Fossil leaves could reveal the climate of the last dinosaur era


WASHINGTON – Richard Barclay opens a metal drawer in the Smithsonian Natural History Museum archives containing fossils nearly 100 million years old. Despite their age, these rocks are not fragile. The geologist and botanist manipulates them with casual ease, placing one in his palm for further examination.

Embedded in the ancient rock is a triangular leaf with rounded upper lobes. This leaf fell from a tree during the time when T-rex and triceratops roamed prehistoric forests, but the plant is instantly recognizable. “You can tell it’s ginkgo, it’s a unique form,” Barclay said. “It hasn’t changed much in several million years.”

What’s also special about ginkgo trees is that their fossils often preserve actual plant material, not just the impression of a leaf. And that thin layer of organic matter may be the key to understanding the old climate system – and the possible future of our warming planet.

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But Barclay and his team must first crack the code on the plant to read the information from the old leaf.

“Ginkgo is a pretty unique time capsule,” said Peter Crane, a paleobotanist at Yale University. As he wrote in “Ginkgo,” his book on the plant, “It’s hard to imagine that these trees, which now dominate cars and commuters, grew up with dinosaurs and have come down to us almost unchanged for 200 years. million years ago. “

If a tree fell in an ancient forest, what can it tell scientists today?

“The reason scientists are looking back is to understand what happens in the future,” said Kevin Anchukaitis, a climate researcher at the University of Arizona. “We want to understand how the planet has responded to large-scale climate change in the past – how ecosystems have changed, how ocean chemistry and sea levels have changed, how forests used to function.”

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Scientists are particularly interested in “greenhouse” periods when they think carbon levels and temperatures were significantly higher than they are today. One of these events occurred at the end of the Cretaceous Period (66-100 million years ago), the last dinosaur era before a meteor crashed into Earth and most species do not disappear.

Learning more about greenhouse climates also gives scientists valuable data to test the accuracy of climate models for projecting the future, says Kim Cobb, climatologist at Georgia Tech University.

But climate information on the distant past is limited. Air bubbles trapped in ancient ice cores allow scientists to study ancient levels of carbon dioxide, but these only date back to about 800,000 years.

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That’s where the Smithsonian’s ginkgo leaf collection comes in. In a maze of corridors, Barclay spans millennia – as is only possible in a museum – until the 19th century, when the Industrial Revolution began to change the climate.

From a cabinet he pulls out sheets of paper where Victorian-era scientists taped and tied ginkgo leaves picked from botanical gardens of their time. Many specimens have labels written in beautiful cursive, including one dated August 22, 1896.

The shape of the leaf is virtually identical to the fossil from around 100 million years ago and a modern leaf that Barclay holds in his hand. But one key difference can be seen under a microscope: how the leaf reacted to the change of carbon in the air.

Tiny pores on the underside of a leaf are arranged to absorb carbon dioxide and breathe water, allowing the plant to turn sunlight into energy. When there is a lot of carbon in the air, the plant needs fewer pores to absorb the carbon it needs. When carbon levels drop, the leaves produce more pores to compensate.

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Today scientists know the global average level of carbon dioxide in the atmosphere is around 410 parts per million – and Barclay knows what the leaf looks like. Thanks to Victorian botanical leaves, he knows what ginkgo leaves looked like before humans dramatically transformed the planet’s atmosphere.

Now he wants to know what the pores of fossilized ginkgo leaves can tell him about the atmosphere of 100 million years ago.

But first he needs a decryptor, a translation sheet – a sort of Rosetta Stone to decipher the writing of the ancient atmosphere.

This is why he is conducting an experiment in a clearing in Maryland.

One morning earlier this year, Barclay and his project assistant Ben Lloyd tended rows of ginkgo trees in open enclosures of plastic sheeting that exposed them to rain, sun and the changing seasons. “We grow them that way so that the plants go through natural cycles,” Barclay said.

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Researchers adjust the carbon dioxide pumped into each chamber, and an electronic monitor outside flashes the levels every five seconds.

Some trees grow at current levels of carbon dioxide. Others are growing at significantly high levels, approaching levels of the distant past, or perhaps the future.

“We’re looking for analogues – we need something to compare with,” Barclay said. If there is a match between the appearance of the leaves in the experiment and the appearance of the fossil leaves, this will give researchers a rough guide to the ancient atmosphere.

They’re also studying what happens when trees grow in crowded environments, and they’ve found that more carbon dioxide makes them grow faster.

But Barclay adds: “If plants grow very quickly, they are more likely to make mistakes and be more susceptible to damage. … it’s like a race car driver who is more likely to derail at high speed. “

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Follow Christina Larson on Twitter: @larsonchristina


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