Influencing factors and significance of organic and inorganic nitrogen isotopic compositions in lacustrine sedimentary rocks
Juan Chen, Juan Chen, Jianfa Chen, Jianfa Chen, Lipeng Yao, Qingyong Luo, Shengbao Shi, Jianping Chen, Jianping Chen, Lei Zhu, Zeya Chen
Abstract
• Thermal evolution does not significantly modify the δ 15 N of bulk rock and kerogen. • The δ 15 N sil was slightly lower than the δ 15 N ker of the sediments in the YF. • The δ 15 N sil was obviously lower than the δ 15 N ker of the sediments in the QF. • Organic and inorganic nitrogen isotope values are sensitive to redox conditions. 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 ( δ 15 N bulk ) in the non-metamorphic stage are significantly different from that of kerogen, which challenge the conceptual framework of sedimentary nitrogen isotope interpretation. The δ 15 N bulk from the YF and QF were lower than their respective the nitrogen isotope values of kerogen ( δ 15 N ker ), with offsets up to ∼5.1‰, which have the inverse relationship for the metamorphosed rock. Thermal evolution did not significantly modify the δ 15 N of bulk rock and kerogen. The δ 15 N of sediments from the YF ( δ 15 N bulk , 1.6‰–5.6‰) were lower than that of rock from the QF ( δ 15 N bulk , 10.2‰–15.3‰). The nitrogen isotope values of silicate incorporated nitrogen ( δ 15 N sil ) were slightly lower than those of the δ 15 N ker 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 N 2 loss leads to the relatively light δ 15 N bulk . In the stratified water for the QF, redox transition zone promotes denitrification and anammox, resulting in the heavy δ 15 N bulk of rock and promotes the DNRA, resulting in heavy δ 15 N ker and low δ 15 N sil .