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In a review of hundreds of paleoceanographic studies, researchers found evidence for only one period in the last 300 million years when the oceans changed as fast as today: the Paleocene-Eocene Thermal Maximum, or PETM. In ocean sediment cores, the PETM appears as a brown mud layer flanked by thick deposits of white plankton fossils. About 56 million years ago, a mysterious surge of carbon into the atmosphere warmed the planet and turned the oceans corrosive. [Source: National Science Foundation]
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