Radial profiles of O/H, Li/H, Be/H and B/H (from top to bottom) at time t = 7.5 Gyr (solar system formation, dotted curves) and at t = 12. Gyr (now, solid curves). Present-day profiles between 4 and 12 kpc are fitted with exponentials (thick segments) and the corresponding slopes appear in the lower left corner of each panel.