This project will apply ion microprobe analyses to examine variation of trace elements (B, F, Mg, Sr) and oxygen isotopes at sub-monthly resolution in Porites corals from Johnston Atoll (central Pacific) and Two-Mile Reef (South Africa). The goals are twofold: to determine covariation among trace elements in order to assess their relationship to water temperature, and to examine the relationship between coral growth geometry and the geochemical tracers. Several colonies will be analyzed at each site to examine local variability and instrumental records of water temperature are available.