PureInsight | December 10, 2006
{The Geological Society of America} A new theory on just what causes
Earth's worst mass extinctions may help settle the endless scientific
dust-up on the matter. Whether you favor meteor impacts, volcanic
eruptions, cosmic rays, epidemics, or some other cause for the worst
mass extinction events in Earth's history, no single cause has ever
satisfied all scientists all the time for any extinction event. That
may be because big extinctions aren't simple events.
The new Press/Pulse theory gets around the controversy by rejecting the
all-or-nothing approach to mass extinction, calling instead on a
combination of deadly sudden catastrophes - "pulses" - with longer,
steadier pressures on species - "presses."
"What we wanted to do is move away from the idiosyncratic approach to
extinction mechanisms and look for what these intervals had in common.
If you have A and B you will get a mass extinction," said Ian West, a
2006 graduate of Hobart and William Smith Colleges in Geneva, NY.
West and Hobart and William Colleges paleontology professor Nan Crystal
Arens are scheduled to present their work on the Press/Pulse theory on
Wednesday, 25 October, at the Annual Meeting of the Geological Society
of America in Philadelphia.
Using databases that chart genera of marine organisms and their
extinctions through the fossil record, West and Arens divided the last
488 million years of geologic history into four groups: times of
suspected impact events (Pulses), times of massive volcanic eruptions
(Presses), times when neither Presses nor Pulses occurred, and times
when Press and Pulse coincided. They compared average extinction rates
in geologic stages in each of these groups.
During stages when only impacts occurred, an average of 7.3% of genera
became extinct every million years; 8.3% of genera became extinct in
stages characterized by flood volcanism alone. When neither Press nor
Pulse were active, 8.2% of genera became extinct. These averages are
statistically indistinguishable. "Statistically speaking, extinction
rates are not significantly higher at times of impact or volcanism vs.
no geologic events," West said.
In contrast, when Press and Pulse events coincided, an average of 12.8%
of genera became extinct per million years, statistically higher than
the rate observed during other geologic stages.
"The goal of our work was to come up with a unifying theory of mass
extinctions. We also wanted to make it applicable to what's going on
now," said West, referring to rapid losses of biodiversity worldwide
now underway as a result of climate change and destruction of habitats
by human activities.
"Is this model, which seems to work for the big five mass extinction events in Earth's history, applicable today?" West asked.
At first glance the answer would appear to be 'no.' There is, after
all, no massive flood basalt eruption underway today, nor have there
been any recent meteor impacts. On the other hand, some very similar
effects are being seen on Earth.
"We came up with the idea that humans themselves act as both Press and
a Pulse," said West. "Humans began manipulating the environment - the
Press - from the advent of agriculture. However, that alone did not
trigger the current mass extinction. That seems to have been triggered
by the pulse of industrialization and the demands for energy and
resources that came with it."
The bottom line, says West is that it's extremely hard to pinpoint simple causes for Earth's great periods of extinction.
"We sought to rephrase the question," said 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."
Source: http://www.geosociety.org/news/pr/06-52.htm
