Calcium isotopes link ocean acidification to Aptian–Albian foraminiferal extinctions

The second-largest extinction event in the evolutionary history of planktic foraminifera occurred at the Aptian–Albian boundary. This extinction may reflect ocean acidification (OA) associated with Oceanic Anoxic Event 1b. As calcium isotope ratios (δ 44/40 Ca) can track how biocalcification rates respond to OA, we measured δ 44/40 Ca records for planktic and benthic foraminifera, bulk carbonates, and authigenic calcite across the Aptian–Albian boundary in the South Atlantic. Benthic and bulk δ 44/40 Ca data display a distinct sequence of negative and positive excursions, similar to δ 44/40 Ca variations across other OA events. Planktic δ 44/40 Ca values increase markedly, tracking a reduction in calcification rates coincident with decreases in the size, diversity, and shell thickness of planktic foraminifera. These results suggest that OA drove extinctions of planktic foraminifera at the Aptian–Albian boundary.