Azolla event

About 49 million years ago (roughly 50 million by some dating) the Arctic Ocean was warm, almost enclosed and covered by a layer of fresh water from rivers and rain. In that setting the small floating fern Azolla grew in huge mats. Sediment cores drilled from the Lomonosov Ridge in 2004 by the Arctic Coring Expedition, reported by Jan Backman, Henk Brinkhuis and colleagues in 2006, revealed Azolla remains in layers that built up over about 800,000 years.¹

When dead ferns sank into the stagnant bottom water, which held little oxygen, they were buried with their carbon. Some modelling suggests the burial could have removed a large amount of carbon dioxide from the air, adding to a long Eocene cooling trend. It is likely that other changes mattered more, including continental movements, and the fern's exact role is debated. Even so, the story shows how living organisms can change the climate. Azolla is a tiny plant, now a weed of rice paddies, and the fossil mats make it a prime example of a planet-scale effect.