COMPARISON STUDY OF ELEVATIONS OF ANCIENT SEA LEVEL DURING TRANSGRESSIONS OF THE WEST COAST OF BOHAI SEA IN THE PAST 40ka
近4万年渤海西岸海侵时古海面的现代标高对比研究*

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 2~3m. 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 modern 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.