Gas hydrates are ice-like solids composed of natural gas, usually methane in marine environments, and water. Hydrates are known to exist in the Cascadia margin, west of Vancouver Island, beneath the seafloor. Sediment stiffness is increased by frozen hydrates, like ice in winter mud. The degree of stiffness is an indicator of the amount of hydrate present per unit volume. Gas hydrate outcrops, venting and topography in the Cascadia margin have been intensively studied and are observed to change over time. Does the volume of hydrates also change with time? University of Toronto researchers Lisa Roach and Nigel Edwards are trying to find out.
An uncommon, specialized exploration technique, known as seafloor compliance is used to probe beneath the seafloor, by examining the deflection of the seafloor caused by waves on the ocean surface. Waves on the surface create pressure changes on the seafloor; as they “push” the seafloor down, it deflects them. The amount of seafloor deflection depends on the stiffness of the sediment, which, in turn, depends on hydrate content. In areas where there are fewer buried hydrates, the seafloor is more compliant (less stiff). The depth at which hydrates are buried beneath the seafloor can be inferred from gravity wave frequency at the seafloor. A compliance value in the higher frequency range describes shallower hydrates, while lower frequencies describe deeper deposits.
Roach and Edwards are using this technique to study changes in buried gas hydrates in a place called Bullseye Vent at our Clayoquot Slope location (depth 1260m). To determine the change in compliance, pressure and velocity data were recorded at 1 second intervals for a total of 228 24-hour long records between 1 October 2010 and 16 May 2011. Pressure was measured with a differential pressure gauge (DPG), capable of recording pressure changes of 1Pa in a background of 1MPa, while very small velocities, associated with seafloor wave deflections as small as the radius of an iron atom, are measured by a broadband ocean-bottom seismometer (OBS). These instruments are part of the NEPTUNE Canada ODP 889 seismic station.
A trend in the compliance over the 228 days was revealed (shown above). The trend represents a -2.88% change in the compliance of the sediments over the study period. This decrease in the compliance corresponds to an increase in the stiffness in sediments (indicating an increase of gas hydrate amount) between 0-600mbsf (metres below seafloor). Hydrates are stable within the top 225m of sediments, so a change in compliance could have been caused by property changes over this depth range. To match the decrease in compliance to the increase in hydrate content, the stiffness of the layer between 0-100mbsf (initial hydrate content of 22%) was varied until the -2.88% change was observed following a model suggested by the IODP drilling. A rather surprising three fold increase in hydrate concentration of the 100m layer, from 22% to 64% is predicted.
Assuming a 100m diameter hydrate mass, the change in hydrate concentration is equivalent to a change in hydrate mass of 600 million kg, which poses the question where does this mass come from? If this decrease is distributed over the entire hydrate zone (~250mbsf) the change in hydrate concentration and mass would be smaller. Maybe the zone of increased stiffness extends to greater depths and is in a layer associated with a hydrate production mechanism?
As more high-resolution data are collected in the future, scientists will gain increased understanding of how fluctuations in compliance relate to the dynamics of the hydrate system and its evolution. There is still much to be learned!
