Red circles show the location and size of many dead zones. Black dots show Ocean dead zones of unknown size.
The size and number of marine dead zones—areas where the deep water is so low in dissolved oxygen that sea creatures can't survive—have grown explosively in the past half-century. – NASA Earth Observatory
This world perspective on oceanic currents demonstrates the interdependencies of transnational regions on circulating currents.
Oceanic anoxic events or anoxic events (anoxia conditions) were intervals in the Earth's past where portions of oceans become depleted in oxygen (O2) at depths over a large geographic area. During some of these events, euxinia, waters that contained H
2S hydrogen sulfide, developed. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events coincided with several mass extinctions and may have contributed to them. These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating. Many geologists believe oceanic anoxic events are strongly linked to slowing of ocean circulation, climatic warming, and elevated levels of greenhouse gases. Researchers have proposed enhanced volcanism (the release of CO2) as the "central external trigger for euxinia".
2S hydrogen sulfide, developed. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events coincided with several mass extinctions and may have contributed to them. These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating. Many geologists believe oceanic anoxic events are strongly linked to slowing of ocean circulation, climatic warming, and elevated levels of greenhouse gases. Researchers have proposed enhanced volcanism (the release of CO2) as the "central external trigger for euxinia".
Background
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 plateaus (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
- particularly in the geological record of the otherwise
limestone-dominated Apennines chain in Italy - that led to the realization that these widespread similar strata recorded highly unusual oxygen-depleted conditions in the world ocean during several 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 and green sulfur bacteria: organisms that required both light and free hydrogen sulfide (H2S), illustrating that anoxic conditions extended high into the illuminated upper water column.
There are currently several places on earth that are exhibiting
the features of anoxic events on a localized scale such as
algal/bacterial blooms and localized "dead zones". Dead zones exist off the East Coast of the United States in the Chesapeake Bay, in the Scandinavian strait Kattegat, the Black Sea
(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 worldwide is 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. A 2008 study counted 405 dead zones worldwide.
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 in the geologic record. Likewise the high relative temperatures believed linked to so called "super-greenhouse events".
Euxinia
Oceanic anoxic events with euxinic
(i.e. sulfidic) conditions have been linked to extreme episodes of
volcanic outgassing. Thus, volcanism contributed to the buildup of CO2
in the atmosphere, increased global temperatures, causing an
accelerated hydrological cycle that introduced nutrients to the oceans
to stimulate planktonic productivity. These processes potentially acted
as a trigger for euxinia in restricted basins where water-column
stratification could develop. Under anoxic to euxinic conditions,
oceanic phosphate is not retained in sediment and could hence be
released and recycled, aiding continued high productivity.
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 (less than a million years) 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 sank to form an intermediate
layer, at 500 to 1,000 m (1,640 to 3,281 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.
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.
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 intense 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 due
to both burial of organic matter and weathering of silicate rocks
(inverse greenhouse effect); global temperatures fall, and the
ocean–atmosphere system returns to equilibrium (OAE ends).
In this way, an oceanic anoxic event can be viewed as the Earth’s
response to the injection of excess carbon dioxide into the atmosphere
and hydrosphere. One test of this notion is to look at the age of large igneous provinces
(LIPs), the extrusion of which would presumably have been accompanied
by rapid effusion of vast quantities of volcanogenic gases such as
carbon dioxide. 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.
Occurrence
Oceanic anoxic events most commonly occurred during periods of very warm climate characterized by high levels of carbon dioxide (CO2) and mean surface temperatures probably in excess of 25 °C (77 °F). The Quaternary levels, the current period,
are just 13 °C (55 °F) in comparison. Such rises in carbon dioxide may
have been in response to a great outgassing of the highly flammable natural gas (methane) that some call an "oceanic burp". Vast quantities of methane are normally locked into the Earth's crust on the continental plateaus in one of the many deposits consisting of compounds of methane hydrate,
a solid precipitated combination of methane and water much like ice.
Because the methane hydrates are unstable, except at cool temperatures
and high (deep) pressures, scientists have observed smaller "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 many times more powerful than 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 Periods, when numerous examples have been documented, but earlier examples have been suggested to have occurred in the late Triassic, Permian, Devonian (Kellwasser event), Ordovician and Cambrian.
The Paleocene–Eocene Thermal Maximum
(PETM), which was characterized by a global rise in temperature and
deposition of organic-rich shales in some shelf seas, shows many
similarities to oceanic anoxic events.
Typically, oceanic anoxic events lasted for less than a million years, before a full recovery.
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 and Mesozoic. The early Toarcian and Cenomanian-Turonian anoxic events correlate with the Toarcian and Cenomanian-Turonian extinction events
of mostly marine life forms. Apart from possible atmospheric effects,
many deeper-dwelling marine organisms could not adapt to an ocean where
oxygen penetrated only the surface layers.
An economically significant consequence of oceanic anoxic events
is the fact that the prevailing conditions in so many Mesozoic oceans
has helped produce most of the world's petroleum and natural gas
reserves. During an oceanic anoxic event, the accumulation and
preservation of organic matter was much greater than normal, allowing
the generation of potential petroleum source rocks
in many environments across the globe. Consequently, some 70 percent of
oil source rocks are Mesozoic in age, and another 15 percent date from
the warm Paleogene: only rarely in colder periods were conditions
favorable for the production of source rocks on anything other than a
local scale.
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 probably 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-Triassic extinction event show deformities consistent with UV radiation. This evidence, combined with fossil biomarkers of green sulfur bacteria, indicates that this process could have played 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.
Ocean chemistry effects
Reduced
oxygen levels are expected to lead to increased seawater concentrations
of redox-sensitive metals. The reductive dissolution of iron–manganese
oxyhydroxides in seafloor sediments under low-oxygen conditions would
release those metals and associated trace metals. Sulfate reduction in
such sediments could release other metals such as barium. When heavy-metal-rich anoxic deep water entered continental shelves and encountered increased O2
levels, precipitation of some of the metals, as well as poisoning of
the local biota, would have occurred. In the late Silurian mid-Pridoli
event, increases are seen in the Fe, Cu, As, Al, Pb, Ba, Mo and Mn
levels in shallow-water sediment and microplankton; this is associated
with a marked increase in the malformation rate in chitinozoans and other microplankton types, likely due to metal toxicity. Similar metal enrichment has been reported in sediments from the mid-Silurian Ireviken event.
Anoxic events in Earth's history
Cretaceous
Sulfidic (or euxinic) conditions, which exist today in many water bodies from ponds to various land-surrounded mediterranean seas such as the Black Sea, were particularly prevalent in the Cretaceous
Atlantic but also 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 mountains 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
(OAEs) were particularly significant in terms of their impact on the
chemistry of the oceans, one in the early Aptian (~120 Ma), sometimes called the Selli Event (or OAE 1a) after the Italian geologist, Raimondo Selli (1916–1983), and another at the Cenomanian–Turonian boundary (~93 Ma), sometimes called the Bonarelli Event (or OAE 2) after the Italian geologist, Guido Bonarelli (1871–1951). OAE1a lasted for ~1.0 to 1.3 Myr.
The duration of OAE2 is estimated to be ~820 kyr based on a
high-resolution study of the significantly expanded OAE2 interval in
southern Tibet, China.
- 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, Hauterivian, Albian and Coniacian–Santonian
stages), but their sedimentary record, as represented by organic-rich
black shales, appears more parochial, being dominantly represented in
the Atlantic and neighboring areas, and some researchers relate them to
particular local conditions rather than being forced by global change.
Jurassic
The only oceanic anoxic event documented from the Jurassic took place during the early Toarcian (~183 Ma). Because no DSDP (Deep Sea Drilling Project) or ODP (Ocean Drilling Program)
cores have recovered black shales of this age – there being little or
no Toarcian ocean crust remaining 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.
Paleozoic
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 CO
2 levels were high), in the midst of a glaciation.
2 levels were high), in the midst of a glaciation.
Jeppsson (1990) proposes a mechanism whereby the temperature of
polar waters determines the site of formation of downwelling water.
If the high latitude waters are below 5 °C (41 °F), they will be dense
enough to sink; as they are cool, oxygen is highly soluble in their
waters, and the deep ocean will be oxygenated. If high latitude waters
are warmer than 5 °C (41 °F), their density 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 CO
2 that the oceans can hold in solution, which makes the oceans release large quantities of CO
2 into the atmosphere in a geologically short time (tens or thousands of years). The warm waters also initiate the release of clathrates, which further increases atmospheric temperature and basin anoxia. Similar positive feedbacks operate during cold-pole episodes, amplifying their cooling effects.
2 that the oceans can hold in solution, which makes the oceans release large quantities of CO
2 into the atmosphere in a geologically short time (tens or thousands of years). The warm waters also initiate the release of clathrates, which further increases atmospheric temperature and basin anoxia. Similar positive feedbacks operate during cold-pole episodes, amplifying their cooling effects.
The periods with cold poles are termed "P-episodes" (short for primo), and are characterised by bioturbated deep oceans, a humid equator and higher weathering rates, and terminated by extinction events - for example, the Ireviken and Lau events. The inverse is true for the warmer, oxic "S-episodes" (secundo), where deep ocean sediments are typically graptolitic black shales.
A typical cycle of secundo-primo episodes and ensuing event typically lasts around 3 Ma.
The duration of events is so long compared to their onset because
the positive feedbacks must be overwhelmed. Carbon content in the
ocean-atmosphere system is affected by changes in weathering rates,
which in turn is dominantly controlled by rainfall. Because this is
inversely related to temperature in Silurian times, carbon is gradually
drawn down during warm (high CO
2) S-episodes, while the reverse is true during P-episodes. On top of this gradual trend is overprinted the signal of Milankovic cycles, which ultimately trigger the switch between P- and S- episodes.
2) 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.
