Geochemical and Microstructural Signals in Giant Clam Tridacna maxima Recorded Typhoon Events at Okinotori Island, Japan

To validate the usability of the giant clam shell as a recorder of short-term environmental changes such as typhoons, we collected a live Tridacna maxima from Okinotori Island, Japan, on 15 June 2006. Growth increment thickness, stable isotope ratio (δ¹⁸O_shell, δ¹³C_shell), and the barium/calcium ratio (Ba/Ca) in the T. maxima shell sample were measured and compared to Okinotori Island instrumental environmental data. In the outer layer of the shell sample, there were 365 ± 6 growth increments per year, as estimated by the δ¹⁸O_shell profile compared with sea surface temperature. The growth increments in the specimen were formed daily, and thus, we can determine the date of the sampling points of δ¹⁸O_shell, δ¹³C_shell and the Ba/Ca ratio by counting growth increments. After typhoon approach, there is a decrease in increment thickness and some disturbed growth increments. The positive peaks in the shell Ba/Ca ratio and δ¹⁸O_shell corresponded to lower sea surface temperature caused by typhoons. These results indicated that the microstructural and geochemical record in Tridacna maxima shells could be useful for detecting past typhoon events.