用岩屑样定量研究南海西北陆架古环境

QUANTITATIVE ANALYSIS OF PALAEOENVIRONMENT IN NORTHWESTERN SHELF OF SOUTH CHINA SEA BASED ON CUTTING SAMPLES

对南海西北陆架23口浅海钻孔的101个岩屑样及西太平洋2个深海钻探(DSDP)钻孔的10个岩芯样中大于0.154mm的浮游有孔虫进行定量分析,共得浮游有孔虫12属47种.采用微体古生物学、生物地层学与数理统计(聚类分析、目标因子分析、目标转换函数法)和同位素地球化学相结合及时间面与时间系列相结合的方法,研究了南海西北陆架Globoquadrina altispira末现面和Globorotaliamulticamerata末现面之上两个时间面的古环境,再造了南海西北陆架上新世末至更新世初古海洋变迁的历史.

Quantitative analysis of planktonic foraminifera (with test size>0.154mm) has been carried out for 101 cutting samples from 23 wells in the northwestern shelf of the South China Sea (SCS), as well as for 10 core samples from 2 DSDP sites in the Western Pacific. Planktonic foraminifera belonging to 47 species and 12 genera have been identified in the samples from the SCS. In order to study the palaeoenvironmental changes from the LAD of Globoquadrina altispira to the LAD of Globorotalia multicamerata, multidisciplinary approaches of mieropaleontology, biostratigraphy, statistics (cluster analysis, target factor analysis, target transfer function) and isotopic geochemistry have been applied. As a result, the following conclusions have been reached: 1. Well Cuttings from the SCS can be used with satisfaction to reveal the evolutionary trends of palaeoenvironments through quantitative ananlysis of microfossils at specific datums. 2. The surface water covering the present northwestern shelf of the SCS during the maximum transgression of Pliocene can be divided into the Coast Water Mass, with shallower depth, lower temperature and lower salinity, and the SCS Offshore Water Mass, with greater depth, higher temperature and higher salinity. In the earliest Pleistocene, the sea surface temperature decreased and the SCS Offshore Water Mass receded completely from the northwestern shelf. 3. A 2.0℃ decrease of winter sea surface temperature (SST) and 3℃-4℃ difference in the seasonal SST during the onset of the Northern Hemisphere glaciation (2.5-2.0Ma) can be found in the northwestern shelf of the SCS, which exceeded those in the Western Pacific at a similar latitude. A remarkable regression occurred in the SCS at the same time.