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

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 (δ¹⁵N_DCN) and kerogen extracts (δ¹⁵N_kerogen). However, some studies have shown that there are differences between these two proxies. In addition, differences were found between δ¹⁵N_DCN and the nitrogen isotopic compositions of untreated samples (δ¹⁵N_bulk). 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) δ¹⁵N_bulk is greater than δ¹⁵N_DCN in most samples by 0.6‰ on average, indicating that acid-soluble organic N with higher δ¹⁵N values was lost preferentially during decarbonization; (2) δ¹⁵N_DCN is greater than δ¹⁵N_kerogen 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) δ¹⁵N_kerogen greater than δ¹⁵N_DCN 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 δ¹⁵N_kerogen and increase δ¹⁵N_DCN. 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.