位于陆地与海洋交界的海岸系统,对环境变化反应敏感,海岸沉积是记录区域及全球环境变化的重要载体.但海岸沉积动力环境复杂,多存在沉积物的侵蚀、搬运与再沉积现象,或缺乏14C测年所需的有机质材料,因此年代问题是海岸第四纪地层研究的难点.光释光测年技术(OSL)主要通过石英或长石矿物的释光信号,确定沉积物的埋藏年龄,测年范围从近百年到几十万年.OSL的快速发展为海岸第四纪地层年代确定提供了有利条件,特别是单片再生剂量法的提出,提高了光释光测年结果的准确度和精度.OSL测年需要根据样品的年龄与性质,进行测年矿物、测量程序条件及粒级的选择.文章基于近年来石英和长石光释光测年技术的发展,结合我国海岸第四纪地层断代中已发表的500多个光释光年代数据,探讨光释光测年技术在海岸第四纪沉积物定年中遇到的常见问题及应对策略,包括测年矿物的选择(石英vs.长石)、样品的晒退、剂量率、石英信号的组分、长石包裹体等.对这些问题的认识有助于对光释光测年结果准确度的评估,可以更好地服务于海岸晚第四纪沉积研究.%The coastal system,lying at the interface between the land and the ocean,is sensitive to environmental changes.Coastal deposits are archives of regional and global environment changes.However,due to the complex sedimentary processes,such as sediment reworking and redeposition,and lack of suitable material for 14C dating,accurate age determination remains a challenge for Quaternary stratigraphic study in coastal system.Optically stimulated luminescence(OSL) dating,which normally uses quartz and feldspar as the dating minerals,can be applied to obtain depositional age or burial age of the sediments in a variety of sedimentary environments.The technique provides a wide age range from nearly a hundred years to several hundred thousand years.Recent development in the luminescence methodology has led to great improvement in the applicability and precision of luminescence dating,especially the improved single-aliquot regenerative-dose(SAR) protocol for OSL,and its rapid development has offered an invaluable technique for dating late Quaternary coastal deposits.Here we compiled more than forty papers published since 2002 in China,twenty-seven of which are written in English and contain more than five hundred OSL data.From the statistical analysis,we can found that OSL dating technique is mainly applied to the Huanghe(Yellow)/Changjiang(Yangtze)/Zhujiang(Pearl) River deltas and their adjacent continental shelfs,and the coastal plains on both side of Taiwan Strait.Based on a review of recent progress of OSL dating technique and its application in coastal regions of China,this paper discusses the common issues and coping strategies in establishing reliable chronology for coastal sediments using OSL method,including selection of dating minerals (quartz vs.feldspar),incomplete bleaching,dose rate,components of quartz OSL signal and feldspar inclusion.An understanding of these issues is critical to the assessment of OSL dating accuracy,which would provide good support to Quaternary environment change study in coastal region.It has been found that OSL technique is mainly applied to sediments deposited during the second half of Late Pleistocene and Holocene in China,and quartz is the main choice for measurement.Quartz from different coastal areas show different bleaching characterizes,which may be connected with various sediment provenance and transport history in the coastal zones.Through the investigation of the samples from the Yangtze River delta,we found that medium-grained quartz samples generally appeared to be well bleached than coarse-grained quartz.Incomplete bleaching,dose rate,purity of quartz,different grain-size fractions (quartz or feldspar),different protocols can affect the accuracy of OSL dating results.Therefore,it is needed to choose suitable grain-size fractions,minerals,and protocols according to the characteristics of the sediment.Meanwhile,when analyzing the data,we should consider the possibility of incomplete bleaching,dose rate and characteristics of quartz OSL signal components to obtain accurate ages for the samples.Quartz is normally an ideal choice for dating relative young sample.However,there has been an apparent age limit around 70～80 ka,i.e.samples from obviously older layers tend to yield luminescence ages around 70～80 ka.Feldspar should be the first choice for determining the age of relative old samples.At present,there are limited studies on feldspar OSL dating for old (＞70～ 80 ka)coastal sediments in China.A few relative old OSL ages were mainly obtained using the traditional quartz OSL signal and feldspar infrared stimulated luminescence (IRSL)signal without anomalous fading correction,which may underestimate the OSL ages.According to the above considerations,an integrated study of various dating techniques(including the internal comparison of OSL results obtained from different particle-size fractions,different minerals and different protocols),as well as the consideration of stratigraphic sequence and geomorphological evolution,will improve the reliability of dating results.
展开▼
