Anoxic event - AbsoluteAstronomy.com

Oceanic anoxic events or anoxic events occur when the Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's ocean

Ocean

An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

s become completely depleted of oxygen (O₂)

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

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. Anoxic events may have caused mass extinctions. These mass extinctions were so characteristic, they include some that geobiologists

Geobiology

Broadly defined, geobiology is an interdisciplinary field of scientific research that explores interactions between the biosphere and the lithosphere and/or the atmosphere....

uses as time markers

Global Boundary Stratotype Section and Point

A Global Boundary Stratotype Section and Point, abbreviated GSSP, is an internationally agreed upon stratigraphic section which serves as the reference section for a particular boundary on the geologic time scale. The effort to define GSSPs is conducted by the International Commission on...

in biostratigraphic

Biostratigraphy

Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period...

dating. It is believed oceanic anoxic events are strongly linked to lapses in key oceanic current circulations, to climate warming and greenhouse gas

Greenhouse gas

A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

es.

Background

Analysis of the geologic record

Geologic record

The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata — deposits laid down in volcanism or by sediment deposition of weathering detritus including all its fossil content and the information it yields about the history...

s occurring before and after the affected ages suggest that onsets are rapid and so are recoveries. Both sets of data suggest that a sudden climate threshold

Tipping point (climatology)

A climate tipping point is a point when global climate changes from one stable state to another stable state, in a similar manner to a wine glass tipping over. After the tipping point has been passed, a transition to a new state occurs...

or tipping point

Tipping point

In sociology, a tipping point is the event of a previously rare phenomenon becoming rapidly and dramatically more common. The phrase was coined in its sociological use by Morton Grodzins, by analogy with the fact in physics that adding a small amount of weight to a balanced object can cause it to...

occurs at about four times the Earth's mean carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

levels relative to the baseline concentrations of about 280 ppmv in circa 1750. This date is significant in that it is regarded as the beginning of the Industrial age

Industrial Age

Industrial Age may refer to:*Industrialisation*The Industrial Revolution...

. Strata analysis suggests that in the era when Earth had a predominantly overheated climate, with heavy daily rains and violent storms, name="Six steps"
> the global climate of the time resulted in far heavier erosion, which in turn fed more nutrients into the world's waters. At the same time it caused deep water circulation between the poles and the equator to stop in a cataclysmic fashion. This obstruction in oceanic circulation led to 'death in the depths' from oxygen deprivation. The stagnation caused by this lack of circulation could not be offset by natural processes and became a source of mildly poisonous hydrogen sulfide

Hydrogen sulfide

Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

s. The stratified waters would support life in the oxygenated surface layer but the deeper layers became a lethal mixture where life was impossible. The toxic lower layers halted scavenger activity along the organically rich ooze, or sapropel

Sapropel

Sapropel is a term used in marine geology to describe dark-coloured sediments that are rich in organic matter...

, and all creatures that died in it drifted down and accumulated on the abyssal basins and bottoms. All these life forms unwarily drifting into the anoxic or toxic layers would have died and contributed to the continual accumulation of unicellular microorganisms. The surface layer benefited from an explosion in life, spurred by the increased nutrients from the super-greenhouse conditions, which was then killing itself in waste products. Ironically these deposits of sedimentary organic materials may have accumulated into lipid

Lipid

Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...

rich deposits. It is now widely believed that most of today's fossil oil reserves formed in several distinct anoxic events in earth's geologic history.

There are currently several places on earth that are exhibiting the features of anoxic events on a localized level such as algae blooms and localized "dead zones". Dead zones exist off the East Coast of the United States in the Chesapeake Bay

Chesapeake Bay

The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West...

, in the Scandinavian strait Kattegat

Kattegat

The Kattegat , or Kattegatt is a sea area bounded by the Jutland peninsula and the Straits islands of Denmark on the west and south, and the provinces of Västergötland, Scania, Halland and Bohuslän in Sweden on the east. The Baltic Sea drains into the Kattegat through the Øresund and the Danish...

, the Black Sea

Black Sea

The Black Sea is bounded by Europe, Anatolia and the Caucasus and is ultimately connected to the Atlantic Ocean via the Mediterranean and the Aegean seas and various straits. The Bosphorus strait connects it to the Sea of Marmara, and the strait of the Dardanelles connects that sea to the Aegean...

(which may have been anoxic in its deepest levels for millennia, however), in the northern Adriatic as well as a dead zone off the coast of Louisiana. The current surge of jellyfish

Jellyfish

Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

worldwide are sometimes regarded as the first stirrings of an anoxic event. Other marine dead zones have appeared in coastal waters of South America, China, Japan, and New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

. A 2008 study counted 405 dead zones worldwide.http://www.sciencedaily.com/releases/2008/08/080814154325.htm

This is a recent understanding. This picture was only pieced together during 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

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

in the geologic record

Geologic record

. Likewise the high relative temperatures believed linked to so called "super-greenhouse events" name="3 degrees C"
>
Oceanic anoxic events were in all likelihood caused or stimulated by extreme episodes of volcanic outgassing. These events contributed to the characteristic elevated carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

levels four to six times current levels that are attributed to these periods. At even a few degrees warmer, rain forests are extremely vulnerable to fire hazards. These forests have little natural resistance to fires, and some conjecture a critical tipping point. Practically overnight the increase of temperature might have been reached and triggered a huge burn-off of planetary forests. This would have released unprecedented amounts of carbon dioxide into the atmosphere. With a change of mean temperatures of three degrees Celsius, the ice caps melted. This triggered a runaway effect. In the super-greenhouse ecologies—the term meaning average temperature rose to or beyond six degrees above today—the seas were so warm, it is believed the water temperatures at the two poles name="3 degrees C-2"
> were in the lower 80s°F (i.e. above 27 °C). The Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

and Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

periods' world ecologies were essentially ice free, had massive storms driven by warm oceans, and were dying from the double hit of lack of oxygen and toxic hydrogen sulfide

Hydrogen sulfide

accumulations at lower layers because of a shut down in the ocean conveyor belt

Thermohaline circulation

The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....

s. In this time, most of the world would experience the highly noxious scent of rotten eggs and the seas would have slowly acquired a deep green hue from the high amounts of algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

Occurrence

Oceanic anoxic events most commonly occurred during periods of very warm climate

Climate

Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

characterized by high levels of carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

and mean surface temperatures probably in excess of 25 °C (77 °F). The Quaternary

Quaternary

The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...

levels, our current period, are just 13 °C (55.4 °F) in comparison. Such rises in carbon dioxide may have been in response to a great outgassing of the highly flammable natural gas

Natural gas

Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

(methane) some have christened an "oceanic burp". Vast quantities of methane

Methane

Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

are normally locked into the Earth's crust on the continental plateaus in one of the many deposits consisting of compounds of methane hydrate, a solid precipitated combination of methane and water much like ice. Because the methane hydrates are unstable, save at cool temperatures and high (deep) pressures, scientists have observed smaller "burps" due to tectonic events. Studies suggest the huge release of natural gas could be a major climatological trigger, methane itself being a greenhouse gas

Greenhouse gas

many times more powerful than carbon dioxide. However, anoxia was also rife during the Hirnantian

Hirnantian

The Hirnantian is the seventh and final internationally-recognized stage of the Ordovician Period of the Paleozoic Era. It was of short duration, lasting about 1.9 million years, from 445.6 ± 1.5 to 443.7 ± 1.5 Ma . The early part of the Hirnantian was characterized by cold temperatures, major...

(late Ordovician) ice age.

Oceanic anoxic events have been recognized primarily from the already warm Cretaceous

Cretaceous

and Jurassic

Jurassic

Periods, when numerous examples have been documented, but earlier examples have been suggested to have occurred in the late Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

, Permian

Permian

The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

, Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

(Kellwasser event/s), Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

and Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

.

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.

Consequences

Oceanic anoxic events have had many important consequences. It is believed that they have been responsible for mass extinctions of marine organisms both in the Paleozoic

Paleozoic

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

and Mesozoic

Mesozoic

The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

. The early Toarcian and Cenomanian-Turonian anoxic events correlate with the Toarcian

Toarcian turnover

The term Toarcian turnover, alternatively the Toarcian extinction, the Pliensbachian-Toarcian extinction, or the Early Jurassic extinction, refers to the wave of extinctions that marked the end of the Pliensbachian stage and the start of the Toarcian stage of the Early Jurassic period, c...

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

Petroleum

Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...

and natural gas

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

Source rock

In petroleum geology, source rock refers to rocks from which hydrocarbons have been generated or are capable of being generated. They form one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been deposited in a variety of environments including...

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.

Jurassic and Cretaceous

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 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

Volcanic plateau

A volcanic plateau is a plateau produced by volcanic activity. There are two main types: lava plateaus and pyroclastic plateaus.-Lava plateau:...

s (Shatsky Rise, Manihiki Plateau

Manihiki Plateau

The Manihiki Plateau is an oceanic plateau in the southwest Pacific Ocean. The Manihiki Plateau was formed by volcanic activity 125 to 120 million years ago during the mid-Cretaceous period at a triple junction plate boundary called the Tongareva triple junction....

), coupled with the fact that they were identical in age with similar deposits cored from the Atlantic Ocean and known from outcrops in Europe - particularly in the geological record of the otherwise limestone-dominated Apennines

Apennine mountains

The Apennines or Apennine Mountains or Greek oros but just as often used alone as a noun. The ancient Greeks and Romans typically but not always used "mountain" in the singular to mean one or a range; thus, "the Apennine mountain" refers to the entire chain and is translated "the Apennine...

chain in Italy - that led to the realization that these widespread intervals of similar strata

Stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....

recorded highly unusual oxygen-depleted conditions in the world ocean during several distinct discrete periods of 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. Furthermore, detailed organic geochemical studies have recently revealed the presence of molecules (so-called biomarkers) that derive from both purple sulfur bacteria

Purple sulfur bacteria

The purple sulfur bacteria are a group of Proteobacteria capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in hot springs or stagnant water. Unlike plants, algae, and cyanobacteria, they do not use water as their...

and green sulfur bacteria

Green sulfur bacteria

The green sulfur bacteria are a family of obligately anaerobic photoautotrophic bacteria. Most closely related to the distant Bacteroidetes, they are accordingly assigned their own phylum....

: organisms that required both light and free hydrogen sulfide (H₂S)

Hydrogen sulfide

, illustrating that anoxic conditions extended high into the illuminated upper water column.

Such sulfidic (or euxinic) conditions, which exist today in many water bodies from pond

Pond

A pond is a body of standing water, either natural or man-made, that is usually smaller than a lake. A wide variety of man-made bodies of water are classified as ponds, including water gardens, water features and koi ponds; all designed for aesthetic ornamentation as landscape or architectural...

s to various land-surrounded mediterranean sea

Mediterranean Sea

The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...

s name="dicdef-1">definition of mediterranean sea; "6. surrounded or nearly surrounded by land." such as the Black Sea

Black Sea

of today, were particularly prevalent in the Cretaceous

Cretaceous

Atlantic but also characterized other parts of the world ocean. In an ice-free sea of these supposed super-greenhouse worlds, oceanic waters were as much as 200 meters higher, in some eras. During the time spans in question, the continental plates are believed to have been well separated, and the mountain

Mountain

Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...

s we know 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 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

Aptian

The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...

(~120 Ma), sometimes called the Selli Event (or OAE 1a) after the Italian geologist, Raimondo Selli (1916–1983), and another at the Cenomanian

Cenomanian

The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding...

–Turonian

Turonian

The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous epoch, or a stage in the Upper Cretaceous series. It spans the time between 93.5 ± 0.8 Ma and 89.3 ± 1 Ma...

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 (in the Valanginian

Valanginian

In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 140.2 ± 3.0 Ma and 136.4 ± 2.0 Ma...

, Hauterivian

Hauterivian

The Hauterivian is, in the geologic timescale, an age in the Early Cretaceous epoch or a stage in the Lower Cretaceous series. It spans the time between 136.4 ± 2 Ma and 130 ± 1.5 Ma...

, Albian

Albian

The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous epoch/series. Its approximate time range is 112.0 ± 1.0 Ma to 99.6 ± 0.9 Ma...

and Coniacian

Coniacian

The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series and spans the time between 89.3 ± 1 Ma and 85.8 ± 0.7 Ma...

–Santonian

Santonian

The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series. It spans the time between 85.8 ± 0.7 mya and 83.5 ± 0.7 mya...

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

Toarcian

The Toarcian is, in the ICS' geologic timescale, an age or stage in the Early or Lower Jurassic. It spans the time between 183.0 Ma and 175.6 Ma...

(~183 Ma). Because no DSDP (Deep Sea Drilling Program) or ODP (Ocean Drilling Program

Ocean Drilling Program

The Ocean Drilling Program was an international cooperative effort to explore and study the composition and structure of the Earth's ocean basins. ODP, which began in 1985, was the direct successor to the highly successful Deep Sea Drilling Project initiated in 1968 by the United States...

) 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 to 1000 m (1,640.4 to 3,280.8 ft) depth, with a temperature of 20 °C (68 °F) to 25 °C (77 °F).

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

Foraminifera

The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...

.

Such an accelerated flux of organic matter would have expanded and intensified the oxygen minimum zone

Oxygen minimum zone

The Oxygen minimum zone , sometimes referred to as the shadow zone, is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about 200 to 1,000 metres, depending on local circumstances....

, 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

Upwelling

Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...

, requiring major climate change on a global scale. Geochemical data from oxygen-isotope

Isotope

Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

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.

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

Greenhouse effect

The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

; 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

Hydrosphere

A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....

. One test of this notion is to look at the age of large igneous province

Large igneous province

A Large Igneous Province is an extremely large accumulation of igneous rocks—intrusive, extrusive, or both—in the earth's crust...

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

Karoo-Ferrar

Karoo and Ferrar denote a major geologic province consisting of flood basalt, which mostly covers South Africa and Antarctica, although portions extend further into southern Africa and into South America, India, Australia and New Zealand...

flood basalt, Caribbean large igneous province

Caribbean large igneous province

The Caribbean large igneous province consists of a major flood basalt, which created this large igneous province. It is the source of the current large eastern Pacific oceanic plateau, of which the Caribbean-Colombian oceanic plateau is the tectonized remnant. The deeper levels of the plateau have...

, Ontong Java Plateau

Ontong Java Plateau

The Ontong Java Plateau is a huge oceanic plateau located in the Pacific Ocean, lying north of the Solomon Islands. The plateau covers an area of approximately , or roughly the size of Alaska, and reaches a thickness of up to . The plateau is of volcanic origin, composed mostly of flood basalts...

) correlates uncannily well with that of the major Jurassic

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 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 a time of low global temperatures (although 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 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 the amount of that the oceans can hold in solution, which makes the oceans release large quantities of into the atmosphere in a geologically short time (tens or thousands of years). The warm waters also initiate the release of clathrates

Clathrate gun hypothesis

The clathrate gun hypothesis is the popular name given to the hypothesis that rises in sea temperatures can trigger the sudden release of methane from methane clathrate compounds buried in seabeds and permafrost which, because the methane itself is a powerful greenhouse gas, leads to further...

, 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

Bioturbation

In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...

deep oceans, a humid equator and higher weathering rates, and terminated by extinction events - for example, the Ireviken

Ireviken event

The Ireviken event was a minor extinction event at the Llandovery/Wenlock boundary .The event is best recorded at Ireviken, Gotland, where over 50% of trilobite species go extinct; 80% of the global conodont species also become extinct in this interval.-Anatomy of the event:The event lasted around...

and Lau event

Lau event

The Lau event was the last of three relatively minor mass extinctions during the Silurian period, having a major effect on the conodont fauna...

s. 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 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 ) 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.

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.

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 released 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

Ozone layer

The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...

, which normally blocks the deadly ultraviolet radiation of the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. The increased UV radiation caused by this ozone depletion

Ozone depletion

Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere , and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon...

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 biomarker

Bioindicator

Biological indicators are species used to monitor the health of an environment or ecosystem. They are any biological species or group of species whose function, population, or status can be used to determine ecosystem or environmental integrity. An example of such a group are the copepods and other...

s of green sulfur bacteria

Green sulfur bacteria

The green sulfur bacteria are a family of obligately anaerobic photoautotrophic bacteria. Most closely related to the distant Bacteroidetes, they are accordingly assigned their own phylum....

, 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.

Background

Occurrence

Consequences

Jurassic and Cretaceous

Mechanism

Paleozoic anoxia

Atmospheric effects

See also

Further reading

External links