Partitioning Methane Sources and Diffusive Flux Along the US Cascadia Margin Reveals Dominant Influences of Localized and Ephemeral Features

Abstract

Coastal ocean methane (CH ₄ ) emissions remain one of the most uncertain natural CH ₄ sources to the atmosphere, largely due to the scarcity of high resolution CH ₄ measurements needed to capture sharp concentration gradients. This study deployed a newly developed system to map the concentration and stable carbon isotopic composition of dissolved CH ₄ at high resolution in surface waters along the US Cascadia margin. Three regions of elevated dissolved concentration with distinct isotopic signatures were observed, which can be attributed to different processes based on their covariance with other physical and biogeochemical properties. The highest CH ₄ concentrations and lightest isotopic composition align with the Columbia River plume (CRP). South of the CRP, moderate concentrations and heavy isotopic composition are attributed to upwelling of residual unoxidized CH ₄ from offshore seepage. Finally, elevated concentrations and moderate isotopic composition are observed north of the CRP following upwelling but cannot be explained by the upwelling of local seafloor CH ₄ alone. Instead, we find evidence for in situ CH ₄ production associated with an ongoing phytoplankton bloom due to strong covariance with elevated chlorophyll and low nitrate. Our data reveal that point sources and ephemeral events contributed significantly to coastal CH ₄ emissions: the CRP contributed ∼24% of emissions during our study period, despite occupying <10% of the region's area, while emissions during upwelling conditions are over three times higher than during neutral conditions on a per day basis. These findings have important implications for quantifying global coastal CH ₄ emissions and monitoring their response to environmental change.