Mid-Miocene warmth pushed fossil coral calcification to physiological limits in high-latitude reefs

The history of resilience of organisms over geologic timescales serves as a reference for predicting their response to future conditions. Here we use fossil <i>Porites</i> coral records of skeletal growth and environmental variability from the subtropical Central Paratethys Sea to assess coral resilience to past ocean warming and acidification. These records offer a unique perspective on the calcification performance and environmental tolerances of a major present-day reef builder during the globally warm mid-Miocene CO₂ maximum and subsequent climate transition (16 to 13 Ma). We found evidence for up-regulation of the pH and saturation state of the corals' calcifying fluid as a mechanism underlying past resilience. However, this physiological control on the internal carbonate chemistry was insufficient to counteract the sub-optimal environment, resulting in an extremely low calcification rate that likely affected reef framework accretion. Our findings emphasize the influence of latitudinal seasonality on the sensitivity of coral calcification to climate change.