The calibration of D[Sr/Ca] versus sea surface temperature relationship for Porites corals

Taking advantage of the availability of a continuous sea surface temperature (SST) record at Kenting, southern Taiwan, we have carried out a calibration of D[Sr/Ca]-SST (D: distribution coefficient) relationships for Porites lobata and P. lutea. Between 22 degrees C similar to 28 degrees C, the best fitting linear relationships for the two species agree within their respective errors with a maximum deviation less than 0.3 degrees C. Our calibration overlaps with the only published calibration for Porites (deVilliers et al., 1994) but is somewhat less temperature sensitive. The maximum temperature differences for the two calibrations between 21 degrees C and 27 degrees C are +/- 1.2 degrees C. The D-SST relationship calculated using the data of Beck et al. (1992, 1994) and their unpublished seawater data lies between these two calibrations. Our calibration removes some discrepancies previously described in the studies of McCulloch et al. (1994a) on the Great Barrier Reef and of Min et al. (1995) on New Caledonia. We found little growth rate effect on the calibration for P. lutea between 18 mm/yr and 23 mm/yr. We have also monitored the seawater [Sr/Ca] for the entire 1993. The observed variation of 0.033 mmol/mol can cause a temperature artifact of 0.7 degrees C and is thus a dominant error in using this new paleo-thermometer. To carry out this experiment, we have developed a microsurgical technique that can be used to sample corals at better than 0.15 mm resolution. The analytical precision should ultimately allow SST reconstruction of better than 0.2 degrees C. Furthermore, the insensitivity of the calibration against growth rate variation and species differences suggests a promising future for this new thermometer, in contrast to the pessimism of deVilliers et al. (1995). However, inter-laboratory differences and the question of seawater [Sr/Ca] variation need to be addressed first. We recommend a single SST calibration for the Sr/Ca thermometer on coral Politest Sr/Ca-N = 10.286-0.0514 x SST (N: normalized to Hawaii seawater [Sr/Ca] = 8.510 mmol/mol) or D[Sr/Ca] = 1.2077 - 0.006011 x SST.