Anoxic event Totally Explained

Everything about Anoxic Event totally explained

Oceanic 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.

Occurrence

Oceanic anoxic events most commonly occur during periods of very warm climate characterised by high levels of carbon dioxide (CO₂) and mean surface temperatures probably in excess of 25 °C (Quaternary levels are 13 °C). However, anoxia was also rife during the Hirnantian (late Ordovician) ice age.
   Oceanic anoxic events have been recognised primarily from the 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 in the Jurassic and Cretaceous

The concept of the oceanic anoxic event (OAE) was first proposed in 1976 by Seymour Schlanger (1927–1990) and 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, that led to the realization that these widespread intervals recorded highly unusual conditions in the world ocean during 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. Furthermore, detailed organic geochemical studies have recently revealed the presence of molecules (so-called biomarkers) that derive from green sulfur bacteria: organisms that required both light and free hydrogen sulfide (H₂S), illustrating that anoxic conditions extended high into the water column. Such sulfidic (or euxinic) conditions, which exist today in the Black Sea, were particularly prevalent in the Cretaceous Atlantic but also characterized other parts of the world ocean.
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's 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).
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, that'll be dense enough to sink; as they're 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.), 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.

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.

Consequences

Oceanic anoxic events have had many important consequences. It is believed that they've 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 couldn't 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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