DISTINCTIVE GEOMORPHOLOGY OF GAS VENTING AND NEAR SEAFLOOR GAS HYDRATE-BEARING SITES

High-resolution multibeam bathymetry and chirp seismic profiles collected with an Autonomous Underwater Vehicle (AUV) complemented by Remotely Operated Vehicle (ROV) observations and sampling reveal the fine scale geomorphology associated with gas venting and/or near subsurface gas hydrate accumulations along the Pacific North American continental margin (Santa Monica Basin, Hydrate Ridge, Eel River, Barkley Canyon, and Bullseye Vent). At 1 m multibeam grid resolution, distinctive features and textures that were previously undetected show the impact of gas venting, gas hydrate development, and related phenomena on the seafloor morphology. Together a suite of geomorphic characteristics illustrates different stages in the development of seafloor gas venting systems. The more mature and/or impacted areas are associated with widespread exposures of methane-oxidation-derived carbonates, which form broken and irregular seafloor pavements with karst-like voids in between the cemented blocks. These mature areas also contain elevated features >10 m high and circular craters with diameters of 3-50 m that appear to be associated with missing sections of the original seafloor. Smaller mound-like features (<10 m in diameter and 1-3 m higher than the surrounding seafloor) reoccur at multiple sites. Solid lenses of gas hydrate are occasionally exposed along fractures on the sides of these mounds and suggest that these are push-up features associated with gas hydrate growth within the near seafloor sediments. The youngest appearing features are associated with more-subtle (<3 m in diameter and ~0.5 m high) seafloor mounds, with their crests crossed with small cracks lined with white bacterial mats. ROV-collected (<1.5 cm long) cores obtained from these subtle mounds encounter a hard layer at 30-60 cm sub-bottom. When this layer is penetrated, methane bubbles may gush out and continue to flow out for over an hour. These observations suggest that these small mounds are young features that trap considerable volumes of gas near the seafloor. Together these observations reveal the integrated effect that gas and/or gas hydrate occurrences can have on the seafloor. The existence of gaseous gas within ~1 m of the seafloor has intriguing implications as to the geo-hazard potential of such sites.