Carbon and Oxygen Isotope Records from Tridacna derasa Shells: Toward Establishing a Reliable Proxy for Sea Surface Environments

We report the carbon (δ(13)C) and oxygen (δ(18)O) isotope records of three modern Tridacna derasa shells from Ishigaki-jima, southwestern Japan. The high-resolution δ(13)C profiles of samples from the inner shell layer on cross-sections fall within similar narrow ranges and display no regular variations or trends, such as an ontogenetic trend or abrupt short-term drops likely to be related to reproductive activity. This suggests that the calcification site of this species is not likely affected by photosynthetic CO(2) uptake or CO(2) incorporation during respiration. The δ(18)O profiles show distinct seasonal cycles. The intraspecific variability in the δ(18)O values is small in parts of the shell precipitated in the adult stage, but is not negligible in the juvenile and senescent stages. The differences in the monthly and seasonally resolved δ(18)O values among shells are less than 0.51‰ and 0.76‰, respectively. The shell δ(18)O values are nearly identical or close to the δ(18)O values for aragonite precipitated in oxygen isotope equilibrium with ambient seawater (δ(18)O(EA)). The largest differences between the shell δ(18)O and δ(18)O(EA) values calculated from the monthly and seasonally resolved data correspond to an overestimate of the seawater temperature by as much as 1.7°C and 2.3°C, respectively. However, these differences are smaller in the adult stage (<0.25‰) than in the other stages. This small difference allows an accurate reconstruction of the seawater temperature with an error of <1.1°C. Consequently, we recommend that multiple shell records be obtained because of the non-negligible intraspecific variations in their δ(18)O values. Growth banding, composed of alternating narrow white bands and wide light-grey bands, is discernible on cross-sections of the inner shell layer. The δ(18)O(shell) data indicate that they were formed in winter and the other seasons, respectively.