冰川沉积释光测年:采样策略与测试选择

Luminescence dating of glacial sediments:samplingstrategies and measuring choices

释光测年是可对冰川地貌进行直接定年的一种测年技术,已被广泛应用于冰川沉积测年中,推动了第四纪冰川研究的深入发展。但冰川沉积释光测年还没有达到标准化的程度,实际应用中仍有不少问题需要探究,其中最受关注的是冰川沉积物释光信号晒退不完全的问题,即样品在埋藏前因曝光机会有限导致信号没有归零或仅部分归零。冰川沉积释光信号晒退程度与地貌部位和沉积环境密切相关。冰川沉积释光采样需注意几个方面:(1)详细的地貌学和沉积学调查及对采样点的选择;(2)较适合释光测年的冰水沉积和冰缘风成沉积采集及其与冰川作用期次的联系;(3)冰碛夹层中的冰水砂透镜体的选取;(4)冰碛垄采样时垂直与水平方向上的考量;(5)岩石释光测年的发展使砾石成为当前第四纪冰川释光测年采样的一种选择。室内进行释光等效剂量测试时,也有几个关键的选择:(1)粗颗粒石英光释光测年是末次冰期以来冰川作用的首选方法;(2)如果样品年代老于石英测年上限,或者石英不适合测试,则可考虑钾长石后红外高温释光测年方法;(3)单颗粒、小测片和岩石释光埋藏测年技术可以鉴别样品的晒退情况,是目前最适合冰川沉积释光测年的几种选择;(4)如有条件,尝试用不同矿物、不同粒径、不同方法进行测试对比和交叉检验。要获得第四纪冰川释光测年的最佳年代学结果,地貌学、沉积学和年代学的结合是非常必要的。

As one of the few dating techniques which able to date glacial landforms directly,luminescence dat⁃ing has been widely applied on glacial sediments,which in turn promoted great development of Quaternary glaci⁃ation.However,so far there is not a standardized protocol for glacial sediment luminescence dating,and thus lots of investigations are required in practical application.The most concerned issue is incomplete bleaching or partial bleaching of luminescence signal,i.e.,the signal was not completely zeroed due to insufficient exposure prior to burial.For glacial sediments,the degree of bleaching of luminescence signal is closely related to geo⁃morphological position and sedimentary environment.When sample glacial sediments for luminescence dating,several issues should be paid attention to:1)Detailed geomorphological and sedimentological surveys,and se⁃lection of sampling sites;2)Melt water deposits and periglacial eolian deposits,which are more suitable for lu⁃minescence dating than tills,but their relationship with glaciation has to be considered firstly;3)Selection of glaciofluvial lens in moraines,which are currently considered as the best choice for determining chronology of Quaternary glaciation;4)Vertical and horizontal distance consideration when sampling on moraines;5)thanks to the development of rock luminescence dating,cobble is now one of the options for sampling for dating glacial landforms.There are also several key choices for laboratory luminescence(equivalent dose)measurement:1)Sand size(coarse grain)quartz OSL dating is currently the preferred method for determining chronology of glaci⁃ation since the Last Glacial.2)If the sample is older than the upper dating limit of quartz,or quartz is not suit⁃able for measurement,K-feldspar elevated temperature post-IR infrared stimulated luminescence(post-IR IRSL)would be a good choice.3)Single grain,small aliquot and rock luminescence burial dating technique have the potential to identify the bleaching condition,thus are most recommended options for glacial sediments at present.4)If possible,try to use different minerals,different grain size,different methods for comparison and cross checking.Combination of geomorphology,sedimentology and chronology is strongly suggested to achieve the best luminescence ages of Quaternary glaciation.