Top panels: abundance profiles obtained from the model (solid lines), and from the model with constant ζ₂ = 3 × 10⁻¹⁷ s⁻¹ (dashed lines). The different colours refer to the four time steps investigated in this work (labelled in the first panel, in year). Bottom panels: ratios between the abundance profiles from model and from themodel with constant ζ₂ = 3 × 10⁻¹⁷ s⁻¹. The colour code is the same as for the upper panels.