The largest mass extinction in geological history, the one that occurred 250 million years ago did not occur as a result of a single catalytic event
Avi Blizovsky
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A Chinese-British team tells in Nature magazine that the extinction that happened 250 million years ago had to come in waves. Their conclusion is based on the wealth of "organic fossils" found in the rocks at Xi'an in southern China. According to these rocks at least two stages of the mass extinction at the end of which 95 percent of the life forms of that time were destroyed. "And this corresponds to a large number of articles according to which a series of complex changes occurred on Earth" Mr. Richard Pancost (Pancost) from the University of Bristol in the UK.
A complex path to death
The scientists proposed the idea that the Great Dying at the boundary of the Permian to Triassic period could not have occurred suddenly enough as a result of environmental changes caused by a giant celestial rock. This is the argument that explains the extinction of the dinosaurs in an event that occurred much later - 65 million years ago.
A geological structure known as the Bedout High on the sea floor near what is now Australia has been suggested as the remains of the impact crater. However, there are not many supporters of this claim. The accepted opinion is that several factors - including a massive volcanic eruption and massive global warming - combined over thousands of years to create the deterioration of biodiversity. It is possible that the Earth was hit by objects from space, but it is unlikely that there were fatal hits from space. The British and Chinese scientists claim.
The ring of existence
The new data from China supports this view. They are based on the traces left in the rocks by cyanobacteria. This single-celled, photosynthesizing algae existed in huge quantities in the sea during the Permian period. They are one of the groups of phytoplankton, which is the base of the marine food chain. However, the phytoplankton that is not eaten by higher creatures sinks to the bottom of the sea and it combines there with the sedimentary rocks that we see today. The chemical components in the cell membrane have left an identifiable imprint of their existence. In particular, a lipid molecule known as 2-methylhopane has left a ring structure in Mischian rocks. "These ring structures are the 'carbon skeleton' - that's how we refer to them - and they can be preserved for a very long time," explains Dr. Pancost.
The research teams saw two peaks of plankton abundance in the Chinese rocks that they believe are a sign of periods immediately after the biotic crisis in the oceans - times when the collapse of higher organisms in the sea allowed the cyanobacteria population to flourish. "What we believe happened was that the pressure of the food chain changed," explains Dr. Pancost. "Most of the extinct fauna reached the larval stage because otherwise they would have fed on the phycoplankton.
In the great extinction of the Permian-Triassic, 95 percent of all marine species were killed and about three quarters of all terrestrial families. This is the limit where the best-known sea-dwelling arthropods - the trilobites - ceased to exist.
In the Permian, all the Earth's continents merged into the supercontinent Pangea, and the geological evidence indicates that this land mass experienced massive volcanic activity.
The Siberian traps were built at the same time millions of cubic kilometers of basaltic lava flowed over the surface of the earth. "This is the first time we know what happened at the base of the food chain at that time. We don't have an estimate of the state of the plankton population before because they don't snack," explained Dr. Paul Wignall from the University of Leeds, who studied the Permian-Triassic extinction. "This is very interesting because the impact on the base of the food chain is extremely important. It indicates a global crisis."
For news at the BBC
Earth scientist - natural phenomena
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