Mineral deposits in fossilized brachiopod shells allow geochemists to plot changes in sea temperatures over millions of years. These patterns of climate change serve as tests for climate models used to predict global warming.
Grossman researches the isotopic make-up of fossilized brachiopods, sea creatures whose distant relatives – such as lampshells – are still found in today's oceans.
"We drill tiny holes in the fossilized shells, then through isotope-ratio mass spectrometry, analyze the extracted powder for different forms of oxygen," said Grossman of the Department of Geology and Geophysics in the College of Geosciences. "Oxygen always has eight protons but can have different numbers of neutrons. Regular oxygen [O-16] has 8 neutrons in its nucleus, but a nonradioactive isotope with 10 neutrons [O-18] is naturally present in seawater in measureable amounts. The O-18 has a temperature-dependent affinity for the calcium carbonate which forms the shells of many sea creatures.
"By analyzing the ratio of O-18 to regular oxygen in the shells, we can tell the temperature of the ancient seawater. Higher values of O-18 correspond with periods of colder temperatures, and fluctuations in the isotopic ratios of oxygen in the shells over time reflect transitions between icehouse and greenhouse periods in Earth history."
Grossman, along with collaborators David Pollard, Pennsylvania State University; Christopher Scotese, University of Texas at Arlington; and William Hyde, Duke University, presented a paper on their latest analysis of relationships between O-18 distribution and global paleoclimate models at the October annual meeting of the Geological Society of America.
"Oxygen isotope estimates of paleotemperature show good agreement with computerized global climate models for some intervals in the past but not with others," Grossman noted. "We're in the process of investigating possible reasons for this lack of agreement. But study of the patterns of transition during ancient times from periods of cold temperatures, with widespread glaciation, to warmer periods of a more tropical Earth, can lend support to current computer models projections that the Earth has entered another warming period."
Grossman's oxygen isotope research also correlates with studies indicating that increased carbon dioxide in the atmosphere coincides with higher temperatures. This work relies on the measurement of stable carbon isotopes (C-13/C-12) in the shells, a proxy indicator for the carbon cycle.
"Research indicates that drawdown of carbon dioxide, with sequestration of carbon into biomass and sediments, can trigger periods of glaciation," Grossman observed. "Correlating fluctuations of carbon dioxide and carbon and oxygen isotopes in the past may help us better calibrate computer models of future global climate change.
"Our research doesn't prove that increased CO2 is currently causing global warming, but oxygen isotopic measurements of ancient Earth temperatures suggest that this has caused global warming in Earth's past."
Contact: Keith Randall, 979-845-4644, kr@univrel.tamu.edu;
Ethan Grossman, 979-845-0637, e-grossman@tamu.edu.