Investigating spatio-temporal variability of initial 230Th/232Th in intertidal corals

One of the key factors in obtaining precise and accurate ²³⁰Th ages of corals, especially for corals with ages less than a few thousand years, is the correction for non-radiogenic ²³⁰Th based on an initial ²³⁰Th/²³²Th value (²³⁰Th/²³²Th₀). Studies that consider coral ²³⁰Th/²³²Th₀ values in intertidal environments are limited, and it is in these environments that corals have Th concentrations 100–1000 times greater than open-ocean corals. Here we present 66 reconstructed U–Th isochrons of modern and Holocene Porites lutea and lobata corals from throughout the Sumatran forearc islands (SFI). Mean square of weighted deviates (MSWD) is used to evaluate the variabilities of 28 location-specific ²³⁰Th/²³²Th0 values before the regional mean. Results show no significant spatial difference between the means of 3.1 ± 3.2 × 10⁻⁶ for the northern SFI and 4.7 ± 1.3 × 10⁻⁶ for the southern SFI. Using the weighted mean of 4.3 ± 2.5 × 10⁻⁶ for all islands has the advantage that no data need be subjectively excluded in the calculation of reliable ²³⁰Th ages, where a local initial value is unknown. There were no clear centennial to millennial time-scale changes in ²³⁰Th/²³²Th₀ in the past 2500 years. This study, therefore, suggests a reliable value of initial ²³⁰Th/²³²Th ratio in intertidal environments.