Ocean acidification at the Toarcian Anoxic Event captured by boron isotopes in the lime mud record

The Toarcian Oceanic Anoxic Event (ca. 183 million years ago) marks a global mass extinction coincident with dramatic changes in climate and ocean circulation, likely driven by large igneous province emplacement. Rapid carbon dioxide release may have induced global warming, widespread ocean deoxygenation, and ocean acidification. To constrain the magnitude of ocean acidification, we present boron isotope data from three different carbonate components, lime mud (micrite), brachiopods, and bivalves, from two marine sections in SW Europe. Only data from micrite shows a temporary decrease in boron isotope composition during the Toarcian Oceanic Anoxic Event, recording an ocean acidification event, which we reproduce using a coupled biogeochemical model. The contrasting stability of boron isotope values shown by bivalves and brachiopods suggests that the investigated taxa may have been able to physiologically buffer changes in ocean pH, and are therefore poor targets for the interrogation of pH changes in Earth history.