| The study of Quaternary climate change in the Tibetan Plateau is the key to understand the Northern hemisphere and global environment variations. The Quaternary glaciations are the most direct and convincing evidence of climate fluctuations. Accurate chronology of these paleo-glaciations on the Tibetan Plateau is highly demanded. Since the1980s, development of terrestrial in situ cosmogenic nuclide (TCN) exposure method has provided a robust technique to date glacial landforms. The temperate paleo-Daocheng ice cap, located in the southeast Tibetan Plateau, is sensitive to climate change. Reconstruction of its evolution is of great significance to reveal the period when Tibetan Plateau achieved its elevation into cryosphere and understand the coupling mechanism between Tibet uplift and climate change. The well preserved glacial remnants (including the oldest moraines in the Hengduan Mountains), as well, provide a natural laboratory to develop TCN method and other methods in the field of dating glacial landforms. Although there has been a lot of work in this area, consensus has not reached on the time of the maximum glaciation and whether glacier advance during MIS3. We sampled the glacial debris of the moraine surface, roche moutonnee and polished surface and reconstructed the glaciation history of paleo-Daocheng ice cap using the methods of TCN10Be and26A1exposure dating and optically stimulated luminescence (OSL). The main conclusions are as follows:Firstly, The TCN10Be and26A1exposure ages of glacial remnants of paleo-Daocheng ice cap show that there were at least five major glaciations in this region. The results are sequentially:765.8±167.4ka-477.3±123.7ka,187.5±23.6-97.4±10.6ka,49.4±4.7ka and39.2±3.8ka,25.7±2.4ka and19.3±1.8ka-12.8■±1.2ka, corresponding to MIS-16-12, MIS6, MIS3, MIS2(including global last glacial maximum and last deglaciation).Secondly,10Be concentrations in the samples (s7-s13) from a vertical profile, collected from a big erratic boulder in Haizishan summit, decrease firstly and then increase from top to bottom. This fact is contradicted with the theory that the concentration of cosmogenic nuclide should decrease exponentially with the depth. We suggest the boulder has experienced a re-transportion or rolling over history. Assuming the re-transportion history of the boulder, we evaluate the formation age of the boulder was at about45-46ka, corresponding to MIS3.Thirdly, based on the dating data and the relative geomorphologic and stratigraphic relationship of the moraines, we concluded that the paleo-Daocheng ice cap area entered the cryosphere at0.8-0.6Ma. The extents of subsequent glacier advance were smaller than the previous one. This is because of the uplift of the southeastern part of the Tibetan Plateau, which gradually blocked southwest monsoon water vapor from entering the Hengduan Mountains. The extent of glacier advance during MIS3is larger than that during the traditional LGM in MIS2in this region, which is probably due to the glacier advance is related to abundant precipitation during MIS3. Moreover, there were two obvious climatic fluctuation events at around19ka and15ka during the last deglaciation. The two events may be the response to the global uniformity and are relevant to the19ka meltwater pulse (19ka-MWP) and Boiling warming event at around15ka respectively.Furthermore, the erosion rates of bedrock and exposure time-scale of samples importantly impacted on the application of TCN exposure dating method to glacial landforms. The results indicated that the age for1X10a is underestimated approximately0.5%,1%and2%for erosion rates of0.5,1and2mm/ka, respectively. For age of10X104a, the underestimate is approximately should be5%,7%and20%, and for50X104a approximately40%,70%and even more than100%, respectively. When using TCN dating method to estimate the maximum erosion rate, the maximum erosion rate increases with the decrease of exposure time, which is about1mm/ka in500ka time scale, while the erosion rate in100ka and10ka time scale are about5mm/ka and40mm/ka respectively.Finally, the OSL ages of the moraines in Kuzhaori of the south edge paleo-Daocheng ice cap were not consistent with previously ages and some of the OSL ages do not agree with the stratigraphic relationship of the moraines. The main reasons were the partial bleaching prior to burial (overestimating the true age) and difficulty of purifying quartz (underestimation of the true age causing by anomalous fading of feldspar) for the moraine samples. So, great consideration should be given on these two issues during the future OSL dating. |
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