Scientists Uncover Causes of Mass Extinction in the Ashes
By TANG YUANKAI
DRAGONFLIES as big as a man hover overhead in the oxygen-rich atmosphere and mammal-like reptiles called pelycosaurs roam the land, some of them reaching up to six meters in length. Underwater, we find coral reefs of rugosa, also known as “horn coral” for its often remarkable resemblance to rhino horns, and honeycomb-like tabulate corals. These exotic scenes are some that you might have seen during the Permian Period, which existed around 295 milion to 250 million years ago. But at the end of this period, a mass extinction occurred resulting in 95 percent of all marine species and 75 percent of all land species being wiped out.
Scientists including Chinese paleontologists have made breakthroughs in determining the timing of the Permian-Triassic mass extinction, and come closer to finding its cause. Since the appearance of organisms on earth 3.8 billion years ago, at least five mass extinctions have occurred. But that at the end of the Permian Period was the most devastating.
Shen Shuzhong took the lead in the research that involved more than 20 specialists from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences (NIGPAS), the Massachusetts Institute of Technology (MIT), the American National Museum of Natural History and several other research institutes. Their 10-year research overturned previous estimates by Chicago experts in 1984 of the mass extinction lasting 10 million years, coming up with the new figure of just 200,000 years, about 252.28 million years ago.
Shen Shuzhong and his research team traveled to 20 strata boundary sections in southern China, Tibet, Pakistan and Kashmir to collect the fossils needed for analysis. They found particularly large deposits of volcanic ash in Meishan in Zhejiang Province, the world’s most famous record of the Permian-Triassic mass extinction, and Guangyuan in Sichuan Province. These and other samples were analyzed at MIT, where scientists employed the most advanced dating methods.
The paper announcing their results, produced by Chinese scientists in conjunction with 14 scientific research institutes, was published in the authoritative U.S. Science magazine. Shen Shuzhong commented, “The mass extinction had been studied for decades, yet the timing and duration of the extinction and recoveries were not pinpointed.”
The significance of this breakthrough is that a more accurate time scale for the extinction and the recovery that followed will help scientists further understand the physical, ecological, and chemical changes that occurred during the event, and to test possible causes.
Research also found evidence of the causes of the extinction using a computerized ratio optimization method, information collected from the 29 volcanic ash beds and a database of 1,450 types of fossils from the period.
Previous theories include that published in Science magazine in 2000 by world-renowned Chinese paleontologist Jin Yugan, who postulated that an instantaneous event led to the mass extinction. Some American and Japanese scientists presume that intensive seismic waves from the collision between asteroids and the earth exterminated almost all species within a span of thousands of square kilometers. This view remained popular for a time.
The scientists involved in the recent research, however, believe that the massive volcanic eruptions and the release of methane from the surface of the earth led to increased amounts of carbon dioxide in the atmosphere that intensified the greenhouse effect. Shortage of oxygen in the sea led to the mass extinction of marine species. Meanwhile rapid global warming and aridity gave rise to frequent large-scale forest fires that were disastrous for the tropical forests in equatorial regions. They led to further aridity and sped up the collapse of the soil system.
These events completely changed the appearance of the world’s surface. “If there had been no such mass extinction during the Permian Period, people would have seen two-meter-long dragonflies in real life,” Shen said with a sigh.
Paleobotanist Wang Jun, a colleague of Shen Shuzhong, revealed that few fossils of living things from right after the mass extinction were found. “The recovery of life on the earth took more than five million years, significantly longer than after any other mass extinction,” said Wang. Fortunately a few marine and terrestrial species survived. Shen said that these remaining species went on to populate the earth during the Triassic Period and included what would soon evolve into one of paleontology’s most iconic creatures, dinosaurs.
Shen Shuzhong sees these findings as an important message for today’s world. “The depraved situation of the earth’s surface has a long-term impact on the ecosystem. But if it goes beyond the environment’s capabilities, the ecosystem can collapse in a short time,” he said. “This enlightens our environmental awareness.”