Pliensbachian-Toarcian extinction
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The Toarcian Oceanic Anoxic Event (TOAE), also known as the Jenkyns Event, was a global
anoxic event Oceanic anoxic events or anoxic events ( anoxia conditions) describe periods wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic (anoxic and sulfidic) waters. Although anoxic events have not ...
during the early part of the
Toarcian The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toarcian ...
age, approximately 183 million years ago, during the
Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-J ...
. The TOAE is believed to be possibly the most extreme case of widespread ocean deoxygenation in the entire Phanerozoic eon. It is documented by a high amplitude negative
carbon isotope Carbon (6C) has 15 known isotopes, from to , of which and are stable. The longest-lived radioisotope is , with a half-life of years. This is also the only carbon radioisotope found in nature—trace quantities are formed cosmogenically by t ...
excursion, as well as the widespread deposition of black shales and a major extinction event of marine life associated with a major rise in global temperatures. This anoxic event was responsible for the deposition of commercially extracted oil shales, particularly in China.


Timing

The TOAE lasted for approximately 500,000 years. The extinction event occurred in two distinct pulses. The first, more recently identified pulse occurred during the ''mirabile'' subzone of the ''tenuicostatum'' ammonite zone, coinciding with a slight drop in oxygen concentrations and the beginning of warming following a late Pliensbachian cool period. This first pulse, occurring near the Pliensbachian-Toarcian boundary, is sometimes classified as a separate extinction event and referred to as the Pliensbachian-Toarcian boundary event (PTo-E). The main extinction pulse occurred during the ''elegantulum'' subzone of the ''serpentinum'' ammonite zone, during a marked, pronounced warming interval.


Causes

The eruption of the Karoo-Ferrar Large Igneous Province and the resulting surge in atmospheric carbon dioxide levels are often attributed as the mainspring of the TOAE. The large igneous province also intruded into coal seams, releasing even more carbon dioxide and methane than it otherwise would have. Magmatic sills are also known to have intruded into shales rich in organic carbon, causing additional venting of carbon dioxide into the atmosphere. In addition, possible associated release of
methane clathrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amo ...
s has been potentially implicated as yet another cause of global warming, though other studies contradict this viewpoint, concluding that the isotopic record is too incomplete to conclusively attribute the isotopic excursion to methane hydrate dissociation, that carbon isotope ratios in belemnites and bulk carbonates are incongruent with the isotopic signature expected from a massive release of methane clathrates, that much of the methane released from ocean sediments was rapidly sequestered, buffering its ability to act as a major positive feedback, and that methane clathrate dissociation occurred too late to have had an appreciable causal impact on the extinction event. Geological, isotopic, and palaeobotanical evidence suggests the late Pliensbachian was an icehouse period. A warming trend lasting from the latest Pliensbachian to the earliest Toarcian was interrupted by a "cold snap" in the middle ''polymorphum'' zone, equivalent to the ''tenuicostatum'' ammonite zone, which was then followed by the abrupt warming interval associated with the TOAE. It has been hypothesised that the release of cryospheric methane trapped in permafrost amplified the warming and its detrimental effects on marine life. Seawater warmed by anywhere between 3 °C and 7 °C, depending on latitude. Geochemical evidence from what was then the northwestern European epicontinental sea suggests that a shift from cooler, more saline water conditions to warmer, fresher conditions prompted the development of significant density stratification of the water column and induced anoxia. Further consequences resulting from
large igneous province A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive (sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The formation ...
activity included increased
silicate weathering In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...
and an acceleration of the hydrological cycle, as shown by a greatened amount of terrestrially derived organic matter found in sedimentary rocks of marine origin during the TOAE. The enhanced continental weathering in turn led to increased eutrophication that helped drive the anoxic event in the oceans. Continual transport of continentally weathered nutrients into the ocean enabled high levels of primary productivity to be maintained over the course of the TOAE. A 2019 geochronological study found that the emplacement of the Karoo-Ferrar large igneous province and the TOAE were not causally linked, and simply happened to occur rather close in time, contradicting mainstream interpretations of the TOAE. The authors of the study conclude that the timeline of the TOAE does not match up with the course of activity of the Karoo-Ferrar magmatic event. The early stages of the TOAE were accompanied by a decrease in the acidity of seawater following a substantial decrease prior to the TOAE. Seawater pH then dropped close to the middle of the event, strongly acidifying the oceans. The sudden decline of carbonate production during the TOAE is widely believed to be the result of this abrupt episode of
ocean acidification Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
. Additionally, the enhanced recycling of phosphorus back into seawater as a result of high temperatures and low seawater pH inhibited its mineralisation into apatite, helping contribute to oceanic anoxia. The abundance of phosphorus in marine environments created a positive feedback loop whose consequence was the further exacerbation of eutrophication and anoxia.


Effects on biogeochemical cycles


Carbon cycle

Occurring during a broader, gradual positive carbon isotope excursion, the TOAE is associated with a global negative carbon isotope excursion recognised in fossil wood, organic carbon, and carbonate carbon in the ''tenuicostatum'' ammonite zone of northwestern Europe. The global ubiquity of this negative excursion has been called into question, however, due to its absence in certain deposits from the time, such as the Bächental bituminous marls. The negative excursion has been found to be up to -8% in bulk organic and carbonate carbon, although analysis of compound specific biomarkers suggests a global value of around -3% to -4%. In addition, numerous smaller scale carbon isotope excursions are globally recorded on the falling limb of the larger TOAE carbon isotope excursion. A positive carbon isotope excursion, likely resulting from the mass burial of organic carbon during the anoxic event, is known from the subsequent ''falciferum'' ammonite zone.


Sulphur cycle

A positive sulphur isotope excursion in carbonate-associated sulphate occurs synchronously with the positive carbon isotope excursion in carbonate carbon during the ''falciferum'' ammonite zone. This positive sulphur isotope excursion has been attributed to the depletion of isotopically light sulphur in the marine sulphate reservoir that resulted from microbial sulphur reduction in anoxic waters.


Effects on life


Marine invertebrates

The extinction event associated with the TOAE primarily affected marine life as a result the collapse of the carbonate factory.
Brachiopods Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, wh ...
were particularly severely hit, with the TOAE representing one of the most dire crises in their evolutionary history. Ostracods also suffered a major diversity loss, with almost all ostracod clades’ distributions during the time interval corresponding to the ''serpentinum'' zone shifting towards higher latitudes to escape intolerably hot conditions near the Equator. Other affected invertebrate groups included bivalves,
corals Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secr ...
,
ammonoids Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...
, echinoderms,
radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The el ...
ns,
dinoflagellates The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates a ...
, and
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...
.


Marine vertebrates

The TOAE had minor effects on marine reptiles, in stark contrast to the major impact it had on many clades of marine invertebrates.


Terrestrial vertebrates

The TOAE is suggested to have caused the extinction of various clades of dinosaurs, including coelophysids, dilophosaurids, and many basal
sauropodomorph Sauropodomorpha ( ; from Greek, meaning "lizard-footed forms") is an extinct clade of long-necked, herbivorous, saurischian dinosaurs that includes the sauropods and their ancestral relatives. Sauropods generally grew to very large sizes, had lon ...
clades, as a consequence of the remodelling of terrestrial ecosystems caused by global climate change. In the wake of the extinction event, many derived clades of ornithischians, sauropods, and theropods emerged, with most of these post-extinction clades greatly increasing in size relative to dinosaurs before the TOAE.


Terrestrial plants

The volcanogenic extinction event initially impacted terrestrial ecosystems more severely than marine ones. A shift towards a low diversity assemblage of cheirolepid conifers,
cycads Cycads are seed plants that typically have a stout and woody ( ligneous) trunk with a crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants of a species are either male o ...
, and ''Cerebropollenites''-producers adapted for high aridity from a higher diversity ecological assemblage of
lycophyte The lycophytes, when broadly circumscribed, are a vascular plant (tracheophyte) subgroup of the kingdom Plantae. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina. They are one of the oldes ...
s, conifers, seed ferns, and wet-adapted ferns is observed in the palaeobotanical and
palynological Palynology is the "study of dust" (from grc-gre, παλύνω, palynō, "strew, sprinkle" and ''-logy'') or of "particles that are strewn". A classic palynologist analyses particulate samples collected from the air, from water, or from deposit ...
record over the course of the TOAE.


Palaeogeographic changes

During the anoxic event, the
Sichuan Basin The Sichuan Basin (), formerly transliterated as the Szechwan Basin, sometimes called the Red Basin, is a lowland region in southwestern China. It is surrounded by mountains on all sides and is drained by the upper Yangtze River and its tributar ...
was transformed into a giant lake, which was believed to be approximately thrice as large as modern-day
Lake Superior Lake Superior in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh wa ...
. Lacustrine sediments deposited as a result of this lake's existence are represented by the Da’anzhai Member of the Ziliujing Formation. Roughly ∼460 gigatons (Gt) of organic carbon and ∼1,200 Gt of inorganic carbon were likely sequestered by this lake over the course of the TOAE.


Comparison with present global warming

The TOAE and the Palaeocene-Eocene Thermal Maximum have been proposed as analogues to modern
anthropogenic global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
based on the comparable quantity of greenhouse gases released into the atmosphere in all three events. Some researchers argue that evidence for a major increase in Tethyan tropical cyclone intensity during the TOAE suggests that a similar increase in magnitude of tropical storms is bound to occur as a consequence of present climate change.


See also

* Bonarelli Event * Selli Event


References

{{reflist Extinction events Toarcian Stage