Anoxic event

An oceanic anoxic event, OAE abbreviated, always takes place when the world's oceans below the surface layer is completely depleted of oxygen. Euxinisches (English Euxinia ) Event describes an anoxic event with formation of H ₂ S hydrogen sulphide. Even if such an event did not take place in the last millions of years, as can be found in sediments of the more distant geological past is clear evidence of several such incidents. Possibly caused anoxic events and mass extinction. It is believed that oceanic anoxic events are very likely with disorders of the great ocean currents, with greenhouse gases and global warming are directly related.

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Basic level

A careful study of the deposited before and after OAE sediments suggests that such an event may set up very quickly, the marine ecosystem afterwards but mostly recovered again very quickly. The tipping point seems to lie in a four-fold concentration of carbon dioxide from pre-industrial value of 270 ppmv by 1750 (ie, at ~ 1100 ppmv ). The prevailing climate with oceanic anoxic event was inserting an apparently anomalous warm with water vapor-saturated rainforests, heavy daily rains and destructive thunderstorms. However, most important result of this greenhouse climate was an enormous, newly ascended erosion rate, which overloaded the world's oceans with continental weathering products and less " over-fertilized ." At the same time the deep-water circulation between the poles and the equator came to a virtual standstill. The immediate consequence of this was a depletion in the deep water of oxygen and the onset of " Tiefentod ". At the same time was toxic hydrogen sulfide. The surface layer ( photic zone) was still well ventilated and due to the increased nutrient full of life, however briefly reigned among them already hostile conditions. Even the activity of destructive organisms in the muddy seabed, the sapropel, came to a halt. Organisms that get into the anoxic, toxic zone died and sank into the abyssal basins - and increased along with the ever trickling down unicellular microorganisms the entry of organic carbon in the forming seabed sediments. The result was a world ocean, the formal explosion of life experienced because of the greenhouse climate prevailing in the surface layer, but at the same time threatened to choke a little deeper in his own waste. Ironically, just made this organic waste hydrocarbon-rich sediments. So it is today regarded as fairly certain that can be the most fossil oil reserves attributable to anoxic events in the geological history.

This characterization of an oceanic anoxic event is the result of research over the past three decades. All known and suspected anoxic events have been able to be with the parent rocks of the large oil reservoirs, the world's widespread black shales correlated. Similarly, the presumed relatively high temperatures could be associated with so-called " super greenhouse events" in conjunction.

Oceanic anoxic events were most likely triggered by volcanic eruptions extremely strong, who had introduced vast amounts of volcanic gases into the atmosphere. These emissions contributed to oceanic anoxic event that during the carbon dioxide concentration compared to the current level reached a four to six times higher value. It is also assumed that at the elevated temperatures a huge fire hazard to the tropical rain forests existed and even in this case a critical tipping point has been reached, which led to a massive burning of forests. Characterized in addition large quantities of carbon dioxide released into the atmosphere. With an increase in average temperatures by three degrees Celsius then the ice caps began to melt. The warming has become uncontrollable continued, and it installed eventually super greenhouse conditions with rise in average temperatures of about six degrees compared to today's value. As a corollary then heated the oceans, even for the polar seas here in temperatures above 27 ° C will be accepted.

During the Cretaceous and Jurassic the earth was essentially free of ice, but was it hit by severe storms. The oceans were suffering because of the failure of the thermohaline circulation in the deeper sections of periodically occurring oxygen deficiency and toxic hydrogen sulphide accumulations. The smell of rotten eggs was probably present everywhere and because of the heavy algae growth, the seas gradually took a dark green color.

Geological enthusiasts occurrence and duration

Oceanic anoxic events are geologically mainly bound to very warm climate periods with very high carbon dioxide concentrations, global average temperatures at the surface were doing probably above 25 ° C. The values ​​for our present Holocene are relatively low, reaching 13 ° C. The high carbon dioxide concentrations may be associated with greater Erdgasemanationen (especially the highly flammable methane) in context. Huge amounts of methane are usually found as methane hydrates in the sediments of the continental shelf, usually in the form of clathrates, ice-like, the precipitated solid mixtures of methane and water. Methane hydrates are stable only at low temperatures and high pressures. Due to the large energy released in the tectonic quake hydrates become unstable and it can then (as already observed) quite a release of methane come. Scientific studies have concluded that large Erdgasemanationen can certainly exert a climate-influencing function, since methane is a greenhouse gas and also releases carbon dioxide when burned.

Anoxic events may but strangely also occur during an ice age, as for example during the Upper Ordovician Hirnantiums.

However, the majority of oceanic anoxic events occurred mainly during the Cretaceous and Jura - both very warm sections in the Earth's history. But even earlier, it was probably already too oceanic anoxic events, possibly in the Upper Triassic, Permian, Carboniferous ( Crenistria horizon), in Devon with the Kellwasser event, already mentioned in the above Ordovician and Cambrian.

Also the Paleocene / Eocene Thermal Maximum ( PETM ) - a global temperature increase with concomitant deposition of carbon-rich shales in some shelf seas - shows strong similarities with oceanic anoxic events.

Oceanic anoxic events usually last about 500,000 years, until the ocean regenerated.

Significant oceanic anoxic events

Discovery

The term oceanic anoxic event (English Oceanic anoxic event, OAE) in 1976 for the first time by Seymour Schlanger ( 1927-1990 ) and the geologist Hugh Jenkyns coined. It is based on discoveries of the Deep Sea Drilling Project ( DSDP ) in the Pacific. With holes in the submarine plateau basalts of the Shatsky Rise and the Manihiki Plateau were passed in the overlying Cretaceous Hüllsedimenten black, carbon-rich shales. Similar black shales of comparable age had previously been encountered already in the Atlantic, there was also other examples in outcrop on the European mainland, such as the otherwise strong kalkbetonten Apennines in Italy. Gradually, then sat down by the realization that these intervals very similar layers (ie, narrowly limited in time ) reflected very unusual and " selective " deposition conditions in the world's oceans.

Sedimentological characteristics

Sedimentological investigations of this very carbon-rich sediments indicate an undisturbed benthic fine stratification, which suggests on the seabed to anoxic conditions associated with a toxic hydrogen sulfide layer. Further geochemical studies have recently demonstrated molecules (called biomarkers), which are due to sulfur purple bacteria and green sulfur bacteria. Both groups of organisms need light the way and freely available hydrogen sulfide to survive - an indication that anoxic conditions had to spread widely in higher water layers up.

Such sulphide (or euxinische ) ratios can be found even today in many waters, the spectrum ranges from ponds to inland seas such as the Black Sea. In the Cretaceous Atlantic they occurred particularly frequently, but were also present in other oceans.

Temporal distribution

The following table gives an overview of Mesozoic oceanic anoxic events known so far:

Detailed stratigraphic investigations Cretaceous black shales in different regions underline the particular importance of two oceanic anoxic events on the marine chemistry:

• The Selli Event ( OAE 1a), named after the Italian geologist Raimondo Selli ( 1916 to 1983 ) in the Lower Aptian (~ 124 Ma BP)
• The Bonarelli event (OAE -2), named after the Italian geologist Guido Bonarelli ( 1871-1951 ) at the turn of Cenomanian / Turonian ( ~ 93 Ma BP)

More oceanic anoxic events ( eg, Valanginian, Hauterivian ) reported also for other Cretaceous stages. But your black shale sediments are more spatially limited nature and mainly to be found in the Atlantic region and its neighboring areas; some researchers believe that this is more likely to regional phenomena as to global climate disasters.

If ever a type locality for oceanic anoxic events in the Cretaceous period are selected, then the choice should fall on Gubbio in the Apennines. Here are laminated black shales at within different colored mudstones and pink to white limestones. This only one meter thick black shale band is on the border Cenomanian / Turonian and is named after its first described from the year 1891 as " Livello Bonarelli ".

The only known oceanic anoxic event from the Jurassic took place in the Lower Toarcian (~ 183 Ma BP). Neither the DSDP nor the ODP ( Ocean Drilling Program) found in their drilling campaigns black shale sediments from this time step. In the oceans oceanic crust from the Toarcian is preserved only in fragments, so the question black shales therefore originate all from mainland outcrops. You have now been detected on all continents and in some commercial oil wells. The Jurassic event is quite comparable with the two main events in the Cretaceous.

Theories on the mode of origin

Temperatures during the Jurassic and Cretaceous are generally viewed as being relatively warm, hence the dissolved oxygen concentration in seawater was lower than now, and it could therefore also significantly more likely to occur anoxic events. However, it requires much more specific conditions in order to explain the geological sense, only relatively short ( 500,000 years and less) persistent oceanic anoxic events. The following two hypotheses (and their variants) here have emerged:

• The abnormal accumulation of organic matter in the sediment is due to a better preservation mode under restricted and poorly ventilated conditions, which in turn were themselves dependent on the particular nature of the oceanic depositional environment.

This hypothesis is well suited for the young and relatively narrow Cretaceous Atlantic (then in a sense an over-sized black sea with only poorly connected to the rest of the world ocean ), but can for the black shales occurring simultaneously on the open plateau of the Pacific and the different shelf lakes provide no explanation. For the Atlantic, for example, there is evidence that a change in ocean circulation took place, as already warm, salty waters of the tropics were hypersaline, and sank in 500-1000 meters depth, a 20-25 ° C were warm intermediate layer.

• Oceanic anoxic events reflect a fundamental change in the biological production of the ocean, (including bacteria) resulted in a tremendous increase in not wired plankton at the expense of Kalkschaler as coccoliths and foraminifera.

An accelerated turnover of organic matter causes an expansion and strengthening of the oxygen minimum zone, and thereby indirectly increasing the organic carbon entry into the bottom sediment. However, this is a greater availability of dissolved nutrients such as nitrates, phosphates, and possibly iron for the living in the photic zone phytoplankton. This in turn could only be made possible with a larger hard land supply in combination with increased buoyancy - both indicators of global climate change. Oxygen isotope ratios in carbonates and fossils as well as magnesium / calcium ratios in fossils demonstrate that all major oceanic anoxic events associated with temperature maxima are. It is therefore very likely that the global erosion rates and nutrient inputs were increased in the oceans during these events. Furthermore, a decreased oxygen solubility causes the release of phosphates, which in turn stimulates the biological production in the oceans and in turn attracts an increased oxygen demand for themselves - a positive feedback loop that keeps the event alive.

• An alternative explanation for oceanic anoxic events is based on the following scenario: Above average strong volcanism releases huge amounts of carbon dioxide into the earth's atmosphere; due to the greenhouse effect increase the average global temperatures; Fluvial erosion rates and nutrient inputs are increasing in importance; organic production in the ocean increases; the sedimentation of organic carbon is picking up speed - the OAE begins; Carbon dioxide is sequestered from the atmosphere (inversion of the greenhouse effect ); increases in global average temperatures fall again and the ocean-atmosphere system returns to its equilibrium state back - the OAE ends.

This hypothesis considered an oceanic anoxic event in the response of our planet to an exaggerated injection of carbon dioxide into the atmosphere and the hydrosphere. A verification possibility is the age of the giant igneous provinces ( the Large Igneous Provinces or LIPs ) upon its formation undoubtedly enormous amounts of volcanic gases such as carbon dioxide were released. Three LIP- age ( Karoo - Ferrar flood basalt, Caribbean and Ontong Java Plateau LIP ) agree remarkably well with the oceanic anoxic events in the Toarcian, in the Lower Aptian and the Cenomanian / Turonian boundary, so that a relationship appears to be possible.

Anoxic events in the Paleozoic

The boundary between the Ordovician and Silurian has several anoxic events, alternating with oxic conditions. Also in the Silurian anoxic events took place. In contrast, however, to the Mesozoic events they originated, even though the carbon dioxide concentrations had high values ​​at low average global temperatures in the middle of an ice age.

In 1990, Jeppsson suggested a scenario in which the temperatures of the polar water masses determine the location of the sinking. If the temperatures of the water masses at high latitudes below 5 ° C, thus causing a decrease in their high density. Because of their low temperature oxygen can be well resolved and there are very oxygen-rich deep waters. If the initial temperatures above 5 ° C, its density is not enough to dive under the deep waters. In this case, a thermohaline circulation only come there in transition, where the density of the water masses is increased by higher salt concentration - this is the case in warm seas with increased evaporation rate. In comparison to the polar cold water abtauchendes warm water is quantitatively insignificant and can also only relatively little oxygen hold in solution; the circulation of this oxygen-poor deep water just goes tough vonstatten. Nevertheless, the effects of the warm water masses over the entire ocean will spread significantly. Due to their low absorption capacity for carbon dioxide greater amounts of this gas must be vented to the atmosphere, therefore, in a relatively short time - this process is expected to take approximately several tens to thousands of years to complete. Warm water masses have probably also released clathrates, thus additionally can increase the temperatures in the atmosphere and at the same time enhances the anoxic conditions in the ocean basins.

The cold- water periods are referred to as P- episodes of Jeppsson ( for primo ), they are characterized by bioturbation in deep-sea floor sediment, humid tropics and higher erosion rates. You have a cooling -reinforcing feedback mechanism and usually end with the extinction of species such as the Ireviken event and the Lau Event. The reverse is true for the warmer, south - oxic episodes ( for secundo ), the deep-water sediments characteristically consist of graptolithenhaltigen black shales.

A typical Secundo -primo cycle with the following anoxic event lasts about three million years.

This relatively long period of time finds an explanation in the positive feedback mechanisms that need to be overcome. The carbon content in the ocean-atmosphere system is influenced by changing erosion rates, which in turn are themselves affected by the rainfall. Since during the Silurian is an inverse temperature dependence, while carbon is warm, rich in carbon dioxide S episodes retrieved from the atmosphere, but during the cool, low-carbon P- episodes released as a greenhouse gas. This very gradually cycle run trend is but additionally superimposed by Milankovitch cycles, ultimately cause the overturning of P- to S- events.

In Devon, the Secundo -primo cycles lengthen. Probably the rapid growth of land plants has buffered the carbon dioxide concentrations.

The event resulted in Hirnantium possibly from massive algal blooms. They are due to a sudden input of nutrients, caused by wind-driven buoyancy or by the influx of nutrient-rich glacial meltwater. The supply of fresh water of the glaciers also slowed the oceanic circulation.

Effects on the atmosphere

In 2005, Lee Kump, Alexander Pavlov and Michael Arthur submitted a scenario whereby oceanic anoxic events are characterized by the buoyancy of toxic, schwefelwasserstoffbeladenem deep water. This released hydrogen sulfide then released into the atmosphere, which plants and animals are poisoned and it comes to mass extinction. He even rises into the upper atmosphere and starts to attack the ozone layer, which normally restrains the deadly ultraviolet radiation from the sun. The reduction of ozone, the UV radiation with additional destructive consequences for animal and plant life intensified. Fossil spores from layers that were deposited at the time of the mass extinction at the Permian - Triassic boundary, exhibit deformations that can certainly be attributed to increased UV radiation. In addition, biomarkers were found for green sulfur bacteria - a further indication that the aggressive UV radiation in this and possibly other mass extinction could well have played a role. However, the ultimate trigger for the mass extinction was a warming of the ocean, caused by an increase in carbon dioxide concentration to about 1000 ppmv.

Follow

Oceanic anoxic events had many important consequences. It is believed that they are responsible for the mass extinction of marine organisms in the Paleozoic and Mesozoic. Anoxic events in the Lower Toarcian and at the boundary Cenomanian / Turonian correlates remarkably well with the respective mass extinction mainly marine organisms at these levels. Since oxygen was only present in the surface layers, it is no longer managed many marine deep-water forms to adapt to the changing situation. Possible effects on the atmosphere should remain undecided.

An important consequence for the global economy of the oceanic anoxic events is the fact that it came to the formation of significant oil and gas deposits in many Mesozoic ocean basins due to their. During an oceanic anoxic event collection and storage of organic matter in sediments was greatly increased, so that potential petroleum source rocks were sedimented in different Faziesräumen. Approximately 70 percent of petroleum source rocks are Mesozoic age, and another 15 percent comes from the warm Paleogene. It is rare that conditions were realized in cooler stages to produce cross-regional petroleum source rocks.

In the ice-free oceans of the hypothetical super greenhouse world of the sea level is estimated to have higher levels in some time around 200 meters. Among the periods in question, the continental plates were separated, there was no mountain building processes and therefore only relatively flat and low-lying landscapes. Even in less extreme greenhouse conditions still prevailed strong erosion rates and there were huge amounts of nutrients washed into the seas - Therefore, in the oxygen-rich upper class, there was an explosive growth of micro- plankton and the entire dependent food chain.