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The significant attenuation of the cosmic-ray flux above $\sim 5 10^{19}$ eVsuggests that the observed high-energy spectrum is shaped by the so-called GZKeffect. This interaction of ultra-high-energy cosmic rays (UHECRs) with theambient radiation fields also affects their composition. We review the effectof photo-dissociation interactions on different nuclear species and analyze thephenomenology of secondary proton production as a function of energy. We showthat, by itself, the UHECR spectrum does not constrain the cosmic-raycomposition at their extragalactic sources. While the propagated composition(i.e., as observed at Earth) cannot contain significant amounts of intermediatemass nuclei (say between He and Si), whatever the source composition, and whileit is vastly proton-dominated when protons are able to reach energies above$10^{20}$ eV at the source, we show that the propagated composition can bedominated by Fe and sub-Fe nuclei at the highest energies, either if thesources are very strongly enriched in Fe nuclei (a rather improbablesituation), or if the accelerated protons have a maximum energy of a few$10^{19}$ eV at the sources. We also show that in the latter cases, theexpected flux above $3 10^{20}$ eV is very much reduced compared to the casewhen protons dominate in this energy range, both at the sources and at Earth.

Implications of the cosmic ray spectrum for the mass composition at the highest energies