Time-Resolved Two Million Year Old Supernova Activity Discovered in the Earth’s Microfossil Record

Massive stars (M≥M⊙), which terminate their evolution as core collapse supernovae, are theoretically predicted to eject >10⁻⁵M⊙ of the radioisotope ⁶⁰Fe (t_1/2=2.6 Ma). If such an event occurs sufficiently close to our solar system, traces of the supernova debris could be deposited on Earth. Herein, we report a time-resolved ⁶⁰Fe signal residing, at least partially, in a biogenic reservoir. Using accelerator mass spectrometry, this signal was found through the direct detection of live ⁶⁰Fe atoms contained within secondary iron-oxides, among which are magnetofossils; the fossilized chains of magnetite crystals produced by magnetotactic bacteria. The magnetofossils were chemically extracted from two Pacific Ocean sediment drill cores. Our results show that the ⁶⁰Fe signal onset occurs around 2.6−2.8 Ma, near the lower Pleistocene boundary, terminates around 1.7 Ma, and peaks at about 2.2 Ma.