Scientists studying ice cores from Tibet’s Guriya Plateau have found evidence that some of the region’s ice may be more than 128,000 years old. The finding extends the known glacier record there by over 100,000 years and suggests that high-altitude ice survived major climate shifts, including the transition out of the last ice age. It could provide a rare record of Earth’s ancient climate outside the polar regions.
Dating ancient ice from the Guriya Plateau
The international research team analyzed ice cores recovered from the Guriya Plateau in northwestern Tibet in 1992 and 2015. Although the cores were drilled 23 years apart, they contained strikingly similar oxygen isotope patterns. Scientists use these patterns to reconstruct past environmental conditions, and the agreement between the samples indicates that the ice preserves a reliable and reproducible climate record.
“Reproducibility of records within a particular ice sheet is extremely important,” said Lonnie Thompson, the study’s lead author, a professor of geosciences and senior research fellow at The Ohio State University’s Byrd Polar Research Center. “If we get the same records a quarter of a century later, we’ll learn a lot about the behavior of ice over time.”
Establishing the age of ice that has remained frozen for tens of thousands of years is challenging. To build a more accurate chronology, the researchers used radioactive isotopes of beryllium and chlorine. These naturally occurring elements decay at predictable rates, allowing scientists to identify events preserved in Earth’s environmental record.
In the Guriya cores, the researchers looked for chemical evidence of the Reshan geomagnetic displacement, a major disturbance in Earth’s magnetic field that occurred more than 41,000 years ago and left distinctive signatures in the ice.
The team also compared the ice-core results with oxygen isotope records preserved in cave deposits elsewhere in Tibet. Taken together, the evidence suggests that the Guriya ice record may extend well beyond 128,000 years into the past.
“This is a really important discovery because outside of the polar regions, this is the only ice core taken from a mountaintop that goes that far,” Thompson said. “The Huascaran records only go back over 30,000 years, not over 100,000 years ago, which makes Guriya a very unique record.”
The research was published in the journal Science Advances.
A record of climate during the last ice age
Ice cores are among the most valuable sources of information about ancient climate. As snow accumulates and is gradually compressed into ice, it can preserve chemical evidence of atmospheric and environmental conditions at the time. By studying successive layers, researchers can reconstruct climate changes over thousands of years.
That interpretation depends on knowing when each layer formed. Without a dependable timeline, even a well-preserved ice core can be difficult to understand.
The lack of a precise chronology complicated earlier interpretations of the Guriya Ice Sheet. Previous research indicated that its record extended only to the mid-Holocene, a geological epoch that began about 12,000 years ago after the last ice age.
The new evidence indicates that at least some of the ice is substantially older than the Holocene. It also affects understanding of whether large ice masses remained on the Tibetan Plateau as the region emerged from the last ice age.
“If Tibet only had Holocene ice, it would mean that the large ice sheets from the last ice age did not remain until the early Holocene,” Thompson said. “But the evidence shows that it happened.”
Why the ice survived
The unusual topography of the Guriya Ice Sheet may help explain why it contains such an old climate record. Its high elevation and surrounding terrain likely created conditions that allowed ancient ice to remain in place through major environmental changes.
A more precise chronology could help researchers investigate fundamental questions about the history of Earth’s glaciers, including when major ice ages began, how long they lasted and when glaciers retreated. Establishing those timescales is essential for reconstructing the planet’s natural climate cycles.
“When did ice start forming on Earth? And what was its natural ice loss process?” Thompson said. “We need to know about them, and the only way we can really demonstrate that is by showing the exact time scales in these records.”
The revised timeline also creates opportunities to examine the Guriya ice core in greater detail. Researchers plan to study additional samples to determine how other indicators of past climate and environmental conditions fit into the longer history.
Future analyses may reveal information that was previously overlooked or interpreted differently because the age of the ice was not fully understood.
For Thompson, the consistent agreement among the different measurements is one of the most encouraging aspects of the discovery.
“The big picture is that our results tell a very consistent story,” he said. “If you get the science right, it’s consistent. What we need to catch up to is our understanding of what the science is trying to tell us.”
The research was supported by the National Science Foundation. Co-authors from Ohio State University include Mary Davis and Ellen Moseley Thompson. Other contributors were Juerg Beer from the Swiss Federal Institute of Fisheries Science and Technology; Christoph Wockenhuber and Markus Kristol of ETH Zurich, Switzerland; Tandong Yao of the Chinese Academy of Sciences; and Ninglian Wang and Ling Fang of Northwestern University, China.
Source: www.sciencedaily.com
