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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.
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Oceanic Anoxic Events occurred only 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).
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.
Major oceanic anoxic events occurred around 183 million years ago (early Toarcian stage of the Early Jurassic), and many times in the period from 140 to 84 million years ago during the Cretaceous. Typically, each oceanic anoxic event lasted for half a million years or less and then oxygenation of the oceans would return to pre-event levels.
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).
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.
The mechanism by which major oceanic anoxic events took place remains controversial. Temperatures throughout the Jurassic and Cretaceous are generally thought to have been relatively warm so that 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.
The first of these suggests that the anomalous accumulation of organic matter relates to its enhanced preservation under restricted and poorly ventilated conditions, which themselves reflected the particular geometry of the ocean basin: such a hypothesis, although readily applicable to the young and relatively narrow Cretaceous Atlantic, fails to explain the occurrence of coeval black shales on open-ocean Pacific plateaus and shelf seas around the world.
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 and 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.
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.
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. 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.
Categories: Chemical oceanography | Environmental chemistry | Bioindicators
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Anoxic_event". A list of authors is available in Wikipedia.|