Anoxic event

Oceanic anoxic events or anoxic events (anoxia conditions) were intervals in the Earth's past where portions of oceans became depleted in oxygen (O2) at depths over a large geographic area. During some of these events, euxinia, waters that contained hydrogen sulfide, H
, developed.[2] Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events coincided with several mass extinctions and may have contributed to them.[3] These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating.[4] Many geologists believe oceanic anoxic events are strongly linked to slowing of ocean circulation, climatic warming, and elevated levels of greenhouse gases. Researchers have proposed enhanced volcanism (the release of CO2) as the "central external trigger for euxinia".[5]

Aquatic Dead Zones
Red circles show the location and size of many dead zones.
Black dots show Ocean dead zones of unknown size.
The size and number of marine dead zones—areas where the deep water is so low in dissolved oxygen that sea creatures can't survive—have grown explosively in the past half-century.NASA Earth Observatory[1]
Thermohaline Circulation 2
This world perspective on oceanic currents demonstrates the interdependencies of transnational regions on circulating currents.


The concept of the oceanic anoxic event (OAE) was first proposed in 1976 by Seymour Schlanger (1927–1990) and geologist Hugh Jenkyns[6] 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 geological record of the otherwise limestone-dominated Apennines[6] chain in Italy – that led to the realization that these widespread similar strata recorded highly unusual oxygen-depleted conditions in the world ocean during several 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.[7] Furthermore, detailed organic geochemical studies have recently revealed the presence of molecules (so-called biomarkers) that derive from both purple sulfur bacteria[7] and green sulfur bacteria: organisms that required both light and free hydrogen sulfide (H2S), illustrating that anoxic conditions extended high into the illuminated upper water column.

There are currently several places on earth that are exhibiting the features of anoxic events on a localized scale such as algal/bacterial blooms and localized "dead zones". Dead zones exist off the East Coast of the United States in the Chesapeake Bay, in the Scandinavian strait Kattegat, the Black Sea (which may have been anoxic in its deepest levels for millennia, however), in the northern Adriatic as well as a dead zone off the coast of Louisiana. The current surge of jellyfish worldwide is sometimes regarded as the first stirrings of an anoxic event.[8] Other marine dead zones have appeared in coastal waters of South America, China, Japan, and New Zealand. A 2008 study counted 405 dead zones worldwide.[9]

This is a recent understanding. This picture was 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 in worldwide bands of black shale in the geologic record. Likewise the high relative temperatures believed linked to so called "super-greenhouse events".[7]


Oceanic anoxic events with euxinic (i.e. sulfidic) conditions have been linked to extreme episodes of volcanic outgassing. Thus, volcanism contributed to the buildup of CO2 in the atmosphere, increased global temperatures, causing an accelerated hydrological cycle that introduced nutrients to the oceans to stimulate planktonic productivity. These processes potentially acted as a trigger for euxinia in restricted basins where water-column stratification could develop. Under anoxic to euxinic conditions, oceanic phosphate is not retained in sediment and could hence be released and recycled, aiding continued high productivity.[5]


External image A flow chart of magma sourcing trace metals, ocean fertilization, stratification, and anoxia.

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 (less than a million years) 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 sank to form an intermediate layer, at 500 to 1,000 m (1,640 to 3,281 ft) depth, with a temperature of 20 °C (68 °F) to 25 °C (77 °F).[10]

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.[11]

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 intense 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 due to both burial of organic matter and weathering of silicate rocks (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.


Oceanic anoxic events most commonly occurred during periods of very warm climate characterized by high levels of carbon dioxide (CO2) and mean surface temperatures probably in excess of 25 °C (77 °F). The Quaternary levels, the current period, are just 13 °C (55 °F) in comparison. Such rises in carbon dioxide may have been in response to a great outgassing of the highly flammable natural gas (methane) that some call an "oceanic burp".[7][12] Vast quantities of methane are normally locked into the Earth's crust on the continental plateaus 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, except at cool temperatures and high (deep) pressures, scientists have observed smaller "burps" due to tectonic events. Studies suggest the huge release of natural gas[7] could be a major climatological trigger, methane itself being a greenhouse gas many times more powerful than 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,[13][14] but earlier examples have been suggested to have occurred in the late Triassic, Permian, Devonian (Kellwasser event), 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 lasted for less than a million years, before a full recovery.


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.[11] 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.

An 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.

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 released into the atmosphere. This phenomenon would probably 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-Triassic extinction event 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.[15]

Ocean chemistry effects

Reduced oxygen levels are expected to lead to increased seawater concentrations of redox-sensitive metals. The reductive dissolution of ironmanganese oxyhydroxides in seafloor sediments under low-oxygen conditions would release those metals and associated trace metals. Sulfate reduction in such sediments could release other metals such as barium. When heavy-metal-rich anoxic deep water entered continental shelves and encountered increased O2 levels, precipitation of some of the metals, as well as poisoning of the local biota, would have occurred. In the late Silurian mid-Pridoli event, increases are seen in the Fe, Cu, As, Al, Pb, Ba, Mo and Mn levels in shallow-water sediment and microplankton; this is associated with a marked increase in the malformation rate in chitinozoans and other microplankton types, likely due to metal toxicity.[16] Similar metal enrichment has been reported in sediments from the mid-Silurian Ireviken event.[17]

Anoxic events in Earth's history


Sulfidic (or euxinic) conditions, which exist today in many water bodies from ponds to various land-surrounded mediterranean seas[18] such as the Black Sea, were particularly prevalent in the Cretaceous Atlantic but also characterized other parts of the world ocean. In an ice-free sea of these supposed 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[7] 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 (OAEs) 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) [19] after the Italian geologist, Raimondo Selli (1916–1983), and another at the CenomanianTuronian boundary (~93 Ma), sometimes called the Bonarelli Event (or OAE 2)[19] after the Italian geologist, Guido Bonarelli (1871–1951). OAE1a lasted for ~1.0 to 1.3 Myr.[20] The duration of OAE2 is estimated to be ~820 kyr based on a high-resolution study of the significantly expanded OAE2 interval in southern Tibet, China.[21]

  • 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 (in the Valanginian, Hauterivian, Albian and ConiacianSantonian 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).[13][14] Because no DSDP (Deep Sea Drilling Project) 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.


The boundary between the Ordovician and Silurian periods is marked by repetitive periods of anoxia, interspersed with normal, oxic conditions. In addition, anoxic periods are found during the Silurian. These anoxic periods occurred at a time of low global temperatures (although CO
levels were high), in the midst of a glaciation.[22]

Jeppsson (1990) proposes a mechanism whereby the temperature of polar waters determines the site of formation of downwelling water.[23] If the high latitude waters are below 5 °C (41 °F), 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 than 5 °C (41 °F), 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.[23] The effect of this warm water propagates through the ocean, and reduces the amount of CO
that the oceans can hold in solution, which makes the oceans release large quantities of CO
into the atmosphere in a geologically short time (tens or thousands of years).[24] The warm waters also initiate the release of clathrates, which further increases atmospheric temperature and basin anoxia.[24] Similar positive feedbacks operate during cold-pole episodes, amplifying their cooling effects.

The periods with cold poles are termed "P-episodes" (short for primo[24]), 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.[23] A typical cycle of secundo-primo episodes and ensuing event typically lasts around 3 Ma.[24]

The duration of events is so long compared to their onset because the positive feedbacks must be overwhelmed. Carbon content in the ocean-atmosphere system is affected by changes in weathering rates, which in turn is dominantly controlled by rainfall. Because this is inversely related to temperature in Silurian times, carbon is gradually drawn down during warm (high CO
) 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.[24]

These events become longer during the Devonian; the enlarging land plant biota probably acted as a large buffer to carbon dioxide concentrations.[24]

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 of nutrient-rich meltwater from melting glaciers, which by virtue of its fresh nature would also slow down oceanic circulation.[25]

Archean and Proterozoic

It has been thought that through most of Earth's history, oceans were largely oxygen-deficient. During the Archean, euxinia was largely absent because of low availability of sulfate in the oceans,[5] but during the Proterozoic, it would become more common.

See also


  1. ^ Aquatic Dead Zones NASA Earth Observatory. Revised 17 July 2010. Retrieved 17 January 2010.
  2. ^ Timothy W. Lyons; Ariel D. Anbar; Silke Severmann; Clint Scott & Benjamin C. Gill (January 19, 2009). "Tracking Euxinia in the Ancient Ocean: A Multiproxy Perspective and Proterozoic Case Study". Annual Review of Earth and Planetary Sciences. 37 (1): 507–53. Bibcode:2009AREPS..37..507L. doi:10.1146/
  3. ^ Wignall, Paul B.; Richard J. Twitchett (24 May 1996). "Oceanic Anoxia and the End Permian Mass Extinction". Science. 5265. 272 (5265): 1155–1158. Bibcode:1996Sci...272.1155W. doi:10.1126/science.272.5265.1155. PMID 8662450.
  4. ^ Peters, Walters; Modowan K.E. (2005). The Biomarker Guide, Volume 2: Biomarkers and Isotopes in the Petroleum Exploration and Earth History. Cambridge University Press. p. 749. ISBN 978-0-521-83762-0.
  5. ^ a b c Katja M Meyer; Lee R Kump (January 9, 2008). "Oceanic euxinia in Earth history: Causes and consequences". Annual Review of Earth and Planetary Sciences. 36: 251–288. Bibcode:2008AREPS..36..251M. doi:10.1146/ Retrieved April 11, 2014. The central external trigger for euxinia is proposed to be enhanced volcanism (release of volcanic CO2), although other external forcings of the climate system could be imagined (changing solar luminosity, changes in continental configuration affecting ocean circulation and the stability of ice sheets.
  6. ^ a b History Channel, "The History of Oil" (2007), Australian Broadcasting System, Inc., aired: 2:00–4:00 pm EDST, 2008-07-08; Note: Geologist Hugh Jenkyns was interviewed in the History Channel's (re: footnote:3 History Channel, "The History of Oil" (2007)) documentary "The History of Oil" and attributed the matching occurrence high in the Apennine Mountains' meter thick black shale band put together with the findings from the Deep Sea Drilling Project as triggering the theory and work that followed from a beginning ca 1974.
  7. ^ a b c d e f "What would 3 degrees mean?". Archived from the original on 19 July 2008. Retrieved 2008-07-08. [At plus] Six degrees [i.e rise of 6 degrees Celsius] * At the end of the Permian period, 251 million years ago, up to 95% of species became extinct as a result of a super-greenhouse event, resulting in a temperature rise of six degrees, perhaps because of an even bigger methane belch that 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 probably 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)
  8. ^ Raquel Vaquer-Sunyer & Carlos M. Duarte (October 7, 2008). "Thresholds of hypoxia for marine biodiversity". Proceedings of the National Academy of Sciences of the United States of America. 105 (40): 15452–15457. Bibcode:2008PNAS..10515452V. doi:10.1073/pnas.0803833105. PMC 2556360.
  9. ^ "Study Shows Continued Spread of 'Dead Zones'; Lack of Oxygen Now a Key Stressor on Marine Ecosystems".
  10. ^ Friedrich, Oliver; Erbacher, Jochen; Moriya, Kazuyoshi; Wilson, Paul A.; Kuhnert, Henning (2008). "Warm saline intermediate waters in the Cretaceous tropical Atlantic Ocean". Nature Geoscience. 1 (7): 453. Bibcode:2008NatGe...1..453F. doi:10.1038/ngeo217.
  11. ^ a b Meyer, K. M.; Kump, L. R. (2008). "Oceanic Euxinia in Earth History: Causes and Consequences". Annual Review of Earth and Planetary Sciences. 36: 251–288. Bibcode:2008AREPS..36..251M. doi:10.1146/
  12. ^ Mark Lynas (May 1, 2007). "Six Steps to Hell: The Facts on Global Warming". Archived from the original on May 2, 2009. Retrieved 2008-07-08. With extreme weather continuing to bite – hurricanes may increase in power by half a category above today’s top-level Category Five – world food supplies will be critically endangered. :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.
  13. ^ a b Gronstal, A. L. (2008-04-24). "Gasping for Breath in the Jurassic Era". Imaginova. Archived from the original on 29 April 2008. Retrieved 2008-04-24.
  14. ^ a b Pearce, C. R.; Cohen, A. S.; Coe, A. L.; Burton, K. W. (March 2008). "Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the Early Jurassic". Geology. 36 (3): 231–234. Bibcode:2008Geo....36..231P. doi:10.1130/G24446A.1. Archived from the original on 29 April 2008. Retrieved 2008-04-24.
  15. ^ Ward, Peter D. "Impact from the Deep". Scientific American. 2006 (October): 64–71.
  16. ^ Vandenbroucke, T. R. A.; Emsbo, P.; Munnecke, A.; Nuns, N.; Duponchel, L.; Lepot, K.; Quijada, M.; Paris, F.; Servais, T.; Kiessling, W. (2015-08-25). "Metal-induced malformations in early Palaeozoic plankton are harbingers of mass extinction". Nature Communications. 6: 7966. Bibcode:2015NatCo...6.7966V. doi:10.1038/ncomms8966. PMC 4560756. PMID 26305681.
  17. ^ Emsbo, P.; McLaughlin, P.; Munnecke, A.; Breit, G. N.; Koenig, A. E.; Jeppsson, L.; Verplanck, P. L. (November 2010). "The Ireviken Event: A Silurian OAE". 2010 GSA Denver Annual Meeting. 238-8. Retrieved 2015-09-19.
  18. ^ definition of mediterranean sea; "6. surrounded or nearly surrounded by land."
  19. ^ a b Leckie, R.; Bralower, T.; Cashman, R. (2002). "Oceanic anoxic events and plankton evolution: Biotic response to tectonic forcing during the mid-Cretaceous" (PDF). Paleoceanography. 17 (3): 1–29. Bibcode:2002PalOc..17.1041L. doi:10.1029/2001pa000623.
  20. ^ Li, Yong-Xiang; Bralower, Timothy J.; Montañez, Isabel P.; Osleger, David A.; Arthur, Michael A.; Bice, David M.; Herbert, Timothy D.; Erba, Elisabetta; Premoli Silva, Isabella (2008-07-15). "Toward an orbital chronology for the early Aptian Oceanic Anoxic Event (OAE1a, ~ 120 Ma)". Earth and Planetary Science Letters. 271 (1–4): 88–100. Bibcode:2008E&PSL.271...88L. doi:10.1016/j.epsl.2008.03.055.
  21. ^ Li, Yong-Xiang; Montañez, Isabel P.; Liu, Zhonghui; Ma, Lifeng (2017). "Astronomical constraints on global carbon-cycle perturbation during Oceanic Anoxic Event 2 (OAE2)". Earth and Planetary Science Letters. 462: 35–46. Bibcode:2017E&PSL.462...35L. doi:10.1016/j.epsl.2017.01.007.
  22. ^ Page, A. (2007). "Deglacial anoxia in a long-lived Early Palaeozoic Icehouse." (PDF). In Budd, G.E.; Streng, M.; Daley, A.C.; Willman, S. (eds.). Programme with Abstracts. Palaeontological Association Annual Meeting. 51. Uppsala, Sweden. p. 85.
  23. ^ a b c Jeppsson, L. (1990). "An oceanic model for lithological and faunal changes tested on the Silurian record". Journal of the Geological Society. 147 (4): 663–674. Bibcode:1990JGSoc.147..663J. doi:10.1144/gsjgs.147.4.0663.
  24. ^ a b c d e f Jeppsson, L. (1997). "The anatomy of the Mid-Early Silurian Ireviken Event and a scenario for P-S events". In Brett, C.E.; Baird, G.C. (eds.). Paleontological Events: Stratigraphic, Ecological, and Evolutionary Implications. New York: Columbia University Press. pp. 451–492. ISBN 978-0-231-08250-1.
  25. ^ Lüning, S.; Loydell, D.K.; Štorch, P.; Shahin, Y.; Craig, J. (2006). "Origin, sequence stratigraphy and depositional environment of an Upper Ordovician (Hirnantian) deglacial black shale, Jordan—Discussion". Palaeogeography, Palaeoclimatology, Palaeoecology. 230 (3–4): 352–355. doi:10.1016/j.palaeo.2005.10.004.

Further reading

  • Kashiyama, Yuichiro; Nanako O. Ogawa; Junichiro Kuroda; Motoo Shiro; Shinya Nomoto; Ryuji Tada; Hiroshi Kitazato; Naohiko Ohkouchi (May 2008). "Diazotrophic cyanobacteria as the major photoautotrophs during mid-Cretaceous oceanic anoxic events: Nitrogen and carbon isotopic evidence from sedimentary porphyrin". Organic Geochemistry. 39 (5): 532–549. doi:10.1016/j.orggeochem.2007.11.010.
  • Kump, L.R.; Pavlov, A. & Arthur, M.A. (2005). "Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia". Geology. 33 (5): 397–400. Bibcode:2005Geo....33..397K. doi:10.1130/G21295.1.
  • Hallam, A. (2004). Catastrophes and lesser calamities: the causes of mass extinctions. Oxford [Oxfordshire]: Oxford University Press. pp. 91–607. ISBN 978-0-19-852497-7.
  • Demaison G.J. and Moore G.T., (1980),"Anoxic environments and oil source bed genesis". American Association of Petroleum Geologists (AAPG) Bulletin, Vol.54, 1179–1209.

External links


The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts:

Anoxic waters, sea water, fresh water or groundwater that are depleted of dissolved oxygen

Anoxic event, when the Earth's oceans become completely depleted of oxygen below the surface levels

Euxinic, anoxic conditions in the presence of hydrogen sulfide

Hypoxia (environmental), low oxygen conditions

Hypoxia (medical), when the body or a region of the body is deprived of adequate oxygen supply

Cerebral anoxia, when the brain is completely deprived of oxygen, an extreme form of cerebral hypoxia


The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding age. Both age and stage bear the same name.

As a unit of geologic time measure, the Cenomanian age spans the time between 100.5 ± 0.9 Ma and 93.9 ± 0.8 Ma (million years ago). In the geologic timescale it is preceded by the Albian and is followed by the Turonian. The Upper Cenomanian starts approximately at 95 M.a.

The Cenomanian is coeval with the Woodbinian of the regional timescale of the Gulf of Mexico and the early part of the Eaglefordian of the regional timescale of the East Coast of the United States.

At the end of the Cenomanian an anoxic event took place, called the Cenomanian-Turonian boundary event or the "Bonarelli Event", that is associated with a minor extinction event for marine species.

Cenomanian-Turonian boundary event

The Cenomanian-Turonian boundary event, or the Cenomanian-Turonian extinction event, the Cenomanian-Turonian anoxic event (OAE 2), and referred also as the Bonarelli Event, was one of two anoxic extinction events in the Cretaceous period. (The other being the earlier Selli Event, or OAE 1a, in the Aptian.) The OAE 2 occurred approximately 91.5 ± 8.6 Ma, though other estimates are given as 93–94 Ma. The Cenomanian-Turonian boundary has recently been refined to 93.9 ± 0.15 Ma There was a large carbon disturbance during this time period. However, apart from the carbon cycle disturbance, there were also large disturbances in the oxygen and sulfur cycles of the ocean.

The event brought about the extinction of the Pliosauridae, and most Ichthyosauria. Coracoids of Maastrichtian age were once interpreted by some authors as belonging to ichthyosaurs, but these have since been interpreted as plesiosaur elements instead. Although the cause is still uncertain, the result starved the Earth's oceans of oxygen for nearly half a million years, causing the extinction of approximately 27 percent of marine invertebrates, including certain planktic and benthic foraminifera, mollusks, bivalves, dinoflagellates and calcareous nannofossils. The global environmental disturbance that resulted in these conditions increased atmospheric and oceanic temperatures. Boundary sediments show an enrichment of trace elements, and contain elevated δ13C values.The Cenomanian and Turonian stages were first noted by D'Orbigny between 1843 and 1852. The global type section for this boundary is located in the Bridge Creek Limestone Member of the Greenhorn formation near Pueblo, Colorado, which are bedded with the Milankovitch orbital signature. Here, a positive carbon-isotope event is clearly shown, although none of the characteristic, organic-rich black shale is present. It has been estimated that the isotope shift lasted approximately 850 kyrs longer than the black shale event, which may be the cause of this anomaly in the Colorado type-section. A significantly expanded OAE2 interval from southern Tibet documents a complete, more detailed, and finer-scale structures of the positive carbon isotope excursion that contains multiple shorter-term carbon isotope stages amounting to a total duration of 820±25 kyrs.The boundary is also known as the Bonarelli event because of 1- to 2-meter layer of thick black shale that marks the boundary and was first studied by Guido Bonarelli in 1891. It is characterized by interbedded black shale, chert and radiolarian sands is estimated to span a 400,000-year interval. Planktic foraminifera do not exist in this Bonarelli level, and the presence of radiolarians in this section indicates relatively high productivity and an availability of nutrients.

One possible cause of this event is sub-oceanic volcanism, possibly the Caribbean large igneous province, with increased activity approximately 500,000 years earlier. During that period, the rate of crustal production reached its highest level for 100 million years. This was largely caused by the widespread melting of hot mantle plumes under the oceans at the base of the lithosphere. This resulted in the thickening of the oceanic crust in the Pacific and Indian Oceans. This volcanism would have sent large quantities of carbon dioxide into the atmosphere, leading to global warming. Within the oceans, the emission of SO2, H2S, CO2, and halogens would have increased the acidity of the water, causing the dissolution of carbonate, and a further release of carbon dioxide. When the volcanic activity declined, this run-away greenhouse effect would have likely been put into reverse. The increased CO2 content of the oceans could have increased organic productivity in the ocean surface waters. The consumption of this newly abundant organic life by aerobic bacteria would produce anoxia and mass extinction. The resulting elevated levels of carbon burial would account for the black shale deposition in the ocean basins.


The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series and spans the time between 89.8 ± 1 Ma and 86.3 ± 0.7 Ma (million years ago). The Coniacian is preceded by the Turonian and followed by the Santonian.

Cretaceous Thermal Maximum

The Cretaceous Thermal Maximum (CTM), also known as Cretaceous Thermal Optimum, was a period of climatic warming that reached its peak approximately 90 million years ago (90 Ma) during the Turonian age of the Late Cretaceous epoch. The CTM is notable for its dramatic increase in global temperatures characterized by high carbon dioxide levels.

Early Cretaceous

The Early Cretaceous (geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 146 Ma to 100 Ma.

Exshaw Formation

The Exshaw Formation is a stratigraphic unit in the Western Canada Sedimentary Basin. It takes the name from the hamlet of Exshaw, Alberta in the Canadian Rockies, and was first described from outcrops on the banks of Jura Creek north of Exshaw by P.S. Warren in 1937. The formation is of Late Devonian (late Famennian) to Early Mississippian (middle Tournaisian) age as determined by conodont biostratigraphy, and it straddles the Devonian-Carboniferous boundary.The Exshaw strata were deposited in a marine setting during the Hangenberg event, an oceanic anoxic event associated with the Late Devonian extinction. The black shales of the Exshaw Formation are rich in organic matter and are one of the most important petroleum source rocks of the Western Canada Sedimentary Basin.

Hangenberg event

The Hangenberg event is a bioevent that occurred at the end of the Famennian stage (late Devonian) associated with the Late Devonian extinction (roughly 358.9 ± 0.4 million years ago); it was an anoxic event marked by a black shale. It has been proposed that this was related to a rapid sea-level fall due to the last phase of the Devonian Southern Hemisphere glaciation. It has also been suggested that it was linked to an increase in terrestrial plant cover, leading to increased nutrient supply in rivers. This may have led to eutrophication of semi-restricted epicontinental seas and could have stimulated algal blooms.It is named from the Hangenberg Shale, part of a sequence that straddles the Devonian-Carboniferous boundary, from the Rhenish Massif in Germany.Following the extinction, vertebrates experienced reduced body size for the following 36 million years, at least in part because smaller taxa diversified more successfully.

Ireviken event

The Ireviken event was the first of three relatively minor extinction events (the Ireviken, Mulde, and Lau events) during the Silurian period. It occurred at the Llandovery/Wenlock boundary (mid Silurian, 433.4 ± 2.3 million years ago). The event is best recorded at Ireviken, Gotland, where over 50% of trilobite species became extinct; 80% of the global conodont species also become extinct in this interval.


The Karoo and Ferrar Large Igneous Provinces (LIPs) are two large igneous provinces in Southern Africa and Antarctica respectively, collectively known as the Karoo-Ferrar, Gondwana, or Southeast African LIP, associated with the initial break-up of the Gondwana supercontinent at c. 183 Ma.

Its flood basalt mostly covers South Africa and Antarctica but portions extend further into southern Africa and into South America, India, Australia and New Zealand.Karoo-Ferrar formed just prior to the breakup of Gondwana in the Lower Jurassic epoch, about 183 million years ago; this timing corresponds to the early Toarcian anoxic event and the Pliensbachian-Toarcian extinction. It covered about 3 x 106 km2. The total original volume of the flow, which extends over a distance in excess of 6000 km (4000 km in Antarctica alone), was in excess of 2.5 x 106 km³ (2.5 million cubic kilometres).The Ferrar LIP is notable for long distance transport and the Karoo LIP for its large volume and chemical diversity.The igneous activity of the Karoo LIP began c. 204 Ma at the northern margin of the province. The long-lasting Chon-Aike Province in Patagonia, the Antarctic Peninsula, and Ellsworth Land was activated c. 190 Ma in an unstable tectonic environment in which both extension and subduction occurred. Chon-Aike had a peak between 183 to 173 Ma but produced continued magmatism between 168 to 141 Ma. By 184 to 175 Ma the Karoo magmatism had spread to Namibia, Lesotho, Lebombo, and the Ferrar province in Antarctica. The Karoo LIP ended 145 Ma with peripheral eruptions in Patagonia, the Antarctica Peninsula, northern South Africa, Kerala in India, and southeast Australia. The Karoo Province uplifted southern Africa c. 1.5 km (0.93 mi) and broke East Gondwana (India, Antarctica, and Australia) away from West Gondwana (South America and Africa) beginning in the opening of the Weddell Sea.In the Cretaceous, some 15 million years after the last Karoo eruption, renewed magmatism was initiated between Mary Byrd Land in Antarctica and New Zealand from where it spread along Gondwana's southern margin, from eastern Australia to the Antarctic Peninsula. Isotopic dating suggests a series of igneous events at 133–131, 124–119, and 113–107 Ma in Australia; 110–99 Ma in Mary Byrd Land; 114-109 and 82 Ma in New Zealand; and 141 and 127 Ma in the Antarctic Peninsula. This phase of magmatism resulted in extension and rift between Australia and Antarctica, Australia and Lord Howe Rise, and Mary Byrd Land and New Zealand.

Kačák Event

The Kačák Event (Czech pronunciation: [ˈkatʃaːk]) or Kačák-otomari Event is a widely recognised bioevent or series of events that occurred close to the end of the Eifelian Age of the Middle Devonian Epoch. It involved a global eustatic rise in sea level. It was named for the Kačák Member of the Srbsko Formation in Bohemia, where it is represented by a black shale interval within a sequence of limestone. In marine environments, this appears as an anoxic event, often forming potential hydrocarbon source rocks such as the Marcellus Shale. Within the Old Red Sandstone continent, it is represented by the Achanarras lake, the deepest and most widespread lake that developed within the Orcadian Basin. The event is associated with significant extinctions, particularly amongst the Ammonoidea.

Mulde event

The Mulde event was an anoxic event, and marked the second of three1 relatively minor mass extinctions (the Ireviken, Mulde, and Lau events) during the Silurian period. It coincided with a global drop in sea level, and is closely followed by an excursion in geochemical isotopes. Its onset is synchronous with the deposition of the Fröel formation in Gotland. Perceived extinction in the conodont fauna, however, likely represent a change in the depositional environment of sedimentary sequences rather than a genuine biological extinction.


OAE could refer to:

Oceanic Anoxic event, in which the Earth's oceans become completely depleted of oxygen below the surface levels.

Orchestra of the Age of Enlightenment, a British period instrument orchestra.

Otoacoustic emissions, involved in testing hearing.

Omni Air Express, United States (ICAO operator designator)

Operation Active Endeavour

Oddworld: Abe's Exoddus a platform game made by Oddworld Inhabitants released in 1998OAE also can refer to Old Antarctic Explorer, anyone who has worked or spent significant time on the continent of Antarctica. Term coined in or around the International Geophysical Year.

Ontong Java Plateau

The Ontong Java Plateau (OJP) is a huge oceanic plateau located in the southwestern Pacific Ocean, north of the Solomon Islands.

The OJP was emplaced around 120 million years ago (Ma) with a much smaller volcanic event around 90 Ma. Two other southwestern Pacific plateaus, Manihiki and Hikurangi, now separated from the OJP by Cretaceous ocean basins, are of similar age and composition and probably formed as a single plateau and a contiguous large igneous province together with the OJP.

When emplaced this Ontong Java–Manihiki–Hikurangi plateau covered 1% of Earth's surface and represented a volume of 80 million km3 (19 million cu mi) of basaltic magma.

This "Ontong Java event", first proposed in 1991, represents the largest volcanic event of the past 200 million years, with a magma emplacement rate estimated at up to 22 km3 (5.3 cu mi) per year over 3 million years, several times larger than the Deccan Traps.

The smooth surface of the OJP is punctuated by seamounts such as the Ontong Java Atoll, the largest atoll in the world.

Shutdown of thermohaline circulation

A shutdown or slowdown of the thermohaline circulation is a hypothesized effect of global warming on a major ocean circulation.

Data from NASA in 2010 suggested that the Atlantic Meridional Overturning Circulation (AMOC) had not slowed down, but may have actually sped up slightly since 1993. A 2015 study suggested that the AMOC has weakened by 15-20% in 200 years.

Toarcian turnover

The term Toarcian turnover, alternatively the Toarcian extinction, the Pliensbachian-Toarcian extinction, or the Early Jurassic extinction, refers to the wave of extinctions that marked the end of the Pliensbachian stage and the start of the Toarcian stage of the Early Jurassic period, c. 183 million years ago.

The Toarcian turnover was most strongly manifested in aquatic lifeforms, notably in mollusk groups like ammonites. Its reach was global in extent, as evidenced by research in European (peninsula of Peniche, Portugal) and Japanese waters, the Andean basin, the floor of the former Tethys Sea. Evidence points to anoxic bottom waters as the probable cause of these marine extinctions, linked in turn to the massive volcanism of the Karoo-Ferrar eruptions in the relevant era. The Toarcian turnover was the seventh-largest mass extinction in Earth's history.


The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous epoch, or a stage in the Upper Cretaceous series. It spans the time between 93.9 ± 0.8 Ma and 89.8 ± 1 Ma (million years ago). The Turonian is preceded by the Cenomanian stage and underlies the Coniacian stage.At the beginning of the Turonian an anoxic event took place which is called the Cenomanian-Turonian boundary event or the "Bonarelli Event".

Western Interior Seaway anoxia

Three Western Interior Seaway anoxic events occurred during the Cretaceous in the shallow inland seaway that divided North America in two island continents, Appalachia and Laramidia (see map). During these anoxic events much of the water column was depleted in dissolved oxygen. While anoxic events impact the world's oceans, Western Interior Seaway anoxic events exhibit a unique paleoenvironment compared to other basins. The notable Cretaceous anoxic events in the Western Interior Seaway mark the boundaries at the Aptian-Albian, Cenomanian-Turonian, and Coniacian-Santonian stages, and are identified as Oceanic Anoxic Events I, II, and III respectively. The episodes of anoxia came about at times when very high sea levels coincided with the nearby Sevier orogeny that affected Laramidia to the west and Caribbean large igneous province to the south, which delivered nutrients and oxygen-adsorbing compounds into the water column.

Most anoxic events are recognized using the 13C isotope as a proxy to indicate total organic carbon preserved in sedimentary rocks. If there is very little oxygen, then organic material that settles to the bottom of the water column will not be degraded as readily compared to normal oxygen settings and can be incorporated into the rock. 13Corganic is calculated by comparing the amount of 13C to a carbon isotope standard, and using multiple samples can track changes (δ) in organic carbon content through rocks over time, forming a δ13Corganic curve. The δ13Corganic, as a result, serves as a benthic oxygen curve.

The excellent organic carbon preservation brought about by these successive anoxic events makes Western Interior Seaway strata some of the richest source rocks for oil and gas.


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