中国南方上山岗和乌鸦山旧石器旷野遗址光释光测年研究

A lot of Paleolithic open-air sites, including human fossil sites which are very important to understand the evolution and migration of ancient human, have been found and excavated in South China. These sites indicate that South China is a very important region in which ancient human lived and migrated. However, accurate dating of most of such sites have been difficult. Even for the chronologies published for some sites, their reliabilities have been argued. The problem with accurate dating of such sites has hindered archaeologists to further understand the significance and interpretation of the sites. The development of optically stimulated luminescence (OSL) technique has made it to be one of the most important dating methods in Quaternary and archaeology. However, its application to such sites has been difficult. This is because chemical weathering of deposits resulted in the mobilization and migration of some elements, making it difficult to accurately estimate average annual dose during the burial period based on the measured U, Th and K contents of the dating samples. In this project, we plan to investigate the effect of chemical weathering on the annual dose for the samples from the Paleolithic open-air sites of Shangshanggang in Zhejiang province and Wuyashan in Hunan province. Geochemical methods will be employed to study the elemental mobility and radioactive equilibrium caused by chemical weathering. We will try to establish a method to estimate the annual dose for the weathering samples, and resolve the problem with the dating of the Paleolithic open-air sites in South China.

中国南方大量的旧石器洞穴和旷野遗址表明该地区是古人类长期生活和迁徙的重要地区。然而准确测量该地区大多数遗址的年代仍然很困难，尤其是旷野遗址。年代问题困扰考古学家们对这些遗址重要性的进一步认识和更深入的研究。光释光技术现已成为第四纪和旧石器考古测年中的一个非常重要的工具。然而我国南方旷野遗址的光释光测年因化学风化作用而变得比较困难，因为风化作用导致一些元素在样品埋藏期间发生活化迁移，使我们难以根据现在测量的U、Th和K含量准确估算样品在埋藏期间的平均年剂量。为此，本项目准备以浙江上山岗和湖南乌鸦山两个典型的南方旧石器旷野遗址剖面研究为例，研究剖面中易活化迁移元素和不易活化迁移元素在剖面中的分布规律和放射性同位素的分馏和放射性平衡，探讨因风化作用引起的元素活化和迁移对光释光测年中年剂量的影响，尝试建立这类样品的年剂量评估方法，解决类似沉积物的光释光测年和一些南方旷野遗址的年代问题。

中国南方第四纪地表堆积物以红色为特征，因而统称其为红土，其成因和来源一直是第四纪地质研究的重要内容，但其研究受到测年困难的阻碍，尤其是在红土中发现了大量旧石器，使解决红土的测年问题变得越来越急迫。在现有第四纪测年方法中，释光技术可能是解决红土测年最有希望的一个方法，因为释光测年物质是无处不在的石英颗粒，但红土的风化作用可能使其中的放射性元素发生迁移，从而影响剂量率的准确估算。为此，本项目对湖南北部8个旧石器遗址中的红土进行了成因和释光测年研究。对研究区的红土的地貌背景和粒度分析（包括端元分析等）表明，这些红土与中国黄土高原和南方下蜀黄土具有相似的粒度分布和组分（端元EM1 和EM2的众数峰粒径分别为5.6 μm和端元二为11.2 μm），且其分布不受地形限制，因而推测这些红土是风成成因，这一结论有助于理解人类活动的古环境背景。通过测量遗址区红土剖面常量、微量和稀土元素含量、矿物成分和放射性平衡，分析了剖面中化学风化指数变化规律，及化学风化作用引起的元素迁移和，并由此引起的沉积物在埋藏期间剂量率变化，并提出了适用这种环境剂量率变化的一个校正模型，有助于得到准确的光释光年代， 这一模型也有助于解决南方红土的光释光测年问题。对湖南北部8个旧石器遗址中的红土进行了释光测年中的各种方法的探讨，包括石英颗粒的光释光信号（OSL）和热转移光释光（TT-OSL）标准曲线的建立，最后得到了这8个遗址的年代，其文化层年代为1.5—13.9 万年，跨越了氧同位素MIS 2-6阶段，揭示了古人类早期在这一地区活动的历史，填补了古人类在这一地区活动的时间空白。