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.