地球磁场变化对珠江三角洲万顷沙沉积钻孔年代地层的约束

Dating the core sediments from Wanqingsha area of Pearl River Delta using geomagnetic field variations

珠江三角洲晚更新世海侵沉积的历史,是始于氧同位素三阶段(MIS3)还是五阶段(MIS5)已争议几十年,严重制约了该区域海平面升降对全球变化响应方式的讨论和区域地质演化历史的认识。本文以位于珠江三角洲腹地伶仃洋河口地区万顷沙的GZ-2钻孔(22°42.339′N,113°30.831′E)岩心为研究对象,对沉积物进行岩石磁学和古地磁学研究,提取沉积物中记录的地球磁场极性漂移和相对强度变化信息。在AMS;C和光释光年龄限定之下,将地球磁场相对强度曲线与高分辨率的、具有精确年代控制的标准曲线进行对比,尝试确定晚更新世细粒沉积物的年代框架。结果表明钻孔细粒沉积物发生在约43 ka以来,海侵历史始于MIS3阶段。这一认识表明中国东南沿海可能普遍存在MIS3阶段的海侵,其控制因素在于在该时期构造沉降和全球相对海平面上升时,海水沿部分深切河谷入侵,形成局部海侵沉积层。

There is a great argue about the ages of the late Pleistocene sediments in the Pearl River delta.The significant uncertainties,the transgression initiated during MIS3(Marine Oxygen Isotope Stage 3)or MIS5,restrained discussing the corresponding pattern of the sea level to global change and evolution history of the delta.In this article,the core GZ-2(22°42.339′N,113°30.831′E)near the Lingdingyang estuary,located on the Wanqingsha area of the delta,were selected to reconstruct the geomagnetic records based on the study of the sediments magnetic properties.The rock magnetic experiments illustrate that the fine mag‐netite contributes the main changes of the nature remanent magnetization of the sediments.The reliable inclination and relative paleointensity(RPI)of the geomagnetic field can be reconstructed from the sam‐ples.Comparing the relative intensity curve(PRI=NRM_((20-40)mT)/ARM_((20-40)mT))constrained by the AMS^(14)C and Optically Stimulated Luminescence(OSL)ages,with the standard high-resolution intensity proxies from the GLOPIS-75 and the north Atlantic ocean,the chronology the sediments can be recovered.The results show the age of the fine sediments is younger than~43 ka,and the transgression initiate at the MIS3 stage.The marine sediments during the MIS3 stage in the Pearl River delta as well as in the southeast coastal areas of China may be raised from the tectonic subsidence dominated the relative sea level rising,which intrude along the river valley and formed the marine sediments.