The Great Unknowns in Gulf Oil Spill

The deep water of the ocean is the largest habitat on earth but it’s also the least understood, making the effects of this deep-sea spill without precedent.

The oil spill in the Gulf of Mexico falls into a distinct category from any other oil catastrophe; it’s the first blowout in history to release oil in such deep waters, nearly a mile below the surface.
As a result, scientists say, the impacts of this spill are likely to go far beyond the oiled birds and dead sea turtles, spoiled beaches and wetlands that we think of when we think “oil spill.” A substantial piece of the total impact is likely occurring under the sea, invisible (for now at least) but no less ominous than the more traditional shoreline effects. Far below the sea, the spill threatens organisms of all kinds and, indirectly, the ecosystem at large, though the extent of the danger is still obscured.
Oil on the surface of the ocean is a known quantity, says Ed Overton, an oil-spill expert at the Louisiana State University who is analyzing water, sediment, and other samples for NOAA’s scientific-support team. “It’s going to cause very substantial and noticeable damage—but it won’t take very long to find the marsh loss and coastal erosion and impact on fisheries,” he says. The effects of oil in the water column and at the sea floor, on the other hand, remains a mystery.
The first scientific mission to assess deepwater impacts of the Deepwater Horizon blowout, conducted from the research ship Pelican and funded by NOAA, discovered massive plumes of dispersed oil up to 30 miles long by seven miles wide and hundreds of feet thick. Though the data collected by the Pelican was criticized by NOAA as being too preliminary to draw conclusions from, scientists say the finding is not surprising and is in line with the results of previous studies.
One such study, a 2003 report by the National Research Council, considered what the effects of a deepwater well blowout might be and predicted that such an event, particularly of a reservoir rich in gas (as the Deepwater Horizon reservoir appears to be) would generate diffuse underwater plumes of microaerosolized oil much like what the Pelican scientists found.
A few years earlier, the U.S. Minerals Management Service (MMS) organized a study in 2000 in which scientists released oil into deep seas off the coast of Norway, but could only account for a small amount of it on the surface—suggesting that much of it remained in the water column. (While the conditions of this study aren’t identical to the conditions of the current spill, Overton says the general findings could be expected to apply).
Conventional wisdom suggests that oil is lighter than water and therefore floats, but that’s not entirely the case when a complex mixture of crude oil and natural gas is gushing from a well a mile below the surface, at high temperature and pressure, as is happening right now in the gulf. In this case, the gas can effervesce out of the oil, aerosolizing it into tiny droplets, much the way a fine mist emerges from the top of an aerosol can. Some of these droplets may be neutrally buoyant, meaning they move to a point in the water column where they neither rise nor sink, possibly resulting in underwater “plumes” like the ones reported. Adding subsea dispersants, which similarly break up the oil and are intended to prevent it from reaching the surface, may exacerbate this and could have toxic effects themselves.