Monsoon-driven export fluxes and early diagenesis of particulate nitrogen and its δ15N across the Somalia margin

被引:8
作者
Brummer, GJA [1 ]
Kloosterhuis, HT [1 ]
Helder, W [1 ]
机构
[1] Netherlands Inst Sea Res, Dept Marine Chem & Geol, NIOZ, NL-1790 AB Den Burg, Netherlands
来源
TECTONIC AND CLIMATIC EVOLUTION OF THE ARABIAN SEA REGION | 2002年 / 195卷
关键词
D O I
10.1144/GSL.SP.2002.195.01.19
中图分类号
P5 [地质学];
学科分类号
0709 ; 081803 ;
摘要
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 +/- 0.2parts per thousand in their stable isotope composition. Seasonal minima in delta(15)N of 3.7parts 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 muM of NO(3)(-) in the photic layer, maxima in the N export flux, and an increasing delta(15)N by Rayleigh distillation. Yet as stratification develops, nutrient exhaustion follows and export production collapses as the delta(15)N increases to over 7parts per thousand. Cyanobacterial N(2) fixation probably diminishes the delta(15)N by 0.4-1.6parts 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(15)N of up to 7.4parts per thousand. Annual N fluxes have virtually the same delta(15)N of 6.0parts per thousand in all traps despite considerable differences in both N flux and delta(15)N between the traps during the year and at different depths. In comparison with the annual delta(15)N of 6.0parts per thousand arriving on the sea floor, core-top sediments are enriched by +0.6parts per thousand on the upper slope (at 487 m) increasing to +2.9parts 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 d15N 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(15)N down continental margins in general.
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页码:353 / 370
页数:18
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