Stable isotope geochemistry of nitrogen in marine particulates

Abstract

Isotope studies of nitrogen and carbon were undertaken to investigate the fate of particulate organic matter (POM) during its residence in the water column and after deposition on the seafloor. The processes focused on were water-column transformations and sedimentary diagenesis. Sampling sites were chosen to provide POM subject to different specific mineralization processes (nitrification, denitrification, and sulfate reduction), different lengths of water column (duration of the mineralization process), and differences in the size of the organic-matter flux. The δl5N and δ13C of plankton, POM, and sediments from several oceanic sites were related to biological and hydrographic processes identified from nutrient, temperature, and salinity profiles. This was done to determine what effect these processes have on the δ15N of POM. Four stations were studied in the upwelling area off the coast of Peru and one station was studied in the Gulf of Maine. Important factors controlling the δ15N of plankton appear to be the concentration and δl5N of nitrate in the surface waters, and the relative zooplankton and phytoplankton abundances. Plankton from the Peru Upwelling Area are enriched in 15N as compared to plankton from other parts of the world's oceans where denitrification is absent. This enrichment may be due to the assimilation of 15N-enriched nitrate, produced by the selective reduction of 14N during denitrification. Zooplankton are 3 to 4% enriched in 15N as compared with phytoplankton. Production of 14N -enriched fecal pellets is suggested as a mechanism for this trophic enrichment. In the surface waters, the δl5N of POM is similar to that of plankton. In the Peru Upwelling Area, the δ15N of POM from the oxygen-deficient waters decreases with increasing depth. In the Gulf of Maine, below the euphotic zone in the oxic deep waters, the δ15N of POM increases with increasing depth. The difference in isotopic alteration may be due to the effect of different redox conditions on the mechanism and sequence by which specific organic nitrogen compounds, variably enriched in 15M, undergo degradation. Furthermore, bacterial growth on nitrogen-poor particles in the deep waters of the Peru Upwelling Area may contribute to the low δ15N of POM. In contrast to the large range in δ15N (-2 to +17%) of the POM, the range of δ15N in the sediments is small (+5 to +9%). Within a core, the average variation in δ15N was only 1.8%. Temporal variability in the δ15N of sedimenting POM and benthic activity appear to be important in determining the δ15N of the sediments. The large changes in POM concentration and isotope content at the sediment/water interface as compared with the more constant values found down-core, suggest that processes occuring at the sediment/water interface are critical, although bioturbation may also be important in determining the δ15N of oxic sediments. If diagenesis causes a significant loss of organic matter, profiles of organic carbon and nitrogen contents should show decreases with increasing depth and C/N ratios should increase with increasing depth (Reimers, 1981). Since none of the sedimentary profiles exhibited such trends, it is concluded that diagenesis was insufficient to erase the percent carbon, nitrogen and C/N ratio signatures generated by the POM flux and alterations at the sediment/water interface. Temporal variability in the δ15N of bottom-water POM may be caused by changes in deep-water currents which transport POM horizontally and to changes in bacterial and possibly other biological activity in the water column. This thesis work suggests that δ15N may be a useful tool in studying the geochemistry of POM in the marine environment. In addition, this research has shown that interpretation of the sedimentary 15N record must include consideration of isotopic alteration associated with bacterial remineralization of POM and benthic activity.