蓟县剖面中—新元古代沉积物的稳定碳同位素生物地球化学研究  

A Carbon Isotopic Biogeochemical Study of Meso- to Neoproterozoic Sediments from the Jixian Section, North China

作　　者：李超[1] 彭平安[1] 盛国英[1] 傅家谟[1] 阎玉忠[2] 

机构地区：[1]中国科学院广州地球化学研究所,510640 [2]中国地质调查局天津地质矿产研究所,300170

出　　处：《地质学报》2002年第4期433-440,共8页Acta Geologica Sinica

基　　金：本文为国家杰出青年基金项目(编号001202); 国家自然科学基金项目(编号 49321003;4945003)资助的部分成果。

摘　　要：本文报道了蓟县剖面约1 Ga的中—新元古代沉积物中干酪根、可溶单体烃类等沉积有机质与共生碳酸盐的稳定碳同位素特征,并结合剖面古环境和古生物资料对这些同位素数据进行了初步讨论。干酪根和共生碳酸盐碳同位素数据表明,约1.55 Ga以前地表环境中CO_2浓度曾迅速下降,而之后趋于稳定。沉积碳酸盐和共生干酪根之间的碳同位素差值(△_C)记录了燕山盆地元古宙演化中海侵和海退的交替以及随之具有不同适应性生物群落的更迭。剖面1.4 Ga以前的干酪根和正构烷烃、类异戊二烯烷烃的稳定碳同位素特征符合正构烷烃和类异戊二烯烷烃主要起源于喜盐古细菌类脂的假设,而之后,三者之间的碳同位素特征表明正构烷烃主要起源于原始藻类有机质,而类异戊二烯烷烃则保持着喜盐古细菌类脂的输入。正构烷烃生物起源的转变表明,1.4 Ga以后,古燕山盆地水体中原始类脂物质的异养降解程度明显降低。这一降低可能与在此阶段形成的稳定浅水陆表海环境有关。蓟县剖面干酪根与可溶单体烃类的稳定碳同位素关系显示了具有局限海特征的元古宙燕山海盆独特的生物地球化学过程。This paper preliminary reports the carbon isotopic characteristics of kerogen, individual soluble hydrocar-bons and associated carbonate preserved in Meso- to Neoproterozoic (1. 8 - 0. 85 Ga) sediments from the Jixian section, North China. A comparison was carried out between these isotopic data and independent palaeoenvi-ronmental and palaeontologic information from these strata. The carbon isotopic characteristics of kerogen and coexisting carbonate show that the atmospheric CO2 concentration declined rapidly before 1. 55 Ga and then re-mained stable. The difference between the carbon isotopic values of kerogen and carbonate (△C) shows the cyclic variation along the strata sequence, and suggests the alternation of marine transgression and regression that took place in the Proterozoic Yanshan basin and the ensuing changes of biocommunities adaptable to the changing environments. The carbon isotopic relationships among kerogen and n-alkane and isoprenoid from these strata are consistent with the hypothesis that both the n-alkanes and isoprenoids were mainly derived from heterotrophic halophilic archaeobacteria before 1. 4 Ga, and after the age only the isoprenoids kept its input from these archaea, while n-alkanes were mainly derived from primitive organic matter of photosynthetic organisms such as cyanobacteria. The change in biological origin of n-alkanes implied the marked decline in heterotrophic reworking of primitive lipids after 1. 4 Ga. This decline might be related to the formation of a stable epicontinental sea with a shallow water environment. The isotopic relationships between kerogen and individual soluble hydrocarbons suggest a special biogeochemical cycle existed in the Proterozoic Yanshan Basin with the characteristics of a restricted sea.

关 键 词：蓟县剖面 元古宙 碳酸盐 干酪根 可溶单体烃 碳同位素 有机质 生物地球化学 

分 类 号：P597[天文地球—地球化学]