Reconstructing past disturbance in coral communities using U-Th dating of dead coral skeletons

In the absence of detailed broad-scale studies, both spatially and temporally, the overall status (disturbed, recovering, or in decline) of many of the reefs that make up the Great Barrier Reef remains uncertain. Moreover, of the numerous and varied threats, their relative role in impacting individual reefs is generally unclear. Here, we adopt a novel approach to reliably reconstruct historical disturbance events at Rib and Davies Reefs, two mid-shelf reefs, using uranium-thorium (U-Th) dating of dead corals. Corrected 230Th ages obtained from dead Acropora spp. bracket time periods of lowest coral cover observed during independent ecological surveys in the late 1980s and 2000s at 1988.2 ± 1.3 and 2003.9 ± 6.2 CE, shortly after the arrival of crown-of-thorns starfish (CoTS) at Rib Reef in 1983−1985 and 2000−2002, respectively. At Davies Reef, 230Th ages dated to 1999.2 ± 1.2 CE when coral cover was halved as a result of Cyclone Tessi and an “unknown” disturbance in 2000−2001. Prior to modern surveys, there is remarkable overlap in 230Th ages between reefs, with repeated peaks in the age distribution having a return period of ∼10−15 years, akin to the periodicity of modern CoTS outbreaks. Our findings suggest that U-Th dating of dead corals can provide a robust foundation for understanding disturbance history and show promise in contributing to the effective monitoring of coral communities by providing a reliable benchmark with which to assess recovery.