Fe-60 and Pu-244 deposited on Earth constrain the r-process yields of recent nearby supernovae

Abstract

Half of the chemical elements heavier than iron are produced by the rapid neutron capture process (r-process). The sites and yields of this process are disputed, with candidates including some types of supernovae (SNe) and mergers of neutron stars. We search for two isotopic signatures in a sample of Pacific Ocean crust-iron-60 (Fe-60) (half-life, 2.6 million years), which is predominantly produced in massive stars and ejected in supernova explosions, and plutonium-244 (Pu-244) (half-life, 80.6 million years), which is produced solely in r-process events. We detect two distinct influxes of 60Fe to Earth in the last 10 million years and accompanying lower quantities of Pu-244. The Pu-244/Fe-60 influx ratios are similar for both events. The Pu-244 influx is lower than expected if SNe dominate r-process nucleosynthesis, which implies some contribution from other sources.