Nan Crystal Arens, associate professor of geoscience, believes that her new theory about Earth’s worst mass extinctions may help settle decades of scientific debate. While major cataclysms, such as meteor impacts or dramatic climate change, are often the focus of research related to extinction, these may be the exception rather than the rule. Mass extinctions aren't simple events with simple explanations.
The new theory: “Press/Pulse,” developed by Arens and research assistant Ian West ’06, gets around the scientific 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.”
Professor Arens and West are scheduled to present their work on the Press/Pulse theory on Wednesday, Oct. 25 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, Arens and West divided the past 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 percent of genera became extinct every million years; 8.3 percent of genera became extinct in stages characterized by flood volcanism alone. When neither Press nor Pulse was active, 8.2 percent 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 percent of genera became extinct per million years, statistically higher than the rate observed during other geologic stages.
“Is this model, which seems to work for the big five mass extinction events in Earth’s history, applicable today?” they 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 in the modern day.
“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, they concluded, 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.”
Their research has already received considerable press attention, including coverage in ABC News' “Double Whammy Causes Mass Extinction,” Discovery News' “Mass Extinctions Caused by One-Two Punch, ZeeNews.com's Meteorite-greenhouse gases combo linked to Mass Extinction, Cosmos' “New Theory for Mass Extinctions,” SpaceDaily's “New Theory for Mass Extinction” and NewKerala.com's “Meteorite-greenhouse gasses combo linked to Mass Extinction”