The Guadalupian Mass Extinction
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Roughly 260 million years ago a mass extinction of life on Earth preceded, by nine million years, the Great Dying of the Permian-Triassic mass extinction, which exterminated ninety percent of life in the seas and seventy percent of life on land. Scientists possess plenty of evidence for what caused the Permian-Triassic mass extinction. Geologists and paleontologists have inferred that the Great Dying came about when two small asteroids hit Antarctica and the ocean off the west coast of Australia, thereby creating tremendous explosions and instigating flood vulcanism in Siberia, all of which made a major mess of the biosphere. But for the Guadalupian mass extinction (also called Olsonís extinction), which occurred at the boundary between the Guadalupian and Lopingian epochs of the Permian period, science has only half of the evidence for a similar series of events.
In southwestern China, centered roughly 500 to 600 kilometers south-southwest from Chengdu, the Emeishan lava beds, in some places 200 meters deep, sprawl over more than two million square kilometers. Though far inland today, the lava beds originally formed near the shore of a shallow sea, so limestone deposits formed between successive layers of hardened lava, the limestone coming from reefs that grew after each episode of vulcanism and were buried by the next one. Fossils extracted from the limestone have enabled paleontologists to track the extinctions caused by half a million years of flood vulcanism and to conclude that over half the life on Earth was killed, making the Guadalupian the second worst mass extinction in Earthís history.
On the opposite side of the planet from Emeishan, roughly 2000 kilometers west of what is now Southern California, we might expect to find a crater, perhaps as wide as 300 kilometers or more. The flood vulcanism at Emeishan occurred because an asteroid or comet hit that antipodal point and thereby created a shock wave that came up under the Emeishan area and cracked Earthís crust. The center of the impact zone lay in the waters of the Panthalassic Ocean, so no one will ever find the crater, which was long ago subducted into Earthís mantle. Thus we have only half of the evidence for what happened.
Dust and steam lofted high into the atmosphere by the impact began the process of extinguishing life. In thermal events greater than any firestorm that humans have ever witnessed, sheets of lava surging onto the surface added carbon dioxide and sulfur oxides to the stratospheric mix, replenishing them periodically over a span of half a million years. Darkness, cold, and acid rain swept over Earth, followed by heat (due to the greenhouse effect) and intense ultraviolet radiation (due to loss of the ozone layer). Repetitions of that cycle over half a million years stressed half of the species on Earth to the extinction point. Then the lava stopped flowing and over the next half million years, as inferred from fossils, the biosphere recovered its health, evolving new species to replace the ones lost.
We may also discern one lasting consequence of that impact. The asteroid or comet struck the ocean roughly 2000 kilometers west of what became California and left a crater that would span half the distance between Los Angeles and San Francisco. The impact certainly rippled the surrounding seabed and that fact is relevant because geologists have inferred that, starting some time before 250 million years ago, the Panthalassic Ocean plate began to subduct eastward, sliding under the North America plate, and over the next 190 million years created a volcanic island arc that evolved into the Sierra Nevada and Mojave regions in what is now eastern California. If the impact started the subduction, then the mountains that I can see out my window stand as the lasting remnants of that impact.
Thus we see how giant impacts can not only shape the evolution of life, but also help shape the landscape in which mutation and selection play out the evolution of life.
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