Late Devonian extinction
[7] Although it is well established that there was a massive loss of biodiversity in the Late Devonian, the timespan of this event is uncertain, with estimates ranging from 500,000 to 25 million years, extending from the mid-Givetian to the end-Famennian.
[13] The extinction event was accompanied by widespread oceanic anoxia; that is, a lack of oxygen, prohibiting decay and allowing the preservation of organic matter.
Plants, which had been on land in forms similar to mosses and liverworts since the Ordovician, had just developed roots, seeds, and water transport systems that allowed them to survive away from places that were constantly wet—and so grew huge forests on the highlands.
Several clades had developed a shrubby or tree-like habit by the Late Givetian, including the cladoxylalean ferns, lepidosigillarioid lycopsids, and aneurophyte and archaeopterid progymnosperms.
[28] The most hard-hit biological category affected by the Kellwasser event were the calcite-based reef-builders of the great Devonian reef-systems, including the stromatoporoid sponges and the rugose and tabulate corals.
Atrypid and strophomenid brachiopods became rarer, replaced in many niches by productids, whose spiny shells made them more resistant to predation and environmental disturbances.
The shape of conodonts' feeding apparatus varied with the oxygen isotope ratio, and thus with the sea water temperature; this may relate to their occupying different trophic levels as nutrient input changed.
True tetrapods (defined as four-limbed vertebrates with digits) survived and experienced an evolutionary radiation following the Kellwasser extinction,[1] though their fossils are rare until the mid-to-late Famennian.
[39] The Kellwasser event, named for its type locality, the Kellwassertal in Lower Saxony, Germany, is the term given to the extinction pulse that occurred near the Frasnian–Famennian boundary (372.2 ± 1.6 Ma).
Possible triggers for the Kellwasser event are as follows: During the Late Silurian and Devonian, land plants, assisted by fungi,[45][46] underwent a hugely significant phase of evolution known as the Silurian-Devonian Terrestrial Revolution.
[24] The relatively sudden input of nutrients into river water as rooted plants expanded into upland regions may have caused eutrophication and subsequent anoxia.
[24][53] Anoxic conditions correlate better with biotic crises than phases of cooling, suggesting anoxia may have played the dominant role in extinction.
[62] Photic zone euxinia, documented by concurrent negative ∆199Hg and positive δ202Hg excursions, occurred in the North American Devonian Seaway.
[15] Anoxia was not omnipresent across the globe; in some regions, such as South China, the Frasnian-Famennian boundary instead shows evidence of increased oxygenation of the seafloor.
[65] Trace metal proxies in black shales from New York state point to anoxic conditions only occurring intermittently, being interrupted by oxic intervals, further indicating that anoxia was not globally synchronous,[66] a finding also supported by the prevalence of cyanobacterial mats in the Holy Cross Mountains in the time period around the Kellwasser event.
This oxygen isotope excursion is known from time-equivalent strata in South China and in the western Palaeotethys, suggesting it was a globally synchronous climatic change.
[71] The "greening" of the continents during the Silurian-Devonian Terrestrial Revolution that led to them being covered with massive photosynthesizing land plants in the first forests reduced CO2 levels in the atmosphere.
[24] The biological sequestration of carbon dioxide may have ultimately led to the beginning of the Late Palaeozoic Ice Age during the Famennian, which has been suggested as a cause of the Hangenberg event.
[73] The weathering of silicate rocks also draws down CO2 from the atmosphere, and CO2 sequestration by mountain building has been suggested as a cause of the decline in greenhouse gases during the Frasnian-Famennian transition.
[75] This reduction in atmospheric CO2 would have caused global cooling and resulted in at least one period of late Devonian glaciation (and subsequent sea level fall),[24] probably fluctuating in intensity alongside the 40ka Milankovic cycle.
[78] The end of the Devonian Period had extremely widespread trap magmatism and rifting in the Russian and Siberian platforms, which were situated above the hot mantle plumes and suggested as a cause of the Frasnian / Famennian and end-Devonian extinctions.
[80] Volcanic rocks, dyke belts, and sills that cover more than 320,000 km2, and a gigantic amount of magmatic material (more than 1 million km3) formed in the Viluy branch.
[79] Viluy magmatism may have injected enough CO2 and SO2 into the atmosphere to have generated a destabilised greenhouse and ecosystem, causing rapid global cooling, sea-level falls, and marine anoxia to occur during Kellwasser black shale deposition.
[83] Recent studies have confirmed a correlation between Viluy traps in the Vilyuysk region on the Siberian Craton and the Kellwasser extinction by 40Ar/39Ar dating.
[86] However, not all sites show evidence of mercury enrichment across the Frasnian-Famennian boundary, leading other studies to reject volcanism as an explanation for the crisis.
[102] A recent explanation suggests that a nearby supernova explosion was the cause for the specific Hangenberg event, which marks the boundary between the Devonian and Carboniferous periods.
This could offer a possible explanation for the dramatic drop in atmospheric ozone during the Hangenberg event that could have permitted massive ultraviolet damage to the genetic material of lifeforms, triggering a mass extinction.
Recent research offers evidence of ultraviolet damage to pollen and spores over many thousands of years during this event as observed in the fossil record and that, in turn, points to a possible long-term destruction of the ozone layer.
Thus, supernovae have also been speculated to have been responsible for the Kellwasser event, as well as the entire sequence of environmental crises covering several millions of years towards the end of the Devonian period.
