Isotopic Differences Of Various Forms Of Nitrogen In Sedimentary Rocks And Its Applications In Paleoenvironment

In recent years,the stable nitrogen isotopic composition(δ¹⁵N)in sedimentary rocks is an important tool in reconstructing the paleo-oceanic nitrogen cycle and has been widely used in palaeoenvironmental studies of the Precambrian and Phanerozoic Era.Currently,the commonly used nitrogen proxies are the nitrogen isotopic compositions of decarbonated samples(δ¹⁵N_DCN)and kerogen extracts(δ¹⁵N_kerogen).However,some studies have shown that there are differences between the two nitrogen proxies as well as theδ¹⁵N_DCN and the nitrogen isotopic composition of untreated samples(δ¹⁵N_bulk).But the reason for this difference is not clear,which affects the acquisition of the original nitrogen signal.Based on previous studies on kerogen extraction,a method ofδ¹⁵N analysis for different nitrogen forms/speciation in sedimentary rocks was established in this paper.A set of geological samples of different ages were analyzed to find the pattern between the nitrogen isotope values of different forms of sedimentary rocks.Finally,the difference ofδ¹⁵N and its paleoenvironmental significance during the Smithian/Spathian boundary（late Early Triassic）is discussed under the rules discovered in this dissertation.In this paper,we firstly extracted the bulk sample,DCN sample and kerogen sample from sedimentary rock samples from six sections,and performed high-precision TOC,TN,δ¹⁵N and elemental content analysis.Then the reasons for the differences inδ¹⁵N of different forms in the shale samples from the Smithian/Spathian boundary in the Shanggang section were also analyzed.Four conclusions are drawn in this paper,which are as follows.（1）Theδ¹⁵N_bulk in most of the Phanerozoic sedimentary rock samples is larger thanδ¹⁵N_DCN,with an average difference of 0.6‰.The reason is that the preferential release of ¹⁵N-depleted nitrogen during the thermal maturation of organic matter makes theδ¹⁵N in asphaltene higher,and some of the soluble organic N will be lost in the process of acid treatment.（2）The difference betweenδ¹⁵N_kerogen andδ¹⁵N_DCN,with an average difference of0.6‰,suggests a different nitrogen isotopic composition between kerogen-bound N and NH₄⁺-N in the rocks.The reason for the higherδ¹⁵N_kerogen thanδ¹⁵N_DCN is mainly the presence of ¹⁴NH₄⁺,which may come from the preferential release of ¹⁴NH₄⁺from organic matter during decomposition or the preferential production of ¹⁴NH₄⁺during DNRA,when it is immobilized by clay minerals to make the inorganic nitrogen in the sediment have a lower isotopic composition.For the phenomenon thatδ¹⁵N_DCN is higher thanδ¹⁵N_kerogen,it may be due to the anammox that enriched ¹⁵NH₄⁺in the water column is fixed by clay minerals on the one hand,and on the other hand,it may be due to the organic matter synthesized after the preferential absorption of ¹⁴NH₄⁺in assimilation has a lowerδ¹⁵N,and the sediment fixes the ¹⁵NH₄⁺in the water column.（3）Nitrogen isotope values of three nitrogen forms extracted from shale at the Smithian/Spathian boundary in the Shanggang section are significantly different.In most samples,δ¹⁵N_DCN is higher thanδ¹⁵N_bulk,which may be due to the existence of an ammonium-rich ocean in South China in the Early Triassic,the nitrogen in rocks mainly exists in the form of inorganic nitrogen,and part of exchangeable ammonium rich in¹⁴NH₄⁺is lost during the acid treatment process.Whileδ¹⁵N_DCN is generally higher thanδ¹⁵N_kerogen,the reason may be that the organic matter with lowδ¹⁵N was synthesized by microbial assimilation in the anoxic ocean with high productivity in the Early Triassic,and the rich ¹⁵NH₄⁺was fixed by clay minerals in the sediments.