Fate of deep methane bubble plumes at the Haima cold seeps in the South China Sea

Submarine methane seepage constitutes the primary pathway for carbon transfer from Earth's subsurface to the ocean. This study investigates the fate of deep methane bubble plumes at the Haima cold seeps (1408-1522 m depth)-the largest active seep area in the South China Sea (SCS)-through integrated field observations and numerical simulations. Methane bubbles, encased by hydrate shells, ascended at velocities of 11.1-33.4 cm/s near the seabed. Local hydrographic conditions generated distinct plume morphologies: a sinuous flare at seep HM01 and an eastward-tilted conical plume at seep HMS18, with maximum observed heights of 797 m and widths of 244 m. Plumes terminated near the hydrate stability zone (HSZ) upper boundary, which oscillated between 580 and 645 m depth due to temperature modulation by mesoscale eddies (vertical amplitude: 32.5 m) and tides (5.6 m). Enhanced currents or weakened eruptions can intermittently suppress plume below this boundary. Nine-month moored current observations constrained the maximum lateral bubble displacement to 727 m. Dissolved methane reached 5.3 μM at seabed vents but declined by 1-2 orders of magnitude within 20-40 m above the vents, demonstrating that high-concentration anomalies due to dissolved-phase emissions were trapped near the seabed by density stratification. Numerical simulations predicted a secondary methane concentration anomaly (1.5 nM increase) at the HSZ upper boundary, resulting from hydrate shell dissociation of big bubbles. This study provides critical insights for designing methane leakage monitoring strategies during natural gas hydrate exploitation in the SCS.