近4万年渤海西岸海侵时古海面的现代标高对比研究

COMPARISON STUDY OF ELEVATIONS OF ANCIENT SEA LEVEL DURING TRANSGRESSIONS OF THE WEST COAST OF BOHAI SEA IN THE PAST 40ka

文章基于众多钻孔海相层中有孔虫、介形类等海相微体生物化石的组合特征恢复海水深度,重建了海侵最大时的古海面的现代标高。结果显示:40~28kaB.P.海侵,海面的现代标高最高可达-11^-5m;10~4kaB.P.海侵则为2~3m。后者同众多研究所认为的中全新世存在高海面,海面高度为2~3m的结论大致吻合,前者则与全球气候尚处在间冰阶,冰川部分消融,世界洋面处在-50m的大背景不协调。而辽东与山东半岛沿海众多钻孔揭示,40~28kaB.P.渤海地区并没有高于-50^-20m海面存在的证据。通过区域环境的综合分析,认为40~28kaB.P.渤海西岸的海侵,是早玉木冰期持续4~5万年之久的冰期低海面环境,这种特殊的环境使现代渤海西岸的大部分区域远离沉积环境,成为冲刷侵蚀区,这种效应叠加在冰期边缘海式构造下沉与弧后盆地性质的构造下沉背景之上造成区域性异常地面低洼;渤海西岸异常地面低洼在间冰阶全球趋暖,冰川型海侵的过程中形成的区域性强烈“视”海侵(指示当时海侵时海水深度很大,而不是海水的陆泛范围大)。

Marine foraminifera and ostracoda are not only significant for identifying marine sediments but also used to estimate the depth of sea water quantitatively. Since 40kaB. P., corresponding to the warm and humid stages with summer high solar radiation of precessional cycle, the present west coast area of the Bohai Sea has experienced two sea water encroachments： the 40 ～ 28kaB. P. and 10 ～ 4kaB. P. transgression events. Based on sea water depth calculated by combined characteristics of marine microbiological fossils, such as foraminifera, ostracoda, etc., from many drilling cores, elevations of sea level relative to modern sea level during transgression climax are reconstructed. The result shows that the sea level elevation is - 11 ～ -5m during 40 ～ 28kaB. P. transgression climax and 2 ～ 3m during 10～4kaB. P. The latter is in consistence with many previous researches that there exits a high sea level stage in Holocence with a height of 2 ～ 3 m. The former is out of line with previous results of that time span with the global climate still in the last glacier period, i. e., although probably 30 ～ 50 percent of the ice sheet was melted, the world sea level was up to -40 ～ -50m low. Drilling data from the coasts of Liaodong and Shandong peninsulas reveal that there is no evidence of the existence of sea level higher than - 50 ～ - 20m during 40 ～ 28kaB. P. in the Bohai Sea region, which is largely different from the current result. Integrated analysis of regional environment indicates that the marine transgression on the west coast of the Bohai Sea during 40～28kaB. P. was a global marine transgression for glacier melt in the interstadial stage. At that time, the present west coast of the Bohai Sea was a regional abnormal low-lying erosive-alluvial plain, which was formed by： 1 ） island-arc-behind basin tectonic subsidence of the Bohai Sea which was a far-forth marginal marine basin, 2 ） far-forth marginal marine tectonic subsidence in the ice age with continental direction of the upper mantle masses movement,and especially 3 ）the low sea level in the ice age from ca. 80kaB. P. to ca. 35kaB. P., lasting for 40 ～ 50 thousands years, which made most part of modem west coast of the Bohai Sea far from the depositional environment and became an erosion area. All these factors formed a more intensive ＂ visional＂ regional transgression with a background of global transgression during the global warming interstadial stage.