An integrated view of inorganic and organic biogeochemical indicators of palaeoproductivity changes in a coastal upwelling area

In order to evaluate the role of the continental margin areas in the changes of atmospheric CO2 during past climatic cycles, it is crucial to reconstruct reliably export palaeoproductivity changes in such areas. This paper examines and discusses the specific relationships between various productivity-dependent geochemical signals recorded in a single sediment core from the northwestern Africa continental slope, including both major and trace parameters of inorganic and organic origin. This paper clearly shows that the export productivity flux, as well as its composition (e.g. the relative contribution of calcareous, siliceous and mineral-free producers) have changed. These changes have subsequently modified benthic conditions, which can have had, in turn, a retroaction on major biogenic element contents. This effect seems, however, to be only minor compared to that one directly induced by changes in biogenic fluxes. These results indicate that direct palaeoproductivity transfer functions, which are based on present-day statistical calibrations of one single parameter and which are valid to some extent for sediments from abyssal open ocean, cannot be properly applied in marginal areas which are much more complex and, in particular, highly affected by sea level changes. In such areas, quantitative palaeoproductivity reconstructions should be only considered at a regional scale including off-shoreward, on-shoreward and latitudinal migration or extension of the productive areas, as well as the palaeoproductivity intensity and composition.