As part of MBARI's 2009 northern expedition, Charlie Paull’s group focused its efforts on the north and south summits of Hydrate Ridge off Oregon, and on Bullseye Vent and Barkley Canyon off Canada. The goal was to better elucidate the nature and origin of the small scale (1 to 10 meters high) topography associated with gas-rich seafloor environments. Historically, it has been difficult to resolve features of this scale using surface-ship bathymetric surveys. The combination of high resolution mapping survey data collected by D. Allan B., MBARI’s mapping AUV , followed by sampling from the remotely operated vehicle Doc Ricketts provided new insights about the impact that gas venting exerts on seafloor morphology.
The seafloor surrounding Bullseye Vent and Hydrate Ridge have been the focus of scientific ocean drilling, numerous ROV and HOV dives, sediment coring, seismic reflection surveys, and surface-ship multibeam bathymetric surveys. Previous observations from human-occupied vehicles (HOVs), ROVs, and towed camera sleds had shown that chemosynthetic biological communities, methane gas hydrates in the near subsurface, and methane gas vents occur in these areas.
However, it has been difficult to assess the impact of the gas venting on the seafloor topography. For example, data from the surface-ship surveys gave the impression that Bullseye Vent was a broad subtle (approximately two meters) topographic high. However, the significantly improved resolution of the bathymetry obtained using MBARI’s state-of-the-art mapping AUV showed the seafloor shape in unprecedented detail and clearly demonstrated that Bullseye Vent is a distinct, 350-meter-long, 50-meter-wide, and approximately 6-meter-deep topographic depression.
The new high-resolution multibeam maps and the realizations that come from seeing the seafloor in such unprecedented detail have fundamentally changed the perception of what processes shape the seafloor. Instead of considering processes that produce local topographic highs, processes that excavate the seafloor are required. Thus, simple discoveries can still be made that radically change our point of view, even in some of the best-studied seafloor environments.
Notable features revealed by the mapping AUV data and sampled with the Doc Ricketts included widespread areas where methane-derived carbonates are exposed on the seafloor, circular seafloor depressions with diameters of three to 50 meters excavated into the seafloor, and smaller mound-like features one-to-three-meters higher than the surrounding seafloor. Thin lenses of solid gas hydrate exposed along fractures on the sides of the mounds suggest that these mounds are push-up features created by gas hydrate growth in the sediments just below the seafloor.