Historical mortality in massive Porites from the central Great Barrier Reef, Australia: Evidence for past environmental stress?

Two hiatuses in coral skeleton growth, associated tissue death and subsequent regrowth, were discovered while dating eight multi-century Parites coral cores collected from the central Great Barrier Reef (GBR), Australia. Cross-dating of characteristic annual luminescent lines visible in the coral core slices under UV-light (Hendy et al. 2003) accurately dated the two events to 1782-85 and 1817 A.D., Die-off scars were observed in only one core for each event. X-radiographs and photographs taken under UV-light show the pattern of regrowth and the period taken by the coral to recover. Bioerosion, predominately by boring sponges (Cliona spp.), of the exposed coral surface following the 1782-85 event caused a hiatus of up to 14 years' growth, with the coral taking 7-8 years to reclaim the whole surface contained within the 9-cm-diameter core. Contemporary historical and proxy-climate records indicate that El Niño climatic conditions occurred at the time of both growth discontinuities. Intense luminescence observed in corals growing continuously during the 1817 event suggests that low salinity from river runoff was a contributing factor, analogous environmental conditions to those that were associated with the 1998 bleaching event in the GBR.