Oxidation of methane in seawater - Laboratory experiments and the use of models - OC2022 A-114

Oxidation of methane was determined in this project by two methods: Stable isotope analyses (13CH4) and by the use of tritium-labelled methane (3H-CH4). The intentions of the project were to obtain methane oxidation rates for the marine water column, which may be of relevance for the Norwegian Continental Shelf. Experimental, sampling, and analytical methods were established for both methods. Relatively few experiments were of a quality considered to be acceptable. The oxidation data determined by the two methods resulted in half-lives of 21.6 ± 6.2 days at 5°C incubation temperature with the stable isotope method, and 5.6 ± 0.7 and 4.6 ± 2.5 days with the tritium method at 5 to 8.5°C incubation temperature, respectively. Based on the comparison to data for field studies, it seemed that the data obtained by the tritium methods represented an overestimation of methane oxidation rates, while the stable isotope data were more in agreement with field data. The oxidation data were used in model simulations to determine atmospheric releases of methane from seeps at different seep depths.