In this paper we point out a previously unnoticed anticorrelation between the
observed abundance ratio [X/Zn] (where Zn is assumed to be undepleted and X
stands for the refractories Fe, Cr and Ni) and metal column density
([Zn/H]+log(N{HI})) in DLAs. We suggest that this trend is an unambiguous sign
of dust depletion, since metal column density is a measure of the amount of
dust along the line of sight. Assuming that DLAs are (proto-)galactic disks and
using detailed chemical evolution models with metallicity dependent yields we
study chemical evolution and dust depletion patterns for alpha and iron-peak
elements in DLAs. When observational constraints on the metal column density of
DLAs are taken into account (as suggested in Boisse et al. 1998) we find that
our models reproduce fairly well the observed mild redshift evolution of the
abundances of 8 elements (Al, Si, S, Cr, Mn, Fe, Zn and Ni) as well as the
observed scatter at a given redshift. By considering the aforementioned
dependence of abundance ratios on metal column density, we further explore the
general dust depletion pattern in DLAs, comparing to our model results and to a
solar reference pattern. We suggest that further measurements of the key
elements, i.e. Zn, S and Mn, will help to gain more insight into the nature of
DLAs. In any case, the presently uncertain nucleosynthesis of Zn in massive
stars (on which a large part of these conclusions is based) should be carefully
scrutinised.Comment: 12 pages, 4 figures, Astronomy and Astrophysics, in pres
