Science1 publisher3 min readPublished
Ohio State-led team traces Tibet's Guliya ice cap back into the last ice age
Lonnie Thompson's team estimates Tibet's Guliya ice cap holds a climate record reaching back well over 128,000 years, into the last ice age. Thompson says that makes it the only mountaintop ice core outside the polar regions to reach that far.
The Scientist · Science desk
What happened
- Cores drilled on the Guliya Plateau in 1992 and 2015 showed similar oxygen isotope patterns, which the researchers take as evidence that the ice cap's climate signal is reproducible.
- Beryllium and chlorine isotopes let the team locate the Laschamp geomagnetic excursion, a magnetic field reversal that left chemical traces in the ice more than 41,000 years ago.
- Earlier work had suggested Guliya's record ended in the mid-Holocene, part of an epoch that began nearly 12,000 years ago.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint The radiometric marker dates only about the youngest third of the claimed span. If the Tibetan cave chronology is revised, Guliya's older ages move with it.
- decision Researchers working from Guliya samples now have to pick between the Holocene-only chronology and this one. The two differ by more than 100,000 years in the age they give the deepest ice.
- capability A mountain ice record older than 100,000 years gives researchers an archive outside the poles for dating when glaciations began and ended. Thompson says that question cannot be answered without accurate timescales.
Agreement between cores drilled 23 years apart [1] shows that Guliya's oxygen isotope signal belongs to the ice cap itself and repeats from one borehole to the next [2]. Lonnie Thompson, the lead author and a senior research scientist at Ohio State's Byrd Polar Research Center [3], puts the weight there. "Reproducibility of the records within a given ice cap is extremely important," he said. "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." [4]
Matching cores cannot date themselves, though. Two cores from one cap would carry a misdated layer as consistently as a correctly dated one. For the clock, the international team needed an independent event it could find in the ice. It used the Laschamp excursion, picked out through beryllium and chlorine and placed at more than 41,000 years ago [5].
I think the matched cores are the firmest result in the study and the oldest ages are the least certain. Past the Laschamp marker the method changes. The estimate that the record may stretch back well over 128,000 years rests on lining up Guliya's oxygen isotopes against records from nearby Tibetan cave deposits [6]. The Laschamp tie point sits at roughly a third of that age, so about two-thirds of the timeline is dated by matching one curve to another [2]. The thing the report doesn't tell you is how the cave records are dated or how closely the two curves agree. The report also says the cores "appear to preserve fragments" of ice-age ice [1], so the old section may be patchy.
Thompson cares about the earlier mid-Holocene reading [8] because of what it would mean for the ice sheets. "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," he said. "But evidence shows that they did." [9] The paper, in Science Advances, is titled "Additional evidence supports a timescale exceeding 100,000 years for the Guliya ice cap, Tibetan Plateau" [11].
He also compared Guliya with other mountain archives. "Records out of Huascarán only go back over 30,000 years, not over 100,000, so that makes Guliya a unique record," Thompson said [7]. On those figures, Guliya reaches about four times further back [3]. The report says the cap's topography likely helps explain why such old ice survived there [10].
The team plans to use future samples to test how other climate and environmental indicators look under the revised timescale [13]. "The big picture is that our results tell a very consistent story," Thompson 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." [14]
What to watch
- A second radiometric marker found deeper in the Guliya ice, which would test the 128,000-year estimate independently of the cave records.
- The team's planned measurements of other climate and environmental indicators on future samples, read against the revised timescale.
- Whether reconstructions built on the Holocene-only Guliya chronology are revised to use the older timescale.