Description: This paper presents a comprehensive, multidisciplinary study of cold vents associated with near-seafloor gas hydrate. Several cold vents characterized by seismic blank zones have been identified on the northern Cascadia margin near Ocean Drilling Program (ODP) Site 889/890. The most prominent vent site (Bullseye vent) has been the subject of intense geophysical and geochemical studies, including two- and three-dimensional (2D/3D) seismic imaging, heat flow measurements, piston coring with measurements of sediment physical properties and pore-fluid geochemistry, seafloor video observation, and sampling with the unmanned submersible ROPOS. The main seismically derived constraining observations are: (1) blanking increases with seismic frequency, (2) at low frequencies, layers can be traced through the zones, (3) blank zones widen with depth, (4) blank zones are underlain by a bottom simulating reflector (BSR), and (5) no velocity anomalies were detected across the vents. Constraints from piston core and thermal probe analyses are: (1) massive hydrate was recovered just below the seafloor at Bullseye vent, and (2) chemical alteration of sediments was observed by reduced magnetic susceptibility, increased thermal conductivity, and an elevated sulfate/methane interface. Additional constraints are: (1) no thermal anomaly was observed, (2) widespread carbonates and active chemosynthetic communities were found, and (3) elevated levels of methane were detected in the water column above Bullseye vent. We present a model for the seismic blanking at Bullseye vent that honors the constraints from all observations. The cold vents represent channels or networks of filamentous fractures containing hydrate and/or free gas. Free gas can be present within the hydrate stability field only in fractures, which may be coated with hydrate that prevents the inflow of water. The overall concentration of hydrate or gas within the vent must be small, because there was no observable velocity anomaly.

Description: High-resolution multibeam bathymetry data collected with an autonomous underwater vehicle (AUV) complemented by compressed high-intensity radar pulse (Chirp) profiles and remotely operated vehicle (ROV) observations and sediment sampling reveal a distinctive rough topography associated with seafloor gas venting and/or near-subsurface gas hydrate accumulations. The surveys provide 1 m bathymetric grids of deep-water gas venting sites along the best-known gas venting areas along the Pacific margin of North America, which is an unprecedented level of resolution. Patches of conspicuously rough seafloor that are tens of meters to hundreds of meters across and occur on larger seafloor topographic highs characterize seepage areas. Some patches are composed of multiple depressions that range from 1 to 100 m in diameter and are commonly up to 10 m deeper than the adjacent seafloor. Elevated mounds with relief of 〉10 m and fractured surfaces suggest that seafloor expansion also occurs. Ground truth observations show that these areas contain broken pavements of methane-derived authigenic carbonates with intervening topographic lows. Patterns seen in Chirp profiles, ROV observations, and core data suggest that the rough topography is produced by a combination of diagenetic alteration, focused erosion, and inflation of the seafloor. This characteristic texture allows previously unknown gas venting areas to be identified within these surveys. A conceptual model for the evolution of these features suggests that these morphologies develop slowly over protracted periods of slow seepage and shows the impact of gas venting and gas hydrate development on the seafloor morphology.