The Biomarkers in the Mesoproterozoic Organic-rich Rocks of North China Craton: Implication for the Precursor and Preservation of Organism in the Prokaryotic Realm

Mesoproterozoic marine organic-rich rocks are widely distributed in the North China Craton,include the Gaoyuzhuang(GYZ),the Hongshuizhuang(HSZ),and the Xiamaling(XML)formations.According to the T;value and isomerisation ratio of C;homohopanes,the XML,HSZ,and GYZ samples were in low mature,mature and high mature stage,respectively.Biomarker distribution in extractable organic matter(EOM)of three Mesoproterozoic organic-rock samples in different maturity were analysed to reveal the organic precursor and preservation pathway of in the Mesoproterozoic Combined with gold-tube pyrolysates of three Mesoproterozoic samples,it could further illuminate the chemical composition of Mesoproterozoic kerogen,given excluding.The results indicated that the three formations were all deposited under reducing condition and their organic precursors mainly were some aquatic organisms.High content of rearranged hopanes was detected in EOM of XML and HSZ samples,whereas they were relatively low in the high mature GYZ sample.Contrast to that in EOM,the relative concentration of rearranged hopanes sharply decreased in the gold-tube pyrolysates of the XML kerogen,then slightly increased but was still significantly lower than the EOM of XML sample,which indicated that catalysis of clay minerals in the early diagenesis only changed the chemical composition of the unstable functional groups of the kerogen during the preservation.Due to the thriving heterotrophic microbes and low sink rate of particulate organic matter during the Mesoproterozoic,primary producers suffered extensive degradation during sinking process,only some resistant biopolymers lacking of lipid compounds survived from heterotrophic degradation,while heterotrophic microbes contained more proportion of organic precursors.Abundant pristane(Pr)and phytane(Ph)were only released in high mature stage because of the protection of the macromolecular structure of resistant biopolymers which prevented biomarkers from being altered by the thermal stress.The absence of 13α(n-alkyl)-tricyclic terpanes in the high matured hydrocarbon products also indicated the different precursors between different parts of Mesoproterozoic kerogen.The evolution of the biomarker composition and content of Mesoproterozoic kerogen showed some special characteristics differing from those of Phanerozoic kerogen.The total concentrations of hopanes displayed with an order of low mature stage>high mature stage>mature stage.Relative content of rearranged hopanes in the hydrocarbon generated in high mature stage was significantly lower than that in the low maturity stage.The ratios of Pr/n-C₁₇ and Ph/n-C₁₈ increased with thermal maturity,and the ratio of nC_21-/nC₂₂₊ decreased in the high maturity stage,thus displaying another order of mature stage>high maturity stage>low maturity stage.The unique preservation pathway of Mesoproterozoic organisms was attributed to the special evolution characteristics of biomarker distributions,which should be considered in the Mesoproterozoic marine environment and biological studies.