Spatial variations in nutrient utilization, production and diagenesis in the sediments of a coastal upwelling regime (NW Africa): Implications for the paleoceanographic record

A biogeochemical study of recent (multicores) sediments of the northwest Af rican slope was undertaken to understand how the sediment composition varie s with respect to the location of core sites relative to the centers of coa stal upwelling, and how this has affected the palaeoceanographic record. Se dimentary organic carbon contents are inversely correlated with the nitroge n isotopic composition (delta N-15), high C-organic concentrations and low delta N-15 occurring at proximal (shallow) sites and the opposite at distal (deep) ones. These spatial differences are interpreted to result from high er relative nutrient utilization and a decrease in production as waters are advected offshore from the zone of upwelling. Highest C-organic contents a lso correlate positively with highest concentrations of redox-sensitive ele ments (U, Mo and S) that are fixed diagenetically in the sediments. These results suggest that the sedimentary regime at a fixed position depen ds on the spatial location of the productive areas relatively to a given co re site. Downcore records of Zr/Al, Ti/Al, mean grain size of the terrigeno us fraction, delta N-15, C-organic biogenic Ba, U, Mo and sulfur at a singl e site on the slope are interpreted to reflect glacial-interglacial changes in the core location relative to the coastline (sea-level effect), and hen ce changes in production as the area of coastal upwelling moved on- and off shore as sea-level changed, as well as undoubtedly changes in upwelling int ensity through wind forcing. Further studies are needed to fully understand the interrelationships of all these processes, which are required for buil ding more reliable paleoceanographic-paleoclimatic records.