The Dirty Blizzard: how oil from the Deepwater Horizon spill reached the seafloor

The paper:

Yan B, Passow U, Chanton JP, Nöthig EM, Asper V, Sweet J, Pitiranggon M, Diercks A, Pak D. Sustained deposition of contaminants from the Deepwater Horizon spill. Proceedings of the National Academy of Sciences. 2016 May 31:201513156.

Oil, oil, everywhere

Figure 1: The Deepwater Horizon oil spill (Creative Commons)
Figure 1: The Deepwater Horizon oil spill (Creative Commons)

The BP Deepwater Horizon oil spill—the largest in history—dumped over 200 million gallons of oil into the Gulf of Mexico (Figure 1). It captured the world’s attention and triggered a massive cleanup operation. But after the spill was finally contained, visible signs of the spill disappeared amazingly quickly. A research cruise 3 weeks after found little trace of the oil left in the surface layer of the ocean. However, it left an unexpected legacy. Organisms that live on the seafloor of the Gulf of Mexico, like corals and some fish, were impacted more than anyone thought possible. In addition to the direct effects of exposure to toxic oil chemicals, microbes eating the oil caused depletion of oxygen near the seafloor, harming the marine life that relies on it. But how did so much end up at the seafloor? Since oil floats on water, the spill should only affect the surface ocean, right?

A new study investigates how the oil got from the surface to the deep ocean. The authors hypothesized that oil got there by hitching a ride on marine snow, which is made of clumps of dead algae, poop from larger plankton, and other tiny particles that rain down through the water column after a plankton bloom. It is quite sticky, so chemicals and pollutants at the surface can glob on and sink with the marine snow particle. If oil absorbed onto sinking marine snow, this could explain how it was quickly removed from the surface but accumulated on the seafloor.

To determine how much oil might have reached the seafloor via marine snow, the authors set up a sediment trap near the site of the spill (Figure 2). A sediment trap is a giant funnel suspended in the water column to capture any sinking particles. The trap automatically directs the falling particles into one of 20 collection bottles, changing about every month, which lets them look at how the falling particles change with time. The trap was set up in the weeks after the spill, and left out for the next year, so the aftermath of the spill could be compared with more normal conditions the next year. Ideally there would be multiple traps to compare but since the spill was so unexpected there was only time to put out one.

Figure 2: location of the sediment trap and the site of the spill (left) and a picture of the sediment trap (right).
Figure 2: location of the sediment trap and the site of the spill (left) and a picture of the sediment trap (right) (Yan et al. 2016).


Oil and marine snow: a dirty blizzard

Figure 3: Skeletonema under a microscope
Figure 3: Skeletonema under a microscope (Creative Commons)

Skeletonema (Figure 3), a type of algae that does well in oil spill conditions, formed a huge bloom right after the spill. At its peak, over 8 billion dead Skeletonema cells per square meter were sinking through the water column- and apparently taking oil with them! The marine snow found in the sediment traps during the bloom contained oil degradation products directly traceable to the spill. Some of this could have been from the algae themselves, since they produce their own oil. However, some detective work showed it was mostly from the spill, since petroleum has a slightly different molecular signature compared to freshly produced algal oils. In particular, the material in the sediment trap contained polyaromatic hydrocarbons (also known as PAHs), toxic compounds found in oil but not algae. The pattern of alkanes (similar to saturated fats) is also different in petroleum and algae, and a parameter known as the carbon preference index (CPI) can be used to identify the “fingerprint” of petroleum from oil spills. The CPI in the sediment trap after the oil spill matched the petroleum fingerprint, pointing squarely at Deepwater Horizon as the likely source (Figure 4).

Figure 4: composition of PAHs (left, red), and the alkane distribution (carbon preference index; right and in blue). For 5 months after the spill, the sediment trap contained oil from the spill. After that most came from plants and algae.
Figure 4: composition of PAHs (left, red), and the alkane distribution (carbon preference index; right and in blue). For 5 months after the spill, the sediment trap contained oil from the spill. After that most came from plants and algae (Yan et al. 2016).

Another pollutant found in the sediment traps was black carbon. This is like charcoal, the remains left behind after a fire. It could be either from the burning of the oil itself during the explosion and spill (there were about 400 controlled burns to remove some of this oil, and these fires leave behind about 5% of the original material), or associated with the oil burning by all the ships that responded to the spill. About 20% of the total char produced during these burns sank from the surface in association with the diatom bloom. Finally, there was evidence of the drill mud used to cap the spill found its way into the food chain. Barite, a mineral used in the mud mixture, was found in the sediment traps with the marine snow, indicating it was absorbed by the algae before they sank.

The long legacy of oil spills

This study illustrates why the surface appeared to be cleaned up rather quickly. The oil and other pollutants attached to sinking particles, and they sank to the deep ocean. That means the problem was out of sight, but this just moved the oil to the seafloor where it remained toxic to deep sea life. In addition, it shows how dangerous oil products can make their way into fish and other marine organisms. The algae that absorbed the oil form the base of the food chain in the Gulf of Mexico, and organisms that eat them will also absorb the oil. Since some oil products bioaccumulate and are difficult for organisms to eliminate, this can lead to toxic concentrations in predators like fish higher up the food chain. The study is a reminder that even after the obvious effects of a spill, like oils slicks on the surface and tar balls washing up on the beach, are no longer visible, the oil can continue to wreak havoc on marine ecosystems beneath the waves.

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