Micro-analysis of a western equatorial Pacific coral across two annual cycles reveals seasonal variation in the boron isotope composition. A heavy boron isotope spike is found coincident with a heavy carbon isotope spike in samples collected from the high density bands in the coral head. The heavy isotope values in both carbon and boron are consistent with high productivity of coral symbionts during periods of high insolation. During these high productivity periods, the symbionts fix carbon and thus produce a residual heavy carbon pool and CO2 depletion. Calcification from the resulting high pH pool results in heavier boron isotope compositions in the coral as the isotopic composition of the aqueous boron species that is incorporated in the coral skeleton (BOH4-) increases with pH. This study suggests that coupled carbon and boron isotope studies may lead to a better understanding of coral calcification processes and a more lucid understanding of coral &dgr;13C records. ¿ 1998 American Geophysical Union