青藏高原年保玉则山末次冰期中期以来的湖泊演化与古冰川发育

冰蚀湖中的沉积物不仅记录了湖泊演化及气候变化的历史,还间接地反映冰川发育过程。年保玉则山(果洛山)下的希门错是在冰川强烈退缩后形成的一个典型冰蚀湖。从地域分布上讲,该区冰川发育遗迹分为4个部分,即上希门错以上、上希门错和希门错之间、希门错出口附近和希门错以北6～10km处。前人依据冰碛物的暴露年龄和上覆黄土的热释光年代将上述4组冰碛物划分为4次冰进,包括形成于MIS3阶段的尼格曲冰期、形成于末次盛冰期的希门错冰期、以上希门错和希门错之间的冰碛物作为标志的末次冰消期和形成于全新世的新冰期。通过对采自希门错长达12.8m的岩芯研究发现,希门错自约38kaB.P.以来有连续的湖泊沉积,这一地质证据说明在湖泊存在的这段时间内,冰川规模都没有超过现在湖泊所在位置。希门错冰期的形成时代应该至少老于湖泊沉积物底界的年龄,即大于38kaB.P.,而并非末次冰盛期的产物;尼格曲冰期的形成时代则应该更老。文章还初步分析了湖泊沉积物与冰碛物测年结果代表的不同意义和差别及其造成不一致的原因,认为冰碛物的暴露年代和热释光年代均可能代表了冰川退缩后的年代,而不能代表冰川的形成和发育年龄,冰川存在的时间要比通过冰碛物所测的年代更老。进一步深入研究青藏高原典型冰蚀湖的形成时代、演化过程不仅有利于对湖泊气候环境记录的全面认识,也将为冰川形成与演化历史、特别是冰川形成年代和其他测年资料的科学解释提供可靠的依据。

LAKE EVOLUTION AND GLACIATION OF THE NIANBAOYUZE MOUNTAIN IN THE TIBETAN PLATEAU SINCE THE MIDDLE OF THE LAST GLACIAL

Lacustrine sediment of glacial lakes can not only record the history of regional environmental change,but also reflect glacial succession.The Nianbaoyuze Mountains is located in Jiuzhi County,Qinghai Province and Aba County,Sichuan Province in Eastern Tibetan Plateau.The forming time of moraine in this area has been studied extensively using various techniques,including 14C,CRN,OSL etc.The cosmogenic radionuclide(CRN)dating and optically stimulated luminescence(OSL)ages indicate that the Nigequ glaciation was formed during MIS 3 and the Ximen Co glaciation was formed during the Last Glacial Maximum.Yet this result contradicts with record from the Ximen Co core.The Ximen Co,located in the north part of the Nianbaoyuze Mountains,is a glacial lake which is formed after the glaciation retreated intensively in glacial stage.A continuous 12.80m long core at the water depth of 50m from the Ximen Co was obtained in 2004.There are ten AMS ages of the core,three of which at the bottom are converted.The dead carbon effect of the Ximen Co is 1300a.In this article we reconstruct the glacial succession and environmental change based on analysis of organic content,grain size,and magnetic susceptibility.Continuous lake sediment indicates that the lake have existed since 38kaB.P.and the glaciation did not exceed the modern lake during the existence of the lake.The lake sediment also indicates that Ximen Co existed during the LGM,which contradicts with the CRN dating of the Ximen Co glaciation.The reasons of the discrepancy between moraine dating and sediment are discussed.We believe that the CRN and TL datings of the moraine represent the age of ice retreating,not the age of glaciation.We thus conclude that the ages of the Nigequ glaciation and the Ximen Co glaciation are both beyond 38kaB.P.The moraine of LGM is probably in the south of the Ximen Co.The result also shows that many work about the glaciation in the LGM and MIS 3 need to be done further through lake sediments.