The Discovery of Deep Oil Plumes at the Deepwater Horizon Oil Spill Site (Invited)

In May 2010, the National Institute for Undersea Science and Technology (NIUST), a partnership of the University of Mississippi, the University of Southern Mississippi and NOAA, had a 17-day research cruise aboard the UNOLS vessel R/V Pelican scheduled. Two weeks before departure, the Deepwater Horizon oil platform burned and sank, resulting in an uncontrolled oil spill at a depth of ~1500 m at Mississippi Canyon Block 252. The initial mission plan to do AUV surveys of wrecks and hydrate outcrops in the northern Gulf of Mexico, some of them very close to the site of the accident, was abandoned in favor of responding to the still uncontrolled oil spill. The primary goals of the redefined cruise were to acquire baseline and early impact data for seafloor sediments and subsurface distribution of oil and gas hydrates as close as possible in time and space to the origin of the oil spill. Investigating an oil spill nearly a mile deep in the ocean presents special benthic sampling and subsurface oil detection challenges. NIUST’s AUV’s were unloaded from the ship and a large main winch installed to allow operation of a full ocean depth box corer for collecting sediment samples in water depths up to 2000 m. During the first five-day leg of the cruise, a total of 28 box cores were collected. The Pelican returned to port (Cocodrie, LA) to drop off sediment and water samples for immediate analyses, and to take on more sampling gear and supplies for the second leg of the cruise, including an Acrobat, a CDOM fluorometer, a Video Ray ROV, and a CO2 sensor in addition to the already installed CTD Rosette with O2 sensor and beam transmissometer. During Leg 2, CTD stations were plotted to cover the area surrounding the wreck site and at various water depths to map the subsurface water column structure and chemistry as baseline values for future investigations and especially to look for submerged oil and/or gas hydrates. Early in the water column sampling, a subsurface feature was discovered at 1200 to 1400 m depth. This layer was detected by three independent sensors, CDOM (colored dissolved organic matter) fluorometer, beam transmissometer, and dissolved oxygen sensor. All three instruments responded in unison with greater fluorescence and beam attenuation and decreased O2 concentration. These signals were first observed at a station 5 miles from the accident site. Second and third station measurements at 2.5 miles, and at 1.25 miles from the spill site, showed the same signal but with significantly greater magnitude. Following this discovery, the sampling plan for the remaining days of the cruise was changed to map the newly discovered feature. This paper will discuss our data acquired during this cruise aboard the RV Pelican and its original discovery of the deep oil plumes from the Deepwater Horizon well.