Azolla event

Azolla event

The Azolla event occurred in the middle Eocene period,citation
author = Henk Brinkhuis, Stefan Schouten;
year = 2006
title = Episodic fresh surface waters in the Eocene Arctic Ocean
journal = Nature
volume = 441
issue = 7093
pages = 606–609
doi = 10.1038/nature04692
url =
] around Ma|49, when blooms of the freshwater fern "Azolla" occurred 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 from the atmosphere 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.cite journal
author = Waddell, L.M.
coauthors = Moore, T.C.
year = 2006
title = Salinity of the Early and Middle Eocene Arctic Ocean From Oxygen Isotope Analysis of Fish Bone Carbonate
journal = American Geophysical Union, Fall Meeting 2006, abstract# OS53B-1097
url =
accessdate = 2007-10-16
] 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 [Parts per million. Today's concentration is 380 ppm, and the concentration between the last glacial and the industrial revolution peaked at 280 ppm.] during this event.


The fossil ferns are morphologically indistinguishable from modern examples of the genus, which has led to the formation of a working group to better understand the physiology of the organism. "Azolla" today is a freshwater fern which forms a symbiotic relationship with a cyanobacterium, "Anabaena"; this organism fixes nitrogen very efficiently. "Azolla" has been deemed a "super-plant" as it can draw down as much as a tonne of nitrogen per acre per yearcite journal
author = Belnap, J.
year = 2002
title = Nitrogen fixation in biological soil crusts from southeast Utah, USA
journal = Biology and Fertility of Soils
volume = 35
issue = 2
pages = 128–135
url =
accessdate = 2007-10-17

doi = 10.1007/s00374-002-0452-x ] (0.25kg/m²/yr); this is matched by 6 tonnes per acre of carbon drawdown (1.5kg/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.cite journal
author = Brinkhuis, H.
coauthors = Schouten, S.; Collinson, M.E.; Sluijs, A.; Sinninghe Damsté, J.S.; Dickens, G.R.; Huber, M.; Cronin, T.M.; Onodera, J.; Takahashi, K. "et al".
year = 2006
title = Episodic fresh surface waters in the Eocene Arctic Ocean
journal = Nature
volume = 441
pages = 7093
url =
accessdate = 2007-10-17

doi = 10.1038/nature04692 ]

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.cite journal
author = Stein, R.
year = 2006
title = The Paleocene-Eocene (“Greenhouse“) Arctic Ocean paleoenvironment: Implications from organic-carbon and biomarker records (IODP-ACEX Expedition 302)
journal = Geophysical Research Abstracts
volume = 8
pages = 06718
url =
accessdate = 2007-10-16
format = abstract
] 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.cite web| first1 = J.D. | last1 = Gleason | first2 = D.T. | last2 = Thomas | first3= T.C. last3= Moore | first4= J.D. | last4= Blum | first5= R.M. | last5= Owen | title = Water column structure of the Eocene Arctic Ocean from Nd-Sr isotope proxies in fossil fish debris | year = 2007 | accessdate = 2007-11-03 | url= | quote = 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 ] Even a few centimetres of fresh water would be enough to allow the colonisation of "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) and accessible nitrogen in the atmosphere are known to have been high at this time.cite journal
author = Pearson, P.N.
coauthors = Palmer, M.R.
year = 2000
title = Atmospheric carbon dioxide concentrations over the past 60 million years
journal = Nature
volume = 406
issue = 6797
pages = 695–699
url =
accessdate = 2008-03-14
doi = 10.1038/35021000

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 eventually fossilised. 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.

Global effects

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 a global temperature decline which continued for millions of years; 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, [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.] the planet had ice caps at both of its poles. A geologically rapid decrease in temperature between 49 and Ma|47, 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 Ma|15 that evidence for widespread polar freezing is common.

Alternative explanations

Whilst a verdant Arctic Ocean is a viable working model, sceptical scientists point out that it would be possible for "Azolla" colonies in deltas or freshwater lagoons to be swept into the Arctic Ocean by strong currents, removing the necessity for a freshwater layer.cite journal|journal=National Geographic|date=May 2005|url=|title=Great green north|author=Tim Appenzeller]

Economic considerations

Much of the current interest in oil exploration in the Arctic regions, ironically made possible by global warming, 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.cite news|journal=New York Times|date=2004-11-20|title="Under all that ice, maybe oil|author=ANDREW C. REVKIN|url=|accessdate=2007-10-17] While this does mean that much money is available for the study of this event — a centre having been set up in the Netherlands devoted to "Azolla" [] — the use of these fossil fuels may reverse the climate change caused by "Azolla".

Horizontal timeline
title=The Azolla event in geological time
caption=Million years ago. Age of Earth = 4,650
Row1=Note row
1-text=Azolla event

Row3=Geological eons|-600
Row2=Geological periods|-600
Row4=Scale row

Notes and references

ee also

* Paleocene-Eocene Thermal Maximum

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