Azolla Event

The Azolla event was a period roughly 49 million years ago, in the middle Eocene epoch, when blooms of the carbon fixing freshwater fern Azolla proliferated across the Arctic Ocean, then a low salinity, landlocked sea. Over about 800,000 years the fern's remains sank and were buried in seafloor sediment, drawing down atmospheric carbon dioxide from an estimated 3,500 parts per million in the early Eocene to about 650 parts per million by the event's end. The resulting drop in greenhouse gas is credited with helping tip the planet's climate from the hothouse conditions of the Paleocene-Eocene Thermal Maximum toward the cooler icehouse state that eventually allowed polar ice caps to form at both poles.