关键词: 中国南海, 大陆架, 更新世晚期, 磁化率, 古环境

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