They can't talk. They're not measurably intelligent. They can't even move on their own. Yet Katherine Freeman of Penn State University's geosciences department has been learning something from common marine algae.
"We're trying to look back in time," says Freeman, "to see if the level of carbon dioxide in the atmosphere was higher when the temperature was higher." By understanding Earth's ancient atmosphere, scientists hope to predict the consequences of modern-day greenhouse gas emissions.
Creating a computer model of the current climate system and running it back through time, as Eric Barron of Penn State has done, can simulate ancient conditions, but it's up to others, like Freeman, to check the computer's results with direct observations. To do so, she is looking at biomarkers. "A biomarker," she says, "is simply a relatively unique chemical that an organism leaves behind in measurable quantities." Countless organic compounds made from what was once atmospheric carbon dioxide are present in rock and sediment. The trick is to find one that can be traced to a specific, known organism.
Freeman's chemicals of choice are alkenones, made by only one class of algae. "They're called prymnesiophytes, and they're found in all the world's oceans," she says. Freeman chose this algae because its biomarkers preserve well, and because it has been thoroughly studied as a means for determining ancient temperatures. By examining how the contemporary prymnesiophytes behave in different environments, Freeman can guess at how their ancestors acted.
Since algae settle to the sea floor when they die, those ancestors are buried in ocean mud and sedimentary rocks in areas like the American Midwest, which were underwater during ancient warm spells. Freeman and her students, who are studying the boundaries between warm and cool periods, collected 94-million-year-old mud samples from Colorado and Utah last year, and 400-million-year-old rock cores from exposed outcroppings near State College, PA, this past summer. She collected algae and mud from Peru's coast in 1992, and other researchers have provided her with samples from the equatorial Pacific, and soon from the Arabian Sea, in an effort to construct a global picture.
The samples themselves aren't pretty. "I know what it looks like," Freeman laughs: sewage. Freeman's research assistants (five graduate and three undergraduate students who share a variety of tasks) soak the mud or powdered rock samples in solvents for at least 24 hours, filter the solution, and separate the chemicals by various chromatographic methods. The alkenones are then fed into a mass spectrometer, giving Freeman the ratio of the two stable isotopes of carbon, carbon-12 and carbon-13, that are present in the alkenones. This approach is called Compound-Specific Isotope Analysis (CSIA). As a graduate student in the late 1980s, Freeman was one of four investigators who developed the technique and the necessary hardware and software. Apart from listening to elderly algae, CSIA can be used to "fingerprint" oils to see how similar they are and to learn where to find other deposits, to trace digestion and absorption of food with stable (non-radioactive) isotopes, to determine the fates of man-made compounds in groundwater, and to check if flavorings are natural or synthetic, or if honey has been spiked with corn syrup. In Freeman's case, CSIA can show what quantity of carbon dioxide was available to ancient algae.
As Freeman explains, the lazy algae prefer carbon-12 atoms to carbon-13 atoms: "It's easier for their enzymes to transport the lighter isotope," she says. When carbon dioxide is abundant, and the algae's enzymes can pick and choose among carbon atoms, they wind up with more of the choice carbon-12. Likewise, when carbon dioxide is scarce, and the algae's enzymes have to take what they can get, the alkenones contain more carbon-13.
Similarly, the ancient sea temperature is determined by studying the ratio of oxygen isotopes in calcium carbonate, found in the shells of fossil organisms. Evaporation preferentially removes water containing the lighter oxygen- 16, and in cool periods much of this water, taken out of the oceans, falls and remains on the polar ice caps as snow rather than returning to the sea as rain. The oceans and the shells of animals that live in them are thus left enriched with the heavier oxygen-18.
The two isotope analyses allow algae to speak about both the temperature (via the calcium carbonate in their shells) and the air quality (via the alkenones). Through her method, Freeman in cooperation with Michael Arthur of Penn State, who provided the temperature data has indeed found a correlation between global warming and increased levels of atmospheric carbon dioxide.
During the Cretaceous period, 90 to 100 million years ago, tropical sea temperatures averaged 25 degrees C (as opposed to the chillier 20 degrees C of today). "It was so warm then," she says, "that there were no polar ice caps." From the differences in carbon isotope levels, she estimates that the atmosphere and oceans held three to four times more carbon dioxide than today.
So what do the algae say about the computer models? It seems they have no major objections. "The level of carbon dioxide in our findings is consistent with some models and lower than others," Freeman says.
Katherine H. Freeman, Ph.D., is assistant professor of geosciences in the College of Earth and Mineral Sciences, 209 Deike Building, University Park, PA 16802; 814-863-8177. Michael Arthur is head of the department of geosciences, 503 Deike Building, University Park, PA 16802; 814-863-6054. Eric Barron is professor of geosciences and director of the Earth Systems Science Center, 248 Deike Building, University Park, PA 16802; 814-865-1619. Also working on this project are doctoral students Robert Dias, Timothy Filley, and Richard Pancost; master's degree students Fran Cooper and Adel Saleh; undergraduates Melissa Casey, John Darcy, and Eunice Huang; and lab technician Denny Walizer.
Samples were provided by the United States Global Ocean Flux Study and the United States Geological Survey in Denver; funding came from the National Science Foundation, the American Chemical Society, the U.S. Department of Energy, and by private investors.