The R/V Cape Hatteras, on a port call after two weeks zig-zagging around the northern Gulf of Mexico on the trail of oil from the Deepwater Horizon spill, seemed quiet when I came aboard Friday afternoon. Some of her eight scientists and technicians (and ten crew members) are enjoying a day off, while others prepare for the next stretch of a 27-day cruise. I watch food and sodas go onto the ship and trash come off, and overhear snatches of cell phone calls home. Chief scientist Tracy Villareal of the University of Texas at Austin contemplates a Google map of the floor of the Gulf.
The existence and location of an oil plume rising from the ruptured well-head has been debated for months (see ‘The mystery of the missing oil plume’ and related stories at Nature’s collection of oil spill stories).
Measurements of hydrocarbons and depleted oxygen at 38 stations on the first leg of the Cape Hatteras’ cruise have already revealed a plume of oil from MC252, better known as Deepwater Horizon. The plume is about 1,000 meters deep, up to several hundred meters thick, and 60 miles long. It extends southwest from the blown well, its movement affected by the Gulf’s complex bottom topography. The data do not indicate oil to the southeast, as predicted by NOAA models, Villareal said.
The main order of business on the day that I show up is determining measurement stations for the second leg of the cruise. As one of the scientists says, locating the oil is a bit like finding a needle in a haystack – only the haystack in question is hundreds of square miles of water up to several thousand meters deep.
Station selection is based on the results so far, plus a grid developed by the National Oceanic and Atmospheric Administration (NOAA) that aggregates data from a number of research cruises, and consultation with the crew of the expedition’s partner ship, the Oceanus. Four stations are selected along DeSoto canyon northeast of the well site (see map), a handful near the well, and a few others on the northern edge of NOAA’s grid. That grid already shows early data from this cruise – the yellow and orange dots at S15 and 17 and T15 and 16 that aren’t quite on the corners of the grid.
We leave Gulfport Saturday morning and sail through calm seas, escorted by sea birds and bow-surfing dolphins then, farther out, scattering flying fish and a continuous stream of dinner-plate-sized jelly fish. By 8pm, the ship stops at Station 39, some 91 nautical miles from Gulfport. The Conductivity-Temperature-Depth (CTD) array drops over the side at 9:15 pm, disappearing below the surface in the glare of the ship’s lights. In the tech room, Villareal, intern Emily Gyde, and technician Tina Thomas monitor real-time data transmitted from the CDT on a bank of computer screens.
The cruise was originally an NSF-funded project to examine nitrogen fixation in shallow northern Gulf waters, but it was revised after the Deepwater Horizon blew. That resulted in scrambling for equipment, some of it begged or borrowed. “Usually, these cruises are planned for a year or more,” Villareal says. “This one was put together in about 3 months.” The Cape Hatteras (owned by NSF and assigned to the Duke/University of North Carolina Oceanographic Consortium) is working in tandem with chief scientist Joe Montaya on the Oceanus (a Woods Hole Oceanographic Institution vessel). The former does surveys to locate the plume, and the latter then does additional studies. Only the first half-dozen stations have been set in stone, so the rest can account for what the crews might, or might not, find tonight and in the days to come.
Oil on the surface can usually be mapped via satellite and visual imagery, but subsurface oil presents more of a challenge. Since hydrocarbons have fluorescent properties, certain sensors can detect different organic compounds, including crude oil. Oxygen depletion, known in the business as dissolved oxygen anomaly, provides a measure of metabolism of hydrocarbons by bacteria. The cruise collects real-time data on these two indicators.
After about 45 minutes, the CDT returns to the surface and the ship’s wet lab is full of activity. Water collected in the CDT’s niskins, or bottles, is drained and parceled out. Ellen Knapke, a graduate student at the University of Texas, measures bacteria and Katie Swanson, a technician working for Villareal, is in charge of nutrient analysis (see picture). Antje Vossmeyer and Kamrun Zargar, from University of Georgia Athens, measure methane. Within an hour, the ship arrives at Station 40, 11.6 nautical miles along the transect, and the process starts all over again. Another hour, and the Hatteras moves on to Station 41. It’s going to be a long night.
Posted on behalf of Melissa Gaskill
Map credit: NOAA c/o Tracy Villareal
Picture credits: Melissa Gaskill