Calibration of stable oxygen isotopes in Siderastrea radians (Cnidaria:Scleractinia): Implications for slow-growing corals

Geochemical proxies in the skeletons of corals used for the purpose of reconstructing environmental records have typically been obtained from relatively fast-growing corals (usually >8 mm yr^-1) and from only a few key genera (most commonly Porites and Montastraea). In many areas, however, there are no suitable fast-growing corals available for such reconstructions. Here, we investigate the potential of Siderastrea radians, a slow-growing Atlantic and Caribbean zooxanthellate coral, as an archive of sea surface temperature (SST) and salinity over the period from 1891 to 2002. Sampling the skeleton of three corals from the Cape Verde Islands, we were able to reproduce a clear seasonal signal, but with limited correlation to monthly SST, arising from inadequate chronologic constraint of the individual samples. The SST-δ¹⁸O calibration slopes for different sampling scales on several cores can range from about-9°C%^-1 to +2°C% ^-1 (compared to other published values of around-5 to-4°C% ^-1). Careful treatment produced a δ¹⁸O-SST calibration equation where SST(°C) = 12.56(±1.20)-3.86(±0.39) *(δ_c-(δ_w). The recognition of the limitations of calibration at such small growth rates due to skeletal complexity and suspicion of environmental interferences suggests the need for careful consideration in the interpretation of climate proxy results from S. radians and other slow-growing corals.