Variability of Marine Methane Bubble Emissions on the Clayoquot Slope, Offshore Vancouver Island, Between 2017 and 2021

Seabed methane gas emissions occur worldwide at cold seeps located along most continental margins. Fluxes of methane gas released from the seabed in the form of bubbles can be extremely variable even over short time intervals. Some factors controlling the variability are still poorly understood. Here, we report on the results of continuous long-term sonar monitoring of bubble emissions at a depth of 1,260 m on the Clayoquot Slope, northern Cascadia margin. With a total monitoring duration of 4 years and a sampling period of 1 h, this is by far the longest high temporal resolution monitoring of seabed methane gas release ever conducted. Our results provide evidence that the diurnal and semi-diurnal tides influence the timing of the onset and cessation of bubble emissions. However, gas emissions within the monitoring area are active more than 84% of the time, indicating that tides alone are not sufficient to make venting pause. We hypothesize that the gas fluxes are transient but generally sufficiently high to maintain ebullition independently of the tidally-induced bottom pressure variations. Results also show that the tides do not seem to modulate the vigor of active gas emissions

Measuring Methane in the Arctic Ocean - From legal framework to time series analysis via technology innovation

Understanding how the earth system interacts with ongoing climate change is important to find a realistic route towards a sustainable future. The impact of Arctic seabed methane seepage on contemporary and future climate is still poorly constrained, described, and quantified. An important limiting factor in our understanding of seabed seepage in the Arctic is a lack of in situ measurements; however, remoteness and harsh environmental conditions make data acquisition difficult. The aim of this thesis is to improve understanding of and ability to measure methane in the Arctic Ocean via inter-disciplinary work, method development and time-series analysis. To fill crucial data gaps and increase the general data coverage in the region demands implementation of innovative technology and increased research activity. Legal scholars have identified emerging legal gaps associated with this increased activity and regulation of marine scientific research. However, our inter-disciplinary assessment indicates that an evolutionary interpretation of the legal framework is currently adequate to regulate and facilitate current conduct of marine scientific research in the Arctic Ocean. We obtained a unique data set from two intense seep sites (at 91 and 246 meter depth) offshore West Spitsbergen by deploying two autonomous ocean observatories which recorded respectively 10 and 3 month time-series of bottom water physical and chemical parameters between July 2015 and May 2016. High short term variability (−1 on hourly time-scales) were observed which were partly explained by changing ocean currents and location of nearby seeps. A seasonal variation with lower (∼halved) concentrations and variability in winter season was coupled with increased water column mixing. No clear effect of tidal hydrostatic pressure changes were observed, but a negative correlation between methane and temperature at the deepest seep site aligns well with hypothesized seasonal blocking of lateral sedimentary methane pathways. We highlighted and quantified potential uncertainties that can arise from high short-term variability in budget estimates. To enable direct observations of bubble release, we developed a method for using ADCP to monitor seabed seepage. The method makes it possible to integrate all backscatter data from the ADCP and monitor seepage activity on the seafloor by modeling bubble transport in the water column. Using this model, the ADCP at the 91 meter observatory uncovered continuous ongoing seepage to the north of the observatory and a stationary seep configuration. Several chemical sensors, including conventional dissolved methane sensors, rely on separating the medium of interest (e.g. methane) from the measured medium (e.g. water) using equilibrium partitioning across a membrane. This process causes slow response times, which is problematic for applications where steep gradients are expected such as at our observatory location, in profiling or other highly dynamic domains. We developed a new technique to deconvolve slow response signals and obtain fast response data by using the theoretical framework of statistical inverse theory. This method provides an explicit uncertainty estimate, quality assessment of the result and no extra input parameters other than what already provided in standard calibration procedures. There is a vast range of questions that are relevant to pursue to increase our understanding of seabed methane seepage in the Arctic Ocean. In light and line of this work, future efforts to improve quantification of methane and methane seepage could focus on assessing uncertainty in various approaches to budget estimates, further validate new methodology presented herein and use these on e.g. autonomous vehicles capable of providing large volumes of high resolution data within short time spans

Interannual variations of freshwater content in Hornsund

Salinity measurements have been used to calculate the freshwater content in Hornsund for the years 2001 to 2014. In 2011 there was significantly higher freshwater content in the fjord, compared to the other years, with a total freshwater content of 1.08 km3, when calculated with a reference salinity of 34.2. The high freshwater content in 2011 was attributed to the inflow of sea ice and sea ice meltwater of Barents Sea origin, that had been advected into the fjord from the shelf. The lowest amount of freshwater was found in July 2014. No good relashionship between estimations of variations in terrestial runoff and freshwater content in the fjord was found. The distribution of freshwater in Hornsund seems to be governed by wind conditions and the rotational dynamics in the fjord. The fractional contributions of meteoric water, sea ice meltwater, and seawater was calculated based on δ^18O and salinity measurements in September 2013. Significant amounts of meteoric water was found in the surface waters of the bays Burgerbukta and Brepollen. The δ^18O in Brepollen, the innermost bay of Hornsund, showed unusually high values compared to other δ^18O measurements taken in and around Svalbard. This was attributed to the possible existence of a hydrothermal vent or significant amounts of sea ice meltwater in the bay

The law of the sea and current practices of marine scientific research in the Arctic

The rapid changes in both climate and human activity occurring in the Arctic Ocean demands improved knowledge about this region. Combined with eased accessibility due to reduced sea ice cover and new technologies, this has led to increased research activity in the region. These circumstances put pressure on the applicable legal framework, i.e. the United Nations Convention on the Law of the Sea. Therefore, a conversation is needed between legal and marine scientists to promote the alignment between the legal framework and current practices of marine scientific research in the Arctic. This article showcases three current practices of marine scientific research in the Arctic, which are subsequently analysed in light of the existing legal framework, highlighting the legal questions arising from the use of these three technologies. The three technologies analysed here are seabed structures off Svalbard, floating ice-tethered observatories deployed across the marine Arctic, and remote sensing activities paired with in situ measurements

A new numerical model for understanding free and dissolved gasprogression toward the atmosphere in aquatic methane seepage systems

We present a marine two‐phase gas model in one dimension (M2PG1) resolving interaction between the free and dissolved gas phases and the gas propagation toward the atmosphere in aquatic environments. The motivation for the model development was to improve the understanding of benthic methane seepage impact on aquatic environments and its effect on atmospheric greenhouse gas composition. Rising, dissolution, and exsolution of a wide size‐range of bubbles comprising several gas species are modeled simultaneously with the evolution of the aqueous gas concentrations. A model sensitivity analysis elucidates the relative importance of process parameterizations and environmental effects on the gas behavior. The parameterization of transfer velocity across bubble rims has the greatest influence on the resulting gas distribution, and bubble sizes are critical for predicting the fate of emitted bubble gas. High salinity increases the rise height of bubbles; whereas temperature does not significantly alter it. Vertical mixing and aerobic oxidation play insignificant roles in environments where advection is important. The model, applied in an Arctic Ocean methane seepage location, showed good agreement with acoustically derived bubble rise heights and in situ sampled methane concentration profiles. Coupled with numerical ocean circulation and biogeochemical models, M2PG1 could predict the impact of benthic methane emissions on the marine environment and the atmosphere on long time scales and large spatial scales. Because of its flexibility, M2PG1 can be applied in a wide variety of environmental settings and future M2PG1 applications may include gas leakage from seafloor installations and bubble injection by wave action

Response time correction of slow-response sensor data by deconvolution of the growth-law equation

International audienceAbstract. 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

Autonomous methane seep site monitoring offshore Western Svalbard: Hourly to seasonal variability and associated oceanographic parameters

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 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 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

Contrasting Methane Seepage Dynamics in the Hola Trough Offshore Norway: Insights From Two Different Summers

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