High-resolution under-water laser spectrometer sensing provides new insights to methane distribution at an Arctic seepage site

Abstract

Abstract. Methane (CH4) 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 CH4 in oceans and lakes. Detailed observations of aquatic CH4 concentrations are required for a better understanding of CH4 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 CH4 throughout the water column over a 400 m deep CH4 seepage area at the continental slope west of Svalbard. A new fast-response under-water membrane-inlet laser spectrometer sensor demonstrates technological advances and breakthroughs for ocean measurements. We reveal decametre-scale variations of dissolved CH4 concentrations over the CH4 seepage zone. Previous studies could not resolve such heterogeneity in the area, assumed smoother distribution and therefore lacked both details and insights to ongoing processes. We show good repeatability of the instrument measurements, which are also in agreement with discrete sampling. New numerical models, based on acoustically evidenced free gas emissions from the seafloor, support the observed heterogeneity and CH4 inventory. We identified sources of CH4, undetectable with echosounder, and rapid diffusion of dissolved CH4 away from the sources. Results from the continuous ocean laser-spectrometer measurements, supported by modelling, improve our understanding of CH4 fluxes and related physical processes over Arctic CH4 degassing regions.

Biogeochemistry and timing of methane-derived carbonate formation at Leirdjupet fault complex, SW Barents sea

“…90% of the methane seeping from marine sediments (Hinrichs and Boetius, 2002;Reeburgh, 2007;Knittel and Boetius, 2009) is microbially consumed close to the seafloor by the anaerobic oxidation of methane (AOM) consortium (Boetius et al, 2000). The remaining small fraction of methane escaping the sedimentary bio-filter is almost entirely oxidized by aerobic methane-oxidizing bacteria in the watercolumn before reaching the sea surface (McGinnis et al, 2006;Sparrow et al, 2018;Jansson et al, 2019). Methane-derived carbon is sequestered into authigenic carbonates precipitating within the sediment due to an alkalinity increase produced by the AOM consortium (CH 4 + SO 4 2− → HCO 3 − + HS − + H 2 O).…”

No increasing risk of a limnic eruption at Lake Kivu: Intercomparison study reveals gas concentrations close to steady state

“…The extraction system does not rely on gas equilibration across the membrane, but the dry side of the membrane is maintained at low pressure while continuously flushing it with dry zero air. This allows achieving fast response times < 30 sec, making the technique adapted for fast 3D mapping of water masses [21]. The accuracy (standard deviation at the three sigma level) of the measurements was quantified to be ± 33% from repeated measurements at the same depth.…”