Azolla event
The Azolla event is a
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+[
Azolla
Azolla has been deemed a "super-plant" as it can draw down as much as a tonne of nitrogen per acre per year[4] (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.[1] This rate of growth pushes the plants deep under away from sunlight where death and carbon sequestration occur.
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.[5] High temperatures and winds led to high evaporation, increasing the density of the ocean, and — through an increase in rainfall[6] — high discharge from rivers which fed the basin. This low-density freshwater formed a nepheloid layer, floating on the surface of the dense sea.[7] 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.[3]
Blooms alone are not enough to have any geological impact; to permanently draw down CO2 and cause
Global effects
With 800,000 years of Azolla bloom episodes and a 4,000,000 km2 (1,500,000 sq mi) 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.[
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 deltas or freshwater lagoons to be swept into the Arctic Ocean by strong currents, removing the necessity for a freshwater layer.[9][10]
Economic considerations
Much of the current interest in oil exploration in the Arctic regions is directed towards the Azolla deposits [citation needed]. 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.[11] In 2008 a research team was set up in the Netherlands devoted to Azolla.[12]
See also
- Paleocene-Eocene Thermal Maximum
References
- ^ S2CID 4412107.
- hdl:2027.42/95320.
- ^ S2CID 205008176.
- S2CID 41924008.
- ^ Stein, R. (2006). "The Paleocene-Eocene ("Greenhouse") Arctic Ocean paleoenvironment: Implications from organic-carbon and biomarker records (IODP-ACEX Expedition 302)" (abstract). Geophysical Research Abstracts. 8: 06718. Retrieved 2007-10-16.
- doi:10.1130/G30218.1
- ^
Gleason, J.D.; Thomas, D.T.; Moore, T.C.; Blum, J.D.; Owen, R.M. (2007). "Water column structure of the Eocene Arctic Ocean from Nd-Sr isotope proxies in fossil fish debris" (PDF). Retrieved 2007-11-03.
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
[permanent dead link] (full text of a similar article at doi:10.1029/2008PA001685) - Snowball earth" is a matter of debate.
- ^ a b Tim Appenzeller (May 2005). "Great green north". National Geographic. Archived from the original on March 26, 2008.
- ^ ANDREW C. REVKIN (2004-11-20). "Under all that ice, maybe oil". New York Times. Retrieved 2007-10-17.
- ^ The Azolla Research Team