Early Holocene Coral δ¹⁸O-based Sea Surface Temperature

Abstract

Carbon and oxygen stable isotope measurements were conducted on Holocene (9.2kyrs BP) and present-day hermatypic corals (Porites sp.) collected at Shidooke, Kikai Island, Ryukyu Islands. Those in modern corals show annual cycles which can be converted into time-series oscillations based on annual bands and temperature-dependency of δ¹⁸O values. The data clearly show that δ¹³C and δ¹⁸O negatively covariance, occurring approximately 120-150° out of phase (negative peaks of δ¹³C precede negative ones of δ¹⁸O). Also distinguished are distinctive negative covariance between δ¹⁸O and sea surface temperatures (SSTs) and indistinctive positive covariance between δ¹³C and solar radiation. Based on the relationship between δ¹⁸O and SST, a δ¹⁸O-based thermometer is proposed for present-day corals. This thermometer is modified to be applicable for Holocene forms by taking account of ¹⁸O-enrichment caused by increased polar ice volumes. The SSTs calculated at 9.2kyrs BP by using a modified thermometer indicate that annual mean SST at that time was about 2.7°C higher than at present. Our result is significantly higher than generally-accepted estimates (0 to 2°C lower than the present). It is highly probable that the conflict is ascribed mainly to disequilibrium aragonite precipitation by corals from seawater (vital effect).