Ancient Ice on the Roof of the World
AIce cores are among the most valuable records of the Earth's past climate. When snow falls on a glacier year after year, it is slowly pressed into ice, and each layer keeps a trace of the conditions at the time it formed. By drilling down through an ice cap and lifting out long cylinders of ice, scientists can read this history in much the same way that geologists read layers of rock. Most of the oldest ice cores have come from Antarctica and Greenland, but glaciers high in the mountains of other regions also contain important records.
BOne such glacier lies on the Guliya Plateau in north-western Tibet. A team led by Lonnie Thompson, an earth scientist at the Byrd Polar Research Center of Ohio State University in the United States, has now reported that some of the ice there may be more than 128,000 years old. If this is correct, it extends the known glacial history of the region by over 100,000 years, and it makes these the oldest ice cores ever recovered from this part of Tibet. The study, which was funded by the US National Science Foundation, was published in the journal Science Advances. Researchers from Switzerland and China also took part in the work.
CThe age of the Guliya ice has been a matter of debate for some time. Earlier research had suggested that the record went back only as far as the middle of the Holocene, the relatively warm period that began almost 12,000 years ago. The new work indicates that at least part of the ice is much older than this. According to Thompson, the question is about more than a single number. If Tibet held only ice that had formed during the Holocene, it would mean that the great ice masses of the last ice age had disappeared by the early part of that period. The evidence from Guliya, he says, shows that they survived.
DTo build a reliable timescale, the researchers combined several kinds of evidence. First, they compared two sets of cores that had been drilled from the ice cap 23 years apart, in 1992 and 2015. Both showed remarkably similar patterns of oxygen isotopes, a chemical signal that reflects the climate at the time the snow fell. Thompson believes that being able to reproduce a record in this way is essential. Matching cores that were taken a quarter of a century apart, he explains, can reveal a great deal about how the ice has behaved over time.
ESecond, the team measured radioactive forms of two elements, beryllium and chlorine. These allowed them to identify a well-known event called the Laschamp geomagnetic excursion, a disturbance in the Earth's magnetic field that took place more than 41,000 years ago. The disturbance left a distinctive chemical mark in the ice, giving the scientists a fixed point in time against which other layers could be compared. Finally, they compared the Guliya record with oxygen isotope data from cave deposits in nearby parts of Tibet, which helped them to push the timeline further back.
FThese results make Guliya unusual among mountain glaciers. Thompson says it is the only mountaintop ice core outside the polar regions that is known to reach back this far. By comparison, the ice cores from Huascarán, a mountain in Peru, cover only around 30,000 years. The researchers also think that the shape of the local landscape may help the ice cap to preserve such old layers.
GFor climate scientists, an accurate timeline is the key to answering larger questions, such as when periods of intense glaciation in the region began and ended. Knowing exactly when ice formed, and when it was lost, depends on having dates that can be trusted. "Our results tell a very consistent story," Thompson says. The team now plans to study further samples to see how other indicators of past climate and environment fit into the longer record.