Dynamics of the monsoon in the equatorial Indian Ocean over the last 260,000 years

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 coccolith 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 delta(18)O 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 coccolith 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 delta(18)O stages. This may indicate that the productivity events are related to increase of the winter monsoon. However the analysis of the phase between the palaeoproductivity 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 degrees N is in perfect phase with the paleoproductivity record for the last 260,000 years.