MICROMORPHOLOGY AND TRACE METAL CONTENT AS INDICATORS OF BLEACHING IN SKELETONS OF MODERN AND HOLOCENE CORALS

Morphology and trace metal content of scleractinian corals have previously been used as proxies for past environmental conditions, but no proxy for the health of ancient corals currently exists. Skeletal material from Recent Montastrea franksii bleached in the 1987 Caribbean Warm Event was analyzed with SEM and ICPAES. SEM analysis was inconclusive for morphological differences between M. franksii samples known to have been bleached and non-bleached in 1987. Significantly higher trace metal/Ca ratios were found with ICPAES for Ag, As, P, and Zn in bleached M. franksii.

Differences in both morphology and trace metal content were previously found in skeletal material associated with bleached and non-bleached tissue in Porites divaricata. SEM analysis showed skeletal corallites associated with unbleached tissue had well-defined septal and columellar denticles. Skeletal corallites associated with bleached tissue had weakly-defined denticles, many appearing rudimentary. Significantly higher trace metal/Ca ratios Ag, As, Cd, and Co between skeletal material associated with bleached and non-bleached tissue in P. divaricata. The presence of these differences suggests the processes of skeletogenesis and the uptake and deposition of trace metals in the scleractinian skeleton are affected by bleaching.

Mid-Holocene (~5,000 ybp) Porites sp. were tested with ICPMS and SEM to determine if such indicators might be observed in the fossil record. Ag was present in 7 of 16 colonies tested, and SEM analysis revealed septal and columellar denticles similar to those of Recent bleached P. divaricata.