MECHANISMS FORCING ABRUPT FLUCTUATIONS OF THE INDIAN-OCEAN SUMMER MONSOON DURING THE LAST DEGLACIATION

A piston core from the Somali upwelling area has been studied at high resolution for its dinoflagellate cyst content. Variations in cyst ass ociation are inferred to reflect changes in Indian Ocean summer monsoo n intensity. Several abrupt fluctuations in monsoon intensity are dete cted for the interval between 20 and 10 ka BP. Comparison of these flu ctuations with changes in contrast between 30 degrees N and 30 degrees S July insolation and the delta(18)O GRIP ice-core values suggests th at different mechanisms influenced monsoon intensity at different time intervals. A general trend in monsoon intensity follows variations in insolation contrast, lagging minimum contrast by 7.5 ka and maximum c ontrast by 4 ka. This phase lag difference can be explained by assumin g the existence of an ice/snow cover over central Asia/Tibet during gl acial times. Between 18.7 and 12.5 ka BP fluctuations in SW-monsoon in tensity may be forced by variations in the thermohaline ocean circulat ion. A rapid transition towards strong SW-monsoons at 12.5 ka BP is po ssibly the result of variations in atmospheric circulation and melting of the snow/ice fields in central Asia/Tibet. Variations in glacial-i nterglacial boundary conditions related to temperature change at north ern latitudes are likely to have influenced SW-monsoon intensity betwe en 12.5 and 10 ka BP, whereas after 10 ka BP variations in tropical la nd surface boundary conditions may be the dominant forcing factor. (C) 1997 Elsevier Science Ltd.