Nature, not humans, may cause mass extinctions

Tavurvur, Papua-New Guinea: Volcanic eruptions may go a long way to explain mass extinctions. Image: By Richard Bartz, via Wikimedia Commons

Life on Earth has been through mass extinctions before − every 27 million years. Blame it on celestial clockwork.

LONDON, 18 December, 2020 − US scientists believe they have identified a recurring pattern of mass extinctions and catastrophic climate change − and this time humans really are not to blame.

Instead, the planet and the solar system could be caught up in some deadly astronomical cycle.

They argue that every 27 million years, a high proportion of land-dwelling species − birds, mammals, reptiles and amphibians − disappear from the fossil record at around the same time.

And this disappearance seems to coincide, again according to geological evidence, with devastating eruptions of volcanic lava and violent asteroid collisions that would have had the effect of darkening the skies, lowering the temperature, depleting the ozone layer, then stimulating a greenhouse effect and starting extensive fire and acid rain.

“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,” said Michael Rampino, a biologist at New York University.

Serial crises

He and colleagues used statistical analysis to identify, in the journal Historical Biology, an ominous rhythm of catastrophe in the Earth’s deep history.

Such research highlights the extraordinary nature of the present life-on-Earth survival crisis. Earth is now undergoing what naturalists and geologists see as the Sixth Great Extinction in its 500-million-year fossil history, and potentially calamitous climate change, not because of any shift in planetary orbit or galactic traffic accident, but because of the human population explosion and the 200-year-long addiction to fossil fuels.

But researchers know the present crisis to be the latest of a series of crises in the long history of life on Earth only because of the capricious evidence of the fossil record, and they have spent the last half century trying to decipher some reason how and why these might have happened.

More than 40 years ago geologists began to see what they argued seemed to be cycles of destruction, followed by the slow restoration of the biodiversity of Earth.

“The global mass extinctions were apparently caused by the largest cataclysmic impacts and massive volcanism, perhaps sometimes working in concert”

Around that time, earth scientists found evidence of an asteroid impact that appeared to have wiped out the entire dinosaur lineage, along with seven-tenths of all species on land and sea. And some began to argue that mass extinctions might not be random events, but happen according to some kind of heavenly timetable.

Most of the evidence for such happenings comes from marine sediments: evidence of extinction in the oceans every 26 million years or so. Now Professor Rampino and his colleagues have looked at the record of mass extinctions on land, and found that these seem to follow a similar cycle spaced 27.5 million years apart.

In which case, there might be an agency to take the blame: the Milky Way Galaxy, of which the solar system and planet Earth is a very small part, moving at colossal speed.

Not only does the Earth orbit the Sun at 30 kms a second, the Sun and its planets waltz around the Galaxy at 220 kms a second, and make a complete revolution, astronomers think, about every 26 million to 30 million years. That means that any galactic traffic accident guarantees a collision at very high speed.

Suspect eruptions

And it could also mean that on every round trip, the Solar System passes through some kind of unidentified hazard zone that triggers showers of comet collisions and asteroid impacts.

“In fact three of the mass annihilations of species on land and in the sea are already known to have occurred at the same times as the three largest impacts of the last 250 million years, each capable of causing disaster and resulting mass extinctions,” Professor Rampino said.

But there is another more down-to-earth factor. All eight of those episodes of mass death on land and parallel extinction in the seas also matched periods of eruptions in which hot basalts flooded across the landscape.

All volcanic eruptions release carbon dioxide: in such cases enough to create conditions of intense cold followed by greenhouse warming,  acidification of the oceans and acid rain on land, destruction of the ozone layer that normally screens the planet from dangerous ultra-violet radiation, and even marine oxygen depletion. In which case life on the planet would have to withstand a kind of double assault.

“The global mass extinctions were apparently caused by the largest cataclysmic impacts and massive volcanism, perhaps sometimes working in concert,” Professor Rampino said. − Climate News Network