Publication Date:
2002-01-01
Description:
Settling nitrogen fluxes intercepted by sediment traps on the mid-slope and in the deep basin off Somalia show a consistent annual range of 3.4 {+/-} 0.2{per thousand} in their stable isotope composition. Seasonal minima in {delta}15N of 3.7{per thousand} are associated with the moderate N fluxes derived from coastally upwelled water, which is rapidly carried offshore along eddy margins passing over the mooring sites during the SW monsoon (June-September). Coastal upwelling, offshore transport and deep wind mixing cease at the end of the SW monsoon, leading to enhanced utilization of the up to 20 {micro}M of NO3- in the photic layer, maxima in the N export flux, and an increasing {delta}15N by Rayleigh distillation. Yet as stratification develops, nutrient exhaustion follows and export production collapses as the {delta}15N increases to over 7{per thousand}. Cyanobacterial N2 fixation probably diminishes the {delta}15N by 0.4-1.6{per thousand} during the autumn intermonsoon (November-December) when settling N fluxes are lowest. Nutrient utilization remains high during the NE monsoon (January-March), when nutrient entrainment by deep wind mixing results in enhanced N export with maxima in {delta}15N of up to 7.4{per thousand}. Annual N fluxes have virtually the same {delta}15N of 6.0{per thousand} in all traps despite considerable differences in both N flux and {delta}15N between the traps during the year and at different depths. In comparison with the annual {delta}15N of 6.0{per thousand} arriving on the sea floor, core-top sediments are enriched by +0.6{per thousand} on the upper slope (at 487 m) increasing to +2.9{per thousand} in the deep basin (at 4040 m), whereas the N sediment burial efficiency declines from about 17% to 3%. It appears that the extent of oxic decomposition at the sediment-water interface is the most likely cause of such isotope enrichment. Similar positive gradients in {delta}15N with bottom depth have been reported from other continental margin transects and are generally attributed to increased nutrient utilization in the photic ocean with distance offshore. As for Somalia, nitrogen isotope fractionation as a result of oxic decomposition on the bottom rather than nutrient utilization at the ocean surface may account for the observed increase of sedimentary {delta}15N down continental margins in general.
