Diatom assemblages are vital components of the Antarctic ecosystem and nutrient supply chain, and they are often utilized as paleoclimate proxies to better understand past climatic changes. We demonstrate enhanced diatom production and accumulation in the outer Amundsen Sea during a Mid-Pliocene interglacial that coincides with pulses of ice-rafted terrestrial debris, providing compelling evidence that iceberg calving seeds diatom productivity in the Southern Ocean.