Environment & Science

Off Santa Barbara, oil stays in sediment, study finds

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Oil seeps have long left their mark on the Santa Barbara coast with slicks on the sea, and tar balls on the beach. New research published in the journal Environmental Science and Technology aims to explain where and how the seeping oil settles into the marine environment. KPCC’s Molly Peterson talked to one scientist on the project.

Molly Peterson: Dave Valentine says he was the kid who always played in the mud. Now a chemical oceanographer at UC Santa Barbara, he studies it. He's interested in a reservoir of oil at Coal Oil Point, a half mile or more below the sea floor, where 20 to 25 tons a day of sticky oil seep up to the surface, slicking off the central coast.

Dave Valentine: The reservoir is in many ways a black box where we know the chemical components that are there, but we don't know what happens in the cracks and fissures up to the surface.

Peterson: On its way up, Valentine says, the oil evaporates and gets weathered. Then part of it sinks down to the sea floor again. There, tiny organisms like bacteria start eating at the oil, and breaking it down. Valentine's team first took core samples, to figure out where the oil landed.

Valentine: You track the currents and you look at the pattern of oil in the sediment and you just have this tongue of oil in the sediments. And we basically nailed the plume where we see what we're calling a tar shadow.

Peterson: His calculations show a million tons of oil could sit in the sands, in that tar shadow. Valentine says the amount is somewhere between 8 to 80 spills of the Exxon Valdez, depending how far down the oil's penetrated. It's from a natural process. But Valentine says some of the same things happen to oil spilled accidentally on the open ocean.

Valentine: We think that as the economy moves towards heavier and heavier oils – which is a natural process out there, and we're running out of the light stuff – there's going to be a lot more transport of those things on the water in tankers.

Peterson: UCSB's Valentine worked with scientists at the Woods Hole Oceanographic Institute to identify the chemicals in the oil using a two-dimensional process – making a sort of fingerprint for what's in the tar shadow. That fingerprint, Valentine says, tells them how much the oil's changed moving up to the surface and back down again, and how far it moves from its source.

Valentine: We find here that the stuff, that it's about half a day to five days is how long it lasts before it finds its apparently permanent home in the sediments.

Peterson: It also says something about those tiny organisms that eat the oil. Valentine says after a while, they lose steam – they can't break down the compounds in the oil any further. Valentine says they don't know why yet. But they do know that the oil left in the floor of the ocean is going to stay there for a very long time.