High-resolution seismo-acoustic characterization of Green Canyon 600, a perennial hydrocarbon seep in Gulf of Mexico deep water

Green Canyon Lease Block 600 is a well-documented cold seep site characterized by near-perennial oil and gas slicks large enough to reach the sea surface and to be detected on satellite imagery. This site has been selected by the Ecosystem Impact of Oil and Gas Inputs to the Gulf consortium to study the impacts of natural hydrocarbon seepage versus anthropogenic inputs of hydrocarbons into the Gulf of Mexico ecosystem following the Deepwater Horizon oil spill disaster that occurred in 2010. Here we present the first high-resolution characterization of the site. We investigated the seafloor and shallow subsurface combining autonomous underwater vehicle-borne seismo-acoustics with photo data. Our records show vigorous hydrocarbon flux occurring over a wide area of the complex seafloor morphology. Several mounds, which are the most prominent features of the site, are aligned along a ridge structure and are actively venting hydrocarbons. Detailed bathymetry of the mound’s surface highlights the presence of numerous pockmarks, cones, and micro-mounds of variable sizes and shapes. High reflectivity seafloor backscatter characterizes the entire ridge surface; ground-truthing photo surveys confirm the presence of authigenic carbonate hardgrounds, gas hydrates, and chemosynthetic communities. The shallow stratigraphy indicates that the ridge represents the seafloor expression of normal faults, probably rooted to a deep salt body. Salt tectonics generated a complex network of normal faults, many of which displace the upper part of the Holocene sediments up to the seafloor, forming long linear scars. Acoustic anomalies, interpreted as active migration pathways, underlie mounds and venting structures, suggesting a link between the deep hydrocarbon reservoir and seafloor expulsion features.