STABLE NITROGEN ISOTOPES IN ANGOLA BASIN SURFACE SEDIMENTS

Sedimentary delta(15)N contains a record of biogeochemical processes o ccurring in the water column. Stable nitrogen isotopes were measured i n the bulk sedimentary matter of 18 surface sediment samples collected in three transects in the eastern Angola Basin, perpendicular to the coast, at 6 degrees S, 12 degrees S and 17 degrees S and two samples f rom the Zaire estuary. Relative enrichment in N-15 in sediments with d istance from the coast was seen in each of the three transects off nor thern, middle and southern Angola. Values in the Zaire Fan (northern A ngola) and southern Angola increased by 1.9 parts per thousand and 1.6 parts per thousand, respectively, from shallow (similar to 100 m) to deep (similar to 4500 m) water sediments and in middle Angola by 2.5 p arts per thousand (water depth range 73-3809 m). We propose that in th e Angola region the degree of fractionation of nitrogen isotopes in or ganic matter is a function of nutrient supply to sunlit waters. The sh ift towards lighter isotopic values in shallow water signifies that th e larger nitrate pool is not utilized as extensively as in water farth er away from the coastal upwelling centers. Southern Angola sediments were more than 1 parts per thousand lower (average delta(15)N, 5.4 par ts per thousand) than the middle and northern transects (average delta (15)N, 7.2 parts per thousand and 6.5 parts per thousand, respectively ). This difference is attributed to higher nitrate concentrations in t he surface water at 17 degrees S, for which there are two reasons. One is that these sediments are located south of the Angola/Benguela fron t at similar to 16 degrees 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 delta(15)N With delta(1 3)C(org) and C/N ratios show that the observed sedimentary delta(15)N variations with water depth are due neither to water column diagenesis nor to mixing of terrestrial material with marine-derived organic mat ter. The two estuary sediment samples were mainly terrigenous in origi n,based on delta(13)C(org)and C/N ratios. delta(15)N values of these t wo samples were 7.0 parts per thousand and 7.6 parts per thousand.