Nitrogen Isotopes in Sinking Particles and Surface Sediments in the Central and Southern Atlantic

Abstract

This manuscript provides an overview of sedimentary nitrogen isotope records in the tropical and southern Atlantic Ocean. Sedimentary δ15N in most of this region reflects the extent of surface water nitrate depletion. Nitrogen isotopes in sediments from the coastal upwelling regions of the Angola Basin and Benguela region off of Africa ranged from 5 to 12‰ and were negatively correlated with averaged near surface (0 – 50 m) historical nitrate concentrations. Coincidence of low δ15N in sinking particles (2 – 5‰) with low sea surface temperatures and high fluxes confirm the importance of relative nitrate utilization for the isotopic composition of organic matter in the Benguela during modem times. Off the Brazilian coast, isotope ratios were 5 – 7‰ and showed weak correspondence to surface nitrate concentrations, which are low. The expected relationship between nitrate and δ15N may be obscured here and in other oligotrophic regions because the δ15N of the small pool of nitrate can be readily altered by the advection or diffusion of even low levels of nitrate with a different isotopic composition. In the tropical and central South Atlantic, sediment isotopic values were between 6 and 11‰. The lack of any apparent relationship between δ15N and surface nitrate may be partly due to a paucity of nitrate concentration data, but δ15N near the equator also may be influenced by nitrogen fixation. In these oligotrophic waters, the input of iron via aeolian transport of African dust may support the fixation of N2 into organic matter, thereby lowering δ15N. Higher input of dust to the surface waters north of the equator is hypothesized to be the cause of the southward δ15N increase in sinking particles revealed by a north-south transect of moored sediment traps. In the southern South Atlantic, relative nitrate utilization is evidently the main control on sedimentary nitrogen isotopes between around 35°S and 50°S, where average near surface nitrate concentrations were strongly correlated with δ15N. South of the Polar Front, at around 50°S, this relationship is not observable in our data and there is an apparent switch from nitrate-based primary production to production based on ammonium. Sinking particles at the Polar Front are enriched in 15N in austral winter and show δ15N minima when fluxes are high, but because of the consistently low relative nitrate utilization in the Southern Ocean, this pattern is likely caused by changes in the plankton community or to increased degradation during times of low flux. δ15N values throughout the tropical and southern Atlantic are correlated with sediment organic carbon content and also generally mirror primary productivity patterns, with low δ15N associated with areas of high productivity and vice versa. Sediment trap data indicate that sediments are enriched in 15N relative to sinking particles by up to 4‰. The offset (−2.0 to 4.3‰) does not vary greatly between the Polar Front, the productive Benguela upwelling area and the oligotrophic tropical Atlantic, in spite of the vastly different environmental conditions prevailing in these three regions.