Surface-water acidification precedes deep-water carbonate dissolution and anoxia during Oceanic Anoxic Event

The Early Aptian (~120 Ma) Oceanic Anoxic Event (OAE)1a was caused by massive addition of volcanic CO2 to ocean and atmosphere that should have lowered seawater pH. Calcareous nannofossils trace early acidification of surface-water in Tethys and Pacific Oceans. The demise of heavily calcified nannoconids and reduced calcite paleofluxes marks the beginning of the pre-OAE1a calcification crisis, while ephemeral coccolith dwarfism and malformation represent remarkable species-specific adjustments to survive maximum acidity. Abundance peaks of peculiar nannoliths trace intermittent alkalinity recovery. Deepwater acidification occurs with a delay of 25-30kyr. Onset of OAE1a is preceded by a dissolution event recording 1 to 2km shallowing of the CCD not related to anoxia. After dissolution climax, nannoplankton and carbonate recovery developed over ~160 kyr, under persisting global dysoxia-anoxia.