A biomarker perspective on paleoproductivity variations in two Late Quaternary sediment sections from the Southeast Atlantic Ocean

Sedimentary inventories of selected molecular biomarkers in two sediment sections from the African continental margin below highly productive surface waters in the Southeast Atlantic ocean, each spanning the last 76 kyr, were investigated and compared to each other. A principal component analysis of the stratigraphic data separated groups of biomarkers, representing different assemblages of biological sources and/or different diagenetic histories, related to their chemical structures and their metabolic role in the food chain. The concentrations of long-chain n-alkenones, n-diols, the major n-ketool, and loliolide and its epimer isololiolide, are correlated with paleoproductivity estimates that were calculated from organic carbon contents. This group of relatively refractory compounds very likely represents those planktonic communities, which contributed significantly to surface water productivity and subsequent burial of organic carbon during the considered time interval. Concentrations of (dust-transported) terrigenous long-chain fatty acids (C24 to C28) are interpreted to be related to the intensity of upwelling-driving winds and are also significantly correlated with high contents of organic matter. Concentrations of cholesterol and the major short-chain fatty acids are not related to the marine paleoproductivity signal, most likely as a consequence of their nonspecific origin and their metabolic role during heterotrophic feeding processes. Abundances of 24-ethylcholest-5-en-3β-ol and its saturated counterpart, 24-ethyl-5α-cholestan-3β-ol, the major steroid alcohols in both sedimentary environments, appear not to be related to any of the above mentioned components and probably represent the contribution of algal species adapted to nutrient conditions different from those of other major primary producers. Differences between both environments, the Angola Dome and the Benguela coastal upwelling, are evident in the relative distribution of major components and the stratigraphic behavior of selected compounds relative to other variables. These differences are discussed in relation to specific characteristics of both oceanographic environments.© 1999 Elsevier Science Ltd. All rights reserved.