Toarcian Oceanic Anoxic Event

The Toarcian extinction event, also called the Pliensbachian-Toarcian extinction event,

The Early Toarcian extinction event occurred in two distinct pulses,[4] with the first event being classified by some authors as its own event unrelated to the more extreme second event.^[19] 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.^[20] This first pulse, occurring near the Pliensbachian-Toarcian boundary,^[21] is referred to as the PTo-E.^[8][9] The TOAE itself occurred near the tenuicostatum–serpentinum ammonite biozonal boundary,^[22] specifically in the elegantulum subzone of the serpentinum ammonite zone, during a marked, pronounced warming interval.^[20] The TOAE lasted for approximately 500,000 years,^[23][24][25] though a range of estimates from 200,000 to 1,000,000 years have also been given.^[26] The PTo-E primarily affected shallow water biota, while the TOAE was the more severe event for organisms living in deep water.^[27]

Causes

Geological, isotopic, and

The large igneous province also intruded into coal seams, releasing even more carbon dioxide and methane than it otherwise would have.[56]^[57][16] Magmatic sills are also known to have intruded into shales rich in organic carbon, causing additional venting of carbon dioxide into the atmosphere.^[58] Carbon release via metamorphic heating of coal has been criticised as a major driver of the environmental perturbation, however, on the basis that coal transects themselves do not show the δ¹³C excursions that would be expected if significant quantities of thermogenic methane were released, suggesting that much of the degassed emissions were either condensed as pyrolytic carbon or trapped as coalbed methane.^[59]

In addition, possible associated release of deep sea methane clathrates has been potentially implicated as yet another cause of global warming.^[60][61][62] Episodic melting of methane clathrates dictated by Milankovitch cycles has been put forward as an explanation fitting the observed shifts in the carbon isotope record.^[63][26][64] Other studies contradict and reject the methane hydrate hypothesis, however, concluding that the isotopic record is too incomplete to conclusively attribute the isotopic excursion to methane hydrate dissociation,^[65] that carbon isotope ratios in belemnites and bulk carbonates are incongruent with the isotopic signature expected from a massive release of methane clathrates,^[66] that much of the methane released from ocean sediments was rapidly sequestered, buffering its ability to act as a major positive feedback,^[67] and that methane clathrate dissociation occurred too late to have had an appreciable causal impact on the extinction event.^[68] Hypothetical release of methane clathrates extremely depleted in heavy carbon isotopes has furthermore been considered unnecessary as an explanation for the carbon cycle disruption.^[69]

It has also been hypothesised that the release of cryospheric methane trapped in permafrost amplified the warming and its detrimental effects on marine life.^[70][71] Obliquity-paced carbon isotope excursions have been interpreted as some researchers as reflective of permafrost decline and consequent greenhouse gas release.^[72][73]

Major drops in marine oxygen content during the Early Toarcian were among the most lethal killers of life. A positive δ¹³C excursion, likely resulting from the mass burial of organic carbon during the anoxic event, is known from the falciferum ammonite zone, chemostratigraphically identifying the TOAE.

Euxinia occurred in the northwestern Tethys Ocean during the TOAE, as shown by a positive δ³⁴S excursion in carbonate-associated sulphate occurs synchronously with the positive δ¹³C excursion in carbonate carbon during the falciferum ammonite zone. This positive δ³⁴S 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.^[92] Similar positive δ³⁴S excursions corresponding to the onset of TOAE are known from pyrites in the Sakahogi and Sakuraguchi-dani localities in Japan, with the Sakahogi site displaying a less extreme but still significant pyritic positive δ³⁴S excursion during the PTo-E.^[93] Euxinia is further evidenced by enhanced pyrite burial in Zázrivá, Slovakia,^[94] enhanced molybdenum burial totalling about 41 Gt of molybdenum,^[95] and δ^98/95Mo excursions observed in sites in the Cleveland, West Netherlands, and South German Basins.^[96] Valdorbia, a site in the Umbria-Marche Apennines, also exhibited euxinia during the anoxic event.^[26] There is less evidence of euxinia outside the northwestern Tethys, and it likely only occurred transiently in basins in Panthalassa and the southwestern Tethys.^[97] Due to the clockwise circulation of the oceanic gyre in the western Tethys and the rough, uneven bathymetry in the northward limb of this gyre, oxic bottom waters had relatively few impediments to diffuse into the southwestern Tethys, which spared it from the far greater prevalence of anoxia and euxinia that characterised the northern Tethys.^[98] The Panthalassan deep water site of Sakahogi was mainly anoxic-ferruginous across the interval spanning the late Pliensbachian to the TOAE, but transient sulphidic conditions did occur during the PTo-E and TOAE.^[99] In northeastern Panthalassa, in what is now British Columbia, euxinia dominated anoxic bottom waters.^[100]

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.^[13] The sudden decline of carbonate production during the TOAE is widely believed to be the result of this abrupt episode of ocean acidification.^{[101][102][103]} 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.^[104]

The extreme and rapid global warming at the start of the Toarcian promoted intensification of tropical storms across the globe.^[105][106]

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.

Carbonate platforms collapsed during both the PTo-E and the TOAE. Enhanced continental weathering and nutrient runoff was the dominant driver of carbonate platform decline in the PTo-E, while the biggest culprits during the TOAE were heightened storm activity and a decrease in the pH of seawater.[27]

The recovery from the mass extinction among benthos commenced with the recolonisation of barren locales by opportunistic pioneer taxa. Benthic recovery was slow and sluggish, being regularly set back thanks to recurrent episodes of oxygen depletion, which continued for hundreds of thousands of years after the main extinction interval.^[128] Many marine invertebrate taxa found in South America migrated through the Hispanic Corridor into European seas after the extinction event, aided in their dispersal by higher sea levels.^[129]

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. In fact, in the Southwest German Basin, ichthyosaur diversity was higher after the extinction interval, although this may be in part a sampling artefact resulting from a sparse Pliensbachian marine vertebrate fossil record.^[130]

Terrestrial vertebrates

The TOAE is suggested to have caused the extinction of various clades of dinosaurs, including

The TOAE was associated with widespread phosphatisation of marine fossils believed to result from the warming-induced increase in weathering that increased phosphate flux into the ocean. This produced exquisitely preserved lagerstätten across the world, such as Ya Ha Tinda, Strawberry Bank, and the Posidonia Shale.^[136]

As is common during anoxic events, black shale deposition was widespread during the deoxygenation events of the Toarcian.^{[137][18][138]} Toarcian anoxia was responsible for the deposition of commercially extracted oil shales,^[139] particularly in China.^[140][141]

Enhanced hydrological cycling caused clastic sedimentation to accelerate during the TOAE; the increase in clastic sedimentation was synchronous with excursions in ¹⁸⁷Os/¹⁸⁸Os, ⁸⁷Sr/⁸⁶Sr, and δ^44/40Ca.^[142]

Additionally, the Toarcian was punctuated by intervals of extensive kaolinite enrichment. These kaolinites correspond to negative oxygen isotope excursions and high Mg/Ca ratios and are thus reflective of climatic warming events that characterised much of the Toarcian.^[143] Likewise, illitic/smectitic clays were also common during this hyperthermal perturbation.^[144]

Palaeogeographic changes

The intertropical convergence zone migrated southwards across southern Gondwana, turning much of the region more arid. This aridification was interrupted, however, in the spinatus ammonite biozone and across the Pliensbachian-Toarcian boundary itself.^[52]

The large rise in sea levels resulting from the intense global warming led to the formation of the Laurasian Seaway, which enabled the flow of cool water low in salt content to flow into the Tethys Ocean from the Arctic Ocean. The opening of this seaway may have potentially acted as a mitigating factor that ameliorated to a degree the oppressively anoxic conditions that were widespread across much of the Tethys.^[145]

The enhanced hydrological cycle during early Toarcian warming caused lakes to grow in size.^[45] During the anoxic event, the Sichuan Basin was transformed into a giant lake,^[146][147] which was believed to be approximately thrice as large as modern-day Lake Superior.^[148] Lacustrine sediments deposited as a result of this lake's existence are represented by the Da’anzhai Member of the Ziliujing Formation.^[149] 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.^[148]

Comparison with present global warming

The TOAE and the

• Weissert Event
• Selli Event
• Bonarelli Event

References