<p>A phytoplankton bloom spanning 1 million km² is consistently observed in the western Indian Subantarctic Zone. This oceanic region between the Subtropical and Subantarctic fronts in the Indian sector of the Southern Ocean contributes 20–40% of Southern Ocean carbon export to the deep ocean. However, aeolian inputs of iron, the key limiting nutrient for primary production, support only half of the phytoplanktonic iron demand of this bloom. Here we show that primary production in the western Indian Subantarctic Zone is sustained by long-range transport of iron via the Agulhas Current, one of the strongest ocean currents, which flows southwestward along the southern African coast. Float trajectories and high-resolution model diagnostics indicate that these waters, enriched with iron over the African margin, cross the Subtropical Front via the region’s intense mesoscale eddy variability. Removing the African sedimentary iron source in the model decreases surface iron concentrations in the western Indian Subantarctic Zone by 55%, reducing annual primary production and carbon export by 25% and 26%, respectively. Strengthening of the Agulhas Return Current over the past 130 kyr may therefore have increased iron supply and palaeoproductivity in the Indian Subantarctic Zone, potentially leading to enhanced atmospheric CO₂ drawdown.</p>