ONSET OF AN ARID CLIMATE AT 3.5 KA IN THE TROPICS - EVIDENCE FROM MONSOON UPWELLING RECORD

Studies on the variability of Southwest (SW) monsoon strength using th e monsoon upwelling indices (fluxes of total planktonic foraminifera a nd Globigerina bulloides) from the western Arabian Sea reveal that the weakening phase of the SW monsoon started from 5 ka (ka = 1000 years) . The intensity off monsoon returned to glacial strength at 3.5 ka, co inciding with the onset of arid climate elsewhere in the tropics. The onset of the weak phase of the monsoon and arid climate at 3.5 ka appe ars to be a primary reason for the decline of Indus Valley Civilizatio n, major change in vegetation along the Western Chats and decrease of river discharge from all major rivers during that period.