Influencing factors and significance of organic and inorganic nitrogen isotopic compositions in lacustrine sedimentary rocks

Comprehensive nitrogen biogeochemical cycle has been reconstructed for representative lacustrine organic-rich sedimentary rock in China,namely the Triassic Yanchang Formation(YF,199-230 Ma)in Ordos and the Cretaceous Qingshankou Formation(QF,86-92 Ma)in Songliao basins,by evaluating the organic and inorganic nitrogen isotopic compositions rather than only organic or bulk nitrogen iso-topic compositions.The results indicate that the nitrogen isotope values of bulk rock(δ15Nbulk)in the non-metamorphic stage are significantly different from that of kerogen,which challenge the conceptual framework of sedimentary nitrogen isotope interpretation.Theδ15Nbulk from the YF and QF were lower than their respective the nitrogen isotope values of kerogen(δ15Nker),with offsets up to～5.1‰,which have the inverse relationship for the metamorphosed rock.Thermal evolution did not significantly mod-ify the δ15N of bulk rock and kerogen.The δ15N of sediments from the YF(δ15Nbulk,1.6‰-5.6‰)were lower than that of rock from the QF(δ15Nbulk,10.2‰-15.3‰).The nitrogen isotope values of silicate incorporated nitrogen(δ15Nsil)were slightly lower than those of the δ15Nker in the YF and obviously lower for the QF.The fact that different nitrogen cycles occur in the YF and QF due to the different depositional redox conditions leads to different isotopic results.The YF water environment dominated by oxic condi-tions is not conducive to the occurrence of denitrification and anammox,and no abundant N2 loss leads to the relatively light δ15Nbulk.In the stratified water for the QF,redox transition zone promotes denitrifica-tion and anammox,resulting in the heavy δ15Nbulk of rock and promotes the DNRA,resulting in heavyδ1 5Nker and low δ15Nsil.