  Massive stars, which terminate their evolution as core collapse supernovae,
are theoretically predicted to eject more than 1E-5 solar masses of the
radioisotope 60Fe. 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 Ma to 2.8 Ma, near the lower
Pleistocene boundary, terminates around 1.7 Ma, and peaks at about 2.2 Ma.
