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

Abstract

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.