Coccolithophore response to oceanographic changes in the Equatorial Atlantic during the last 200,000 years
A deep-sea core from the eastern equatorial Atlantic (Meteor core 16772, 1°21′S, 11°58′W) revealed strong primary productivity variations (70–230 gC/m2/year) based on the relative abundance (%) of the deep living coccolithophore Florisphaera profunda with productivity maxima following temperature minima during the last 200,000 years. The response of the coccolithophore flora was investigated by relative and absolute abundance records of coccoliths. The wind strength and the resulting equatorial upwelling intensity are thought to comprise the most important control of coccolith abundance and species composition, which reflects the different abundance of coccolithophores living in the upper or lower photic zone. Relative abundances of the different species varies widely throughout the core. Although a wide range of species were present in the different samples, three different taxa dominated the assemblage. The lower photic zone species F. profunda was the most abundant, with a mean relative abundance of 49.4% throughout the core. The upper photic zone flora were divided into three groups. Emiliania huxleyi together with small Gephyrocapsa (<3 μm) was the second most abundant species group (mean 20.8%). Gephyrocapsa oceanica was the third most important species in the core and had a mean relative abundance of 16.8%. All other species together represented a mean of 13%. Highest absolute abundance of coccoliths (number/gram sediment) occurs in sediments deposited during periods of high SST, mainly in warm isotope stages 1 and 5. The accumulation rates of coccoliths (number/cm2/ky) show peaks in isotope stages 1 and 5, but also in some parts of cool isotope stages 4 and 6. High relative and absolute abundance of F. profunda were found in warm isotope stages, which suggests that a deep nutricline existed and that primary productivity was low. Absolute and relative abundance records of E. huxleyi and small Gephyrocapsa (<3μm) mainly showed higher values in sediments from warm periods. The other upper photic zone coccolith species were generally more abundant during periods of low SST, particularly in isotope stages 2, 4, and 6, which were times of high primary productivity and a shallow nutricline. This suggest that E. huxleyi and small Gephyrocapsa probably were able to live deeper in the water column than most other species during periods of low nutrient supply to the upper photic zone. In summary, the composition of the coccolith flora can be explained by the nutricline depth level with three different assemblages being recognized, a deep photic zone assemblage consisting of F. profunda, a middle-upper photic zone assemblage of E. huxleyi and small Gephyrocapsa and an upper photic zone assemblage consisting of all other species.