Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon
Elizabeth A. Bell a,1, Patrick Boehnke a, T. Mark Harrison a,1, and Wendy L. Mao b
aDepartment of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095;
bSchool of Earth, Energy, and Environmental Sciences, Stanford University, Stanford, CA 94305
Contributed by T. Mark Harrison, September 4, 2015 (sent for review July 31, 2015)
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.
Freely available online through the PNAS open access option.
FREE PDF GRATIS: PNAS
