Ancient ice trapped within an ice cap in Northwestern Tibet appears to preserve fragments from the last ice age, extending Earth's glacial record by over 100,000 years and making them the oldest ice cores ever recovered from the region, according to a new study.
The analysis, comparing ice cores drilled from the Guliya Plateau in 1992 and 2015, shows that both samples contained similar oxygen isotope patterns, suggesting that the environmental signals they contain are consistent and reproducible.
"Reproducibility of the records within a given ice cap is extremely important," said Lonnie Thompson, lead author of the study, professor of earth sciences and senior research scientist at the Byrd Polar Research Center at The Ohio State University. "If you get an identical record a quarter of a century later, it can tell you a lot about the behavior of the ice over time."
To measure the true age of the ice, researchers employed advanced dating techniques to detect two radioactive isotopes, beryllium and chlorine, whose half-lives make them reliable tools for capturing large-scale environmental changes. In this paper, the international team used these elements to mark the Laschamp Geomagnetic Excursion, a reversal of Earth's magnetic field that left major chemical traces in the ice more than 41,000 years ago.
By comparing these results with oxygen-isotope records from nearby Tibetan cave deposits, this team estimates that the Guliya record's age may stretch back well over 128,000 years, said Thompson.
"It's a really critical finding, because outside of the polar regions, this is the only ice core we have from the mountaintops that go back that far," he said. "Records out of Huascarán only go back over 30,000 years, not over 100,000, so that makes Guliya a very unique record."
The paper was published today in Science Advances.
Ice cores serve as vital climate archives of the locations and time periods they represent. Yet deciphering these flash-frozen treasure troves can be a challenge, as it requires an accurate knowledge of the ice core's timescale, said Thompson.
For example, while previous work suggested that Guliya's record stopped at the Mid-Holocene, an era that began nearly 12,000 years ago, evidence from this work lays to rest doubts about its chronology, confirming that some of its ice is, in fact, much older than the Holocene.
"If we only had Holocene-era ice in Tibet, that would mean that the big ice sheets from the last ice age didn't persist through the early Holocene," said Thompson. "But evidence shows that it did."
While Guliya's topography likely plays a role in its ability to preserve such old paleoclimate records, constructing a more accurate timescale for the ice cap may also offer answers to bigger environmental questions, such as when Earth's intense periods of glaciation began and ended.
"When did ice start forming on the planet and what did those natural processes of loss look like?" said Thompson. "We need to know those things, and the only way you can really demonstrate it is with accurate time scales on these records."
Because these results open new pathways for other types of detailed scientific investigations on Guliya ice cores, the team plans to use future samples to determine how other key climate and environmental indicators may change with this new acknowledgment of Earth's geologic history.
"The big picture is that our results tell a very consistent story," he said. "If you get the science right, it's consistent. It's our understanding of what it's trying to tell us that has to catch up."
This research was supported by the U.S. National Science Foundation. Ohio State co-authors include Mary Davis and Ellen Mosley-Thompson, as well as Juerg Beer from the Swiss Federal Institute of Aquatic Science and Technology, Christof Vockenhuber and Marcus Christl from ETH Zurich in Switzerland, Tandong Yao from the Chinese Academy of Sciences, and Ninglian Wang and Ling Fang from Northwest University in China.
