That was the smoking gun linking ocean acidification to the severe biocalcification stress that ultimately led to their extinction.” A Cretaceous crisisAt the boundary between the Aptian and Albian ages within the Early Cretaceous period, the second largest extinction event in the history of planktic foraminifera swept through the oceans. By locking away carbon inside their shells, foraminifera play a key role in natural carbon sequestration. Across the extinction interval, calcium isotope values in planktic foraminifera increased dramatically, indicating a major reduction in calcification rates. “We’ve applied these tools at a different number of places where there was reason to suspect ocean acidification occurred,” Sageman said.