Oceanic anoxic event

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The main ocean currents were already mapped in 1888 - the understanding of the time did not, however, recognize their interaction with regional and global climatic conditions.
This graphic shows the worldwide movement of ocean currents (" Thermohaline Circulation ")

An oceanic anoxic event , OAE for short , always occurs when the world's oceans are completely depleted of oxygen below the surface layer . A euxinic (English Euxinia) event describes an anoxic event with the formation of hydrogen sulfide (H₂S). Even if such an event did not take place in the last millions of years, there are clear indications of several such incidents in sediments of the more distant geological past. Possibly caused anoxic events and mass extinctions . It is believed that oceanic anoxic events are very likely to be directly related to major ocean current disturbances , greenhouse gases and global warming .

Introductory description

A careful study of the sediments deposited before and after an OAE shows that such an event can set in very quickly, but that the marine ecosystem usually recovers very quickly afterwards, ie over a period of a few 100,000 years. The tipping point appears to be at a carbon dioxide concentration of ~ 1100 ppmv , four times the pre-industrial value from the year 1750 of 270 ppmv. The prevailing climate at the onset of an oceanic anoxic event was obviously abnormally warm, with rainforests saturated with water vapor, heavy daily downpours and devastating thunderstorms. The most important result of this greenhouse climate, however, was an enormously increased rate of erosion , which overloaded the world's oceans with continental weathering products and effectively “over-fertilized” them. At the same time, the deep water circulation between the poles and the equator practically came to a standstill. The immediate consequence of this was a depletion of oxygen in the deep water and the onset of "deep death". At the same time, toxic hydrogen sulfide was produced . The surface layer ( photic zone ) was still well ventilated and full of life due to the increased nutrient input, but shortly below there were already hostile conditions. Even the activity of corrosive organisms in the muddy seabed, the sapropel , came to a standstill. Organisms that got into the anoxic, toxic zone died and sank into the abyssal basin - and together with the continuously trickling down unicellular microorganisms increased the entry of organic carbon into the seabed sediments that formed. The result was a world ocean that experienced a literal explosion of life due to the prevailing greenhouse climate in its surface layer, but at the same time threatened to suffocate a little deeper on its own waste materials. Ironically, it was precisely this organic waste that formed hydrocarbon-rich sediments. It is now considered fairly certain that most fossil oil deposits can be traced back to anoxic events in geological history.

This characterization of an oceanic anoxic event is the result of research results over the past three decades. All known and suspected anoxic events have so far been correlated with the bedrocks of the large oil deposits, the black schists that are widespread worldwide . Similarly, the suspected relatively high temperatures could be associated with so-called “super greenhouse events”.

Oceanic anoxic events were likely caused by extremely powerful volcanic eruptions that injected massive amounts of volcanic gases into the atmosphere. These emissions contributed to the fact that during an oceanic anoxic event, carbon dioxide concentrations reached four to six times higher than pre-industrial levels. It is also assumed that the elevated temperatures posed an enormous fire hazard for the tropical rainforests and that in this case too, a critical tipping point was reached, which led to an enormous burning of the forests. This also released huge amounts of carbon dioxide into the atmosphere. When the average temperature increased by three degrees Celsius, the ice caps began to melt. The warming, which had become uncontrollable, continued, and ultimately super greenhouse conditions with increased average temperatures of over six degrees compared to today's value set in. As a consequence, the oceans then also warmed up, temperatures of over 27 ° C are assumed even for the polar seas.

During the Cretaceous and Jurassic , the earth was essentially free of ice, but was ravaged by strong storms. The oceans suffered from periodic oxygen deficiency and toxic hydrogen sulfide accumulations because of the failure of the thermohaline circulation in deeper sections. The smell of rotten eggs was probably everywhere, and because of the strong growth of algae, the seas gradually turned a dark green color.

Geological occurrence and duration

In terms of geological history, oceanic anoxic events are mainly linked to very warm climatic periods with very high carbon dioxide concentrations; the global average temperatures on the earth's surface probably rose to over 25 ° C during the climatic optimum of the Upper Cretaceous . The values ​​for the geological presence of the Holocene are comparatively low, they are in the range of 14 to 15 ° C. The high carbon dioxide concentrations may be available with larger natural gas - emanations (particularly methane ) related. Huge amounts of methane are usually found as methane hydrate in the deposits of the continental shelf, mostly in the form of clathrates , ice-like, precipitated solid mixtures of methane and water. Methane hydrates are only stable under low temperatures and high pressures. Due to the large amount of energy released during tectonic earthquakes, the hydrates become unstable and, as already observed, methane can then be released. Scientific studies have come to the conclusion that large natural gas emanations can definitely have a climate-influencing function, since methane is a greenhouse gas and, moreover, releases carbon dioxide when burned.

Strangely enough, anoxic events can also take place during an Ice Age, for example during the Hirnantium in the Upper Ordovician .

However, the majority of oceanic anoxic events occurred mainly during the Cretaceous and Jurassic - both very warm periods in the earth's history. But even earlier there were probably oceanic anoxic events, possibly in the Upper Triassic , in the Permian , in the Carboniferous ( Crenistria horizon ), in the Devonian with the Kellwasser event , in the Ordovician and in the Cambrian already mentioned .

The Paleocene / Eocene maximum temperature (PETM) - a global temperature increase with the accompanying deposition of carbon-rich clay schists in some shelf seas - shows strong similarities with oceanic anoxic events.

Oceanic anoxic events usually take around 500,000 years to regenerate the ocean.

Significant oceanic anoxic events

discovery

The term oceanic anoxic event ( English Oceanic anoxic event , OAE ) was coined in 1976 for the first time by Seymour Schlanger (1927-1990) and the geologist Hugh Jenkyns. It is based on discoveries made by the Deep Sea Drilling Project (DSDP) in the Pacific . When drilling in the submarine plateau basalts of the Shatsky Rise and the Manihiki Plateau , black, carbon-rich slates were driven through in the overlying Cretaceous covering sediments. Similar black slates of comparable age had previously been found in the Atlantic; there were also other examples in outcrops on the European mainland, e.g. B. in the otherwise heavily calcareous Apennines in Italy. Gradually, it was recognized that these intervals of very similar layers reflect very unusual and “punctual” (i.e., time- limited) deposit conditions in the world's oceans.

Sedimentological characteristics

Sedimentological investigations of these very carbon-rich deposits reveal a fine stratification undisturbed by the benthos , which suggests anoxic conditions in connection with a toxic hydrogen sulfide layer on the sea floor. Furthermore, geochemical studies have only recently shown molecules (so-called biomarkers) that can be traced back to purple sulfur bacteria and green sulfur bacteria . Both groups of organisms need light and freely available hydrogen sulfide to survive. This is an indication that anoxic conditions had spread far up into higher water levels.

Such sulphidic (or euxinic) conditions can still be found today in many bodies of water. The spectrum ranges from ponds to inland seas such as z. B. the Black Sea . They were particularly common in the Cretaceous Atlantic, but were also present in other oceans.

Temporal distribution

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

event designation Geological stage Absolute age (Ma BP) Duration (million years)
OAE-3 Coniacium to Santonium 87.3 to 84.6 2.7
OAE-2 Bonarelli Upper Cenomanium 93.8 to 93.5 0.3
OAE-1d Pulp case Upper Albium 100.6 to 100.2 0.4
OAE-1c Great book Upper Albium 103.7 to 103.4 0.3
OAE-1b Urbino Lower Albium 110.9 to 110.6 0.3
Paquier Lower Albium 112.0 to 111.6 0.4
Jacob Upper aptium 113.6 to 113.2 0.4
OAE-1a Selli Lower aptium 124.2 to 123.4 0.8

Detailed stratigraphic investigations of Cretaceous black slate in different regions underline the special importance of two oceanic anoxic events for marine chemistry:

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

Further oceanic anoxic events have also been reported for other chalk levels (e.g. Valanginium , Hauterivium ). Their black slate sediments are, however, of a more spatially limited nature and mainly to be found in the Atlantic area and its neighboring areas; some researchers believe that these are regional phenomena rather than global climate disasters.

If a type locality for oceanic anoxic events in the Cretaceous period should be selected, the choice should fall on Gubbio in the Apennines. Laminated black slate within differently colored clay stones and pink to white limestone stand here. This black slate band, only one meter thick, lies on the border between the Cenomanian and Turonian and is called “Livello Bonarelli” after its first description from 1891.

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 black slate sediments from this period in their drilling campaigns. In the oceans, oceanic crust from the Toarcian is only fragmentarily preserved, which is why the black slates in question all come from mainland outcrops. They have now been detected on every continent and in some commercial oil wells. The Jurassic event can be compared with the two main events in the Cretaceous.

Theories of the origin

The temperatures during the Jura and the Cretaceous are generally considered to be relatively warm, consequently the oxygen concentration dissolved in the sea water was lower than now, and anoxic events could therefore also occur much more easily. However, far more specific conditions are required in order to be able to explain the oceanic anoxic events, which in the geological sense are only relatively brief (500,000 years and less). The following two hypotheses (and their variants) have emerged:

  • The anomalous accumulation of organic matter in the sediment is due to a better conservation mode under restricted and poorly ventilated conditions, which in turn were dependent on the respective nature of the oceanic deposition area.

This hypothesis is well suited for the young and relatively narrow Cretaceous Atlantic (at that time, to a certain extent, an oversized Black Sea with only poor connections to the rest of the world), but cannot provide an explanation for the black shale occurring at the same time on the open plateaus of the Pacific and the different shelf lakes . For the Atlantic, for example, there is evidence that a change in ocean circulation took place when the warm, salty water masses of the tropics became hypersaline, sank and formed a 20–25 ° C intermediate layer at a depth of 500–1000 meters.

  • Oceanic anoxic events reflect a fundamental change in the biological production of the ocean, which led to an enormous increase in unworked plankton (including bacteria) at the expense of calcareous shells such as coccoliths and foraminifera .

An accelerated turnover of organic matter causes an expansion and strengthening of the oxygen minimum zone , and thus indirectly an increase in the organic carbon input into the bottom sediment. A prerequisite for this, however, is a greater availability of dissolved nutrients such as nitrates, phosphates and possibly iron for the phytoplankton living in the photic zone. This, in turn, could only be made possible by a larger continental supply combined with increased upwelling - both indicators of global climate change. Oxygen isotope ratios in carbonates and fossils and magnesium / calcium ratios in fossils show that all significant oceanic anoxic events are related to temperature maxima. It is therefore very likely that global erosion rates and nutrient input into the oceans were increased during these events. Furthermore, decreased oxygen solubility causes the release of phosphates, which in turn stimulates bioproduction in the oceans and in turn leads to increased oxygen demand - a positive feedback that keeps the event alive.

  • an alternative attempt to explain oceanic anoxic events is based on the following scenario: above-average volcanism releases huge amounts of carbon dioxide into the earth's atmosphere; global average temperatures are rising due to the greenhouse effect; Erosion rates and fluvial nutrient input are increasing in importance; bioproduction in the ocean is increasing; the sedimentation of organic carbon gets going - the OAE begins; Carbon dioxide is sequestered from the atmosphere (reversing the greenhouse effect); global mean temperatures are falling again and the ocean-atmosphere system returns to its equilibrium - the OAE ends.

This hypothesis regards an oceanic anoxic event as the reaction of our planet to an excessive injection of carbon dioxide into the atmosphere and into the hydrosphere. One possibility for checking lies in the age of the huge igneous provinces (the Large Igneous Provinces or LIPs), when they form, undoubtedly, enormous amounts of volcanic gases such as B. carbon dioxide were released. Three LIP ages ( Karoo-Ferrar flood basalt, Caribbean LIP and Ontong Java Plateau ) agree surprisingly well with the oceanic anoxic events in the Toarcian, Lower Aptian and at the Cenomanian / Turonian boundary, so that a connection seems possible.

Anoxic Events in the Paleozoic

The border between Ordovician and Silurian shows several anoxic events alternating with oxic conditions. Anoxic events also took place in the Silurian. In contrast to the Mesozoic events, although the carbon dioxide concentrations were high, they arose under low global average temperatures in the middle of an ice age.

In 1990 Jeppsson proposed a scenario in which the temperatures of the polar water masses determine the location of the sinking. If the temperatures of the water masses in the high latitudes are below 5 ° C, their high density causes a decrease. Because of its low temperature, oxygen can be easily dissolved and deep water that is extremely rich in oxygen is created. If the original temperatures are above 5 ° C, their density is not sufficient to submerge under the deep water. In this case, a thermohaline circulation can only get going where the density of the water masses is increased by higher salt concentration - this is the case in warm seas with increased evaporation rate. Compared to polar cold water, submerged warm water is less important in terms of quantity and can only hold relatively little oxygen in solution; the circulation of this oxygen-poor deep water is slow. Nevertheless, the effects of the warm water masses will spread noticeably across the entire ocean. Because of their lower absorption capacity for carbon dioxide, larger quantities of this gas have to be released into the atmosphere in a relatively short period of time - this process should take several tens to thousands of years. Warm water masses have probably also released clathrates, thus increasing the temperatures in the atmosphere and at the same time increasing the anoxic conditions in the sea basins.

The cold water periods are referred to by Jeppsson as P episodes (for primo ), they are characterized by bioturbation in the deep sea bed sediment, humid tropics and higher erosion rates. They have a cooling-enhancing feedback mechanism and usually end in species extinction such as B. the Ireviken event and the Lau event . The reverse is true for the warmer, oxic S episodes (for secundo ), the deep water sediments of which typically consist of black schists containing graptolites .

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

This relatively long period is explained by 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 depend on the amount of precipitation. Since there is an inverse temperature dependency during the Silurian, carbon is drawn back from the atmosphere during warm, carbon dioxide-rich S episodes, but is released as a greenhouse gas during the cool, low carbon P episodes. However, this very gradual cycle trend is also superimposed by Milanković cycles , which ultimately trigger the overturning from P to S events.

In the Devonian, the secundo-primo cycles are lengthened. The rapid growth of land plants has probably buffered carbon dioxide levels.

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

Effects on the atmosphere

In 2005, Lee Kump, Alexander Pavlov and Michael Arthur submitted a scenario according to which oceanic anoxic events are characterized by the upwelling of toxic, hydrogen sulfide-laden deep water. This released hydrogen sulfide then enters the atmosphere, poisoning plants and animals and leading to mass extinction. It even rises into the higher atmosphere and there begins to attack the ozone layer , which normally holds back the deadly UV radiation from the sun. The reduction in ozone increases UV radiation with additional destructive consequences for flora and fauna. Fossil spores from layers that were deposited at the time of the mass extinction at the Permian-Triassic border show deformations that can certainly be attributed to increased UV radiation. Biomarkers for green sulfur bacteria were also found - another indication that the aggressive UV radiation could have played a role in this and possibly other mass extinctions. The ultimate trigger for the mass extinction, however, was a warming of the world's oceans, brought about by an increase in the carbon dioxide concentration to over 1000 ppmv.

consequences

Oceanic anoxic events had many significant consequences. It is believed that they were responsible for the mass extinction of marine organisms in both the Paleozoic and Mesozoic Era . The anoxic events in the Lower Toarcian and at the Cenomanian-Turonian border correlate surprisingly well with the simultaneous mass extinctions of mainly marine organisms. Since oxygen was only available in the uppermost mixed water layer in the oceans, many organisms living in the deep sea could not adapt to the changed marine environment. Possible effects on the atmosphere are still largely unclear in terms of their scope and duration.

A consequence of the oceanic anoxic events that was significant for the global economy was the formation of extensive oil and gas deposits in many Mesozoic sea basins. During an oceanic anoxic event, the accumulation and storage of organic matter was greatly increased, so that potential petroleum mother rocks were sedimented in different facies. About 70 percent of the petroleum mother rocks come from the Mesozoic Era, and another 15 percent were formed in the warm climate of the Paleogene . In cooler geological periods, the deposition of supra-regional petroleum mother rocks took place only rarely.

During the Mesozoic tropical climate with its ice-free polar regions, the sea level was at times 200 meters higher than it is now. In the later Jurassic, the supercontinent Pangea , which had existed since the Carboniferous, was already largely fragmented, there were no ore formation processes and therefore relatively low-lying mainland areas that were inundated by extensive flat seas. Even under less extreme greenhouse conditions, erosion rates were still strong, and significant amounts of nutrients were washed into the oceans, causing microplankton and the entire food chain that depends on it to explode in the oxygen-rich upper layers of water.

See also

Web links

swell

  • Yuichiro Kashiyama, Nanako O. Ogawa, Junichiro Kuroda, Motoo Shiro, Shinya Nomoto, Ryuji Tada, Hiroshi Kitazato, Naohiko Ohkouchi: Diazotrophic cyanobacteria as the major photoautotrophs during mid-Cretaceous oceanic anoxic events: Nitrogen and carbon isotopic evidence from sedimentation . In: Organic Geochemistry . 39, No. 5, 2008-05, pp. 532-549. doi : 10.1016 / j.orggeochem.2007.11.010 .
  • Kump, LR, Pavlov, A., and Arthur, MA (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

References and comments

  1. ^ Kemp et al .: Temporal responses of coastal hypoxia to nutrient loading and physical controls . 2009. Retrieved on October 16, 2013: “Depending on the physical characteristics of the coastal system, this may initiate periodic or permanent water column anoxia and euxinia, with the latter term implying the presence of free sulfide (Kemp et al., 2009) . "
  2. ↑ a b c d e f g h i History Channel, "The History of Oil" (2007), Australian Broadcasting System, Inc., broadcast: Jul 8, 2008 2: 00-4: 00 pm EDST
  3. ^ A b Mark Lynas, Oneworld.net: Six Steps to Hell: The Facts on Global Warming . May 1, 2007. Archived from the original on May 2, 2009. Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Accessed July 8, 2008: “Extreme weather situations continue to advance - should cyclones increase their strength from category 5 to 5.5 - the food supply could be seriously jeopardized. In addition: The greenhouse event from the Eocene fascinates scientists not only because of its effects (large mass extinction in the ocean), but also because of its cause, the methane hydrates . These ice-like compounds of methane and water are only stable at low temperatures and high pressures. They may have reached the atmosphere through an immense “ oceanic eruption ” from the ocean floor, causing the global average temperatures to skyrocket there (methane is a far more powerful greenhouse gas than carbon dioxide). Today we find enormous amounts of methane hydrates on the seabed of the continental shelves. There is a real risk that these compounds could pave their way to the surface again as the water temperature rises, similar to 55 million years ago. " @1@ 2Template: Webachiv / IABot / www.stwr.org
  4. ^ A b Friedrich, Oliver: Warm saline intermediate waters in the Cretaceous tropical Atlantic Ocean . In: Nature Geoscience . 1, 2008, p. 453. doi : 10.1038 / ngeo217 .
  5. ↑ a b c d e What would 3 degrees mean? . Archived from the original on January 23, 2009. Retrieved July 8, 2008: “A temperature rise of six degrees Celsius:
    • 251 million years ago at the end of the Permian , up to 95% of all species became extinct due to a super greenhouse event . The average temperature had risen by six degrees, possibly caused by a methane release that far exceeded a similar event 200 million years later in the Eocene .
    A temperature rise of five degrees Celsius:
    • Reached during the Paleocene / Eocene temperature maximum 55 million years ago. At that time, breadfruit trees grew on the coast of Greenland, and the Arctic Ocean had water temperatures of 20 ° C, 200 kilometers south of the North Pole. Both poles were ice-free, and central Antarctica must have been forested.
    The greenhouse event in the Eocene was probably caused by methane hydrates (an ice-like compound of methane and water), which rose from the sea floor in a gigantic “oceanic burp” and suddenly increased global average temperatures in the atmosphere. Even now there are huge amounts of methane hydrates on the continental shelves. It certainly took 10,000 years for the greenhouse climate to develop in the Lower Eocene. We might be able to do this in just under a hundred years. "
  6. ↑ What would 3 degrees mean? . Archived from the original on January 23, 2009. Retrieved July 8, 2008: “A temperature rise of five degrees Celsius was achieved during the Paleocene / Eocene temperature maximum 55 million years ago. Breadfruit trees grew on the coast of Greenland and the Arctic Ocean had water temperatures of 20 ° C 200 kilometers south of the North Pole. Both poles were ice-free and central Antarctica was probably forested. "
  7. ↑ a b AL Gronstal: Gasping for Breath in the Jurassic Era . In: http://www.space.com/ . Imaginova . April 24, 2008. Retrieved April 24, 2008.
  8. ↑ a b Pearce, C. R., Cohen, AS; Coe, AL; Burton, KW: Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the Early Jurassic . In: Geological Society of America (Ed.): Geology . 36, No. 3, March 2008, pp. 231-234. doi : 10.1130 / G24446A.1 . Retrieved April 24, 2008.
  9. ^ A b History Channel, "The History of Oil" (2007), Australian Broadcasting System, Inc., broadcast July 8, 2008, 2: 00-4: 00 pm EDST. Geologist Hugh Jenkyns was interviewed by the History Channel on the documentary "The History of Oil". In his opinion, the occurrence of a meter-thick black slate layer high in the Apennines together with the results of the Deep Sea Drilling Project from 1974 onwards led to the theory of oceanic anoxic events and then triggered further research.
  10. ↑ definition of mediterranean sea Definition of a Mediterranean sea: almost completely surrounded by land
  11. ^ A b Katja M. Meyer & Lee R. Kump: Oceanic Euxinia in Earth History: Causes and Consequences . In: Annual Review of Earth and Planetary Sciences . 36, 2008, p. 251. doi : 10.1146 / annurev.earth.36.031207.124256 .
  12. ↑ Page, A., Zalasiewicz, J. & Williams, M: Deglacial anoxia in a long-lived Early Palaeozoic Icehouse. Archived from the original on March 27, 2009. Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. In: Paleontological Association 2007 . August, p. 85. Retrieved April 1, 2009. @1@ 2Template: Webachiv / IABot / downloads.palass.org
  13. ↑ a b c Jeppsson, L .: An oceanic model for lithological and faunal changes tested on the Silurian record . In: Journal of the Geological Society . 147, No. 4, 1990, pp. 663-674. doi : 10.1144 / gsjgs.147.4.0663 .
  14. ↑ a b c d e f Jeppsson, L: The anatomy of the Mid-Early Silurian Ireviken Event and a scenario for PS events . In: Brett, CE, Baird, GC (Eds.): Paleontological Events: Stratigraphic, Ecological, and Evolutionary Implications . Columbia University Press, New York 1997, pp. 451-492.
  15. ↑ Lüning, S., Loydell, DK; Štorch, P .; Shahin, Y .; Craig, J .: Origin, sequence stratigraphy and depositional environment of an Upper Ordovician (Hirnantian) deglacial black shale, Jordan-Discussion . In: Palaeogeography, Palaeoclimatology, Palaeoecology . 230, No. 3-4, 2006, pp. 352-355. doi : 10.1016 / j.palaeo.2005.10.004 .
  16. ↑ Peter D. Ward : Impact from the Deep . In: Scientific American . 2006, No. October, August, pp. 64-71. Retrieved September 26, 2006.