摘要
香港是受台风暴潮影响最严重的地区之一。本项研究是根据近岸及浅海地区沉积物的特性来分析和获取全新世时期的台风暴潮信息。根据沉积结构和构造特征，发现有５种类型的沉积层：粉砂质粘土、砂质粒序层、水平或波状层理层、贝屑富集层和含贝屑粉砂层，其中后４种类型与台风暴潮有关。根据沉积物的磁化率分析，上部深厚的高磁化率层可能与台风暴潮对现代受污染沉积物的改造致使新老沉积物的混合所致。有孔虫分析显示，风暴沉积层中含大量外来有孔虫，而且壳体小，壳径分布与沉积物粒度分布很接近。根据沉积物的成分以及有孔虫壳的破碎程度分析，可推断台风暴潮沉积以悬浮方式搬运为主。风暴层的保存主要与沉积速率和生物扰动有关。
Hong Kong, located on the north coast of the South China Sea, is under the influence of tropical storms. In order to examine storm signatures in the Holocene sediments of Hong Kong, offshore cores made for engineering site investigation and sand search were selected for the present study. Instrumental records of storm events since 1884 were also studied to provide an explanation of forcing mechanism for storm sedimentation. Detailed sedimentological study of four offshore cores, VB1,PV3,PV18 and PEW42, located in eastern, central and western waters of Hong Kong, revealed five types of sedimentary beds: ① structureless silty clay bed; ② graded sand bed; ③ horizontal or wavy laminated bed; ④ shell rich bed which can be subdivided into graded shell bed and ungraded shell bed, and ⑤ structureless shelly silt bed. The graded sand beds, laminated beds and shell rich beds are indicative of the dynamic sedimentary environment during storms. Their sedimentological characteristics are similar to modern storm sequences found in other inner shelves. Under normal conditions, the moderate tidal currents are incapable of transporting and enriching coares grained materials such as sand and shells. However, during extreme storms, both currents and waves were greatly enhanced by strong winds with bottom current velocity reaching over several times above normal. It is only under such conditions that coarse sediment can be mobilized. The foraminiferal content in sediments is found to be particularly useful in indicating the presence of storm deposits through a greater abundance of exotic species in comparison to native species. Because of the low specific gravity of the foraminiferal tests, they are significantly exchanged between different sedimentary facies during extreme storms. At least four storm beds have been identified in the upper 7 m of core VB1 based on the increase in diversity of exotic foraminifera species. However, in the eastern waters of Hong Kong which is located in the open shelf foraminifera are not useful for recognition of an individual storm bed. This is because low sedimentation rate and high post depositional reworking in this area makes the storm deposits totally mixed with the normal, leading to a homogenous foraminiferal distribution like in core DEW42. Magnetic susceptibility is also used to assist the identification of sediment mixing caused by storms. Contaminated and uncontaminated sediments are found to have a high and a low magnetic susceptibility respectively. The high content of contaminated sediment down to a depth of about 2 m confirms that mixing through storm is an important process. Because of this, the dating of surficial sea floor sediment down to a depth of about 2 m is problematical. The well preservation of storm beds in the western waters of Hong Kong is explained by the low rates of bioturbation and the high rate of sedimentation due to the influence of the Zhujiang River. In the eastern waters, the poor preservation of storm beds in core DEW42 is explained by low rate of sedimentation and high rate of bioturbation.
出处
《地理学报》
EI
CSSCI
CSCD
北大核心
1998年第3期216-227,共12页
Acta Geographica Sinica
基金
国家自然科学基金
关键词
香港
全新世
沉积物
风暴潮
Holocene environment, storm sedimentation, Hong Kong waters, offshore sedimentology
