Calibration of seawater temperature and δ 18Oseawater signals in Tridacna maxima’s δ 18Oshell record based on in situ data

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The giant clam Tridacna maxima presents a strong potential for paleoclimatic reconstructions but its use remains limited by the fact that the relationship between the shell stable oxygen isotopes ratio (δ 18Oshell), the sea surface temperature (SST), and the stable oxygen isotopes ratio of seawater (δ 18Osw) has not been calibrated yet. In this study, a T. maxima specimen was stained with calcein and grown in a tank for 1.5 yr. The tank seawater was monitored weekly for sea surface salinity, and the stable oxygen isotopes ratio of seawater (δ 18Osw) and SST was recorded hourly. Microscopic observations of outer shell layer thin sections revealed micrometric, simple and complex, growth increments. The staining experiment demonstrated that these growth increments were deposited on a daily basis. The weekly δ 18Oshell record showed that the shell was precipitated in isotopic equilibrium with seawater and that 63 % of the (δ 18Oshell − δ 18Osw) variations were explained by SST variations. A conventional linear function (1) linking T. maxima high-resolution δ 18Oshell record, in situ δ 18Osw, and SST data was calculated as: $$ {\text{SST}} = - 3.96 \pm 0.59 *{\Delta \delta }^{18} {\text{O}} + 20.12 \pm 0.65\quad (1) $$ where SST is in degree Celsius and Δδ 18O is the difference between the stable oxygen isotopes ratio of the shell and of the seawater (δ 18Oshell − δ 18Osw), in ‰ VPDB. The root mean square error of the SST reconstructed by this equation is ±1.41 °C revealing the potential of T. maxima for paleoclimatic studies. An average SST = f(δ 18Oshell − δ 18Osw) equation for giant clam is proposed by compiling the equations from the present study and previous works $$ {\text{SST}} = - 3.79 \pm 0.65 *{\Delta \delta }^{18} {\text{O}} + 21.31 \pm 0.93\quad (2) $$ This equation permits one to obtain SST records from isotopic values of giant clams species that have not been calibrated yet or from fossil giant species that do not have living modern conspecifics.