Isotopic Composition Characteristics Of Organic Nitrogen And Inorganic Nitrogen In Source Rocks

Nitrogen is the key element of life macromolecules and plays an important role in the earth environment.In recent years,with the development of nitrogen isotope determination technology,the research on the characteristics of nitrogen isotope composition in sediments and its geological significance has made remarkable progress.However,most of the nitrogen isotopes in the sediments studied by many scholars are determined by the whole rock.However,the nitrogen in source rocks exists in two forms,organic nitrogen and inorganic nitrogen.Only analyzing the total nitrogen isotope can not obtain the characteristics of nitrogen isotope composition in different occurrence states,and can not more carefully explore the geological information of early diagenesis process.In this study,sodium hypochlorite and sample mixture heating boiling method were used to separate inorganic nitrogen isotope samples.Soxhlet extraction method was used to separate soluble organic matter.Based on Dumas combustion method,EA-IRMS was used to determine the source rock of marine sediments(niubutang formation of Cambrian in Chongqing area)and continental freshwater lake sediments(Yanchang Formation of Triassic in Ordos basin)Nitrogen isotopic composition of organic nitrogen and inorganic nitrogen.Compared with the corresponding geochemical parameters and sedimentary environment,the distribution characteristics and geological significance of nitrogen isotopes are discussed.The results are as follows:(1)the organic and inorganic nitrogen isotopic compositions of Xiasiwan area,the source rock of the Triassic continental fresh water lake in Ordos Basin,are quite different;the inorganic and whole rock nitrogen isotopic compositions of the source rock with buried depth greater than 3040m in Tianhuan depression are similar,and the inorganic and whole rock nitrogen isotopic compositions of the source rock with buried depth less than 3040m are heavier than that of the whole rock.The geochemical parameters show that there are relatively more nitrogen losses in the early deposition,indicating that the oxygen content is relatively increased in the upper part.(2)The total nitrogen isotopic composition of the source rocks of the Cambrian niuditang formation in Chongqing area is similar to that of the inorganic nitrogen.The whole isotopic composition is lighter when the burial depth is greater than 1150m.The geochemical parameters show that the nitrogen fixation is the main stage in the early diagenetic stage,which is consistent with the anoxic sedimentary environment caused by the hydrothermal activity in the lower part of 1150m.(3)Sedimentary environment is an important factor affecting the nitrogen isotopic composition of source rocks.In different sedimentary settings,the nitrogen isotopic composition of continental fresh water lakes is more important than that of marine sediments,including whole rocks and inorganic nitrogen isotopes.Under the same sedimentary facies,the nitrogen isotope has a strong correlation with the change of micro oxygen content in the early diagenetic sedimentary environment.The oxygen content and nitrogen cycling mechanism of early deposition can be well characterized by TOC,C/N and other geochemical parameters.(4)In this paper,the standard experimental process of using sodium hypochlorite reagent to remove organic matter from source rock to obtain inorganic nitrogen samples is proposed.The experimental results have been well verified by the determination of TOC content in the treated samples,and the experimental scientificity has been well guaranteed by the analysis and research.(5)The best experimental process for obtaining inorganic nitrogen samples was explored.The results showed that sodium hypochlorite mixed with samples was the best.