Calibration of seawater temperature and d18Oseawater signals in Tridacna maxima’s d18Oshell record based on in situ data

Abstract

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 (d 18 O shell ), the sea surface temperature (SST), and the stable oxygen isotopes ratio of seawater (d 18 O sw ) 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 (d 18 O sw ) and SST was recorded hourly. Micro- scopic observations of outer shell layer thin sections revealed micrometric, simple and complex, growth incre- ments. The staining experiment demonstrated that these growth increments were deposited on a daily basis. The weekly d 18 O shell record showed that the shell was precipi- tated in isotopic equilibrium with seawater and that 63 % of the (d 18 O shell - d 18 O sw ) variations were explained by SST variations. A conventional linear function (1) linking T. maxima high-resolution d 18 O shell record, in situ d 18 O sw , and SST data was calculated as: SST 1⁄4 À3:96 Æ 0:59 Ã Dd 18 O þ 20:12 Æ 0:65 ð1Þ where SST is in degree Celsius and Dd 18 O is the difference between the stable oxygen isotopes ratio of the shell and of the seawater (d 18 O shell - d 18 O sw ), in % VPDB. The root mean square error of the SST reconstructed by this equa- tion is ±1.41 °C revealing the potential of T. maxima for paleoclimatic studies. An average SST = f(d 18 O shell - d 18 O sw ) equation for giant clam is proposed by compiling the equations from the present study and previous works SST 1⁄4 À3:79 Æ 0:65 Ã Dd 18 O þ 21:31 Æ 0:93 ð2Þ