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

Pär Jansson (1) , Jack Triest (2) , Roberto Grilli (2) , Benedicte Ferré (1) , Anna Silyakova , Jurgen Mienert (1) , Jérôme Chappellaz (2)

1 CAGE - Centre for Arctic Gas Hydrate, Environment and Climate
2 IGE - Institut des Géosciences de l’Environnement

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.

Domaines: Sciences de l'environnement

Pär Jansson, Jack Triest, Roberto Grilli, Benedicte Ferré, Anna Silyakova, et al.. High-resolution underwater laser spectrometer sensing provides new insights into methane distribution at an Arctic seepage site. Ocean Science, 2019, 15 (4), pp.1055-1069. ⟨10.5194/os-15-1055-2019⟩. ⟨hal-02366680⟩