The Azolla Event

Sunday, 07 March 2010 14:18 Galaxy Science - Climate

Azolla has been deemed a "super-plant" as it can draw down as much as a tonne of nitrogen per acre per year^[5] (0.25 kg/m²/yr); this is matched by 6 tonnes per acre of carbon drawdown (1.5 kg/m²/yr). Its ability to use atmospheric nitrogen for growth means that the main limit to its growth is usually the availability of phosphorus: carbon, nitrogen and sulphur being three of the key elements of proteins, and phosphorus being required for DNA, RNA and in energy metabolism. 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.

Geological evidence of the event

Image: δ¹⁸O – a proxy for temperature – over the past 65 million years. The Azolla event marks the end of the Eocene optimum and the beginning of a long-term decline in global temperatures.

In sedimentary layers throughout the Arctic basin, a unit reaching at least 8 m in thickness^[2] 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.^[3] 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.^[1] 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.