Large-scale climatic forcing is impacting oceanic biogeochemical cycles and is expected to influence the watercolumn 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 toPublished by Copernicus Publications on behalf of the European Geosciences Union. 5892 S. T. Wilson et al.: An intercomparison of oceanic methane and nitrous oxide measurements 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.
The amount of primary production fueled by upwelled “new” nitrate can be used to estimate the amount of organic carbon available for export to the deep ocean. Nitrate production in the euphotic zone from the microbial process of nitrification affects these estimates, yet the controls on nitrification in the upper ocean are debated. This study examines how seasonal cycles in primary production influence rates of nitrification fueled by both ammonia and urea‐derived nitrogen (N), and how these processes relate to the distribution of the greenhouse gas nitrous oxide (N2O) using monthly rate measurements from the San Pedro Ocean Time‐series (SPOT) station. Nitrification rates were highest at the onset of upwelling and were correlated with depth‐integrated primary production in the lower euphotic zone. Similar ammonia and urea‐derived N oxidation rates suggest urea is a significant N source fueling nitrification, particularly below the euphotic zone. Nitrification supplied a large proportion of phytoplankton N demand in the lower euphotic zone, implying significant regenerated production. The Southern California Bight was always a source of N2O to the atmosphere, which likely was advected into the system from the eastern tropical North Pacific, rather than produced locally from nitrification, and ventilated to the atmosphere during upwelling. Together, the results suggest the coupling of N remineralization and primary production in the upper ocean have important implications for the amount of organic carbon available for export out of the surface ocean, but that transport may dominate over local production in explaining local N2O dynamics.
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