Left panel: same as Fig. 9, but for ζ = 1.3 × 10⁻¹⁶ s⁻¹. In both panels, the vertical purple solid line represents the early chemistry time as defined in the text, while the two vertical orange solid lines represent the two times (5 × 10³ yr, and 8 × 10⁴ yr) that we analyse and discuss in the text. The trends obtained with the standard ζ of the fiducial model are superimposed with dashed lines. The black horizontal dashed line represent the initial ¹²C/¹³C ratio of 68.