Microscopic bitumen nodules in Australian rocks provide clues about early life.

There's Life in That Oil!

The world's oldest oil contains clues that early Earth may have been a livelier place than previously believed. The findings indicate that the ocean was teeming with primitive organisms 3.2 billion years ago, just over a billion years after the planet formed.

The first traces of life appear in the fossil record around 3.5 billion years ago in the form of microbial mounds in Western Australia known as stromatolites. But most rocks this age have been heated and reheated so much that any fossils they may have contained are gone. So just how extensive life was during this period remained a mystery. In 1998, a team led by geologist Birger Rasmussen of the University of Western Australia discovered microscopic drops of fluid oil preserved within mineral grains (similar to gas bubbles trapped in an ice cube) in rocks more than 3 billion years old in Australia's Pilbara region. But scientists suggested that unlike the oil on Earth today, which formed from dead organisms, the ancient oil could have formed through nonbiological processes.

Now, Rasmussen has found evidence that oil in 3.2-billion-year-old rocks in Western Australia was produced from decayed organic matter. He studied two well-preserved sequences of black shale, a type of rock that often contains organic matter and can produce oil under the right conditions. One shale is 3.2 billion years old, and the other 2.6 billion years old. In both sequences, he found thin, discontinuous streaks of kerogen, a waxy precursor to fossil fuels formed from organic matter and commonly found in much younger shales that are known oil producers. Both shales also contained microscopic nodules of bitumen, a tarlike remnant left behind when oil migrates out of the shale. Rasmussen, who reports his results in the current issue of Geology, believes the abundance and extent of the kerogen in the shales indicates the ocean was already teeming with enough single-celled life 3.25 billion years ago to support widespread oil generation.

"If we take this as globally representative, then it is impressive," says biogeochemist John Hayes of Woods Hole Oceanographic Institution in Massachusetts. But don't go pointing your drilling rigs at older rocks just yet, he cautions. Hayes believes that a fundamental change in the carbon cycle around 570 million years ago made younger rocks better at turning dead organisms into oil.

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