We consider the stochastic propagation of high-energy protons and nuclei in
the cosmological microwave and infrared backgrounds, using revised photonuclear
cross-sections and following primary and secondary nuclei in the full 2D
nuclear chart. We confirm earlier results showing that the high-energy data can
be fit with a pure proton extragalactic cosmic ray (EGCR) component if the
source spectrum is \propto E^{-2.6}. In this case the ankle in the CR spectrum
may be interpreted as a pair-production dip associated with the propagation. We
show that when heavier nuclei are included in the source with a composition
similar to that of Galactic cosmic-rays (GCRs), the pair-production dip is not
present unless the proton fraction is higher than 85%. In the mixed composition
case, the ankle recovers the past interpretation as the transition from GCRs to
EGCRs and the highest energy data can be explained by a harder source spectrum
\propto E^{-2.2} - E^{-2.3}, reminiscent of relativistic shock acceleration
predictions, and in good agreement with the GCR data at low-energy and holistic
scenarios.Comment: 4 pages, 4 figures, submitted to A&A Letters (minor changes, two
figures replaced, two references added