Stable nitrogen isotopes in Angola Basin surface sediments

Abstract

Sedimentary δ 15 N contains a record of biogeochemical processes occurring in the water column. Stable nitrogen isotopes were measured in the bulk sedimentary matter of 18 surface sediment samples collected in three transects in the eastern Angola Basin, perpendicular to the coast, at 6 °S, 12 °S and 17 °S and two samples from the Zaire estuary. Relative enrichment in 15N in sediments with distance from the coast was seen in each of the three transects off northern, middle and southern Angola. Values in the Zaire Fan (northern Angola) and southern Angola increased by 1.9%. and 1.6%., respectively, from shallow (~100 m) to deep (~4500 m) water sediments and in middle Angola by 2.5%. (water depth range 73–3809 m). We propose that in the Angola region the degree of fractionation of nitrogen isotopes in organic matter is a function of nutrient supply to sunlit waters. The shift towards lighter isotopic values in shallow water signifies that the larger nitrate pool is not utilized as extensively as in water farther away from the coastal upwelling centers. Southern Angola sediments were more than 1%. lower (average δ 15 N, 5.4%.) than the middle and northern transects (average δ 15 N, 7.2%. and 6.5%., respectively). This difference is attributed to higher nitrate concentrations in the surface water at 17 °S, for which there are two reasons. One is that these sediments are located south of the Angola/Benguela front at ~16 °S which demarcates the convergence of warm equatorial water with cold, nutrient-rich Benguela Current water. The second cause is the intense Namibian coastal upwelling zone extending northward up to the front. Comparisons of bulk δ 15 N with δ 13 C org and C N ratios show that the observed sedimentary δ 15 N variations with water depth are due neither to water column diagenesis nor to mixing of terrestrial material with marine-derived organic matter. The two estuary sediment samples were mainly terrigenous in origin, based on δ 13 C org and C N ratios. δ 15 N values of these two samples were 7.0%. and 7.6%.