Metallicity distribution as a function of H I column density (left panel) and dust-phase iron column density, N(Fe)_dust (right panel). Only the subsets of the full GRB sample with both a metallicity and N(H I) measurements (left) and depletion values (right) are shown. Blue filled circles again denote the GRBs with C I detected in absorption (empty blue circles show the GRBs with C I detected below the completeness limit). Black empty circles mark the GRBs in our sample without C I, where triangles represent bursts with lower limits on [X/H]. In the left panel, the dashed line corresponds to logN(H I)/cm⁻² + [X/H] = 20.7, above which all the observed systems with detected C I are located and in the right panel the dashed line mark logN(Fe)_dust/cm⁻² = 16.2. At the top of both panels there are histograms of H I column density (left) and dust-phase iron column density (right) of the GRB absorbers with (filled blue) and without (black line) C I detected in absorption.