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

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 δ(13)CPDB 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.

Keywords: Hadean; carbon isotopes; early Earth; origin of life; zircon.

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.

Fig. 2.

δ¹³C for Eoarchean–Hadean carbon samples measured via SIMS vs. host mineral age compared with inorganic and organic carbon (organic carbon values from ref. ; inorganic from ref. 14).

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 Ce_N/(La_N × Pr_N)^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 T^xlln 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.