Methane at the ocean-atmosphere interface, from temperate to polar regions: an isotopic approach

• Main Author: Jacques, Caroline
• Other Authors: Pattyn, Frank, Tison, Jean-Louis, Fripiat, François, Sapart, C. J., Delille, Bruno, Niemann, Helge, Roeckmann, T., Heeschen, Katja
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universite Libre de Bruxelles 2021
• Subjects: Glaciologie; Methane; Stable isotopes; Sea ice; Ocean; Atmosphere; Kongsberg
• Online Access: http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/325149; https://dipot.ulb.ac.be/dspace/bitstream/2013/325149/3/Thesis_CJacques_revised.pdf; https://dipot.ulb.ac.be/dspace/bitstream/2013/325149/5/ContratDiJacques.pdf; https://dipot.ulb.ac.be/dspace/bitstream/2013/325149/4/Thesis_CJacques_revised_ToC.pdf

Given its crucial role in atmospheric chemistry and its global warming potential, methane(CH4) deserves to be accurately budgeted. However, the recent renewed rise in atmosphericCH4 growth rates from 2007 on, after a few years of slow-down, attests that sources are notcompensated anymore by sinks, and calls for a better assessment of the processes contributingto the global CH4 budget. Among natural sources, oceanic emissions are still subject tomany uncertainties, due to the lack of sampling. This is particularly relevant in polar regions,where the role of sea ice on CH4 sea-air fluxes is largely unknown.In an effort to contribute to a better characterisation of CH4 dynamics in oceanic environments,we investigated very contrasted settings during a journey from temperate to polarregions and applied the concentration and stable isotope approach.We start by evaluating the performance of a commercially available in situ CH4 sensor(CONTROS HydroC® CH4 from Kongsberg Contros) in controlled and natural environments,with the hope of using it in the framework of our various field campaigns. Although thissensor has the potential to significantly increase the spatial and temporal resolution comparedto discrete sampling, the long response time prevents from using its measurements as absolutevalues in dynamic natural environments and calls for progress in the field of technologies forcontinuous in situ dissolved CH4 measurements. However, the sensor turns out to be veryuseful during cruises to observe relative changes in dissolved CH4 concentrations and guidethe discrete sampling episodes.Our journey starts in the Scheldt estuary, at the transition between land and sea. Stableisotope analyses reveal that the unusual enrichment of dissolved CH4 in 13C and D in theupper estuary could result from intense microbial oxidation or an unknown source upstream.In the lower part of the estuary, this enriched CH4 mixes with depleted CH4 produced bymethanogenesis in the sediments, before entering the North Sea.In the shallow coastal ...