Behavior of methane seep bubbles over a pockmark on the Cascadia continental margin

Abstract

A newly modified acoustic method was used to derive time-dependent bubble emission size distributions and to monitor associated zooplankton behavior at a methane seep emitted from the northeast Pacific continental shelf in 150 m water depth near Grays Harbor, Washington State, USA. Instrumentation consisted of a seafloor mooring with an upward-oriented 200 kHz sonar that imaged the column's lower 100 m for 33 h during September 2009. The profiler observed several highly variable methane bubble streams venting from a large carbonate-lined pockmark. Other acoustic data and visual observations confirmed that the gas bubbles reached the sea surface and were highly variable in nature. Individual bubble traces in the acoustic sonar images were used to derive vertical bubble velocities with a mean value of 24.6 ± 2.5 cm s−1 over the entire depth range. Some bubbles entering the acoustic image at shallower water depths exhibited a slower rise velocity of 22.2 ± 2.4 cm s−1 and likely originated from adjacent emission sites. Measured rise velocities were too slow to be clean, uncoated bubbles. We therefore assumed that the bubbles were surfactant coated with a Gaussian-shaped size distribution peaking at an observed radius of 7500 ± 100 µm. If the flux derived from these measurements was assumed to be relatively constant over time, total methane issuing from only one of the ∼20 active bubble vents at the pockmark site is estimated as ∼9 kg yr−1, similar to the flux from other reported marine CH4 vent sites.