PALEOHYDROGRAPHIC CHANGES IN THE SOMALI BASIN (5-DEGREES-N UPWELLING AND EQUATORIAL AREAS) DURING THE LAST 160 KYR, BASED ON CORRESPONDENCE-ANALYSIS OF FORAMINIFERAL AND RADIOLARIAN ASSEMBLAGES

This study documents the biological signatures impressed upon the sedi mentary record underlying both the 5 degrees N upwelling system of the Somali Current and the equatorial area of the Somali Basin out of the upwelling influence. The evolution of these two distinct hydrographic systems is compared for the last 160 kyr. Correspondence and cluster analyses are performed on combined radiolarian and planktonic foramini feral quantitative data in order to study the changes of the planktoni c assemblages through time and space. The Upwelling Radiolarian Index (URI) is used as a productivity proxy, The water temperature and hydro graphic structure of the upper water masses appear to be the major fac tors controlling the distribution patterns of the fauna. The relative abundances of three groups of foraminifera, cold water form (dextral N . pachyderma), mixed layer dwellers (G. trilobus, G. ruber, G. sacculi fer, G. conglobatus, and G. glutinata), and thermocline dwellers (G. m enardii, G. tumida, N. dutertrei, G. crassaformis, and P. obliquilocul ata), follow distinct evolutionary patterns at the two sites during th e last 160 kyr, At the equatorial site (core MD 85668), downcore fluct uations in the relative abundances of the three groups are closely rel ated to the glacial/interglacial cyclicity and provide some insights i nto the interpretation of hydrographic changes, The dominance of the m ixed layer foraminifera at the transition intervals between isotope st ages 6/5 and 2/1, combined with weak URI values, is thought to reflect the reorganization of the oceanographic circulation. These short-term events (with a duration of < 5000 year) could be related to the rapid inflow of oxygen-depleted water through the Indonesian straits as a r esult of sea level rise during deglaciation, Underneath the 5 degrees N gyre (core MD 85674), the response to global climatic changes is ove rprinted by the regional effect of the Somalian upwelling, which has b een persistent over the last 160 kyr.