Muons penetrate much further than X-rays, they do essentially zero damage, and they are provided for free by the cosmos.
Image credit: Science Source/Digital Globe.
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.
Life on Earth is an ancient phenomenon, with the earliest identified microfossils at nearly 3.5 billion years before present (Ga) (1) and the earliest potential chemofossils at 3.83 Ga (2, 3). Investigation of older materials is limited by the increasingly sparse and metamorphosed rock record, with the oldest rock age at 4.0 Ga (ref. 4; cf. ref. 5). Given the temporal limits of the rock record, it has been difficult to assess terrestrial habitability or document a biosphere before ∼3.8 Ga. Despite the lack of a rock record before 4.0 Ga, detrital zircons as old as 4.38 Ga have been documented (6). About 5% of the zircons from a ∼3-Ga metaconglomerate at Jack Hills, Western Australia yield ages greater than 3.8 Ga (6). Their geochemistry and mineral inclusions have been interpreted to indicate their derivation largely from hydrous, sediment-derived granitic magmas (7⇓⇓⇓⇓–12). The relatively clement conditions implied suggest a potentially habitable planet and leave open the possibility of a Hadean (>4-Ga) biosphere.
The stable isotope ratio 13C/12C (herein defined relative to the Pee Dee Belemnite standard, i.e., δ13CPDB) provides a potential biosignature due to the isotopic fractionation that occurs during carbon fixation. As a consequence, biogenically derived kerogens yield an average of −25 ± 10‰ across the sedimentary rock record from 3.4 Ga to the present (13), whereas carbonates and mantle values are consistently offset with averages of 0‰ (13) and −5‰ (14), respectively. Thus, the discovery of isotopically light graphite in Eoarchean metasediments from southern West Greenland (2, 15) was proposed as evidence of a 3.7- to 3.8-Ga biosphere.
The >3.8-Ga Jack Hills zircons contain abundant mineral inclusions, mostly of a granitic character (11). Abundant and intimately associated diamond and graphite (4% of each in the zircons investigated) were reported in previous studies (16, 17) but subsequently shown to be diamond polishing debris and epoxy that had lodged in cracks during sample preparation (18). This left the true occurrence and abundance of carbonaceous materials in the Jack Hills zircons uncertain. We imaged a large number of >3.8-Ga Jack Hills zircons in search of carbonaceous inclusions and made carbon isotopic measurements of primary graphite found in one grain. To our knowledge, we report here the first unambiguous carbon isotopic measurements of terrestrial Hadean material.
Results
From an initial population of over 10,000 Jack Hills zircons (6), we examined 656 grains with ages over 3.8 Ga for the presence of graphitic inclusions. The zircons were mounted in epoxy and polished to expose their interiors. The search protocol included an initial screening for opaque inclusions using transmitted light microscopy. Seventy-nine candidates thus identified were then targeted for Raman spectroscopy from which we documented two zircons containing partially disordered graphite (Fig. 1, Inset) beneath their polished surfaces (RSES 81-10.14 in a cracked region; RSES 61-18.8 in a crack-free region). We did not consider RSES 81-10.14 further due to the potential for contamination via ingress on cracks.
Fig. 1.
Transmission X-ray image of RSES 61-18.8 with graphite indicated. (Inset) Raman spectra for the top inclusion and for an epoxy “inclusion” from another investigated zircon. The broadened “D-band” at ∼1,400 cm−1 indicates disordered graphite (39); C–H stretch bands at ∼2,800–3,100 cm−1 (39) are observed in epoxy but not graphite.
A concordant U-Pb age of 4.10 ± 0.01 Ga was obtained on a polished internal surface of zircon RSES 61-18.8 (6). Its low U content (∼100 ppm; Supporting Information) minimizes the potential for radiation damage and is a contributing cause for its 99% U-Pb concordancy (6). A roughly 30 × 60 × 20-µm sliver containing two carbonaceous phases was milled using a Ga+ focused ion beam (FIB) and attached to a tungsten needle via a platinum weld for synchrotron transmission X-ray microscopy (19) at beam line 6-2c of the Stanford Synchrotron Radiation Lightsource (SSRL). The 40-nm spatial resolution of this imaging method revealed no through-going cracks or defects associated with these inclusions that exhibit the graphite crystal habit (Fig. 1; also Supporting Information). Due to their isolation within a crack-free region of an isotopically undisturbed zircon, conditions shown to preserve primary inclusion assemblages (20), we interpret these graphitic inclusions to have been incorporated during crystallization of this igneous zircon.
The zircon sliver was remounted in indium, coated with gold, and analyzed for δ13C on a CAMECA ims1270 secondary ion mass spectrometer (SIMS) using a 5-nA Cs+ primary beam (21) with static multicollection following a 2-min cleaning via rastering of the primary beam. Note that the two inclusions had not previously been exposed to epoxy or any other source of laboratory carbon contamination and were revealed only by drilling with the primary Cs+ ion beam. During analysis of the smaller inclusion, we found that count rates increased to a maximum value as the inclusion was breached followed by a tailing to background (Supporting Information). This precludes the possibility that our data were affected by surface contamination.
SIMS measurement of C isotopes in carbonaceous materials is known to be little affected by instrumental mass fractionation. For example, a previous study found less than 2‰ variation in an intercomparison of hydrocarbon standards that varied in H/C by a factor of 7 (21). Thus, we used epoxy, with δ13C of −26.8‰ (21), as a standard material for instrumental mass fractionation corrections and Escherichia coli with δ13C of −24‰ (21) as a secondary standard. The two inclusions yielded similar results in both carbon abundance and δ13C. Maximum signals of ∼105 counts per second (cps) of 12C2+ for the inclusions was at least an order of magnitude higher than for the C background in the indium mounting material (∼104 cps) and much higher than a clean area in the zircon (∼103 cps). The δ13C values of the two inclusions are indistinguishable within error, and thus we combined the results to obtain an average of −24 ± 5‰. This is notably light with respect to terrestrial inorganic carbon and most meteoritic carbon, but consistent with terrestrial biogenic carbon (Fig. 2). Following carbon isotopic analysis, we measured rare earth element (REE) and Ti abundances using the ims1270 with a 15-nA O− primary beam. Two spots yielding measurable Ti correspond to an average apparent crystallization temperature of ∼660 °C, similar to the average Hadean Jack Hills value (9), with REE consistent with crystallization under reduced conditions (using the method of ref. 22) and a continental trace element signature (following the discriminant diagrams of ref. 23).
Discussion
Given the isolation of these inclusions provided by their entrapment in a crack-free and isotopically undisturbed zircon crystal, as well as the vanishingly slow self-diffusion of carbon in graphite under crustal conditions (24), we interpret them to be petrologically primary and isotopically closed systems. Because these inclusions are at least as old as their 4.10-Ga host, the observation of isotopically light carbon raises the possibility that biologic processes were operating during the Hadean. To be sure, not all light carbon isotopic signals are biogenic.
Abiotic processes that could produce light δ13C during the Hadean include Fischer–Tropsch mechanisms (25) and carbon isotopic fractionation by diffusion (26), incorporation of meteoritic (ranging from +68‰ to −60‰ δ13C) materials, mid-ocean ridge basalt degassing (27), and high-temperature disproportionation of siderite (28). However, in the absence of experimental evidence for particulate carbon formation via a Fischer–Tropsch process (ref. 25; cf. ref. 27) or diffusive or mineral disproportionation mechanisms that could selectively lead to light carbon condensates that were ultimately incorporated in a low-temperature (i.e., ∼650 °C) granitoid zircon, a biogenic origin seems at least as plausible. The quantity of extraterrestrial carbon necessary to dominate the carbon budget of a felsic magma protolith would be significant and profoundly influence the chemistry of that magma away from its granitoid character unless separated from the host meteorite. A possible mechanism to effect this on ancient Earth might be through sedimentary processing and concentration of meteoritic carbon in pelagic sediments. Even if meteoritic carbon could be concentrated in this fashion, our observed isotopic signal is uncharacteristic of the most likely candidates, carbonaceous chondrites, which contain on average 3.5 wt% C (29). Although δ13C of various components ranges between +68‰ and −34‰ (30), only 10% of bulk carbon measurements have δ13C less than −20‰ (31), and thus this class of meteorites would be a fortuitous, although possible, source for both inclusions. Our view that isotopically light C in ancient terrestrial materials may be of biogenic origin is consistent with interpretations of light δ13C in mantle diamonds (e.g., ref. 32). Although diamonds as low as approximately −10‰ are able to be accounted for by isotopic fractionation of abiogenic precursors, more negative values down to −23‰ do not appear to be explicable by such processes and strongly suggest an origin from subducted organic matter (33). Although we cannot rule out an abiogenic source, the eventual acquisition of a large database of Hadean carbon isotope measurements will permit the plausibility of hypothetical abiogenic origins to be more closely scrutinized.
By contrast, biogenic organic matter is incorporated into the vast majority of preserved clastic sediments, can reach 10% by weight in Phanerozoic sediments (34), and has averaged δ13C approximately −25‰ over the past 3.5 Ga (13). Given this historical trend and the several lines of evidence indicative of sediment involvement in the Jack Hills zircon magmas (8, 10⇓–12), the simplest interpretation of our data are that the carbonaceous inclusions represent graphitized organic carbon present during melting of a pelitic protolith at 4.10 Ga. This interpretation is entirely consistent with the observed low crystallization temperature, continental affinity, and reduced environment (35) of zircon formation.
The isolated, primary nature of the graphite in sample RSES 61-18.8, along with the lack of diamond features in its Raman spectrum, differs from the abundant graphite previously reported (16, 17) in Jack Hills zircons, which was found often intersecting cracks in the host zircon, and all of which were reported to contain diamond (17). As noted, the diamonds were later found to be contamination (18). The rare occurrence of primary graphite in this study (in <0.2% of >3.8-Ga zircons) also contrasts with these earlier results (graphite in 4% of zircons of all ages), further suggesting that much or all of the earlier-purported graphite identified was contamination.
There are considerable limitations of basing any inference regarding early Earth on a single zircon containing primary carbonaceous inclusions. Instead, we see this contribution as demonstrating the feasibility of perhaps the only approach that could lead to establishing a Hadean carbon isotope record. In this regard, we emphasize that because >3.8-Ga grains make up only ∼5% of the Jack Hills zircon population (6), RSES 61-18.8 represents an abundance of carbon-bearing Jack Hills zircons of only about 1-in-10,000.
Conclusions
This study extends the terrestrial carbon isotope record ∼300 My beyond the previously oldest-measured samples from southwest Greenland. Our interpretation that the light C isotope signature in primary graphitic inclusions could reflect biologic processes is consistent with an estimate from molecular divergence in prokaryote phylogenetic relationships that a terrestrial biosphere had emerged by 4.1 Ga (36). Confirming such a connection would represent a potentially transformational scientific advance. However, given the low occurrence of carbon-bearing Hadean zircons, establishing a Hadean carbon cycle and its possible bearing on the origin of life will require enormous and sustained efforts.
Methods
Zircons were mounted in epoxy and polished to expose their interiors, typically involving loss of approximately one-third of the zircon during polishing. Polishing was accomplished with 1,200-grit silicon carbide paper and 1-µm diamond paste. The search protocol included an initial screening for opaque inclusions using transmitted light microscopy and a 40× objective lens.
Raman Spectroscopy.
Confocal Raman spectroscopy was carried out on zircons mounted in epoxy in several laboratories, using a green laser and 20× or 40× objective lenses. Opaque inclusions noted by transmitted light microscopy were analyzed for carbonaceous material. This screening procedure includes some inherent bias toward zircons clear enough for effective transmitted light microscopy, although this appears to include most of the Hadean Jack Hills population imaged in this study. We also note that our screening protocol does not completely preclude some of the other opaque inclusions from containing some carbonaceous material—the Raman signal could be masked by high noise.
FIB Milling.
Sample RSES 61-18.8 was coated in gold and a ∼60 × 30 × 20 triangular prismatic region containing the identified carbonaceous inclusion was milled from the larger zircon sample using a Nova 600 SEM/FIB system at the University of California, Los Angeles (UCLA) Nanoelectronics Research Facility. The section of zircon was attached to a tungsten needle using a platinum weld.
X-Ray Microscopy.
The FIBed section of RSES 61-18.8 was imaged by transmission X-ray microscopy at beam line 6-2c of the SSRL (19), at a spatial resolution of 40 nm. An energy of 7,160 eV was used for imaging. One X-ray image is seen in Fig. 1, while movies constructed from the X-ray slices are presented in Supporting Information.
Carbon Isotopic Measurements.
We used the method of ref. 3, modified for static multicollection. We also included a 2-min precleaning of analysis surfaces with a 20 × 20-µm raster of the primary beam. We used a Cs+ primary beam of 5 nA for measurements on E. coli and zircon samples and of 2 pA for measurements on the epoxy. We used epoxy as our primary standard (−26.8‰ PDB; ref. 21) for all instrumental mass fractionation corrections and E. coli as a secondary standard (−24.1; our analyses averaged −19 ± 4‰).
Error bars on our graphite inclusions incorporate the internal error bars for the analyses, the reproducibility of the epoxy, and quadratic addition of a 4‰ uncertainty due to potential matrix effects between epoxy and graphite (21). The latter is included even though negligible matrix effects were noticed in an earlier study, where less than 2‰ variation was observed in an intercomparison of hydrocarbon standards that varied in H/C by a factor of 7 (21). Considering these error bars, normalization to either the primary epoxy or secondary E. coli standard produces statistically indistinguishable δ13CPDB values for our inclusions.
Trace Element Measurement.
These analyses were carried out using a CAMECA ims1270 ion microprobe at UCLA, with primary O− beam intensity of ∼15 nA and a spot size of 60 µm. Secondary ions were detected at high energy offset (−100 eV) to suppress molecular interferences. Further peak stripping for background and isobaric interference corrections was accomplished off-line. Standardization was to the National Institute of Standards and Technology 610 standard glass, with the 91500 zircon (37) used as a secondary standard. Additional correction of [Ti] used a value for 91500 of 5.2 ± 0.3 ppm (38).
Fig. S1.
Carbon signals for background measurements made on other zircons (some Jack Hills zircons, some more recent granite zircons). Carbon signal on y axis, analysis number within each group of analyses on x axis. “Surficial graphite inclusions” are regions on the polished surfaces of Hadean Jack Hills zircons identified as disordered graphite by Raman analysis. There is little evidence in the ion microprobe data for significant graphite in these regions, suggesting they were surficial contamination from previous sample preparation procedures. Data are presented in Table S1.
Fig. S2.
Carbon signals for analyses on the mount RSES 61-18.8. Carbon signal on y axis, analysis number within each group of analyses on x axis. “B.g.,” background, taken after presputtering to clean the analysis surface of potentially contaminating carbon. Data are presented in Table S2.
Fig. S3.
Electron images taken in between analyses on RSES 61-18.8. (A) Secondary electron (SE) image after first measurements that did not find inclusions (“zircon,” “zircon_try2” in Tables S2 and S3). Initial analysis pit is indicated by “1,” and large inclusion is indicated by “2.” Note the larger rastered area that was precleaned before isotopic analysis (dark area). The hexagonal spot in analysis pit 1 is one of the smaller, noncarbonaceous inclusions evident in Movies S1 and S2. (B) SE image taken after measurement of small inclusion in “3” (“zircon_try3”). Remnant of “1” pit is also seen. (C) SE image of zircon after measurement of large inclusion (“zircon_try8”). Note the larger precleaned area from a second rastering before measurement of the large inclusion, which cleaned the surface surrounding “2.” “3*” is the carbon background measurement in a slightly different location than “3,” but partially erasing the earlier pit. (D) Backscattered electron image (BSE) after all carbon isotope and some trace element measurements. “2*” indicates the pit used for analyzing the large inclusion. Trace element analysis spot 2b is shown by “4.” (E) BSE image showing a wider view of the zircon mounted in In after carbon isotopic and trace element analyses. Analysis spots are unlabeled.
Fig. S4.
Corrected δ13C of epoxy and inclusions on mount RSES 61-18.8. 1σ error bars are shown. Secondary standard E. coli is also shown. All analyses are corrected to the epoxy standard (−26.8‰ PDB; ref. 21). The dashed line is at −26.8; corrected epoxy measurements mostly plot within error of the correct value. E. coli should be at −24.1‰ (21). E. coli measurements average −19 ± 4‰.
Fig. S5.
Rare earth elements (REEs) in sample RSES 61-18.8, normalized to chondritic abundances. The cerium anomaly is calculated as CeN/(LaN × PrN)1/2 (N indicating a chondrite-normalized value) and is calculable for one point. Due to the high light REE (LREE) and light element signal on the indium mounting medium, we were unable to calculate La concentrations for two of the three points and unable to calculate Ti concentrations for one point. Using the Ce/Ce* from spot 2b and using the average Txlln for the two points with usable Ti data (∼660 ± 80 °C), we calculate redox conditions ∼9 ± 1 log units below FMQ (after ref. 22). Data are shown in Table S4. Spot 2b, blue circles; spot 4, red triangles; spot 5, white stars. Spot 5 was collected after spot 4 in the same location.
View this table:
Table S1.
Measurements on the 12C2+ species on various materials, using either a 5-nA (zircon, inclusions, E. coli) or 2-pA (epoxy) Cs+ primary beam
View this table:
Table S2.
Raw carbon isotopic measurements on mount RSES 61-18.8 (including epoxy, zircon background, indium background, and inclusion measurements) and E. coli secondary standard
View this table:
Table S3.
Corrected δ13C for isotopic measurements in Table S2
View this table:
Table S4.
Trace element data for RSES 61-18.8
Acknowledgments
Raman spectroscopy by Anatoliy Kudryavtsev, Anke Watenpuhl, Olivier Beyssac, and Daniel Wilkinson, using confocal Raman systems in the laboratories of William Schopf, Andrew Steele, Olivier Beyssac, and Bruce Dunn, made possible our search for carbonaceous inclusions. FIB milling by Noah Bodzin, X-ray imaging by Wendy Mao and Crystal Shi [who are supported by National Science Foundation (NSF)–Division of Earth Sciences (EAR) Grant 1055454], and sample mounting by Christopher Snead were also essential. We thank Elizabeth Boehnke for X-ray image reconstruction and for Movie S2. We thank the three reviewers, whose comments greatly improved our manuscript. This project was funded by a Simons Collaboration on the Origin of Life Postdoctoral Fellowship (to E.A.B.) and NSF-EAR Grant 0948724 (to T.M.H.). The UCLA ion microprobe facility is partially supported by a grant from the Instrumentation and Facilities Program of EAR, NSF. Data reported in this manuscript are presented in full in Supporting Information. Portions of this research were carried out at the SSRL, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the US Department of Energy, Office of Science by Stanford University.
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.
References
Potentially biogenic 4.1 billion-year-old carbon
Article Classifications
- Physical Sciences
- Earth, Atmospheric, and Planetary Sciences
Sign up for Article Alerts
Jump to section
You May Also be Interested in
Researchers are exploring whether these ubiquitous fluorinated molecules might worsen infections or hamper vaccine effectiveness.
Image credit: Shutterstock/Dmitry Naumov.
One protein seems to play a key role in touch sensitivity for flytraps and other meat-eating plants.
Image credit: Shutterstock/Kuttelvaserova Stuchelova.
Adam Mastroianni and Daniel Gilbert explore why conversations almost never end when people want them to.
A study finds that giant pandas roll in horse manure to increase their cold tolerance.
Image credit: Fuwen Wei.
See related content: