Is There Methane Reflux from Bottom Waters into Sediments on the East Siberian Arctic Shelf?

Large fractions of the global subsea permafrost are located on the shallow East Siberian Arctic Shelf (ESAS), characterized by high methane ebullition and concentrations found over extensive scales, with atmospheric methane release estimated to be of the same scale as emissions from the rest of the World Oceans. Current research generally focuses on pinpointing the sources of subsea methane and understanding the release processes, by which methane migrates from the sediments, through the shallow water column and in part escapes to the atmosphere. A potentially neglected, yet critical question regarding the fate of the methane released from the subsea permafrost system is: can dissolved methane in the water column diffuse back down into sediment porewater with lower dissolved methane concentration?

Here, we investigate methane concentration gradients and estimate diffusive fluxes across the sediment–water interface on the ESAS. We compiled an extensive dataset of CH₄ in sediments and overlying bottom water (n>100 stations), including both unpublished and published measurements collected during multiple expeditions between 2012 and 2020. Approximately 25% of the stations exhibit reversed concentration gradients, with higher CH₄ concentrations in bottom waters than in surface sediments, indicating the potential for downward CH₄ diffusion.

We propose that a substantial fraction of methane is released from the sediments to the seawater as bubbles, which then dissolves in the bottom water. A fraction of this methane diffuses back down into the surface sediment, where it may possibly be degraded by microbes – a process that mitigates how much of the total initial sediment release of methane that escapes to the atmosphere. We asses the magnitude of diffusive CH₄ fluxes across the sediment water interface using Fick’s law. Aside from gradient strength, flux magnitude also depends on site specific conditions and properties such as sediment porosity, temperature, salinity, and bottom shear stress which controls the diffusive boundary layer thickness. The flux calculations indicate that the observed reversed gradients can result in a net diffusive flux of methane from the water column into the sediments, with the highest reflux estimated to be 11 mmol m^-2 day^-1. 

Our results suggest that ESAS sediments can alternately function as both a source and a sink for methane, challenging the prevailing view of a one-directional sediment-to-water flux. These findings highlight the need to further explore the potential of Arctic Shelf sediments acting as both a sink and source of methane as part of the dynamic sediment–water methane exchange processes.