<jats:p> The iron-limited Southern Ocean plays an important role in regulating atmospheric CO <jats:sub>2</jats:sub> levels. Marine mammal respiration has been proposed to decrease the efficiency of the Southern Ocean biological pump by returning photosynthetically fixed carbon to the atmosphere. Here, we show that by consuming prey at depth and defecating iron-rich liquid faeces into the photic zone, sperm whales ( <jats:italic>Physeter macrocephalus</jats:italic> ) instead stimulate new primary production and carbon export to the deep ocean. We estimate that Southern Ocean sperm whales defecate 50 tonnes of iron into the photic zone each year. Molar ratios of C <jats:sub>export</jats:sub> ∶Fe <jats:sub>added</jats:sub> determined during natural ocean fertilization events are used to estimate the amount of carbon exported to the deep ocean in response to the iron defecated by sperm whales. We find that Southern Ocean sperm whales stimulate the export of 4 × 10 <jats:sup>5</jats:sup> tonnes of carbon per year to the deep ocean and respire only 2 × 10 <jats:sup>5</jats:sup>  tonnes of carbon per year. By enhancing new primary production, the populations of 12 000 sperm whales in the Southern Ocean act as a carbon sink, removing 2 × 10 <jats:sup>5</jats:sup> tonnes more carbon from the atmosphere than they add during respiration. The ability of the Southern Ocean to act as a carbon sink may have been diminished by large-scale removal of sperm whales during industrial whaling. </jats:p>