Abstract:

As the key area of interaction between land and sea, continental shelf is important for the tectonic evolution of continent, sea-land change, sea level eustacy and climate change. Due to the limits of different methods, the understanding of the chronology and potential geological information of the sediments on the continental shelf is not enough. The South China Sea, as the largest marginal sea of the West Pacific, is not only one of the most active areas of marine sedimentation in the world, but also the typical region of the interaction between land and sea. As the main sedimentary area of the East Asia, the South China Sea has received increasing academic research attention. At present, the researches mostly focus on the deep-sea sediments because they are continuous and can record stable signals, even though the relative slow deposition and low resolution. Comparatively, the shallow continental shelf deposits with faster sedimentary rate and higher resolution can provide important geological materials for studying the high-resolution chronology and paleoenvironment. However, the sedimentary signals recorded by the continental shelf sediments are unstable and even missing due to the turbulence of the sedimentary environment of the continental shelf. There are relatively few studies on the continental shelf sediments of the South China Sea, especially the high-resolution chronology of cores, thus limiting the understanding of tectonic and climate evolution of the South China Sea. In order to better constrain the geological chronology of the Late Pleistocene continental shelf sediments in northern South China Sea, study the paleoenvironmental signals in the continental shelf sediments and discuss the driving mechanism of the climate changes in East Asia and provide the chronological framework for the study of marine active tectonics in the South China Sea, the comparison between magnetic susceptibility and Marine Oxygen Isotope based on microscopic paleonotological fossils and carbon isotopic age(¹⁴C)was studied on the Core DG in this paper. Additionally, the results of sediments color and pollens were used to study the paleoclimatic implications. The results of magnetic susceptibility suggest that the chronology of the sediments of Core DG can be constrained from MIS 1 to MIS 9, with the age of the bottom being about 300ka. The relative high and low values of magnetic susceptibility correspond to interglacial and glacial periods, respectively. This is consistent with the paleoclimatic signals evidenced by the changes of pollen and color parameters in the DG core sediments. Therefore, we suggest that the magnetic susceptibility of continental shelf sediments can be affected by the changes of climate. During glacial periods, the relative cold weather, shallow water and increased transportation distance of the sediments resulted in the enhanced oxidation and the formation of minerals with weak magnetic susceptibility(such as hematite), thus the magnetic susceptibility decreased and the redness increased in the sediments. However, during interglacial periods, the relative warm and wet climate, together with the decreased transportation distance of the sediments, led to the formation of minerals with strong magnetic susceptibility(such as magnetite), thus the magnetic susceptibility enhanced significantly and the redness decreased in the sediments. Therefore, the variations of the magnetic susceptibility in the continental shelf sediments in the northern part of the South China Sea can reflect the glacial-interglacial cycles in the East Asia since the late Pleistocene. In conclusion, as a relative dating method used in the unconsolidated sediments in the late Quaternary, the comparison between magnetic susceptibility and Marine Oxygen Isotope is applicative and reliable in constraining the chronology of the Late Pleistocene continental shelf sediments in northern South China Sea, thus providing a new reference for studying and correlating the continental shelf sediments, which can be used reasonably in the Quaternary chronology.

Key words: South China Sea, continental shelf, late Pleistocene, magnetic susceptibility, paleoclimate

摘要：

大陆架作为海陆相互作用的关键地区, 对于研究大陆的构造演化、 海陆变迁、 海平面升降以及气候变化具有重要意义。然而由于不同研究方法的局限性, 目前对大陆架沉积物年代学及其蕴含地质信息的认识仍然不足。南海是西太平洋最大的边缘海, 是全球海洋沉积作用最为活跃的地区之一, 也是海陆相互作用最为典型的区域。作为东亚大陆物质的主要沉积区, 南海已经受到了学术界越来越多的关注。然而, 目前的研究工作主要集中于沉积连续、 信号记录稳定但沉积速率较慢、 总体分辨率较低的深海区沉积物。相对而言, 沉积速率较快、 分辨率较高的浅海大陆架沉积为高分辨率年代学和古环境的研究提供了重要的地质材料, 但由于大陆架沉积环境动荡导致沉积信号记录不稳定甚至缺失。针对南海大陆架沉积, 尤其是对钻孔沉积物高分辨率年代学研究仍相对较少, 限制了对南海构造与气候演化过程的认识。为了更好地限定南海北部陆架区更新世晚期沉积物的年代, 研究其中蕴含的古环境信息, 探讨东亚地区气候变化的驱动机制问题, 同时为南海海域活动构造研究提供年代学框架, 文中以南海北部DG钻孔为研究对象, 在微体古生物化石和碳同位素年龄(¹⁴C)数据的基础上, 利用大陆架沉积物磁化率与深海氧同位素的对比对其沉积物年代学进行了系统研究。基于此, 结合色度和孢粉结果, 对其古气候意义进行了初步探讨。结果表明, 该钻孔沉积物的磁化率可对应于深海氧同位素的阶段1—阶段9(MIS 1—MIS 9), 底部年龄约为300ka, 磁化率低值区间对应于冰期, 高值区间对应于间冰期。这与该钻孔沉积物中的孢粉和色度所记录的古环境信息相吻合。冰期时气候较为寒冷, 水体变浅, 沉积物搬运距离相对增大, 矿物以氧化作用为主, 主要形成弱磁性的磁性矿物(如赤铁矿), 导致磁化率较低; 间冰期时, 气候相对暖湿, 水体变深, 沉积物搬运距离相对缩短, 矿物以还原作用为主, 主要形成强磁性的磁性矿物(如磁铁矿等), 导致沉积物的磁化率显著增强。因此, 南海大陆架北部更新世晚期沉积物的磁化率变化可以反映东亚地区更新世晚期以来冰期—间冰期气候旋回。磁化率与深海氧同位素的对比作为一种晚第四纪松散沉积物的相对定年方法, 在南海北部陆架区更新世晚期沉积物定年方面是适用且可靠的, 可为海洋大陆架沉积物定年和对比研究提供新的参考。

关键词: 中国南海, 大陆架, 更新世晚期, 磁化率, 古环境