Australian shale tells tale of layered seas

Harvard researchers have found important clues about the Earth's environment 1.5 billion years ago. Their results present quite a different picture from present times, in which oceans have oxygen-rich waters from top to bottom and are capable of supporting large animal life even in their depths. The findings are important not just for what they tell us about prehistoric oceans, but also for what they tell us about oxygen in the air at the time. Scientists believe oxygen in the Earth's atmosphere rose to today's levels in two distinct jumps, 2.3 billion years ago and at about 800 million years ago.

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