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

Abstract

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.

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

“…We observed high CH4 concentrations up to 75-100 masf, which is in agreement with bubble models (e.g. McGinnis et al, 2006;Jansson et al, 2019), highlighting that bubbles of observed sizes (~3 mm average equivalent radius) are fully dissolved within this range. Density stratification plays an important role in the vertical distribution of dissolved CH4 because turbulent energy is required to mix solvents across isopycnals.…”

“…1a. FlareHunter derived flow rates within 50 m from line 3 were projected into the model domain, and the source of 175 dissolved CH4, mediated by bubbles, was distributed vertically by applying a non-dimensional source-function similar to the approach of Jansson et al (2019): 0( ) = 6.6 × 10 −2 × −0.066× , where z is the vertical distance from the seafloor in metres.…”

“…Average concentrations were calculated in a "box" volume equivalent to MILS line 3 (4.5 km long (x-direction), 50 m wide (y-direction), equivalent to the echosounder beam width, 75 m high (z-direction) corresponding to the most dynamic and CH4 310 enriched zone (e.g. McGinnis et al, 2006;Graves et al, 2015;Jansson et al, 2019). Box averages were derived as follows:…”

Compositional Differences in Dissolved Organic Matter Between Arctic Cold Seeps Versus Non-Seep Sites at the Svalbard Continental Margin and the Barents Sea

“…MOx was 10 times more efficient at SP (rate constant k MOx 6 × 10 −3 day −1 ) than in PKF (k MOx 6 × 10 −4 ay −1 ), presumably due to the difference in the hydrography or the size of methanotrophic community. At SP, a well-defined pycnocline ( Figure 4A SP-3 panel) hindered the transport of gas bubbles to upper layers (Damm et al, 2005;Damm et al, 2008;Gentz et al, 2014;Jansson et al, 2019) whereas the vertical transport of methane was easier in uniform and shallow water column at PKF ( Figure 4A PKF-2 panel).…”

“…Seeping gases form bubbles and rise up through the water column (Westbrook et al, 2009;Berndt et al, 2014). Along its trajectory towards the upper layers, methane exchanges with more abundant dissolved gases such as nitrogen, enriched in the surrounding water and promotes MOx (James et al, 2016;Steinle et al, 2016;Jansson et al, 2019). Hydroacoustic surveys demonstrated that seeps can stay active for weeks to decades (Gentz et al, 2014;Veloso-Alarcón et al, 2019) however, the fate of methane at active flares sites is highly dependent on bubble size, salinity, and water velocity (Jansson et al, 2019).…”