14Comments
For decades scientists have toyed with the idea mass extinctions are not timed randomly, instead operating on a cycle of around 27 million years. A new paper puts more substance behind the idea, but still lacks detail on how the proposed cause might operate.
As soon as the theory an asteroid caused the extinction of the non-avian dinosaurs took hold, people started to wonder if this was a one-off. After all, the end-Cretaceous event is only the most recent of five (or maybe six) periods of catastrophic species loss, prior to the one humans have initiated. Adding in some episodes with somewhat lower death counts led people to notice 25-30 million year cycle, with some gaps where the timing would have predicted an extinction.
The idea of a distant planet, nicknamed Nemesis or Shiva, became briefly popular and is occasionally revived. The idea is that every 30 million years this object's orbit causes it to disturb the path of many comets in the Oort cloud, sending so many of them plunging into the inner solar system that the odds of one hitting Earth skyrocket.
However, once most geologists concluded the bulk of mass extinctions were caused by enormous volcanic eruptions producing massive basalt provinces, with comet strikes the exception, the idea was largely dropped. Now, however, a cross-disciplinary team have revived it, connecting death from the skies and beneath our feet.
Previous claims of cyclic mass extinctions primarily relied on large-scale disappearances of ocean life, where the fossil record is easier to read than on land. Professor Michael Rampino of New York University searched scientific publications for reports of terrestrial extinctions. In Historical Biology, he identifies ten events over the last 300 million years all of which match a 27.5 million-year cycle. Eight of these line up with previously noted marine extinctions.
“It seems that large-body impacts and the pulses of internal Earth activity that create flood-basalt volcanism may be marching to the same 27-million-year drumbeat as the extinctions, perhaps paced by our orbit in the Galaxy,” Rampino said in a statement.
Of Rampino's ten mass extinctions, three have dates matching the largest impact craters of the last 300 million years, fitting the cometary disturbance theory. On the other hand, eight coincide with basalt province-producing eruptions that would have wrecked havoc on the Earth's atmosphere and temperature. (The ends of the Jurrasic and Cretaceous had both.)
The challenge is to explain what mechanism could make these eruptions so regular, let alone line up with impacts from space.
Rampino and co-authors note the Solar System passes through the mid-plane of the galaxy approximately every 26-30 million years. They speculate this could cause increased encounters with dark matter, disturbing cometary orbits, and perhaps have some effect on the Earth's interior processes. Although we don't really understand how, they propose the latter might stimulate mantle plumes that cause massive volcanism.
