In a High School Lab, Glimpses of an Ancient Climate

Elias De Leon and Brian Zhou on Wednesday, Aug. 17th

Sometimes you achieve something that’s significant only to the few people who know just how hard a thing it is to do. In our case, the thing that was hard to do was also hard to see. But in the end — on our second-to-last day of school in June, as our classmates signed yearbooks and talked about summer plans — we did see it. There it was, like a tiny, open mouth calling at us through the microscope. A stomate.

Stomates are tiny holes on the underside of leaves that allow plants to exchange gases with the air around them – exhaling oxygen and water vapor and inhaling carbon dioxide. They’re as common to plants as mouths are to mammals. Only more so, since a single leaf can contain hundreds of thousands of stomates. But what was so uncommon about the stomates under our microscope was their age: about 15 million years old.

The stomates of a red maple leaf at 400x magnification. (Photo: John Cunningham)

Also unusual in this context is our age. We are both teenagers, recently graduating seniors at Brooklyn Technical High School where this past school year we took a class called Environmental Inquiry. Neither of us had much hands-on research experience, but with the guidance of our teacher, John Cunningham, our study is truly cutting-edge. It may even help to advance the emerging field of paleoclimatology.

A big part of studying global warming is comparing today’s climate to that of the past. If you’ve ever seen the movie An Inconvenient Truth, you’d recognize a chart that shows how carbon dioxide (CO2) levels in the atmosphere have fluctuated along with global temperatures. You may recall that climatologists got those historic CO2 measurements from bubbles of air trapped in the ice of very old glaciers. But those ice core samples go back only 650,000 years. What we’re trying to do is extend that timeline much further back into the past.

In theory, our experiment is easy to grasp. Researchers have evidence that, for many plants, the number of stomates per leaf changes in response to how much carbon is in the air. By comparing modern leaves to leaves preserved in 15-million-year-old clay deposits, we can estimate how much CO2 was in the air when the ancient leaves were alive. It’s like solving a simple algebra problem.

A 15 million-year-old leaf separates from the clay and disintegrates in a dish of hydrogen peroxide. (Photo: Charlie Foster)

The not-so-simple part is getting a good look at the stomates of the preserved leaves in order to count them. For one thing, the leaves are extremely delicate. To view the lower epidermis of living leaves, we simply painted the leaf’s underside with nail polish, covered it with cellophane tape when it dried, and peeled off an imprint that clearly showed each stomate under the microscope. Using the peel method on an ancient leaf destroys it.

The leaves are also rare. They were sent to us by a professor of paleobotany at the University of Idaho named William Rember, who dug them out of his backyard pit near Clarkia, Idaho. During the Miocene epoch, when strange-looking mammals like the hornless rhino and the three-toed horse roamed North America, much of Idaho was underneath a large lake whose oxygen-deprived clay bottom mummified the leaves that fell into it. There are few sites in the world with such well-preserved leaves. (Mr. Cunningham calls it a “Shangri-La” for Miocene “mummies.”) So while we found the nail polish-tape technique for fresh leaves after a quick Google search, there are few documented procedures to observe the stomates of Miocene leaves.

At least not with the materials at our disposal. For a high school, we have a top-of-the-line laboratory at Brooklyn Tech. But because we’re high school students, we’re forbidden from handling hydrofluoric acid, which according to other studies will reliably separate a mummy’s epidermis from the rest of the leaf. So we had to experiment with other chemicals.

First we tried soaking a leaf overnight in a dish of hydrogen peroxide. The next morning, it was destroyed — nothing but floating debris by the naked eye and indistinguishable cellular bits under the microscope. We adjusted the length of time and the strength of the peroxide. None of them worked – the solution either destroyed the leaves or had no effect on them.

One of the authors, Elias De Leon, at work in his classroom lab at Brooklyn Technical High School. (Photo: Charlie Foster)

Usually, the science experiments teachers assign us work perfectly. You get a lab sheet with the exact measurements of the chemicals you need to use and an estimate down to the minute of how long the procedure should last. It’s like a tried and true cooking recipe. And it almost always gets you the result and the conclusion you (and, more importantly, the teacher) expects. All over the course of a single lab period. Two periods, max.

Our experiment took much longer: four months of trial and error with different chemical solutions and different lengths of time, diligently recording every step but never catching a glimpse of a Miocene stomate. We pushed on. But with each day closer to the end of the school year we began suspecting that we’d never get past our study’s Materials and Methods section to its Results.

Then a last-minute surprise. Mr. Cunningham got a tip that Clorox bleach might work, and during the last week of class we started experimenting with it. Ten minutes after dropping a piece of leaf into a test tube of undiluted bleach, a white piece of debris floated to the top. We pulled it out with tweezers, placed it in distilled water and then on a glass slide. It was a lot lighter in color than the previous debris we’d put under the microscope. We turned the side knob and it slowly came into focus…

Ultimately, we ran out of time to do the modern leaf-ancient leaf comparison that could have added a 15-million-year mark on the timeline of the Earth’s CO2 levels. But what might look anticlimax to some, was a triumph for us. We started from scratch on this research and we took it as far as we could so that next year’s Environmental Inquiry class can go further. They’ll have a guideline to the nitty-gritty of how climate science gets done, the methods they’ll need to get the results. And, we hope, the knowledge that when you finally do get them, it’s pretty satisfying.

More Info:

Youth Radio Investigates is an NSF-supported science reporting series in which young journalists collect and analyze original data with professional scientists, and then tell unexpected stories about what they discover.

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