Geochemistry of surface sediment samples from the South Atlantic Ocean

The aim of the present study is an evaluation of the applicability of biogenic barium as a proxy for productivity. For this purpose, 190 surface sediment samples from the South Atlantic Ocean were analysed for their barium and aluminium concentrations. Biogenic barium is estimated by subtracting the calculated terrigenous barium (obtained from the terrigenous Ba/Al ratio and the amount of Al in the sample) from the total Ba content in the sample. Based on the accumulation rates of biogenic barium, export production is estimated using three different algorithms proposed by [Paleoceanography 7 (1992) 163, doi:10.1029/92PA00181; Global Biogeochem. Cycles 9 (1995) 289, doi:10.1029/95GB00021; Geomar. Report 38 (1995) 105]. Primary productivity was calculated from these different export productions and compared with measurements of recent primary productivity in the overlying surface waters. Only the primary productions calculated on the basis of the algorithm of [Paleoceanography 7 (1992) 163, doi:10.1029/92PA00181] yield productivity values comparable to those existing in ocean surface waters. This study further reveals that it is not sufficient to use a constant, generally applicable organic carbon/biogenic barium ratio, as is postulated by [Global Biogeochem. Cycles 9 (1995) 289, doi:10.1029/95GB00021]. This ratio has to be assessed regionally. For the sediments of the Cape Basin in the eastern South Atlantic Ocean, a new algorithm is developed which gives plausible primary productivities for the overlying surface waters.