Seafloor geomorphic manifestations of gas venting and shallow subbottom gas hydrate occurrences

Abstract

High-resolution multibeam bathymetry data collected with an autonomous underwater vehicle (AUV) complemented by compressed high-intensity radar pulse (Chirp) profi les and remotely operated vehicle (ROV) observations and sediment sampling reveal a distinctive rough topography associated with seafl oor 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 Pacifi c margin of NorthAmerica, which is an unprecedented level of resolution. Patches of conspicuously rough seafl oor that are tens of meters to hundreds of meters across and occur on larger seafl oor 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 seafl oor. Elevated mounds with relief of >10 m and fractured surfaces suggest that seafl oor 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 profi les, ROV observations, and core data suggest that the rough topography is produced by a combination of diagenetic alteration, focused erosion, and infl ation of the seafl oor. This characteristic texture allows previously unknown gas venting areas to be identifi ed 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 seafl oor morphology.

Active mud volcanoes on the continental slope of the Canadian Beaufort Sea

“…Also notably absent were visible accumulations of methane‐derived authigenic carbonates which are commonly exposed on the seafloor around methane seepage sites elsewhere, including the surface of some mud volcanoes [e.g., Lance et al ., ]. Such phenomena create distinctive seafloor morphologies easily identifiable in AUV survey data [e.g., Paull et al ., ]. The observed high alkalinity and depletions in pore waters calcium and magnesium (Figure c) is consistent with authigenic carbonate formation.…”

Submarine Mass Movements and their Consequences

“…Alternatively, the sediment slide opened pathways for fluid flow, which subsequently derived into gas-hydrate formation, authigenic carbonate precipitation and the blistering that produced the mound (Paull et al 2015a). One possibility is that the active gas and oil flow compromised the sediment strength and facilitated the sediment failure(s) that generated the large-scale morphology of Eel Slump.…”

“…41.1 and 41.2) that stands nearly 60 m higher than the surrounding seafloor. The mound displays a distinctive rough topography known to develop at methane seeps where methane-derived authigenic carbonates overlay and intercalate with gas-hydrate deposits (Paull et al 2015a). Chirp profiles over the mound do not resolve sub-bottom reflectors (Fig.…”

Bayesian inversion of marine controlled source electromagnetic data offshore Vancouver Island, Canada

“…Comparing the vertical position of the elevated resistivities at WP 18 within the seismic section suggests that the higher resistivities are within the shallow, younger sediments, rather than in the older, accreted sediments. Additionally, sub-bottom profiler data obtained with an autonomous underwater vehicle (Paull et al 2009) show seismic blank zones at WP 18 that may indicate the presence of fluid venting and/or gas hydrate occurrences.…”