Fossil leaves could reveal the climate of the last era of dinosaurs

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

Embedded in the ancient rock is a triangular leaf with rounded upper lobes. This leaf fell from a tree back when T-rex and triceratops roamed prehistoric forests, but the plant is instantly recognizable. “You can tell it’s ginkgo, it’s a unique shape,” Barclay said. “It hasn’t changed much for millions of 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 this thin layer of organic matter may be key to understanding the ancient climate system — and the possible future of our warming planet.

But Barclay and his team must first crack the plant’s code to read the information in the old sheet.

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

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

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

Of particular interest to scientists are ” periods when they think carbon levels and temperatures were significantly higher than today. One of these periods occurred at the end of the Cretaceous (66 to 100 million years ago), the last era of the dinosaurs. before a meteor hits Earth and most species become extinct.

Learning more about hothouse climates also gives scientists valuable data to test the accuracy of climate models for projecting the future, says Kim Cobb, a climatologist at the Georgia Institute of Technology.

But climate information about 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 around 800,000 years.

That’s where the Smithsonian’s Ginkgo Leaf Collection comes in. In a maze of corridors, Barclay hops through the millennia – as only possible in a museum – to the 19th century, when the Industrial Revolution began to change the climate.

From a cupboard, he pulls out sheets of paper where Victorian 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 to a modern leaf that Barclay holds in his hand. But one key difference can be seen under the microscope – how the leaf reacted to the change in 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 convert 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.

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

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

But first, he needs a decryptor, a translation sheet—a kind of Rosetta Stone to decipher the ancient atmosphere’s handwriting.

That’s why he’s conducting an experiment in a glade in Maryland.

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

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 dramatically high levels, approaching levels of the distant past, or perhaps the future.

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

They’re also studying what happens when trees grow in overloaded environments, and they’ve found that more carbon dioxide speeds them up.

But adds Barclay, “If plants are growing very quickly, they are more likely to make mistakes and to be more susceptible to damage. … It’s like a race car driver who is more likely to go off the rails high speed.

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Follow Christina Larson on Twitter: @larsonchristina

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The Associated Press Health and Science Department is supported by the Howard Hughes Medical Institute Department of Science Education. The AP is solely responsible for all content.

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This story has been corrected to state that the school’s name is Georgia Institute of Technology, not Georgia Tech University.