Testing the precision and accuracy of the U-Th chronometer for dating coral mortality events in the last 100 years

To assist with our understanding of reef dynamics prior to modern monitoring programs and recent observations of coral decline, a robust dating technique is required to place coral mortality events and historical changes in community structure in an accurate chronological framework. In this study we adopted a refined Uranium-Thorium (U-Th) isotope measurement protocol using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for rapid, precise and accurate age determination of a large branching Acropora coral death assemblage from an inshore reef of the Great Barrier Reef (GBR) where the timing of mortality is independently constrained. To achieve this, we developed a vigorous sample cleaning/treatment procedure to remove most non-carbonate detritus from the coral skeleton, and a correction scheme that accounts for initial 230Th sources in the dead coral skeletons. Using this method, the 230Th ages (with 2σ errors of 1-5 years) from 41 individual dead Acropora branches precisely bracket the timing of a documented ∼100% loss of hard coral cover, primarily Acropora, that was caused by increased sea-surface temperatures during the 1997-1998 mass bleaching event. Our results demonstrate the applicability of U-Th dating in accurately determining the timing of previous disturbance events in coral reef communities, as well as identifying potential drivers. This approach provides a powerful tool to researchers and managers in assessing the current status of reefs and identifying areas vulnerable to degradation where long-term monitoring data are absent or too recent.