单项选择题

In an event known as the "Great Dying", some 250 million years ago, 90 percent of all marine life and nearly three-quarters of land-based plants and animals went extinct. Scientists have long debated the cause of this calamity--which occurred before the era of dinosaurs--with possibilities including such disasters as meteor impacts. Researchers led by Peter Ward of the University of Washington now think the answer is global warming caused by volcanic activity. Their findings are ’’reported in Thursday’’s online edition of the journal Science. They studied the Karoo Basin of South Africa, using chemical, biological and other evidence to relate layers of sediment there to similar layers in China that previous research has tied to the marine extinction at the same period. Studying a 1,000-foot thick section of exposed sediment, Ward’’s team found evidence of a gradual extinction over about 10 million years followed by a sharp increase in extinction rate that lasted another 5 million years. They believe that the extinctions were caused by global warming and oxygen deprivation over long periods of time. Massive volcanic flows in what is now Siberia brought on the warming while, at the same time, geologic action caused global sea levels to drop, Ward explained. "Once you expose a huge amount of underwater sediment to the atmosphere, two very bad things happen--a huge amount of carbon in the sediments is released and also methane. Once (methane) hits the atmosphere it’’s the most efficient greenhouse gas on the planet," he said. That provided a one-two punch of warming and a decline in oxygen levels, he said. "Some of us have been toying with the idea that dinosaurs evolved to be a low-oxygen adaptation," resulting from this era, Ward said, "We know birds can live at much lower oxygen concentrations than we do, and we think there were similar lung adaptations in dinosaurs." Currently the atmosphere consists of about 21 percent oxygen, but the addition of gases at that time could have lowered levels to 16 percent or less, Ward said. "If you didn’’t live on the sea level you didn’’t live," he commented, reflecting the fact that oxygen concentrations decline with altitude. The result would have been to eliminate half the living space on the planet, said Ward. It is implied in the last paragraph that _________.