In Situ Observations Reveal That Internal Solitary Waves Promote Methane Release from the Seabed on the Northern Slope of the South China Sea

As one of the most powerful marine hydrodynamic phenomena, internal solitary waves (ISWs) profoundly modulate the circulation of materials and energy transfer in the ocean. Huge amounts of dissolved methane are released into seawater from the surface sediments of the seabed, promoted by ISWs; however, the influence of ISWs on methane release has not been quantitatively studied. We designed an observation system and carried out in situ observations of the northern slope of the South China Sea (SCS). It was observed that during the passage of ISWs, the pressure of bottom seawater can be reduced by 472 Pa. We propose the following mechanism for this phenomenon: ISWs cause an instantaneous pressure decrease in seabed surface sediment, which will form an upward pressure gradient, thereby accelerating the release of dissolved methane in pore water into seawater. Based on the observational data, the ISW-promoted methane release flux was estimated to be 6.34–34.43 μmol·m−2·d−1, corresponding to an annual release of 1.85–10.05 × 104 Mg·yr−1 from the northern slope of the SCS, equivalent to roughly 0.13% of the global seabed methane release. These results demonstrate that ISWs constitute one of the most significant oceanic dynamic processes promoting seabed methane release and provide quantitative constraints on the associated emissions. This work offers new perspectives for understanding the role of ISWs in the global ocean carbon cycle.