MAGNETIC-PROPERTIES OF SEDIMENTS IN THE BAY OF BENGAL AND THE ANDAMANSEA - IMPACT OF RAPID NORTH-ATLANTIC OCEAN CLIMATIC EVENTS ON THE STRENGTH OF THE INDIAN MONSOON

The results of a high-resolution mineral magnetic study combined with major element geochemistry analysis, oxygen isotopes and C-14 AMS stra tigraphy are reported for deep-sea gravity cores MD77-169 and MD77-180 located in the Andaman Sea and the Bay of Bengal, respectively. Core MD77-169 covers the last 280 kyr and core MD77-180 covers the last 160 kyr. In both cores, rock magnetic parameters indicate that the magnet ic assemblage is dominated by pseudo-single domain titanomagnetite gra ins, with grain-size variations following a strong 23 kyr periodicity. Smaller magnetic grain sizes are observed during periods characterize d by a strong summer monsoon. In addition, in core MD77-180, we observ e a correlation between magnetic grain size and a chemical index of al teration. This suggests that these magnetic grain-size changes are rel ated to chemical weathering driven by summer monsoon rainfall. A compa rison of the GISP2 ice core isotopic record and the magnetic grain-siz e record of the Bay of Bengal shows that rapid temperature variations documented in the ice core (Dansgaard-Oeschger cycles and Heinrich eve nts), during the last glacial period are also present in the magnetic grain-size record. Heinrich events and cold stadial events are charact erized by relatively large magnetic grain sizes. Furthermore, Heinrich events are characterized by lower values of the chemical index of alt eration implying a lower degree of chemical weathering related to sign ificantly drier conditions on the continent, We suggest that rapid col d events of the North Atlantic (Heinrich events) during the last glaci al stages are characterized by a weaker summer monsoon rainfall over t he Himalaya via an atmospheric teleconnection. (C) 1998 Elsevier Scien ce B.V. All rights reserved.