Abstract .Neutron-star mergers were recently confirmed as sites of rapid-neutron-capture (r-process) nucleosynthesis1 , 2 , 3. However, in Galactic chemical evolution models, neutron-star mergers alone cannot reproduce the observed element abundance patterns of extremely metal-poor stars, which indicates the existence of other sites of r-process nucleosynthesis4 , 5 , 6. These sites may be investigated by studying the element abundance patterns of chemically primitive ...