Cool Stars 14 - Submitted Abstract # 8 This version created on 05 October 2006 Atomic diffusion : helium, lithium and heavy elements Andreas Korn, Uppsala Astronomical Observatory (UAO), Sweden Frank Grundahl, University of Aarhus, Denmark Olivier Richard, GRAAL, University of Montpellier II, France Paul Barklem, UAO Lyudmila Mashonkina, Institute of Astronomy, Moscow, Russia Remo Collet, UAO Nikolai Piskunov, UAO Bengt Gustafsson, UAO The measurement of the cosmic microwave background by experiments like WMAP has strongly constrained the cosmological parameters of the Universe. When the measured density of baryons is combined with standard Big-Bang nucleosynthesis (BBN) calculations, the amounts of hydrogen, helium and lithium produced shortly after the Big Bang can be predicted with unprecedented precision. The predicted primordial lithium abundance is a factor of 2 ~ 3 higher than the value measured in the atmospheres of old stars. With estimated errors of 10 to 25%, this cosmological lithium discrepancy seriously challenges our understanding of stellar physics, BBN or both. Certain modifications to BBN have been proposed, but found experimentally not to be viable. Diffusion theory, however, predicts atmospheric abundances of stars to vary with time. We present spectroscopic observations of stars in the metal-poor globular cluster NGC 6397 which reveal trends of atmospheric abundance with evolutionary stage for various elements. These element-specific trends are reproduced by stellar-evolution models with diffusion and turbulent mixing. The most important implication concerns the primordial lithium abundance: diffusive processes are predominantly responsible for the low apparent stellar lithium abundance in the atmospheres of old stars. ----------------------------------