When did Oxygen appear on Earth?

oxygen, which makes up about 21 per cent of the Earth's atmosphere, might have made its appearance on the planet million of years earlier than what scientists had earlier thought. An analysis of a deep rock core from Australia indicates the presence of some amount of oxygen 50 to 100 million years before the 'great oxidation event'--the earliest indications of oxygen on earth--occurred.

The 'great oxidation event' is believed to have occurred about 2.3 to 2.4 billion years ago. Scientists claim the sudden evolution of organisms that produce oxygen by photosynthesis had caused the event. But the new findings suggest significant changes were occurring much before it.Two teams of us researchers--from the University of Maryland and the Arizona State University--analysed layers of sedimentary rock in a 3,000-ft-long core sample from the Hamersley Basin in Western Australia and found evidence of oxygen in the oceans and possibly Earth's atmosphere 2.5 billion years ago.

Recovered from deep underground, the shale retained evidence of oxidation. This "whiff" of oxygen, the scientists believe, was produced via photosynthesis by some simple organism, possibly a type of aquatic bacteria, which inhabited the planet at the time. In their studies published in the journal Science (Vol 317, No 5846), the researchers doubt the Great Oxidation Event theory and say the oxygen buildup in Earth's atmosphere was gradual than sudden. They also suggest free oxygen radicals were undetectable just before that time.

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