Internal solitary waves (ISWs) are ubiquitous in the stratified ocean and can induce significant seabed pressure disturbances during propagation, leading to pore pressure responses within marine sediments and subsequent pore fluid seepage. However, direct in situ observations of ISW-induced pore pressure responses in deep-sea environments have been lacking, leaving the physical mechanisms largely speculative. Here we present the first long-term, high-frequency in situ pore pressure observations conducted on the Dongsha Slope of the northern South China Sea at a water depth of 740 m, as part of the Internal Wave–Benthic Boundary Layer Experiment (IWBBLE). Our results reveal that large-amplitude depression-type ISWs generate distinct negative pore pressure responses, with a maximum amplitude of approximately 0.69 kPa. The pore pressure responses exhibit transient characteristics without significant accumulation, and a clear hysteresis is observed relative to the seafloor pressure disturbances, with the negative pore pressure peak shifting toward the upwelling side of the ISW. The estimated penetration depth of pore pressure response is approximately 0.5 m below the seafloor during the observation period, and may extend to within 1.5 m for an extreme-amplitude ISW of 173 m. The induced vertical pore pressure gradients drive pore fluid seepage beneath the pressure disturbance peak, with maximum seepage velocities approaching 6 × 10⁻³ cm s⁻¹ at 1 m depth. The seepage paths are nearly vertical beneath the pressure peak and gradually transition to horizontal along the flanks. These findings demonstrate that ISWs drive significant pore fluid circulation across the sediment–water interface, promoting upward migration of dissolved solutes and fine particles. This study provides crucial observational evidence for understanding seabed dynamic responses to ISWs and their implications for sediment resuspension, material transport, and biogeochemical cycling on continental margins.

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