I'm starting to think that there is a typo in that graphic. That is to say, what they should've labled "low-mass stars" should have been labled "high-mass stars" and vice-versa. Then that graphic makes more sense.Without a doubt neutron star mergers were much more common in the early universe than they are today. Population III stars were much larger than Pop. II and I stars, so the percentage of neutron stars would have been proportionally increased - as would the number of supernovae in the early universe. Both combined could explain the elements heavier than iron, but neutron mergers on their own wouldn't cut it. I also don't understand how "low-mass stars" can contribute any element heavier than iron since they do not eject their outer envelope at relativistic speeds and it is not degenerate material being ejected.