128,000-year-old ice in Tibet could rewrite climate history

Deep within an ice cap in northwestern Tibet, scientists have uncovered evidence of a remarkable frozen record that may stretch back more than 128,000 years. The discovery suggests that ice high in the mountains has survived since the last ice age and could provide an extraordinary window into Earth's ancient climate.

The findings extend the known glacial record from the region by more than 100,000 years, making these the oldest ice cores ever recovered from this part of Tibet. They also offer rare evidence of how mountain glaciers endured major changes in Earth's climate.

Researchers reached their conclusions by examining ice cores collected from the Guliya Plateau in 1992 and 2015. Despite being drilled 23 years apart, the samples revealed strikingly similar patterns of oxygen isotopes, which scientists use to reconstruct past environmental conditions. The close match indicates that the ice has preserved a reliable and reproducible record of climate history.

"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."

Determining the age of ice that has remained frozen for tens of thousands of years is no simple task. To establish a more accurate timeline, the international research team turned to advanced dating methods involving radioactive isotopes of beryllium and chlorine.

These naturally occurring radioactive forms of elements decay at predictable rates, making them valuable tools for identifying events deep in Earth's environmental history. In this case, researchers used them to detect evidence of the Leschamp Geomagnetic Excursion, a dramatic disturbance in Earth's magnetic field that occurred more than 41,000 years ago and left distinctive chemical signatures in the ice.

The team then compared its findings with oxygen isotope records preserved in nearby Tibetan cave deposits. Together, the evidence suggests that the Guliya ice record may extend well beyond 128,000 years into the past.

"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."

Ice cores are among scientists' most valuable sources of information about ancient climates. As snow accumulates and gradually becomes compressed into ice, it can preserve chemical clues about the atmosphere and environmental conditions at the time. By examining different layers, researchers can piece together how climate changed over thousands of years.

However, interpreting these records depends on knowing when each layer formed. Without a reliable timeline, even an exceptionally well-preserved ice core can be difficult to understand.

That uncertainty had complicated earlier interpretations of the Guliya ice cap. Previous research suggested that its record reached back only to the middle of the Holocene, the geological epoch that began nearly 12,000 years ago following the last ice age.

The new evidence establishes that at least some of the ice is considerably older than the Holocene. This also has important implications for understanding whether large ice masses in Tibet persisted as the planet emerged from the last glacial period.

"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."

The Guliya ice cap's unusual geography may help explain why it has preserved such an exceptionally old climate record. Its high elevation and surrounding terrain likely contributed to conditions that allowed ancient ice to remain in place through major environmental changes.

Beyond establishing the ice's age, researchers hope that a more precise timeline will help answer fundamental questions about the history of Earth's glaciers. Knowing when major periods of glaciation began, how long they lasted, and when the ice retreated is essential for reconstructing the planet's natural climate cycles.

"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."

The revised timeline also creates new opportunities to investigate the Guliya ice cores in greater detail. Researchers plan to study additional samples to determine how other indicators of past climate and environmental conditions fit into this much longer history.

For Thompson, one of the most encouraging aspects of the discovery is how consistently the different measurements support the same interpretation.

"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."

The research received support from the U.S. National Science Foundation. Ohio State co-authors include Mary Davis and Ellen Mosley-Thompson. Other contributors were Juerg Beer of the Swiss Federal Institute of Aquatic Science and Technology; Christof Vockenhuber and Marcus Christl of ETH Zurich in Switzerland; Tandong Yao of the Chinese Academy of Sciences; and Ninglian Wang and Ling Fang of Northwest University in China.