Higher surface production is generally reflected in higher organic carbon content of the underlying sediments. The bulk of the accumulating carbon is believed to be exported by aggregation and sinking of intact cells and chains in the aftermath of diatom blooms. Less is known about the factors triggering the sinking process and the mechanisms of aggregate formation. We studied the growth and demise of individual diatom species populations during an in situ phytoplankton bloom induced by artificial iron fertilization (EIFEX: European Iron Fertilization Experiment). A massive flux event, which appeared as spikes in vertical transmissometer profiles and was confirmed by discrete water-column sampling, started on day 26 during which about half the suspended particulate organic carbon was exported out of the surface mixed layer. A significant fraction of the sinking particles reached the sea floor at 3700 m depth within 10 days. These speeds can only be reached by prior formation of large aggregates. The species composition of the diatoms in the deep water column indicated that some species sank out quantitatively, others seemed to have been entrained in the sinking aggregates and yet others escaped entrainment and sinking and continued growth in the surface layer after the sinking event. Species of the long-spined diatom genus Chaetoceros appear to have played a major role in the aggregate formation and sinking event. The results of the experiment suggest that large-scale fertilization of the Southern Ocean could well function as one of several strategies to combat ongoing global warming and surface ocean acidification.