稻城冰帽区更新世冰川测年研究

Dating of the Pleistocene Glaciations around the Daocheng Ice Cap

通过ESR和14C测年对典型冰碛物进行直接和间接定年,并结合冰碛地貌形态和位置,确定稻城冰帽区自中更新世以来经历了6次较大规模的冰川前进,依次与MIS 16、MIS 6、MIS 3中期、MIS 2早期、全球意义上的末次盛冰期(MIS 2)和冰后期对应,冰川作用规模基本上逐渐减小.最早、规模最大的冰川作用发生在570kaBP前后,意味着本段高原面在570kaBP之前已进入了冰冻圈,且自那时起,该区经历了较大规模的抬升作用.全新世到来之前,稻城冰帽全部消失了,它是一个随着气候冷暖波动逐渐后退的过程,不是以死冰的方式突然消亡.值得注意的是,末次冰期大间冰阶中期(MIS 3b)的冰川前进规模超过了末次盛冰期(MIS 2),表明末次冰期最盛期在全球范围内的不同步性.其不同步的原因可能是:MIS 3中期,北半球夏季太阳辐射相对较低,但其海陆分布状况能够诱发较强的南亚夏季风,它给以季风降水为主要补给的海洋性冰川区带来较多的降水,结合该时段较低的温度,有利于冰川较大规模前进;末次盛冰期时,气候严寒,但夏季风微弱,降水稀少,冰川平衡线下降程度反而不及MIS 3b.

Plenty of glacial sediments since the earliest glaciation during the mid-Pleistocene are well preserved in the Daocheng Ice Cap area, southeastern Tibetan Plateau, where South Asian monsoon is active. Based on dating results of glacial deposits in this area using Electron Spin Resonance (ESR) and (()^(14)C) techniques, combined with glacial landforms and weathering differences of tills, it is believed that there were six major glaciations in the Daocheng Ice Cap area, corresponding to MIS-16, MIS-6, mid-MIS-3, early MIS-2, global LGM and postglacial period, respectively, with gradually decreasing glaciated extents. The earliest glaciation in this area occurred at about 571.2 ka BP, which means that the southeastern part of the Tibetan Plateau was uplifted above the snow line and began to develop glaciers at least 571.2 ka BP ago, and since then this region has been significantly uplifted. Before the Holocene, the glaciers in this region had ablated away completely and they did retreat gradually with several fluctuations.A noticeable result in this study is that during the last glacial cycle, the glaciers around the Daocheng Ice Cap advanced during the mid-Megainterstadial (MIS-3b), with a considerable extent, larger than that during the global LGM. Other researchers also have drawn similar conclusions about the Quaternary glaciers in the Himalayas. From these results, it is concluded that the glacial maximum during the last glacial period may not have occurred at the same time worldwide, contrary to what has usually been assumed. During the middle MIS-3, summer insolation was lower than that in the early and the late MIS-3, but the difference in insolation between land and sea could still induce a stronger summer monsoon. It brought heavy precipitation to the glaciers in the monsoon regions. Combined with the lower temperature, the heavier precipitation was favorable for most glaciers advancing in MIS-3b. During the Last Glacial Maximum, temperature was extremely low, but the summer monsoon was considerably weaker with little precipitation, which resulted in a depression of ELA less than that during MIS-3. The result further demonstrates that no unified ice sheet occurred in the Tibetan Plateau during the Quaternary.