by: Matthew Holm
From Research/Penn State, Vol. 16, no. 3 (September, 1995)
hey 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.
imilarly, 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.
