Catch single atoms in a "trap" to measure the age of ice cores

The scientific expedition team drilled ice cores in the Qiangtang Glacier on the Qinghai-Tibet Plateau. Image provided by Polar Future

What kind of sparks will be created when a 109-meter ice core drilled at an altitude of 5,900 meters on the Qinghai-Tibet Plateau meets a new dating method based on quantum precision measurement?

Recently, the single-atom detection team led by Professors Lu Zhengtian and Jiang Wei of the University of Science and Technology of China (USTC) and the glaciology team led by Researcher Tian Lide of Yunnan University (YUNU) published their research results in the Proceedings of the National Academy of Sciences of the United States. The team conducted the first argon-39 isotope dating measurement of ice cores in the world, establishing an accurate chronological scale for thousands of years for the ice cores of the Qiangtang Glacier on the Qinghai-Tibet Plateau.

A 'unique archive' of atmospheric conditions

An ice core is a core obtained from the interior of a glacier by drilling. Generally speaking, the younger the ice core is, the younger it is from the bottom to the top.

"These ice cores record a variety of indicators of climate and environmental changes, which are usually divided into three categories: the first category is the ice itself. The ratio of hydrogen and oxygen isotopes in water molecules can reflect temperature changes; the second category is the atmospheric composition and content in the ice cores, such as greenhouse gases such as carbon dioxide and methane, which can reveal the process of changes in atmospheric composition." Tian Lide introduced that the third category is various impurities contained in the ice cores, such as dust visible to the naked eye, which can be inferred that there were more sandstorm activities at the time; there are also various chemical components detected by laboratory instruments, which can provide relevant information on natural and human activities.

Therefore, ice cores can be called "unique archives" that preserve the atmospheric environment, and mastering the accurate age information of ice cores is the first step in decoding the "archives". my country's Qinghai-Tibet Plateau is known as the world's third pole and is a treasure trove of paleoclimate research in mid- and low-latitudes.

"Unlike the Antarctic and the Arctic, the Qinghai-Tibet Plateau has a large amount of accumulated snow and the ice core resolution is higher. Humans live at its latitude and have many activities. The history recorded in the ice cores is closely related to the human living environment." Tian Lide said that this makes ice core research on the Qinghai-Tibet Plateau particularly important. It is precisely because of this that the Qinghai-Tibet Plateau has attracted scientists from all over the world and has become a "battlefield" for international ice core research.

In May 2014, Tian Lide and his colleagues worked continuously for more than ten nights at the top of the "Qiangtang No. 1" glacier on the Qinghai-Tibet Plateau at an altitude of 5,900 meters (the temperature was high during the day and the melted ice chips could easily jam the drill rig), and successfully drilled two 109-meter-long ice cores through the bottom, which are stored in the Yunnan University cold storage at -17°C all year round.

What age do these two ice cores belong to, what information do they contain, and how to decode this hard-won "archive" are the issues faced by Tian Lide's team. Lu Zhengtian's team at USTC just happens to have the "key" to open this "archive".

Catching Argon-39 in an Atomic Trap

There are three rare radioactive gas isotopes in the atmosphere, namely krypton-85, argon-39, and krypton-81. As early as 1969, Swiss geoscientists Hans Oeschger and Hugo Loosli proposed that argon-39 and other isotopes are ideal dating isotopes for mountain glaciers.

However, it is extremely difficult to detect them. "The abundance of argon-39 isotope is extremely low, as low as one part per billion. And these atoms are mixed in argon atoms that are 17 orders of magnitude more numerous than them." Lu Zhengtian said that the difficulty of detection is like finding a special grain of sand on the beach. Therefore, for nearly half a century, the quantitative analysis of argon-39 in ice cores has been a difficult problem.

In this study, Lu Zhengtian's team used the "Atomic Trap Trace Analysis (ATTA)" method. This method was invented by Lu Zhengtian when he worked at Argonne National Laboratory in the United States in his early years. The principle is to use precisely controlled lasers to manipulate argon-39 atoms and capture them in an "atomic trap" composed of six laser beams. The atoms will emit fluorescence in the trap, and the sensitive EMCCD camera will detect individual argon-39 atoms and "count" them one by one.

Argon-39 has a half-life of 268 years and can be used to date environmental samples from 1,800 to 50 years ago.

Lu Zhengtian took 1 kilogram of modern ice as an example. "It contains about 10,000 argon-39 atoms. After one half-life, the number of argon-39 atoms will be reduced by half to 5,000; after another half-life, it will be reduced by half again to 2,500. As time goes by, the number of argon-39 atoms will become less and less. Therefore, the abundance of argon-39 in the ice core can tell us when the ice was formed, that is, its age."

So, here comes the question: How can the 109-meter-long, 700-kilogram ice core stored in Yunnan University's cold storage be transported to the USTC laboratory for research?

"At that time, my colleague Dr. Florian Ritterbusch went to Professor Tian Lide's laboratory at Yunnan University with a device that looked like a pressure cooker, extracted the gas from the ice core and brought it back to USTC." Jiang Wei said that this was the first step in dating the ice core.

Why is it called a "pressure cooker"? "Because it has good sealing performance, and we actually light a fire under the pot to melt the ice and take out the gas." Jiang Wei said with a smile, "Don't underestimate this 'pressure cooker'. In order to take samples, it has been to the Qinghai-Tibet Plateau, Shanghai, Paris, France, and Seoul, South Korea."

The second step is purification. Jiang Wei explained: "Because the retrieved gas contains various chemical components, we need to react with other gases first, leaving only argon."

Finally, the separated argon gas is placed in an atomic trap trace analyzer to measure the abundance of the argon-39 isotope and calculate the age of the sample.

In this study, the team from USTC and Yunnan University used argon-39 dating method and finally obtained the age distribution of the entire ice core, with the age of the bottom reaching 1,300 years.

"The latest collaborative results have confirmed for the first time the huge potential of argon-39 in the absolute dating research of millennium ice cores." Tian Lide said that argon-39 dating technology can also be used to date other ice cores on the Qinghai-Tibet Plateau, solving the problem of absolute dating of ice cores that earth scientists have been unable to overcome for many years.

Plans to establish a global isotope detection center

In the Laser Trace Detection and Precision Measurement Laboratory of USTC, the optical platform of the atomic trap trace analysis instrument is filled with a dazzling array of optical components.

Jiang Wei introduced: "These optical components have fixed positions and are not placed randomly. Students spent several months installing and debugging them so that the complex optical path can produce lasers of a specific frequency for capturing and detecting argon-39 atoms."

Moreover, the efficiency of the latest instruments has been greatly improved. "In 2010, we conducted an experiment in the United States using naturally abundant argon gas, and at that time, we could only see one argon-39 atom in five hours," said Lu Zhengtian. Now at USTC, using the latest instruments to measure the same abundance of argon gas, we can detect 10 argon-39 atoms per hour, a counting rate that is about 50 times higher than that of the time.

In this study, the researchers also compared the argon-39 dating results with the ice core age scale constructed based on the several-year layer method, corrected it, constrained the glacier flow model, and finally established a new ice core age scale based on the argon-39 results.

"This article will arouse widespread interest among ice core scientists, paleoclimatologists and radioisotope dating experts," said another reviewer.

The team led by Lu Zhengtian and Jiang Wei has long been committed to developing ultra-sensitive detection technology for rare gas isotopes such as krypton-81 and argon-39, and applying it to cutting-edge earth science research. The cutting-edge measurement technology has attracted scientists from home and abroad to collaborate, and has made a series of advances in research fields such as groundwater, glaciers and oceans, demonstrating the role of new technologies in promoting innovative research.

Lu Zhengtian said: "In the next step, the team will continue to develop atomic trap trace analysis instruments and improve the performance of various indicators to make it an indispensable tool in the field of earth science. On the other hand, the team plans to establish an international isotope detection center in Hefei to cooperate with research groups from all over the world, hoping to help Chinese earth scientists achieve major original results and play a leading role in international cooperation."