Correlations between Arabian Sea organic carbon and GISP2 δ18O records indicate a pronounced oxygen minimum zone (OMZ) during interstadials, whereas well-oxygenated conditions prevailed during stadials. Local deep winter mixing ventilated intermediate water during the coldest stadials, corresponding to North Atlantic Heinrich events. Here we show that in the Arabian Sea periods of climatic warming following Heinrich events H6--H4 and the Younger Dryas (YD) are characterized by dominant Polysphaeridium zoharyi (dinoflagellate) cysts. The finding of assemblages dominated by P. zoharyi in the open ocean is unusual because today similar assemblages are restricted to lagoonal settings. It is postulated that the highly saline mixed layer and the strong density gradient which characterized Arabian Sea hydrography after H6--H4 and the YD simulated a shallow seafloor, thereby enabling germination of cysts prior to sinking. The strong density gradient following cold stadials should have facilitated the rapid reestablishment of a pronounced OMZ during interstadials.