Biogeophysical Classifìcation of Seafìoor Seeps at a Carbonate-Hydrate Mound, Northern Gulf of Mexico

Located on the continental slope in 900m of water, Woolsey Mound dominates seafloor morphology at Mississippi Canyon 118. The carbonate-hydrate mound is the site of the Gulf of Mexico Hydrates Research Consortium’s seafloor observatory to investigate and monitor hydrographic, geophysical, geological, geochemical and biological processes of the hydrocarbon system, northern Gulf of Mexico. Spatial and temporal variability of processes that produced the mound - venting fluids, formation/dissociation of hydrates, formation of authigenic carbonate and of micro and macrofaunal communities - are unknown and form the basis of several investigations at the site. Innovative survey and monitoring systems, sensors, and tools have been developed to extract samples and data to unravel the history, character and composition of the site. This study represents an attempt to integrate results of extensive geophysical studies with recent studies of the macrofauna thriving at the site, and to use the results to develop a system of vent classification for use in evaluating the subseafloor hydrocarbon system. Seafloor morphology and geology have been characterized integrating high resolution swath bathymetry, acoustic imagery, seafloor video and sediment, water column, and pore-water samples. AUV bathymetric surveys were completed in 2005 and 2009; video images, photographs, core samples and water samples were collected during cruises from 2002 - 2010. Seismo-acoustic data have been directed at maximizing definition at various depths. Deep data show a salt body underlying Woolsey Mound; crestal faults emanating from the salt body infrequently but notably intersect the seafloor. High frequency chirp and surface-source-deep-receiver data reveal many intersections of antithetical faults with the seafloor. These have been mapped over seafloor bathymetry determined from multibeam surveys. Outcropping hydrates, fluid-migration features and seafloor communities- identified and described from numerous types of imagery - have been mapped. These maps have been combined/overlain on the bathymetry/fault maps to produce a biotypes-seep map from which we have identified and differentiated types of seeps. Community complexity is used as a proxy for seep duration/age while specific community components are believed to reflect composition of seep fluids. Although preliminary, this approach represents a novel classification system for seafloor hydrocarbon seeps.