Pergamon 1040-6182(95)00017-8 QuaternaryInternational, Vol. 31, pp. 13-18, 1996. Copyright© 1995INQUA/Eisevier ScienceLtd Printed in Great Britain.All rights reserved. 1040-6182196$29.00 DYNAMICS OF THE MONSOON IN THE EQUATORIAL INDIAN OCEAN OVER THE LAST 260,000 YEARS Luc Beaufort Laboratoire de G~ologie du Quaternaire, CNRS-CEREGE, 13549 Aix-en-Provence, France The coccolithophorids are excellent indicators of the variations in ocean productivity. As an example, the relative abundance of Florisphaera profunda, a species which lives exclusively in the deepest part of the photic zone, constitutes a reliable monitor of the depth of the nutricline as a function of climatic variability. Productivity in the Indian Ocean is closely linked with the monsoon. Its intensity affects the depth of the mixing zone which deepens as the wind stress increases. During the monsoon season, the nutricline is shallow resulting in a higher productivity. The variations in composition of eoccolith assemblages in well-dated oceanic sediments allows to describe the dynamics of productivity and therefore the fluctuation of the monsoon intensity. Core MD900963 was retrieved in the equatorial Indian Ocean, East of the Maldives at a water depth of 2400 m. A precise chronology was established by fine-tuning the high resolution 81sO record with the SPECMAP Stack. Coccoliths were counted from samples taken at 10 cm intervals in the core, providing a resolution of 2000 years for the last 260,000 years. The productivity estimates made from coccolith counts show that productivity varied greatly in the Maldives area during this time. The variations of the coceolith productivity index match the variations of the organic carbon content in the sediment. Spectral analysis reveals strong precessional cycles, and weak obliquity and eccentricity cycles. This implies that the solar radiation has a dominant effect on the monsoon variability South of India. The productivity maxima occur during even-numbered 81sO stages, This may indicate that the productivity events are related to increase of the winter monsoon. However the analysis of the phase between the palaeowoductivity and seasonal solar radiation curves calculated for the past 260,000 years suggest an alternative hypothesis: the westerlies which blow in April in the Maldives area (there, the onset of the Monsoon is in advance on the Arabian Sea) would have a major effect on productivity, since the insolation curve of the end of March at 5"N is in perfect phase with the paleoproductivity record for the last 260,000 years. INTRODUCTION The Asian Monsoon climate, driven by differential land-ocean sensible heating, produces seasonal reversals in the wind direction which affect strongly the Northern Indian Ocean. Palaeoelimate studies have shown that the intensity of the monsoon has varied through time, during the Last Glacial Maximum the summer monsoon was weaker (Prell et al., 1980; Duplessy, 1982), and the winter monsoon was stronger (Duplessy, 1982; Fontugne and Duplessy, 1986; Sarkar et al. 1990). Clemens et al. (1991) suggested that the variations of the summer monsoon intensity are associated with changes in the seasonal pattern of the solar radiation, whereas variability in global ice volume is not a primary factor controlling this intensity, which is not in agreement with general circulation model. The summer monsoon variability is recorded particularly well in the Arabian Sea where the strongest winds appears during summer (e.g. Knox, 1987). The seasonal variations of the wind stress over the surface water induce dramatic changes on oceanic productivity (e.g. Ittekkot et al., 1992; Curry et al., 1992) which can be identified in the fossil records from palaeoproductivity proxies (e.g. Cullen, 1981; Prell and Curry, 1981; Clemens and Prell, 1990; Canlet et al., 1992). The effects produced by the summer monsoon are particularly well observed in the areas of the Somalian and Arabian margins where seasonal upwelling are induced by the SW (summer) monsoon while it is stopped by the NE (winter) monsoon. The record of palaeoproductivity variations in these upwelling systems corresponds essentially to the dynamic of the summer monsoon. At the opposite, records of the variability of the winter monsoon were found in the Bay of Bengal and Andaman Sea (Fontugne and Duplessy, 1986) where this monsoon season is predominant. Today, very little is known about the variability of the seasonal winds over the rest of the Indian Ocean, where seasonal contrasts are less pronounced (regions where both monsoon seasons are recorded). The giant piston core MD900963 retrieved during the SEYMAMA expedition of the French R/V Marion Dufresne in 1990, at the junction between the Bay of Bengal and the Arabian Sea, provides an excellent record for investigating the dynamics of the monsoon in the equatorial Indian Ocean. COCCOLITHS AS PALAEOPRODUCTIVITY INDICATORS The coccolithophores (Primnesiophyceae) constitute one of the major phytoplanktonic group. As primary producers their growth depend on the amount of light and nutrients available in the photic zone. The photic zone in a stratified ocean presents strong contrasts of these two parameters on a vertical scale: the upper photic zone (0 to -60 m) is depleted in nutrients, whereas light is strongly limited in the lower photic zone (-60 to -180 m). The phytoplanktonic communities are adapted to these physicochemical differences and in consequence, their composition differs with depth (Venriek, 1982, 1990). At low latitude, the lower- photic-zone coccolithophore communities are dominated by Florisphaera profunda and Thorosphaera flabellata while most of the other species live in the upper part of the photic zone (Okada and Honjo, 1973). That aspect of the coccolith vertical zonation has been used successfully for palaeoproductivity studies by Molfino and McIntyre (1990), 13