晚第四纪湖泊演化光释光测年

Luminescence dating of Late Quaternary lake-levels in Northern China

在过去的20年里，光释光（optically stimulated luminescence，简称OSL）测年技术发展迅速，已经被广泛应用于第四纪沉积物的测年研究中。近年来，以OSL技术为主要定年手段，利用其测年范围大、测年材料（石英和长石）丰富且易于提取等优势，在我国干旱/半干旱区（腾格里沙漠古湖、内蒙古黄旗海等湖泊）开展了一系列晚第四纪湖泊演化和古气候环境变化的研究工作。高水位湖相阶地的OSL测年结果表明，晚更新世高湖面事件发生时间在MIS 5阶段甚至更老，对基于14C数据建立的“MIS 3a大湖期”假说的年代框架提出了质疑；通过东北兴凯湖晚第四纪连续湖泊沉积物的高分辨率OSL和14C测年对比研究，从年代学机理上揭示晚更新世高湖面的年代学争论主要源自14C技术测定较老样品时的局限性，14C测年手段及易造成老于30000年的沉积物年龄的低估；来自青藏高原东北缘的秦王川盆地和东北地区兴凯湖湖岸的风成沉积也记录了MIS 3a阶段可能不存在气候湿润和高湖面事件；另外，通过对白碱湖古湖岸堤的OSL测年，建立了湖面波动过程和全新世区域湿度变化历史，揭示了季风边缘区早中全新世湿润期和中晚全新世以来逐渐干旱化的气候环境变化过程。综上所述，相较于传统的14C技术，OSL测年技术在湖泊沉积物的应用中具有诸多优势和潜力。尽管如此，OSL测年方法在实际应用过程中会遭遇一些会影响测年结果的潜在的技术问题。因而本文也从石英组分的长石污染、沉积物沉积前OSL信号的曝光程度、铀系不平衡和含水量以及宇宙射线对剂量率的动态影响等多方面，综述了水成沉积释光研究中面临的具体问题，并提出了初步解决方案和建议。

During the last two decades, the optically stimulated luminescence（OSL） dating techniques have been significantly developed and extensively applied to a variety of Quaternary sediments.This method has much wider dating range（up to 300ka） compared with commonly used 14C dating technique, and the great advantage of OSL dating over other methods is the direct association of the depositional event, and its dating material mainly includes quartz and feldspar minerals, which are very easily to extract from sediments.Recently, we have carried out a series of studies on Late Quaternary lake-levels reconstruction using luminescence dating of paleoshoreline and terrace sediments from several lakes （e.g., Baijianhu Lake from the Tengger Desert and Huangqihai Lake from the Inner Mongolia, etc.）in arid and semiarid China.The results can be summarized into：（1）The highstands from Baijianhu Lake and Huangqihai Lake were optically dated to the marine isotope stage 5 （MIS 5） or even older.This is obviously challenging the radiocarbon-based hypothesis of ＂MIS 3a mega-lake period＂, which had been reiterated for many years and widely reported across Northern and Western China; （2）In order to explore the reason why the chronology of the Late Pleistocene high lake-level event according to luminescence dating is so different from that derived from 14C dating, a Late Quaternary sediment sequence from Xingkai Lake was chosen for a combined OSL and 14C study, suggesting that the radiocarbon dating technique may significantly underestimate the age of sediments for samples older than 30ka; （3）In addition, a couple of aeolian sand records from Qinwangchuan Basin and Xingkai Lake also do not support the paleoclimatic interpretation of the ＂MIS 3a mega-lake period＂ hypotheses, which claimed very humid environment during the late MIS 3; （4）Regarding the Holocene period, OSL dating of shoreline sediments around Baijian Lake suggested a lake highstand during 8~5ka, and a shrinking process since 5ka.This may indicate that the wettest stage of the Holocene occurred in the Mid-Holocene and a drying trend appeared during the Late Holocene in the East Asian monsoon marginal area.In the last part of this paper, we listed a few challenges and problems which we have to face for luminescence dating application in lake sediments （such as impact of feldspar-contaminated quartz on De measurement, evaluation on bleaching conditions of fine grained materials, impacts of uranium disequilibrium, water content and cosmic contribution on dose rate calculation）, as well as their corresponding solutions and recommendations.