LATE QUATERNARY SURFACE CIRCULATION IN THE EAST EQUATORIAL SOUTH-ATLANTIC - EVIDENCE FROM ALKENONE SEA-SURFACE TEMPERATURES

Angola Basin and Walvis Ridge records of past sea surface temperatures (SST) derived from the alkenone U-37(k) index are used to reconstruct the surface circulation in the east equatorial South Atlantic for the last 200,000 years. Comparison of SST estimates from surface sediment s between 5 degrees and 20 degrees S with modern SST data suggests tha t the alkenone temperatures represent annual mean values of the surfac e mixed layer. Alkenone-derived temperatures for the warm climatic max ima of the Holocene and the penultimate interglacial are 1 to 4 degree s C higher than latest Holocene values. All records show glacial to in terglacial differences of about 3.5 degrees C in annual mean SST, whic h is about 1.5 degrees C greater than the difference estimated by CLIM AP (1981) for the eastern Angola Basin. At the Walvis Ridge, significa nt SST variance is observed at all of the Earth's orbital periodicitie s. SST records from the Angola Basin vary predominantly at 23- and 100 -kyr periodicities. For the precessional cycle, SST changes at the Wal vis Ridge correspond to variations of boreal summer insolation over Af rica and lead ice volume changes, suggesting that the east equatorial South Atlantic is sensitive to African monsoon intensity via trade-win d zonality. Angola Basin SST records lag those from the Walvis Ridge a nd the equatorial Atlantic by about 3 kyr. The comparison of Angola Ba sin and Walvis Ridge SST records implies that the Angola-Benguela Fron t (ABF) (currently at about 14-16 degrees S) has remained fairly stati onary between 12 degrees and 20 degrees S (the limits of our cores) du ring the last two glacial-interglacial cycles. The temperature contras t associated with the ABF exhibits a periodic 23-kyr variability which is coherent with changes in boreal summer insolation over Africa. The se observations suggest that surface waters north of the present ABF h ave not directly responded to monsoon-modulated changes in the trade-w ind vector, that the central field of zonally directed trades in the s outhern hemisphere was not shifted or extended northward by several de grees of latitude during glacials, and that a cyclonic gyre circulatio n has existed in the east equatorial South Atlantic over the last 200, 000 years. This scenario contradicts former assumptions of glacial int ensification of the Benguela Current into the eastern Angola Basin and increased coastal upwelling off Angola.