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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.
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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 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
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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 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
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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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... Nonetheless, these values are within the range of organic carbon (Schidlowski, 1987;, but, given the potential for carbon isotope fractionation during high-temperature metamorphism, it is unclear if these values represent bona fide organic signatures or fractionation of a non-biogenic source (e.g. Ohtomo et al., 2014). This is explored in the discussion. ...
... Graphite in metamorphic rocks can be precipitated from carbonic fluids or melts (Pasteris, 1999;Luque et al., 2014) or due to the metamorphism of organic matter (e.g. Ohtomo et al., 2014;Taner et al., 2017). Carbon in metamorphic fluids can be derived from decarbonation reactions in carbonates or calc-silicate rocks associated with prograde metamorphism (e.g. ...
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... Some evidence suggests that life might have originated while impacts were still intense and frequent (e.g. Ohtomo et al. 2014;Dodd et al. 2017;Tashiro et al. 2017), which, combined with genetic evidence that the last universal common ancestor might have been thermophilic and lived in a hydrothermal setting (e.g. Nisbet and Sleep 2001;Weiss et al. 2016), raises the question of whether impacts could have facilitated the origin of life rather than frustrated it (e.g. ...
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As Mars transitioned from an early Earth-like state to the cold desert planet it is today, it preserved a near pristine record of surface environments in a world without plate tectonics and complex life. The records of Mars’ Earth-like surfaces have remained largely untouched for billions of years, enabling space exploration to provide critical insights about the early days of our own planet. Here, we first review what Mars has taught us about volcanic, tectonic, and metamorphic processes in the absence of discrete plates, drawing comparisons to the terrestrial and venusian records. Then, we summarize advances in understanding its early surface environments, including impact cratering, hydrological, sedimentary, and geochemical processes. Altogether, the martian record provides a picture of early environments that were similar to modern terrestrial ones in many respects, with sediment and geochemical cycling, hydrothermal systems capable of hosting life, but with the exception that topography, sediment, and heat sources were provided by volcanoes and impact cratering rather than plate tectonics. Mars thus offers a lens through which one might catch a glimpse of Earth's infancy, provided exploration efforts continue to refine our understanding of the similarities between Earth and Mars as well as the specificities of each planet.
... Hence, the impact flux was inadequate to sterilize the surface zone. Perhaps it comes as no surprise that different lines of evidence for a biosphere at ca. 3.8 Ga from characteristic stable isotope fractionation patterns of Carbon (72)(73)(74)(75)(76)(77), Iron (78), Nitrogen (79) and Sulfur (80,81) seem to show that life has existed on Earth since the late Hadean, perhaps emerging within the first ~200 Myr of the solar system (82). It is interesting to speculate about what this first life could have been like. ...
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Life on Earth emerged at the interface of the geosphere, hydrosphere and atmosphere. This setting serves as our basis for how biological systems originate on rocky planets. Often overlooked, however, is the fact that the chemical nature of a rocky planet is ultimately a product of galactic chemical evolution. Elemental abundances of the major rock-forming elements can be different for different stars and planets formed at different times in galactic history. These differences mean that we cannot expect small rocky exoplanets to be just like Earth. Furthermore, age of the system dictates starting nuclide inventory from galactic chemical evolution, and past, present and future mantle and crust thermal regimes. The bulk silicate mantle composition of a rocky planet modulates the kind of atmosphere and hydrosphere it possesses. Hence, the ingredients of a rocky planet are as important for its potential to host life as proximity to the so-called habitable zone around a star where liquid water is stable at the surface. To make sense of these variables, a new trans-disciplinary approach is warranted that fuses the disciplines of Geology and Astronomy into what is here termed, Geoastronomy.
... Such investigations reveal the possible pathways of formation of different carbon allotropes due to tectonic movements, sudden changes in the temperatures, meteorite impacts, high-pressure and temperature as well as any other extreme conditions. Formation of graphite in the sedimentary rocks is believed to have originated from the organic matter trapped within rocks, where each pore of the rock may have served as a "reaction chamber", thus facilitating pyrolysis over millions of years [46,47]. Various carbon allotropes are present at varying depths under the Earth's crust, depending upon the different temperature and pressure conditions experienced by the initial organic matter. ...
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Thermally induced chemical decomposition of organic materials in the absence of oxygen is defined as pyrolysis. This process has four major application areas: (i) production of carbon materials, (ii) fabrication of pre-patterned micro and nano carbon-based structures, (iii) fragmentation of complex organic molecules for analytical purposes and (iv) waste treatment. While the underlying process principles remain the same in all cases, the target products differ owing to the phase and composition of the organic precursor, heat-treatment temperature, influence of catalysts and the presence of post-pyrolysis steps during heat-treatment. Due to its fundamental nature, pyrolysis is often studied in the context of one particular application rather than as an independent operation. In this review article an effort is made to understand each aspect of pyrolysis in a comprehensive fashion, ensuring that all state-of-the-art applications are approached from the core process parameters that influence the ensuing product. Representative publications from recent years for each application are reviewed and analyzed. Some classical scientific findings that laid the foundation of the modern-day carbon material production methods are also revisited. In addition, classification of pyrolysis, its history and nomenclature and the plausible integration of different application areas are discussed.
... Anaerobic heterotrophic bacteria were the first organisms to thrive on the primordial oceans after the formation of the Planet Earth [1,2]. In these oxygen-free environments, bacteria used specific compounds, such as nitrate, sulphate or fumarate as terminal electron acceptors of their respiratory chains [3]. ...
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Iron is the most versatile of all biochemically active metals, with variability encompassing its electronic configuration, number of unpaired electrons, type of ligands and iron-complexes stability. The versatility of iron properties is transposed to the proteins it can be associated to, especially relevant in the case of heme proteins. In this Review, the structural and functional properties of heme proteins are revisited, with particular focus on c-type cytochromes. The genome of Geobacter bacteria encodes for an unusually high number of assorted c-type cytochromes and, for this reason, they are used in this Review as a showcase of the cytochrome diversity. In the last decades, a vast portfolio of cytochromes has been revealed in these bacteria, with most of them defining new classes, ranging from monoheme to the recently identified polymeric assembly of multiheme cytochromes that forms micrometer-long electrically conductive filaments. These discoveries were on pace with the development of modern NMR equipment and advances in protein isotopic labeling methods, which are also revisited in this Review. Finally, following the description of the current state of the art of Geobacter cytochromes, examples on how the available structural and functional information was explored to structurally map protein–protein and protein–ligand interacting regions in redox complexes, and hence elucidate Geobacter’s respiratory pathways, are presented.
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Natural graphite forms in a range of metamorphic and hydrothermal environments across timelines spanning from the birth of the solar system, to the evolution of early Precambrian life, and the development of contemporary geotectonic cycles. A precise timeline of these and other graphite-forming events, however, has hitherto been obscured by a lack of radiometric ages and as such, chronologies are inferred from host-rock or hydrothermal mineral ages. Herein we examine the Re-Os systematics and chronology of graphite formed in a suite of terrestrial and extraterrestrial environments (n = 17) with the principal aim of establishing the viability of Re-Os geochronology of natural graphite. Graphite Re and Os contents and isotopic ratios exhibit a wide range of values that extend up to 1520 ppb Re, 4639 ppt Os, and 4101 and 42.18 for ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios, respectively. These values are broadly comparable to those reported for crustal sulfides, organic-rich sediments, and hydrocarbons. X-ray diffraction crystallinity data reveals that graphite Re abundances show a broadly inverse correlation with graphite formation temperature and crystallinity (d002 and Lc(002)) with interplanar spacing (d002) having the strongest anti-correlation with graphite Re contents. Graphite Re-Os geochronology is demonstrated with two independent case studies (Wollaston-Mudjatik Transition shear zones, Saskatchewan, Canada and Merelani Hills, Tanzania) yielding precise (<1%) Re-Os isochron dates of 1731.52 ± 7.48 Ma (2σ; MSWD = 1.3) and 586.89 ± 2.39 Ma (2σ; MSWD = 1.2) that are consistent, within uncertainty, to their mineralization ages constrained by other radiometric methods. These data confirm that graphite mineralization was synchronous with Trans-Hudsonian exhumation and tsavorite-tanzanite gemstone mineralization, respectively. Method accuracy, however, appears contingent on the analytical protocols used to isolate graphite, e.g. handpicking vs. heavy liquids (SPT) and water, with the latter perturbing graphite Re-Os systematics by as much as 20%. We, therefore, recommend handpicking paired with magnetic separation and grain mount examination. Our Re-Os age results are then coupled with new SIMS carbon isotope data (Wollaston-Mudjatik Transition graphite: δ¹³C = -21.64 to -15.28 ‰; Merelani Hills graphite: δ¹³C = -25.90 to -24.36 ‰) and ¹⁸⁷Os/¹⁸⁸Osi isotope data (Wollaston-Mudjatik Transition graphite = 0.3119 ± 0.0037; Merelani Hills graphite = 1.680 ± 0.038) to constrain graphitic carbon to sedimentary carbonate/organic (Wollaston-Mudjatik Transition graphite) and organic (Merelani Hills graphite) carbon sources. This unique pairing of isotope systems in graphite provides the first detailed chronology of localized carbon mobility in the Earth’s crust. Re-Os graphite geochronology likely has wide applications in ore-deposit and metamorphic geology with the potential to reshape our understanding of carbon cycling in the crust-mantle system, and for graphite exploration initiatives that are critical for a global transition to a green economy.
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It is 50 years since the landmark paper where Black et al. (1971) presented whole-rock Pb-Pb and Rb-Sr isotopic evidence for some rocks in Greenland surviving from Earth’s first billion years; the ≥ 3700 Ma Amîtsoq gneisses. This overturned ideas prevalent at that time that the young Earth was far too violent for such ancient rocks to survive. In the following few years it emerged how ‘normal’ this early Earth appeared to be, with a retained hydrosphere (oceans) by 3700 Ma and ‘continental’ crust dominated by rather normal granitic sensu lato rocks. By several decades ago this led to a vision of Eoarchean lithosphere development being controlled by a geodynamic regime with definite similarities to that in the Phanerozoic, particularly with evidence for lateral lithosphere movement (mobile lid) such as exemplified by arc-like magmatism and Barrovian metamorphism (the latter requiring tectonic crustal thickening). However, the literature in recent years is increasingly emphasising model-driven non-uniformitarian stagnant lid geodynamic scenarios to explain the Eoarchean geological record. This paper reviews the broad range of field and laboratory evidence extracted from the Eoarchean geological record in Greenland. We argue that the non-uniformitarian sagduction within a stagnant lid regime interpretation does not fit Eoarchean facts.
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The number of day/night cycles available for the origin of life on Earth is O (100 billion). This would seem more than enough for any hypothesized mechanism requiring an alternation of day/light or correlated environmental parameters for any model of origin of life. Thus, the concerns of “not enough time” seem unwarranted, and panspermia may not have to be invoked.
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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.
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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.
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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.
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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.
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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.
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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.
Book
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.
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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.
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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.
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Organic material in sediments is progressively altered during diagenesis and metamorphism, leading to the formation of kerogen and ultimately crystalline graphite. Bulk carbonaceous material in metamorphic terrains typically has attained an overall degree of structural order that is in line with peak metamorphic temperature. On a micron- to nano-scale, however, carbonaceous material can display strong structural variation. The main factor that drives this variation is the chemical and molecular heterogeneity of the precursor biologic material. Specific conditions during metamorphism, however, can also play a role in shaping the microstructure of carbonaceous material. Here we describe the structural variation of carbonaceous material in rocks of the 2.0 Ga Zaonega Formation, Karelia, Russia. Raman spectroscopy indicates that bulk carbonaceous matter in these rocks has experienced peak temperatures between 350 and 400 °C consistent with greenschist-facies metamorphism. On a nano-scale, however, a strong structural heterogeneity is observed. Transmission electron microscopy (TEM) reveals the occurrence of thin films of highly ordered graphitic carbon at mineral surfaces. These graphite films – consisting of 20–100 individual layers – completely envelop quartz crystals and occur on specific crystal surfaces of chlorite. It is proposed that minerals can act as templates for the parallel ordering of carbon crystallites causing enhanced graphitization within narrow zones at mineral surfaces. Alternatively, oriented organic precursor molecules could have been adsorbed onto charged mineral surfaces, leading to thin graphitic films during later metamorphic heating episodes. Overall the presented observations demonstrate that mineral surfaces can initiate and accelerate localized graphitization of sedimentary organic material during metamorphism, and therefore cause distinct nano-scale variation in structural order.