10Be for Surface exposure dating (SED)

Raised beaches at Turakerai Head that were dated by surface exposure dating

Raised beaches at Turakerai Head that were dated by surface exposure dating

The relatively new technique of surface exposure dating (SED) utilises primarily the build-up of 10Be in rock materials over time rather than its radiometric decay: Its amount and that of other cosmogenic isotopes (e.g., 26Al, 21Ne, 3He, 36Cl), produced in-situ within a rock matrix due to exposure of the rock to cosmic rays, contains information about the exposure history. Analytical results may only be interpreted geologically if the 10Be production rate is carefully calibrated, for example by correcting for partial attenuation and complete shielding effects. Also, the exposure assumptions must be justified through other means, for example by taking into account clear signs of surface erosion and information consistent with (or suggestive of) 100% shielding prior the exposure event to be dated. So-called ‘inherited’ 10Be from earlier exposure invalidate a single age determination, but there are ways to check for ‘prior exposure’ for example by measurement on another cosmogenic isotope, utilising the difference in half-lives. SED is now an established tool for geomorphology and landscape change studies.

Surface exposure age dating requires intensive chemistry. Our samples are now pre-treated at the University of Canterbury. Quartz is separated from up to several kg of rock and then processed, with 9Be carrier added, to recover the 10Be. The end product, pure BeO, is sent to our AMS facility at the National Isotope Centre for target preparation and 10Be measurement.