The BP oil spill left an oily "bathtub ring" on the sea floor that's about the size of U.S. State of Rhode Island or a little larger than Canada's Manitoulin Island, new research shows.

The study by David Valentine, the chief scientist on the federal damage assessment research ships, estimates that about 37 million litres (10 million gallons) of oil coagulated on the floor of the Gulf of Mexico around the damaged Deepwater Horizons oil rig.

Controlled burning oil slicks

Scientists are still trying to figure where all the oil went and what effects it had. Some of the surface slicks were managed with controlled burning. (David Valentine)

Valentine, a geochemistry professor at the University of California Santa Barbara, said the spill from the Macondo well left other splotches containing even more oil. He said it is obvious where the oil is from, even though there were no chemical signature tests because over time the oil has degraded.

"There's this sort of ring where you see around the Macondo well where the concentrations are elevated," Valentine said. The study, published in Monday's Proceedings of the National Academy of Sciences, calls it a "bathtub ring."

Oil levels inside the ring were as much as 10,000 times higher than outside the 3100-square-kilometre ring, Valentine said. A chemical component of the oil was found on the sea floor, anywhere from two-thirds of a mile to a mile below the surface.

The rig blew on April 20, 2010, and spewed 651 million litres of oil into the Gulf through the summer. Scientists are still trying to figure where all the oil went and what effects it had.

Oil from spill grossly overstated, BP says

BP questions the conclusions of the study. In an email, spokesman Jason Ryan said, "the authors failed to identify the source of the oil, leading them to grossly overstate the amount of residual Macondo oil on the sea floor and the geographic area in which it is found."

It's impossible at this point to do such chemical analysis, said Valentine and study co-author Christopher Reddy, a marine chemist at Woods Hole Oceanographic Institute, but all other evidence, including the depth of the oil, the way it laid out, the distance from the well, directly point to the BP rig.

Outside marine scientists, Ed Overton at Louisiana State University and Ian MacDonald at Florida State University, both praised the study and its conclusions.

The study does validate earlier research that long-lived deep water coral was coated and likely damaged by the spill, Reddy said. But Reddy and Valentine said there are still questions about other ecological issues that deep.

Hydrocarbon contamination Deepwater Horizon seafloor

The 'bathtub ring' of hydrocarbon contamination is seen on the seafloor near the Macondo Well, with dots indicating affected coral communities. The lower right inset depicts the molecular structure of hopane, a chemical component of the oil found on the seafloor. ( Image courtesy of G. Burch Fisher.)