[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.
[32] This global warming, driven by rising atmospheric carbon dioxide, was the mainspring of the early Toarcian environmental crisis.
[3] The eruption of the Karoo-Ferrar Large Igneous Province is generally attributed to have caused the surge in atmospheric carbon dioxide levels.
[16] Although the PTo-E is not associated with a decrease in δ13C analogous to the TOAE's, volcanism is nonetheless believed to have been responsible for its onset as well, with the carbon injection most likely having an isotopically heavy, mantle-derived origin.
[49] The values of 187Os/188Os rose from ~0.40 to ~0.53 during the PTo-E and from ~0.42 to ~0.68 during the TOAE, and many scholars conclude this change in osmium isotope ratios evidences the responsibility of this large igneous province for the biotic crises.
[59] 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.
[61][62][63] 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.
[71][72] Obliquity-paced carbon isotope excursions have been interpreted as some researchers as reflective of permafrost decline and consequent greenhouse gas release.
[10] 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.
[14][84][85] Continual transport of continentally weathered nutrients into the ocean enabled high levels of primary productivity to be maintained over the course of the TOAE.
[87] An alternate model for the development of anoxia is that epicontinental seaways became salinity stratified with strong haloclines, chemoclines, and thermoclines.
This caused mineralised carbon on the seafloor to be recycled back into the photic zone, driving widespread primary productivity and in turn anoxia.
[88] The freshening of the Arctic Ocean by way of melting of Northern Hemisphere ice caps was a likely trigger of such stratification and a slowdown of global thermohaline circulation.
[93] Extensive organic carbon burial induced by anoxia was a negative feedback loop retarding the otherwise pronounced warming and may have caused global cooling in the aftermath of 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.
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.
[134] Evidence from the Cleveland Basin suggests it took ~7 Myr for the marine benthos to recover, on par with the Permian-Triassic extinction event.
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.
[138] However, amidst the hyperthermal warmth of the latter half of the Toarcian in the TOAE's aftermath, plesiosaurids, microcleidids, leptonectids, temnodontosaurids, and basal parvipelvians vanished.
[143] Insects may have experienced blooms as fish moved en masse to surface waters to escape anoxia and then died in droves due to limited resources.
[84] The coincidence of the zenith of Classopolis and the decline of seed ferns and spore producing plants with increased mercury loading implicates heavy metal poisoning as a key contributor to the floristic crisis during the Toarcian mass extinction.
[145] Poisoning by mercury, along with chromium, copper, cadmium, arsenic, and lead is speculated to be responsible for heightened rates of spore malformation and dwarfism concomitant with enrichments in all these toxic metals.
This produced exquisitely preserved lagerstätten across the world, such as Ya Ha Tinda, Strawberry Bank, and the Posidonia Shale.
[154] 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.
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.
[161] The TOAE and the Palaeocene-Eocene Thermal Maximum have been proposed as analogues to modern anthropogenic global warming based on the comparable quantity of greenhouse gases released into the atmosphere in all three events.