Understanding the Long-term Tidal Modulation on Methane Bubble Emissions at Southern Hydrate Ridge using Acoustics

Abstract

Methane release from seafloor reservoirs is thought to have caused paleo-climate warming and have the potential to accelerate global warming. Methane reservoirs along the upper continental slope are especially susceptible to destabilization due to the narrow margin of hydrate stability within the seafloor. At Southern Hydrate Ridge (SHR), discontinuity in methane bubble emissions was observed, but still not well understood. Our study uses acoustic data archived by the Ocean Observatories Initiative (OOI) Cabled Array to investigate the potential tidal impact on methane bubble emissions at SHR. We detect methane plume structures based on the proxy of echo contrasts caused by acoustic-bubble interaction. Through analyzing the cumulative frequency of methane bubble emissions over a tidal cycle, we deduce a sinusoidal bubble plume response curve with ~120-degree lag from a cosine curve of tidal forcing. Our result reveals significant tidal modulation on bubble emissions with an abundance difference up to ~2.5 times over ~3.5 months. During the phase of falling tides, frequency of methane emissions is higher. Tidally induced pressure change could modulate the pore fluid pressure system by promoting or inhibiting fracturing that could cause bubble emissions. We propose a model for the dynamic equilibrium between the seafloor pressure and the pore fluid pressure for SHR. Our study has demonstrated the application of high-temporal resolution Acoustic Doppler Current Profiler (ADCP) in studying methane bubbles.