Members of a U.N.-sponsored research team with members from Appalachian State University's Department of Geology have found evidence for catastrophic oceanographic events associated with climate change and a mass extinction 375 million years ago that devastated tropical marine ecosystems.
"The Late Devonian mass extinction was one of the five largest mass extinction events in the history of life," said Professor Johnny Waters, who is a co-leader of the five-year, U.N. International Geoscience Programme project that began in 2011. The research team, which includes Assistant Professor Sarah Carmichael, is examining the relationship between climate change and changes in the ecosystems in the Devonian period, from 419 to 359 million years ago.
"This is the third most significant mass extinction and it was caused by plants," Waters said. "Unlike the dinosaur mass extinction, which was related to an asteroid impact, this one was environmentally related."
In the Devonian period, Waters explained, the world was experiencing super greenhouse climate conditions. This means that it was very warm, there probably were no ice caps, there was a lot carbon dioxide in the atmosphere (with estimates of 4,000 parts per million).
"As plant communities expanded onto land to form the first forests, they depleted the carbon dioxide (CO2) that was in the atmosphere," Waters said. "CO2 levels dropped to 400 ppm toward the end of the Devonian. It got colder. There were glaciation events and the rapid change in the climate caused severe extinction in the tropics and the existing coral reefs became extinct." By comparison, the world's current CO2 level is very close to 400 ppm.
Most of the knowledge that geologists have about this mass extinction comes from North America and Europe. Although these two land masses are far apart now, in the Devonian they were very close to each other. Scientists have tried to make inferences about worldwide events based on sample locations that are really quite limited in terms of their geographic history, or paleogeography. Therefore, it is vitally important to obtain samples from locations outside this region for understanding global climate change during this time period.
Waters' international team of geoscientists has conducted field work in remote areas of western China for many years, in addition to two recent field seasons in western Mongolia near the Russian and Chinese borders. The changing political climate in China, Russia and Mongolia in recent years has now made it possible to do fieldwork in these locations. The strength of these field collaborations is that they draw on the expertise of scientists from a variety of disciplines to add critical climatic information to a limited database. U.N. researchers associated with this project are also collecting related data in Thailand, Myanmar, Vietnam and Northern China.
"The reason we are working in central Asia is that there is a lot of good evidence of what happened at and after this mass extinction – this is an area that has not been well studied," Waters said. "It's all a part of our work finding the places that give us the best information in sorting out what happened in the extinction event and in its aftermath."
Answers about the earth's climate during and after this mass extinction are contained within rock samples from these new field sites, which were once part of the ocean floor, as geochemical signals preserved in the rocks record devastating climate change. The paleogeography of the field sites indicate that Devonian climate change not only had environmental impacts on life associated with large land masses, but also on life in the open ocean.
"We now have evidence that the radiation of surviving life following the mass extinction was centered in Central Asia," Waters said.
The geochemistry of the samples is being analyzed primarily by students in Appalachian's Department of Geology under Carmichael's supervision, with additional analyses being conducted at UNC-Chapel Hill and a university in Austria. "We are using geochemistry to tie it all together all across Central Asia, which used to be an open ocean, and compare our new data to established sequences in Europe and North America, in order to develop a global understanding of the climate change associated with this mass extinction," Waters said.
"Today we are looking at increases in carbon dioxide causing warming and the negative impacts to the ecosystem. In the Devonian period, we are looking at a rapid loss of carbon dioxide, which in geologic time occurred over millions of years rather than hundreds of years," Waters said. "But the lessons are actually quite similar. We clearly are concerned today about climate change and its impact on the environment and its effect on the ecosystem, and the geologic record is really the only record where we can see these events and compare what happened before and after."
Waters and Carmichael will present the preliminary results of their research at the Geological Society of America's Annual Meeting in Denver in October and at the American Geophysical Union's annual meeting in San Francisco in December.
Next summer, Waters will lead a 20-member team, including Dr. Sarah Carmichael and two students from Appalachian's Department of Geology, for continued field work in Mongolia.
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