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The Azolla event is a scenario hypothesized to have occurred in the middle Eocene epoch,[1] around 49 million years ago, when blooms of the freshwater fern Azolla are thought to have happened in the Arctic Ocean. As they sank to the stagnant sea floor, over a period of about 800,000 years, they were incorporated into the sediment; the resulting draw-down of carbon dioxide has been speculated to have helped transform the planet from a "greenhouse Earth" state, hot enough for turtles and palm trees to prosper at the poles, to the current icehouse Earth known as the Late Cenozoic Ice Age.

The modern fern Azolla filiculoides. Blooms of a related species may have pulled the Earth into the current icehouse world.

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