Enhanced aeolian supply of iron to the biota of the Southern Ocean during glacial periods is suspected to be an important contributory mechanism to the concurrently low observed mixing ratios of atmospheric CO2. Declining rates of dust deposition prior to the glacial terminations may be critical in driving the initial deglacial rise in CO2, but the reasons behind the dust decline itself are as yet unknown. Here we show that the dust record from the Vostok ice core can be qualitatively derived from a few general assumptions regarding the formation and aging of Patagonian sources of aeolian material and the efficiency with which it is transported through the atmosphere. We suggest that during glacial periods this dust supply becomes particularly sensitive to changes in global climate and that, in turn, climate is responsive to the dust due to iron fertilization. This positive feedback may mean that during glacial periods the carbon cycle exhibits two quasi steady states, characterized by distinct CO2 concentrations. Recognition of this glacial subcycle may help to account for the timing and sequence of events at the terminations.