text: 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 δS, it was determined that hydrogen sulfide was more prevalent than 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, 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 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, 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 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 areas, but stable, gyre-like areas remained oxic.