Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks

Abstract

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.

Full text

LETTERS
PUBLISHED ONLINE: 8 DECEMBER 2013 | DOI: 10.1038/NGEO2025
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.
*e-mail: ohtomoy@jamstec.go.jp
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
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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO2025
65° 09‘ N
50° 10‘ W
Nuuk
Greenland
ISB
Amitsoq gneiss
ISB
Proterozoic dyke
Lake
N
km
50° 09‘ 70.00‘‘ W
65° 09‘ 10.00‘‘ N
N
50 m
Metabasalts
BIFs
Quartz dyke
Graphite-rich schist
Carbonate vein
A
A‘
B
B‘
C
C‘
10 cm
Graphite-rich schist
BIFs
BIFs
BIFs
Graphite-rich schist
BIFs
Quartz-rich BIFs
Graphite-rich schist
Quartz-rich BIFs
Metabasalts
Metabasalts
(m)
2
4
6
8
0
Metabasalts
BIFs
Quartz-rich BIFs
Graphite-rich schist
Metabasalts
(m)
Fine laminated
BIFs
Quartz-rich BIFs
Graphite-rich schist
(m)
Metabasalts
A
A‘
B
B‘
C
C‘
b
River
051234
11.5
2
4
6
8
10
0
2
4
6
8
10
0
ab
cd
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.
10.00
1.00
0.10
0.01
Rock/PAAS ratio
3072303
4062308
4062309
5080501
6072905
6072906
Average of BIF samples
in northern section
La Ce Pr Nd Sm Eu Gd Tb
REE
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 ⇥5⇥0.5 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
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2025 LETTERS
Frequency
Frequency
Frequency
Frequency
Laser microprobe analyses, sample 3072303
n
= 8
ave. = ¬20.9
Laser microprobe analyses, sample 5080603
n = 13
ave. = ¬17.4
6
8
4
2
0
Bulk analyses
n = 58
ave. = ¬17.9
8
6
4
2
0
Previous studies
n = 82
ave. = ¬10.8
% PDB)(%
%
%
%%
%%
%
%
δ13Cgraphite
% PDB)(%
δ13Cgraphite
% PDB)(%
δ13Cgraphite
% PDB)(%
δ13Cgraphite
¬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
16
14
12
10
8
6
4
2
0
2
0
a
b
c
d
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
±0.2h.
50 nm 50 nm 50 nm
20 nm
d
e
g
002
004
100
002
004
100
abc
defg
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.
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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO2025
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 h001i⇤streaks 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, h001i⇤streaks 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
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Acknowledgements
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 www.nature.com/reprints.
Correspondence and requests for materials should be addressed to Y.O.
Competing financial interests
The authors declare no competing financial interests.
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