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[1] Laser ablation ICP/MS was used to analyze Mg, Sr and U at fine (hundreds of microns) and seasonal spatial scales in the reef coral Porites and the non-photosynthetic “deep-sea” corals Lophelia, Oculina, and Desmophyllum. Tropical corals display strong seasonal correlations between Mg, Sr and U, even when normal cycles are perturbed by unknown factors. An inverse relationship between Mg and U is a universal feature of all the corals, but is much larger in deep corals despite stable environmental conditions. This correlation originates from two aragonitic skeletal types: Mg-rich opaque centers of calcification, and U-rich large translucent crystals. The Mg-rich material probably precipitates rapidly and corresponds to materials strongly deficient in 18O and 13C compared to isotopic equilibrium. Combined, these observations imply that varying proportions of the two skeletal types may account for most of the “temperature” or “climate” signal in tropical corals, as well as anomalous “vital effects”.