Oct. 20, 2006 — If you can't decide if you prefer meteor impacts, volcanoes or some other explanation for Earth's biggest mass extinction events, take heart: You no longer have to choose.
A new statistical study of mass extinctions throughout the history of life on Earth is backing up the idea that no single meteor, volcanic eruption or other lone gunman is ever to blame, even in the case of the Cretaceous-Tertiary event that brought the end of dinosaurs 65 million years ago.
Instead, the worst die-offs happen when some sort of interminable, multi-generational pressure on life is combined with a few powerful blows. It's what is now being called the press/pulse theory of mass extinctions.
Reading the Cretaceous-Tertiary extinction literature conversations with colleagues "made me wonder whether the simplistic scenario of 'Everything's fine until one day in June when the asteroid hits and everything goes to hell-in-a-hand-basket' really explains the diversity of data," said plant fossil expert Nan Arens of Hobart and William Smith Colleges in New York.
Wouldn't it make more sense, she surmised, if certain species were already vulnerable when the triggering event happened?
To test the idea, she and then-undergraduate student Ian West compiled a large database of marine organisms and their extinctions through geological time.
They divvied up the last 488 million years into four groups: Suspected meteor impacts (pulses), gigantic volcanic flood basalt eruptions (presses), periods with neither presses nor pulses, and times when press and pulse coincided. They compared average extinction rates in each of these groups.
Flood basalt eruptions are considered "presses" because they release vast amounts of greenhouse gases into the atmosphere and can change the Earth's climate.
