- Peter Ludwiga,
- Shawn Bishopa,1,
- Ramon Eglib,
- Valentyna Chernenkoa,
- Boyana Denevaa,
- Thomas Faestermanna,
- Nicolai Famuloka,
- Leticia Fimiania,
- José Manuel Gómez-Guzmána,
- Karin Haina,
- Gunther Korschineka,
- Marianne Hanzlikc,
- Silke Mercheld, and
- Georg Rugeld
- aPhysik Department, Technische Universität München, 85748 Garching, Germany;
- bGeomagnetism and Gravimetry, Central Institute for Meteorology and Geodynamics, 1190 Vienna, Austria;
- cChemie Department, Fachgebiet Elektronenmikroskopie, Technische Universität München, 85748 Garching, Germany;
- dHelmholtz-Zentrum Dresden-Rossendorf, Helmholtz Institute Freiberg for Resource Technology, 01328 Dresden, Germany
Edited by Edouard Bard, Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement, Aix-en-Provence, France, and accepted by Editorial Board Member Anthony Leggett June 22, 2016 (received for review January 28, 2016)
Significance
Massive stars, which terminate their evolution in a cataclysmic explosion called a type-II supernova, are the nuclear engines of galactic nucleosynthesis. Among the elemental species known to be produced in these stars, the radioisotope 60Fe stands out: This radioisotope has no natural, terrestrial production mechanisms; thus, a detection of 60Fe atoms within terrestrial reservoirs is proof for the direct deposition of supernova material within our solar system. We report, in this work, the direct detection of live 60Fe atoms in biologically produced nanocrystals of magnetite, which we selectively extracted from two Pacific Ocean sediment cores. We find that the arrival of supernova material on Earth coincides with the lower Pleistocene boundary (2.7 Ma) and that it terminates around 1.7 Ma.
Abstract
Massive stars (<mml:math><mml:mrow><mml:mi>M</mml:mi><mml:mo>≳</mml:mo><mml:mn>10</mml:mn><mml:mo> </mml:mo><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:mrow></mml:math>M≳10 M⊙), which terminate their evolution as core-collapse supernovae, are theoretically predicted to eject <mml:math><mml:mrow><mml:mo>></mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>5</mml:mn></mml:mrow></mml:msup><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:mrow></mml:math>>10−5M⊙ of the radioisotope 60Fe (half-life 2.61 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 Ma to 2.8 Ma, near the lower Pleistocene boundary, terminates around 1.7 Ma, and peaks at
about 2.2 Ma.
Footnotes
Author contributions: S.B. designed research; P.L., S.B., R.E., B.D., T.F., N.F., L.F., J.M.G.-G., K.H., G.K., M.H., S.M., and G.R. performed research; P.L., V.C., M.H., S.M., and G.R. developed chemical techniques/protocols; P.L. and R.E. analyzed data; and P.L., S.B., and R.E. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. E.B. is a Guest Editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1601040113/-/DCSupplemental.