William G. Thompson
The delicate and open structure of the coral skeleton, and the small size of the aragonite crystals make corals particularly susceptible to the movement of the U-series isotopes used in dating. As a result, many corals behave as open-systems, a problem that gets worse with increasing coral age. There are two fundamental processes that can affect coral isotope systematics: the gain or loss of daughter products during radioactive decay related to alpha recoil and the gain or loss of U or Th due to diagenetic processes.
Screening criteria used to identify so-called ‘reliable’ or coral ages reduce, but do no eliminate the impact of daughter addition, so that the reported ages are often older than the true age. Age correction approaches do not take into account U or Th gain or loss and typical screening criteria are also not very sensitive to this process, so that reported ages scatter around the true age. The combined impact of these effects on coral-based sea level reconstructions is to make sea level events appear to be older and of greater duration than they actually are.Developing accurate sea level reconstructions by dating older corals is a real challenge, and both sample screening and age correction approaches offer the promise of progress. However, at the present time, the variety of approaches that are used to deal with open system effects produce ages that are not necessarily comparable. What can be done to improve the reliability of coral based sea level estimates?