Carbon and Oxygen Isotope Records of Tridacna squamosa Shells from two Different Latitudes in the Ryukyu Islands

We report carbon (I13C) and oxygen (I18O) isotope records of two modern giant clam (Tridacna squamosa) shells from two sites (Ishigaki-jima and Okinoerabu-jima) at different latitudes in the Ryukyu Islands, Japan. The I13C profiles of samples from the inner shell layer on cross-sections along the maximum growth axis display no ontogenetic trends or seasonal variations. This finding suggests that the calcification site is likely to be unaffected by CO2 uptake and release resulting from the metabolic activity of the molluscan host and algal symbionts. The I18O profiles show distinct seasonal cycles. After accounting for the influence of seawater I18O, the time-series variations are consistent with variations in sea surface temperature, and the temperature dependency of oxygen isotope fractionation is nearly identical to previously published I18O-Temperature relationships for biogenic and synthetic aragonite. We conclude that I18O records from pristine fossils of this species will enable accurate paleoenvironmental reconstructions at high temporal resolution.