Anoxic event 1632974 224585934 2008-07-09T15:11:53Z Fabartus 281293 polish up my prose {{FixHTML|beg}} [[Image:Ocean currents 1911.jpg|thumb|right|200px|As early as 1911, major oceanic currents were well mapped and understood, albeit without today's understanding of how they affect regional and global [[climatology|climatological]] conditions.]] [[Image:Thermohaline Circulation 2.png|thumb|right|200px|This world perspective on oceanic currents demonstrates the interdependencies of transnational regions on circulating currents.]] {{FixHTML|end}} '''Oceanic anoxic events''' or '''Anoxic events''' occur when the [[Earth]]'s [[oceans]] become completely depleted of [[oxygen|oxygen (O<sub>2</sub>)]] 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]]&mdash;including some of those which [[geobiology|geobiologists]] use to as a [[benchmark|time marker]] in [[biostratigraphy|biostratigraphic]] dating. It is believed they are strongly linked to lapses in key oceanic current circulations, to climate warming and greenhouse gases. {{TOCnestright}} What is clear from careful analysis of the [[geologic record]]s occurring before and after the affected {{gAge}}s that onsets are rapid and so are recoveries&mdash;both data suggesting a sudden [[Tipping point (climatology)|climate threshold]] or [[tipping point]] believed to occur at about four times the Earth's mean [[carbon dioxide]] levels relative to the baseline concentrations of circa [[1750]], the beginning of the [[Industrial age]]. The strata analysis suggest a Earth wide overheated climate<ref name="HistOfOil"/> of steaming rain forests, heavy daily rains, terrible storms<ref name="HistOfOil"/><ref name="Six steps" >{{Cite web |url=http://www.stwr.org/climate-change-environment/six-steps-to-hell-the-facts-on-global-warming.html |accessdate=2008-07-08 |title=Six Steps to Hell: The Facts on Global Warming |date=May 1st, 2007 |author=Mark Lynas, Oneworld.net |quote=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.{{I}}And:{{I}}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. }}</ref >, heavy erosion feeding nutrients into the [[water bodies|world's waters]] and a complete or at least large scale stoppage of water circulation between the poles and equator<ref name=ngeo217 >{{cite journal |doi=10.1038/ngeo217 |title=Warm saline intermediate waters in the Cretaceous tropical Atlantic Ocean|year=2008|author=Friedrich, Oliver|journal=Nature Geoscience|volume=1|pages=453 }}</ref >&mdash; leading to 'death in the depths' from oxygen deprivation and failure to offset natural sources of mildly poisonous [[hydrogen sulfide]]s. The stratified waters would support life where oxygenated in a surface zone and become rapidly more inimicable to life at increasing depths, leading to a lack even of scavenger activity along the accumulating organically rich ooze on layering the basins and bottoms. Life forms unwarily drifting into anoxic and toxic layers contributed to the continual rain of unicellular microorganisms from the bioexplosion spurred by the increased nutrients from the super-greenhouse accelerated weathering and erosion and so the anoxic tipping events are believed to be the triggers to millions of years of accumulations of the carbohydrate rich deposits which became today's diminishing [[oil reserve]]s. In a picture only pieced together during the last three decades, the handful of known and suspected anoxic events are tied geologically to large scale production of the world's oil reserves, widespread (i.e. world wide) large bands of black shale in the [[geologic record]] and high relative temperatures believed linked to so called "[[super-greenhouse event]]s"<ref name="3 degrees C" >{{cite web |url=http://www.carbonequity.info/docs/3degrees.html |author= |title=What would 3 degrees mean? |accessdate=2008-07-08 |quote=[At plus] '''Six degrees''' [i.e rise of 6 degrees centigrade]{{I}}• At the end of the [[Permian]] period, 251 million years ago, up to 95% of species were extinct as a result of a '''super-greenhouse event''', resulting in a temperatures rise by six degrees, perhaps because of an even bigger [[methane]] belch than happened 200 million years later in the [[Eocene]].{{I}} and also:{{I}}•Five degrees of warming occured during the Eocene, 55 million years ago: 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.{{I}}• The Eocene greenhouse event was likely 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. {{I}}• 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) }} </ref > Oceanic '''anoxic events'''<ref name="HistOfOil" >{{Cite Sm |History Channel, "The History of Oil" (2007), Australian Broadcasting System, Inc.<!--may be out of context -->, aired: 2:00-4:00 pm EDST, 2008-07-08 }} </ref >stimulated by extremes of possible volcanic outgassing or at least of the characteristic elevated [[carbon dioxide]] levels four (or five) to six times current levels. At even a few degrees warmer, rain forests are extremely vulnerable to fire hazards, having little natural resistance to fires,<ref name="3 degrees C"/> and some conjecture a tipping point might have been reached effectively overnight in a huge burn-off<ref name="3 degrees C"/> releasing extraordinary amounts of carbon-dioxide into the atmosphere. With a change of mean temperatures of three degrees centigrade, the ice caps melted, in the super-greenhouse ecologies&mdash;the term meaning average temperature rose to or were beyond six degrees above today&mdash;the seas were so warm, it is believe the water temperatures at the two poles<ref name="3 degrees C-2" >{{cite web |url=http://www.carbonequity.info/docs/3degrees.html |author= |title=What would 3 degrees mean? |accessdate=2008-07-08 |quote=[At plus] '''Five degrees C'''{{I}}• Five degrees of warming occurred during the Eocene, 55 million years ago: 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. }}</ref > were in the lower 80's°F (i.e. above 26.7°C)<ref name="HistOfOil"/>. The [[Cretaceous]] and [[Jurassic]] {{gEra}}s world ecology's were essentially ice free<ref name="HistOfOil"/>, had massive storms driven by Oceans were dying from double hit<ref name="HistOfOil"/> of lack of oxygen and [[toxic]] [[hydrogen sulfide]] accumulations at lower layers because of a shut down in the [[thermohaline circulation|ocean conveyor belt]]s<ref name="HistOfOil"/>. ==Occurrence== Oceanic anoxic events most commonly occur during periods of very warm [[climate]] characterised by high levels of [[carbon dioxide|carbon dioxide (CO<sub>2</sub>)]] and mean surface temperatures probably in excess of {{C to F|25|wiki=yes|abbr=no}}. The [[Quaternary]] levels, our current {{gPeriod}}, are just {{C to F|13|wiki=no|abbr=yes}} in comparison. Such rises in carbon-dioxide may have been in response to a great out gasing of the highly flammable [[natural gas]] (methane) some have christened a "oceanic burb".<ref name="Six steps"/><ref name="3 degrees C"/> Vast quantities of [[methane]] are normally locked into the Earth's crust on the continental plateau's 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, save at cool temperatures and high (deep) pressures scientists have observed smaller "burps" due to [[tectonic]] events and studies suggest the huge release of natural gas<ref name="3 degrees C"/> could be a major climatological trigger, methane itself being a [[greenhouse gas]] which when burned releases 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]] [[Period (geology)|Periods]], when numerous examples have been documented,<ref name="Gronstal 2008">{{cite web | last = Gronstal | first = A. L. | title = Gasping for Breath in the Jurassic Era | work = http://www.space.com | publisher = [[Imaginova]] | date = 2008-04-24 | url = http://www.space.com/scienceastronomy/080424-am-jurassic-warming.html | accessdate = 2008-04-24}} </ref><ref name="Cohen 2008"> {{cite journal | last = Pearce | first = C. R. | coauthors = Cohen, A. S.; Coe, A. L.; Burton, K. W. | title = Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the Early Jurassic | journal = Geology | volume = 36 | issue = 3 | pages = 231–234 | publisher = [[Geological Society of America]] | date = March 2008 | url = http://geology.geoscienceworld.org/cgi/content/abstract/36/3/231 | doi = 10.1130/G24446A.1 | accessdate = 2008-04-24}}</ref> 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== ;The timeline data of the Jurassic and Cretaceous The concept of the oceanic anoxic event (OAE) was first proposed in 1976 by Seymour Schlanger (1927–1990) and geologist Hugh Jenkyns<ref name="HistOfOil-2">{{Cite Sm |History Channel, "The History of Oil" (2007), Australian Broadcasting System, Inc.<!--may be out of context -->, aired: 2:00-4:00 pm EDST, 2008-07-08 |q= [[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 occurence 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. }}</ref > 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 plateau]]s ([[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&mdash;particularly in the geologic record of the otherwise limestone dominated [[Apennine Mountains|Apennies]]<ref name="HistOfOil-2"/> chain in Italy, that led to the realization that these widespread intervals of similar [[stratigraphy|strata]] recorded highly unusual "punctuational" conditions in the world ocean during the several distinct discrete periods of [[geological time scale|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<ref name="HistOfOil"/>. Furthermore, detailed organic geochemical studies have recently revealed the presence of molecules (so-called biomarkers) that derive from both [[purple sulfur bacteria]]<ref name="HistOfOil"/> and [[green sulfur bacteria]]: organisms that required both light and free [[hydrogen sulfide|hydrogen sulfide (H<sub>2</sub>S)]], illustrating that anoxic conditions extended high into the upper water column. Such sulfidic (or euxinic) conditions, which exist today in many [[water bodies]] from [[pond]]s to various land surrounded [[mediterranean sea]]s<ref name="dicdef-1">{{Cite Sm |[http://dictionary.reference.com/browse/mediterranean definition of mediterranean sea] |quote=6. surrounded or nearly surrounded by land. }}</ref > such as the [[Black Sea]] of today, were particularly prevalent in the [[Cretaceous]] Atlantic but also characterized other parts of the world ocean. In an ice free sea of these believed super-greenhouse worlds, oceanic waters were as much as 200 meters higher, in some eras. During the time spans in question, the [[continental plate]]s are believed to have been well separated, and the [[mountain]]s we know today were (mostly) future [[tectonic]] events&mdash;meaning the overall landscapes were generally much lower&mdash; and even the half super-greenhouse climates would have been eras of highly expedited water erosion<ref name="HistOfOil"/> 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 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 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 (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).<ref name="Gronstal 2008"/><ref name="Cohen 2008"/> 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. === 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 (half a million years or less) 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 sunk to form an intermediate layer, at 500-1000m depth, with a temperature of 20-25{{oC}}.<ref name=ngeo217/> 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.<ref>{{cite journal|doi=10.1146/annurev.earth.36.031207.124256|title=Oceanic Euxinia in Earth History: Causes and Consequences|year=2008|author=Meyer, Katja M.|journal=Annual Review of Earth and Planetary Sciences|volume=36|pages=251}}</ref> 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 province]]s (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. ==Paleozoic anoxia== The boundary between the Ordovician and Silurian periods marked by repetitive periods of anoxia, interspersed with normal, oxic conditions. In addition, anoxic periods are found during the Silurian. These anoxic period occurred at a time of low global temperatures (although {{co2}} levels were high), in the midst of a glaciation.<ref name=Page2007>{{palAss2007|author=Page, A., Zalasiewicz, J. & Williams, M|title=Deglacial anoxia in a long-lived Early Palaeozoic Icehouse. |pages=85}}</ref> Jeppsson (1990) proposes a mechanism whereby the temperature of polar waters determines the site of formation of downwelling water.<ref name=Jeppsson1990>{{cite journal | author = Jeppsson, L. | year = 1990 | title = An oceanic model for lithological and faunal changes tested on the Silurian record | journal = Journal of the Geological Society | volume = 147 | issue = 4 | pages = 663–674 | doi = 10.1144/gsjgs.147.4.0663 }}</ref> If the high latitude waters are below 5{{oC}}, 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 then 5{{oC}}, 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.<ref name=Jeppsson1990/> The effect of this warm water will propagate through the ocean, and the warmer water has the additional effect of reducing the amount of {{co2}} which can be stored in the oceans - causing the release of large quantities to the atmosphere in a short time - tens or thousands of years.<ref name=Jeppsson1997/> The warm waters would also initiate the release of clathrates, further increasing both atmospheric temperature and basin anoxia.<ref name=Jeppsson1997/> Similar positive feedbacks operate during P-episodes, amplifying their cooling effects. The periods with cold poles are termed "P-episodes" (short for ''primo''<ref name=Jeppsson1997>{{cite book | author = Jeppsson, L | year = 1997 | chapter = The anatomy of the Mid-Early Silurian Ireviken Event and a scenario for P-S events | editor = Brett, C.E., Baird, G.C. | title = Paleontological Events: Stratigraphic, Ecological, and Evolutionary Implications | publisher = Columbia University Press | location = New York | pages = 451-492 }}</ref>), and are characterised by bioturbated deep oceans, a humid equator and higher weathering rates, and terminated by extinction events - for example, the [[Ireviken event|Ireviken]] and [[Lau event]]s. The inverse is true for the warmer, oxic "S-episodes" (''secundo''), where deep ocean sediments are typically [[graptolit]]ic black shales.<ref name=Jeppsson1990/> A typical cycle of ''secundo-primo'' episodes and ensuing ''event'' typically lasts around 3&nbsp;Ma.<ref name=Jeppsson1997/> The duration of events is so long compared to their onset because the positive feedbacks must be overwhelmed. Carbon is only removed from the ocean-atmosphere system by changes in weathering rates, which are dominantly controlled by rainfall. Because this is inversely related to temperature in Silurian times, carbon is gradually drawn down during warm (high {{co2}}) 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.<ref name=Jeppsson1997/> These events become longer during the Devonian; the enlarging land plant biota probably acted as a large buffer to carbon dioxide concentrations.<ref name=Jeppsson1997/> 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 nutrient-rich meltwater from melting glaciers, which by virtue of its fresh nature would also slow down oceanic circulation.<ref name=Lüning2006>{{cite journal | author = Lüning, S. | coauthors = Loydell, D.K.; Štorch, P.; Shahin, Y.; Craig, J. | year = 2006 | title = Origin, sequence stratigraphy and depositional environment of an Upper Ordovician (Hirnantian) deglacial black shale, Jordan—Discussion | journal = Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 230 | issue = 3-4 | pages = 352–355 | doi = 10.1016/j.palaeo.2005.10.004 }}</ref> ==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 [[Bioindicator|biomarker]]s 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.<ref>{{cite journal | last = Ward | first = Peter D | authorlink = Peter Ward (paleontologist) | coauthors = | title = Impact from the Deep | journal = [[Scientific American]] | volume = 2006 | issue = October | pages = 64–71 | publisher = | location = | url = http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=00037A5D-A938-150E-A93883414B7F0000 | doi = | id = | accessdate = 2006-09-26}}</ref> ==Consequences== Oceanic anoxic events have had many important consequences. It is believed that they have been responsible for mass [[extinction events|extinctions]] of marine organisms both in the [[Paleozoic]] and [[Mesozoic]].<ref name=10.1146.2Fannurev.earth.36.031207.124256>{{subst:cite doi|10.1146/annurev.earth.36.031207.124256}}</ref> The early Toarcian and Cenomanian-Turonian anoxic events correlate with the [[Toarcian turnover|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. 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 rock]]s 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. ==See also== * [[Anoxic sea water]] * [[Hypoxia (environmental)|Hypoxia]] for links to other articles dealing with environmental hypoxia or anoxia. ==Notes, links and references== ==References== ===External links=== *[http://ethomas.web.wesleyan.edu/ees123/hotstinky.htm Hot and stinky: The oceans without oxygen] *[http://www.geology.yale.edu/~ajs/2001/Feb/qn020100112.pdf Seawater strontium isotopes, oceanic anoxic events, and seafloor spreading] *[http://www.usssp-iodp.org/PDFs/Workshop_PDFs/cretaceous_climate_report.pdf Cretaceous climate-ocean dynamics] *[http://eprints.ouls.ox.ac.uk/archive/00000808/01/Jenkyns_2004.pdf Further evidence for photic-zone euxinic conditions during Mesozoic oceanic anoxic events] *[http://abc.net.au/science/crude/ Crude - the incredible journey of oil, Australian Broadcasting Corporation] *{{Cite journal | last = Kashiyama | first = Yuichiro | coauthors = Nanako O. Ogawa, Junichiro Kuroda, Motoo Shiro, Shinya Nomoto, Ryuji Tada, Hiroshi Kitazato, Naohiko Ohkouchi | title = Diazotrophic cyanobacteria as the major photoautotrophs during mid-Cretaceous oceanic anoxic events: Nitrogen and carbon isotopic evidence from sedimentary porphyrin | journal = Organic Geochemistry | volume = 39 | issue = 5 | pages = 532–549 | accessdate = 2008-05-10 | date = 2008-05 | doi = 10.1016/j.orggeochem.2007.11.010 | url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V7P-4R98K6R-1&_user=1080547&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000051389&_version=1&_urlVersion=0&_userid=1080547&md5=49204479929f0c87061bf3c69d7b1949 }} *Kump, L.R., Pavlov, A., and Arthur, M.A. (2005). "Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia". ''Geology'', v. 33, pp.397–400 *Hallam, Tony (2004) Catastrophes and lesser calamities, Oxford University Press. pp.91-607 ===Footnotes=== {{reflist}} [[Category:Chemical oceanography]] [[Category:Environmental chemistry|Anoxic event]] [[Category:Oceanography|Anoxic event]] [[Category:Bioindicators]] [[it:evento anossico]] [[ja:海洋無酸素事変]]