ABSTRACT
Radioactiveisotopesareextremelyusefulfordeterminingtheageofcorals,whichcanalsoprovideextremelyuseful archives of past climate change and environmental pollution. In this study, we present a novel approach for dating young corals (<10 a), by applying the228Th/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. The228Th/228Ra dating method was used to estimate the228Th/228Ra disequilibrium-derived age based on the assumption of the ingrowth of228Th from228Ra in coral skeletons. To validate the228Th/228Ra dating method, radiogenic (228Th/228Ra) and stable isotopes (δ18O) were
measured for the calculations and comparison of the absolute age and relative age inPoritescoral skeletons. The228Th/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 of228Th and228Ra activities and228Th/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 the228Th/228Ra dating method were constrained by the acceptable value of uncertainty of age. The228Th/228Ra datingmethodmayprovideasupplementaryapproachtotheexistingcoralchronologicaltoolboxof210Pb/226Ra,14C,231Pa/235U, and230Th/238U on a time scale from decades and hundreds of years to thousands of years.
