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A common feature in δ15N profiles downcore in continental margin sediments is that the heaviest δ15N values are frequently observed during the deglaciation (i.e., between 12,000 calendar years BP and the climatic optimum at 6000 yrs BP), not at the warmest stage. Using a conceptual model across the northwestern Africa margin, a region of pronounced modern upwelling, as well as data from a core in the area, we show that this feature can be explained as a consequence of postglacial sea level rise. The model is based on a simplified twodimensional physical circulation scheme orthogonal to the margin and uses the topographic profile at the latitude of the core as well as a simplified biological model for nitrate utilization and nitrogen isotope fractionation. Shore-parallel influences are ignored. The most recently published age model of sea level rise for the last deglaciation is used [Bard et al., 1996]. The trangression causes a progressive increase in the area of shallow regions where large amounts of nutrients are recycled relative to deep regions, to which a significant portion of the nutrients is exported. This causes first an increase and then a decrease in the δ15N of the organic matter accumulating at a fixed point on the upper slope. Although the deglacial δ15N maximum is more pronounced in areas where there is not a marked oxygen minimum layer [Holmes et al., 1997; this paper], it does exist in areas where an oxygen minimum layer is present in the water column [Altabet et al., 1995; Ganeshram et al., 1995]. In such areas, the major δ15N contrast between glacial and interglacial episodes is explained by higher denitrification during interglacial stages, but it is probable that transgressing sea level contributes to this effect. The model has implications for the changes of vertical oceanic nutrient fractionation [Boyle, 1988] and the respiratory dissolution of deep carbonates [Archer and Maier-Reimer, 1994] and hence could have important potential implications for the timing of global CO2 exchanges between ocean and atmosphere and their feedback to climate. |
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