Glacier advance on the Qinghai-Xizang Plateau during MIS 4,evidence from terrestrial cosmogenic nuclide exposure dating using 10Be
Geomorphological and stratigraphic evidence,as well as absolute dating results,show that the Qing-hai-Xizang Plateau and its surrounding mountains,which currently hold the most widely distributed area of mod-ern glaciers on Earth (excluding the polar regions),experienced several significant glacier advances and retreats during the Quaternary. Multiple paleoclimatic proxies reveal that global average temperature decreased,conti-nental ice volume increased,and sea level decreased during marine oxygen isotope stage 4 (MIS 4),which was an ice stage of the last glacial period. Sporadic dating results suggest that individual locations on the Qinghai-Xizang Plateau have experienced glacier advances during MIS 4. In this article,the term "MIS 4 glacier ad-vance" refers to the glacier advance that has been dated to MIS 4 based on boulder surface exposure ages. It is important to note that the glacier advance itself may not have taken place entirely during MIS 4. However,con-sidering that the Qinghai-Xizang Plateau is home to numerous high-altitude mountains,it remains unclear where else MIS 4 glacier advances have occurred across the entire plateau. Therefore,it is also uncertain whether the MIS 4 glaciation was widespread throughout the plateau. Moreover,the scale of the MIS 4 glaciation has not been thoroughly investigated. To address the aforementioned questions,this study examines and reassesses the 10Be surface exposure ages of boulders from the Qinghai-Xizang Plateau during the MIS 4 and neighboring ice stages. The analysis is further refined using the P-CAAT method,with particular emphasis on the spatial distri-bution of MIS 4 moraine ridges,the climate-driven mechanisms behind the glaciation during MIS 4,and the scale of glaciers at various locations across the Qinghai-Xizang Plateau. Through statistical analysis and recalcu-lation,the study identifies twelve groups (i. e.,twelve sites) with 10Be surface exposure ages falling within MIS 4. These groups are predominantly found in the western Qinghai-Xizang Plateau,followed by the southern and central regions of the plateau. The 10Be surface exposure ages of boulders,recalculated using the CRONUS-Earth online exposure age calculator (version 3),were analyzed using the P-CAAT method. Various techniques were utilized to estimate the bandwidth. The formation ages of the moraine ridges were determined by interpret-ing the peak age and its uncertainty from the Gaussian-component with the highest probability height derived from the program run. Combined with the LR04 curve and a range of paleoclimatic proxies,the study's findings suggest that the formation ages of these twelve moraine ridges correspond to the period when glacier retreat commenced. This indicates that these abandoned and steadily deposited moraine ridges were formed during the MIS 4 glacier advance. The formation ages of the moraine ridges in the twelve groups varied to some extent,ranging from slightly younger than 60 ka to nearly 70 ka. In summary,this paper presents the following conclu-sions:(1) During MIS 4,glacier advances took place across various regions of the Qinghai-Xizang Plateau,in-cluding the western,southern,eastern,and inner regions. The largest extent of glaciation was observed in the western region,followed by the southern region. In contrast,the inner region experienced the smallest scale of glaciation,which can be mainly related to the divergent amount of glacier recharge. (2) The MIS 4 glaciation on the Qinghai-Xizang Plateau was primarily influenced by variations in summer solar insolation and atmospher-ic CO2 concentration,and by the migration of mid-latitude westerlies in the Northern Hemisphere. These factors played crucial roles in shaping the dynamics and distribution of glaciers on the plateau. From MIS 5a to the early MIS 4,solar insolation gradually decreased to its minimum value. Consequently,the concentration of CO2 also significantly decreased,resulting in a corresponding decrease in temperature on the Qinghai-Xizang Plateau. Si-multaneously,the temperature gradient between high and low latitudes in the Northern Hemisphere increased,leading to a southward migration of the strengthened westerlies. This shift in wind patterns likely brought an in-creased amount of solid precipitation during the winter months. The combination of lower temperature and in-creased winter precipitation resulted in a greater accumulation of glacier mass,ultimately driving the advance of glaciers. During the middle and late MIS 4,as the summer solar insolation continued to increase and the concen-tration of CO2 rapidly rose,the glaciers on the Qinghai-Xizang Plateau commenced a rapid retreat.
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