Slow-growing reef corals as climate archives: A case study of the Middle Eocene Climatic Optimum 40 Ma ago

The skeletons of stony corals on tropical shallow-water reefs are high-resolution climate archives. However, their systematic use for unlocking climate dynamics of the geologic past is limited by the susceptibility of the porous aragonite skeleton to diagenetic alterations. Here, we present oxygen and carbon isotope time series (monthly resolution) from reef corals with an unusual unaltered preservation from the Middle Eocene Climatic Optimum (MECO) hyperthermal (40 million years ago). Annual extension of the corals at the studied midlatitude site (France) was remarkably low (0.2 cm). Nonetheless, isotope signatures display no evidence for kinetic disequilibria that discredit their use as climate archive, but growth rate-dependent annual signal amplitude attenuations need corrections using an innovative sampling approach. Thereafter, we present evidence of symbiotic zooxanthellae in reef corals of the Paleogene and subdued sea surface temperature seasonality of only 7 degrees to 8 degrees C during the MECO, consistent with the globally equant climate of the hothouse.

Automated summary

The stony coral skeletons can serve as high-resolution climate archives, but corrections need to be made for the growth rate-dependent annual signal attenuation. The study found evidence of symbiotic zooxanthellae in Paleogene reef corals and subdued sea surface temperature seasonality during the Middle Eocene Climatic Optimum, indicating a globally equant climate.