LATE QUATERNARY EVOLUTION OF FERTILITY IN DICATORS AND MONSOON, IN THE SOMALIAN BASIN, NORTHWEST INDIAN-OCEAN

Time series of geochemical and biological markers in pelagic sediments , such as partial derivative(13)C N.dutertrei and URI (Upwelling Radio larian Index), have been used to reconstruct upwelling changes in the Somalian upwelling area (NW Indian Ocean). Other fertility indicators, such as G. bulloides, SiO2 and Ba fluxes, have been used to monitor m onsoon changes in the Arabian Sea. Here we compare time series of upwe lling proxies from two cores located under the Equator (Core MD 85668) and under the Somalian gyre (Core MD 85574) to data previously publis hed for the Arabian Sea. To quantify relationships between fertility a nd climatic indicators, cross spectral comparisons with ETP (eccentric ity + tilt + precession composite signal) have been used to estimate c oherences in conjunction with phase relationships. Strong coherencies are recorded over the excentricity band (100 kyr) by fertility tracers from both Somalian sites, which highlight the influence, in that area , of large scale climate changes associated with glacial-interglacial variability. The fertility markers demonstrate significant coherence w ith precession and obliquity at the equatorial sire whereas under the upwelling, coherence is good for URI in the obliquity band and for G. bulloides % and partial derivative(13)C N.dutertrei minima, in the 23 kyr (precessional) band. Phase relationships show, however, that radia tive forcing alone cannot account for the timing of stronger upwelling and/or fertility. Responses of these markers in the precessional band lead monsoon indicators from the Arabian Sea. Both Somalian and Arabi an upwelling systems being mostly induced by the Indian Summer Monsoon , the results demonstrate that fertility or upwelling tracers cannot b e used to reconstruct palaeomonsoon changes in the Somalian area. As i t has been already observed in the Arabian sea, control on the nutrien t supply may not necessarily be related only to monsoon intensity. Var iations of geochemical and biological markers in the Somalian basin ap pear to be mostly related to water mass reorganization induced by glob al climate changes and in some cases to the distribution of solar radi ation.