Organic-walled dinoflagellate cyst assemblages in the tropical Atlantic Ocean and oceanographical changes over the last 140 ka

A palaeoceanographic reconstruction of the Late Quaternary tropical Atlanti c Ocean has been made on the basis of dinoflagellate cyst associations of t wo sediment cores: the first core was recovered from below the highly produ ctive waters of the equatorial divergence and the second from the oligotrop hic western tropical Atlantic Ocean. Palaeoenvironmental indicators for pro ductivity, sea surface temperature (SST) and salinity (SSS) based on select ed organic-walled dinoflagellate cyst species have been established. On the basis of these palaeoenvironmental indicators, a strengthened intensity of the equatorial divergence in the eastern region during glacials and cold p eriods of interglacials has been reconstructed. The highest SST probably oc curred around substage 5.5 and might refer to weakest upwelling intensity. In comparison, SST and SSS appear to have been generally higher in the west ern tropical Atlantic Ocean, with probably enhanced values during glacial i ntervals. Pronounced differences in accumulation rates and relative abundan ces of cysts formed by congruentidiacean dinoflagellates and relative abund ances of oligotrophic cyst species between the eastern and the western regi on can be related to differences in palaeoproductivity, suggesting much hig her values in the eastern area. The coherence between variation in frequenc y of the indicators for productivity and the boreal summer insolation and m onsoon intensity in the eastern tropical Atlantic Ocean suggests an oceanog raphic reflection of regional intertropical, rather than boreal, dynamics. (C) 2000 Elsevier Science B.V. All rights reserved.