Increased supply of the micronutrient iron promotes export production in the iron-limited Southern Ocean, thus acting as a dynamic sink of atmospheric CO2 that has amplified past climate variations. This mechanism is typically considered to be regulated by the amount and solubility of iron delivered by aeolian transport. Here we use sedimentological and geochemical tracers to investigate iron input and carbon uptake in the largest sector of the Southern Ocean Antarctic Zone. Our data show that millennial-scale variations in West Antarctic Ice Sheet dynamics controlled both the supply of particulate iron and lithogenic particle composition (affecting particle solubility) in the Pacific Antarctic Zone over the last 500,000 years. Rather than the total iron input, a higher abundance of chemically more pristine glaciomarine particles (high particle solubility) was critical for providing bioavailable iron, which enhanced export production. High lithogenic iron fluxes are characterized by chemically mature particles (low particle solubility), in particular during phases of pronounced ice loss in West Antarctica. The corresponding export production was low, indicating that this ‘ice-sheet–iron feedback’ is positive during these retreat phases. Accordingly, future West Antarctic Ice Sheet retreat is likely to decrease carbon uptake in the large Pacific sector of the Southern Ocean.