Skeletal 618O Records of Coral Bleaching

Extensive coral bleaching was witnessed in the GBR, Australia, during the austral summer of 1997-1998 and appeared later in the Northern Hemisphere (Wilkinson, 1998). For the first time on record, corals around the Ryukyu Islands, Japan, exhibited severe bleaching which resulted in extensive coral mortality (Fujioka, 1999). The most likely cause of the high-SST event around the Ryukyus in the summer of 1998 was the lack of typhoons in the Ryukyu Islands region (Fujioka, 1999). Low-pressure systems, including tropical typhoons, bring high cloudiness and stimulate vertical mixing of the surface water, which may result in an SST decrease. In the absence of typhoons during the summer of 1998, SST continued to rise because of solar heating during the same year and resulted in an extraordinarily high SST around Ishigaki Island (daily mean SST of 31.5 °C; Fig. 4) which caused the mass bleaching event.

While there has been remarkable progress in understanding the pho-tophysiological processes of coral bleaching (Lesser, 1996; Brown, 1997), less is known about the effect of bleaching on coral skeletogenesis. Moreover, the influence of bleaching on the geochemical composition (818O, 813C, Sr/Ca) of coral skeletons has been controversial. Some studies reported clear changes associated with bleaching events, while in others changes were less evident (Porter et al., 1989; Leder et al., 1991; Alison et al., 1996). However, very fine-resolution sampling is required to detect isotopic changes in the coral skeletons corresponding to brief bleaching events. Recently, Suzuki et al. (2003) introduced a high-resolution skeletal isotope microprofiling technique for examining the skeletal records of coral bleaching (Fig. 9). When this technique was used, bleached corals from Pandora Reef, GBR, and Ishigaki Island, Japan, showed a dramatic decrease in skeletogenesis that was coincident with the worldwide mass coral-bleaching event of 1997-1998. These results indicate that isotopic microprofiling may be the key to identifying gaps in coral growth that are diagnostic of past bleaching events.

Ishigaki Island Pandora Reef , GBR

Ishigaki Island Pandora Reef , GBR

Distance from surface of colony (cm)

Figure 9: Skeletal S18Oc records of Porites spp. corals from Ishigaki Island and Pandora Reef, plotted against distance from the surface of the colonies along the microsampling transects. Regional weekly SSTs (IGOSS NMC SSTs) are shown in the top-most panels for comparison with the shape of the coral annual S18Oc cycles. The year shown above the S18Oc minimum indicates boreal summer for the Ishigaki corals, and austral winter for the Pandora Reef corals. The intervals of growth discontinuity are shown by shaded vertical bars. Data are replotted from Suzuki et al. (2003).

Distance from surface of colony (cm)

Figure 9: Skeletal S18Oc records of Porites spp. corals from Ishigaki Island and Pandora Reef, plotted against distance from the surface of the colonies along the microsampling transects. Regional weekly SSTs (IGOSS NMC SSTs) are shown in the top-most panels for comparison with the shape of the coral annual S18Oc cycles. The year shown above the S18Oc minimum indicates boreal summer for the Ishigaki corals, and austral winter for the Pandora Reef corals. The intervals of growth discontinuity are shown by shaded vertical bars. Data are replotted from Suzuki et al. (2003).

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