Abstract

The δ15N signal in marine sediments appears to be a good palaeoceanographic tracer. It records biological processes in the water column and is transferred to and preserved in the sediments. Changes in forcing factors in upwelling systems may be recorded by δ15N. These forcing conditions can be of a biogeochemical nature, such as the initial isotopic signal of the nutrients or the trophic structure, or of a physical nature, such as wind stress, insolation, temperature or dynamic recycling. A simple nitrogen-based trophic chain model was used to follow the development of the nitrogen isotopic signal in nutrients, phytoplankton, zooplankton and detritus. Detrital δ15N, influenced by the isotopic signature of the upwelled nutrients and isotopic fractionation along the trophic chain (photosynthesis and zooplankton excretion), was then compared to the sedimentary signal measured off Mauritania. In our model, the biological variables are transported at shallow depths by a simple circulation scheme perpendicular to the coast depicting a continental shelf recirculation cell. Because cell length depends on the extension of the continental shelf, modifications of the cell length mimic sea level changes. Long cell length (high sea level) scenarios produce higher δ15N values whereas short cell length scenarios result in lower values as in the glacial low sea level periods. Despite changes in many climatic parameters throughout this period, our results show that changing the sea level is sufficient to reconstruct the main pattern of the sedimentary δ15N variations offshore of the Mauritanian upwelling, i.e. an increase from about 3‰ to 7‰ during the deglaciation, without invoking any change in nitrogen fixation or denitrification.

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