The presence of free oxygen in the oceans occurred at least 200 million years before oxygen began to build up in the Earth's atmosphere, reports a paper online in Nature Geoscience. This finding supports previous controversial studies that have suggested that organisms capable of producing oxygen through photosynthesis appeared hundreds of millions of years before the accumulation of free oxygen in the atmosphere 2.3 billion years ago.
Linda Godfrey and Paul Falkowski reconstructed the ancient nitrogen cycle using nitrogen isotopes from the organic matter preserved in two- to three-billion-year-old rocks from South Africa. They found evidence of nitrogen cycles that could have taken place only in the presence of free oxygen, beginning about 2.7 billion years ago, with definitive evidence appearing 2.5 billion years ago.
They conclude that oxygen-producing organisms had evolved by at least 2.5 billion years ago, and that there was a few-hundred-million-year delay between the time oxygen was available in the oceans and its build-up in the early Earth's atmosphere.