Intercomparison exercise

During 2013-2018, a series of interlaboratory comparisons of oceanic methane and nitrous oxide measurements was conducted.  The work was conducted as Working Group #143 of the Scientific Commission on Ocean Research (SCOR).  Discrete seawater samples were collected from the North Pacific Ocean and the Baltic Sea, and distributed to twenty participating laboratories.  Seawater collections from different locations and varying depths provided a range of methane and nitrous concentrations with which variability between laboratories was assessed.  Several prevalent trends were evident in both methane and nitrous oxide measurements for both low and high concentrations of the dissolved gases.  A full description of the intercomparison work, including recommendations and next steps, is being prepared for publication.  In addition to the intercomparison exercise and to facilitate methane and nitrous oxide measurements in the future, compressed gas standards were distributed to twelve laboratories globally.  Each laboratory received a near-atmosphere standard and an elevated standard with which to calibrate their personal gas standards. The workshop will provide an update on ongoing efforts to improve and standardize methane and nitrous oxide measurements including the production of reference material.  In addition to discrete water-column measurements, we will also discuss other relevant analytical techniques.

See: Wilson, S. T., Bange, H. W., Arévalo-Martínez, D. L., Barnes, J., Borges, A. V., Brown, I., Bullister, J. L., Burgos, M., Capelle, D. W., Casso, M., de la Paz, M., Farías, L., Fenwick, L., Ferrón, S., Garcia, G., Glockzin, M., Karl, D. M., Kock, A., Laperriere, S., Law, C. S., Manning, C. C., Marriner, A., Myllykangas, J.-P., Pohlman, J. W., Rees, A. P., Santoro, A. E., Tortell, P. D., Upstill-Goddard, R. C., Wisegarver, D. P., Zhang, G.-L., and Rehder, G.: An intercomparison of oceanic methane and nitrous oxide measurements, Biogeosciences, 15, 5891-5907, https://doi.org/10.5194/bg-15-5891-2018, 2018.

Nitrous oxide

Nitrous oxide is a by-product of microbial metabolism. In the majority of the global oceans, the metabolic activity of specific microorganisms cause nitrous oxide concentrations to exceed the values that would otherwise occur from gaseous exchange with the overlying atmosphere.  The two major microbial pathways that produce nitrous oxide, nitrification and denitrification, are sensitive to the concentration of dissolved oxygen.  A strong relationship between nitrous oxide and oxygen is observed with increasing production of nitrous oxide with decreasing oxygen concentrations.  This occurs until a threshold value of oxygen is reached at which point net microbial consumption of nitrous oxide occurs as the final steps of denitrification are favored.  The balance between net production and net consumption of nitrous oxide and the overall relationship with dissolved oxygen is not well defined at present.  Furthermore, the correlation between nitrous oxide and oxygen varies depending on the microorganisms present, nutrient concentrations, and other environmental variables.  Predictions about the effect of ocean deoxygenation on nitrogen cycling indicate that water-column inventories of nitrous oxide might decrease.  However due to the uncertainty associated with our understanding of the relevant processes, there is low confidence in these predictions at present.  The workshop will specifically focus on our comprehension, analytical capabilities, and modeling capacity of nitrous oxide production and consumption processes in relation to oxygen and other environmental variables. Predictions about the effect of ocean deoxygenation on nitrogen cycling indicate that water-column inventories of nitrous oxide might decrease. However due to the uncertainty associated with our understanding of the relevant processes, there is low confidence in these predictions at present.

Methane

There are many facets to methane in the ocean environment. New microbial pathways that produce and consume methane continue to be discovered and microorganisms that were previously thought incapable of producing methane have been shown to produce this virulent greenhouse gas. In the open ocean, away from benthic or coastal influences, the aerobic production of methane is now considered primarily responsible for the supersaturation that occurs in the euphoric zone. The open ocean methane production and consumption processes are in sharp contrast to the high concentrations and strong gradients that are associated with methane vents, seeps, and other emissions from the seafloor. Much of the methane released to the overlying water-column does not reach the ocean surface due to microbial oxidation which represents a highly effective filtering process. However, ocean warming is expected to increase the release of methane from the seafloor and it is not clear how the microbial oxidation will be affected by increased water-column temperatures.

Bubbles of methane underwater and at the sea-air interface at the Santa Barbara Gas Seeps. The photos were taken during the Bubble Shuttle project (DFG SCHM 2530/7-1, PIs: Schmale, Schulz-Vogt, Treude)