SCIENCE

The analysis was based on a recent survey of 9,708 adults and a comparable survey in the early 1990s. It was primarily funded by the National Institute of Mental Health.

-- Shankar Vedantam

Scientists Devise Way to Date Oil

Oil is a fossil, the transformed remains of plants and animals that lived millions of years ago. How many millions? Curiously, that is a question scientists have not been able to answer very well.

Now, a pair of geochemists at the University of Alberta have devised a way to estimate the age of oil much more accurately. Their findings were reported in the May 27 issue of the journal Science.

David Selby and Robert A. Creaser used the rate of decay of two extremely rare metals -- rhenium and osmium -- as their clock. The elements are present in seawater but under certain conditions concentrate in bottom sediments, including ones loaded with decaying organic matter that ultimately ends up as oil.

Normally, there's much less of the osmium isotope Os-187 than of its slightly heavier sister, Os-188. In ancient seawater, the ratio was about 1 to 3.

This ratio isn't necessarily constant, however. That's because one way that Os-187 forms is by the decay of a radioactive isotope of rhenium, Re-187. This doesn't happen fast; the half-life of rhenium is 42 billion years. Nevertheless, if there is rhenium around, the amount of Os-187 can rise very slowly.

The researchers took samples from the oil sand deposits of Alberta, believed to have been formed from organic matter laid down about 112 million years ago in the Cretaceous Period, and measured the ratio of Os-187 to Os-188. It was 1.4 to 1.

Using the known rate of radioactive rhenium decay, they calculated that the plants and animals that became the Canadian oil may have been deposited much earlier -- as early as the Devonian Period, which ended 360 million years ago.

Although oil's age alone is not of great interest to petroleum companies, the light it sheds on the history of oil-bearing formations is.

"One of the things they want to know is when [oil] reservoir-filling occurred, what the actual trapping mechanism was, and what has allowed the oil to move. This will help them understand the whole system better," Selby said.

-- David Brown