Chipping at Fossils for Global Warming’s Toll
Jennifer McElwain will spend the next year chipping her way through more than a ton of sediment and plant fossils at the Field Museum, hoping to find rock-solid evidence of global warming’s ecological toll.
McElwain, a paleobotanist at the natural history museum, led a team of scientists who collected more than 1,000 fossils during a one-month expedition to Greenland funded by the National Geographic Society.
The fossils span a period of mass extinction and recovery that started more than 200 million years ago -- a period that some scientists blame in part on high levels of carbon dioxide that led to global warming. By examining the pores on leaves, she hopes to predict roughly what would happen to today’s plants and animals if carbon dioxide in the atmosphere doubles during this century, as some predict.
“It could be that we find plants are really happy and they sail through extreme warming,” McElwain said. “I don’t think that’s going to be the case.”
A former researcher at the University of Sheffield, England, McElwain previously studied a lesser collection of ancient ferns, ginkgoes and other fossils from before the age dominated by dinosaurs. She found that as the years progressed during the mass extinction between the Triassic and Jurassic periods, the number of pores per leaf declined.
Laboratory research on modern plants shows that they respond to high levels of carbon dioxide by developing fewer pores to suck up the gas. And many scientists believe that carbon dioxide spewed from volcanoes in the Atlantic Ocean as the continents drifted apart.
Surviving plants also developed smaller or segmented leaves, which McElwain believes indicates major global warming as the gas trapped heat. Small leaves release more heat, helping plants stay cool.
The specimens were hacked centimeter by centimeter out of Hurry Fjord in Greenland. They await unpacking in a museum storage room. The fjord’s 990-foot cliff is a complete record of Triassic-Jurassic plant life, laid down in layers by stream sediments and cut open by glaciers.
A protective resin helps leaf fossils retain some of their original material instead of being replaced by rock entirely. As McElwain removes their paper wrapping and brushes their surface, some flake off the rock altogether, making them ready for the microscope. Some of them came loose as soon as the rock was hammered from the fjord.
“When it was windy, you opened the rock and there were 200-million-year-old leaves flying around,” she said.
Others will have to be removed with acid before McElwain can count pores.
She and a student will spend the next year studying fossils from increments of time to see how many species disappeared or lost dominance -- and which others took their places. If she can determine the changes wrought by a gas increase similar to the current one, it may indicate how the world will change this century.
“That’s the idea,” she said. “The problem with the idea is that the plants that were around 200 million years ago are very different.”
There are competing theories on what led to the disappearance of life in the period that McElwain is studying.
Some scientists believe that a layer of iridium around the Earth from the period of the extinction indicates that a meteor impact blacked out the sun -- much like the one often blamed for the later extinction of dinosaurs.
Lawrence Tanner, geology professor at Bloomsburg University in Pennsylvania, acknowledged that carbon dioxide buildup and global warming are the prevailing theories for what’s viewed as Earth’s third-largest extinction. But he believes that it’s just as likely that the opposite -- global cooling -- killed off plants and the animals that relied on them.
