Can sediment geochemistry record changes in coastal upwelling palaeoproductivity? Evidence from northwest Africa and the Arabian Sea

Abstract

Abstract The causes and consequences of organic carbon (C org ) burial in sediments underlying regions of coastal upwelling are of profound importance to studies of palaeoceanography and palaeoclimatology alike. The primary control on coastal upwelling, wind stress and resulting Ekman transport, is linked intimately to variation in the Earth’s insolation record via a variety of feedback mechanisms. In comparison, the nutrients required to sustain productivity, and the factors affecting the C org content of the sediment, are related to variation in global glacial boundary conditions. Using a variety of geochemical indicators, it is apparent that the record of upwelling off northwest Africa, driven by the northeast trade winds, is characterized by arid glacial conditions and high wind stress sustaining elevated euphotic zone productivity. The resulting C org accumulation record reflects higher palaeoproductivity during glacial episodes. In contrast, the Arabian Sea upwelling is seasonal. Faunal and geochemical (Ba/Al, U/Th ratios) indicators suggest strong interglacial (23 ka) cyclicity in the Southwest Monsoon. The C org record is spatially and temporally consistent, but in disagreement with other palaeoproductivity indicators. These results point to the decoupling of surface ocean productivity in areas of coastal upwelling from deep-ocean circulation and nutrient supply. This result has important consequences for driving atmospheric p CO 2 changes on timescales less than that of oceanic mixing. Further applications of geochemical tracers to areas of coastal upwelling, such as northwest Africa and Peru, are needed urgently.