在福建和江苏沿海平原下的第四纪倒数第二海侵层,一般埋深在18~32m以下。研究表明主要是30~40 ka B.P.时期气候-冰川-海面作用下的沉积产物,沉积后期受到了海陆构造沉降、后期沉积物压实以及陆源河流泥沙充填等多重海陆升降效应的改造。为了认识MIS 3晚期海面变化下海侵层特征、后期沉积变化以及复杂动力效应,本文采用气候驱动冰川型海面变化-东黄海地海系统响应多层次模式,对福建和江苏沿海30~40ka B.P.时期的气候驱动的海侵层进行模拟,根据8个钻孔重建的晚更新世海侵层资料对比,分析海侵层沉积后期的构造沉降、沉积物压实、陆源河流泥沙充填等复杂效应用来模拟相对海面变化。模拟结果表明,MIS 3晚期海面变化主要由冰川型控制,该时期在东海和南黄海由气候-冰川驱动的海面高度在-19.2~-22.2m a.s.l.,相对海面高度在-26.3~-29.9m a.s.l.;江苏沿海平原长期沉降和福建沿海断裂带间断性沉降(模拟平均下沉6.7m),同时东黄海第四纪以来持续沉降(模拟平均下沉9.0m)。由于后期的上覆沉积物压实作用,MIS 3晚期海侵层以及上覆沉积物孔隙的压缩,使得沉积层厚度减少,这个作用使海侵层相对下降0.93m;此外,中国大陆河流泥沙向陆架充填造成海底底面的增高,使海洋容积减少、海面相对升高,从而使得陆地海侵层的相对位置降低(模拟平均增加8.2m)。模拟相对海面的结果在8个典型钻孔的海侵层位上进行对比,误差在-3.1%(模拟偏低)~+6.4%(模拟偏高),模拟结果可以接受。本研究对认识多重海陆升降复杂效应作用下的海侵层特征,揭示南黄海和东海MIS 3海面变化的性质有重要意义。
英文摘要:
Abundant evidences of marine sediments from Jiangsu and Fujian coasts have confirmed a marine transgression event during 30~40ka B.P., which suggested that there was a stage of the higher sea level and a warm climate when ice sheets were shrinking in the Northern Hemisphere. The duration of 30~40ka B.P. spanned a period in the later Marine Isotope Stage (MIS) 3 and was an interstadial period during the Last Glacial period of the Quaternary. Different from the glacial with cold climate, this marine transgression considered as a penultimate higher sea level during the Quaternary remains a puzzle: what are properties of this transgression in terms of sea level height,post-sedimentary changes and dynamic effects on? and why the evidence is contrary to the Quaternary glacial theory? It is important to understand sea level rise for these areas sensitively responding to the global changes in the future. The sediment records of eight cores in Jiangsu and Fujian coastal plains were sourced from the four published cores and from the present study of the other four cores. The records showed that the marine transgression strata were located at -18~-32m a. s. 1. during the later MIS 3,indicated by the stratigraphical sequence, marine sedimentary facies,various saline microfossil assemblages, and chorology of radiocarbon dating. The depths of the marine transgression strata were analyzed which is the result of multiple land and sea system effects, including neotectonic subsidence, glacier meltwater, sediment compaction and terrestrial sediment filling since 35ka B.P. To recognize the key issues on the Late Pleistocene sea level changes,the eustatic sea level (H_S) was defined as the glaciation-climate-forced sea levels, and the relative sea level change (H_R) was defined as palaeo-sea levels that the record was preserved in sediments which have experienced multiply secondary actions of land and sea effects. On these bases, we constructed multi-level models of climate-driven glacio-eustatic change and land-sea system to simulate the MIS 3 transgression of the eastern coasts of China. By integrating data sets from eight sedimentary cores and prescribing the boundary conditions of the model,simulations of the H_S and H_R have been performed for East China Sea and Southern Yellow Sea areas. Tectonic subsiding of coastal land caused the relative sea level 6.7m lowing and the sea subsiding caused 9.0m lowing. Sediment porosity compression made a decrease of 0.93m and the sediment filling made an increase of 8.2m in the sediment thickness. Compared with sedimentary evidence, the simulated relative sea-levels are between -26.3~-29.9m a.s.l. The simulated margins of error in varying localities are between (2.5~4.5) m that are lower than the total 10% (i. e. -3.1%~+ 6.4%), suggesting that the margins of error are relatively small and the simulation results can be accepted. The simulations also showed that the later MIS 3 sea-level changes were dominated by glacier effects, in which the eustatic sea-level was between -19.2~-22.2m a.s.l. The study is of significance to understand the characteristics of marine transgression under the effect of multiple complex land-sea activity and to illustrate the nature of sea-level changes of the East-Yellow Seas during the later MIS 3. This sedimentary model in the vertical dimension should be further improved by using a 3-D model in the near future.