  Lithium and beryllium are destroyed at different temperatures in stellar
interiors. As such, their relative abundances offer excellent probes of the
nature and extent of mixing processes within and below the convection zone. We
determine Be abundances for a sample of eight solar twins for which Li
abundances have previously been determined. The analyzed solar twins span a
very wide range of age, 0.5-8.2 Gyr, which enables us to study secular
evolution of Li and Be depletion. We gathered high-quality UVES/VLT spectra and
obtained Be abundances by spectral synthesis of the Be II 313 nm doublet. The
derived beryllium abundances exhibit no significant variation with age. The
more fragile Li, however, exhibits a monotonically decreasing abundance with
increasing age. Therefore, relatively shallow extra mixing below the convection
zone is necessary to simultaneously account for the observed Li and Be behavior
in the Sun and solar twins.
