Biogeochemistry of Nitrogen Isotopes of Ammonia in the Pore Water of Sediments in the Eastern Tropical North Pacific; Implications for the Problem of Isotopic Fractionation of Sedimentary Nitrogen during Early Diagenesis

Previous studies have indicated that the process of diagenesis alters the isotopic ratio in preserved organic matter. Published work by others has shown that: (1) in an oxygenated environment, the δ15N of preserved material is about 3 to 4 \permil heavier than original isotopic composition of marine organic matter; and (2) bacterial degradation of organic matter leads to the preferential break down of ¹⁴ N containing bonds in peptides and amino acids. We address the issue of diagenetic alteration of the isotopic ratio of degrading organic matter by comparing the isotope composition of ammonium in the pore water to that of N in bulk sediments. Profiles of ammonium δ15N were measured in pore waters of several cores obtained from the middle of the OMZ in the eastern Tropical North Pacific and the Gulf of California. Our prediction was that δ15N of ammonia in the pore water should be lighter than bulk nitrogen isotopic ratio. However, the opposite was observed. At depth of several meters, δ15N of ammonia is about 2-3 \permil heavier then bulk N, and becomes up to 10-12 \permil heavier than bulk N near the sediment-water interface. A diffusion-reaction-advection modeling was applied, and results indicate that observed pattern might be explained by a combination of three factors: lower diffusivity of ¹⁵NH₄+ relative to ¹⁴ NH₄+, isotopic fractionation during bacterial degradation of organic matter, and preferential sink for lighter N isotopes in the upper 50-70 cm of sediment. The last factor might be related to either bacterially mediated anaerobic oxidation of ammonia, which would cause residual ammonia to become heavy, or bacterial uptake of ammonium during chemosynthesis. The extent of fractionation during diagenetic release of ammonium cannot be determined until relative diffusivities of ¹⁵ NH₄+ and ¹⁴ NH₄+ are established. Further modeling efforts and analytical work is being currently performed to address these problems. Preliminary analysis of δ15N in bulk nitrogen show little change with depth, suggesting that the net diagenetic fractionation of bulk nitrogen isotopes is small at these sites.