Isotopic differences and paleoenvironmental significance of nitrogen contained in bulk sedimentary rocks, decarbonated aliquots and kerogen extracts

Abstract

The stable nitrogen isotopic composition (δ15N) of sedimentary rocks is an important tool for reconstructing the paleo-oceanic nitrogen cycle and has been widely used in palaeoenvironmental studies of the Precambrian and Phanerozoic. Currently, the commonly used parameters are the isotopic compositions of decarbonated samples (δ15NDCN) and kerogen extracts (δ15Nkerogen). However, some studies have shown that there are differences between these two proxies. In addition, differences were found between δ15NDCN and the nitrogen isotopic compositions of untreated samples (δ15Nbulk). In order to explain these phenomena, we selected sedimentary rock samples from different time periods in the Phanerozoic and analyzed nitrogen isotopes after all three treatments (bulk rock, decarbonated rock, and kerogen extracts). We find that (1) δ15Nbulk is greater than δ15NDCN in most samples by 0.6‰ on average, indicating that acid-soluble organic N with higher δ15N values was lost preferentially during decarbonization; (2) δ15NDCN is greater than δ15Nkerogen mainly in anoxic environments, which may be linked to ammonium accumulation in pore fluids or in the water column during deposition and/or diagenesis. For example, the anaerobic ammonia oxidation process and partial assimilation preferentially consume 14NH4+ and enrich 15NH4+ in seawater. Therefore, NH4+ fixed by clay minerals may have a higher δ15N than organic nitrogen; (3) δ15Nkerogen greater than δ15NDCN was observed in a subset of samples and may reflect either preferential release of 14NH4+ from organic matter or biological production of isotopically light NH4+, followed by adsorption to clay minerals. In this case, the δ15N of residual organic nitrogen would be relatively high compared to the released NH4+. However, this scenario is only retained in samples that have not undergone significant post-depositional alteration, which tends to lower δ15Nkerogen and increase δ15NDCN. Differences between the three nitrogen parameters are in many cases small and would not alter overall paleoenvironmental interpretations; however, in some cases, they can be significant and carry additional information about diagenetic conditions that may be unlocked with additional studies on wider range of deposition environments.