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Research Article

Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon

Elizabeth A. Bell, View ORCID ProfilePatrick Boehnke, T. Mark Harrison, and Wendy L. Mao
  1. aDepartment of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095;
  2. bSchool of Earth, Energy, and Environmental Sciences, Stanford University, Stanford, CA 94305

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PNAS November 24, 2015 112 (47) 14518-14521; first published October 19, 2015; https://doi.org/10.1073/pnas.1517557112
Elizabeth A. Bell
aDepartment of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095;
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  • For correspondence: ebell21@ucla.edu tmark.harrison@gmail.com
Patrick Boehnke
aDepartment of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095;
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  • ORCID record for Patrick Boehnke
T. Mark Harrison
aDepartment of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095;
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  • For correspondence: ebell21@ucla.edu tmark.harrison@gmail.com
Wendy L. Mao
bSchool of Earth, Energy, and Environmental Sciences, Stanford University, Stanford, CA 94305
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  1. Contributed by T. Mark Harrison, September 4, 2015 (sent for review July 31, 2015)

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Significance

Evidence for carbon cycling or biologic activity can be derived from carbon isotopes, because a high 12C/13C ratio is characteristic of biogenic carbon due to the large isotopic fractionation associated with enzymatic carbon fixation. The earliest materials measured for carbon isotopes at 3.8 Ga are isotopically light, and thus potentially biogenic. Because Earth’s known rock record extends only to ∼4 Ga, earlier periods of history are accessible only through mineral grains deposited in later sediments. We report 12C/13C of graphite preserved in 4.1-Ga zircon. Its complete encasement in crack-free, undisturbed zircon demonstrates that it is not contamination from more recent geologic processes. Its 12C-rich isotopic signature may be evidence for the origin of life on Earth by 4.1 Ga.

Abstract

Evidence of life on Earth is manifestly preserved in the rock record. However, the microfossil record only extends to ∼3.5 billion years (Ga), the chemofossil record arguably to ∼3.8 Ga, and the rock record to 4.0 Ga. Detrital zircons from Jack Hills, Western Australia range in age up to nearly 4.4 Ga. From a population of over 10,000 Jack Hills zircons, we identified one >3.8-Ga zircon that contains primary graphite inclusions. Here, we report carbon isotopic measurements on these inclusions in a concordant, 4.10 ± 0.01-Ga zircon. We interpret these inclusions as primary due to their enclosure in a crack-free host as shown by transmission X-ray microscopy and their crystal habit. Their δ13CPDB of −24 ± 5‰ is consistent with a biogenic origin and may be evidence that a terrestrial biosphere had emerged by 4.1 Ga, or ∼300 My earlier than has been previously proposed.

  • Hadean
  • carbon isotopes
  • early Earth
  • zircon
  • origin of life

Footnotes

  • ↵1To whom correspondence may be addressed. Email: ebell21{at}ucla.edu or tmark.harrison{at}gmail.com.
  • Author contributions: E.A.B., P.B., and T.M.H. designed research; E.A.B., P.B., and T.M.H. performed research; W.L.M. contributed new reagents/analytic tools; E.A.B., P.B., and T.M.H. analyzed data; and E.A.B., P.B., and T.M.H. wrote the paper.

  • The authors declare no conflict of interest.

  • See Commentary on page 14410.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1517557112/-/DCSupplemental.

Freely available online through the PNAS open access option.

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Potentially biogenic 4.1 billion-year-old carbon
Elizabeth A. Bell, Patrick Boehnke, T. Mark Harrison, Wendy L. Mao
Proceedings of the National Academy of Sciences Nov 2015, 112 (47) 14518-14521; DOI: 10.1073/pnas.1517557112

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Potentially biogenic 4.1 billion-year-old carbon
Elizabeth A. Bell, Patrick Boehnke, T. Mark Harrison, Wendy L. Mao
Proceedings of the National Academy of Sciences Nov 2015, 112 (47) 14518-14521; DOI: 10.1073/pnas.1517557112
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