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New Insights into the Chronology of Tibetan Ice Cores

From September to October of 2015, a 60-person team were gathering on the Guliya ice cap in the Kunlun Mountains of the Tibetan Plateau, with the purpose to retrieve the world's oldest ice. In a FEATURES article published in Science (29/01/2016) entitled "Tibet's Primeval Ice: the quest for the world's oldest ice could yield a Rosetta Stone for how Asia responds to a changing climate", it was written that "in faint layers of dust and gas bubbles and subtle variations in chemicals within the ice, the glistening cylinder holds part of a record of ancient climate on the Tibetan Plateau that could stretch back nearly a million years".

In fact, this is not the first try for retrieving ice cores from the Guliya ice cap. In 1992, a Guliya ice core to bedrock (308.6 m in length) was recovered at an elevation of 6 200 m above sea level. The 1992 Guliya core was featured on the cover of Science published on 6/20/1997 for its extraordinary long-time scale with the following introduction: "……The Guliya ice core to bedrock contains a regional climate history for the entire last glacial cycle, and the basal ice may exceed 500 000 years in age".

The dating results have made the 1992 Guliya core the oldest non-polar core. In comparison, the Vostok ice core (3 623 m in length) from the central East Antarctica yielded a climate history of past 420 000 years, and the EPICA Dome C ice core (3 260 m in length) also from the central East Antarctica yielded so far the longest ice core record of 800 000 years.

The Guliya ice core temperature record has been extensively used as a paleoclimatic reference since its publication in 1997. However, its chronology was recently challenged by several independent studies. Cheng et al. argued that the chronology of the upper 266 m of the Guliya ice core needed to be compressed by half in order to reconcile the stable isotopic profiles of the Guliya ice core and the Kesang stalagmite records.

In a recent paper published in 2022, Wang et al. further suggested that the Guliya ice core may have a basal age of ∼70 000 years in order to reconcile the stable isotopic variations between speleothem and Tibetan ice cores.

The radiometric 81Kr dating of several ice samples collected from the edge of the Guliya ice cap yielded upper limits for the age of Guliya ice in the range of 15 000–74 000 years. It's apparent that these dating results were one order of magnitude younger than the original bottom age estimates of the Guliya ice core. Furthermore, Hou et al. compared the stable isotopic profiles of the Guliya and its nearby Chongce ice cores based on their relative depth, and found significant positive correlation between the two records, implying a similar timescale for both ice cores. Located only ~30 km away from the Guliya ice core site, the Chongce ice cores had the estimated bottom age of 9 000 and 8 300 years for the 135.8 m and the 216.6 m cores respectively. Moreover, the first optically stimulated luminescence (OSL) dating of the basal sediment collected from the bottom segment of the Chongce 216.6 m ice core resulted in an age estimation of 42 000 years, providing an upper limit for the age of the Chongce ice cores.

Given the close proximity between the Chongce and Guliya ice core drilling sites and the remarkable similarity between their stable isotopic profiles, the Holocene origin of the Chongce ice cores casts significant doubt on the exceptional length of the Guliya ice core record. Moreover, all the other Tibetan ice cores, including the Dunde and Shulenanshan in the Qilian mountains, Zangser Kangri and Puruogangri in the central Tibetan Plateau, and Dasuopu and East Rongbuk in the Himalayas, were also suggested to be of Holocene origin.

Dating alpine ice cores is always a challenging task. Usually, layer counting based on seasonal signals in the physical and chemical properties of the ice core is only reliable for the upper part of the core due to rapid thinning of ice layers in deeper sections.

For the deeper (older) part of the ice cores, absolute dating methods, usually based on the decay of radioactive elements preserved in the ice cores, are needed to establish the core chronology. Therefore, the new Guliya ice cores drilled in 2015 provide a valuable opportunity to directly verify the chronology of the 1992 Guliya ice core, particularly with the help of modern dating techniques such as 14C, 36Cl, 40Ar and 81Kr. Such an effort is extremely necessary and urgent not only for establishing accurate chronologies of Tibetan ice cores, but also for developing reliable reconstructions of past climate and testing important climate concepts and hypotheses, such as asynchronous glaciation and Holocene temperature conundrum on the Tibetan Plateau.

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