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We analyse the abundance ratios of the light elements Mg, Ca, C and N, relative to Fe, for 147 red‐sequence galaxies in the Coma cluster and the Shapley Supercluster. The sample covers a 6 mag range in luminosity, from giant ellipticals to dwarfs at   M  ★ + 4 . We exploit the wide mass range to investigate systematic trends in the abundance ratios Mg/Fe, Ca/Fe, C/Fe and N/Fe. We find that each of these ratios can be well modelled using two‐parameter relations of the form  [X/Fe]= a  0 + a  1  log σ+ a  2 [Fe/H] , where σ is the velocity dispersion. Analysing these ‘X‐planes’ reveals new structure in the abundance patterns, beyond the traditional one‐parameter correlations (Mg/Fe–σ, etc.). The X‐planes for the α elements, Mg and Ca, indicate a positive correlation with velocity dispersion, and simultaneously an anticorrelation with Fe/H (i.e.   a  1  &gt; 0  and   a  2  &lt; 0 ). Taking both effects into account dramatically reduces the scatter, compared to the traditional  X/Fe−σ  relations. For C and N, a similar correlation with velocity dispersion is recovered, but there is no additional dependence on Fe/H (i.e.   a  1  &gt; 0  and   a  2 ≈ 0 ). The explicit dependence of X/Fe on two parameters is evidence that at least two physical processes are at work in setting the abundance patterns. The Fe/H dependence of Mg/Fe and Ca/Fe, at fixed σ, may result from different durations of star formation, from galaxy to galaxy, leading to a range in the degree of Type Ia supernova contribution to the chemical enrichment. The absence of corresponding Fe/H dependence for C and N is consistent with these elements being generated in intermediate‐ and low‐mass stars, on time‐scales similar to the Fe production. The σ dependence, at fixed Fe/H, is similar for elements Mg, C and N, despite their likely origin in stars of very different masses and lifetimes. Ca/Fe is positively correlated with σ, at fixed Fe/H, but its dependence is significantly less steep than that of Mg, C and N. We discuss possible origins for this pattern of trends and find no simple prescription that reproduces the observations. In particular, if C and N are produced on time‐scales comparable to that of Fe, then the X/Fe–σ trends cannot be explained solely by a systematic variation of star formation time‐scale with σ.

Abundance ratios in red‐sequence galaxies over a wide mass range: the ‘X‐planes’ for magnesium, calcium, carbon and nitrogen