Scientists document fate of oil slicks from natural seeps

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David Valentine, UCSB

Twenty years ago, the oil tanker Exxon Valdez was exiting Alaska's Prince William Sound when it struck a reef in the middle of the night. What happened next is considered one of the nation's worst environmental disasters: 10.8 million gallons of crude oil spilled into the pristine Alaskan waters, eventually covering 11,000 square miles of ocean.

Now, imagine 8 to 80 times the amount of oil spilled in the Exxon Valdez accident.

According to new findings by scientists from the University of California at Santa Barbara (UCSB) and the Woods Hole Oceanographic Institution (WHOI), that's how much oil has made its way into sediments offshore from petroleum seeps near Coal Oil Point off Goleta, Calif., in the Santa Barbara Channel.

These natural seeps release some 20 to 25 tons of oil daily, "providing an ideal laboratory to investigate the fate of oil in the coastal ocean," says oceanographer David Valentine of UCSB.

The team's research, reported in a paper published in the May 15 issue of the journal Environmental Science & Technology, documents how the oil is released by the seeps, carried to the surface along a meandering plume, then deposited on the ocean floor in sediments that stretch for miles northwest of Coal Oil Point.

The findings also reveal that the oil is so degraded by the time it gets buried in the sea bed that it's a mere shell of the petroleum that initially bubbles up from the seeps.

"These were spectacular findings," said Christopher Reddy, a marine chemist at WHOI and, along with Valentine, one of the co-authors of the paper.

Other co-authors include UCSB's Libe Washburn; and Emily Peacock and Robert Nelson, both of WHOI.

"Whether it comes from a natural seep or from human industry, oil entering the ocean has a 'life story,'" says Don Rice, program director in the National Science Foundation (NSF)'s Division of Ocean Sciences, "one with many chapters after the early ones we all see--the surface slicks, tar balls on the beach and affected marine animals. This team of scientists set about constructing the rest of the story--and succeeded.

"In an energy-hungry world, it's a tale we're going to need to know a lot about."

The lead author is Christopher Farwell, who at the time of the research was an undergraduate studying chemistry at UCSB. Inspired by this project, Farwell has changed his career path and is now a graduate student at UCSB studying marine science and earth science.

"It was a great opportunity," Farwell said. "I was able to cross over into a different discipline that allowed me to make a contribution and understand the process of science as a whole."

Valentine, who supervised Farwell's research, said, "It's unusual to have an undergraduate take the lead in such a significant study, and its success is a testament to Chris's perseverance."

In an earlier paper published in 2008, Valentine and Reddy documented how microbes devour many of the compounds in the oil emanating from the seeps.

The new study examines the final step in the life cycle of the oil.

"One of the natural questions is: What happens to all this oil?" Valentine said. "So much oil seeps up and floats on the sea surface. It's something we've long wondered.

"We know some of it will come ashore as tar balls, but it doesn't stick around. And then there are massive slicks. You can see them, sometimes extending 20 miles from the seeps. But what is really their ultimate fate?"

Based on previous research, Valentine and Reddy surmised that the oil was sinking "because the oil is heavy to begin with," Valentine said.

"It's a good bet that it ends up in the sediments because it's not ending up on land. It's not dissolving in ocean water."

An all-night sampling marathon on the research vessel (R/V) Atlantis, funded by NSF, provided the means to test the hypothesis. With Farwell and Reddy leading the way, the team took 16 sediment samples from the ocean floor, following a carefully calculated path mapped out by Farwell.

The researchers were hoping that their route, described by Farwell as a "rectangle along the coast from Santa Barbara to Point Conception," would match the trail of the plume.

Farwell's calculations were perfect, Valentine said. The 16-point route yielded an unmistakable pattern of oil-saturated sediment all along the ship's path.

The scientists then painstakingly analyzed the samples using Reddy's comprehensive two-dimensional gas chromatograph. "What we saw is that we can link the seep oils to the oils in the sediment," Valentine said.

"We can do that through the composition of molecules that are specific to the oils from the seeps. So, being able to link them, and being able to quantify how much is there, we can see the pattern of the oil. It's coming from the seeps."

Washburn, who has been using radio waves to map ocean currents off Santa Barbara, provided additional evidence. "Libe took a seven-year average of surface current flow in the region, and plotted that out," Valentine said. "It matched perfectly with our plume."

This research proved to be an extension of the 2008 study by Valentine and Reddy: that the oil has indeed been degraded, largely eaten away by microbes, before it settles back to the ocean floor and becomes buried.

"For all these samples, the bacteria seem to hit a common 'wall' where they don't eat anymore," Valentine said. "In the previous study, we were looking at subsurface biodegradation where there is no oxygen.

"You still have thousands of compounds in that oil, but now we're seeing the evaporation and dissolution that happens to the slick, and then the biodegradation that happens in the slick with oxygen present.

"When it finally falls to the sea floor, it continues to be biodegraded. It seems to be biodegraded to the same point--and then it just stops."

"It's dramatic how much the oil loses in this life cycle," Reddy said. "It's almost like someone who has lost 400 pounds."

Source: National Science Foundation