RESEARCH HIGHLIGHT

Prehistoric climate change

David Barbeau

Why did a once ice-free Antarctica suddenly become a frozen continent halfway through the Cenozoic Era? One prominent hypothesis suggests that the progressive decrease of atmospheric carbon dioxide inhibited the greenhouse effect that had previously kept Earth warm—effectively the opposite of the climate change that is occurring today. Barbeau’s research is testing an alternative hypothesis first proposed 30 years ago by scientists at the University of Birmingham and the University of Rhode Island.

"Geologic evidence suggests that southern South America used to be connected to the Antarctic Peninsula," Barbeau said. "At some point in the past 50 million years, these tectonic plates shifted enough to separate the two land masses, creating a pathway for a cold, fast ocean current that developed around Antarctica."

Today, this Antarctic Circumpolar Current acts as a barrier to warm pole-ward moving ocean currents that would otherwise keep the continent ice-free, much in the same way that the Gulf Stream keeps northwestern Europe surprisingly warm despite its high latitude.

"Although the modern thermal effect of the Antarctic Circumpolar Current is fairly well understood, there is great debate in the scientific community about when this current was established," Barbeau said. "Our research is using a variety of geochemical techniques to determine whether this plate tectonic activity occurred in time to have caused the initial growth of ice on Antarctica. If not, we need to more closely consider the roles of other phenomena in the associated climate change and increase in polar ice volume, such as a decreasing concentration of greenhouses gases."

Toward evaluating this polar gateway hypothesis, Barbeau and his students are using rocks they collected in Tierra del Fuego and Antarctica to reassemble the land masses into their positions prior to the opening of Drake Passage, which now separates the two continents. To augment this data, Barbeau’s team is also conducting radioactive dating of granitic rocks in both locations to determine the timing of the plate tectonic activity. Co-principal investigators Bob Thunell and Howie Scher—the latter an incoming assistant professor at South Carolina—are using marine geochemical data to further constrain the relationships between circulation of the Southern Ocean, Antarctic glaciation, and global cooling.

"Although the time scales we’re looking at are much longer than the time frame of modern global warming," Barbeau said, "understanding the causes of glaciation is fundamentally important. Bob and Howie’s work is providing vital information toward understanding how ocean circulation affects the global climate system, which we know to be important regardless of the climate state or time period in Earth’s history."

Barbeau’s research began with a $20,000 seed grant from the University’s research office and Department of Geological Sciences that produced enough data to secure funding from NSF for a larger proposal.