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The early Earth was essentially anoxic. A number of indicators suggest the presence of oxygenic photosynthesis ∼2700–3000 million years (Ma) ago, but direct evidence for molecular oxygen (O 2 ) in seawater has remained elusive. Here we report rare earth element (REE) analyses of ∼2800 million year old shallow-marine limestones and deep-water iron-rich sediments at Steep Rock Lake, Canada....
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