The sources and sinks of atmospheric carbon dioxide over glacial/interglacial cycles are under debate. Variation in productivity of the Antarctic Circumpolar Current (ACC) could potentially play a significant role, but current interpretations of sedimentary geochemical proxies suggest that glacial productivity was not higher than today. We present areal and down-core distribution patterns of previously overlooked diatom resting spores that indicate the occurrence of extensive phytoplankton blooms across the entire Atlantic sector of the ACC, particularly in the seasonal ice zone (SIZ), linked to higher iron input during the last glacial. Sea ice acts as an effective transporter of iron and enhances its bioavailability. The dominance of the deep living radiolarian Cyladophora davisiana in glacial SIZ sediments indicates that organic carbon export to mesopelagic depths was at least tenfold higher than today.