广西香田弗拉斯-法门阶界线事件标志——碳、氧稳定同位素异常特征  

FRASNIAN-FAMENNIAN BOUNDARY EVENT MARKERS AT XIANGTIAN, GUANGXI——THE CHARACTERISTIC OF STABLE CARBON AND OXYGEN ISOTOPE ANOMALIES

作　　者：严正 叶莲芳 侯鸿飞[2] 刘荣谟[3] 

机构地区：[1]国家地震局地质研究所,北京100029 [2]中国地质科学院地质研究所,北京100037 [3]中国科学院西安黄土与第四纪研究室,710061

出　　处：《地质科学》1993年第2期135-144,共10页Chinese Journal of Geology(Scientia Geologica Sinica)

基　　金：国家自然科学基金4870088

摘　　要：本文对广西香田晚泥盆世弗拉斯-法门阶界线剖面进行了系统的碳、氧稳定同位素研究。发现随着弗拉斯-法门期之交的生物大量死亡和绝灭,在弗拉斯-法门阶界线层中,碳、氧稳定同位素组成出现了明显的异常。这和白垩—第三系界线灾变事件中出现的异常颇为相似。碳同位素异常达-6.6‰左右,氧同位素出现正异常达2‰。此外,在界线层中出现铱异常为230×10^(-12)g/g。这些表明,上述同位素地球化学异常是地球历史中生物绝灭灾变事件的重要标志。The Fr/Fa secton, which is located in Xiangtian, Guangxi, China, is well developed. In the top part of Frasnian stage is Xiangtian Fm. composed of is carbonaceous limestone and calcareous shale, and under the Xiangtian Fm. laid Liujian Fm. is composed of siliceous limestone. In the lowest part of Famennian stage is Wuzhizhan Fm. composed of thin bedded grey nodular limestone. Intensive sampling was carried out across the boundary and the results of the isotope analyses are shown in Table 1 and Figure 1. Clearly the δ13 C values of Carbon isotope at this Fr/Fa boundary section can be classified into three parts:The lowest part of carbon isotope curve (Xiangtian Fm.), samples D-lb to D-12 are collected from the strata below the boundary belonging to Frasnian Stage with average δ13 C value of about l%o that is characterized by narrow variation of about 1.5‰. At the top of Frasnian Stage, the δ13 C values suddenly grow negative near the boundary within the 3-centemeter thickness of the same rock. That means the δ13 C variation of carbon isotope composition of strata is independent on rock.The middle part of the curve, samples E-O to E-6C are gathered from the strata near the boundary, which are 20-centemeter-thick marlstone. The δ13 C values are almost negative, ranging from 0.10 ‰ to-6.57% with an average of-3.0‰, showing a complex peak of negative values, which is compounded by a large negative peak with two secondary one. This behaviour is similar to the curve of oxygen isotope at deep sea drilling hole v28-238 in that a large signal is overprited with some secondary 'noise', reflecting the violent change in environment. This complex peak is also quite similar to the anomalous peak of carbon isotope at P/Tr boundary in Guangyuan, Sichun, China.Moreover, the δ18O values of oxygen isoto pe tend to positive direction, and its value reaches to 2‰. It indicates there was a lower temperture anomaly at the transition of Fr/Fa boundary. Besides, a Ir anomaly of 230×10-12 g/ghad been found in the Fr/Fa bound

关 键 词：事件标志 碳 氧 稳定同位素 广西 

分 类 号：P597.2[天文地球—地球化学]