestimation of the total N2O emission by about a third, if concentrations at 5 or 10 m depth are used as surrogate for bulk water N2O concentration. Locally, gradients exist which would lead to emission rates overestimated by a factor of two or more. As the Peruvian upwelling region is an N2O source of global importance, and other strong N2O source regions could tend to develop multi-day near-surface stratification as well, the bias resulting from multi-day near-surface stratification may also impact global oceanic N2O emission estimates.

Description: The open ocean is a major source of nitrous oxide (N2O), an atmospheric trace gas attributable to global warming and ozone depletion. Intense sea-to-air N2O fluxes occur in major oceanic upwelling regions such as the eastern tropical South Pacific (ETSP). The ETSP is influenced by the El Niño–Southern Oscillation that leads to inter-annual variations in physical, chemical, and biological properties in the water column. In October 2015, a strong El Niño event was developing in the ETSP; we conduct field observations to investigate (1) the N2O production pathways and associated biogeochemical properties and (2) the effects of El Niño on water column N2O distributions and fluxes using data from previous non-El Niño years. Analysis of N2O natural abundance isotopomers suggested that nitrification and partial denitrification (nitrate and nitrite reduction to N2O) were occurring in the near-surface waters; indicating that both pathways contributed to N2O effluxes. Higher-than-normal sea surface temperatures were associated with a deepening of the oxycline and the oxygen minimum layer. Within the shelf region, surface N2O supersaturation was nearly an order of magnitude lower than that of non-El Niño years. Therefore, a significant reduction of N2O efflux (75 %–95 %) in the ETSP occurred during the 2015 El Niño. At both offshore and coastal stations, the N2O concentration profiles during El Niño showed moderate N2O concentration gradients, and the peak N2O concentrations occurred at deeper depths during El Niño years; this was likely the result of suppressed upwelling retaining N2O in subsurface waters. At multiple stations, water-column inventories of N2O within the top 1000 m were up to 160 % higher than those measured in non-El Niño years, indicating that subsurface N2O during El Niño could be a reservoir for intense N2O effluxes when normal upwelling is resumed after El Niño.