In this paper we present the results of the first high resolution morpho-acoustic study of OC26. Seafloor morphology, seafloor sediment reflectivity, water column data from multibeam echosounders, shallow subsurface
seismic images, and photographs have been integrated to characterize the natural hydrocarbon seepage system and help the ECOGIG scientists to build a baseline dataset for this natural seep site.
OC26 is located on the Louisiana continental slope, approximately 100 km southeast of the Birdsfoot delta of the Mississippi River in ~1600 m of water. The region is known for abundant hydrocarbon seeps and gas hydrates (Garcia-Pineda et al., 2010). The study area is located on the western part of the Gloria Dome, one of the many dome structures in the northern GOM.
2.1. Geological context, natural seeps, and gas hydrates in the Gulf of Mexico
Despite its nature as a passive continental margin, the GOM is a complex geodynamic province. It is largely characterized by faulting, folding, seismic activity and other deformational processes related to the layering of thick sedimentary units over buoyant salt deposits. The continuous BMS265246
of salt domes and connected salt withdrawal basins produces complex faulting patterns that serve as conduits for migrating fluids, including gases, liquid hydrocarbons, and brines, from the deep oil reservoirs (Sassen et al., 2004). This geodynamic framework affects GOM natural hydrocarbon seepage and gas hydrates distribution/occurrence. The GOM is different from many other continental margins (e.g. Blake Ridge, Hydrates Ridge, Costa Rica Margin, and Nankai accretionary margin) where the top of the hydrates stability zone is tens of meters or more below the seafloor and well-marked by the presence of a Bottom Simulating Reflector (BSR). Rather, hydrates in the GOM may be found at and near the seafloor in addition to many meters sub-seafloor. In areas dominated by salt tectonics, the presence of high salinity pore fluids alters the HSZ due to both the high thermal conductivity of salt and the inhibitory effect of dissolved salt on hydrate formation (Ruppel et al., 2005). Such thermal and chemical perturbations, which are particularly prevalent above and on the flanks of salt diapirs, tend to compress the gas hydrate zone and shift it closer to the seafloor. At OC26, outcropping hydrates and active seeps occur together and support microbial communities, chemosynthetic communities and communities of benthic fauna no longer dependent upon hydrocarbon seeps for their sustenance. As illustrated in Fig. 1, the lower continental slope is populated by salt domes.
Fig. 1. Location of MC297 (OC26) in the Northern Gulf of Mexico.Figure optionsDownload full-size imageDownload as PowerPoint slide
Often these structures are subject to gravity slides that disturb the local bathymetry. (Note the slump scar upslope from the Whiting Dome where the resulting canyon is 60-80 m deep.)
3. Materials and methods
This study is based on an extensive, new, wide range of acoustic and photodata collected by the National Institute for Undersea Science and Technology (NIUST). The NIUST-owned and operated AUV, Eagle Ray (ER), was used to collect multibeam swath bathymetry, multibeam backscatter, and chirp subbottom profiles. Previously collected water-column echosounder data (R/V Falkor, 2012) were used to target water column plumes related to active gas venting, while the NIUST AUV, Mola Mola (MM), was employed to collect video images to ground-truth acoustic data and to investigate benthic fauna
In October 2012, the Eagle Ray was launched at OC26 to perform a MultiBeam Echo Sounder (MBES) survey of a 27.7 km2 area. The vehicle surveyed from a constant altitude of 50 m above the seafloor, mapping 175 m wide swaths of the seafloor, using a Kongsberg EM2000 (200 kHz) MBES and a Konsberg GeoAcoustics subbottom profiler (SBP). This survey produced high-resolution bathymetry, acoustic backscatter data, and along the nadir of the vehicle’;s track, high resolution sub-bottom profiles. Of paramount importance in AUV surveying, is the ability to acquire accurate geospatial data in the deep sea. This ability is