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http://www.sciencenews.org/view/access/id/44415
THE IRON RECORD OF EARTH'S OXYGEN
Among Earth's oldest rocks are banded iron formations, named for the layers rich in iron that alternate with layers hosting little iron. In some places, these layers extend for kilometers. The formations remain puzzling. The most extensive ones probably stopped forming around the time the atmosphere became oxygen-rich. Credit: Mikeuk/iStockphoto


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