We measured Sr/Ca ratios by thermal ionization mass spectrometry in radial and axial growth of modern Caribbean Acropora palmata corals. Comparison of our results with sea surface temperature (SST) allows radial and axial Sr/Ca-SST calibrations of Sr/Ca (mmol/mol) = 11.30 - 0.07072 ¿ SST (¿C) and Sr/Ca (mmol/mol) = 11.32 -- 0.06281 x SST (¿C), respectively. Application of the calibrations to fossil Acropora palmata from the last glacial maximum in Barbados (Guilderson et al., 1994) imply ~7¿C cooler conditions than the present, much larger than the 1--1.5¿ cooling suggested by modern analog technique foraminifera-based estimates (Trend-Staid and Prell, 2002). If the foraminifera-based estimates are correct, then the excess cooling suggested by the Barbados corals could be explained by a 5% shift in the marine Sr/Ca ratio or an addition of ~20% abiotic secondary aragonite.