An ice core retrieved from the Antarctic has been dated to approximately five million years ago, allowing researchers to study Earth’s ancient atmosphere. Describing the sample in the journal The Cryosphere, the study authors explain that the ice dates back to the Pliocene epoch, when global temperatures and carbon dioxide levels were higher than they are today.
Analyzing Earth’s climatic conditions in the distant past allows scientists to make informed predictions about how the environment is likely to change in the future. For this reason, researchers regularly pull cores from the ice sheets in eastern Antarctica, hoping to find air bubbles that have been trapped in the ice for millions of years.
However, the study authors explain that “the oldest ice that has been recovered from the thickest parts of the Antarctic ice sheets is 800 000 years old.”
“The lack of ice older than [one million years old] severely limits our direct paleoclimate record and creates uncertainties when modeling future climate predictions which include modeled configuration of the past Antarctica Ice Sheet,” they say.
Scientists are particularly keen to obtain ice samples dating back to the Pliocene, which is seen as an analog for current anthropogenic warming. In search of a sample from this period, the researchers decided to drill in the Ong Valley, located in the Transantarctic mountains and containing ice believed to be older than eastern Antarctica’s ice sheets.
After retrieving a “debris-rich ice core” measuring 944 centimeters (371 inches) in length, the team analyzed cosmogenic nuclide concentrations in the rocky debris to determine the age of the sample. Cosmogenic nuclides are rare isotopes produced by cosmic rays interacting with surface rocks, and can be used to calculate how long material has been buried.
Results indicated that the ice core contained two separate layers, the younger of which was roughly 2.95 million years old. Beneath this, an older section of ice was dated between 4.3 and 5.1 million years old.
“The ages reported here coincide with the Pliocene epoch,” write the study authors.
More specifically, the two layers correspond to two global glaciations, one of which occurred during the early Pliocene (around 4.8 to 4.9 million years ago) while the other took place in the late Pliocene, some 3.3 million years ago.
The latter event was followed by a period in which global temperatures exceeded those of today, which is why scientists are so eager to study the Earth’s atmosphere from this era. Doing so could provide vital clues as to how rising emissions and temperatures are likely to affect the planet in the coming years.
A deeper analysis of the ice core will be needed before any such information can be obtained, although the researchers say their initial findings do at least confirm the timing of the Pliocene’s two glacial advances.
“Collectively our results show that the continental ice sheet advanced into Ong Valley repeatedly, and evidence of at least two such advances at 2.95 and 4.3–5.1 [million years ago],” they write.