The Late Devonian saw sufficient extinctions (around 55 % of all genera) for it to rank among the Big Five, but most genera that disappeared were shallow-water marine, particularly rugose and tabulate corals. Although the Woodleigh impact structure, just north of Perth in Western Australia, has been suggested as a possible culprit, its age is not reliable. Another possible cause is climatic cooling at low-latitudes, because the extinction was followed by the spread to tropical localities of high-latitude faunas. The key to supporting a climatic influence is temperature data from areas most affected by the extinctions. Unusually, a recent study selected phosphatic conodonts (tooth-like microfossils) for oxygen-isotope investigations – carbonate-shelled creatures are the usual choice. Michael Joachimski and Werner Buggisch, of the University of Erlangen in Germany, found prominent oxygen isotope excursions close the Frasnian-famenian boundary (Joachimski, M.M & Buggisch, W. 2002. Conodont apatite d18O signatures indicate climatic cooling as a trigger of the Late Devonian mass extinction. Geology, v. 30, p. 711-714). Their data are well controlled stratigraphically, because the rapid evolution of conodonts in the Devonian allows fine biostratigraphic division.
The extinction event is bracketed by two espisodes of sea-surface cooling, estimated to involve a drop of 6°C from an otherwise constant ambient temperature of around 32°C. They coincide with significant positive shifts in d13C of seawater, interpreted by the authors as evidence of the burial of much organic carbon debris. Therein lies a possible cause for the cooling. Carbon burial would have drawn down atmospheric CO2 levels. The extinction does seem to have been a response to temperature stress, tallying with the colonization of low-latitude seas by high-latitude faunas. However, that still begs the question of why carbon burial underwent two spurts. Was there an increase in sediment supply to the oceans that might augment burial rates, or are the positive carbon-isotope excursions reflections of the extinctions themselves? The second still leaves open the possibility that the undoubted cooling events may have had other causes, such as an increase in stratospheric aerosols, resulting either from major explosive volcanism or perhaps impacts that are yet to be found.