SEA-SURFACE TEMPERATURE AND PRODUCTIVITY RECORDS FOR THE PAST 240 KYRIN THE ARABIAN SEA

Deep-sea sediments of two cores from the western (TY93-929/P) and the southeastern (MD900963) Arabian Sea were used to study the variations of the Indian monsoon during previous climatic cycles. Core TY93-929/P was located between the SW monsoon driven upwelling centres off Somal ia and Oman, which are characterized by large seasonal sea surface tem perature (SST) and particle flux changes. By contrast, core MD900963, was situated near the Maldives platform, an equatorial ocean site with a rather small SST seasonality (less than 2 degrees C). For both core s we have reconstructed SST variations by means of the unsaturation ra tio of C-37 alkenones, which is compared with the delta(18)O records e stablished on planktonic foraminifera. In general, the SST records fol low the delta(18)O variations, with an SST maximum during oxygen isoto pe stage 5.5 (the Last Interglacial at about 120-130 kyr) and a broad SST minimum during isotope stage 4 and 3.3 (approximately 40-50 kyr). The SST difference between the Holocene and the Last Glacial Maximum ( LGM) is of the order of 2 degrees C. In both cores the SSTs during iso tope stage 6 are distinctly higher by 1-2 degrees C than the cold SST minima during the last glacial cycle (LGM and stage 3). To reconstruct qualitatively the past productivity variations for the two cores, we used the concentrations and fluxes of alkenones and organic carbon, to gether with a productivity index based on coccolith species (Florispha era profunda relative abundance). Within each core, there is a general agreement between the different palaeoproductivity proxies. In the so utheastern Arabian Sea (core MD900963), glacial stages correspond to r elatively high productivity, whereas warm interstadials coincide with low productivity. All time series of productivity proxies are dominate d by a cyclicity of about 21-23 kyr, which corresponds to the insolati on precessional cycle. A hypothesis could be that the NE monsoon winds were stronger during the glacial stages, which induced deepening of t he surface mixed layer and injection of nutrients to the euphotic zone . By contrast, the records are more complicated in the upwelling regio n of the western Arabian Sea (core TY93-929/P). This is partly due to large changes in the sedimentation rates, which were higher during spe cific periods (isotope stages 6, 5.4, 5.2, 3 and 2). Unlike core MD900 963, no simple relationship emerges from the comparison between the de lta(18)O stratigraphy and productivity records. The greater complexity observed for core TY93-929/P could be the result of the superimpositi on of different patterns of productivity fluctuations for the two mons oon seasons, the SW monsoon being enhanced during interglacial periods , whereas the NE monsoon was increased during glacial intervals. A sim ilar line of reasoning also could help explain the SST records by the superimposition of variations of three components: global atmospheric temperature, and SW and NE monsoon dynamics. (C) 1997 Elsevier Science Ltd.