Like a detective trying to pick up the trail of a killer by sifting through seemingly innocuous objects left at the scene of the crime, Jennifer McElwain these days is looking closely at old leaves she collected this summer in the frozen wastes of Greenland.
The deadly occurrence that McElwain is investigating happened 206 million years ago, when 95 percent of Earth’s plant species and perhaps 40 percent of its land animals went extinct.
The suspected killer was a buildup of carbon dioxide in the Earth’s atmosphere, the same culprit many biologists believe could trigger new extinctions over the next 100 years by way of global warming.
McElwain, 30, startled the world of paleobotany in 1999 when she ingeniously documented the 206-million-year-old global die-off with a precision no one had thought possible. Still a postgraduate student at England’s Sheffield University at the time, she did it by studying plant fossils collected in Greenland in the 1920s.
In geological history, the extinctions McElwain is studying marked the end of the Triassic period and the beginning of the Jurassic period, when dinosaurs began to dominate.
The raw materials she studies are fossil leaves of plants that grew before, during and after the die-off of Triassic plants and animals. By counting changes in the number of tiny pores on the ancient leaf surfaces, she found evidence of a rapid surge of seven times the normal amount of CO (-2) in the atmosphere.
“CO (-2) is an important greenhouse gas,” said McElwain, an English-born Irish woman the Field Museum eagerly hired 2 1/2 years ago. “It traps heat from the sun in the Earth’s atmosphere. The more CO (-2) there is in the atmosphere, the more heat is trapped, which is why it is called the `greenhouse effect.'”
Heating things up
Global warming in the Triassic period caused by an increase in greenhouse gases caused the leaves of plants growing at that time to burn up, she said.
“If plants couldn’t evade lethal heat, they went extinct. As the climate change resulted in major changes in vegetation, it may have contributed to species extinctions of animals that depended on those plants as food.”
More than mere academic curiosity hovers over her research. CO (-2) levels today, fed by humanity’s insatiable and growing use of fossil fuels, are rising at perhaps an even faster rate than during the so-called Triassic-Jurassic Boundary.
In 1900, the Earth’s atmosphere contained 300 parts per million of CO (-2) . Today it is 360 parts per million and growing at a rate that will put it at 700 parts per million at the end of this century.
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“The work Jenny is involved with is very important,” said Paul Olsen, a mass extinctions expert at Columbia University’s Lamont-Doherty Earth Observatory in New Jersey.
“It will shed light on the mechanism of mass extinctions. Showing this massive increase in CO (-2) is a fascinating revelation. It is not obvious that CO (-2) is a cause or consequence of mass extinction, but there may have been a `super greenhouse’ effect that was the killing mechanism.”
Triassic-Jurassic fossils are found only in a few areas in the world, in rock deposits laid down between 200 and 206 million years ago–in Sweden, Greenland and geologic formations west of New York City, including the Palisades along the Hudson River.
The hardest to get to are in remote, frozen rock faces along the world’s biggest fjord, Scoresby Sund, in northeast Greenland. They contain rich deposits prospected in the 1920s by British paleontologist Thomas Harris, who excavated the plant fossils McElwain used in her 1999 study.
“Harris built up a fantastic picture of 200 species of plants growing in Greenland 200 million years ago,” McElwain said.
When paleontologists break open rocks containing fossilized plants, in many cases they also find, still intact, the “cuticle”, the waxy polymer that coats the leaves of all land plants.
The cuticle is what most interests McElwain because they contain the perfect outlines of the tiny pores, called stomata, that pock the leaf surfaces.
Plants live by combining CO (-2) and water into nourishing sugars, a process powered by the energy of sunlight through photosynthesis. The CO (-2) is absorbed into the plant through the stomata on its leaves.
Researchers growing plants in atmospheres enriched with CO (-2) have noticed that the higher the CO (-2) content, the fewer stomata appear on the plant’s leaves.
By tracking the number of stomata on the fossil plant cuticles, McElwain realized she could gauge the rise and fall of the world’s atmospheric CO (-2) more than 200 million years ago.
The carbon dioxide trail
“We want to see how CO (-2) levels changed over time, and how the plants changed in that same time frame,” she said.
McElwain said she needed to go back to Greenland to more systematically collect fossils through various ages of rock formations leading up to the extinction, at the extinction and afterward.Using a grant from the National Geographic Society, McElwain led a team of five scientists back to Harris’ Greenland fossil beds in July.
The places they visited were inhospitable and dangerous, an hour or two helicopter ride from the nearest settlements, tiny native Inuit villages. In the first week, they were down to four people after a Danish paleontologist suffered a compound fracture in a fall and had to be evacuated.
Still, they managed to find all but one of Harris’ fossil beds and added many new ones of their own.
“It was a treasure mine…,” McElwain said.
“There were high winds all the time, so we had to work in twos. One would crack open the rock, revealing the fossil leaf to sunlight for the first time in more than 200 million years.
“The other was there with a plastic bag and tweezers to capture the cuticle before it lifted off the rock and sailed away in the wind.”In a month, they collected 1.5 tons of rock containing more than 1,000 fossil plants. All of the fossils recently arrived at the Field Museum. McElwain and museum colleagues are in the midst of gingerly unpacking, sorting and storing them.
Her Danish, British and Romanian colleagues who collected the fossils with her will come to Chicago to do their research on the material.
“Within a year, we will have exciting data,” McElwain said, “but we’re going to have to go back to Greenland for more fossils at least one or two more times.”
There have been at least five mass extinctions, when 50 percent or more of the living plant and animal species go extinct in a period of a million years or less, in geological history .
The cause of the Triassic-Jurassic extinction that McElwain is studying is a hotly debated topic.
Paul Olsen at Columbia believes it was a catastrophic meteor collision. McElwain believes it is linked to the giant land mass Pangea that was splitting in two at the time of the Triassic-Jurassic extinction. She reasons that the tectonic forces splitting Pangea set off massive volcanic activity, and that the volcanoes spewed out lethal amounts of CO (-2) into the atmosphere.
Recent studies predict that loss of natural habitat due to expanding human populations and rising global temperatures will send many plant species living today into extinction.
“You can’t totally apply my work to what is happening today, but it lets us see on a large scale how all of this interacts,” McElwain said.
“Atmospheric CO (-2) should double by the end of this century, if we continue in a business-as-usual scenario, using fossil fuel at the rate we do today. That will put us in a realm totally unprecedented in the past 50 million years.”