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Anoxia and euxinia

Evidence for widespread ocean anoxia (severe deficiency of oxygen) and euxinia (presence of hydrogen sulfide) is found from the Late Permian to the Early Triassic. Throughout most of the Tethys and Panthalassic Oceans, evidence for anoxia appears at the extinction event, including small pyrite framboids, negative δ U excursions, negative δ N excursions, positive δ Se isotope excursions, relatively positive δ C ratios in polycyclic aromatic hydrocarbons, high Th/U ratios, positive Ce/Ce* anomalies, depletions of molybdenum, uranium, and vanadium from seawater, and fine laminations in sediments. However, evidence for anoxia precedes the extinction at some other sites, including Spiti, India, Shangsi, China, Meishan, China, Opal Creek, Alberta, and Kap Stosch, Greenland. Biogeochemical evidence also points to the presence of euxinia during the PTME. Biomarkers for green sulfur bacteria, such as isorenieratane, the diagenetic product of isorenieratene, are widely used as indicators of photic zone euxinia because green sulfur bacteria require both sunlight and hydrogen sulfide to survive. Their abundance in sediments from the P–T boundary indicates euxinic conditions were present even in the shallow waters of the photic zone. Negative mercury isotope excursions further bolster evidence for extensive euxinia during the PTME. The disproportionate extinction of high-latitude marine species provides further evidence for oxygen depletion as a killing mechanism; low-latitude species living in warmer, less oxygenated waters are naturally better adapted to lower levels of oxygen and are able to migrate to higher latitudes during periods of global warming, whereas high-latitude organisms are unable to escape from warming, hypoxic waters at the poles. Evidence of a lag between volcanic mercury inputs and biotic turnovers provides further support for anoxia and euxinia as the key killing mechanism, because extinctions would be expected to be synchronous with volcanic mercury discharge if volcanism and hypercapnia was the primary driver of extinction.

The sequence of events leading to anoxic oceans may have been triggered by carbon dioxide emissions from the eruption of the Siberian Traps. In that scenario, warming from the enhanced greenhouse effect would reduce the solubility of oxygen in seawater, causing the concentration of oxygen to decline. Increased coastal evaporation would have caused the formation of warm saline bottom water (WSBW) depleted in oxygen and nutrients, which spread across the world through the deep oceans. The influx of WSBW caused thermal expansion of water that raised sea levels, bringing anoxic waters onto shallow shelfs and enhancing the formation of WSBW in a positive feedback loop. The flux of terrigeneous material into the oceans increased as a result of soil erosion, which would have facilitated increased eutrophication. Increased chemical weathering of the continents due to warming and the acceleration of the water cycle would increase the riverine flux of nutrients to the ocean. Additionally, the Siberian Traps directly fertilised the oceans with iron and phosphorus as well, triggering bioblooms and marine snowstorms. Increased phosphate levels would have supported greater primary productivity in the surface oceans. The increase in organic matter production would have caused more organic matter to sink into the deep ocean, where its respiration would further decrease oxygen concentrations. Once anoxia became established, it would have been sustained by a positive feedback loop because deep water anoxia tends to increase the recycling efficiency of phosphate, leading to even higher productivity. Along the Panthalassan coast of South China, oxygen decline was also driven by large-scale upwelling of deep water enriched in various nutrients, causing this region of the ocean to be hit by especially severe anoxia. Convective overturn helped facilitate the expansion of anoxia throughout the water column. A severe anoxic event at the end of the Permian would have allowed sulfate-reducing bacteria to thrive, causing the production of large amounts of hydrogen sulfide in the anoxic ocean, turning it euxinic. In some regions, anoxia briefly disappeared when transient cold snaps resulting from volcanic sulphur emissions occurred.

This spread of toxic, oxygen-depleted water would have devastated marine life, causing widespread die-offs. Models of ocean chemistry suggest that anoxia and euxinia were closely associated with hypercapnia (high levels of carbon dioxide). This suggests that poisoning from hydrogen sulfide, anoxia, and hypercapnia acted together as a killing mechanism. Hypercapnia best explains the selectivity of the extinction, but anoxia and euxinia probably contributed to the high mortality of the event. The persistence of anoxia through the Early Triassic may explain the slow recovery of marine life and low levels of biodiversity after the extinction, particularly that of benthic organisms. Anoxia disappeared from shallow waters more rapidly than the deep ocean. Reexpansions of oxygen-minimum zones did not cease until the late Spathian, periodically setting back and restarting the biotic recovery process. The decline in continental weathering towards the end of the Spathian at last began ameliorating marine life from recurrent anoxia. In some regions of Panthalassa, pelagic zone anoxia continued to recur as late as the Anisian, probably due to increased productivity and a return of aeolian upwelling. Some sections show a rather quick return to oxic water column conditions, however, so for how long anoxia persisted remains debated. The volatility of the Early Triassic sulphur cycle suggests marine life continued to face returns of euxinia as well.

Some scientists have challenged the anoxia hypothesis on the grounds that long-lasting anoxic conditions could not have been supported if Late Permian thermohaline ocean circulation conformed to the "thermal mode" characterised by cooling at high latitudes. Anoxia may have persisted under a "haline mode" in which circulation was driven by subtropical evaporation, although the "haline mode" is highly unstable and was unlikely to have represented Late Permian oceanic circulation.

Oxygen depletion via extensive microbial blooms also played a role in the biological collapse of not just marine ecosystems but freshwater ones as well. Persistent lack of oxygen after the extinction event itself helped delay biotic recovery for much of the Early Triassic epoch.