Isotope fractionation during nitrogen remineralization (ammonification): Implications for nitrogen isotope biogeochemistry

The 15N/14N isotopic ratio of marine particulate organic matter usually increases as a consequence of preferential 14N-remineralization (ammonification) during decay in the oxic water-column and at the oxic seafloor. However, a fractionation factor 15ε of sediment ammonification in natural environments has not been determined yet. Here, the 15ε of sedimentary ammonification is calculated in a set of seven sapropel containing sediment cores, two subrecent sediment cores and four sediment trap deployments from the Eastern Mediterranean Sea. Data clearly reveal Rayleigh type isotope fractionation and a fractionation factor 15ε between −1.43‰ and −2.3‰. Hence, sediments – and probably sinking particulate organic matter as well – can be important sources of isotopically depleted reactive nitrogen. The knowledge of 15ε further enables to reconstruct paleo export fluxes from sediment cores if original and actual δ15N as well as N accumulation rates are known.