Two newly published studies are helping scientists trace millions of years of Antarctica's climate history, including an age when parts of the continent were as warm as the California coast is today.
One of the studies focuses on an ice core taken from Antarctica's Taylor Glacier, and uses readings of radioactive krypton to confirm that the sample goes back 120,000 years. The researchers behind that study, appearing in the Proceedings of the National Academy of Sciences, say the same technique could provide more accurate dates for ice samples going back as far as 1.5 million years.
"That is very exciting, because a lot of interesting things happened with the earth's climate prior to 800,000 years ago that we currently cannot study in the ice-core record," Christo Buizert, a researcher at Oregon State University who is the study's lead author, said in a news release.
Krypton, a noble-gas element that is arguably most famous for its association with the Superman saga, is much more reliable than carbon-14 for ice-core dating. But it's devilishly difficult to isolate the radioactive krypton-81 atoms from the air bubbles trapped within Antarctic ice.
Buizert and his colleagues used recently developed tools, including the 9.5-inch-wide (24-centimeter-wide) Blue Ice Drill and the ATTA-3 atom counter, to count individual atoms of krypton-81 extracted from the ice. The researchers validated their method by comparing their analysis of Taylor Glacier samples with age estimates derived from measurements of atmospheric methane and oxygen.
Now researchers plan to go after Antarctica's oldest ice.
"Most people assume that it's a question of just drilling deeper for ice cores, but it's not that simple," Edward Brook, an Oregon State University geologist and a co-author of the study, said in the news release. "Very old ice probably exists in small isolated patches at the base of the ice sheet that have not yet been identified, but in many places it has probably melted and flowed out into the ocean."
Antarctica in 40 million B.C.
Other methods are needed to go much further back into Antarctica's climate history. The second study, also appearing in PNAS, focused on two rare chemical isotopes from fossil shells that have been linked to the Eocene epoch, 40 million to 50 million years ago. The shells were collected on Seymour Island, a small island off the coast of the Antarctic Peninsula.
Researchers measured the concentrations of carbon-13 and oxygen-18 in the shells, and fed their findings into computer models to calculate what the temperature was when the shells were formed. This technique is called carbonate clumped isotope thermometry.
The scientists found that the temperatures reached as high as 63 degrees Fahrenheit (17 degrees Celsius) in parts of Antarctica, with an average of 57 degrees F (14 degrees C). That's roughly equivalent to the average temperatures in San Francisco for this time of year.
"This provides strong evidence that global warming is especially pronounced close to the Earth's poles."
A different method known as TEX86 was used to calculate temperature readings for other areas during the same geologic era. In parts of the southern Pacific Ocean, the estimated average temperature was 72 degrees F (22 degrees C), which is similar to seawater temperatures near Florida today. In comparison, the current average South Pacific sea temperature near Antarctica is close to freezing.
Temperatures appeared to be higher on the South Pacific side of Antarctica, and the researchers said ocean currents may have played a role in those differences.
"By measuring past temperatures in different parts of Antarctica, this study gives us a clearer perspective of just how warm Antarctica was when the Earth's atmosphere contained much more CO2 than it does today,” the paper's lead author, Peter M.J. Douglas, said in a Yale news release. "We now know that it was warm across the continent, but also that some parts were considerably warmer than others. This provides strong evidence that global warming is especially pronounced close to the Earth's poles."
Douglas did the research for the PNAS paper, "Pronounced Zonal Heterogeneity in Eocene Southern High-Latitude Sea Surface Temperatures," while he was a graduate student at Yale University. He is now a postdoctoral scholar at Caltech. The paper's other authors include Hagit Affek, Linda Ivany, Alexander Houben, Willem Sijp, Appy Sluijs, Stefan Schouten and Mark Pagani.
In addition to Buizert and Brook, the authors of "Radiometric 81-Kr Dating Identifies 120,000-Year-Old Ice at Taylor Glacier, Antarctica" include Daniel Baggenstos, Wei Jiang, Roland Purtschert, Vasilii Petrenko, Zheng-Tian Lu, Peter Müller, Tanner Kuhl, James Lee and Jeffrey Severinghaus.