铀系和铀铅法联合测定洞穴堆积物年龄：以盘县大洞旧石器遗址为例

U-Series dating of carbonate is an important method to accurately determine the age of Paleolithic cave sites within 500,000 years. Because of the complexity and diversity of the cave natural system, carbonate genesis by changes in geological and climatic conditions in some key speleothem strata may be complicated, and such carbonates may also contain much terrestrial debris. Under these conditions, using traditional U-Series research methods may not provide accurate ages. This project will therefore combine the U-Series method with U-Pb dating of calcite from carbonates to increase the geochronological precision, and will constitute a breakthrough in U-Series dating. The Panxian Dadong Paleolithic site has been chosen as a research object, and samples of flowstone at the bottom of speleothem strata will be examined. Using scanning electron microscopy, Raman spectroscopy, an electron microprobe, energy-dispersive spectroscopy, SHRIMP and Quadrupole-ICP-MS will be employed to observe the morphology, structure and primary-secondary relationship of carbonate in flowstones and to measure the concentrations of major and trace elements as well as oxygen isotopes. Based on these methods, carbonate areas in these samples with pure, high U contents may represent primary accumulations. Such samples extracted by micro-drilling will be dated using high spatial resolution and high-precision U-Series dating and laser-ablation U-Pb dating. This project will provide an improved chronological basis for the Dadong Paleolithic Site and will establish a new method in micro-chronological analysis suitable for middle Pleistocene speleothems. The research results of this project will also be of great significance for the study of the ages of China's early Pleistocene human sites and Australopithecus fossil sites.

碳酸盐铀系测年是精确测定五十万年以内旧石器时代遗址年代的重要方法。但洞穴非完美封闭体系，受地质作用和气候条件的影响，某些关键堆积层位碳酸盐常成因复杂，陆源碎屑物较多，用传统方法制样和测试，难以得到精确的年龄。为提高测试精度，突破铀系测年极限，本课题将结合铀铅法，以贵州盘县大洞旧石器遗址为研究对象，采集夹生于该地点下部堆积层的钙板样品，通过扫描电镜、拉曼光谱、电子探针、能谱仪、二次离子探针、四级杆质谱等微区分析仪器，观测钙板中碳酸盐的形态、结构，原生次生关系；测定其主量、微量元素、氧同位素分布特征。据研究结果选取碳酸盐中纯净、高铀并可代表堆积年龄的区域，进行高空间分辨率高精度的微钻取样铀系法测年和激光铀铅法测年，为盘县大洞旧石器遗址提供进一步的年代学依据，并建立一套适用于中更新世洞穴堆积物的微区年代学分析方法。本课题研究成果对我国早更新世人类遗址和古猿化石地点的年代研究也将有重要意义。

盘县大洞地处贵州省六盘水市所属地区，是中国南方最重要的旧石器遗址，为垂向落水洞，发育有地下河，水系丰富，沉积大量洞穴堆积物。自1992年至2000年开展了六次发掘工作，出土了4颗人类牙齿化石，对比研究表明盘县大洞4颗古人类牙齿化石在形态特征上处于直立人和早期智人之间，已表现出早期现代人和现代人的变异特征，代表了早期智人向现代人过渡的类型。本研究采集了近约100件关键层位的洞穴堆积物，通过扫描电镜、拉曼光谱、电子探针、能谱仪、二次离子探针、四级杆质谱等微区分析仪器，观测碳酸盐与陆源碎屑物的形态、结构，原生次生关系；测定其主量、微量元素、氧同位素分布特征，并使用多接收电感耦合等离子体质谱仪湿化学法U-Th与微区U-Pb对上述样品进行了定年。结果表明第三钙板层结果为327.5±22.0Ka, 第四钙板层结果为379.3±7.6Ka, 盘县大洞发掘出土的人牙P3位于VI-VII层，与第三钙板层同层，且第四钙板层位于文化堆积层中，表明盘县大洞存在40万年以上的人类活动遗迹，且洞中保存了数十万年的人类活动记录。盘县大洞的表层钙板年代约为130Ka，底部最老堆积的年代为542±36.5Ka，综合数据，盘县大洞的洞穴堆积物堆积时间开始于约50-60万年前，期间经历过四期大规模堆积事件，并在约13万年前停止。本研究还建立了碳酸盐中陆源碎屑物的快速识别方法并且重新绘制了盘县大洞的平面图。