Study On Quaternary Glacial Evolution In The Southeastern Tibet And Cosmogenic Nuclides Dating

The southeastern Tibet, a part of the southeastern Qinghai-Tibetan Plateau, is situated in the junction area of the Nyainqentanglha (north), Hengduan (east) and Himalayas (west) Mountains, where numerous monsoonal maritime glaciers develop. In this region, because there is abundant precipitation for glacier development, glaciers are sensitive to temperature changes. Consequently, glacial extent changes most dramatically during the alternation of glacial and interglacial periods, such as the last glacial period. In addition, the glaciers in this region are characterized by high temperature, high accumulation, fast flow, and intensive erosion ability, so abundant, huge, typical and identifiable glacial sediments are remained, especially, multiple sets of moraines since the Last Glaication present. Guxiang and Baiyu Glaciations according to these glacial landforms and sediments are two previously recognized local glaciations of the Tibetan Plateau. They have been widely used as the reference standard for classifying Late Quaternary glaciations on the Tibetan Plateau and its surrounding mountains. However, the numerical chronologies of both glaciations have been lacking. If the glacial relics are determined, we will not only reconstruct the Quaternary glacial evolution sequence, but also study on the difference between the glacial evolutive model since late Pleistocene on the Qinghai-Tibetan Plateau and global climate change.Based on the field investigation of Quaternary glacial relics in Boduizangbu basin, Yigongzangbu basin and Ranwu area, we determined the Quaternary Glaciations sequence with cosmogenic ¹⁰Be, ESR and ¹⁴C technique, and caculated the depression value of ELA during glaciation with AAR methods, futher discussed on the implications climatic changes. In addition, according to published literatures, the author analyzed the timing of glaciations between the hemispheres and the reason of MIS-3 glacial advance on the Qinghai-Tibetan Plateau. The main conclusions are as follows:1. The surface boulders deposited by the glaciers of the Guxiang Glaciation have exposure ages of 112.9±16.7～136.5±15.8 ka BP, which are likely to represent the timing of glacier retreat during the later part of the Guxiang Glaciation, so Guxiang Glaciation should correspond to marine isotope stages 6, so far it is the oldest glaciation found in the southern Tibet. At that time, the paleo-glacier was 6～10 times larger than the present areas, extending more than 100 km long.2. The surface boulders deposited by the glaciers of the Late Baiyu Glaciation have exposure ages of 11.1±1.9～18.5±2.2 ka BP, which are likely to represent the timing of glacier retreat during the post-Last Glacial Maximum, so Late Baiyu Glaciation should correspond to marine isotope stages 2. At that time, the paleo-glacier was 4～7 times larger than the present areas, extending more than 80 km long. The hummocky moraines are dated to ESR 12.3±1.2 ka BP, so it likely correspond to the Boiling or Allerod warming event, the glacial area of 230 km² decreased to 166 km² from Linqiong Stage to Yuren Stage.3. We calculated that the snowlines for Guangxiang and Baiyu Glaciations should be 3800 m and 4000 m, 800 m and 600 m lower than present respectively, and evaluated that the annual air temperature during Guxiang and Baiyu Glaciation should be 6.6℃and 7.8℃below than present. While the snowline raised 400 m from Linqiong Stage and Yuren Stage, so the annual temperature rise should be close to 2.4℃.4. By summarizing and analysing cosmogenic nuclides dating data of Quaternary glacial relics in the global (37 sites, 1168 data), we found that glaciations were synchronous between the hemispheres, and were asynchronous between the high-middle and low latitude.5. It had been found that there were 25 sites of MIS-3 glacial advance on the Qinghai-Tibetan Plateau. Based on the hydro-thermal characteristic analysis of the sub-stages of MIS-3, the author considered that only the MIS-3b cold-humid climate could result in glacial advance, glaciers in MIS-3a, 3c hadn't possess the condition of glacial advace or held at their status quo position, as positive mass balance by rising precipitation could not offset the negative contribution by melting water under higher temperature.