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Earth's History Imbedded in Rocks

Rocks cannot speak. But to geoscientists, rocks in some parts of the world clearly tell the changes the earth has undergone over millions of years.


These rocks have become the yardstick in geoscientists' further studies of the earth.


Take for example the 40-meter thick limestone sequences in Changxing County of east China's Zhejiang Province, about a two-hour car ride west of Shanghai.


The rocks there offer clues about the biotic changes that led to the greatest mass extinction in history, said Wang Xiangdong, a research fellow with the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences (CAS).


Many studies conclude that more than 90 percent of marine organisms and 70 percent of animals and plants on land were wiped out in what the scientists term the late Permian period, the last period of the Paleozoic Era, more than 200 million years ago.


Geoscientists disagree on the cause of the catastrophe, but research of the Changxing rocks suggests that the extinction "occurred rather suddenly, all at once," according to Jin Yugan, an academician of the Chinese Academy of Sciences (CAS) and a geoscientist with the CAS Institute of Geology and Paleontology in Nanjing.


As a result, the Changxing rocks have attracted geoscientists from around the world, becoming a key site for the study of geology and the process of bio-evolution.




With a population of 620,000, Changxing County in the northernmost part of Zhejiang Province claims the country's largest forest of gingko trees, which is over 10 kilometers long.


Dating from the late Paleozoic Era over 250 million years ago, the species, which is unique to China, is known as a "living plant fossil."


"Our gingko forest, with most trees being over 100 years old, is a wonder on the world," said Jin Shuyun, deputy head of the county.


Also home to Chinese alligators, another rare survivor of the late Paleozoic Era, Changxing boasts one of the leading reserves and breeding centers for this endangered reptile.


But what has made Changxing world famous is the limestone section of the Meishan hills.


"Studies chronicling the earth's changes aim to provide better understanding of the changes of Paleozoic climate and the evolution of life and the earth," Wang said. "It is significant to promote the study of other related disciplines."


In order to better understand changes in the Paleozoic climate and the evolutionary process, the Changxing County government, with the support of the provincial government and the Nanjing institute, has preserved the Meishan Formation as a geopark.


The park has won acclamation from international geoscientists.


"The Meishan Section exposed at the new park is truly a world-class site for the geological education of all people," said Samuel Bowering, a geoscientist from Massachusetts Institute of Technology in the United States.


Another group of geoscientists from Australia, Canada, Italy, Japan, Russia, the United Kingdom and the United States visited Meishan before coming to Beijing to attend the ongoing second International Paleontological Congress, which concludes today.


The geoscientists were intrigued by what they found in Changxing.


Sha Jingen, director of the Nanjing institute and chair of the Chinese organizing committee of the congress, said that this interest reflects the appeal of China to geoscientists around the world.


"We expected 200 international participants," he said. "But we actually got more than 500 registered. It is remarkable for a congress on such a specific topic."


The rocks of Meishan, or Coal Hills, contain a great deal of biological fossils featuring the late stage of the Permian Period, the final period of Paleozoic Era, with the discovery of 20 previously unknown species.


US geoscientist Amadeus William Grabau (1870-1946) named limestone typical to the latest chronostratigraphical unit of Permian after Changxing in 1931.


Changxing Limestone, also known as the Changxingian stage, became a special term in stratigraphy to mark the last stage of the Permian Period, according to Jin Yugan.


In the following decades, especially since the 1980s, more intensive work has been done in various sections of the Meishan Formation, which led to more discoveries, said Shen Shuzhong, a research fellow from the institute.


'Golden spike'


The work, jointly conducted by Chinese and foreign geoscientists, convinced the International Union of Geological Sciences (IUGS) in 2000 to designate Section D of the Meishan Formation as the Global Stratotype Section and Point (GSSP) dividing the Permian and Triassic periods, the first period of the Mesozoic Era, more than 250 million years ago.


That is an important boundary on the geological timescale, while GSSP, known as "golden spike," enables global geoscientists to "communicate on the same standard," said Charles Henderson of the Department of Geology and Geophysics at University of Calgary, Canada, who chairs the Subcommission on Permian Stratigraphy under the International Commission on Stratigraphy.


Henderson has made a number of field trips to Changxing since 1987, after hearing about the Meishan sections at an international symposium. Now he can guide fellow geoscientists to the site and indicate where the boundary exists.


The Changxing site, he said, is unusual because it not only defines the upper boundary of the Permian Period, which indicated the beginning of the Mesozoic Era, but it is also the lower boundary of the Changxingian Stage.


Some 100 meters from the first GSSP at Meishan, the IUGS announced last September that it had also designated this area as a "golden spike."


That means Changxing County "has two points" on the geological clock, said Henderson. "This is quite unique."


Among the over 50 "golden spikes" the IUGS has so far ratified, Changxing, as the most intensively studied, is the only site that has two standard boundaries, said Wang.


But only the most-studied sections can rank among the "candidates." And Wang said there has been intense international competition for these "golden spikes," which provide a co-ordinated time frame for the study of geology and the process of bio-evolution.


Competition is intense to secure the 50 "golden spikes" that have yet to be determined, Wang said.


As a late-comer to the competition, China has secured six "golden spikes" since 1997, when a stratotype at Huangnitang in Changshan County, Zhejiang, was ratified as the GSSP defining the Darriwillian Age of the Middle Ordovician Epoch 461 million years ago.


Besides, Chinese geoscientists, all from the Nanjing institute, chair the three sub-commissions on the Cambrian, Ordovician and Silurian periods respectively under the International Commission on Stratigraphy, Wang said.


"This indicates the importance of China's stratotypes and what progress we have made in the field," he said.


Wang and his colleagues have submitted a proposal to the CAS and the Ministry of Science and Technology to further support the campaign for more "golden spikes" over the next two years.


(China Daily June 21, 2006)


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