| The nitrogen isotope composition (delta15N) of sedimentary organic matter (Norg) is a powerful paleoceanographic proxy. However, the processes of early diagenesis may impact the delta15N of preserved organic matter. This work addressed the mechanism of diagenetic fractionation of delta15N in anoxic marine sediments by constructing isotopic mass balances on a variety of time scales, from hundreds to millions of years. The field studies were conducted in the California Borderland Basins, in the Eastern Subtropical North Pacific (ESNP) and along the Peru-Chile margin. The results demonstrated that under anoxic conditions, no isotopic fractionation was observed during decomposition of isotopically homogeneous Norg. However, if isotopically distinct components are present and differ in their labilities, preferential degradation of more labile fraction may alter the delta 15N of the Norg. Laboratory incubation experiments confirmed the field observations for the anoxic organic matter degradation. Experimental results in the presence of oxygen showed that degradation of organic matter leads to enrichment of 15N in the preserved Norg. The degradation process involves release of DON (Dissolved Organic Nitrogen) from organic matter and subsequent hydrolysis and deamination of DON. Fractionation, most likely, occurs at the deamination step, leaving DON heavy. Aerobic heterotrophic bacteria incorporate a portion of this isotopically enriched DON pool, perhaps fractionating delta15N during this step as well. Contrary to that, anaerobic bacteria are not able to assimilate DON in the same way, as aerobic bacteria do. Therefore, fractionation of delta15N occurs only during oxic decomposition of Norg.; Unusual enrichment in 15N of the pore water ammonium was observed in the upper 30 cm of sediments of Eastern Subtropical Pacific. Based on the field observations and the results of reaction-diffusion modeling, the pattern was interpreted as possible evidence for a novel type of chemosymbios between two chemolithotrophic bacteria, Thioploca and Anammox-like bacteria. The proposed chemosymbiosis may significantly impact sedimentary nitrogen cycling on large regional scales. These results provided new insights into the ecogeochemical role of benthic microbial communities in global biogeochemical cycles. |
