Euxinia
Euxinic events in Earth's history
Euxinic events in Earth's history
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 [thumb\|660x660px\|Presence of euxinia in the world's ancient deep oceans. According to Canfield, the deep ocean became sulfitic around 1\.8 billion years ago and stayed that way for much of the boring billion. Periodic euxinia dominated through the Late Devonian Kelwasser events, and then most likely disappeared during the Carboniferous. Euxinia reemerged at the Permian\-Triassic Boundary, and may have been present during the Ocean Anoxia Events of the Mesozoic. Euxinia is rare in Cenozoic oceans. Adapted from Lyons, 2008](/wiki/File:Timeline_of_euxinia_in_ancient_oceans.png "Timeline of euxinia in ancient oceans.png")

### Proterozoic

The Proterozoic is the transition era between anoxic and oxygenated oceans. The classic model is that the end of the [Banded iron formations](/wiki/Banded_iron_formation "Banded iron formation") (BIFs) was due to the injection of oxygen into the deep ocean, an approximately 0\.6 billion year lag behind the [Great Oxygenation Event](/wiki/Great_Oxygenation_Event "Great Oxygenation Event"). Canfield, however, argued that anoxia lasted much longer, and the end of the banded iron formations was due to the introduction of sulfide. Supporting Canfield's original hypothesis, 1\.84 billion year old sedimentary records have been found in the Animike group in Canada that exhibit close to full pyritization on top of the last of the banded iron formations, showing evidence of a transition to euxinic conditions in that basin. In order for full pyritization to happen, nearly all of the sulfate in the water was reduced to sulfide, which stripped the iron from the water, forming pyrite. Because this basin was open to the ocean, deep euxinia was interpreted as being a widespread phenomena. This euxinia is hypothesized to have lasted until about 0\.8 billion years ago, making basin bottom euxinia a potentially widespread feature throughout the [Boring Billion](/wiki/Boring_Billion "Boring Billion").

Further evidence for euxinia was discovered in the McArthur Basin in Australia, where similar iron chemistry was found. The degree of pyritization and the δ34S were both high, supporting the presence of anoxia and sulfide, as well as the depletion of sulfate. A different study found biomarkers for [green sulfur bacteria](/wiki/Green_sulfur_bacteria "Green sulfur bacteria") and [purple sulfur bacteria](/wiki/Purple_sulfur_bacteria "Purple sulfur bacteria") in the same area, providing further evidence for the reduction of sulfate to hydrogen sulfide.

Molybdenum isotopes have been used to examine the distribution of euxinia in the Proterozoic eon, and suggest that perhaps euxinia was not as widespread as Canfield initially postulated. Bottom waters may have been more widely suboxic than anoxic, and there could have been negative feedback between euxinia and the high levels of surface primary production needed to sustain euxinic conditions. Further work has suggested that from 700 million years ago (late Proterozoic) and onward, the deep oceans may have actually been anoxic and iron rich with conditions similar to those during the formation of BIFs.

### Phanerozoic

There is evidence for multiple euxinic events during the Phanerozoic. It is most likely that euxinia was periodic during the Paleozoic and Mesozoic, but geologic data is too sparse to draw any large scale conclusions. In this eon, there is some evidence that euxinic events are potentially linked with mass extinction events including the [Late Devonian](/wiki/Late_Devonian_extinction "Late Devonian extinction") and [Permian–Triassic](/wiki/Permian%E2%80%93Triassic_extinction_event "Permian–Triassic extinction event").

#### Paleozoic

The periodic presence of euxinic conditions in the Lower Cambrian has been supported by evidence found on the Yangtze platform in South China. Sulfur isotopes during the transition from Proterozoic to Phanerozoic give evidence for widespread euxinia, perhaps lasting throughout the Cambrian period. Towards the end of the Lower Cambrian, the euxinic chemocline grew deeper until euxinia was present only in the sediments, and once sulfate became limiting, conditions became anoxic instead of euxinic. Some areas eventually became oxic, while others eventually returned to euxinic for some time.

Geological records from the paleozoic in the Selwyn Basin in Northern Canada have also shown evidence for episodic stratification and mixing, where, using δ34S, it was determined that hydrogen sulfide was more prevalent than [sulfate](/wiki/Sulfate "Sulfate"). Although this was not originally attributed to euxinia, further studies found that seawater in that time likely had low concentrations of sulfate, meaning that the sulfur in the water was primarily in the form of sulfide. This combined with organic\-rich black shale provide strong evidence for euxinia.

There is similar evidence in the black shales in the mid\-continent North America from the Devonian and early Mississippian periods. [Isorenieratene](/wiki/Isorenieratene "Isorenieratene"), a pigment known as a proxy for an anoxic photic zone, has been found in the geological record in Illinois and Michigan. Although present, these events were probably ephemeral and did not last for longer periods of time. Similar periodic evidence of euxinia can also be found in the Sunbury shales of Kentucky.

Evidence for euxinia has also been tied to the [Kellwasser events](/wiki/Kellwasser_event "Kellwasser event") of the Late Devonian Extinction event. Euxinia in basinal waters in what is now central Europe (Germany, Poland, and France) persisted for part of the late Devonian, and may have spread up into shallow waters, contributing to the extinction event.

There was perhaps a period of oxygenation of bottom waters during the [Carboniferous](/wiki/Carboniferous "Carboniferous"), most likely between the Late Devonian Extinction and the Permian\-Triassic Extinction, at which point euxinia would be very rare in the paleo oceans.

The [Permian–Triassic extinction event](/wiki/Permian%E2%80%93Triassic_extinction_event "Permian–Triassic extinction event") may also have some ties to euxinia, with hypercapnia and hydrogen sulfide toxicity killing off many species. Presence of a biomarker for anaerobic photosynthesis by green sulfur bacteria has been found spanning from the Permian to early Triassic in sedimentary rock in both Australia and China, meaning that euxinic conditions extended up quite shallow in the water column, contributing to the extinctions and perhaps even slowed the recovery. It is uncertain, however, just how widespread photic zone euxinia was during this period. Modelers have hypothesized that due to environmental conditions anoxia and sulfide may have been brought up from a deep, vast euxinic reservoir in [upwelling](/wiki/Upwelling "Upwelling") areas, but stable, gyre\-like areas remained oxic.

#### Mesozoic

The Mesozoic is well known for its distinct [Ocean Anoxic Events](/wiki/Ocean_anoxic_event "Ocean anoxic event") (OAEs) which resulted in the burial of layers of black shale. Although these OAEs are not stand alone evidence for euxinia, many do contain biomarkers which support euxinic formation. Again, evidence is not universal. OAEs may have spurred the spread of existing euxinia, especially in upwelling regions or semi\-restricted basins, but photic zone euxinia did not happen everywhere.

#### Cenozoic

Few episodes of euxinia are evident in the sedimentary record during the Cenozoic. Since the end of the Cretaceous OAEs, it is most likely that the oceanic bottom waters have stayed oxic.


In what ways did the Kellwasser events relate to euxinia and extinction?