Contrary to the relatively well understood physical processes that mediate C acquisition, the biological pump remains poorly described due to the high spatial and temporal variability of mechanisms such as uptake, remineralization, or export of key elements, like Fe 15. Despite this limitation, this oceanic region is one of the major sinks of anthropogenic carbon dioxide (CO 2) 8, 9, 10, removing an estimated 10 × 10 13 kg carbon (C) from the pelagic zone each year 11, 12, 13, 14. In the Southern Ocean, several areas such as the Antarctic Circumpolar Current (ACC) or regional zones with seasonally driven iron (Fe) depletions (e.g., Ross Sea polynya) are considered high-nutrient, low-chlorophyll regions 1, 2, 3, where phytoplankton growth is limited by Fe availability 4, 5, 6, 7. With an average guano concentration of 3 mg iron g −1, we estimate that the Chinstrap penguin population is recycling 521 tonnes iron yr −1, representing the current iron contribution half of the amount these penguins were able to recycle four decades ago, as they have declined by more than 50% since then. Our results show that these seabirds are a relevant contributor to the iron remobilization pool in the Southern Ocean. Here, we use breeding site guano volumes estimated from drone images, deep learning-powered penguin census, and guano chemical composition to assess the iron export to the Antarctic waters from one of the most abundant penguin species, the Chinstrap penguin ( Pygoscelis antarcticus). However, penguins have received limited attention despite their representing the largest seabird biomass in the southern polar region. In this area, iron-rich Antarctic krill ( Euphausia superba) and baleen whale species, which are among their main predators, play a large role in the recycling of iron. Iron plays a crucial role in the high-nutrient, low-chlorophyll Southern Ocean regions, promoting phytoplankton growth and enhancing atmospheric carbon sequestration.

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