Finding single reasons for major events is curiously satisfying. Thus the notion that an asteroid strike did away with the dinosaurs — pinning their mysterious demise on one hammerblow from outer space makes sense out of what had seemed inexplicable. But a new theory challenges the single-cause notion of mass extinctions, and questions whether sudden catastrophes in combination aren’t needed to deliver the punch.
The work, to be presented today at the annual meeting of the Geological Society of America in Philadelphia, divides the last 488 million years of geologic history into distinct groups and characterizes each. So-called Pulses are times of sudden, catastrophic events like asteroid impacts, whereas Presses are periods of multigenerational stress on ecosystems, such as massive volcanic eruptions.
Nan Crystal Arens and Ian West (Hobart & William Smith Colleges) chart the history of marine organisms and extinctions through the fossil record to conclude that extinctions in times of Pulse or Press are statistically similar. It is only when Press and Pulse events coincide that a spike in extinctions occurs. “Statistically speaking, extinction rates are not significantly higher at times of impact or volcanism vs. no geologic events,” West said.
Centauri Dreams often cites evidence for extinction events seemingly triggered by incoming debris from the Solar System, such as the Chicxulub crater in the Yucatan associated with the dinosaurs’ demise, or the much larger crater recently found in Antarctica that may have been involved in the Permian-Triassic extinction. Arens and West would doubtless tell me to slow down because extinction events are complicated. Here’s Arens:
“In the modern world, species are commonly endangered by some stress before the final death blow falls. It seems likely that biological systems in the past worked in similar ways. By demonstrating that the coincidence of long-term stress and catastrophic disturbance is needed to produce big extinctions, we hope to break down some of the polarization characteristic of many discussions of extinction. We hope to send people back to the data with a more inclusive hypothesis to test.”
So much the better, and perhaps we can draw some useful inferences for today’s world out of all this. Arens’ comment about modern species relates to her idea that human beings can act as both Press and Pulse, manipulating the environment since the advent of agriculture (the Press) and triggering swift change by industrialization (the Pulse). Are we not entering an era of swifter extinctions by destroying habitats at a record pace? It’s a grim thought, but if Pulse and Press are now working simultaneously, we had better find out whether this link to ancient extinctions has genuine validity for a technological society.
Hi Paul
I just read a piece which discusses the apparent double emergence of life onto land at the end of the Devonian – a whole range of early terrestrial species appeared then there’s Romer’s Gap of 20 Myr before a new set of early tetrapods and other terrestrial species arises. Now some geochemical work has revealed that the first emergence coincided with a rise in O2 to 22%, while the Gap coincided with a low O2 period at 10-13%. This rather nicely parallels the High O2/Low O2 period either side of the Permian/Triassic extinction – O2 rose to 35%, then fell to a mere 10%. According to work on avian respiration the air-sac system of breathing, as seen in birds and dinosaurs, is at an advantage in low O2, thus probably explaining how the dinosaurs came to dominate at the expense of proto-mammals and non-dinosaurian thecodonts. By the end of the Triassic the dinosaurs were the dominant land animal group.
Our recent visitor, Greg Benford, has speculated on intrusions of molecular hydrogen into the inner solar system – this could be one cause of low oxygen periods in Earth’s history, which otherwise are inexplicably long-lived. Since the Sun’s orbit bobs up and down as it circles the Galaxy, and a Galactic plane crossing increases the odds of a gas cloud encounter, the periodicity of extinctions would be neatly explained.
Of course none of these geological or astronomical events parallels our current mass extinction. Do we have the sense to ease back from environmental devastation?
Adam
Adam, that’s a good question indeed (re having the sense to halt current environmental depradations), but it’s also fascinating to hear about gas cloud encounters and their possible effect on these extinctions. I have little knowledge about the double emergence that you cite at the end of the Devonian — maybe you can drop me an e-mail (or post here) the citation for the piece you mention reading. I’d like to look through it as well.
Hi Paul
The gas cloud idea was from Benford’s “The Sunborn” though I’ve read different versions for years.
Peter Ward, of “Rare Earth” fame, is the chief researcher suggesting the low oxygen patch in the Devonian… http://www.physorg.com/news80920576.html
His new book sounds fascinating.
http://www.sciencedaily.com/releases/2008/01/080118101922.htm
Recovering From A Mass Extinction
ScienceDaily (Jan. 20, 2008) — The full recovery of ecological systems, following the most devastating extinction event of all time, took at least 30 million years, according to new research from the University of Bristol.
About 250 million years ago, at the end of the Permian, a major extinction event killed over 90 per cent of life on earth, including insects, plants, marine animals, amphibians, and reptiles. Ecosystems were destroyed worldwide, communities were restructured and organisms were left struggling to recover. This was the nearest life ever came to being completely wiped out.
Previous work indicates that life bounced back quite quickly, but this was mostly in the form of ‘disaster taxa’ (opportunistic organisms that filled the empty ecospace left behind by the extinction), such as the hardy Lystrosaurus, a barrel-chested herbivorous animal, about the size of a pig.
The most recent research, conducted by Sarda Sahney and Professor Michael Benton at the University of Bristol and recently published in Proceedings of the Royal Society B, indicates that specialised animals forming complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to recover.
Sahney said: “Our research shows that after a major ecological crisis, recovery takes a very long time. So although we have not yet witnessed anything like the level of the extinction that occurred at the end of the Permian, we should nevertheless bear in mind that ecosystems take a very long time to fully recover.”
Sahney and Benton looked at the recovery of tetrapods – animals with a backbone and four legs, such as amphibians and reptiles – and found that although globally tetrapods appeared to recover quickly, the dramatic restructuring that occurred at the community level was not permanent and communities did not recover numerically or ecologically until about 30 million years later.
Professor Benton explained: “Diversity is most commonly assessed by tallying the number of taxa on a global scale, but these studies are subject to the vagaries of sampling. By examining well-preserved and well-studied faunas, the taxonomic and ecological recovery of communities after the Permian extinction event can be examined more accurately, and the problems of geological bias are largely avoided.”
The Permian extinctions occurred in three waves, the largest being at the boundary between the Permian and Triassic periods, 251 million years ago. This was the most devastating ecological event of all time, thought to be caused by large-scale volcanism in Russia which produced the ‘Siberian Traps’, covering over 200,000 square kilometers (77,000 square miles) in lava.
Adapted from materials provided by University of Bristol.