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61-540: 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

122-414: A hard, fissile, metamorphic rock known as slate . With continued increase in metamorphic grade the sequence is phyllite , then schist and finally gneiss . Shale is the most common source rock for hydrocarbons ( natural gas and petroleum ). The lack of coarse sediments in most shale beds reflects the absence of strong currents in the waters of the depositional basin. These might have oxygenated

183-480: 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

244-422: 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

305-643: 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

366-548: A platform game made by Oddworld Inhabitants released in 1998 See also [ edit ] Oae (disambiguation) Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title OAE . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=OAE&oldid=1085843560 " Category : Disambiguation pages Hidden categories: Short description

427-723: 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. Reduced oxygen levels are expected to lead to increased seawater concentrations of redox-sensitive metals. The reductive dissolution of iron – manganese 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 O 2 levels, precipitation of some of

488-435: 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

549-510: A time of low global temperatures (although CO 2 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 °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

610-446: Is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g., kaolin , Al 2 Si 2 O 5 ( OH ) 4 ) and tiny fragments ( silt -sized particles) of other minerals, especially quartz and calcite . Shale is characterized by its tendency to split into thin layers ( laminae ) less than one centimeter in thickness. This property is called fissility . Shale is the most common sedimentary rock. The term shale

671-1201: Is accompanied by telogenesis , the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the shale, such as dissolution of some of the cement to produce secondary porosity . Pyrite may be oxidized to produce gypsum . Black shales are dark, as a result of being especially rich in unoxidized carbon . Common in some Paleozoic and Mesozoic strata , black shales were deposited in anoxic , reducing environments, such as in stagnant water columns. Some black shales contain abundant heavy metals such as molybdenum , uranium , vanadium , and zinc . The enriched values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after sedimentation or to slow accumulation from sea water over long periods of sedimentation. Fossils , animal tracks or burrows and even raindrop impressions are sometimes preserved on shale bedding surfaces. Shales may also contain concretions consisting of pyrite, apatite , or various carbonate minerals. Shales that are subject to heat and pressure of metamorphism alter into

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732-677: Is also referred to as Kump's hypothesis. 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. 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

793-638: Is composed of about 58% clay minerals, 28% quartz, 6% feldspar , 5% carbonate minerals, and 2% iron oxides . Most of the quartz is detrital (part of the original sediments that formed the shale) rather than authigenic (crystallized within the shale after deposition). Shales and other mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales, which form in anoxic conditions, contain reduced free carbon along with ferrous iron (Fe ) and sulfur (S ). Amorphous iron sulfide , along with carbon, produce

854-536: Is different from Wikidata All article disambiguation pages All disambiguation pages Anoxic event 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

915-430: Is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts. The fine particles that compose shale can remain suspended in water long after the larger particles of sand have been deposited. As a result, shales are typically deposited in very slow moving water and are often found in lakes and lagoonal deposits, in river deltas , on floodplains and offshore below

976-455: Is more likely to form nonfissile mudstone than shale. On the other hand, black shales often have very pronounced fissility ( paper shales ) due to binding of hydrocarbon molecules to the faces of the clay particles, which weakens the binding between particles. Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of cement that binds the grains together. Pressure solution contributes to cementing, as

1037-452: Is reduced. In addition to this physical compaction, chemical compaction may take place via pressure solution . Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. It is during compaction that shale develops its fissility, likely through mechanical compaction of

1098-839: Is sometimes applied more broadly, as essentially a synonym for mudrock , rather than in the narrower sense of clay-rich fissile mudrock. Shale typically exhibits varying degrees of fissility. Because of the parallel orientation of clay mineral flakes in shale, it breaks into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding planes . Non-fissile rocks of similar composition and particle size (less than 0.0625 mm) are described as mudstones (1/3 to 2/3 silt particles) or claystones (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones . Shales are typically gray in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters

1159-467: 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

1220-419: 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 propagates through the ocean, and reduces

1281-548: The Archean , euxinia was largely absent because of low availability of sulfate in the oceans, but during the Proterozoic, it would become more common. Several anoxic events are known from the late Neoproterozoic , including one from the early Nama assemblage possibly coinciding with the first pulse of the end-Ediacaran extinction . Shale Shale is a fine-grained, clastic sedimentary rock formed from mud that

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1342-592: The Cenomanian – Turonian boundary (~93 Ma), also called the Bonarelli Event (or OAE2 ) after the Italian geologist Guido Bonarelli (1871–1951). OAE1a lasted for ~1.0 to 1.3 Myr. 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. More minor oceanic anoxic events have been proposed for other intervals in

1403-521: 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

1464-399: The U.S. Gulf Coast . As sediments continue to accumulate, the older, more deeply buried sediments begin to undergo diagenesis . This mostly consists of compaction and lithification of the clay and silt particles. Early stages of diagenesis, described as eogenesis , take place at shallow depths (a few tens of meters) and are characterized by bioturbation and mineralogical changes in

1525-469: 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

1586-651: The wave base . Thick deposits of shale are found near ancient continental margins and foreland basins . Some of the most widespread shale formations were deposited by epicontinental seas . Black shales are common in Cretaceous strata on the margins of the Atlantic Ocean , where they were deposited in fault -bounded silled basins associated with the opening of the Atlantic during the breakup of Pangaea . These basins were anoxic, in part because of restricted circulation in

1647-613: 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

1708-476: The Cretaceous (in the Valanginian , Hauterivian , Albian and Coniacian – Santonian stages), but their sedimentary record, as represented by organic-rich black shales, appears more parochial, being dominantly represented in the Atlantic and neighbouring areas, and some researchers relate them to particular local conditions rather than being forced by global change. The only oceanic anoxic event documented from

1769-471: 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

1830-465: 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

1891-461: The Jurassic took place during the early Toarcian (~183 Ma). Since 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—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

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1952-548: The amount of CO 2 that the oceans can hold in solution, which makes the oceans release large quantities of CO 2 into the atmosphere in a geologically short time (tens or thousands of years). The warm waters also initiate the release of clathrates , which further increases atmospheric temperature and basin anoxia. Similar positive feedbacks operate during cold-pole episodes, amplifying their cooling effects. The periods with cold poles are termed "P-episodes" (short for primo ), and are characterised by bioturbated deep oceans,

2013-480: 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

2074-421: The black coloration. Because amorphous iron sulfide gradually converts to pyrite , which is not an important pigment, young shales may be quite dark from their iron sulfide content, in spite of a modest carbon content (less than 1%), while a black color in an ancient shale indicates a high carbon content. Most shales are marine in origin, and the groundwater in shale formations is often highly saline . There

2135-516: The buildup of CO 2 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. Temperatures throughout

2196-506: The clumps of clay particles produced by flocculation vary in size from a few tens of microns to over 700 microns in diameter. The floccules start out water-rich, but much of the water is expelled from the floccules as the clay minerals bind more tightly together over time (a process called syneresis ). Clay pelletization by organisms that filter feed is important where flocculation is inhibited. Filter feeders produce an estimated 12 metric tons of clay pellets per square kilometer per year along

2257-408: The color of the rock. Red, brown and green colors are indicative of ferric oxide ( hematite – reds), iron hydroxide ( goethite – browns and limonite – yellow), or micaceous minerals ( chlorite , biotite and illite – greens). The color shifts from reddish to greenish as iron in the oxidized ( ferric ) state is converted to iron in the reduced ( ferrous ) state. Black shale results from

2318-404: 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". 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 ,

2379-624: 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

2440-409: 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 (CO 2 ) and mean surface temperatures probably in excess of 25 °C (77 °F). The Quaternary levels,

2501-482: 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 . Similar metal enrichment has been reported in sediments from

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2562-570: The mid-Silurian Ireviken event . Sulfidic (or euxinic) conditions, which exist today in many water bodies from ponds to various land-surrounded mediterranean seas such as the Black Sea , were particularly prevalent in the Cretaceous Atlantic but also characterised other parts of the world ocean. In an ice-free sea of these supposed super-greenhouse worlds, oceanic waters were as much as 200 metres (660 ft) higher, in some eras. During

2623-592: The mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. The clay minerals may be altered as well. For example, smectite is altered to illite at temperatures of about 55 to 200 °C (130 to 390 °F), releasing water in the process. Other alteration reactions include the alteration of smectite to chlorite and of kaolinite to illite at temperatures between 120 and 150 °C (250 and 300 °F). Because of these reactions, illite composes 80% of Precambrian shales, versus about 25% of young shales. Unroofing of buried shale

2684-433: The narrow Atlantic, and in part because the very warm Cretaceous seas lacked the circulation of cold bottom water that oxygenates the deep oceans today. Most clay must be deposited as aggregates and floccules, since the settling rate of individual clay particles is extremely slow. Flocculation is very rapid once the clay encounters highly saline sea water. Whereas individual clay particles are less than 4 microns in size,

2745-485: 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 to 25 °C (68 to 77 °F). The second hypothesis suggests that oceanic anoxic events record

2806-434: 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

2867-415: The original open framework of clay particles. The particles become strongly oriented into parallel layers that give the shale its distinctive fabric. Fissility likely develops early in the compaction process, at relatively shallow depth, since fissility does not seem to vary with depth in thick formations. Kaolinite flakes have less tendency to align in parallel layers than other clays, so kaolinite-rich clay

2928-471: 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) after the Italian geologist Raimondo Selli (1916–1983), and another at

2989-479: 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 . Anoxic events with euxinic (anoxic, sulfidic) conditions have been linked to extreme episodes of volcanic outgassing. Volcanism contributed to

3050-463: 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 2 ) S-episodes, while the reverse is true during P-episodes. On top of this gradual trend is overprinted

3111-529: 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, H 2 S . 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 (H 2 S), illustrating that anoxic conditions extended high into

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3172-817: The presence of greater than one percent carbonaceous material and indicates a reducing environment. Pale blue to blue-green shales typically are rich in carbonate minerals . Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely kaolinite , montmorillonite and illite. Clay minerals of Late Tertiary mudstones are expandable smectites , whereas in older rocks (especially in mid-to early Paleozoic shales) illites predominate. The transformation of smectite to illite produces silica , sodium , calcium , magnesium , iron and water. These released elements form authigenic quartz , chert , calcite , dolomite , ankerite , hematite and albite , all trace to minor (except quartz) minerals found in shales and other mudrocks. A typical shale

3233-444: The richest source rocks may contain as much as 40% organic matter. The organic matter in shale is converted over time from the original proteins, polysaccharides , lipids , and other organic molecules to kerogen , which at the higher temperatures found at greater depths of burial is further converted to graphite and petroleum. Before the mid-19th century, the terms slate , shale and schist were not sharply distinguished. In

3294-433: The sediments, with only slight compaction. Pyrite may be formed in anoxic mud at this stage of diagenesis. Deeper burial is accompanied by mesogenesis , during which most of the compaction and lithification takes place. As the sediments come under increasing pressure from overlying sediments, sediment grains move into more compact arrangements, ductile grains (such as clay mineral grains) are deformed, and pore space

3355-698: 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 of nutrient-rich meltwater from melting glaciers, which by virtue of its fresh nature would also slow down oceanic circulation. It has been thought that through most of Earth's history, oceans were largely oxygen-deficient. During

3416-483: The timespans in question, the continental plates are believed to have been well separated, and the mountains as they are known 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 fuelling an overall explosive population of microorganisms and their predator species in

3477-516: The two major Cretaceous examples. The Permian–Triassic extinction event , triggered by runaway CO 2 from the Siberian Traps, was marked by ocean deoxygenation . 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

3538-533: The warm Paleogene: only rarely in colder periods were conditions favorable for the production of source rocks on anything other than a local scale. 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

3599-414: The waters and destroyed organic matter before it could accumulate. The absence of carbonate rock in shale beds reflects the absence of organisms that might have secreted carbonate skeletons, also likely due to an anoxic environment. As a result, about 95% of organic matter in sedimentary rocks is found in shales and other mudrocks. Individual shale beds typically have an organic matter content of about 1%, but

3660-490: 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

3721-414: 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

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