生物标志化合物、早期生命和古环境

Biomarker,Earlier Life and the Concurrent Environment

近年来,早期生命和古环境成为地学研究的热点,生物标志化合物在前寒武纪生命演化和环境研究中发挥着越来越重要的作用。在前寒武系地层中检测到的主要生物标志化合物为:烷烃(包括正构烷烃和异构烷烃、烷基环烷烃)、无环类异戊二烯烃、萜烷(包括三环萜、藿烷、2α-甲基藿烷和3β-甲基藿烷、重排藿烷、新藿烷、降新藿烷)、甾烷(包括C26-C30甾烷、4-甲基甾烷、甲藻甾烷、2-甲基甾烷和3-甲基甾烷、重排甾烷)、类胡萝卜素等。由于不同生物标志化合物的生物前驱物赋存于不同环境之中,并与环境协同演化,分析地层中生物标志化合物组合不仅可得到其前驱物组成信息,还可以获取古环境判断的重要标志。从已知生物标志化合物分析可以发现,在2.7Ga以前就存在生氧光合作用的蓝藻或蓝菌,真核生物可能已经存在,但在古、中元古代,海洋仍以缺氧环境为主,而生物仍以原核生物占优势。新太古代蓝藻或蓝菌的出现虽促进了大气圈的氧化,但海洋氧化的滞后明显制约了真核生物的发展,导致在2.78～0.6Ga长达20多亿年的时间里,生物演化缓慢。

In recent years, research on earlier life and the concurrent environment becomes a hot topic of geosciences. The study of biomarkers plays a more and more important role on the research of earlier life and the concurrent environment. Biomarkers detected from Precambrian include normal and branched alkanes, cyclohexylalkanes, adamantanes, acylic isoprenoids, tricyclic terpanes, hopanes, and a diverse range of steranes and carotenoids. Different biomarker has different biological precursor and indicates different environment. Biomarker analysis can provide critical clues for their precursor composition and diagnostic tools for their environmental interpretation. Known biomarker assemble indicates that oxygen generating cyanobacteria and eukaryotes might exist 2. 7Ga ago. However, the ocean remains oxygen deficiency and organisms are dominated by prokaryotes during Paleo and Mesoproterozoic. The existence of Cyanobacteria in the Archean did boost the atmospheric oxygenation and make eukaryotes be present, but the retention of oxygenation in ocean obviously restrains the development of eukaryotes, leading slow organism evolution in the long period of 2. 0 Ga between 2.78 Ga and 0. 6 Ga.