中元古界沉积物中典型分子标志化合物及其地质意义——以宣隆坳陷下马岭组黑色页岩为例

Typical Molecular Marker Assemblage of the Mesoproterozoic Sediments:A case study of the Xiamaling Formation black shales in the Xuanlong Depression

分子标志化合物在前寒武纪古海洋生命演化和古环境的研究中发挥着越来越重要的作用。在华北克拉通宣隆坳陷中元古界下马岭组黑色页岩中检测到一系列典型分子标志化合物,其特征表现为明显的“UCM”鼓包,高丰度的甲基支链烷烃、C_(24)四环萜烷、C_(19)-C_(20)13β(H),14α(H)-三环萜烷、13α(正烷基)-三环萜烷系列以及四类重排藿烷化合物,甾烷系列化合物的缺失,其组成特征及其地质意义明显不同于显生宙沉积物。甾烷系列化合物缺失可能是异养微生物转化和降解的结果,不能否定真核藻类在14亿年前古海洋的局部繁盛,但生物群落仍以原核生物为主。

Molecular markers play an increasingly important role in the study of early life evolution and paleoenviron⁃ment of the Precambrian ocean.Typical molecular markers have been detected in the black shale of the Mesoprotero⁃zoic Xiamaling Formation in the Xuanlong Depression,the North China Craton,which are characterized by a notably higher base level of the unresolved complex mixture,high abundances of methyl branched alkanes,C_(24) tetracyclic ter⁃pane,C_(19)-C_(20)13β(H),14α(H)-tricyclic terpanes,13α(n-alkyl)-tricyclic terpanes and four types of rearranged hopanes,and the absence of steroids.Their distribution characteristics and geological significance are different from the Phanerozoic sediments.Because the lack of steroids may be the result of heterotrophic microbial transformation and degradation,we cannot exclude that eukaryotic algae may have flourished locally in the paleo-ocean 1.4 billion years ago,but the primary biological community is still dominated by prokaryotes.