Application of Nitrogen Isotopic Systematics in Different N Forms for the Reconstruction of Redox Conditions: An Example From the Songliao Basin

The assessment of nitrogen cycle processes in paleomarine and paleolake systems commonly relies solely on total nitrogen isotopes. If organic nitrogen and adsorbed nitrate can be effectively extracted from samples, the application of nitrogen isotopic systematics in different N forms would be beneficial for reconstructing the nitrogen cycle process in pore water and would mutually complement water chemistry information. According to the variations in δ13Corg, δ15NTN, δ15Norg and δ15Nnitrate data of the Cretaceous Nenjiang Formation in the Songliao Basin, NE China, the profile could be divided into three intervals (intervals A–C). We attribute the anomalously high δ15Nnitrate (∼+20‰) from intervals A and C to the effect of sedimentary denitrification that occurred in the benthic sediments during the early diagenetic stage. We also recognize the intimate relationship between δ15Nnitrate and δ15Norg values, which are interpreted to reflect the effect of water column denitrification in the bottom waters and/or surface pore water. Based on the comparison of nitrogen isotope variability and the evolving environmental context, we interpret the water column denitrification as primarily reflecting anoxic bottom water or surface pore waters, while the sedimentary denitrification indicated by abnormally high δ15Nnitrate can effectively represent oxic bottom water. This study not only reveals that the adsorbed nitrate can be well extracted from mudstones with low thermal decomposition degrees but also suggests that the organic nitrogen‐nitrate isotope proxies can be used as a potential parameter to track the redox structure in water masses.