Friday, April 5, 2013

Castastrophes and Mass Extinctions - Periodic?

From T.S. Elliot's The Hollow Men:
This is the way the world ends
This is the way the world ends
This is the way the world ends
Not with a bang but a whimper.
Actually, it will probably be a slow bang. 

~~~~~

Q: At the Ask-a-Geologist desk, we have received quite a number of end-of-the-world queries. These could be consolidated into a single sentence with two parts: 

Are mass extinctions real, and will another one happen soon?

A: There has been accumulating evidence over the past century that animal life on Earth has been decimated repeatedly. The biggest extinction events:
  • ~440 million years ago (the demise of the Bryozoa, among other fossil species, marking the end of the Ordovician period),
  • 251 million years ago (the “Great Permian Extinction” that saw the disappearance of over 95% of all genera living at the time including the Trilobites),
  • 219 million years ago (the end of the Carnian stage in the late Triassic period, coincident with the appearance of the huge, ~85-km Manicougan craters in Quebec, Canada),
  • 65 million years ago (the end of the Cretaceous period and with it most of the dinosaurs), sometimes called the Chicxulub event for a village in northern Yucatan, Mexico.
  • 33 million years ago (The demise of the Cassidaria family of mollusks near the end of the Eocene, after horse ancestors first appeared),
  • 2.6 million years ago (the boundary between the Pliocene and the Pleistocene epochs)…
  • …and 40,000 and 12,000 years ago.
What could possibly cause all these extinctions?

In the past century geologists have come to realize that the Earth’s crust doesn’t change gradually, either, but instead it apparently evolves episodically. This takes two general forms: asteroid or comet impacts, and episodic convulsions of the Earth’s deep interior. 

The first possible reason for extinctions: asteroids or comets.

For some time astronomers have known about a 26-30 million year cycle of the Solar System, oscillating in and out of the plane of the galaxy as it revolves around a supermassive black hole at its core (Sagittarius-A* in the center of the Milky Way). There is a very rough (in other words, very arguable) periodicity in asteroid impacts mapped in the Earth’s crust. 

The thinking goes something like this: as the Solar System passes through the plane of the Galaxy, there are close approaches by other stars, which disturb the previously-stable orbits of Oort belt objects. These are icy planetesimals orbiting far beyond Kuiper Belt objects such as Pluto and Sedna, reaching out to 50,000 astronomical units from the Sun (up to a light year). The Oort belt is where most of the comets come from. Thus, a disturbance out at this distance could send one or more into the inner Solar System. These may directly impact the Earth, or may disturb or deflect one or more asteroids orbiting between Mars and Jupiter. Asteroids are far more common in the mid-to-inner Solar System, but comets generally have a much high relative velocity with respect to Earth. Since kinetic energy goes the mass times the velocity squared, a comet could potentially do quite a bit more damage for the same size if it impacted the Earth.

For more than a century scientists have been aware of these extinctions in the paleontological record. The cause of the great Permian Extinction of 250 million years ago is still not fully understood, but may be related to huge seafloor craters now known to exist off the northwest coast of Australia (Bedoubt) or the Falkland Islands east of Argentina. The extinction of the dinosaurs 65 million years ago actually has a 'smoking gun': a huge, 150-to-180-km crater now lying beneath the northern edge of the modern Yucatan Peninsula of Mexico. There is other evidence: ginormous tsunami deposits elsewhere in the Caribbean including Haiti, a tektite strewn field throughout the American southeast, and distinctive fragments found in Montana and eastern Pacific ocean deep-sea drill cores.

Keep in mind that the Earth’s crust is a very dynamic place; while we see thousands of craters on the Moon, we see few on the Earth. Careful mapping has identified only 170+ asteroid-impact craters on the Earth, even counting the tiny ones like Wabar in Saudi Arabia, and Henbury in Australia. The Earth’s crust is evolving constantly because of plate tectonics and weathering, so evidence of impacts is steadily being erased.

The second possible reason for mass extinctions: gargantuan volcanic eruptions

A recent article in EOS, the Transactions of the American Geophysical Union (Rampino and Prokoph, EOS 94, No 12, 19 March 2013, p. 113-114), points out that there have been roughly cyclic episodes of large igneous provinces (LIP’s). The Deccan Traps, making up much of western India, is one of these provinces: kilometers-thick, continent-sized basalt flows all erupted over a fairly short window of time. A vast basalt province in Siberia called the Siberian Traps, and the huge Columbia River basalts are among the others. These are thought to be the result of large upwelling mantle plumes; for scale imagine the eastern US being covered by miles-thick flows of basaltic lava.

The geologic record shows these LIP’s to have occurred around
  • 390 million years ago
  • 295 million years ago
  • 250 million years ago (the Siberian Traps)
  • 200 million years ago
  • 185 million years ago
  • 135 million years ago
  • 100 million years ago
  • 65 million years ago (the Deccan Traps occurred close to the Chicxulub impact, causing some confusion about the relative effects of the two events)
  • 30 million years ago
  • 17-14 million years ago (the Columbia River Basalt province).
From these ages frequency-domain filtering (and your eye if you plotted them out) suggests an apparent rough cyclicity of 28-to-35 million years, especially prominent starting 135 million years ago.

When volcanic centers this size erupt, there is a huge degassing process associated with it: sulfur dioxide and vast amounts of carbon dioxide are released. When Mount Pinatubo erupted in 1992, it sent a proportionally smaller cloud of SO2 into the stratosphere – and the Earth’s average temperature cooled for two years afterwards. And that's just from what happens in the stratosphere.

Could there be a third reason for mass extinctions? 

Around 40,000 years ago, most of the large animals of Australia abruptly disappeared. These include the rhino-sized, wombat-like marsupials called Diprotodons, giant 200-kg kangaroos, a goanna bigger than the modern Komodo dragon, a giant goose-like bird twice the size of the emu, and many others. These animals had survived at least two episodes of climate change prior to 40,000 years ago. In North America about 12,000 years ago, most of the large “charismatic megafauna” of North America (mammoths, giant sloths, camels, cave bears, saber-tooth tigers, etc.) suddenly disappeared. In both cases, these mass extinction events (and a more recent event on Madagascar that is still very much on-going) correlate closely with the arrival of the human species in these regions. The implication of overhunting is hard to miss here. As the human population surges past 7 billion today, the largest mass extinction in the past 65 million years is fully underway, and the Passenger Pigeon is just the first and most obvious victim. Habitat loss, overhunting, and accelerating climate change are the proximate mechanisms for this current and stunningly rapid mass extinction event. 

The End of Things As We Know It

There are Near Earth Objects (NEO’s) out there that NASA and the US Air Force are monitoring (the number keeps growing, but at least the search process is now automated). Based on their known sizes (we can generally only see the big ones) and what happened at Chicxulub 65 million years ago, most of these could wipe out human civilization as we currently know it... not if, but when, one hits us. 

If Yellowstone (just one of several known supervolcanoes) unzipped tomorrow, it would cover the eastern two thirds of the United States with a vast blanket of ash, suffocating all living things. The gas and peripheral consequences would devastate the entire planet. To put things in perspective, the last eruption 640,000 years ago left an off-white layer 20 meters (65’) thick called the Pearlette Ash Formation near Colorado Springs… 800 miles away. I have personally pulled a camel’s tooth from the bottom of this formation.

However, the problem may be more imminent. 

As Pogo said, “We has met the enemy, and it is us.”
~~~~~

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