When the Deepwater Horizon oil well in the Gulf of Mexico exploded in 2010, killing the eleven men who were working on the rig, it gushed an estimated 800 million liters of oil into the ocean over a period of three months. In the massive clean-up operation that ensued, some of the oil was recovered, evaporated, or burned, a lot washed ashore all along the Gulf coast, and much was dispersed with chemicals. Yet despite all the action taken, there is still roughly 25 percent of the oil that remains unaccounted for.

It is thought that a large portion of this probably sank to the bottom of the gulf. Researchers, from the Lamont-Doherty Earth Observatory, sampled the sediment thousands of meters down and around 7 kilometers (4.5 miles) from the well after it was capped. They were shocked to find that hydrocarbons and burnt carbon were still falling from the surface months after the spill was stopped and oil supposedly cleaned up in the upper layers. Their results have recently been published in the Proceedings of the National Academy of Sciences. 

It used to be thought that oil from spills didn’t sink, but instead stayed on the surface. We now know that to be wrong, and the new study has looked into exactly how it occurs. Firstly, the oil hydrocarbons were adsorbed to fine particles, which suspended them in the water column for up to five months, much longer than anyone had suspected. These particles were then drawn to blooms of larger diatoms, a type of unicellular algae, which are at the bottom of the food chain. As the diatoms die and sink, they took with them the oil they had gathered, in what is being called a “dirty blizzard.”

There had been some suggestions that the oil still found to be sitting on the ocean floor was simply the result of natural seepages, which do occur in the region. But by analyzing the content of the oil, and looking at the specific compounds present, including barium and the distribution of olefin, the researchers were able to determine that it was indeed derived from the Deepwater Horizon site. “It's kind of like a smoking gun for the source of the contaminants,” says Beizhan Yan, lead author of the study. And the oil that is covering the sea floor is not an insignificant amount.

While hard to calculate, the researchers estimate that as much as 15 percent of the oil that gushed from the Deepwater Horizon well between April 20, 2010, and July 15, has ended up on the sea bed. How long this oily sediment will persist at the bottom of the ocean, and how it will affect the marine ecosystem is still unknown, although it is not exactly expected to have good impacts. Some suggest that it is causing significant harm, and may even affect commercial fisheries for a long time into the future, way beyond when all other evidence of the spill is gone.