ArticlePDF Available

Abstract and Figures

Some graphite contained in the 3.7-billion-year-old metasedimentary rocks of the Isua Supracrustal Belt, Western Greenland, is depleted in 13C and has been interpreted as evidence for early life. However, it is unclear whether this graphite is primary, or was precipitated from metamorphic or igneous fluids. Here we analyse the geochemistry and structure of the 13C- depleted graphite in the Isua schists. Raman spectroscopy and geochemical analyses indicate that the schists are formed from clastic marine sediments that contained 13C-depleted carbon at the time of their deposition. Transmission electron microscope observations show that graphite in the schist occurs as nanoscale polygonal and tube-like grains, in contrast to abiotic graphite in carbonate veins that exhibits a flaky morphology. Furthermore, the graphite grains in the schist contain distorted crystal structures and disordered stacking of sheets of graphene. The observed morphologies are consistent with pyrolysation and pressurization of structurally heterogeneous organic compounds during metamorphism. We thus conclude that the graphite contained in the Isua metasediments represents traces of early life that flourished in the oceans at least 3.7billion years ago.
Content may be subject to copyright.
Evidence for biogenic graphite in early Archaean
Isua metasedimentary rocks
Yoko Ohtomo1*, Takeshi Kakegawa1, Akizumi Ishida1, Toshiro Nagase1and Minik T. Rosing2
Some graphite contained in the 3.7-billion-year-old
metasedimentary rocks of the Isua Supracrustal Belt, Western
Greenland1, is depleted in 13C and has been interpreted as
evidence for early life2. However, it is unclear whether this
graphite is primary, or was precipitated from metamorphic
or igneous fluids3,4. Here we analyse the geochemistry and
structure of the 13C- depleted graphite in the Isua schists.
Raman spectroscopy and geochemical analyses indicate that
the schists are formed from clastic marine sediments that
contained 13C-depleted carbon at the time of their deposition.
Transmission electron microscope observations show that
graphite in the schist occurs as nanoscale polygonal and
tube-like grains, in contrast to abiotic graphite in carbonate
veins that exhibits a flaky morphology. Furthermore, the
graphite grains in the schist contain distorted crystal structures
and disordered stacking of sheets of graphene. The observed
morphologies are consistent with pyrolysation and pressuriza-
tion of structurally heterogeneous organic compounds during
metamorphism. We thus conclude that the graphite contained
in the Isua metasediments represents traces of early life that
flourished in the oceans at least 3.7 billion years ago.
The suggestion that graphite in early Archaean rocks represents
materials of biogenic origin has been met with a degree of
scepticism5,6. Isotopic compositions of graphite in >3.7-billion-
year old (Ga) rocks from the Isua Supracrustal Belt (ISB), Western
Greenland, which are believed to be of sedimentary origin, suggest
that vast microbial ecosystems were present in early Archaean
oceans7,8. However, results of more recent studies suggest that
most of the graphite-bearing rocks formed through interactions
between crustal fluids and surrounding igneous rocks3,4 during later
metasomatic events9, thereby casting doubt on the existence of an
extensive sedimentary sequence in the ISB and on the biogenic
origin of constituents. In contrast, 13C-depleted graphite globules,
which are considered to form from biogenic precursors, have been
reported from metamorphosed clastic sedimentary rocks in the ISB
(ref. 2). However, these globules were found at a single locality, and
it therefore remains unclear whether traces of life at other localities
in the ISB were lost during metamorphism or were originally absent.
The presence of additional clastic sedimentary rocks containing
graphite may provide evidence for the preservation of organic
constituents in early Archaean rocks, thus supporting the notion
that microbes were active in early Archaean oceans.
We conducted a geological survey along the northwestern area
of the ISB (Fig. 1a,b), where the rocks are generally less affected
by deformation than in other areas of the belt10. Metamorphosed
basalts and banded iron formations (BIFs) are dominant in
this area. The BIFs contain interbedded black to grey schist
1Natural Resources and Environmental Geochemistry Research Group, Division of Earth and Planetary Materials Science, Graduate School of Science,
Tohoku University, 980-8578, Japan, 2Nordic Center for Earth Evolution, Natural History Museum of Denmark, University of Copenhagen, 1350, Denmark.
layers (Fig. 1b–d), typically 40–80 cm thick. The schist mainly
comprises chlorite, cummingtonite, quartz and reduced carbon (see
Supplementary Fig. 1 and Table 1). The black–grey schist in the
northern part of the area contains moderate amounts of Al, Ti and
Zr, probably representing detrital components (see Supplementary
Information). The rare earth element (REE) patterns (normalized
by the composition of post-Archaean Australian shale, PAAS;
ref. 11) in the Al-rich rocks (samples 3072303 and 6072905) lie close
to the line defined by a rock/PAAS ratio of 1, with the exception of
slightly lower ratios for light REEs (Fig. 2). These REE patterns and
Al-rich characteristics suggest that the protoliths of the schist were
clastic marine sediments. However, some of the examined rocks
have compositions resembling those of BIFs (for example, samples
4062308, 4062309 and 5080501).
The black–grey schist samples contain abundant reduced
carbon (0.1–8.8 wt%), identified as graphite by X-ray diffraction
(XRD) analysis (Supplementary Tables 3 and 4). The graphite-
rich schist from the northern part of the area is folded, as are
the surrounding BIFs. A sample from this folded area contains
the highest concentrations of Cgraphite (8.8 wt% C) measured in
the present study. At other locations, the schist is concordantly
intercalated in layers of BIFs, and contains moderate concentrations
of Cgraphite (0.1–0.5 wt% C; Supplementary Table 3). In general,
graphite concentrations on microscopic scales vary according to
the compositional layering of the schist, which strikes parallel to
that of the surrounding BIFs. Thus, the moderate to high Cgraphite
concentrations, the correspondence of Cgraphite concentrations with
compositional layering and the concordant layering of schist and
BIF units all support a sedimentary origin of the reduced carbon.
It has been suggested that disproportionation of Fe-carbonate
during metamorphism accounts for the genesis of the graphite in
secondary carbonate veins4. In such veins, substantial amounts of
magnetite and residual Fe-carbonate occur along with the graphite.
However, Fe-carbonate and magnetite are absent from the graphite-
rich schist in this study. Therefore, it is unlikely that the graphite
originated from the thermal disproportionation of Fe-carbonate.
A graphite-rich carbonate vein, located in the northeastern
part of the ISB, contains high concentrations of graphite (sample
4062002y, 4.1 wt% C; see Supplementary Table 3) compared with
other carbonate veins in the ISB, which contain much smaller
amounts of graphite if any (see Supplementary Information).
Such secondary graphite was analysed together with graphite in
the metasediment. The geothermometric signals of the Raman
spectrum in metasediment sample 6072905 indicate that peak
metamorphic temperatures of the graphite were 525±50 C, which
is consistent with prograde metamorphic temperatures reported
for the ISB (500–600 C; ref. 9). This temperature consistency
© 2013 Macmillan Publishers Limited. All rights reserved.
65° 09‘ N
50° 10‘ W
Amitsoq gneiss
Proterozoic dyke
50° 09‘ 70.00‘‘ W
65° 09‘ 10.00‘‘ N
50 m
Quartz dyke
Graphite-rich schist
Carbonate vein
10 cm
Graphite-rich schist
Graphite-rich schist
Quartz-rich BIFs
Graphite-rich schist
Quartz-rich BIFs
Quartz-rich BIFs
Graphite-rich schist
Fine laminated
Quartz-rich BIFs
Graphite-rich schist
Figure 1 |Geologic maps and photos of the study area in the ISB, West Greenland. a, General geology of the ISB, West Greenland. b, Geologic map
produced in this study. c, Geologic cross-sections A–A0(northern section), B–B0(central section) and C–C0(southern section). Points along A–C0are the
same as those presented in b.d, Outcrop photograph of graphite-rich schist intercalated with BIFs.
Rock/PAAS ratio
Average of BIF samples
in northern section
La Ce Pr Nd Sm Eu Gd Tb
Dy Y Ho Er Tm Yb Lu
Figure 2 |REE patterns in the examined graphite-rich schist. The BIF value
in this figure represents the average composition of nine BIF samples from
along the northern section A–A0(Fig. 1b).
suggests that the precursor of the graphite was already present in
the host rocks before prograde metamorphism. Raman spectrum
of secondary graphite (sample 4062002y) is nearly identical to that
of the graphite in the metasediment. The estimated metamorphic
temperatures for the secondary graphite (496 ±50 C) are close to
peak metamorphic temperatures, suggesting a metamorphic origin
of the secondary graphite.
The carbon isotope ratios (13C) of graphite were determined
using the graphite combustion method and the neodymium–
yttrium aluminium garnet (Nd–YAG) laser microprobe technique.
The range of 13C values was 23.8 to 12.5h(average,
17.9h), which is within the range of values reported in previous
studies (Fig. 3). The 13C value of the secondary graphite (sample
4062002y) was 10.5h(Supplementary Table 3). The Nd–
YAG laser microprobe analysis revealed microscale heterogeneities
in the 13C values in single rock chips (10 50.5 mm3;
Fig. 3b,c and Supplementary Fig. 2). For example, the 13C
values for sample 5080603 ranged from 20.9 to 14.7h
(average, 17.4h; Fig. 3c).
Carbonic fluids can precipitate abiogenic graphite during
isobaric cooling, during isothermal increases in pressure12 and
on account of the mixing of CH4- and CO2-rich fluids13.
We modelled the theoretical 13C values of fluid-precipitated
graphites14, assuming that the 13 C value for CO2in the original
metamorphic fluid was 5h, which is similar to the value observed
regionally in carbonate-rich rocks of the ISB (refs 4,7). The
lightest 13C value for graphite precipitated from such fluids under
metamorphic conditions is 16.4h, which is achieved at 400 C
(Supplementary Fig. 5a). The 13C values exceed 16.4hwhen
Rayleigh-type isotope fractionation operates in the fluids. Graphite
that is depleted in13C by more than 16.4hcan form only if the
fluid was derived from a source that already contained abundant
13C-depleted CH4. However, given the possible oxidation state of
infiltrating metamorphic fluids, CH4was most probably absent in
the fluids (see Supplementary Information).
In samples from central to southern sections (B–B0and
C–C0, respectively; Fig. 1), abundances of Cgraphite were low and
13C values were high, suggesting that extensive reactions with
metamorphic fluids shifted 13C of graphite to higher 13 C values
(for example, 12.5h; Supplementary Fig. 3). The heterogeneities
in 13C values (Fig. 3a–c) are explained by exchange of carbon
isotopes with transient carbonic fluids. In other words, all 13C of
graphite were originally lower than those shown in Fig. 3. The more
negative isotopic compositions (for example, 23.8h) represent
less modified components and are closer to premetamorphic
compositions. Therefore, 13C-depleted organic matter in Isua clastic
sediments is postulated as an initial carbon source to explain the less
modified carbon isotope compositions.
We observed the morphologies and nanostructures of graphite
in the metasediment and secondary graphite using scanning
transmission electron microscopy (STEM) and high-resolution
© 2013 Macmillan Publishers Limited. All rights reserved.
Laser microprobe analyses, sample 3072303
= 8
ave. = ¬20.9
Laser microprobe analyses, sample 5080603
n = 13
ave. = ¬17.4
Bulk analyses
n = 58
ave. = ¬17.9
Previous studies
n = 82
ave. = ¬10.8
% PDB)(%
% PDB)(%
% PDB)(%
% PDB)(%
¬24 ¬22 ¬20 ¬18 ¬16 ¬14 ¬12 ¬10 ¬8 ¬6 ¬4 ¬2 0 2
¬24 ¬22 ¬20 ¬18 ¬16 ¬14 ¬12 ¬10 ¬8 ¬6 ¬4 ¬2 0 2
¬24 ¬22 ¬20 ¬18 ¬16 ¬14 ¬12 ¬10 ¬8 ¬6 ¬4 ¬2 0 2
¬24 ¬22 ¬20 ¬18 ¬16 ¬14 ¬12 ¬10 ¬8 ¬6 ¬4 ¬2 0 2
Figure 3 |Carbon stable isotope compositions of graphite in the ISB. a,13C values for graphite in graphite-rich schist extracted from bulk rock powder.
b,13C values for graphite in sample 3072303 obtained using the Nd–YAG laser microprobe technique. c,13C values for graphite in sample 5080603
obtained using the Nd–YAG laser microprobe technique. d, Total range of published 13 C values for graphite in the ISB. Referenced data are listed in
Supplementary Table 6. n, number of analyses; ave., average value; PDB, Pee Dee Belemnite. Reproducibilities and accuracies of measurements are within
50 nm 50 nm 50 nm
20 nm
5 nm 5 nm 5 nm
Figure 4 |Transmission electron microscopy images of graphite. a, STEM image of graphite in metasediment (sample 6072906), showing dominance of
polygonal and tube-like grains. b, STEM image of secondary graphite (sample 4062002y), showing dominance of sheeted flake grains. c, HRTEM image of
graphite in metasediment (sample 6072905). SAED was obtained from area in c.d,e, Magnification of the area marked dand ein c.f, HRTEM image of
sample 4062002y. SAED was obtained from area in f.g, Magnification of the area marked gin f. Arrows are explained in the main text.
transmission electron microscopy (HRTEM). Irrespective of the
sample type, all examined graphite showed highly crystalline
features in HRTEM observations, which is consistent with the
XRD and Raman data. Differences between graphite in the
metasediment and that in the secondary vein samples are reflected
in their respective morphologies, internal graphite nanostructures
and stacking defects. The examined metasediment included
graphitic polygonal grains (white arrows, Fig. 4a) and tube-like
structure (black arrows, Fig. 4a). Some lattice fringes showed
distortion at surfaces and inside graphite grains (onion-like
structures; black arrows, Fig. 4d,e; see Supplementary Information
for details). Such features were found in samples from both
the northern and southern sections (Supplementary Fig. 9a,b).
Sheeted flakes were a dominant morphology of secondary graphite,
whereas polygonal grains and tube-like structures were absent
(Fig. 4b). The sheeted flakes indicate well-layered structures
overall (Fig. 4f,g), although the surfaces and edges of flakes were
sometimes curled. Such curled structures disappear on the inner
portions of sheeted flakes (black arrow, Fig. 4g; white arrows,
Supplementary Fig. 8d,e). The structural changes from the surface
to the inner portions of the sheeted flakes indicate that initial
deposition of distorted graphite (on the current surface) was
followed by ordered deposition of successive layers of graphite
(current inner part)15.
© 2013 Macmillan Publishers Limited. All rights reserved.
In contrast, curled structures are present in the inner portions
of graphite grains in the metasediment (black arrows, Fig. 4d),
suggesting that its origin is different from that of the secondary
graphite. The selected area electron diffraction (SAED) pattern of
graphite in the metasediment has h001istreaks on and near the
h00 reflections (see the bright diffusion line near the 100 plane in
the SAED data, Fig. 4c), caused by a disordered stacking sequence
of the graphene sheets. However, h001istreaks are unclear in the
SAED pattern of Fig. 4f, suggesting a dominance of ordered 2H-type
stacking in the secondary graphite.
The effects of temperature, pressure and mineral surfaces on
the graphitization of biogenic organic compounds in geological
samples have been intensively studied16–22. Distorted structures
and the diffusion of graphene sheet stacking are common in
pyrolysed and pressurized organic compounds. Such precursors
commonly contain non-graphitizing carbon, such as non-planar
carbon ring compounds associated with abundant pores16. At high
pressures, organic matter in the presence of various hydrocarbons
mixed with non-graphitizing carbon is forced into parallelism,
thereby facilitating the formation of hexagonal graphene sheets,
whereas the crystal lattice remains distorted and the graphene
sheet stacking may display diffusion in SAED patterns. Therefore,
biogenic organic matter, which contains various molecules and
functional groups, is favoured as the precursor of the graphite
observed in the metasediment.
Onion-like carbonaceous materials have also been observed
in meteorites23. Some onion-like carbonaceous materials in
meteorites were formed at >1,000 C (ref. 23), which exceeds
general metamorphic temperatures. Furthermore, graphite in the
examined schist has sedimentary characteristics with various
morphologies, suggesting that meteoritic origin is unlikely for the
Isua graphite. Polygonal and tube-like structures in graphite-rich
schist show similarities to those found in artificial nanocarbons
formed under evaporation–condensation conditions and in electric
discharge systems15,24. However, unusual temperature conditions
and chemistry of carbon sources to generate artificial nanocarbons
are difficult to realize in the Earth’s crust. Therefore, we conclude
that polygonal and tube-like structures in the graphite-rich
schist were generated during maturation processes of organic
matter. A minor portion of heterogeneity in the graphitic
structural order could be caused by the secondary effects of
surrounding minerals22 and/or dynamic fluid flow processes15
during metamorphism.
Graphite in the metasediment from the northwest ISB is
distinct from the graphite in vein samples. The combined
information on geological occurrences, graphite morphologies,
nanoscale structures and isotopic compositions of the graphite in
the metasediment suggests a biogenic origin. High concentrations
of 13C-depleted graphite in these rocks would require widespread
biological activity to support the high rate of production and
sedimentary delivery of organic matter to the >3.7 Ga ocean floor.
Received 1 May 2013; accepted 5 November 2013;
published online 8 December 2013
1. Nutman, A. P., Friend, C. R. L. & Paxton, S. Detrital zircon sedimentary
provenance ages for the Eoarchaean Isua supracrustal belt southern West
Greenland: Juxtaposition of an imbricated ca. 3700 Ma juvenile arc against
an older complex with 3920–3760 Ma components. Precambrian Res. 172,
212–233 (2009).
2. Rosing, M. T. 13C-depleted carbon microparticles in >3700-Ma sea-floor
sedimentary rocks from west Greenland. Science 283, 674–676 (1999).
3. Naraoka, H., Ohtake, M., Maruyama, S. & Ohmoto, H. Non-biogenic graphite
in 3.8-Ga metamorphic rocks from the Isua district, Greenland. Chem. Geol.
133, 251–260 (1996).
4. Van Zuilen, M., Lepland, A. & Arrhenius, G. Reassessing the evidence for the
earliest traces of life. Nature 418, 627–630 (2002).
5. Mojzsis, S. J. et al. Evidence for life on Earth before 3,800 million years ago.
Nature 384, 55–59 (1996).
6. Fedo, C. M. & Whitehouse, M. J. Metasomatic origin of quartz–pyroxene
rock, Akilia, Greenland, and implications for Earth’s earliest life. Science 296,
1448–1452 (2002).
7. Schidlowski, M., Appel, P. W. U., Eichmann, R. & Junge, C. E. Carbon
isotope geochemistry of the 3.7109-yr-old Isua sediments, West Greenland:
Implications for the Archaean carbon and oxygen cycles.
Geochim. Cosmochim. Acta 43, 189–199 (1979).
8. Ueno, Y., Yurimoto, H., Yoshioka, H., Komiya, T. & Maruyama, S. Ion
microprobe analysis of graphite from ca. 3.8 Ga metasediments, Isua
supracrustal belt, West Greenland: Relationship between metamorphism and
carbon isotopic composition. Geochim. Cosmochim. Acta 66, 1257–1268 (2002).
9. Rose, N. M., Rosing, M. T. & Bridgwater, D. The origin of metacarbonate
rocks in the Archaean Isua supracrustal belt, West Greenland. Am. J. Sci. 96,
1004–1044 (1996).
10. Rosing, M. T., Rose, N. M., Bridgwater, D. & Thomsen, H. S. Earliest part of
Earth’s stratigraphic record: A reappraisal of the >3.7 Ga Isua (Greenland)
supracrustal sequence. Geology 24, 43–46 (1996).
11. Taylor, S. R. & McLennan, S. M. The Continental Crust: Its Composition and
Evolution (Blackwell, 1985).
12. Luque, F. J. & Rodas, M. Constraints on graphite crystallinity in some Spanish
fluid-deposited occurrences from different geologic settings. Miner. Deposita
34, 215–219 (1999).
13. Chamberlain, C. P. & Rumble, D. Thermal anomalies in a regional
metamorphic terrane: An isotopic study of the role of fluids. J. Petrol. 29,
1215–1232 (1988).
14. Ray, J. S. Carbon isotopic variations in fluid-deposited graphite: Evidence
for multicomponent Rayleigh isotopic fractionation. Int. Geol. Rev. 51,
45–57 (2009).
15. Kuznetsov, V. L., Butenko, Y. V., Zaikovskii, V. I. & Chuvilin, A. L. Carbon
redistribution processes in nanocarbons. Carbon 42, 1057–1061 (2004).
16. Buseck, P. R. & Bo-Jun, H. Conversion of carbonaceous material to graphite
during metamorphism. Geochim. Cosmochim. Acta 49, 2003–2016 (1985).
17. Large, D. J., Christy, A. G. & Fallick, A. E. Poorly crystalline carbonaceous
matter in high-grade metasediments—implications for graphitization
and metamorphic fluid compositions. Contrib. Mineral. Petrol. 116,
108–116 (1994).
18. Deurbergue, A., Oberlin, A., Oh, J. H. & Rouzaud, J. N. Graphitization of
Korean anthracites as studied by transmission electron microscopy and X-ray
diffraction. Int. J. Coal Geol. 8, 375–393 (1987).
19. Rouzaud, J. N. & Oberlin, A. Structure, microtexture, and optical properties of
anthracene and saccharose-based carbons. Carbon 27, 517–529 (1989).
20. Bustin, R. M., Ross, J. V. & Rouzaud, J. N. Mechanisms of graphite formation
from kerogen: Experimental evidence. Int. J. Coal Geol. 28, 1–36 (1995).
21. Beyssac, O. et al. Graphitization in a high-pressure, low-temperature
metamorphic gradient: A Raman microspectroscopy and HRTEM study.
Contrib. Mineral. Petrol 143, 19–31 (2002).
22. Van Zuilen, M. A. et al. Mineral-templated growth of natural graphite films.
Geochim. Cosmochim. Acta 83, 252–262 (2012).
23. Le Guillou, C. et al. High resolution TEM of chondritic carbonaceous
matter: Metamorphic evolution and heterogeneity. Meteorit. Planet. Sci. 47,
345–362 (2012).
24. Horváth, Z. E. et al. Inexpensive, upscalable nanotube growth methods.
Curr. Appl. Phys. 6, 135–140 (2006).
We thank E. Aoyagi for technical assistance with STEM and HRTEM observations. The
isotope ratio mass spectrometer (infrared-MS) analyses were carried out with support
from T. Watanabe and F. W. Nara. The manuscript was improved by discussions with Y.
Furukawa and T. Otake. This study was supported by the Japan Society for the
Promotion of Science (grants 17403011 and 21403009).
Author contributions
Y.O., T.K. and M.T.R. conducted the geological surveys and collected rock samples. Y.O.
carried out the geological and petrographical analyses, carbon stable isotope analyses of
graphite using the graphite combustion method, XRD analyses, HRTEM observations
and thermodynamic/isotopic calculations. A.I. and T.N. contributed to sample
preparation and HRTEM observations. T.K. carried out carbon stable isotope analyses of
graphite using the in situ Nd–YAG laser microprobe technique, STEM observations and
Raman microspectroscopic analyses.
Additional information
Supplementary information is available in the online version of the paper.Reprints and
permissions information is available online at
Correspondence and requests for materials should be addressed to Y.O.
Competing financial interests
The authors declare no competing financial interests.
© 2013 Macmillan Publishers Limited. All rights reserved.
... Some of the oldest putative remnants of life on Earth are preserved in the form of graphite, i.e., crystalline solid carbon with a redox state of zero (C 0 ). Examples include a graphite inclusion in a 4.1 billion year old (Ga) zircon (Bell et al., 2015), graphitic metapelites in northern Labrador from 3.95 Ga (Tashiro et al., 2017) (but see Whitehouse et al., 2019 for a different assessment of the depositional age) and in the >3.7 Ga Isua supracrustal belt (Rosing, 1999;Ohtomo et al., 2014), as well as graphite inclusions within banded iron formation from Isua (Mojzsis et al., 1996) and from Akilia Island at 3.85 Ga (Mojzsis et al., 1996) (but see alternative evidence from Fedo and Whitehouse, 2002;Lepland et al., 2005). Graphite inclusions have also been reported from banded iron formations in the Nuvvuagittuq Supracrustal Belt (>3.75 Ga), but these were concluded to post-date peak metamorphism (Papineau et al., 2011). ...
... This possibility has fuelled debates about the biogenicity of the earliest graphite occurrences in the rock record (van Zuilen et al., 2002;van Zuilen et al., 2005). Additional lines of evidence that have been used in support of a biological origin are the isotopic composition of the graphite (Mojzsis et al., 1996;Rosing, 1999;Ohtomo et al., 2014), in particular relative to co-occurring carbonate (Tashiro et al., 2017), as well as trace amounts of residual N, O and P within the graphite that could be relics of former biomolecules (Hassenkam et al., 2017). However, hydrothermal redox reactions that convert CO 2 , CO and/or CH 4 to graphite can potentially mimic the isotopic composition that would be expected from biological metabolisms (Horita, 2001;Kueter et al., 2019), meaning that carbon isotopes alone are supportive but inconclusive evidence of biogenicity. ...
The oldest remnants of life on Earth from various localities in the Isua supracrustal belt in Greenland date back to >3.7 billion years ago (Ga). They are in the form of graphite, whose biogenicity is controversial. Previous studies used the presence and isotopic composition of nitrogen in graphite from along the Isua belt to argue both for and against biogenicity. To determine if the nitrogen chemistry of graphite can indeed serve as a biosignature, we investigated a hydrothermal graphite deposit from south-east Greenland (1.87–1.82 Ga). We found indications that molar C/N ratios of hydrothermal graphite may be similar to those of biogenic graphite from the Archean rock record, meaning that the nitrogen content of graphite is itself perhaps not diagnostic of ancient life, requiring caution in future studies. However, the hydrothermal graphite deposit also revealed unusually low N concentrations in associated silicates, despite a wide range of K concentrations up to 5 wt%. Using a thermodynamic model of nitrogen speciation in the presence of graphite, paired with previously published partition coefficients for ammonium in K-silicates, we show that abiotic process can explain these low N-concentrations of around 1 μg/g in potassic silicates. Higher concentrations of >10 μg/g, such as those found in graphitic metapelites from the Isua supcracrustal belt, would, however, require an unusually N-rich fluid. Such a N-rich fluid is most easily derived from the breakdown of biomass within sediments prior to graphitization. We therefore conclude that potassic silicates associated with graphite can serve as an indirect biosignature. Our approach supports previous inferences of life on Earth back to at least 3.7 Ga.
... Despite these differences, a comparison with the conditions that dominated the early Earth environments still remains pertinent, both in terms of physical processes and the evolution of life. Organic carbon derived from planktonic microbes has been reported from the c. 3.7 Ga Isua marine successions in Greenland (Rosing, 1999;Ohtomo et al., 2014;Hassenkam et al., 2017), indicating that the early microbial life on Earth was already ecologically diverse (Nutman et al., 2019). Even though the presence of organic matter and fine-grained sediments on Mars is not confirmed yet, there is a possibility that they did accumulate in areas of low energy, such as on basin floors or even within foresets at times of high tide. ...
... The light δ 13 C values, between − 22.1 and − 22.9‰, obtained from purified graphite of the Rio da Areia Formation are still compatible with the geochemical signature of biogenically derived kerogen, more depleted in 13 C than other carbonaceous materials (Fig. 4F) (Hoefs, 1997;Schopf and Kudryavtsev, 2012). These data contrast even with more lightest values of highly crystalline graphite veins precipitated by inorganic mixing of CO 2 -rich and CH 4 -rich fluids under metamorphic conditions (Luque and Rodas, 1999;van Zuilen et al., 2002;Ohtomo et al., 2014;Kakegawa, 2019). However, since different inorganic processes can lead to the formation of isotopically light graphite, a more reliable assessment of biogenic potential depends on complementary techniques (Kakegawa, 2019). ...
The Cryogenian was a period of drastic environmental changes across the entire planet, with glacial erosion limiting the preservation of life forms in most of the geological record. Therefore, to explore more of the biogenic evidence and its implications for this period, here we investigate some metavolcano-sedimentary sequences from the Brusque Metamorphic Complex in Brazil, that record a mid-latitude basin so far unexplored from a pale-ontological point of view. On the one hand, field discoveries reveal the occurrence of Conophyton-type stro-matolites preserved in dolomitic marbles that are isotopically incompatible with cap carbonates. On the other hand, 13 C-depleted graphitic schists interspersed in these marbles indicate an accumulation from exceptional volumes of organic matter. Raman spectroscopy and transmission electron microscopy also highlight disordered structures, heterogeneous morphologies, and structures similar to ultra-small microorganisms in this graphite, in compatibility with a biogenic origin. Combined with geochronological data, these records reflect formation during the late Cryogenian greenhouse, between the Sturtian and Marinoan Snowball Earth glaciations. From this, we suggest deposition under conditions of warmer and brighter waters without ice cover, favoring the growth of some stromatolitic barriers. Sturtian nutrient-rich meltwater pulses may also have led to exceptional phytoplankton blooms followed by mass mortality, promoting the exceptional deposition of organic-rich horizons during the transition window to a Precambrian algae-dominated world.
... As such, Tashiro et al. (2017) found a positive correlation between the δ 13 C org values and the metamorphic grade for the 3.95 Ga metasedimentary rocks from northern Labrador, indicating that metamorphism leads to the variation in carbon isotopes. In addition, recent studies investigated the preservation of biosignatures in Archean-Proterozoic rocks subjected to amphibolite-facies metamorphism (450-700 • C, 2-12 kbar) (Dodd et al., 2019b(Dodd et al., , 2017Lepot, 2020;Ohtomo et al., 2014;Papineau et al., 2019). These studies, among others, indicate that certain diagnostic heteroatoms and functional groups such as aliphatic C-N and C-O can be preserved even when biosignatures have been altered during prograde metamorphism (Cavalazzi et al., 2021;Papineau et al., 2010). ...
It is well-known that multi-stage metamorphism can result in the alteration of indigenous biological molecules, limiting our understanding of early life on Earth. However, the physiochemical mechanisms involved in these processes are still poorly understood. In this study, we present petrographic observations and micro- to nano-geochemical investigations on the carbonaceous matter (CM) in representative Neoarchean banded iron formations (BIFs) from North China, which have undergone significant alteration during lower amphibolite-facies prograde metamorphism, and subsequent retrograde alteration. The CM is in paragenetic equilibrium with prograde mineral phases, and is often associated with apatite that occurs in Fe-rich bands parallel to layering. This implies that the CM is most likely inherited from syn-depositional biomass, as confirmed by the nanoscale infrared spectroscopy, which shows the presence of Cdouble bondC, Csingle bondH, and Csingle bondN/Nsingle bondH bonds. Raman spectroscopic analyses reveal that the maximum metamorphic temperature of CM is 479 ± 50 °C, which is consistent with the metamorphic peak conditions of the host BIFs from petrographic constraints (i.e., garnet-bearing amphibolite-face metamorphism). The BIFs possess average bulk δ13Corg values of −20.0 ± 0.9‰ ( ) and δ13Ccarb values of −12.9 ± 1.8‰ ( ), further indicating syngenetic biomass graphitization during prograde metamorphism. This thermal cracking process may have released gaseous hydrocarbons, as shown by secondary CH4 fluid inclusions in quartz. We further use quantum mechanical simulations to constrain dissociative tendencies for functional groups (Csingle bondC, Csingle bondH, Csingle bondO, and Csingle bondN) of original organic molecules to assess the stability of organic chemical bonds during prograde metamorphism (0–600 °C, 0–15 kbar). The relatively high thermal durability of Csingle bondH and the armoring effects of primary organic-phyllosilicate complexes may account for Csingle bondH preservation in BIFs. Furthermore, the electron microscopy combined with elemental analysis reveals widespread nano-chlorite infiltration into CM during retrograde metamorphism (i.e., partial replacement of garnet by chlorite). The pervasive CM alteration is likely responsible for the absence of Csingle bondO bonds, where nanopore-scale reactions might have played a key role. Altogether, we suggest that multi-stage metamorphic processes, involving mineral-organic reactions and nanoscale interface interactions, may have governed the preservation of ancient biosignatures in BIFs. Our findings highlight the importance of evaluating metamorphic effects when using molecular signals to reconstruct early life behaviors, and shed new light on the study of primordial microorganisms, particularly those found in iron-rich sediments on early Earth.
Microbial Fe(II) oxidation has been proposed as a major source of Fe minerals during deposition of banded iron formations (BIFs) in the Archean and Proterozoic Eons. The conspicuous absence of organic matter or graphitic carbon from BIFs, however, has given rise to divergent views on the importance of such a biologically mediated iron cycle. Here, we present mineral associations, major element concentrations, total carbon contents and carbon isotope compositions for a set of lower amphibolite-facies BIF samples from the Neoarchean Zhalanzhangzi BIF in the Qinglonghe supracrustal sequence, Eastern Hebei, China. Graphite grains with crystallization temperatures (~470 °C) that are comparable to that predicted for the regional metamorphic grade are widely distributed, despite highly variable iron (12.9 to 54.0 wt%) and total organic carbon (0.19 to 1.10 wt%) contents. The crystalline graphite is interpreted to represent the metamorphosed product of syngenetic bio-mass, based on its co-occurrence with apatite rosettes and negative bulk rock δ13Corganic values (–23.8 to –15.4‰). Moreover, the crystalline graphite is unevenly distributed between iron- and silica-rich bands. In the iron-rich bands, abundant graphite relicts are closely associated with magnetite and/or are preserved within carbonate minerals (i.e., siderite, ankerite, and calcite) with highly negative bulk rock δ13Ccarb values (–16.73 to –6.33‰), indicating incomplete reduction of primary ferric (oxyhydr) oxides by organic matter. By comparison, only minor graphite grains are observed in the silica-rich bands. Normally, these grains are preserved within quartz or silicate minerals and thus did not undergo oxidation by Fe(III). In addition, the close association of graphite with iron-bearing phases indicates that ferric (oxyhydr)oxides may have exerted a first order control on the abundance of organic matter. Combined, the biological oxidation of Fe(II) in the oceanic photic zone and subsequent burial of ferric (oxyhydr)oxides and biomass in sediments to form BIFs, suggests that a BIF-dependent carbon cycle was important in the Archean Eon. Although significant re-adsorption of phosphorus to ferric (oxyhydr)oxides and the formation of authigenic phosphate minerals at the sediment-water interface would be expected, oxidation of biomass in BIFs may have recycled at least a portion of the P (and other nutrients) released from reactions between organic matter and ferric (oxyhydr)oxides to the overlying water column, potentially promoting further primary productivity.
Full-text available
Turbiditic and pelagic sedimentary rocks from the Isua supracrustal belt in west Greenland [more than 3700 million years ago (Ma)] contain reduced carbon that is likely biogenic. The carbon is present as 2- to 5-micrometer graphite globules and has an isotopic composition of δ13C that is about –19 per mil (Pee Dee belemnite standard). These data and the mode of occurrence indicate that the reduced carbon represents biogenic detritus, which was perhaps derived from planktonic organisms.
Full-text available
Carbon isotopic composition of fluid‐deposited graphite in a metamorphic terrain can be used as a tracer for sources of fluids, their compositions and temperatures of graphitization. However, δ13C of graphite almost always shows a large spread, mostly because of isotope fractionation during precipitation, making it necessary to understand these processes. In this work, a novel quantitative approach, involving Rayleigh isotope fractionation during formation of graphite from a realistic multicomponent metamorphic fluid, is proposed to explain the observed δ13C variations in fluid‐deposited graphite. Results of this study reveal that most graphite crystallizes from a mixed CO2–CH4 fluid that can acquire its isotopic composition from a variety of sources like organic matter, carbonate, mantle degassing or variable mixtures of these.
The different modes of high-resolution transmission electron microscopy (TEM) were applied to Korean anthracites, semi-graphites and graphites. Whereas X-ray diffraction data yield averaged values, TEM is the only technique able to bring out the heterogeneity of phases which are different in morphology and in microtexture. The thermal behavior of samples was studied using laboratory heat-treatments up to 2800°C. In heat-treated anthracites for example, an increasing degree of graphitization results in phase changes which can be quantified by TEM. The study of a larger sampling appears however necessary to relate crystallographic variations to geological data.
A 3.8-Ga-old metamorphic rock that contains up to ∼ 5.5 wt% reduced carbon as graphite was found in the Isua district, Greenland. This rock exhibits a strong schistosity, comprises mostly talc, contains high MgO (28.0 wt%), Ni (553 ppm) and Cr (267 ppm) and low Al2O3 (2.13 wt%), and bears a high ratio (0.94). The original rock was probably a submarine ultramafic tuff. The carbon content and the δ ¹³C value of the rock vary on a microscopic scale (a few mm³ scale) from 0.7 to 5.5 wt% and from −15 to −12‰, respectively. The second rock, a quartzite, contains 0.13 wt% C as reduced carbon with a δ ¹³C value of −11.25‰. These δ ¹³C values fall within the range −27 to −10‰ reported by previous investigators for graphite from the Isua district.
Metacarbonate rocks, consisting of ankerite and dolomite together with combinations of amphibole, biotite, muscovite, chlorite, quartz, and clinopyroxene, are an important component of the roughly 3.8 Ga Isua supracrustal belt. Along with layers of banded iron formation, felsic schists, basic amphibolites, and variegated amphibolitic schists, they have previously been described as a single supra-crustal suite intruded by ultramafic and Mg-rich basic sills. One of the formal stratigraphic units recognized in this interpretation, the Calc-Silicate Formation, contains abundant metacarbonates and calc-silicates that were regarded as the earliest known examples of metamorphosed calcareous chemical sediments. Field relations and geochemical models, however, suggest that the Isua metacarbonate units are metasomatic in origin and formed where fluids flowed across the contacts between ultramafic bodies and felsic or metabasaltic country rocks at deep crustal levels. Field evidence in support of this includes the common occurrence of metacarbonates at margins of ultramafic rocks, formation of metacarbonate assemblages in veins near ultramafic contact zones, and the replacive nature of the contacts between metacarbonates and felsic or basaltic units. In order to explore possible mechanisms for the origin of the metacarbonates, metasomatic processes accompanying the advection of fluids through ultramafic rocks were simulated numerically using a model configuration in which a column of dunite was sandwiched between layers of host rock. This allowed mineral zonation, bulk composition, and porosity changes to be studied at both upstream and downstream contacts. Fluids entering the ultramafic layer at the upstream contact react with olivine to form talc and magnesite ± chlorite ± phlogopite. The same fluids, modified by passage through the dunite, react with country rock at the downstream contact, forming assemblages analogous to those found in the Calc-Silicate Formation. This process is due primarily to the effect of reduced aSiO2 in changing the saturation state of all host rock minerals at the point where fluids exit the ultramafic rock. Metasomatism of the Isua supracrustals probably took place under amphibolite facies conditions between 500° and 600°C in the presence of water-rich fluids where time integrated fluid flux was between 103 to 104 moles of water cm 2.
Investigations of the Isua supracrustal rocks in West Greenland allow us to identify the protoliths and alteration history for most Isua rocks. The protoliths consisted of alternating basalt and banded iron formation. This sequence was invaded by dunitic sills, metamorphosed, and later intruded by felsic gneisses. Pervasive carbonation and K metasomatism produced a sequence of lithologies, mimicking those found in modern platform deposits. However, the protoliths could have originated in a purely oceanic environment with no sialic detrital components. The Isua sequence probably consists of several tectonic panels, some of which are cut by >3810 Ma felsic intrusive rocks.
This book describes the composition of the present upper crust, and deals with possible compositions for the total crust and the inferred composition of the lower crust. The question of the uniformity of crustal composition throughout geological time is discussed. It describes the Archean crust and models for crustal evolution in Archean and Post-Archean time. The rate of growth of the crust through time is assessed, and the effects of the extraction of the crust on mantle compositions. The question of early pre-geological crusts on the Earth is discussed and comparisons are given with crusts on the Moon, Mercury, Mars, Venus and the Galilean Satellites.
To resolve the role of strain in the formation of natural graphite, a ‘hard’ carbon-based anthracite and a ‘soft’ carbon-based high volatile bituminous coal were deformed in hydrostatic, coaxial and simple shear configurations at temperatures up to 900°C and confining pressures up to 1 GPa. Additional tests were carried out at ambient pressures at temperatures up to 2800°C. In simple shear, graphite appears, with an anthracite starting material at temperatures as low as 600°C; samples tested at 900°C are predominately graphitized, as is evident from optical microscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). In tests on high volatile bituminous coal, graphite first appears in simple shear tests at temperatures of 800°C and is common at 900°C. In TEM observations graphite particles are lamellar, have punctual hkl reflections or Debye-Scherrer hkl rings (triperiodic order) and long, stiff and stacked lattice fringes typical of well crystallized graphite. No graphite was formed in either the hydrostatic or coaxial tests (they remain porous and turbostratic). Micro-Raman spectroscopy of deformed samples indicates the presence of defects (Band at 1350 cm−1) even in samples that prove to be mainly graphite by XRD and TEM.
One hundred and twenty-four carbonate samples from the meta-sedimentary sequence of the 3.7 × 109 yr old Isua supracrustal belt (W-Greenland) have yielded a δ13Ccarb average of −2.5 ± 1.7%. vs PDB and a δ18Ocarb average of +13.0 ± 2.5%. vs SMOW. The oxygen mean comes fairly close to the averages of other early Precambrian carbonates. The carbon average, however, is some 2%. more negative than those of younger marine carbonates. In terms of a simple terrestrial 13C mass balance, if δ13Ccarb values are original sedimentary values, this more negative δ13C average would imply a considerably smaller ratio in the sedimentary shell during Isua times, and would thus support the concept of a gradual buildup of a sedimentary reservoir of organic carbon during the early history of the Earth. Since, however, the Isua supracrustal rocks have experienced amphibolite-grade metamorphism, which in other areas has been shown to lower δ13Ccarb values, it is most likely that the original values of these rocks were approx 0%.. This indicates that Corx and Ccarb were present in the ancient carbon reservoir in about ‘modern’ proportions. Unless this early stabilization of the terrestrial carbon cycle in terms of a constant partitioning of carbon between the reduced and oxidized species is shown to have been caused by some inorganic geochemical process, a considerably earlier start of chemical evolution and spontaneous generation of life must be considered than is presently accepted.