An anoxic event describes a period wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic (anoxic and sulfidic) waters. Although anoxic events have not happened for millions of years, the geologic record shows that they happened many times in the past. Anoxic events coincided with several mass extinctions and may have contributed to them. These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating. On the other hand, there are widespread, various black-shale beds from the mid-Cretaceous which indicate anoxic events but are not associated with mass extinctions. Many geologists believe oceanic anoxic events are strongly linked to the 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."
Human activities in the Holocene epoch, such as the release of nutrients from farms and sewage, cause relatively small-scale dead zones around the world. British oceanologist and atmospheric scientist Andrew Watson says full-scale ocean anoxia would take "thousands of years to develop." The idea that modern climate change could lead to such an event is referred to as Kump's hypothesis.
Background The concept of the oceanic anoxic event (OAE) was first proposed in 1976 by Seymour Schlanger and geologist Hugh JenkynsHistory 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. and arose from discoveries made by the Deep Sea Drilling Project (DSDP) in the Pacific Ocean. The finding of black, carbon-rich shales in Cretaceous sediments that had accumulated on submarine volcanic plateaus (e.g. Shatsky Rise, Manihiki Plateau), coupled with their identical age to similar, cored deposits from the Atlantic Ocean and known outcrops in Europe—particularly in the geological record of the otherwise limestone-dominated Apennines chain in Italy—led to the observation that these widespread, similarly distinct strata recorded very unusual, oxygen-depleted conditions in the world's oceans spanning several discrete periods of geological time.
Modern sedimentological investigations of these organic-rich sediments typically reveal the presence of fine laminations undisturbed by bottom-dwelling fauna, indicating anoxic conditions on the seafloor believed to coincide with a low-lying poisonous layer of hydrogen sulfide, H2S. 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 photic upper-water column.
This is a recent understanding, the puzzle having been pieced slowly together in 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.
Euxinia
Anoxic events with euxinic (anoxic, sulfidic) conditions have been linked to extreme episodes of volcanic outgassing. Volcanism contributed to the buildup of CO2 in the atmosphere and increased global temperatures, causing an accelerated hydrological cycle that introduced nutrients into the oceans (stimulating 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 perpetual high productivity.
Mechanism 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 depth, with a temperature of .
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.
Another way to explain anoxic events is 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. The age of three LIPs (Karoo-Ferrar flood basalt, Caribbean large igneous province, Ontong Java Plateau) correlates 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.
Occurrence 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 . The Quaternary levels, the current period, 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) that some call an "oceanic burp".<ref name="Six steps" >
</ref> 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 outgassing events due to tectonic events. Studies suggest the huge release of natural gas 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, 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.
Consequences 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.
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