Azolla event

As the fern died and sank to the stagnant sea floor, they were incorporated into the sediment over a period of about 800,000 years; the resulting draw-down of carbon dioxide has been speculated to have helped reverse the planet from the "greenhouse Earth" state of the Paleocene-Eocene Thermal Maximum, when the planet was hot enough for turtles and palm trees to prosper at the poles, to the current icehouse Earth known as the Late Cenozoic Ice Age.

[2] This organic matter can also be detected in the form of a gamma radiation spike, that has been noted throughout the Arctic basin, making the event a useful aid in lining up cores drilled at different locations.

The plant can grow at great speed in favourable conditions – modest warmth and 20 hours of sunlight, both of which were in evidence at the poles during the early Eocene – and can double its biomass over two to three days in such a climate.

[3] Blooms alone are not enough to have any geological impact; to permanently draw down CO2 and cause climate change, the carbon must be sequestered by the plants being buried and the remains rendered inaccessible to decomposing organisms.

With 800,000 years of Azolla bloom episodes and a 4,000,000 km2 (1,500,000 sq mi) basin to cover, even by very conservative estimates more than enough carbon could be sequestered by plant burial to account for the observed 80% drop in CO2 by this one phenomenon alone.

This drop initiated the switch from a greenhouse to the current icehouse Earth; the Arctic cooled from an average sea-surface temperature of 13 °C to today's −9 °C,[1] and the rest of the globe underwent a similar change.