證據出土！西藏見證兩百萬年前的氣候變遷

Working 15,000 feet up on the cold, treeless, rocky desert of the Himalayan-Tibetan Plateau, an international research team was surprised to find thick layers of lake sediment filled with plant, fish and animal fossils typical of lower elevations and warmer, wetter climates.

Back at the U.S. National High Magnetic Field Laboratory at Florida State University, analysis of carbon and oxygen isotopes in the fossils has revealed that the animals lived recently in geologic terms - just two to three million years ago - and their diet was made up of abundant plants.

The scientists determined that "a drastic climate change" was the reason for their demise during the late Pliocene era, about two or three million years ago.

The scientific team says that fossil evidence suggests that movements of the Earth's crust caused the Tibetan Plateau to rise to its towering current elevations - making it inhospitable to the plants and animals that thrived there as recently as two million years ago, not millions of years earlier than that, as geologists previously believed.

"The uplift chronology of the Tibetan Plateau and its climatic and biotic consequences have been a matter of much debate and speculation because most of Tibet's spectacular mountains, gorges and glaciers remain barely touched by man and geologically unexplored," said Yang Wang, associate professor in the Florida State University Department of Geological Sciences.

The fossil findings are described in the June 15, 2008 issue of the peer-reviewed journal "Earth and Planetary Science Letters," online at: www.elsevier.com/locate/epsl. Yang co-authored the paper, "Stable isotopes in fossil mammals, fish and shells from Kunlun Pass Basin, Tibetan Plateau: Paleoclimatic and paleoelevation implications" with paleontologists from the Natural History Museum of Los Angeles County, and the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences in Beijing.

The collaborative research project, which since 2004 has featured summer field study on the remote Tibetan Plateau, is funded by a grant from the Sedimentary Geology and Paleobiology Program of the U.S. National Science Foundation.

"So far, my research colleagues and I have only worked in two basins in Tibet, representing a very small fraction of the Plateau, but it is very exciting that our work to date has yielded surprising results that are inconsistent with the popular view of Tibetan uplift," said Yang.

She said the new evidence calls into question the validity of methods currently used by scientists to reconstruct the past elevations of the region.

"Establishing an accurate history of tectonic and associated elevation changes in the region is important because uplift of the Tibetan Plateau has been suggested as a major driving mechanism of global climate change over the past 50-60 million years," said Yang.

"What's more, the region also is thought to be important in driving the modern Asian monsoons, which control the environmental conditions over much of Asia, the most densely populated region on Earth."

This summer, Yang and her colleagues from Los Angeles and Beijing will return for more fieldwork in areas near the Tibetan Plateau.

"The next phase of our work will focus on examining the spatial and temporal patterns of long-term vegetative and environmental changes in and around the region," she said. "Such records are crucial for clarifying the linkages among climatic, biotic and tectonic changes."