| 26 Mar 2025
Modeling water column gas transformation, migration and atmospheric flux from seafloor seepage
Abstract. Understanding the fate of gas seeping from the seafloor is crucial for assessing the environmental impact of natural and anthropogenic seep systems, such as CH4 cold seeps or leaks from gas wells or future carbon capture projects. We present a comprehensive modelling framework that integrates physical, biological and chemical processes to estimate the 3-dimensional dissolved concentration and total atmospheric flux of gas from seafloor seeps. The framework consists of two main steps: 1) A gas phase model that estimates free gas dissolution and direct atmospheric flux at the seep site, and 2) a concentration model that combines particle dispersion modelling, an adaptive-bandwidth kernel density estimator, and customizable process modules. Using this framework, we successfully modeled the concentration field and atmospheric flux of CH4 between May 20 and June 20, 2018, from a natural seep site located at 200 meters depth offshore Northwestern Norway. Results show that dissolved gas is primarily advected northeastward along the coast, spreading effectively across the shelves, reefs, and entering open fjord systems. Within a few days, the vertical CH4 concentration profile is near inverted, with peak concentrations close to the sea surface – facilitating atmospheric exchange. Diffusive emissions are spread out over large areas (>105 km2) and exceeds the local free gas flux by more than threefold (∼0.76 %) during the modeling period while ∼40 % of the CH4 remains in the water column. Although high uncertainties remains regarding microbial oxidation rates, microbes represent the main sink of CH4, converting ∼60 % of dissolved CH4 to CO2 during the modeling period. These findings highlight the importance of accounting for dissolved gas from seeps when evaluating their impact on atmospheric emissions and ecosystem interactions. Our framework provides a globally applicable tool that incorporates free and dissolved gas dynamics and flexible inclusion of chemical and biological processes, supporting improved understanding and ability to quantify environmental impacts of seabed gas seeps in the future.
