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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 a measurement model. The solution is regularized using model sparsity, assuming changes in the measured quantity occur 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 solution 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 are needed to explain variations in atmospheric methane. In polar regions, high uncertainties in current estimates of methane release from the seabed remain. We present unique 10- and 3-month 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 246 m 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 m site fits with hypothesized seasonal blocking of lateral methane pathways in the sediments. Negative correlation between bottom water methane concentration (and variability) and wind forcing, concomitant with signs of weaker water column stratification, indicates increased potential for methane release to the atmosphere in fall and winter. We present new information about short- and long-term methane variability and provide a preliminary constraint on the uncertainties that arise in methane inventory estimates from this variability. [ABSTRACT FROM AUTHOR]

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]

The West Svalbard continental margin is an area of prominent methane seepage where shallow gas accumulation and release of methane gas to the water column have been observed. Methane release has been linked to both gas hydrate dissociation, melting permafrost, and vertical migration from deeper gas reservoirs, and the exact cause of seepage is still not fully understood (e.g. Sahling et al., 2014). Several factors have been suggested to affect the methane release, such as seasonal fluctuations in bottom water temperatures and pressure changes due to ocean tides (Berndt et al., 2014; Römer et al., 2016). Investigations of methane release in the area are mostly based on point measurements acquired when sea and ice conditions allow for ship access, making both long and short term monitoring challenging. To shed more light on the temporal nature of the seep sites offshore Western Svalbard, two ocean observatories equipped with a wide array of instruments were deployed from June 2015 to May 2016 at ~250 and 90 m depth where thousands of methane seeps have been observed. The time-series highlight rapid and drastic changes in methane concentration up to several hundreds of nmol/kg within a few hours. The main driver for these rapid changes is the ocean currents and observations indicate that seepage persisted throughout the whole annual cycle. High concentrations of methane (up to ~1400 nmol/kg) were observed during periods of calm winds and ocean conditions, suggesting that the available energy for ocean mixing impacted the distribution of the methane. A decreased methane concentration in winter was not correlated with decreasing ocean temperatures and no relationship was found between the methane concentration and changes in hydrostatic pressure. The research is part of the Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) and is supported by the Research Council of Norway through its Centres of Excellence funding scheme grant No. 223259 and UiT.Berndt, C., et al. (2014). Temporal Constraints on Hydrate-Controlled Methane Seepage off Svalbard. Science, 343(6168):284–287.Römer, M., et al. (2016). Tidally controlled gas bubble emissions: A comprehensive study using long-term monitoring data from the NEPTUNE cabled observatory offshore Vancouver Island. Geochemistry, Geophysics, Geosystems, 17(9):3797–3814.Sahling, H. et al. (2014). Gas emissions at the continental margin west of Svalbard: mapping, sampling, and quantification. Biogeosciences, 11(21):6029–6046. [ABSTRACT FROM AUTHOR]