structural:summarization
Volcanism

Though more commonly associated with greenhouse gases and global warming, volcanoes may have cooled the planet and precipitated glaciation by discharging sulphur into the atmosphere. This is supported by a positive uptick in pyritic Δ S values, a geochemical signal of volcanic sulphur discharge, coeval with LOMEI-1.

More recently, in May 2020, a study suggested the first pulse of mass extinction was caused by volcanism which induced global warming and anoxia, rather than cooling and glaciation. Higher resolution of species diversity patterns in the Late Ordovician suggest that extinction rates rose significantly in the early or middle Katian stage, several million years earlier than the Hirnantian glaciation. This early phase of extinction is associated with large igneous province (LIP) activity, possibly that of the Alborz LIP of northern Iran, as well as a warming phase known as the Boda event. However, other research still suggests the Boda event was a cooling event instead.

Increased volcanic activity during the early late Katian and around the Katian-Hirnantian boundary is also implied by heightened mercury concentrations relative to total organic carbon. Marine bentonite layers associated with the subduction of the Junggar Ocean underneath the Yili Block have been dated to the late Katian, close to the Katian-Hirnantian boundary.

Volcanic activity could also provide a plausible explanation for anoxia during the first pulse of the mass extinction. A volcanic input of phosphorus, which was insufficient to enkindle persistent anoxia on its own, may have triggered a positive feedback loop of phosphorus recycling from marine sediments, sustaining widespread marine oxygen depletion over the course of LOMEI-1. Also, the weathering of nutrient-rich volcanic rocks emplaced during the middle and late Katian likely enhanced the reduction in dissolved oxygen. Intense volcanism also fits in well with the attribution of euxinia as the main driver of LOMEI-2; sudden volcanism at the Ordovician-Silurian boundary is suggested to have supplied abundant sulphur dioxide, greatly facilitating the development of euxinia.

Other papers have criticised the volcanism hypothesis, claiming that volcanic activity was relatively low in the Ordovician and that superplume and LIP volcanic activity is especially unlikely to have caused the mass extinction at the end of the Ordovician. A 2022 study argued against a volcanic cause of LOME, citing the lack of mercury anomalies and the discordance between deposition of bentonites and redox changes in drillcores from South China straddling the Ordovician-Silurian boundary. Mercury anomalies at the end of the Ordovician relative to total organic carbon, or Hg/TOC, that some researchers have attributed to large-scale volcanism have been reinterpreted by some to be flawed because the main mercury host in the Ordovician was sulphide, and thus Hg/TS should be used instead; Hg/TS values show no evidence of volcanogenic mercury loading, a finding bolstered by ∆ Hg measurements much higher than would be expected for volcanogenic mercury input.