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 impacts of both natural and anthropogenic seep systems, such as CH4 cold seeps, leaking gas wells, and future carbon capture projects. We present a comprehensive modeling framework that integrates physical, chemical, and biological processes to estimate the 3-dimensional water column dissolved gas concentration field and 2-dimensional atmospheric flux field resulting from seafloor seeps. The framework consists of two main components: (1) a gas-phase model that calculates free gas dissolution and direct atmospheric release at the seep site, and (2) a concentration model that combines particle dispersion modeling with an adaptive-bandwidth kernel density estimator and customizable process modules. Applying the framework to a natural CH4 seep at 200 m depth offshore northwestern Norway (20 May–20 June 2018), we found that dissolved methane was advected northeastward along the coast, spreading across shelves, reefs, and into fjord systems. Within days, the vertical CH4 concentration profile was near inverted, with near-surface maxima, facilitating atmospheric exchange. Diffusive emissions covered large areas (>105 km2) and was almost 3 times the local free gas flux. Around 0.7 % of dissolved CH4 reached the atmosphere during a 4 week period, microbial oxidation removed around 65 %, while ∼ 34 % remained in the water column. Uncertainties caused by a range of model framework elements remain substantial, e.g. can estimates of microbial oxidation removal change from 65 % to as low as 5.5 % or as high as 91.4 % depending on rate coefficient assumptions. Our framework provides a globally applicable tool that integrates free and dissolved gas dynamics and accommodates advanced hydrodynamic modeling. Its ability to explicitly resolve spatiotemporal fields enables the inclusion of complex physical and biogeochemical process modules and supports not only the quantification of atmospheric fluxes but also applications that require explicit field representations, such as assessing impacts on local ecosystems.