Ocean acidification induced by atmospheric CO (sub 2) may be a major threat to marine ecosystems, particularly to calcareous nannoplankton. We show that, during the Aptian ( approximately 120 million years ago) Oceanic Anoxic Event 1a, which resulted from a massive addition of volcanic CO (sub 2) , the morphological features of calcareous nannofossils traced the biological response to acidified surface waters. We observe the demise of heavily calcified nannoconids and reduced calcite paleofluxes at the beginning of a pre-anoxia calcification crisis. Ephemeral coccolith dwarfism and malformation represent species-specific adjustments to survive lower pH, whereas later, abundance peaks indicate intermittent alkalinity recovery. Deepwater acidification occurred with a delay of 25,000 to 30,000 years. After the dissolution climax, nannoplankton and carbonate recovery developed over approximately 160,000 years under persisting global dysoxia-anoxia.