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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 underwater membrane-inlet laser spectrometer sensor demonstrates technological advances and breakthroughs for ocean measurements. We reveal decametre-scale variations in dissolved CH4 concentrations over the CH4 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 acoustically evidenced free gas emissions from the seafloor, support the observed heterogeneity and CH4 inventory. We identified sources of CH4 , undetectable with echo sounder, 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Seabed data acquired from the southern Barents Sea continental margin offshore Norway reveal detailed morphology of large sandwave fields. Multibeam echosounder bathymetry and backscatter, shallow seismic, sediment samples and seabed video data collected by the MAREANO program have been used to describe and interpret the morphology, distribution and transport of the sandwaves. The bedforms lie on a slope dominated by relict glacial forms and muddy/sandy/gravelly sediments. Sandwave migration across small gravity mass failures of the glacial mud constrains the field initiation as early post glacial or later. The contour-parallel nature of the fields and crestlines normal to the bathymetry contours and the geostrophic Norwegian Atlantic Current (NwAC) demonstrate that the NNW-flowing oceanographic circulation is the primary driving current. The fields coincide with the depth range at which a transition between warm, saline and underlying cooler, less saline waters fluctuate across the seabed. Statistically rigorous measurements of height, width and various parameters of slope and symmetry confirm a tendency to downstream (NNW) sandwave migration but with significant exceptions. Anomalous bedform symmetry domains within the fields are tuned to meso-scale topography along (relict) glacial debris flow chutes, indicating current focusing. Upstream and upper slope-derived winnowed sand transport eroded from the glacial sediments is the supposed source. Sandwave flank slope values are comparable to the regional slope such that the gravitational vector would have a cumulative downslope migration affect unless balanced by upslope drivers. Perpendicular cross-cutting of stoss face 3-D ripples by linear (2-D) ripples in the sandwave troughs and lee faces is evidence for non-synchronous, episodic current variations. Though deep Ekman transport and internal wave action are unproven here, these could explain chute-related tuning of bedform symmetry through funneling in the debris flow chutes and favor sand recycling, thus contributing to long-term maintenance of the sandwave field. [Copyright &y& Elsevier]