View source for Azolla event

{{short description|Hypothetical geoclimatic event}}
[[Image:Azolla filiculoides MUN.jpg|thumb|The modern fern ''[[Azolla filiculoides]]''. Blooms of a related species may have pulled the [[Earth]] into the current icehouse world.]]

The '''Azolla event''' is a [[paleoclimatology]] scenario hypothesized to have occurred in the middle [[Eocene]] epoch,<ref name=Brinkhuis2006>{{Cite journal
 |vauthors=Brinkhuis H, Schouten S, Collinson ME, Sluijs A, Sinninghe Damsté JS, Dickens GR, Huber M, Cronin TM, Onodera J, Takahashi K, Bujak JP, Stein R, van der Burgh J, Eldrett JS, Harding IC, Lotter AF, Sangiorgi F, van Konijnenburg-van Cittert H, de Leeuw JW, Matthiessen J, Backman J, Moran K
 | 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
 | pmid = 16752440| bibcode = 2006Natur.441..606B 
 | hdl = 11250/174278 
 | s2cid = 4412107 
 | hdl-access = free
 }}</ref> around {{Ma|49}}, when [[algal bloom|blooms]] of the [[carbon fixation|carbon-fixing]] freshwater [[fern]] ''[[Azolla]]'' are thought to have happened in the [[Arctic Ocean]].  As the fern died and sank to the stagnant sea floor, they were incorporated into the [[sediment]] over a period of about 800,000 years; the resulting [[drawdown (climate)|draw-down]] of [[carbon dioxide]] has been speculated to have helped reverse the planet from the "[[greenhouse and icehouse Earth|greenhouse Earth]]" state of the [[Paleocene-Eocene Thermal Maximum]], when the planet was hot enough for turtles and palm trees to prosper at the poles, to the current [[Greenhouse and icehouse Earth|icehouse Earth]] known as the [[Late Cenozoic Ice Age]].

==Geological evidence of the event==
[[Image:65 Myr Climate Change.png|thumb|300px|right|{{delta|18|O|link}} – 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{{Citation needed|date=April 2009}}.]]
In sedimentary layers throughout the Arctic basin, a unit reaching at least 8&nbsp;m in thickness (the bottom of the longest core was not recovered, but it may have reached 20&nbsp;m+{{citation needed|date=December 2014}}) is discernible.  This unit consists of alternating layers; siliceous [[clastic]] layers representing the background sedimentation of [[plankton]]ic organisms, usual to marine sediments, switch with millimetre-thick laminations comprising [[fossil]]ised ''Azolla'' matter.<ref name=Waddell2008>{{cite journal
 | author = Waddell, L.M.
 | author2= Moore, T.C.
 | year = 2008
 | title = Salinity of the Eocene Arctic Ocean from oxygen isotope analysis of fish bone carbonate
 | journal= Paleoceanography
 | volume= 23
 | issue= 1
 | pages= n/a
 | doi = 10.1029/2007PA001451
| bibcode= 2008PalOc..23.1S12W
 | hdl= 2027.42/95320
 | url = https://deepblue.lib.umich.edu/bitstream/2027.42/95320/1/palo1410.pdf
 | doi-access= free
 }}</ref> This organic matter can also be detected in the form of a [[gamma ray logging|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&nbsp;years.<ref name=Brinkhuis2006/>  The event coincides precisely with a catastrophic decline in [[Carbon dioxide in Earth's atmosphere|carbon dioxide levels]], which fell from 3500&nbsp;[[Parts per million|ppm]] in the early [[Eocene]] to 650&nbsp;ppm during this event.<ref name=Pearson2000>{{cite journal
 | author = Pearson, P.N.
 |author2= 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
 | doi = 10.1038/35021000
 | pmid = 10963587
  |bibcode= 2000Natur.406..695P
 |s2cid= 205008176
 }}</ref>

==''Azolla''==
''Azolla'' has been deemed a "super-plant" as it can draw down as much as a tonne of nitrogen per acre per year<ref name=Belnap2002>{{cite 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
 | doi = 10.1007/s00374-002-0452-x
  | bibcode = 2002BioFS..35..128B
 | s2cid = 41924008
 | url = https://zenodo.org/record/1232763
 }}</ref> (0.25&nbsp;kg/m<sup>2</sup>/yr); this is matched by 6&nbsp;tonnes per acre of carbon drawdown (1.5&nbsp;kg/m<sup>2</sup>/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&nbsp;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.<ref name=Brinkhuis2006/> This rate of growth pushes the plants deep under away from sunlight where death and carbon sequestration occur.

==Conditions encouraging the event==
[[Image:Early Eocene Arctic basin.PNG|thumb|The continental configuration during the Early Eocene resulted in an isolated Arctic basin.]]
During the early Eocene, the [[continental drift|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 [[stratification (water)|stratified water column]] resembling today's [[Black Sea]].<ref name=Stein2006>{{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 = http://www.cosis.net/abstracts/EGU06/06718/EGU06-J-06718-1.pdf
 | format = abstract| accessdate = 2007-10-16
  }}</ref>
High temperatures and winds led to high evaporation, increasing the density of the ocean, and — through an increase in rainfall<ref>Greenwood, D.R., Basinger, J.F. and Smith, R.Y. 2010. How wet was the Arctic Eocene rainforest? Estimates of precipitation from Paleogene Arctic macrofloras. Geology, 38(1): 15 - 18. {{doi|10.1130/G30218.1}}</ref> — high discharge from rivers which fed the basin.  This low-density freshwater formed a [[nepheloid layer]], floating on the surface of the dense sea.<ref name=Gleason2007>
{{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=http://www.goldschmidt2007.org/abstracts/A329.pdf
 |quote=The [[Strontium|Sr]]-[[Neodymium|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 
}}{{dead link|date=October 2016 |bot=InternetArchiveBot |fix-attempted=yes }} (full text of a similar article at [https://www.researchgate.net/profile/Ted_Moore/publication/254903697_Early_to_middle_Eocene_history_of_the_Arctic_Ocean_from_Nd-Sr_isotopes_in_fossil_fish_debris_Lomonosov_Ridge/links/53e0bf2d0cf24f90ff60afaa/Early-to-middle-Eocene-history-of-the-Arctic-Ocean-from-Nd-Sr-isotopes-in-fossil-fish-debris-Lomonosov-Ridge.pdf doi:10.1029/2008PA001685])</ref>
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.<ref name=Pearson2000/>

Blooms alone are not enough to have any geological impact; to permanently draw down CO<sub>2</sub> and cause [[climate change (general concept)|climate change]], the carbon must be [[Carbon sequestration|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.

==Global effects==
With 800,000&nbsp;years of ''Azolla'' bloom episodes and a {{cvt|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 CO<sub>2</sub> by this one phenomenon alone.{{citation needed|date=March 2016}} 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&nbsp;°C to today's &minus;9&nbsp;°C,<ref name=Brinkhuis2006/> and the rest of the globe underwent a similar change. For perhaps the first time since "[[Snowball earth]]" (over half a billion years earlier), 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 northern polar freezing is common.<ref name=NatGeo/>

==Alternative explanations==
While a verdant Arctic Ocean is a viable working model, skeptical scientists point out that it would be possible for ''Azolla'' colonies in [[river delta]]s or freshwater [[lagoon]]s to be swept into the Arctic Ocean by strong currents, removing the necessity for a freshwater layer.<ref name=NatGeo>{{cite journal|journal=National Geographic|date=May 2005|url=http://ngm.nationalgeographic.com/ngm/0505/resources_geo.html|archive-url=https://web.archive.org/web/20080326202943/http://ngm.nationalgeographic.com/ngm/0505/resources_geo.html|url-status=dead|archive-date=March 26, 2008|title=Great green north|author=Tim Appenzeller}}</ref><ref>Neville, L.A., Grasby, S.E., McNeil, D.H., 2019, Limited freshwater cap in the Eocene Arctic Ocean.  Scientific Reports (2019)9:4226. {{doi|10.1038/s41598-019-40591-w}}</ref>

==Economic considerations==
Much of the current interest in oil exploration in the Arctic regions is directed towards the ''Azolla'' deposits {{citation needed|date=December 2014}}.  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.<ref name=NYT>{{cite news|journal=New York Times|date=2004-11-20|title=Under all that ice, maybe oil|author=ANDREW C. REVKIN|url=https://www.nytimes.com/2004/11/30/science/earth/30core.html|accessdate=2007-10-17}}</ref>  In 2008 a research team was set up in the Netherlands devoted to ''Azolla''.<ref>[https://web.archive.org/web/20110613171839/http://www.cosis.net/members/meetings/team.php?team=3246 The Azolla Research Team]</ref>
{{Simple Horizontal timeline

|caption=Millions of years ago <br>[[Age of Earth]] = 4,540 million years (off to the left).  Complex life started with the [[Palæozoic]].  <br>The "Age of the Dinosaurs" was the [[Mesozoic]].  [[Homo|Tool-using humans]] have only been around for the last few million years on the far right.

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|row1-1-text=Azolla event

|row2={{Geological periods|-600}}
|row3={{Geological eons|-600}}

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==See also==
* [[Paleocene-Eocene Thermal Maximum]]

==References==
{{Reflist}}

{{DEFAULTSORT:Azolla Event}}
[[Category:Eocene]]
[[Category:Environment of the Arctic]]
[[Category:Prehistory of the Arctic]]
[[Category:Salviniales]]
[[Category:Greenhouse gases]]
[[Category:Events that forced the climate]]
[[Category:Ferns]]
[[Category:Biological hypotheses]]
[[Category:Paleoecology]]