Gas hydrates in shallow sediments as capacitors for cold seep ecosystems: Insights from in-situ experiments

•A deep-sea lander mounted Raman spectrometer was deployed in a cold seep region.•Raman spectral data revealed five distinct phases.•Methane concentration fluctuated minimally over the observation period.•Our observations are consistent with those of hydrate capacitor models.•Hydrates, as methane capacitors, provide energy to chemosynthetic communities. Cold seep ecosystems are characterized by a dense accumulation of chemosynthetic communities that utilize the chemical energy contained in fluids. Due to various technical challenges, the direct monitoring of these communities and their activity shifts during the venting of cold seeps has not been achieved. In this study, an integrated in-situ long-term observation platform was used to monitor seep venting activity, associated gas hydrates, and chemosynthetic communities inhabiting the Formosa Ridge in the South China Sea. In-situ Raman spectral data obtained over 14 days revealed two periods during which cold seep venting formed gas hydrates, interspersed with periods of hydrate decomposition during non-active intervals. The methane concentration in the open seawater column near the cold seep vent fluctuated, with an average of 23.07 μM (variance 28.71 μM). Furthermore, the average coverage ratio of the dominant cold seep macrofauna Shinkaia crosnieri was 22.94 % (variance 0.11 %). We hypothesize that the methane concentrations and biological cover in chemosynthetic communities exhibit stability. This phenomenon may be related to the role of natural gas hydrate deposits as methane capacitors, as proposed by earth scientists.