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Abstract

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‰ in their stable isotope composition. Seasonal minima in δ15N of 3.7‰ 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 μm of NO3 in the photic layer, maxima in the N export flux, and an increasing δ15N by Rayleigh distillation. Yet as stratification develops, nutrient exhaustion follows and export production collapses as the δ15N increases to over 7‰. Cyanobacterial N2 fixation probably diminishes the δ15N by 0.4-1.6‰ 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 δ15N of up to 7.4‰. Annual N fluxes have virtually the same δ15N of 6.0‰ in all traps despite considerable differences in both N flux and δ15N between the traps during the year and at different depths. In comparison with the annual δ15N of 6.0‰ arriving on the sea floor, core-top sediments are enriched by +0.6‰ on the upper slope (at 487 m) increasing to + 2.9‰ 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 δ15N 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 δ15N down continental margins in general.

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