Anoxic event

Anoxic event

Oceanic anoxic events or Anoxic events occur when the Earth's oceans become completely depleted of oxygen (O2) below the surface levels. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past, and may have caused mass extinctions—including some of those which geobiologists employ to serve as a time marker in biostratigraphic dating. It is believed oceanic anoxic events are strongly linked to lapses in key oceanic current circulations, to climate warming and greenhouse gases.

What is clear from careful analysis of the geologic records occurring before and after the affected s is that onsets are rapid and so are recoveries—both data suggesting a sudden climate threshold or tipping point believed to occur at about four times the Earth's mean carbon dioxide levels relative to the baseline concentrations of circa 1750, the beginning of the Industrial age. The strata analysis suggest an Earth wide overheated climate of steaming rain forests, heavy daily rains, violent stormsMark Lynas, Six Steps to Hell: The Facts on Global Warming. Retrieved on 2008-07-08..And:The Eocene greenhouse event fascinates scientists not just because of its effects, which also saw a major mass-extinction in the seas, but also because of its likely cause: methane hydrates. This unlikely substance, a sort of ice-like combination of methane and water that is only stable at low temperatures and high pressure, may have burst into the atmosphere from the seabed in an immense “ocean burp”, sparking a surge in global temperatures (methane is even more powerful as a greenhouse gas than carbon dioxide). Today vast amounts of these same methane hydrates still sit on sub-sea continental shelves. As the oceans warm, they could be released once more in a terrifying echo of that methane belch of 55 million years ago. }}, heavy erosion feeding nutrients into the world's waters and a complete or at least large scale stoppage of deep water circulation between the poles and equatorFriedrich, Oliver (2008). "Warm saline intermediate waters in the Cretaceous tropical Atlantic Ocean". Nature Geoscience 1 453. — leading to 'death in the depths' from oxygen deprivation and failure to offset natural sources of mildly poisonous hydrogen sulfides. The stratified waters would support life where oxygenated in a surface zone and become rapidly more inimicable to life at increasing depths, leading to a lack even of scavenger activity along the organically rich ooze, or sapropel, accumulating on the abyssal basins and bottoms. Life forms unwarily drifting into anoxic and toxic layers would have contributed to the continual rain of unicellular microorganisms from the bioexplosion spurred by the increased nutrients from the super-greenhouse conditions that accelerated weathering and erosion and so the anoxic tipping events are believed to be the triggers to millions of years of accumulations of the carbohydrate rich deposits which became today's fossil oil reserves.

In a picture only pieced together during the last three decades, the handful of known and suspected anoxic events have been tied geologically to large-scale production of the world's oil reserves, world-wide bands of black shale in the geologic record and high relative temperatures believed linked to so called "super-greenhouse events"• At the end of the Permian period, 251 million years ago, up to 95% of species went extinct as a result of a super-greenhouse event, resulting in a temperatures rise of six degrees, perhaps because of an even bigger methane belch than happened 200 million years later in the Eocene. and also:•Five degrees of warming occurred during the Paleocene-Eocene Thermal Maximum, 55 million years ago: during that event, breadfruit trees grew on the coast of Greenland, while the Arctic Ocean saw water temperatures of 20C within 200km of the North Pole itself. There was no ice at either pole; forests were probably growing in central Antarctica.• The Eocene greenhouse event was likely caused by methane hydrates (an ice-like combination of methane and water) bursting into the atmosphere from the seabed in an immense “ocean burp”, sparking a surge in global temperatures. Today vast amounts of these same methane hydrates still sit on subsea continental shelves. • The early Eocene greenhouse took at least 10,000 years to come about. Today we could accomplish the same feat in less than a century. (emphasis, links added) }} Oceanic anoxic events stimulated by extremes of possible volcanic outgassing or at least of the characteristic elevated carbon dioxide levels four (or five) to six times current levels. At even a few degrees warmer, rain forests are extremely vulnerable to fire hazards, having little natural resistance to fires, and some conjecture a tipping point might have been reached effectively overnight in a huge burn-off releasing extraordinary amounts of carbon-dioxide into the atmosphere. With a change of mean temperatures of three degrees centigrade, the ice caps melted, in the super-greenhouse ecologies—the term meaning average temperature rose to or were beyond six degrees above today—the seas were so warm, it is believed the water temperatures at the two poles• Five degrees of warming occurred during the Eocene, 55 million years ago: breadfruit trees grew on the coast of Greenland, while the Arctic Ocean saw water temperatures of 20C within 200km of the North Pole itself. There was no ice at either pole; forests were probably growing in central Antarctica. }} were in the lower 80s°F (i.e. above 26.7°C). The Cretaceous and Jurassic s world ecology's were essentially ice free, had massive storms driven by Oceans were dying from double hit of lack of oxygen and toxic hydrogen sulfide accumulations at lower layers because of a shut down in the ocean conveyor belts.


Oceanic anoxic events most commonly occurred during periods of very warm climate characterised by high levels of carbon dioxide (CO2) and mean surface temperatures probably in excess of . The Quaternary levels, our current , are just in comparison. Such rises in carbon-dioxide may have been in response to a great outgassing of the highly flammable natural gas (methane) some have christened a "oceanic burp". Vast quantities of methane are normally locked into the Earth's crust on the continental plateau's in one of the many deposits consisting of compounds of methane hydrate, a solid precipitated combination of methane and water much like ice. Because the methane hydrates are unstable, save at cool temperatures and high (deep) pressures, scientists have observed smaller "burps" due to tectonic events. Studies suggest the huge release of natural gas could be a major climatological trigger, methane itself being a greenhouse gas which, when burned, releases carbon dioxide. However, anoxia was also rife during the Hirnantian (late Ordovician) ice age.

Oceanic anoxic events have been recognized primarily from the already warm Cretaceous and Jurassic Periods, when numerous examples have been documented, but earlier examples have been suggested to have occurred in the late Triassic, Permian, Devonian (Kellwasser event/s), Ordovician and Cambrian.

The Paleocene-Eocene Thermal Maximum (PETM), which was characterized by a global rise in temperature and deposition of organic-rich shales in some shelf seas, shows many similarities to Oceanic Anoxic Events.

Typically, oceanic anoxic events last for under half a million years, before a full recovery.

Major Oceanic Anoxic Events

The timeline data of the Jurassic and Cretaceous

The concept of the oceanic anoxic event (OAE) was first proposed in 1976 by Seymour Schlanger (1927–1990) and geologist Hugh Jenkyns and arose from discoveries made by the Deep Sea Drilling Project (DSDP) in the Pacific Ocean. It was the finding of black carbon-rich shales in Cretaceous sediments that had accumulated on submarine volcanic plateaus (Shatsky Rise, Manihiki Plateau), coupled with the fact that they were identical in age with similar deposits cored from the Atlantic Ocean and known from outcrops in Europe—particularly in the geologic record of the otherwise limestone dominated Apennines chain in Italy, that led to the realization that these widespread intervals of similar strata recorded highly unusual "punctuational" conditions in the world ocean during several distinct discrete periods of geological time.

Sedimentological investigations of these organic-rich sediments, which have continued to this day, typically reveal the presence of fine laminations undisturbed by bottom-dwelling fauna, indicating anoxic conditions on the sea floor, believed to be coincident with a low lying poisonous layer of hydrogen sulfide. Furthermore, detailed organic geochemical studies have recently revealed the presence of molecules (so-called biomarkers) that derive from both purple sulfur bacteria and green sulfur bacteria: organisms that required both light and free hydrogen sulfide (H2S), illustrating that anoxic conditions extended high into the upper water column.

Such sulfidic (or euxinic) conditions, which exist today in many water bodies from ponds to various land surrounded mediterranean seas such as the Black Sea of today, were particularly prevalent in the Cretaceous Atlantic but also characterized other parts of the world ocean. In an ice free sea of these believed super-greenhouse worlds, oceanic waters were as much as 200 meters higher, in some eras. During the time spans in question, the continental plates are believed to have been well separated, and the mountains we know today were (mostly) future tectonic events—meaning the overall landscapes were generally much lower— and even the half super-greenhouse climates would have been eras of highly expedited water erosion carrying massive amounts of nutrients into the world oceans fueling an overall explosive population of microorganisms and their predator species in the oxygenated upper layers.

Detailed stratigraphic studies of Cretaceous black shales from many parts of the world have indicated that two Oceanic Anoxic Events were particularly significant in terms of their impact on the chemistry of the oceans, one in the early Aptian (~120 Ma), sometimes called the Selli Event (or OAE 1a) after the Italian geologist, Raimondo Selli (1916–1983), and another at the Cenomanian–Turonian boundary (~93 Ma), sometimes called the Bonarelli Event (or OAE 2) after the Italian geologist, Guido Bonarelli (1871–1951).

  • Insofar as the Cretaceous OAEs can be represented by type localities, it is the striking outcrops of laminated black shales within the vari-colored claystones and pink and white limestones near the town of Gubbio in the Italian Apennines that are the best candidates.
  • The 1-meter thick black shale at the Cenomanian–Turonian boundary that crops out near Gubbio is termed the ‘Livello Bonarelli’ after the man who first described it in 1891.

More minor Oceanic Anoxic Events have been proposed for other intervals in the Cretaceous (Valanginian, Hauterivian, Albian, Coniacian–Santonian stages), but their sedimentary record, as represented by organic-rich black shales, appears more parochial, being dominantly represented in the Atlantic and neighboring areas, and some researchers relate them to particular local conditions rather than being forced by global change.

The only Oceanic Anoxic Event documented from the Jurassic took place during the early Toarcian (~183 Ma). Because no DSDP or ODP (Ocean Drilling Program) cores have recovered black shales of this age – there being little or no Toarcian ocean crust remaining in the world ocean – the samples of black shale primarily come from outcrops on land. These outcrops, together with material from some commercial oil wells, are found on all major continents and this event seems similar in kind to the two major Cretaceous examples.


Temperatures throughout the Jurassic and Cretaceous are generally thought to have been relatively warm, and consequently dissolved oxygen levels in the ocean were lower than today - making anoxia easier to achieve. However, more specific conditions are required to explain the short-period (half a million years or less) oceanic anoxic events. Two hypotheses, and variations upon them, have proved most durable.

One hypothesis suggests that the anomalous accumulation of organic matter relates to its enhanced preservation under restricted and poorly oxygenated conditions, which themselves were a function of the particular geometry of the ocean basin: such a hypothesis, although readily applicable to the young and relatively narrow Cretaceous Atlantic (which could be likened to a large-scale Black Sea, only poorly connected to the World Ocean), fails to explain the occurrence of coeval black shales on open-ocean Pacific plateaus and shelf seas around the world. There are suggestions, again from the Atlantic, that a shift in oceanic circulation was responsible, where warm, salty waters at low latitudes became hypersaline and sunk to form an intermediate layer, at 500-1000m depth, with a temperature of 20-25.

The second hypothesis suggests that oceanic anoxic events record a major change in the fertility of the oceans that resulted in an increase in organic-walled plankton (including bacteria) at the expense of calcareous plankton such as coccoliths and foraminifera.

Such an accelerated flux of organic matter would have expanded and intensified the oxygen minimum zone, further enhancing the amount of organic carbon entering the sedimentary record. Essentially this mechanism assumes a major increase in the availability of dissolved nutrients such as nitrate, phosphate and possibly iron to the phytoplankton population living in the illuminated layers of the oceans.

For such an increase to occur would have required an accelerated influx of land-derived nutrients coupled with vigorous upwelling, requiring major climate change on a global scale. Geochemical data from oxygen-isotope ratios in carbonate sediments and fossils, and magnesium/calcium ratios in fossils, indicate that all major oceanic anoxic events were associated with thermal maxima, making it likely that global weathering rates, and nutrient flux to the oceans, were increased during these intervals. Indeed, the reduced solubility of oxygen would lead to phosphate release, further nourishing the ocean and fuelling high productivity, hence a high oxygen demand - sustaining the event through a positive feedback.

Here is another way of looking at oceanic anoxic events. Assume that the earth releases a huge volume of carbon dioxide during an interval of excessive volcanism; global temperatures rise due to the greenhouse effect; global weathering rates and fluvial nutrient flux increase; organic productivity in the oceans increases; organic-carbon burial in the oceans increases (OAE begins); carbon dioxide is drawn down (inverse greenhouse effect); global temperatures fall, and the ocean–atmosphere system returns to equilibrium (OAE ends).

In this way, an oceanic anoxic event can be viewed as the Earth’s response to the injection of excess carbon dioxide into the atmosphere and hydrosphere. One test of this notion is to look at the age of large igneous provinces (LIPs), the extrusion of which would presumably have been accompanied by rapid effusion of vast quantities of volcanogenic gases such as carbon dioxide. Intriguingly, the age of three LIPs (Karoo-Ferrar flood basalt, Caribbean large igneous province, Ontong Java Plateau) correlates uncannily well with that of the major Jurassic (early Toarcian) and Cretaceous (early Aptian and Cenomanian–Turonian) oceanic anoxic events, indicating that a causal link is feasible.

Paleozoic anoxia

The boundary between the Ordovician and Silurian periods marked by repetitive periods of anoxia, interspersed with normal, oxic conditions. In addition, anoxic periods are found during the Silurian. These anoxic period occurred at a time of low global temperatures (although levels were high), in the midst of a glaciation.

Jeppsson (1990) proposes a mechanism whereby the temperature of polar waters determines the site of formation of downwelling water. If the high latitude waters are below 5, they will be dense enough to sink; as they are cool, oxygen is highly soluble in their waters, and the deep ocean will be oxygenated. If high latitude waters are warmer then 5, their density is too low for them to sink below the cooler deep waters. Therefore thermohaline circulation can only be driven by salt-increased density, which tends to form in warm waters where evaporation is high. This warm water can dissolve less oxygen, and is produced in smaller quantities, producing a sluggish circulation with little deep water oxygen. The effect of this warm water will propagate through the ocean, and the warmer water has the additional effect of reducing the amount of which can be stored in the oceans - causing the release of large quantities to the atmosphere in a short time - tens or thousands of years. The warm waters would also initiate the release of clathrates, further increasing both atmospheric temperature and basin anoxia. Similar positive feedbacks operate during P-episodes, amplifying their cooling effects.

The periods with cold poles are termed "P-episodes" (short for primo), and are characterised by bioturbated deep oceans, a humid equator and higher weathering rates, and terminated by extinction events - for example, the Ireviken and Lau events. The inverse is true for the warmer, oxic "S-episodes" (secundo), where deep ocean sediments are typically graptolitic black shales. A typical cycle of secundo-primo episodes and ensuing event typically lasts around 3 Ma.

The duration of events is so long compared to their onset because the positive feedbacks must be overwhelmed. Carbon is only removed from the ocean-atmosphere system by changes in weathering rates, which are dominantly controlled by rainfall. Because this is inversely related to temperature in Silurian times, carbon is gradually drawn down during warm (high ) S-episodes, while the reverse is true during P-episodes. On top of this gradual trend is overprinted the signal of Milankovic cycles, which ultimately trigger the switch between P- and S- episodes.

These events become longer during the Devonian; the enlarging land plant biota probably acted as a large buffer to carbon dioxide concentrations.

The end-Ordovician Hirnantian event may alternatively be a result of algal blooms, caused by sudden supply of nutrients through wind-driven upwelling or an influx nutrient-rich meltwater from melting glaciers, which by virtue of its fresh nature would also slow down oceanic circulation.

Atmospheric effects

A model put forward by Lee Kump, Alexander Pavlov and Michael Arthur in 2005 suggests that oceanic anoxic events may have been characterized by upwelling of water rich in highly toxic hydrogen sulfide gas which was then injected into the atmosphere. This phenomenon would likely have poisoned plants and animals and caused mass extinctions. Furthermore, it has been proposed that the hydrogen sulfide rose to the upper atmosphere and attacked the ozone layer, which normally blocks the deadly ultraviolet radiation of the Sun. The increased UV radiation caused by this ozone depletion would have amplified the destruction of plant and animal life. Fossil spores from strata recording the Permian extinction show deformities consistent with UV radiation. This evidence, combined with fossil biomarkers of green sulfur bacteria, indicates that this process could have played a role in that mass extinction event, and possibly other extinction events. The trigger for these mass extinctions appears to be a warming of the ocean caused by a rise of carbon dioxide levels to about 1000 parts per million.


Oceanic anoxic events have had many important consequences. It is believed that they have been responsible for mass extinctions of marine organisms both in the Paleozoic and Mesozoic. The early Toarcian and Cenomanian-Turonian anoxic events correlate with the Toarcian and Cenomanian-Turonian extinction events of mostly marine life forms. Apart from possible atmospheric effects, many deeper-dwelling marine organisms could not adapt to an ocean where oxygen penetrated only the surface layers.

Another, economically significant consequence of oceanic anoxic events is the fact that the prevailing conditions in so many Mesozoic oceans has helped produce most of the world's petroleum and natural gas reserves. During an oceanic anoxic event, the accumulation and preservation of organic matter was much greater than normal, allowing the generation of potential petroleum source rocks in many environments across the globe. Consequently some 70 percent of oil source rocks are Mesozoic in age, and another 15 percent date from the warm Paleogene: only rarely in colder periods were conditions favorable for the production of source rocks on anything other than a local scale.

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