RECENT SEDIMENTARY RECORDS FROM THE ARABIAN SEA

An attempt is made to understand the redox conditions that prevailed i n the north eastern continental margins of the Arabian Sea and in the nearby deep water regions during the past few centuries using short un disturbed sediment cores. The geochronology is accomplished using Pb-2 10 excess method and the proxy indicators chosen for productivity and associated redox changes are CaCO3, organic matter (OM), Mn and U alon g with major elements Fe and Al. Such changes in principle are related to high productivity in the overlying waters which in turn depend on monsoonal intensity that causes upwelling responsible for increase in productivity. Alongwith the published data on gravity cores from the s ame region, our measurements suggest the following: At approximately 3 00 m water depth, south of 21-degrees-N, the sediment-water interface at depths of approximately 300 m had been anoxic during the time span represented by the presently studied cores for approximately approxima tely 700 y as evidenced by low Mn/Al (< 0.7 x 10(-2)) and high U/Al (> 10(-4)) weight ratios. In some adjacent deeper regions, however, the environment turned oxic around approximately 200 y BP. Whereas both Mn and Ra were lost to the overlying waters in the anoxic regions (depth approximately 340 m), the Mn that diffused from deeper sections appea rs to have mineralized at the sediment-water-interface. Studies of thi s type on long undisturbed cores from the margins of the Arabian Sea a nd the Bay of Bengal, involving several proxies and geochronology by m ore than one method are needed to understand short term environmental (and monsoonal intensity) changes of the recent past with high resolut ion.