Juan Chen, Jianfa Chen, Lipeng Yao, Qingyong Luo, Shengbao Shi, Jianping Chen, Lei Zhu, Zeya Chen. Influencing factors and significance of organic and inorganic nitrogen isotopic compositions in lacustrine sedimentary rocks[J]. Geoscience Frontiers, 2023, 14(2): 101501. DOI: 10.1016/j.gsf.2022.101501
Citation: Juan Chen, Jianfa Chen, Lipeng Yao, Qingyong Luo, Shengbao Shi, Jianping Chen, Lei Zhu, Zeya Chen. Influencing factors and significance of organic and inorganic nitrogen isotopic compositions in lacustrine sedimentary rocks[J]. Geoscience Frontiers, 2023, 14(2): 101501. DOI: 10.1016/j.gsf.2022.101501

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 isotopic 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. The δ15Nbulk 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 modify 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 conditions 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 denitrification and anammox, resulting in the heavy δ15Nbulk of rock and promotes the DNRA, resulting in heavy δ15Nker and low δ15Nsil.
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