Paper: Weber TC, Mayer L, Jerram K, et al. (2014) Acoustic estimates of methane gas flux from the seabed in a 6000 km 2 region in the Northern Gulf of Mexico. Geochemistry, Geophysics, Geosystems 15:1911–1925. doi: 10.1002/2014GC005271
Bubbles elicit scenes of childhood summers playing on the front stoop or backyard. On the other hand, put bubbles at the bottom of the ocean and you will find highly educated adults toiling with complicated mathematical equations and state-of-the-art technology.
The bubbles these scientists are trying to quantify are made of methane gas and are seeping out of sections of seafloor worldwide. Methane has either biological or geological origins, and the pathway from seabed to atmosphere is extremely complex. It is a powerful energy resource and a potent green-house gas, but very little is understood about how oceanic methane gas seeps affect the global carbon cycle.
Methane is undersaturated in the deep ocean, so clean methane bubbles are expected to lose much of their mass over several tens of meters as the bubbles disintegrate into the surrounding water. However, the observations in Weber et al. show that these gas seeps are actually rising several hundreds of meters thorough the water column. This seems likely to be a result of the suggested impediment to gas transfer provided by the bubbles’ skin which acts to slow down the rate of gas transfer. The skin is a boundary between the gas inside the bubbles and the surrounding water; it is made of methane hydrate, a cage-like lattice of ice that is only stable under low temperatures and high pressures.
In Weber et al., many mechanisms that potentially affect gas transfer and the advanced technology used to quantify gas flux are explained in detail. The team collected data from two different mapping systems, a 30kHz multibeam echo sounder (MBES) and an 18 kHz split-beam echo sounder (SBES). The first of these mapping systems is best for efficient mapping and localization of the gas seeps, while the second is best for obtaining absolute acoustic measurements of the seeps. In 2011, the team identified a spatial distribution of a number of seeps in the northern Gulf of Mexico with the MBES, and then acoustically characterized a subset of them with the SBES. The study area was 6000 km2, which is equivalent to less than 0.1% of the current estimate for global seabed methane seepage rates. A year later, Weber et al. returned to the same sites to remap them in case the seeps had undergone any changes.
Additionally, the team used a remotely operated vehicle (ROV) to dive on the seeps to collect footage of bubbles passing a gridded board. The board has squares of a known size which helps better identify the volume of gas in each bubble and the rate at which the bubbles ascend. The bubble video footage was then run through a gamut of computer analyses to produce a precise flux rate. These flux rate measurements were then extrapolated to the entire 2011 survey area, constrained by the SBES acoustic measurements, in order to estimate the bounds on the regional flux of methane gas.
The ROV also physically collected bubbles in a clear container. The bubbles were then filmed during the ascent of the ROV to the surface, expanding in volume along the way as the materials were depressurized and temperature increased. The video footage shows the methane in hydrate form (solid looking blue/white material) at ~1000m, then dissociating of the hydrate into free gas starting at ~600m. By ~420m only water and free gas remained.
You can see a wonderful video of the gas collection, hydrate dissociation, and gas expansion here: Methane collection and gas expansion
The direct comparison of acoustically derived and directly observed flux show that relatively accurate estimates of gas flux can be achieved in the deep ocean using shipboard SBES, although not without several generalizations that can account for both over and underestimations.
Yes, the team had to make several assumptions to generate their estimates, such as spherical bubble geometry, constant discharge rates, and pure methane composition. Ultimately, the study was able to show that acoustics can provide a workable dataset for estimating the amount of methane seeping from the global seafloor.
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