中国东部海岸带现代有孔虫碳同位素及其古环境意义

在第四纪海陆过渡相环境研究中,有没有能够确定古盐度的定量标志?本文作者针对中国东部第四纪海陆过渡相地层中常见的两种有孔虫Ammonia beccarii(Linne)vars.和Protelphidium glabrum(Ho, Hu et K. Wang)进行现代样品的碳同位素分析。在现代样品研究的基础上,确定了碳同位素与盐度存在线性关系,运用这一结果于第四纪几个钻孔样,并综合有孔虫化石群其他特征,推导古环境特点。

THE PRELIMINARY STUDY OF CARBON STABLE ISOTOPE OF RECENT FORAMINIFERA ALONG EAST CHINA COAST AND ITS PALEOENVIRONMENTAL SIGNIFICANCE

In the study of Quaternary sea level changes and transgression in the East China, one of the problems is to recognize the marine-continental environments from the normal ones. This paper puts emphasis on the study of carbon stable isotope of Ammonia beccarii （Linne） vars, and Protelphidium glabrum （Ho, Hu et K. Wang） （Foraminifera）, which are the most predominant species in the marine-continental deposits of the Quaternary strata. Based on the results obtained from 33 recent samples, which were taken from different depositional environments ranging from coastal marshes, estuaries, lagoons to littoral zones, a linear relation has been determined between carbon isotope and salinity. The salinity in low marineness environments are commonly less than 10‰, below which the average δ¹³C value of A beccarii vars. is—6.854 and over which—4.790. This remarked difference of δ¹³C can be used to distinguish lowmarineness environments such as oligohaline estuaries, lagoons and marshes from high-marineness ones meso-or normal marine lagoons, marshes and bays.The results have been applied to interpretation of the paleoenvironments of four typical fossil assemblages from Quaternary cores, that were simply considered as marine-continental deposits. However, according to the analysis of foraminiferal assemblages, carbon stable isotopes and geological backgrounds, two kinds of lagoons could be recognized, one might be oligohaline, the salinity below 3‰ with an enclosed character, another mesohaline having salinity 10—15‰ with a semi-enclosed character. Anyway, a low-salinity estuary （0—3‰） and bay with the salinity 15—20‰ could be realized. Therefore, the δ¹³C of A. beccarii vars. could provide a useful tool for the paleoenvironmental reconstruction, especially in determining the paleo-salinity. But, this method should be used carefully and improved further, as there are many complicated factors in the coastal environments affecting the isotopic contents of foraminifera.