Through a study of data on 176 species of modern-day plants, the authors found evidence that fossil plants can help scientists determine the sources of carbon in the atmosphere hundreds of millions of years ago, a source of useful insight into ancient climates. However, their results, published recently in "Paleobiology," put a potentially more direct link between fossil plants and carbon levels in the prehistoric atmosphere in doubt.

"We have to keep in mind that these ancient organisms operated according to their own ecological agenda," says Hope Jahren, assistant professor of Earth and Planetary Sciences at Johns Hopkins, and an author of the paper. "However, there's still a great deal of meaning there–we just have to look for it from the plant's perspective."

Jahren and the other authors analyzed data from 44 modern plant studies conducted by other scientists. They selected studies of C3 plants, the most prevalent type of plant on Earth now and for the last four hundred million years. C3 plants include all the trees and some grasses.

Scientists found a promising connection between the ratios of carbon isotopes in the air and in the plants. Isotopes are forms of an element that differ only by the addition of one or more subatomic particles known as neutrons. Different isotopes of the same element, identified by a number after the element name, can have different physical properties.

Plants absorb both the isotopes carbon 13 and carbon 12, bound in carbon dioxide, from the atmosphere. As the plant moves carbon from the stomata to sites where it is prepared for use in photosynthesis, carbon 13, heavier by a neutron, moves more slowly, separating the two isotopes.

Given the ratio of carbon 13 to carbon 12 in a plant, scientists found that a few mathematical calculations can produce an accurate picture of the ratio of those isotopes in the atmosphere. The correlation is weaker in arid and desert environments and grasslands, but, according to Jahren, still a potentially useful tool.

"Although this doesn't give us a blanket value for how much CO2 was in the atmosphere, it gives us a strong impression of the status of the carbon cycle at any given time," Jahren explains. "By status I mean which carbon pools are contributing, which pools aren't, have there been any dramatic changes, and which pools might be implicated in those dramatic changes."

Many reservoirs of carbon in the environment are composed of a unique mixture of carbon isotopes. Jahren's lab, for example, has found evidence, now in review for publication, suggesting a release of carbon into the environment 115 million years ago from clathrates, deposits of methane on the bottom of the oceans.

"That's a large reservoir of carbon that we haven't seen change dramatically in human times, but through these means we can identify it as a major player in the carbon cycle through geologic time," she notes. "It's a more subtle question than how much CO2 is in the atmosphere, but it's closer to the heart of what we want to know about how carbon inhabits chemical spheres of the earth, and how that affects climate."

Data on the plants and their environments revealed a number of factors that could disrupt connections between carbon dioxide levels in the atmosphere and in the plant.

"Water is a good example. Plants take in carbon from the atmosphere through openings on their surface called stomata," Jahren says. "But that's also how water gets out of the plant. When there's low humidity in the atmosphere, water at the stomata naturally evaporates up and out, and plants will close their stomata to prevent this, reducing carbon intake."

That finding wasn't too disappointing, Jahren notes, because arid environments normally do not yield plant fossils. But other phenomena more common to fossil-forming environments also change carbon intake, including levels of salt in the soil, light levels, temperature extremes, and humidity.

Although there may be an individual species that shows a less variable intake of atmospheric carbon, the results strongly suggest that plant composition as a whole typically doesn't reflect the level of carbon in the atmosphere.

The lead author on the paper was Nan Crystal Arens, assistant professor of integrative biology at the University of California, Berkeley. Ronald Amundson, professor of soil science at the University of California, Berkeley, was also an author.

Note: If no author is given, the source is cited instead.

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