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This study investigates the temporal variations in methane concentration and flare activity in the Hola trough (offshore Norway) during May 2018 and June 2022. Between these time periods, methane seep activity exhibits 3.5 times increase, as evidenced by hydroacoustic measurements. As the seep area in the Hola trough is constantly within the hydrate stability zone, the observed increase cannot be attributed to migration of its shallow boundary due to temperature increase. However, a combination of low tide conditions resulting in a lower sediment pore pressure and a bottom water temperature increase resulting in a lower methane solubility is likely to explain the increase in the number of seeps observed in June 2022. The hypothesis of tide influence is supported by data collected from a piezometer deployed and recovered during the cruise showing that the tidal effect was observed 3 m below the seafloor. Despite the numerous methane seeps detected, methane concentration and gas flow rates near the seafloor were low (<19 nM and <70 mL min−1, respectively) compared to other areas with methane seep activity. This is likely due to strong currents rapidly dispersing methane in the water column. Sub‐seafloor investigations identified pathways for gas migration in methane seep areas, influenced by topography. This study provides valuable insights into the temporal dynamics of methane concentrations, flare activity, and gas distribution in the Hola trough, contributing to our understanding of offshore methane dynamics in the region. Plain Language Summary: The Hola Trough, offshore Norway's Lofoten‐Vesterålen (LoVe) area, has been of interest for many years due to its rich marine life and potential oil and gas resources. There, coral mounds thrive around methane seepage. The LoVe observatory network monitors this unique environment. Using this observatory platform, associated data set and research expeditions at sea, the project EMAN7 (Environmental impact of Methane seepage and sub‐seabed characterization at LoVe‐Node 7) aims to understand the environmental impact of methane seepage as well as its spatio‐temporal variability. The comparison of methane seep activity during two summers with different environmental conditions revealed 3.5 times more seeps when a combination of warmer bottom water and low tide changes the sediment pore properties. A sensor recording subseafloor pore pressure and bottom temperature supports these findings. Sub‐seafloor investigations identified routes for gas migration in methane seep areas, influenced by topography. Key Points: A combination of warmer bottom water and low tide increases methane seeps 3.5 times by altering sediment gas solubility and pore pressureIntermittent seeps and strong oceanic currents explain the limited microbial mats and the low methane concentration in the water columnWe found sub‐seafloor pathways for gas migration in methane seep areas, influenced by topography [ABSTRACT FROM AUTHOR]

Large amounts of the greenhouse gas methane are released from the seabed to the water column, where it may be consumed by aerobic methanotrophic bacteria. The size and activity of methanotrophic communities, which determine the amount of methane consumed in the water column, are thought to be mainly controlled by nutrient and redox dynamics. Here, we report repeated measurements of methanotrophic activity and community size at methane seeps west of Svalbard, and relate them to physical water mass properties and modelled ocean currents. We show that cold bottom water, which contained a large number of aerobic methanotrophs, was displaced by warmer water with a considerably smaller methanotrophic community within days. Ocean current simulations using a global ocean/sea-ice model suggest that this water mass exchange is consistent with short-term variations in the meandering West Spitsbergen Current. We conclude that the shift from an offshore to a nearshore position of the current can rapidly and severely reduce methanotrophic activity in the water column. Strong fluctuating currents are common at many methane seep systems globally, and we suggest that they affect methane oxidation in the water column at other sites, too.

Accurate, high resolution measurements are essential to improve our understanding of environmental processes. Several chemical sensors relying on membrane separation extraction techniques have slow response times due to a dependence on equilibrium partitioning across the membrane separating the measured medium (i.e., a measuring chamber) and the medium of interest (i.e., a solvent). We present a new technique for deconvolving slow sensor response signals using statistical inverse theory; applying a weighted linear least squares estimator with the growth-law as measurement model. The solution is regularized using model sparsity, assuming changes in the measured quantity occurs with a certain time-step, which can be selected based on domain-specific knowledge or L-curve analysis. The advantage of this method is that it: 1) models error propagation, providing an explicit uncertainty estimate of the response time corrected signal, 2) enables evaluation of the solutions self consistency, and 3) only requires instrument accuracy, response time, and data as input parameters. Functionality of the technique is demonstrated using simulated, laboratory, and field measurements. In the field experiment, the coefficient of determination (R2) of a slow response methane sensor in comparison with an alternative, fast response sensor, significantly improved from 0.18 to 0.91 after signal deconvolution. This shows how the proposed method can open up a considerably wider set of applications for sensors and methods suffering from slow response times due to a reliance on the efficacy of diffusion processes. [ABSTRACT FROM AUTHOR]

Improved quantification techniques of natural sources is needed to explain variations in atmospheric methane. In polar regions, high uncertainties in current estimates of methane release from the seabed remain. We present two unique 10 and 3 months long time-series of bottom water measurements of physical and chemical parameters from two autonomous ocean observatories deployed at separate intense seabed methane seep sites (91 and 246m depth), offshore Western Svalbard from 2015 to 2016. Results show high short term (100-1000 nmol L-1 within hours) and seasonal variation, as well as higher (2-7 times) methane concentrations compared to previous measurements. Rapid variability is explained by uneven distribution of seepage and changing ocean current directions. No overt influence of tidal hydrostatic pressure or water temperature variations on methane concentration was observed, but an observed negative correlation with temperature at the 246 site fits with hypothesized seasonal blocking of lateral methane pathways in the sediments. Negative correlation between bottom water methane concentration/variability and wind forcing, concomitant with signs of weaker water column stratification indicates increased potential for methane release to the atmosphere in fall/winter. We highlight uncertainties in methane inventory estimates based on discrete water sampling and present new information about short- and long-term methane variability which can help constrain future estimates of seabed methane seepage. [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]