High-resolution underwater laser spectrometer sensing provides new insights into methane distribution at an Arctic seepage site

• Published in: Ocean Science
• Main Authors: Jansson, Pär, Triest, Jack, Grilli, Roberto, Ferré, Benedicte, Silyakova, Anna, Mienert, Jurgen, Chappellaz, Jérôme
• Other Authors: Centre for Arctic Gas Hydrate, Environment and Climate (CAGE), The Arctic University of Norway Tromsø, Norway (UiT), Institut des Géosciences de l’Environnement (IGE), Institut de Recherche pour le Développement (IRD)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes 2016-2019 (UGA 2016-2019 )
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2019
• Subjects: [SDE]Environmental Sciences; Arctic; Ocean acidification; Svalbard
• Online Access: https://hal.science/hal-02366680; https://hal.science/hal-02366680/document; https://hal.science/hal-02366680/file/os-15-1055-2019.pdf; https://doi.org/10.5194/os-15-1055-2019

International audience Methane (CH 4) in marine sediments has the potential to contribute to changes in the ocean and climate system. Physical and biochemical processes that are difficult to quantify with current standard methods such as acoustic surveys and discrete sampling govern the distribution of dissolved CH 4 in oceans and lakes. Detailed observations of aquatic CH 4 concentrations are required for a better understanding of CH 4 dynamics in the water column, how it can affect lake and ocean acidification, the chemosynthetic ecosystem, and mixing ratios of atmospheric climate gases. Here we present pioneering high-resolution in situ measurements of dissolved CH 4 throughout the water column over a 400 m deep CH 4 seepage area at the continental slope west of Svalbard. A new fast-response underwater membrane-inlet laser spectrometer sensor demonstrates technological advances and breakthroughs for ocean measurements. We reveal decametre-scale variations in dissolved CH 4 concentrations over the CH 4 seepage zone. Previous studies could not resolve such heterogeneity in the area, assumed a smoother distribution, and therefore lacked both details on and insights into ongoing processes. We show good repeatability of the instrument measurements, which are also in agreement with discrete sampling. New numerical models, based on acousti-cally evidenced free gas emissions from the seafloor, support the observed heterogeneity and CH 4 inventory. We identified sources of CH 4 , undetectable with echo sounder, and rapid diffusion of dissolved CH 4 away from the sources. Results from the continuous ocean laser-spectrometer measurements, supported by modelling, improve our understanding of CH 4 fluxes and related physical processes over Arctic CH 4 de-gassing regions.