ABSTRACT
Radioactive isotopes are extremely useful for determining the age of corals, which can also provide extremely useful archives of past climate change and environmental pollution. In this study, we present a novel approach for dating young corals (<10 a), by applying the 228Th/228Ra method. 228Th/228Ra disequilibrium was widely observed in a variety of coral genera, from the fringing reefs to atoll reefs in the South China Sea using high-purity germanium γ spectrometry in an aboveground laboratory and China Jinping Underground Laboratory with the deepest carbonate rock overburden in the world. The 228Th/228Ra dating method was used to estimate the 228Th/228Ra disequilibrium-derived age based on the assumption of the ingrowth of 228Th from 228Ra in coral skeletons. To validate the 228Th/228Ra dating method, radiogenic (228Th/228Ra) and stable isotopes (δ18O) were
measured for the calculations and comparison of the absolute age and relative age in Porites coral skeletons. The 228Th/228Ra disequilibrium-derived absolute age was found to be in agreement with the relative age based on the δ18O curve. We found that the relative uncertainty of the age is simultaneously determined by the relative un- certainties of 228Th and 228Ra activities and 228Th/228Ra ratio, independent of the analytical methods (α spec- trometry, β counter, or γ spectrometry), and is difficult to control. It was demonstrated that the dating range of the 228Th/228Ra dating method were constrained by the acceptable value of uncertainty of age. The 228Th/228Ra dating method may provide a supplementary approach to the existing coral chronological toolbox of 210Pb/226Ra, 14C, 231Pa/235U, and 230Th/238U on a time scale from decades and hundreds of years to thousands of years.
