EVALUATION OF THE PLATYGYRIAN CORAL AS AN ENVIRONMENTAL RECORDER IN TEMPERATE REGIONS

Platygyra, a hermatypic reef building massive coral, shows a wide geographical distribution from tropics to higher latitude region up to 35°N in the northwest Pacific along the Kuroshio/Tsushima warm currents, where average winter SST (sea surface temperature) drops to 13°C. The occurrences of reef structure coral community suggest Platygyra adapts better in extreme environmental conditions of low SST than other coral species. Up to date, high resolution paleo-SST proxy is not available in middle latitude regions because of limited distribution of common proxy corals such as Porites, Pavona, and Montastrea north of 25°N. In order to test Platygyra as a high-resolution paleo-climate archive, we collected a Platygyra colony from Ishigaki island, Japan, analyzed it for skeletal oxygen (δ¹⁸Oc) and carbon (δ¹³Cc) isotopic ratios, and compared the results with observed environmental data. A Porites colony, grown very next to the sampled Platygyra, was also collected and analyzed together for comparison because it is the most common and faithful coral proxy known up to now. δ¹⁸Oc of Platygyra shows a clear seasonality corresponding to SST variation and that of co-sampled Porites. The temperature dependence was estimated, for the first time with appropriate sampling resolution, at 0.211‰/°C. δ¹³Cc of Porites shows a clear seasonal fluctuation matching that of solar radiation, high in summer and low in winter. In contrast, δ¹³Cc of Platygyra showed an opposite pattern to that of Porites with weaker seasonality although both were exposed to same environmental conditions during growth. The estimated calcification rates (extension rate multiplied by bulk density) of both species were similar, so these opposed trends might be attributed to species-dependent responses against same environmental condition. It is likely caused by different symbiotic algae associations of two species, resulting in different photosynthetic efficiency to the same light stress. Its strong adaptability to colder temperature, strong temperature dependence of δ¹⁸Oc, and seasonality found in δ¹³Cc suggest usefulness of Platygyra as a potential paleo-environmental proxy in mid-latitude region, where high resolution SST proxy is not available.