The Azolla event is a
paleoclimatology
Paleoclimatology ( British spelling, palaeoclimatology) is the scientific study of climates predating the invention of meteorological instruments, when no direct measurement data were available. As instrumental records only span a tiny part of ...
scenario hypothesized to have occurred in the middle
Eocene
The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
epoch,
around , when
blooms of the
carbon-fixing freshwater
fern
The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...
''
Azolla'' are thought to have happened in the
Arctic Ocean
The Arctic Ocean is the smallest and shallowest of the world's five oceanic divisions. It spans an area of approximately and is the coldest of the world's oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, ...
. As the fern died and sank to the stagnant sea floor, they were incorporated into the
sediment
Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...
over a period of about 800,000 years; the resulting
draw-down of
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
has been speculated to have helped reverse the planet from the "
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
icehouse Earth known as the
Late Cenozoic Ice Age.
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
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks by ...
layers representing the background sedimentation of
plankton
Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
ic organisms, usual to marine sediments, switch with millimetre-thick laminations comprising
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
ised ''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
A geomagnetic reversal is a change in the Earth's Dipole magnet, dipole magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with North Pole, geographic north and South Pole, geograp ...
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
The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
to 650 ppm during this event.
''Azolla''
''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.[ 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
The Gulf Stream is a warm and swift Atlantic ocean current that originates in the Gulf of Mexico and flows through the Straits of Florida and up the eastern coastline of the United States, then veers east near 36°N latitude (North Carolin ...
— did not occur, leading to a stratified water column resembling today's Black Sea
The Black Sea is a marginal sea, marginal Mediterranean sea (oceanography), mediterranean sea lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bound ...
.
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.[
(full text of a similar article a]
doi:10.1029/2008PA001685
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
Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
, 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.
Global effects
With 800,000 years of ''Azolla'' bloom episodes and a 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. 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 since "]Snowball earth
The Snowball Earth is a historical geology, geohistorical hypothesis that proposes that during one or more of Earth's greenhouse and icehouse Earth, icehouse climates, the planet's planetary surface, surface became nearly entirely freezing, fr ...
" (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 , 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.[
]
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
A river delta is a landform, archetypically triangular, created by the deposition of the sediments that are carried by the waters of a river, where the river merges with a body of slow-moving water or with a body of stagnant water. The creat ...
s or freshwater lagoon
A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...
s to be swept into the Arctic Ocean by strong currents, removing the necessity for a freshwater layer.
Economic considerations
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. In 2008 a research team was set up in the Netherlands devoted to ''Azolla''.The Azolla Research Team
/ref>
See also
* Paleocene-Eocene Thermal Maximum
References
{{DEFAULTSORT:Azolla Event
Eocene
Environment of the Arctic
Prehistory of the Arctic
Salviniales
Greenhouse gases
Events that forced the climate
Ferns
Biological hypotheses
Paleoecology