Coccolithophores in the equatorial Atlantic Ocean: response to seasonal and Late Quaternary surface water variability

The present study was initiated to ascertain the significance of coccolitho phores as a proxy for paleoceanographic and paleoproductivity studies in th e equatorial Atlantic. Data from a range of different samples, from the pla nkton, surface sediments as well as sediment cores are shown and compared w ith each other. In general, the living coccolithophores in the surface and subsurface water s show considerable variation in cell numbers and distribution patterns. Ce ll densities reached a maximum of up to 300 x 10(3) coccospheres/l in the u pwelling area of the equatorial Atlantic. Here, Emiliania huxleyi is the do minant species with relatively high cell numbers, whereas Umbellosphaera ir regularis and Umbellosphaera tenuis are characteristic for oligotrophic sur face waters. Although they are observed in high relative abundances, these species only occur in low absolute numbers. The lower photic zone is domina ted by high abundances and considerable cell numbers of Florisphaera profun da. The geographical distribution pattern of coccoliths in surface sediments re flects the conditions of the overlying surface water masses. However, abund ances of the oligotrophic species Umbellosphaera irregularis and Umbellosph aera tenuis are strongly diminished, causing an increase in relative abunda nce of the lower photic zone taxa Florisphaera profunda and Gladiolithus fl abellatus. During the past 140,000 years the surface water circulation of the equatori al Atlantic has changed drastically, as can be seen from changes in the coc colithophore species composition, absolute coccolith numbers, as well as co ccolith accumulation rates. Significant increases in coccolith numbers and accumulation rates is observed in the southern equatorial Atlantic during t he last glacial interval (oxygen isotope stages 2-4), which we attribute to enhanced upwelling intensities and advection of cool nutrient rich waters at this site, in the western equatorial Atlantic we observe an opposite tre nd with decreasing numbers of coccoliths during glacial periods, which prob ably is caused by a deepening of the thermocline. (C) 2000 Published by Els evier Science B.V. All rights reserved.