Searching for sudden events that might explain the disappearance of sizeable proportions of fossil taxa is a growing cottage industry among geologists. Until 1980, with Alvarez’ discovery of geochemical evidence for a comet or asteroid impact at the Cretaceous-Tertiary boundary, such tumbles in life’s diversity and volume were merely palaeontological markers which geologists chose to divide the stratigraphic column of the Phanerozoic into Periods and Stages. Mass extinctions now take on a much greater importance through the hunt to explain them. The popular vision of herds of dinosaurs writhing in the inferno following the Chixculub bolide strike at the K-T boundary dwarfs to a large degree the equally certain knowledge that at the same time vast basalt floods in what is now north-western India may have had an equally doleful outcome.
Super-large volcanic events, akin to the Deccan Traps, are a great deal simpler to spot than the subtle signs of impacts in the rock record. Improved precision in dating such basalt piles shows that three of the “Big Five” mass extinctions occurred within the 1 to 2 million-year life spans of flood-basalt paroxysms: the Deccan Traps at the K-T; The Parana Basalts at the Triassic-Jurassic; and the Siberian Traps at the Permian-Triassic boundaries. A similar correlation exists for the lesser Palaeocene-Eocene boundary event at 55 Ma, which implicates the North Atlantic large igneous province responsible for flood basalts in north-west Scotland and Greenland.
The scales have tilted further towards a terrestrial cause for mass death with the recent discovery that the Karoo and Ferrar flood-basalt provinces of South Africa and Antarctica formed at a time (183.6+ 1 Ma) that brackets a lesser extinction event in the early Jurassic Period. Jósef Pálfy of the Hungarian Natural History Museum and Paul Smith of the University of British Columbia (Pálfy, J. and Smith, P.L., 2000. Synchrony between Early Jurassic extinction, oceanic anoxic event, and the Karoo-Ferrar flood basalt volcanism. Geology; v. 28, p. 747–750) use U-Pb dating of thin volcanic ash layers in the Jurassic sedimentary pile of North America to calibrate the ages of individual ammonite Zones of the Pliensbachian and Toarcian Stages of the Jurassic. At that time, about 25 % of organisms at the family level became extinct globally over a period of about 4 million years – the Pliensbachian-Toarcian event was not abrupt. The record in the British Jurassic for extinction of marine animal species shows a marked change at around 183 Ma, within the time span of the Karoo-Ferrar eruptions.
This correlation ties in well with the Toarcian ocean-anoxia event, recorded in the British and Swedish Jurassic (see Earth Pages archives – Methane hydrate – more evidence for the ‘greenhouse’ time bomb) which seems to have coincided with a huge gush of methane into the atmosphere, released by methane hydrate layers in ocean-floor sediments. Methane, a greenhouse gas in its own right, oxidizes to carbon dioxide. What may have happened is that the Karoo-Ferrar volcanism injected massive amounts of CO2, leading to global warming. This, transmitted to deep ocean water, could have triggered breakdown of methane hydrate to give a massive positive feedback to global climate. The heat itself might have driven species and families to extinction, or changed ocean circulation to induce stagnation and anoxia.
Important as Pálfy and Smith’s findings are, they by no means resolve the complexities of interwoven terrestrial events. The 90 million-year old Cenomanian-Turonian ocean-anoxia and extinction event had an associated methane burst, but no flood basalts. That at the Palaeocene-Eocene boundary has no associated anoxia. The largest basalt flood known, beneath the Pacific to form the Ontong-Java Plateau about 120 Ma ago, induced methane release and anoxia, but has no associated extinction peak
Despite well-funded attempts to link mass extinctions, other than the K-T event, to impacts, there is little tangible sign of such a connection using precise radiometric dating. Still, the focus of high-profile stratigraphic research is on boundaries rather than what lies between them.
Putting numbers on ecological effects
In the same issue of Geology a team of American palaeoecologists (Droser, M.L.. Bottjer, D.J., Sheehan, P.M. and McGhee, G.R., 2000. Decoupling of taxonomic and ecologic severity of Phanerozoic marine mass extinctions. Geology; v. 28, p. 675–678) assess the degree to which ecologies change after mass extinctions. They focus on the Late Ordovician and Late Devonian events (two of the “Big Five”). Although both involved similar levels of loss of taxonomic diversity (about 22% decline in marine families), marine ecosystems underwent no significant change after the Ordovician event. Following that towards the end of the Devonian, however, marine ecology changed drastically. One example is reefs colonized by tabulate corals. The early corals were devastated by both extinctions, losing about 75% of taxa. Coral-rich reefs continued after the Ordovician, but virtually disappear from marine ecosystems after the Devonian, until much later in geological time. The most likely explanation for this is that Palaeozoic reefs formed mainly from organisms known as stromatoporoids, which gave the 3-D structure required for tabulate corals. Stromatoporoids lost 50% of their diversity after the Devonian event, and did not recover as reef-formers. The main implication of this study is that the effects of extinctions do not simply depend on the quantity of taxa that are snuffed out, but on specific components of the ecosystems involved.