分析理塘与稻城之间库昭-桑堆处的冰蚀面以及兔儿山北侧宽谷中的羊背石的宇生核素10Be,研究结果表明,库昭-桑堆处的冰蚀面暴露于133.4 ka B.P.,即倒数第二次冰期大约结束于133 ka B.P.,这一时间与深海氧同位素第6期相吻合。兔儿山北侧的冰川作用大约结束于18.5 ka B.P.,即末次冰期大约结束于18.5 ka B.P.,这一时间与深海氧同位素第二期相吻合。由同一地区的两个样品X8、X9得出相同的测年年代,并从样品X6得出的测年年代与野外观测的结论相一致来看,运用宇生核素10Be对冰蚀面的形成年代进行测年是一个行之有效的测年技术,并且还可运用于火山熔岩、断层、侵蚀阶地等其它地貌面的测年,该测年技术具有很大的运用潜力。
With the development of the technique of cosmogenic isotopes dating, the formation age of glacial surface can be calculated now. Its principle is that the cosmogenic isotope concentration in the surface rock is a function of age, erosion rate and uplift rate of plateau. Samples of X8 and X9 are taken from the up-surface of glacial boulder in Kuzhao-Shangdui. Sample of X6 is taken from the up-surface of roche moutonnee in valley of the north of Mountain Tuer. The concentration of the cosmogenic isotope 10Be can be got from measure and calculation. According to field exploration, the rock surface where samples X8 and X9 are taken from is erosed about 9 cm in the exposure age t. It means that the erosion rate is about 9/t (cm·a). Then, the t (133.4 ka) can be calculated. Because having the same geographical environment, the rocke moutonnee has about the same erosion rate with the rock. The t (18.5 ka) can be calculated. The study shows that the glacier boulder was formed in 133.4 ka B.P. and the roche moutonnee was formed in 18.5 ka B.P. It can be concluded that Daocheng ice cap was about current in 133.4 ka B.P. (stage 6 of oxygen isotope of deep sea) and the glaciation in the valley occurred about in 18.5 ka B.P.(stage 2 of oxygen isotope of deep sea). The same result coming from two samples X8 and X9 taking from same glacier boulder, and the result from X6 being in concordance with field exploring show that the cosmogenic isotopes dating technique is a effective method in dating surface of landforms.
[1] Brook J E, Brown E T, Kura, M D, et al. Constraints on age, erosion, and uplift of Neogene glacial deposits in the Transantarctic Mountains determined from in situ cosmogenic 10Be and 26AL[J]. Geology,1995, 23(12): 1063-1066.
[2] Steig E J, Wolfe A P, Miller G H. Wisconsinan refugia and the glacial history of eastern Baffin Island, Arctic Canada:coupled evidence from cosmogenic isotopes and lake sediments[J]. Geology, 1998, 26(9):835-838.
[3] Schafer J M, Ivy-Ochs S, Weiler R, et al. Cosmogenic noble gas srudies in the oldest landscape on earth: surface exposure ages of the Dry Valleys, Antarctic[J]. Earth and Planetary Sci-ence Letter, 1999, 167:215-226.
[4] 徐孝彬,王建, Franoise Yiou, 等.地貌学与第四纪研究的新手段——陆地宇生核素研究[J].地理科学,2002, 22(5):587~591.
[5] Brown E T, Edmond J M, Raisbeck G M, et al. Examination of surface exposure ages of Antarctic moraines using in situ produced 10Be and 26Al[J]. Geochimica et Cosmochimica Acta, 1991, 55:2269-2283.
[6] D Lal. Cosmic ray labeling of erosion surfaces:in situ muclide production rates and erosion models[J]. Earth and Planetary Science Letters, 1991,104: 424-439.
[7] Dunai T J. Scaling factors for production rates of in situ produced cosmogenic nuclides: a critical reevaluation[J]. Earth and Planetary Science Letter, 2000, 176: 157-169.
[8] 周尚哲,李吉均.冰期之青藏高原新研究[J]. 地学前缘,2001,8(1):67~75.
[9] Nishiixumi K, Kohl C P, Arnold J R, et al. Middleton. Cosmic ray produced 10Be and 26Al in Antarctic rocks: exposure and erosion history[J]. Earth and Planetary Science Letter, 1991, 104:440-454.
[10] Brook E, J Nesje A A, Lehman S J, et al. Cosmogenic nuclide exposure ages along a vertical transect in western Norway: Implications for the height of the Fennoscandian ice sheet[J]. Geology, 1996, 24(3):207-210.
[11] Stone J O. Air pressure and cosmogenic isotope production[J]. Journal of Geophysical research,2000,105,B10:23753-23759.
[12] 李吉均,方小敏.青藏高原隆起与环境变化研究[J].科学通报,1998,43(15):1569~1574.
[13] 施雅风,李吉均,李炳元,等.晚新生代青藏高原的隆升与东亚环境变化[J].地理学报,1999,1(3): 5~12.
[14] 施雅风,郑本兴,李世杰,等.青藏高原中东部最大冰期时代高度与气候环境探讨[J].冰川冻土,1995,17(2):97~112.
[15] 吴锡浩,王富葆,安芷生,等. 晚新生代青藏高原隆升的阶段和高度.刘东生,安芷生,吴锡浩(主编).黄土、第四纪地质、全球变化(第3集),北京:科学出版社,1992.1~13.
[16] 潘保田,高红山,李吉均.关于夷平面的科学问题——兼论青藏高原夷平面[J].地理科学,2002, 22(5):520~526.
