Coral Sclerochronology: Similarities and Differences in the Coral Isotopic Signatures Between Mesophotic and Shallow-Water Reefs

Coral sclerochronology is a powerful tool for understanding environmental and ecological changes on coral reefs. Geochemical, isotopic, and skeletal density banding analyses along the major growth axis of massive coral skeletons from tropical shallow-water reefs have been used successfully to reconstruct decadal- to centennial-scale histories of climate and coral growth with annual to seasonal resolution. However, little is known about how coral sclerochronological approaches could capture environmental and/or physiological changes in mesophotic coral ecosystems (MCEs), which occur at depths ranging from 30 to 150 m. We compared the oxygen and carbon isotopes and growth records of coral from upper MCEs with those from adjacent shallow reefs by examining Porites corals collected at 4 and 40 m from Okinawa, Japan, and from 5 and 50 m water depths from the Gulf of Eilat, Red Sea, Israel. Porites corals in MCEs exhibited low calcification rates, but still recorded distinct seasonal to interannual variability in oxygen and carbon isotope signals consistent with coral isotopic records on shallow reefs. The amplitudes of the seasonality in the isotopic records in corals from MCEs were larger than those expected from seasonal environmental variations and those recorded in shallow reefs, indicating that coral growth rate and/or physiological changes affected skeletal isotopic composition. Our results suggest that sclerochronological records have great potential for reconstructing environmental and ecological characteristics of MCEs. The isotopic records of MCE corals are also more influenced by physiological processes such as symbiotic photosynthesis, calcification rates, and trophic levels than their shallow-water counterparts.