Skeletal isotope microprofiles of growth perturbations in Porites corals during the 1997-1998 mass bleaching event

Severe coral bleaching occurred throughout the tropics in 1997/98. We report high-resolution skeletal oxygen isotope (δ¹⁸O) and carbon isotope (δ¹³C) microprofiles for bleached corals from Pandora Reef, Great Barrier Reef, and Ishigaki Island, Japan, in order to examine the ability of Porites corals to record clear signals of bleaching. Analysis of the annual cycle in δ¹⁸O revealed abrupt reductions in skeletal extension immediately after the 1997-98 summer temperature maximum, indicating that bleaching inhibits coral calcification. Skeletal δ¹³C in the Ishigaki corals showed lower values during bleaching, indicating depressed coral metabolism associated with a reduction in calcification. In contrast, microprofiles of skeletal δ ¹³C from the shaded sides of Pandora Reef corals exhibited little change, possibly because algal photosynthesis was already slow prior to bleaching, thus subduing the ¹³C-response to bleaching. Comparison of δ¹⁸O microprofiles from bleached corals with instrumental temperature records showed that Porites corals can recover following 5 months with little skeletogenesis. The results indicate that isotopic microprofiling may be the key to identifying gaps in coral growth that are diagnostic of past bleaching events. We have tested this hypothesis using blue UV fluorescent bands to guide us to coral skeleton where isotope microprofiling identifies bleaching events in 1986, 1989, and 1990. These events, detected by proxy, suggest that coral bleaching may have occurred more commonly on Ishigaki Island than previously recorded.