What observation strongly confirmed the link between neutron star mergers and r-process material expulsion?

Answer

The 2017 gravitational wave detection ($ ext{GW}170817$)

Evidence from the 2017 gravitational wave detection ($ ext{GW}170817$) strongly confirmed that neutron star mergers generate massive quantities of r-process material, evidenced by observed spectra.

What observation strongly confirmed the link between neutron star mergers and r-process material expulsion?
Elementastronomystarsupernovanucleosynthesis