Sr/Ca ratios of modern and fossil Favia corals from the Korea Strait; implications for application of coral Sr/Ca thermometry in a middle latitude region

Favia, a hermatypic reef building coral, is widely distributed from equator to northern latitudinal limit of coral distribution. An 150-mm-long living, grown for 21 years, and 50-mm-long fossil (2600 cal yr BP) Favia specimens, grown for 13 years, were collected from the Iki Island, Japan (34°48'N, 129°39'E) and analyzed for Sr/Ca ratios, in order to test for a high-resolution paleo-SST proxy. The modern and fossil Favia samples reveal clear annual seasonality in density bands and the measured Sr/Ca ratios. In the modern coral, however, the large inter-annual variability of winter Sr/Ca indicates smoothing of environmental signals during cold season due to very slow growth or frequent growth cessations. An SST - Sr/Ca equation, Sr/Ca (mmol/mol) = 10.428 - 0.049 × SST (°C), constructed under the assumption of growth stopping below 14°C, predicts the observed annual summer maximums within an error range of ± 1.2°C. Large inter-annual variability of Sr/Ca is also observed in the fossil coral, but even in the warm season, indicating more frequent coral growth disturbances due to colder condition at 2600 cal yr BP. With an assumption that growth disturbance had the least effect on the lowest Sr/Ca, the equation predicts summer maximum temperature of 24.6°C, ~ 2°C lower than the present. The SST - Sr/Ca equation established in this study successfully reconstructs SSTs of a subtropical region from reported literature values, suggesting the applicability of Sr/Ca thermometry of Favia in a wide range of climate settings.