The magnesium-to-calcium (Mg/Ca) ratio of coral skeletons from Ishigaki Island, Ryukyu Islands, Japan, closely tracked sea surface temperature (SST) over an 8-year period. Measurements were made with the fast technique of inductively coupled plasma-atomic emission spectrometry. The variation of the coral Mg/Ca ratio with SST change is about four times that of the current, widely used coral strontium-to-calcium ratio. The temporal and geographic variation of the seawater Mg2+/Ca2+ ratio probably has little influence on coral Mg/Ca variation. Results indicate that the coral Mg/Ca ratio has the potential to provide fast, precise, high-resolution proxies for past tropical SSTs.

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A surface-water A 14 C record of AD 1948-1999 in the tropical South China Sea (SCS) has been reconstructed from accelerator mass spectrometric radiocarbon measurements of annual bands of a Pontes coral collected from Con Dao Island, Vietnam. Results gave the following A 14 C time series: a steady state of -47.8 ± 2.8%o (mean ± SD, η -8) during 1948-1955 (i.e. in the pre-bomb period); a sharp increase during 1956-1966; a gradual increase during 1967-1973; a relatively high maximum value of ~174%c in 1973; and a gradual decrease for the following period to ~86%c in 1999. This A I4 C record having a sharp increase and a relatively high peak is similar to the records of subtropical corals (latitudes 21-27°) and is distinctly different from the records of equatorial/tropical corals (latitudes <10°), although our coral sample was collected from an equatorial/tropical region (8°39'N, 106°33Έ). This can be explained by the geographic, océanographie, and climatic setting of our study site. The SCS is a semi-enclosed marginal sea in the far western tropical Pacific and is little influenced by equatorial upwelling or related ocean currents. Our study site is located in the southwestern SCS, where an enormous submerged plain (the Sunda Shelf) spreads out with very shallow waters (mean depth <100 m). Furthermore, in the SCS, the East Asian monsoon (a strong, seasonally reversing wind system) enhances air-sea gas exchange especially in the mainland coastal waters, including our study site. Such semi-enclosed shallow waters with enhanced ventilation were probably very sensitive to the atmospheric nuclear explosions in the late 1950s and early 1960s and caused the sharp increase and high peak in the coral A ,4 C record. Our coral A 14 C values in the southwestern SCS are significantly higher than the values in the northwestern SCS (Xisha Islands), which seems to suggest that meridional mixing of surface waters is not active in the SCS and that the openocean water intruding into the northern SCS (i.e. the Kuroshio intrusion) has only a limited influence on the southern SCS.'National Institute for Environmental Studies, Tsukuba 305-8506, Japan. institute for Asian PaleoClimate, Konan 483-8226, Japan. Corresponding author.