Quaternary climate change is characterized by cycles of glaciation and deglaciation, which have significantly contributed to shaping the Earth's surface landforms. The Tibetan Plateau contains the highest number of glaciers outside the Polar Regions. Understanding the initiation of glaciation, its driving forces, and its extent of influence has puzzled scientists for decades. Professor Matthias Kuhle hypothesized that the plateau surface was once covered by an ice sheet during the Marine Isotope Stage 2, and that numerous lakes were formed by glaciation (Kuhle, 1985, 1987). However, this hypothesis was disproved by subsequent studies, which found that only alpine valley glaciers and small, sporadically distributed ice caps were present during the Quaternary period (Shi, 2006). Moraine sediments serve as geological markers for glacial advances. The oldest moraine sediments on the plateau were found at the Kunlun Pass in northwestern Tibet and Mount Shishapangma in the central Himalayas (Figure 1). However, older moraine deposits were dated using ¹⁰Be exposure dating, revealing ages of 561.7 ± 54.8 thousand years ago (kyr) or a simulated age of 835.2 ± 241.0 kyr (Chevalier et al., 2011; Chen et al., 2018). These findings indicate that the moraines are much younger than the final formation chronology (>10 Ma) of the high (>5 km) Tibetan Plateau (Deng et al., 2012; Ding et al., 2022). Thus, seeking original moraine relics and obtaining numerical ages are fundamental for reconstructing the paleo-glacier history of the Tibetan Plateau.

The Yarlung Tsangpo River (~2060 km) is the largest and longest river on the Tibetan Plateau. The Eastern Himalayan Syntaxis is known as Namche Barwa and its highest peak towering at 7,756 meters above sea level (Figure 2). Thick and loose sediment, about 1 km deep, is preserved in the broad Milin Valley, providing an opportunity to investigate the glaciation history of Namche Barwa. Five bedrock-penetrating boreholes were drilled in the Milin Valley. Our findings reveal that fluvial sediment accumulation began over 2.5 Ma, and extensive glacial advances occurred after ~0.75 Ma. Following the initiation of this glacial advance, the Eastern Himalayan Syntaxis experienced rapid exhumation of ~1.3 to 1.6 km, significantly higher than the ~230 m thickness of moraine sediment. This exhumation suggests that rock uplift, rather than glacial damming, played a pivotal role in maintaining the stability of the knickpoints on the southeastern margins of the Tibetan Plateau.

Previous studies indicate a clear trend of coarser grain size in Chinese loess since the Mid-Pleistocene Climatic Transition (MPT), attributed to increased physical weathering in the Tibetan Plateau (e.g. Fang et al., 2020; Ao et al., 2023). Our work revealed that the initiation of extensive glacial advances in the eastern Himalayas occurred at the end of the MPT, significantly enhancing the denudation rate of the Eastern Himalayan Syntaxis. A similar process might have occurred throughout the Tibetan Plateau. Thus, the onset of Mid-Pleistocene glaciation in the Tibetan Plateau may be the primary reason for variations in loess deposition in China.

References

Ao, H., Rohling, E.J., Li, X., Song, Y., Roberts, A.P., Han, Y., Poulsen, C.J., Jonell, T.N., Liebrand, D., Sun, Q., 2023. Northern hemisphere ice sheet expansion intensified Asian aridification and the winter monsoon across the mid-Pleistocene transition. Communications Earth & Environment, 4(1), 36.

Chen, Y., Li, Y., Zhang, M., Cui, Z., Liu, G., 2018. Much late onset of Quaternary glaciations on the Tibetan Plateau: determining the age of the Shishapangma Glaciation using cosmogenic ²⁶Al and ¹⁰Be dating. Science bulletin, 63(5), 306-313.

Chevalier, M., Hilley, G., Tapponnier, P., Van Der Woerd, J., Liu-Zeng, J., Finkel, R.C., Ryerson, F.J., Li, H., Liu, X., 2011. Constraints on the late Quaternary glaciations in Tibet from cosmogenic exposure ages of moraine surfaces. Quaternary Science Reviews, 30(5-6), 528-554.

Deng, T., Li, Q., Tseng, Z. J., Takeuchi, G. T., Wang, Y., Xie, G., Wang, S., Hou, S., Wang, X., 2012. Locomotive implication of a Pliocene three-toed horse skeleton from Tibet and its paleo-altimetry significance. Proceedings of the National Academy of Sciences, 109(19), 7374-7378.

Ding, L., Kapp, P., Cai, F., Garzione, C. N., Xiong, Z., Wang, H., & Wang, C., 2022. Timing and mechanisms of Tibetan Plateau uplift. Nature Reviews Earth & Environment, 3(10), 652-667.

Fang, X., An, Z., Clemens, S. C., Zan, J., Shi, Z., Yang, S., Han, W., 2020. The 3.6-Ma aridity and westerlies history over midlatitude Asia linked with global climatic cooling. Proceedings of the National Academy of Sciences, 117(40), 24729-24734.

Kuhle, M., 1985. Glaciation research in the Himalayas: a new ice age theory. Universitas, 27(1), 281.

Kuhle, M., 1987. Subtropical mountain-and highland-glaciation as ice age triggers and the waning of the glacial periods in the Pleistocene. GeoJournal, 393-421.

Shi, Y.F., 2006. The Quaternary Glaciations and Environmental Variations in China. Hebei Press of Science & Technology, Xi’an.