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Azolla event
The Azolla event occurred in the middle Eocene epoch, around , when blooms of the freshwater fern ''Azolla'' are thought to have happened in the Arctic Ocean. As they sank to the stagnant sea floor, 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 icehouse Earth it has been since.
==Geological evidence of the event==
In sedimentary layers throughout the Arctic basin, a unit reaching at least 8 m in thickness (the bottom of the longest core was not recovered, but it may have reached 20 m+ ) is discernible. This unit consists of alternating layers; siliceous clastic layers representing the background sedimentation of planktonic organisms, usual to marine sediments, switch with millimetre-thick laminations comprising fossilised ''Azolla'' matter. 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. Palynological controls and calibration with the high-resolution geomagnetic reversal record allows the duration of the event to be estimated at 800,000 years.〔 The event coincides precisely with a catastrophic decline in carbon dioxide levels, which fell from 3500 ppm in the early Eocene to 650 ppm during this event.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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