The timing of the last r-process event near Earth from interstellar 60Fe, 244Pu and 247Cm deposition on Earth

Abstract Where are the heavy elements produced in the Universe? While core-collapse supernovae are confirmed sources of 60 Fe, the origin of heavier r-process nuclei such as 244 Pu and 247 Cm remains debated. Here we used single-atom counting of live 60 Fe, 244 Pu and 247 Cm in a deep-ocean ferromanganese crust to trace such nucleosynthesis events near Earth over millions of years. The observed non-correlation of the 244 Pu and 60 Fe profiles supports a continuous r-process influx to Earth that is independent of core-collapse supernovae and constrains the core-collapse supernova r-process yield of actinide isotopes. The non-observation of 247 Cm, produced concurrently with 244 Pu, sets a time limit on the last actinide-producing r-process event near Earth of more than 90–100 Myr ago. Our measured interstellar signatures suggest the occurrence of an old and rare r-process event leading to a diffuse 244 Pu background inside and outside the Local Bubble.