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.
Azolla has been deemed a "super-plant" as it can draw down as much as a tonne of nitrogen per acre per year (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.
Conditions encouraging the event
During the early Eocene, the continental configuration was such that the Arctic sea was almost entirely cut off from the wider oceans. This meant that mixing — provided today by deep water currents such as the Gulf Stream — did not occur, leading to a stratified water column resembling today's Black Sea. High temperatures and winds led to high evaporation, increasing the density of the ocean, and — through an increase in rainfall — high discharge from rivers which fed the basin. This low-density freshwater formed a nepheloid layer, floating on the surface of the dense sea. Even a few centimetres of fresh water would be enough to allow colonization by Azolla; further, this river water would be rich in minerals such as phosphorus, which it would accumulate from mud and rocks it interacted with as it crossed the continents. To further aid the growth of the plant, concentrations of carbon (in the form of carbon dioxide) in the atmosphere are known to have been high at this time.
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. The anoxic bottom of the Arctic basin, a result of the stratified water column, permitted just this: the anoxic environment inhibits the activity of decomposing organisms and allows the plants to sit unrotted until they are buried by sediment and incorporated into the fossil record.
With 800,000 years of Azolla bloom episodes and a 4,000,000 km² 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. It should be stressed that other factors almost certainly played a role. 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, and the rest of the globe underwent a similar change. For perhaps the first time in its history, the planet had ice caps at both of its poles. A geologically rapid decrease in temperature between 49 and , around the Azolla event, is evident: dropstones — which are taken as evidence for the presence of glaciers — are common in Arctic sediments thereafter. This is set against a backdrop of gradual, long-term cooling: it is not until that evidence for widespread northern polar freezing is common.
While a verdant Arctic Ocean is a viable working model, skeptical scientists point out that it would be possible for Azolla colonies in river deltas or freshwater lagoons to be swept into the Arctic Ocean by strong currents, removing the necessity for a freshwater layer.
Much of the current interest in oil exploration in the Arctic regions is directed towards the Azolla deposits. The burial of large amounts of organic material provides the source rock for oil, so given the right thermal history, the preserved Azolla blooms might have been converted to oil or gas. A research team has been set up in the Netherlands devoted to Azolla.
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The Sr-Nd isotopic record is [...] indicative of a poorly mixed ocean and highly stratified water column with anoxic bottom waters. A stable, "fresh" water upper layer was likely a pervasive feature of the Eocene Arctic Ocean
- It is almost certainly the first time the planet had bipolar glaciation during the Phanerozoic; whether or not it was present during the Neoproterozoic "Snowball earth" is a matter of debate.
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- The Azolla Research Team