Abiotic synthesis of graphitic carbons in the Eoarchean Saglek-Hebron metasedimentary rocks

Domain: earth_science
Published: 2024-07-06
Editorial outcome: major_revision

Abstract
Graphite in metasedimentary rocks of the Eoarchean Saglek-Hebron Gneiss Complex (Canada) is depleted in 13C and has been interpreted as one of the oldest traces of life on Earth. The variation in crystallinity of this oldest graphitic carbon could possibly confirm the effect of metamorphism on original biomass, but this is still unexplored. Here, we report specific mineral associations with graphitic carbons that also have a range of crystallinity in the Saglek-Hebron metasedimentary rocks. Petrographic, geochemical and spectroscopic analyses in the Saglek-Hebron banded iron formations suggest that poorly crystalline graphite is likely deposited from C-H-O fluids derived from thermal decomposition of syngenetic organic matter, which is preserved as crystalline graphite during prograde metamorphism. In comparison, in the Saglek-Hebron marble, disseminations of graphite co-occur with carbonate and magnetite disseminations, pointing to abiotic synthesis of graphitic carbons via decarbonation. Our results thus highlight that variably crystalline graphitic carbons in the Saglek-Hebron metasedimentary rocks are potential abiotic products on early Earth, which lay the groundwork for identifying the preservation of prebiotic organic matter through metamorphism on Earth and beyond.

Introduction
The search for evidence of early life is a challenging problem owing to the highly metamorphosed nature of the oldest sedimentary rocks containing organic matter1,2. Metamorphism is known to distort the morphology of microbial remains through the alteration of organic matter to crystalline graphite, and purported microfossils from highly metamorphosed rocks have been reported from the Nuvvuagittuq Supracrustal Belt in northern Canada (3.75–4.3 Ga (gigayears ago))3,4, which has been questioned in origin5. This requires a broader search for direct evidence of life during the Eoarchean (3.6–4.0 Ga) to also include chemical and isotopic biosignatures, which in this context should be defined as the possible signatures of life. Biosignatures of graphitic carbons in >3.6 Ga rocks have thus been reported in the southwest Greenland Isua Supracrustal Belt (>3.7 Ga)6–8 and the Akilia association (>3.83 Ga)9–11, and in the northeast Canada Saglek-Hebron Gneiss Complex (~3.9 Ga)12. Observations show high-grade metamorphism results in biomass losing heteroatoms such as H, N, O and S13 as well as 12C and 14N isotopes, thereby leaving isotopically heavy residual graphitic carbons14. To determine the precursor carbon in such graphitic carbon requires consideration of the effects of metamorphism and various abiotic processes that could potentially explain their composition and crystallinity in metamorphosed sedimentary rocks1,15.

The Saglek-Hebron Gneiss Complex is a granite-greenstone terrane located in the west end of the North Atlantic Craton along the east coast of northern Labrador, Canada (Supplementary Fig. 1). It is mostly composed of orthogneisses from the trondhjemite-tonalite-granodiorite (TTG) series, as old as ~3.9 Ga TTG16,17, but also includes basaltic metavolcanic rocks interbedded with clastic and chemical metasediments18. It is acknowledged that the Saglek-Hebron Gneiss Complex includes multiple generation of TTG gneisses with ages from Eoarchean to Neoarchean17,19, with perhaps two distinct assemblages of Eoarchean to Paleoarchean supracrustal rocks20,21, which have succumbed to a complex and protracted thermal history (see the Supplementary Information; Supplementary Fig. 1)22. Among them, the oldest volcano-sedimentary assemblage, referred to as Nulliak, is interpreted to have an age of nearly 3.8 Ga18,21,23. Tashiro et al. (2017) reported 13C-deleted graphite in some metasedimentary rocks from the Saglek-Hebron Gneiss Complex and interpreted these occurrences as one of the oldest traces of life on Earth12. An increasing number of studies raised doubts about the interpretation of the age and origin of graphitic carbons in these metasedimentary rocks, suggesting that such graphitic carbons could have precipitated from hydrothermal and metasomatic fluids during post-Eoarchean metamorphic episodes24,25. Hence, in this context, the distinction between abiotic and biological processes and sources of organic molecules in graphitic carbons is an imperative goal to achieve accurate identification of traces of early life on Earth.

To address this outstanding problem, correlative petrographic, geochemical and spectroscopic analyses are used to probe specific mineral associations with graphitic carbons in the Eoarchean Saglek-Hebron metasedimentary rocks. Our results suggest that graphite in the Saglek-Hebron banded iron formation (BIF) is likely deposited from C-H-O fluids, while graphite associated with magnetite and carbonate in the Saglek-Hebron marble arises from decarbonation. Hence, the new observations provide a documentation of abiotic transformation of primitive sedimentary organic matter on early Earth, which has implications for identifying early microbial remnants, as well as for understanding geochemical cycling of carbon on early Earth.

Results
Graphitic carbons were studied in three samples from the Eoarchean Nulliak supracrustal assemblage of the Saglek-Hebron Gneiss Complex located in northern Labrador, Canada (Fig. 1a–c). Sample SG-274 is a BIF dominated by grunerite, quartz and variable-sized magnetite, with trace amounts of organic matter, hastingsite, actinolite, apatite, calcite, chlorite, and biotite, whereas the BIF sample SG-236 is dominated by pyroxene + quartz + magnetite (Fig. 1d, e and Supplementary Data 1). In comparison, sample SG-275 is dominated by carbonate + olivine + antigorite + magnetite and is best described as a marble (Fig. 1f and Supplementary Data 1). Mineralogical maps obtained by energy dispersive X-ray spectroscopy (EDS) show that the constituent mineral phases intermix on micron-scale, particularly quartz and magnetite, and magnetite grains are primarily hosted in the grunerite-, olivine- and pyroxene-rich bands in samples SG-274, SG-275 and SG-236, respectively (Fig. 1d–f). This mineral paragenesis represents a typical prograde product under amphibolite-facies metamorphism26,27. Secondary, retrograde chlorite is usually pale green and intrudes the quartz-rich band as linear array from the rims of grunerite-rich band in sample SG-274, representing a prevalent retrograde mineral in these amphibolite-grade BIF (Supplementary Fig. 2). In the selected area of SG-274, 640 calcite grains were identified in quartz-rich band using polarising microscopy and micro-Raman spectroscopy, among which 64 (10%) were associated with graphite (Fig. 1g). This proportion is notably lower than those for apatite + graphite associations in the Eoarchean Akilia quartz-pyroxene rock (about 20%)11 and Nuvvuagittuq Fe-silicate BIF (~25%)28. In the above rocks, the graphite grain sizes vary between 1 and 45 µm and they commonly occur as flakes coating apatite crystals that vary between 3 and 100 µm in size. They also occasionally occur in other mineral assemblages, such as hornblende ± calcite ± magnetite ± pyrrhotite ± chalcopyrite, which are often distributed near the edge of large hornblende grains11. In comparison, graphite in the Saglek-Hebron BIF is generally smaller in size and varies between ~1 and 12 µm, and mainly occurs as thin coatings on microscopic calcite grains inside millimetre-size quartz crystals (Fig. 2).

Graphitic carbons in the Saglek-Hebron metasedimentary rocks can be grouped into four types based on their textural features, petrographic mode of occurrence and associated minerals. The first petrographic type of graphite (GraI) is pure solid inclusion disseminated within coarse quartz grains, while the second (GraII), third (GraIII) and fourth (GraIV) petrographic types of graphite are closely associated with calcite grains, C-H-O fluid inclusions, and magnetite, respectively (Figs. 2–4). In the Saglek-Hebron BIF, the GraI, GraII and GraIII co-occur almost entirely within the central zone of the quartz-rich band, while pure calcite grains without graphite are mainly enriched near the edge of coarse quartz and contact with millimetre size magnetite and grunerite (Fig. 1g). This indicates that graphite could have been oxidised by trivalent iron into CO2 from which calcite formed during retrograde metamorphism, which is consistent with the lower proportion of calcite + graphite associations than those of apatite + graphite associations11,28. The GraI disseminated within the centre of the quartz-rich band commonly has irregular or deformed morphologies with sizes ranging from ~1 to 18 µm (Fig. 2b, d, h, o), which may be derived from the infringement on the surrounding minerals during pre- to syn-metamorphic graphite crystallisation29,30. The GraII is flaky in shape with slightly smaller sizes (~1 to 12 µm) relative to the GraI, and typically appears as thin coatings on ellipsoidal to subhedral calcite grains with area ratios ranging from ~5% to 97% in two dimensions (Fig. 2b, d–o). The GraIII is always mixed with GraII, which is significantly more abundant than GraII, and it is usually surrounded by fluid inclusions within the centre of the quartz-rich band (Fig. 2b, d–n and Supplementary Fig. 3). The GraIII (<3 µm) associated with small micron-sized fluid inclusions (0.7–5.0 µm) are commonly dark in appearance within coarse quartz grains (Figs. 2c, o and 3 and Supplementary Fig. 3). In comparison, the GraIV in the Saglek-Hebron marble occurs as micro-metre size opaque patches closely intertwined with magnetite within and on the edges of carbonate grains (Fig. 4b–i), while it can also display a filamentous texture with ~24 µm in length (Fig. 4j, k). In addition, a large number of graphite-free, primary fluid inclusions that occasionally contain opaque material, occur as trails or linear arrays within coarse quartz grains in the Saglek-Hebron BIF (Supplementary Fig. 3).

Micro-Raman spectroscopy reveals that the four petrographic types of graphite found in the Saglek-Hebron metasedimentary rocks display spectral features varying from poorly to well crystalline graphite, including a narrow and sharp G band between 1583 and 1589 cm−1 (FWHM=18–48 cm−1), a narrow, sharp, but variable-intensity D band between 1352 and 1368 cm−1 (FWHM=21 to 80 cm−1), and a 2D band between 2683 and 2714 cm−1 (FWHM=45–86 cm−1; Supplementary Data 2 and Supplementary Fig. 4). Notably, most GraIII have stronger D- and 2D-bands intensities and lower frequency 2D band (from 2683 to 2703 cm−1) relative to other petrographic types of graphite (Supplementary Fig. 4c, d), while the GraIV shows more variable and larger D- and G-bands FWHM (Supplementary Fig. 4a, b). The occurrence of 2D band collected from graphite in the Saglek-Hebron metasedimentary rocks is markedly different from kerogen’s three peaks in the 2600–3100 cm−1 region, potentially excluding younger infiltrations from elsewhere31. The 2D band may represent turbostratic or curled graphite structures, while D and G bands of graphite represent disordered and ordered graphitic carbon, respectively28,32.

In the Saglek-Hebron metasedimentary rocks, the 2D/G peak intensity ratios for the GraI, GraII, GraIII and GraIV range from 0.22 to 0.67, from 0.29 to 0.83, from 0.44 to 0.97, and from 0.25 to 0.70, respectively, whereas the D/G peak intensity ratios for the GraI, GraII, GraIII and GraIV range from 0.04 to 0.78, from 0.05 to 0.80, from 0.30 to 1.70, and from 0.09 to 1.09, respectively (Supplementary Data 2 and Supplementary Fig. 4d). The large difference in the D/G peak intensity ratios is derived from their heterogeneity in crystallinity rather than from artefacts caused by polishing and graphene sheet orientation33, considering that all graphite was analysed below the thin section surface27,34. The enhancement of the 2D/G peak intensity ratios is similar to those of graphite “whiskers” or cones (Fig. 2m and Supplementary Fig. 4d)35,36, which are attributed to spiral and conical growth patterns11,37,38. High-resolution transmission electron microscopy (HRTEM) images show graphitic lattice fringes with (002) interplanar spacings ranging from 3.23 Å to 3.84 Å (Supplementary Fig. 5b-g), among which expanded lattices larger than the 3.35 Å spacing typical for crystalline graphite are consistent with poorly crystalline graphite (PCG)39. The co-occurrence of multiple crystallinities of graphite has been documented in early Phanerozoic S-type granite from the Lusatian block of eastern central Europe40, Proterozoic gneiss and quartzite from the Iberian metamorphic belt of Spain41, Archean and Paleoproterozoic metamorphosed BIF from Nuvvuagittuq (Canada), Anshan (China), Wutai (China), and Michigamme (USA)27,42, as well as in chert from the Apex Formation in Western Australia43. The degree of graphitic order has been widely used to calculate the crystallisation temperature of graphite under peak metamorphic conditions12,15,28. Based on the geothermometer using Raman spectroscopy of organic matter calibrated for temperatures between 330 °C and 641 °C44, we calculate the crystallisation temperatures with a precision of about ±50 °C for the GraI, GraII, GraIII and GraIV to be between 425 °C and 627 °C, between 415 °C and 610 °C, between 336 °C and 498 °C, and between 368 °C and 583 °C, respectively (Supplementary Data 2). The higher estimated crystallisation temperatures of graphite are consistent with upper amphibolite-facies metamorphism (>500 °C) experienced by the host rocks, potentially indicating an indigenous and syngenetic source of graphitic carbons, while the lower ones point to a secondary origin (Supplementary Fig. 4f).

The carbon isotope values of graphite (δ13Cgra) in the Saglek-Hebron metasedimentary rocks range from −27.0‰ to −22.7‰ (average = −25.5‰, n = 8), while the carbon isotope values of carbonate (δ13Ccarb) show an overall range of 9.8‰ (−11.2‰ to −1.4‰, average = −6.3‰, n = 8; Table 1). The low and large variable δ13Ccarb values indicate that these carbonates formed from the 12C-enriched CO2 produced by the oxidation of organic matter in rocks15,42,45, as suggested by our observation on the petrographic mode of occurrence with more GraI and GraII near the central zone of quartz-rich band and more graphite-free calcite grains near the Fe-silicate + magnetite portions of the Saglek-Hebron BIF (Fig. 1g). This explanation is further supported by recent works that reveal the oxidation of rock organic carbon has released significant amounts of CO2 that offsets the CO2 drawdown by silicate weathering at the global scale46,47. The negative δ13Cgra values in the Saglek-Hebron metasedimentary rocks fall well within the range of δ13C values reported in the Akilia BIF (−49.0‰ to −12.6‰)9,10,27,38, Nuvvuagittuq BIF (−28.1‰ to −18.2‰)27,28, Saglek-Hebron pelitic rocks (−28.2‰ to −11.0‰)12, as well as the average composition of sedimentary organic matter over the last 3.5 Ga48, and are consistent with biological fractionation when assuming that ancient biological processes produced isotopically light carbon deposits similar to modern biological processes. However, high-resolution elemental mappings acquired by NanoSIMS (Supplementary Fig. 6) and elemental compositions acquired by TEM-EDS (Supplementary Data 3) show that graphite in the Saglek-Hebron metasedimentary rocks is comprised of only carbon (C) and hydrogen (H), but lacks in other biologically important elements, such as nitrogen (N), phosphorus (P) and sulfur (S)34,38,42. Such chemical compositions, alongside the absence of apatite + graphite associations representing a common occurrence of the preserved early traces of life in BIF27,38,42, point to abiotic processes that may produce biological signatures, such as decarbonation reaction at high temperature (≥400 °C)49,50, Fischer-Tropsch-type (FTT) reaction at low temperatures (i.e., surface-catalysed reduction of inorganic carbon; 200–350 °C)51–53 and deposition from C-H-O fluids27,28,38,40,54.

Origin of graphite in the Saglek-Hebron BIF
The decarbonation has been considered as a thermal decomposition of Fe-carbonate (FeCO3 → Fe3O4 + CO2 + C), which could produce graphite with similar negative δ13C signatures in carbonate-bearing rocks in Isua, West Greenland (e.g., δ13Cgra −30‰)49,55; however, the lack of the petrographic association of the type of graphite with magnetite clearly excludes this reaction in the Saglek-Hebron BIF. Calcite reduction (or decarbonation: CaCO3 + SiO2 → CaSiO3 + C + 2Ofluid) has also been documented to produce graphite with a maximum fractionation of ~15‰ in carbonate-bearing rocks in Alpine Corsica, France56. However, such a fractionation is notably lower than the carbon isotopic difference between graphite and carbonate in the Saglek-Hebron BIF (δ13Ccarb − δ13Cgra = 17.5‰ to 20.4‰; Table 1). The absence of wollastonite co-produced by calcite reduction further rules out the possibility of calcite reduction in the Saglek-Hebron BIF. The FTT reaction commonly occurs on the surface of metal catalysts in hydrothermal systems to reduce CO or CO2 to relatively simple organic compounds52,53,57, but no association of graphite with catalyst for the FTT reaction (e.g., magnetite) was found in the Saglek-Hebron BIF. The restricted range of δ13Cgra in the Saglek-Hebron BIF is also inconsistent with FTT-derived organic compounds58. In fact, it has been proposed that C-H-O fluids with variable CO2/CH4 ratios can result in the deposition of graphite with δ13C values ranging from −30‰ to −9‰ in metasedimentary rocks from New Hampshire54. Therefore, we suggest that graphite in the Saglek-Hebron BIF is most likely derived from C-H-O fluids considering two lines of evidence: 1) the associations of graphite with C-H-O fluid inclusions are widely found in quartz-rich band (Figs. 2c and 3a–i), and 2) the δ13C values of graphite fall well within the range of δ13C values for fluid inclusions containing mixtures of CO2 and CH4 (δ13C = −21.8‰ and δ13C = −30.3‰; Fig. 3j) in the Saglek-Hebron BIF. As such, the variability of δ13C values in the Saglek-Hebron BIF may arise from the change in the CO2/CH4 ratio in the C-H-O fluids during metamorphism40,54,59.

In the Saglek-Hebron BIF, the co-occurrence of the GraI, GraII and GraIII within micrometre distances (Fig. 1g), together with the absence of graphite veins cross-cutting the Saglek-Hebron BIF, indicates that they have a localised source and were transported together over short distance by metamorphic fluids. The retrograde minerals (such as chlorite; Supplementary Fig. 2) in the Saglek-Hebron BIF suggests the cooling of C-H-O fluids during retrograde metamorphism, which can induce the hydration reactions of the host minerals and decreased carbon solubility, and subsequently result in the precipitation of PCG via the reduction of CO2 and the oxidation of CH4 in aqueous fluids (CO2 + CH4 → 2C + 2H2O)27,28. This model is supported by TEM-EDS images that show the close association of nano-chlorite with PCG (Supplementary Fig. 5h-o). This process has been documented in mafic and ultramafic rock units of the deep-drilling cores in Germany60, granulites from southern India61, as well as basalt and pyroxenite on Mars62, and recently modelled by Rayleigh fractionation equation to confirm that graphite precipitated from a CO2-CH4 fluid could have δ13C values ranging from −44.6‰ to −7.5‰40,54. During the cooling, increasing CO2 disassociation in metamorphic fluids could result in the synchronous precipitation of calcite and graphitic carbon, potentially producing the observed close association of calcite with graphite (Fig. 2)11. The geological CH4 is generally thought to come from three sources63: microbial origin (δ13C < −50‰; emitted by methanogenic archaea), thermogenic origin (δ13C = −50‰ to −20‰; produced by the thermal decomposition of organic matter), and abiogenic origin (δ13C > −20‰; generated from the mantle or non-biological reactions). The δ13C value of CH4 (δ13C = −30.3‰) in fluid inclusions falls well within the range of carbon isotopic compositions for thermogenic CH4, revealing that the CH4 could be a product of thermal decomposition of organic matter in the Saglek-Hebron BIF. The δ13C value of CO2 (δ13C = −21.8‰) in fluid inclusions is typical of that produced by the oxidation of such CH4 rather than carbonates and igneous carbon of mantle origin due to these two carbon sources having δ13C values heavier than −14‰64–66. Therefore, the reactants CO2 and CH4 for the fluid-deposition of PCG could be in-situ liberated from a common syngenetic source of microbial organic matter54, which occurs as crystalline graphite in the Saglek-Hebron BIF27. Such an organically derived C-H-O fluid has been known to result in the deposition of graphite with δ13C values ranging from −30.6 to −20‰ in volcanic rocks of the external zone of the Betic Cordillera, Southern Spain67, basalts and black shales in British Columbia, Canada68, and esites in Borrowdale, England59, metagabbros and amphibolites of the deep-drilling cores in Germany60, as well as Water Hen intrusion in the Duluth Complex, USA64. This indicates that PCG was deposited from C-H-O fluids derived from the retrograde cracking and oxidation of residual organic matter while crystalline graphite stemmed from high-grade metamorphism of syngenetic organic matter. The petrographic observations, nanoscale features and variable peak crystallisation temperatures of graphite in the Saglek-Hebron BIF provide further support for this conclusion (Fig. 1g and Supplementary Data 2). The varying crystallinity of graphitic carbons in the Saglek-Hebron BIF, thus, are sourced from the preservation of fluid-deposited graphite along with syngenetic graphite27,41, which point to primitive organic matter with low δ13C values during prograde and retrograde metamorphism.

Origin of graphite in the Saglek-Hebron marble
In the Saglek-Hebron marble, we observed the Fe-carbonate + graphite + magnetite associations rather than fluid inclusions (Fig. 4 and Supplementary Fig. 7), providing a plausible indication for the occurrences of decarbonation and FTT reactions. Petrographic observations suggest that serpentinization reactions (olivine + water => serpentine + magnetite + hydrogen) in the Saglek-Hebron marble possibly have provided a source of H2, CO and catalyst for the FTT or Friedel–Crafts-type reactions linked to serpentinization51,69,70. However, magnetite within serpentine veins is not found to be associated with graphite in the Saglek-Hebron marble (Supplementary Fig. 8), and the FTT reaction is known to be most effective between 200 and 350 °C to generate methane and short chain hydrocarbons rather than structurally complex graphite12,32,52,53,57,71. Also, such reaction has never been conclusively confirmed to play a significant role in formation of graphite in nature72. Alternatively, the intertwinement of graphite to magnetite within Fe-carbonate grains is most likely a product of decomposition of Fe-carbonate (Fig. 4)49,73, which can be expressed as the following equation74: Fe-dolomite + water => Mg-calcite + magnetite + graphite + methane. High-resolution Raman hyperspectral images reveal that the GraIV in the Saglek-Hebron marble is closely associated with magnetite and calcite and specifically within Fe-dolomite grains (Fig. 4). Such mineral assemblage has been verified to indicate formation of graphite from decarbonation in subduction zone settings by petrological observation and experimental simulation73, as implied by the filamentous graphite in the Saglek-Hebron marble that is comparable to graphite cones and nanotubes from the Kola Peninsula37 and filamentous carbon synthesised by high-temperature, high-pressure treatment under hydrothermal conditions75. Furthermore, the fact that graphite occurs exclusively in association with magnetite within Fe-bearing carbonate grains rather than magnetite within serpentine veins in the Saglek-Hebron marble provides further support for this conclusion (Fig. 4 and Supplementary Figs. 7 and 8). Therefore, our petrographic, spectroscopic and geochemical evidence clearly support abiotic decarbonation reactions as responsible for the formation of graphite in the Saglek-Hebron marble.

In conclusion, we investigate the specific mineral associations with graphitic carbons in BIF and marble from the Saglek-Hebron Gneiss Complex, Canada, and find that graphitic carbons in the Saglek-Hebron BIF originate from metamorphism of syngenetic organic matter and C-H-O fluid deposition during retrograde metamorphism, while graphitic carbons in the Saglek-Hebron marble are ascribed to the decomposition of Fe-carbonate. In this study, a suite of converging and mutually supportive evidence indicative of abiotic synthesis of organic matter was established for graphitic carbons in the Saglek-Hebron metasedimentary rocks, which extends knowledge of origins of organic matter in early Archean rocks. Collectively, our findings provide solid support for previous interpretations that graphite could be precipitated from C-H-O fluids derived from the retrograde cracking and oxidation of syngenetic biomass27,28, and also provide the oldest evidence for abiotic synthesis of graphitic carbons via decarbonation in the Saglek-Hebron metasedimentary rocks. Therefore, this study could pave the way for deciphering the poorly understood effects of metamorphism on prebiotic organic matter and biogeochemical cycling of carbon, as well as the geological transformation of precursor biomass on early Earth.

Methods
Optical microscopy
Graphite was located within the polished thin sections using a BX-51 Olympus microscope in the Department of Earth Sciences at University College London (UCL) equipped with transmitted and reflected light. No immersion oil was used to map petrographic and mineralogical features in thin sections. The thin sections were prepared according to conventional techniques with a final 0.25 mm alumina polish and cleaned with an acetone wetted kimwipe.

TESCAN integrated mineral analyzer
Mineralogical, modal and textural analysis was performed on gold-coated thin sections using a TESCAN Integrated Mineral Analyzer (TIMA FEG3 LMH 119-0205) under liberation analysis mode76. The instrument was operated at an acceleration voltage of 25 kV and a beam intensity of 18.7 mA with a working distance of 15 mm to obtain back-scattered electron (BSE) image and energy dispersive X-ray spectroscopy (EDS) data with point spacings of 2.5 µm and 7.5 µm, respectively. The BSE signal intensity was calibrated on a platinum Faraday cup using the automated procedure, while manganese standard was used for checking EDS performance. The segments (i.e., mineral grains) were produced by merging areas of coherent BSE and EDS data. Individual spectra from points within each segment are summed, and then compared against a classification scheme to identify minerals in thin sections.

Micro-Raman spectroscopy
Micro-Raman spectroscopy was performed with a WITec alpha 300 confocal Raman imaging microscope at the UCL, using a 532 nm wavelength laser and 8 mW power5. Raman spectra and hyperspectral scans were performed with an 100X objective (N.A. = 0.9) using an optical fibre of 50 µm in diameter, to analyze targets below the polished thin section surface, which was meant to minimise spectral artefacts caused by polishing. Hyperspectral images were produced by the main peak intensity mapping for specific minerals, which include quartz (~465 cm−1), carbonate (~1092 cm−1), apatite (~968 cm−1), magnetite (~673 cm−1), crystalline graphite (~1585 cm−1), PCG (~1355 cm−1), and CO2 (~1389 cm−1). Average spectra were corrected with a background subtraction of a polynomial function. Raman spectrum parameters, such as peak positions, Full Width at Half Maximum (FWHM), and areas under the curve were extracted from the background-subtracted spectra, and then modelled with Lorentz-fitted curves. Based on these parameters, the peak crystallisation temperatures of graphitic carbon in the Saglek-Hebron metasedimentary rocks can be calculated using following geothermometer44:

T(°C) = −455 × D1/(D1 + G + D2) + 641

This equation that takes into account peak-broadening effects has been calibrated against metamorphic mineral assemblages in metapelites exposed to the temperature range of 330–641 °C with the error of ±50 °C.

Isotope ratio mass spectrometry
Analyses of carbon isotope compositions of total graphite were carried out on bulk rock powders with a Thermo-Finnigan Flash 1112 EA connected to a ThermoDelta V Isotope Ratio Mass Spectrometer via a Conflo IV gas distribution system in the Bloomsbury Environmental Isotope Facility at UCL27. Prior to analyses, the inorganic carbon component was removed with ultra pure 6N HCl (Sequanal Grade, Pierce). A suite of standard materials that have δ13C values ranging from −26‰ to −11‰, was analysed multiple times through each run to ensure reproducibility better than 0.2‰. Results are quoted as standard δ-notation (in units of permil, ‰), relative to the Vienna Pee Dee Belemnite (VPDB) standard. Analyses of bulk rock powders for carbonate were performed on a Finnigan MAT 253 mass spectrometer in the Laboratory for Stable Isotope Geochemistry at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The CO2 was extracted with pure H3PO4 at 75 °C for δ13C measurements. The results are also reported against VPDB with a standard deviation better than 0.15‰.

The carbon isotopic composition of the extracted CH4 and CO2 in fluid inclusions was determined using a Finnigan MAT 253 mass spectrometer in the Laboratory for Stable Isotope Geochemistry at the IGGCAS. To extract fluid components from fluid inclusions in the quartz, the sample was heated under vacuum at 600 °C for 15 min and then passed through a dry ice + ethanol cold trap under vacuum to remove H2O. The fluid components then passed through a liquid N2 cold trap to separate CH4 and freeze CO2 which was subsequently analysed after heating. The remaining gases (CH4 + N2) were oxidised by reaction with CuO at 780 °C to the reacted gases (CO2 + H2O + N2), which subsequently passed through dry ice + ethanol and liquid N2 cold traps under vacuum to collect pure CO2 representing CH4 carbon composition of fluid inclusions. Carbon isotope compositions of CH4 and CO2 in the fluid inclusions are expressed as δ13C CH4 and δ13C CO2 relative to VPDB with a standard deviation better than ±0.024‰ and ±0.037‰, respectively.

Scanning electron microscopy
The morphology and composition of selected targets were characterised by a JEOL JSM-6480L scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) in the Department of Earth Sciences at UCL. The SEM was operated at standard operating conditions, which include a 15 kV accelerating voltage, 1 nA electron beam current and a working distance of 10 mm.

Focused ion beam milling-transmission electron microscopy
The focused ion beam (FIB) milling was conducted with a Helios 5 CX FIB instrument in the Department of Materials, Imperial College London. In brief, the selected graphite-carbonate was located within the dual-beam FIB by using SEM imaging and a protective Pt shield was deposited on the surface. Then, the material from around the deposited Pt mask was sputtered away by a focused 30 kV Ga+ primary beam, making a 1 µm thick vertical section. After the bottom and sides of the section were removed, it was carefully transferred and welded onto a Cu TEM half-grid. Finally, the section was thinned down to ~100 nm with a 43 pA focused beam of Ga+. High-resolution TEM data were obtained using a JEOL 2100 transmission electron microscopy operating at 200 kV in the Department of Chemistry at UCL.

Nano-scale secondary ion mass spectrometry
Elemental maps were obtained on the polished 30-μm-thick thin sections using a CAMECA NanoSIMS 50L (Paris, France) at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan)34. Each region of interest was pre-sputtered with a 1.5 nA or 3 nA beam current and an ion dose of n = 5 × 1016 ions/cm2 to remove the conductive coating (Au) and the contaminants that were generated during pretreatment. In the multi-collection mode, negative secondary ions (12C−, 12C1H−, 12C14N−, 32O−, 31P−, and 32S−) were sputtered from the sample surface using a Cs+ primary beam with ~1.6 pA intensity. During these analyses, electron gun was used for charge compensation. All of the images referred to in this paper are 512 × 512 pixels, recorded in ~24 min, and processed using CAMECA WINimage software (Version 4.4).

Data availability
All data in this study are included in this published article and supplementary information files, and are available from the corresponding author upon request.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Overall, the paper appears interesting, but it needs significant revisions before publication.\nThe paper provides insights into the origin of graphitic carbon in some of the oldest rocks on Earth.\nHowever, the literature is not appropriately cited, and there are technical aspects that should be\nclarified. In addition, this study builds upon numerous prior investigations, some of which have\nalready documented the variation in crystallinity of graphitic carbons in metasedimentary rocks\nfrom the Nuvvuagittuq Supracrustal Belt. The originality of the paper needs to be clarified.\nThe title needs to be modified. \"Abiotic synthesis of biosignatures\" makes no sense. A biosignature\noriginates from biological processes. If it does not, it is abiotic. It cannot be both.\nThe authors selectively cite some previous work and inappropriately ignore opposing views. The\npoints listed below should be addressed in the main text:\n- L. 19-22; 58-60: The Eoarchean age of the rocks studied by Tashiro et al. (2017) has been\nquestioned (Whitehouse et al., 2019).\n“Unambiguous field relationships between ca. 3.9 Ga tonalitic gneiss and the graphite-bearing\nmetasediments have not been demonstrated in the literature that the Saglek claim relies upon, and\nearlier U-Pb-Hf isotopic studies on zircon from metasediments at one of the localities used in the\nclaim indicate a Mesoarchean to Neoarchean time of deposition.” (Whitehouse et al. 2019).\nThe authors treat this aspect in a superficial and incorrect way in the supplementary information.\nThis should be addressed in the main text.\nSimilarly, the authors fail to acknowledge prior research that has raised doubts about the age and\norigin of graphitic carbon in these rocks (Alleon and Summons, 2019): “Recently, additional\nevidence for Eoarchean life has been claimed based on the presence of 13C-depleted graphite\nparticles within metasedimentary rocks from Isua [33] and from the Saglek block in northern\nLabrador, Canada [34]. Similarly, however, evidence for the antiquity of graphite is lacking. The\ngeological history of these rocks is complex and poorly resolved. All known sequences from these\nlocations have experienced at least two metamorphic events since their formation, the most recent\nreaching at least the upper amphibolite facies around 2.8–2.5 Ga [17–22]. Graphite could thus have\nprecipitated from hydrothermal and metasomatic fluids at any time during post-Eoarchean\nmetamorphic episodes and, at least partially, could be as young as 2.8–2.5 Ga”.\nAnd last, but certainly not least, Alleon and Summons (2019) argued that “sedimentary graphite,\nirrespective of its isotopic composition, does not constitute a reliable biosignature because the\nrocks in which it is found are generally metamorphosed to the point where convincing signs of life\nhave been erased.”.\n- L. 47-48: “purported microfossils from highly metamorphosed rocks have only been reported from\nNuvvuagittuq Supracrustal Belt (3.75 to 4.3 Ga (gigayears ago))4,5”\nThis is not true. Purported microfossils were reported in highly metamorphosed rocks from the Isua\nSupracrustal Belt by Pflug and Jaeschke-Boyer (1979), which were reinterpreted to be abiotic by\nBridgwater et al. (1981) and Roedder (1981).\nFurthermore, the origin of the purported microfossils from Nuvvuagittuq has been questioned\n(McMahon, 2019). This should be acknowledged in the main text.\n- L. 60-63: “Hence, in this context, as well as in the context of exobiology, the distinction between\nabiotic and biological processes and sources of organic molecules in graphitic carbons is an\nimperative goal to achieve accurate identification of traces of early life, prebiotic chemistry and\nextraterrestrial life”.\nThis statement seems to me to be excessive. I am skeptical about the significance of differentiating\n\"between abiotic and biological processes and sources of organic molecules in graphitic carbons\"\nin the context of exobiology or prebiotic chemistry. The search for evidence of extraterrestrial life is\nunlikely to be carried out on rocks that have been heavily metamorphosed. This also applies to\nprebiotic chemistry. Could the authors provide further clarification on these aspects? Without this,\nexobiology and prebiotic chemistry should not be highlighted.\n- Raman identification of CH4 associated with graphitic carbon is not convincing. Assigning the\nband around 2915 cm-1 uniquely to CH4 is inappropriate for disordered graphitic carbons. The\npresence of disorder (defects) activates a broad band around 2900 cm-1, which is usually\nattributed to D + D′ or D + G\" combination modes (e.g., Pimenta et al., 2007; Beyssac and Lazzeri,\n2012).\nThe Raman spectra presented here are too noisy to distinguish a CH4 contribution from the purely\nstructural defects expected in disordered graphitic carbons.\n- L. 147-148: “Based on the Beyssac geothermometer calibrated for temperatures between 330° C\nand 641° C, we calculate the crystallization temperatures with a precision of about ±50° C”.\nThe commonly used term for this geothermometer is Raman spectroscopy of carbonaceous\nmaterial (RSCM) thermometry, rather than the Beyssac geothermometer.\nMore importantly, the RSCM geothermometer developed by Beyssac et al. (2002) is calibrated for a\n514 nm laser source. The defect peaks of graphitic carbons are dispersive, i.e. their frequencies\nand intensities relative to that for the G peak vary with the laser energy (Beyssac and Lazzeri, 2012).\n“This effect has been quantified by Sato et al. (2006) and Cancado et al. (2006) and is significant as\nit may increase the ID/IG ratio for given samples by 12% by simply changing the laser energy from\n2.41 eV (514 nm) to 2.33 eV (532 nm)” (Beyssac and Lazzeri, 2012). The data reported here were\ncollected using a 532 nm laser source. The estimated temperatures presented here are thus likely\nbiased. Geothermometers calibrated for a 532 nm laser source exist and should be used (e.g.,\nKouketsu et al. 2014).\n- L. 272-277: “Collectively, our unprecedented findings provide the oldest evidence for primitive\nsedimentary organic matter in graphitic carbons preserved in the Saglek-Hebron metasedimentary\nrocks, and could pave the way for deciphering the poorly understood effects of metamorphism on\nprebiotic organic matter and biogeochemical cycling of carbon, as well as the geological\ntransformation of precursor biomass on early Earth, or even other habitable planets.”\nThis is highly exaggerated. This study builds upon numerous prior investigations, some of which\nhave already documented the variation in crystallinity of graphitic carbons in metasedimentary\nrocks from the Nuvvuagittuq Supracrustal Belt. “Crystalline graphite is interpreted to represent the\nmetamorphosed product of syngenetic organic carbon deposited in BIF, while poorly crystalline\ngraphite was precipitated from C–O–H fluids partially sourced from the syngenetic carbon, along\nwith fluid-deposited apatite and carbonate.” Dodd et al. (2019). See also Papineau et al. (2011).\nThe originality of the current paper needs to be clarified.", "reviewer_2": "Review report on the manuscript entitled \"Abiotic synthesis of graphitic carbon biosignatures in the\nEoarchean Saglek-Hebron metasedimentary rocks”\nThe study presents new data on metamorphic graphite in samples from rocks of the Saglek-Hebron\nGneiss Complex. The study presents a thorough analysis of graphite using various relevant\ntechniques to deduce the isotopic fractionation and composition and structure of the graphite and\nthe geochemical setting in which it is found. This is then used to deduce a likely metamorphic\ntransformation history, that in several cases involve abiotic processes. The authors argue that while\nthe process that have transformed the carbon material was abiotic, the final state of the carbon\nmaterial is only consistent with the original source material being biogenic in origin. The purported\nclaim is then that biological material was produced sometime during the formation of the rock, 3.9\nbillion years ago, and then this material was transformed by abiotic metamorphic process into the\ncomposition and state observed today.\nThe claim of original bioegnicity is primarily based on carbon isotope fractionation, assuming that\nancient biological processes generated lighter carbon deposits, similar to modern biological\nprocesses. While we agree with this assumption and we think the paper represents a thorough,\nscientific sound treatment of the data, the assumption is not universally accepted.\nAn objection often raised when it comes to conclusions based primarily on carbon isotope\nfractionation in old rocks, is the possibility of later contamination through fluid migration or\nintroduced otherwise, particular for this set of rocks. Nevertheless we think that the data, the\nanalysis and the complex intricate metamorphic picture presented in the manuscript mitigates this\nconcern.\nThe text is well referenced and comments on previous studies by Tashiro, T. et al. (2017) who\nreported similar isotope fractionations in carbon material from the same set of rocks, though the\nissue of later introduction of carbon was more ambiguous in those cases. Nevertheless, the\nmanuscript presents valuable data and analysis deserving of publication.\nSpecific points:\n1)The poorly crystalline graphite (PCG) is identified and suggested to be formed from the\nprecipitation from C-H-O fluids. It would be interesting to see the distribution of PCG on Figure 1.\nDoes it only occur in the fluid-associated graphite (Gra3)?\n2) It is implied that the fluid + graphitic carbon represents a system where syngenetic organic\nmatter is mobilized in C-H-O fluids and redeposited as abiotic PCG and graphite elsewhere (line\n232-239), and that some of the graphite represents the primitive organic matter. Is this directly\nreflected in the Gra1, Gra2, Gra3 association? And does this imply that Gra1 is the graphitized\n‘primitive OM’? The absence of heteroatoms from the Gra1 as suggested by Supplementary Figure 6\ncould be due to loss following metamorphism.\n3) The oxidation of graphite is loosely suggested in the text to explain the patterns of graphite-\ncalcite associations observed in Figure 1. However the mechanism for oxidation of graphite is not\nexplained – is the oxidation a recent happening? Or is the association between graphite and calcite\nbetter explained by synchronous precipitation of graphite and calcite during retrograde\nmetamorphism (line 213-215)?\n4) In line 180 it is stated that ‘lack of magnetite + graphite association clearly excludes’ the\nformation of graphite from decomposition of Fe-carbonates. However, there is magnetite in all the\nsamples studied.\nFig2: I suggest changing the color of the fluid inclusion bar in subfigure O to something else (grey\nperhaps, to match Fig3). At first glance at the Fig2, the reader is lead to think that the blue colors in\nthe Raman maps indicates Fluid inclusions.\nSupplemental information:\nFigS1: Maybe add labels on b, c and d or arrows pointing into a\nFigS3: What is what here? It is not clear to me why and you can distinguish between fluid inclusions\nand graphite inclusions based on this figure. And the Gra I, GraII etc.? Maybe indicate G and D\npeak as well (or lack off).\nFigS5:\nScales are impossible to see and the use of cyan in text makes it unreadable.\nFigS6: Line 107: Why do you want to make this statement here?\nFigS8: What is the scale on a) and b). Again Cyan text on white background is not working well. Was\nthe Raman spectra recorded on this particular sample? I guess they where extracted from h) and i)", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part\nof the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "All of us would like to thank the editors and reviewers for your considerations about this manuscript\nand constructive comments. We have revised it very carefully based on all of the comments. Detailed\nresponses are provided in a point-by-point fashion as follows.\nResponse to Reviewer #1 (Remarks to the Author):\nOverall, the paper appears interesting, but it needs significant revisions before publication.\nThe paper provides insights into the origin of graphitic carbon in some of the oldest rocks on Earth.\nHowever, the literature is not appropriately cited, and there are technical aspects that should be\nclarified. In addition, this study builds upon numerous prior investigations, some of which have\nalready documented the variation in crystallinity of graphitic carbons in metasedimentary rocks\nfrom the Nuvvuagittuq Supracrustal Belt. The originality of the paper needs to be clarified.\nAnswer: We appreciate you for valuable comments, which greatly improved this manuscript. We\nhave responded to your comments point by point below. In brief, our work is original because it\npetrographically and spectroscopically documents different origins of graphitic carbons in the\nEoarchean metasedimentary rocks from the Saglek-Hebron Gneiss Complex. The novelty is now\nclarified in the conclusion.\nThe title needs to be modified. \"Abiotic synthesis of biosignatures\" makes no sense. A biosignature\noriginates from biological processes. If it does not, it is abiotic. It cannot be both.\nAnswer: We have deleted the term “biosignatures” in the title. We would argue however that the\nword “biosignature” in the context of the Eoarchean should be defined as a ‘possible’ signature of\nlife. There are very few unambiguous biosignatures in the geological record and “life” is also not a\nwell-defined concept.\nThe authors selectively cite some previous work and inappropriately ignore opposing views. The\npoints listed below should be addressed in the main text:\n- L. 19-22; 58-60: The Eoarchean age of the rocks studied by Tashiro et al. (2017) has been\nquestioned (Whitehouse et al., 2019).\n“Unambiguous field relationships between ca. 3.9 Ga tonalitic gneiss and the graphite-bearing\nmetasediments have not been demonstrated in the literature that the Saglek claim relies upon, and\nearlier U-Pb-Hf isotopic studies on zircon from metasediments at one of the localities used in the\nclaim indicate a Mesoarchean to Neoarchean time of deposition.” (Whitehouse et al. 2019).\nThe authors treat this aspect in a superficial and incorrect way in the supplementary information.\nThis should be addressed in the main text.\nAnswer: Thank you for providing valuable references. We agree that one of the geochronology\nissues regarding Tashiro et al. (2017) study, is that the pre-3.95Ga age was called into question. This\nage comes from zircons from an orthogneiss sample, which was revised and published to be 3.92\nGa (Shimojo et al., 2016). Using the data from Shimojo et al., (2016), Whitehouse et al. (2019)\nargued that the age of this orthogneiss is closer to 3.865 Ga and the same outcrop was sampled and\nanalysed by Wasilewski et al. (2021) who obtained an age of 3.869 Ga. Moreover, both Whitehouse\net al. (2019) and Wasilewski et al. (2021) questioned the field relationships reported by Shimojo et\nal. (2016). Currently, the best age constraints for the oldest Nulliak supracrustal assemblage (which\nincludes the samples studied here) is arguably from Sm-Nd and Lu-Hf isotopes (Morini et al., 2017;\n2018) and deposition ages of associated detrital metasediments (Nutman et al., 1989) consistent\nwith an age of ~3.8 Ga. Hence, we are no longer relying on the age of the 3.9 Ga tonalitic gneiss to\nconstrain the age of the metasediment samples studied here, but rather on the fact that the\nmetasediments are part of Nulliak supracrustal exposures and therefore best constrained to be ~3.8\nGa. To make this clearer, we have cited some relevant references in the main text to include some\nof the debated point about the Eoarchean to Neoarchean ages of the Saglek-Hebron Gneiss Complex\n(Lines 64-73).\nSimilarly, the authors fail to acknowledge prior research that has raised doubts about the age and\norigin of graphitic carbon in these rocks (Alleon and Summons, 2019): “Recently, additional\nevidence for Eoarchean life has been claimed based on the presence of 13C-depleted graphite\nparticles within metasedimentary rocks from Isua [33] and from the Saglek block in northern\nLabrador, Canada [34]. Similarly, however, evidence for the antiquity of graphite is lacking. The\ngeological history of these rocks is complex and poorly resolved. All known sequences from these\nlocations have experienced at least two metamorphic events since their formation, the most recent\nreaching at least the upper amphibolite facies around 2.8–2.5 Ga [17–22]. Graphite could thus have\nprecipitated from hydrothermal and metasomatic fluids at any time during post-Eoarchean\nmetamorphic episodes and, at least partially, could be as young as 2.8–2.5 Ga”.\nAnd last, but certainly not least, Alleon and Summons (2019) argued that “sedimentary graphite,\nirrespective of its isotopic composition, does not constitute a reliable biosignature because the rocks\nin which it is found are generally metamorphosed to the point where convincing signs of life have\nbeen erased.”.\nAnswer: Thank you for providing this reference and for pointing out this debate. We agree with\nthese authors that metamorphism would result in the variations in carbon isotopic compositions of\norganic matter, thereby leading to blurred carbon isotopic compositions, which cannot be used alone\nto distinguish between abiotic and biogenic sources. However, we respectfully disagree with the\nquoted statement by Alleon and Summons that sedimentary graphitic carbons are always\n‘metamorphosed to the point where convincing signs of life have been erased’ because 1) there are\nseveral reports in the peer-reviewed literature where researchers have used carbon isotopes and\ngraphitic carbons to trace past signs of life including in Hadean zircons, metasedimentary rocks of\nvarious kinds, peridotitic graphite, and eclogitic diamonds, 2) this affirmation is an opinion rather\nthan a demonstrated fact, and 3) it is defeatist and not constructive to give up on this important\nscientific problem because of some people’s opinion. We agree however that the graphite could\npossibly be younger than the host rock’s sedimentary precursor so we have cited this reference in\nthe main text to acknowledge that these authors raised doubts about the age and origin of graphitic\ncarbons within metasedimentary rocks from the Saglek block reported in Tashiro et al. (2017) (Lines\n76-79).\n- L. 47-48: “purported microfossils from highly metamorphosed rocks have only been reported from\nNuvvuagittuq Supracrustal Belt (3.75 to 4.3 Ga (gigayears ago))4,5”\nThis is not true. Purported microfossils were reported in highly metamorphosed rocks from the Isua\nSupracrustal Belt by Pflug and Jaeschke-Boyer (1979), which were reinterpreted to be abiotic by\nBridgwater et al. (1981) and Roedder (1981).\nFurthermore, the origin of the purported microfossils from Nuvvuagittuq has been questioned\n(McMahon, 2019). This should be acknowledged in the main text.\nAnswer: Thank you for sharing valuable references. R1 is correct in pointing out that our statement\nwas incorrect, so we deleted the word “only” from the sentence and also added that the origin of the\nNuvvuagittuq microfossils has been questioned by McMahon (2019). We feel however that it is not\nuseful to cite all these older papers, when 1) there is consensus in the community about the abiotic\nor contamination origin of the purported Isua microfossils, and 2) our new work is neither about\nmicrofossils, nor about the Isua Supracrustal Belt.\n- L. 60-63: “Hence, in this context, as well as in the context of exobiology, the distinction between\nabiotic and biological processes and sources of organic molecules in graphitic carbons is an\nimperative goal to achieve accurate identification of traces of early life, prebiotic chemistry and\nextraterrestrial life”.\nThis statement seems to me to be excessive. I am skeptical about the significance of differentiating\n\"between abiotic and biological processes and sources of organic molecules in graphitic carbons\"\nin the context of exobiology or prebiotic chemistry. The search for evidence of extraterrestrial life\nis unlikely to be carried out on rocks that have been heavily metamorphosed. This also applies to\nprebiotic chemistry. Could the authors provide further clarification on these aspects? Without this,\nexobiology and prebiotic chemistry should not be highlighted.\nAnswer: We have deleted the context of exobiology and prebiotic chemistry here to avoid confusion.\n- Raman identification of CH4 associated with graphitic carbon is not convincing. Assigning the\nband around 2915 cm-1 uniquely to CH4 is inappropriate for disordered graphitic carbons. The\npresence of disorder (defects) activates a broad band around 2900 cm-1, which is usually attributed\nto D + D′ or D + G\" combination modes (e.g., Pimenta et al., 2007; Beyssac and Lazzeri, 2012).\nThe Raman spectra presented here are too noisy to distinguish a CH4 contribution from the purely\nstructural defects expected in disordered graphitic carbons.\nAnswer: We agree with R1 that the disordered graphitic carbon has a broad band around 2900 cm-\n1. However, CH has a very narrow band around 2915 cm-1. To distinguish a CH contribution from\n4 4\nthe purely structural defects in graphite, we have double-checked all our Raman spectra to identify\nCH with a very narrow band at 2915 cm-1 and accordingly modified Figs. 3 and S3 to make the\n4\nwithin fluid inclusions, 2) always with CO , and 3) not systematically associated with graphitic\n2\ncarbons. Hence, the presence of the narrow 2915cm-1 peak is arguably consistent with the presence\nof methane in fluid inclusions.\n- L. 147-148: “Based on the Beyssac geothermometer calibrated for temperatures between 330° C\nand 641° C, we calculate the crystallization temperatures with a precision of about ±50° C”.\nThe commonly used term for this geothermometer is Raman spectroscopy of carbonaceous material\n(RSCM) thermometry, rather than the Beyssac geothermometer.\nMore importantly, the RSCM geothermometer developed by Beyssac et al. (2002) is calibrated for\na 514 nm laser source. The defect peaks of graphitic carbons are dispersive, i.e. their frequencies\nand intensities relative to that for the G peak vary with the laser energy (Beyssac and Lazzeri, 2012).\n“This effect has been quantified by Sato et al. (2006) and Cancado et al. (2006) and is significant as\nit may increase the ID/IG ratio for given samples by ≏12% by simply changing the laser energy\nfrom 2.41 eV (514 nm) to 2.33 eV (532 nm)” (Beyssac and Lazzeri, 2012). The data reported here\nwere collected using a 532 nm laser source. The estimated temperatures presented here are thus\nlikely biased. Geothermometers calibrated for a 532 nm laser source exist and should be used (e.g.,\nKouketsu et al. 2014).\nAnswer: We appreciate this constructive comment on the geothermometer. We have now selected\nthe term ‘the geothermometer using Raman spectroscopy of organic matter’ in the revised paper.\nIn addition, the geothermometers in Kouketsu et al. (2014) were calibrated for low-grade and medium-\ngrade carbonaceous material, which is applicable in range of around 150–400°C. We have used\nthese geothermometers to estimate metamorphic temperatures of Gra and Gra (ID/IG > 0.8;\nIII IV\nplease see Supplementary Data 2), which are consistent with the temperatures estimated by Beyssac\net al. (2002) within uncertainties. This indicates that the small change in the laser energy does not\naffect the applicability of the geothermometer using Raman spectroscopy of organic matter\ndeveloped by Beyssac et al. (2002), thereby causing negligible effect on the temperature ranges\nestimated using R2 area ratios for a large number of specimens in this study.\n- L. 272-277: “Collectively, our unprecedented findings provide the oldest evidence for primitive\nsedimentary organic matter in graphitic carbons preserved in the Saglek-Hebron metasedimentary\nrocks, and could pave the way for deciphering the poorly understood effects of metamorphism on\nprebiotic organic matter and biogeochemical cycling of carbon, as well as the geological\ntransformation of precursor biomass on early Earth, or even other habitable planets.”\nThis is highly exaggerated. This study builds upon numerous prior investigations, some of which\nhave already documented the variation in crystallinity of graphitic carbons in metasedimentary\nrocks from the Nuvvuagittuq Supracrustal Belt. “Crystalline graphite is interpreted to represent the\nmetamorphosed product of syngenetic organic carbon deposited in BIF, while poorly crystalline\ngraphite was precipitated from C–O–H fluids partially sourced from the syngenetic carbon, along\nwith fluid-deposited apatite and carbonate.” Dodd et al. (2019). See also Papineau et al. (2011).\nThe originality of the current paper needs to be clarified.\nAnswer: We agree with R1 and we are acutely aware that Papineau et al., 2011 and Dodd et al.,\n2019 reported the variation in crystallinity of graphitic carbons in metasedimentary rocks from the\nNuvvuagittuq Supracrustal Belt. New observations on the Saglek-Hebron metasedimentary rocks\nfound: 1) there are some graphite associated with fluid inclusions containing CO + CH (Figs. 2c\n2 4\nand 3a-i), and 2) the δ13C values of graphite (δ13C = −27.0‰ to −22.7‰) fall well within the\nGra\nrange of δ13C values for fluid inclusions containing mixtures of CO and CH (δ13C = −21.8‰\n2 4 CO2\nand δ13C = −30.3‰; Fig. 3j). These two lines of evidence lend more convincing support to the\nCH4\nconclusion that poorly crystalline graphite could be precipitated from C–O–H fluids derived from\nthe retrograde cracking and oxidation of sedimentary organic matter. The most important finding of\nthis paper is that graphite could be formed from decarbonation during metamorphism, which has\nnever been demonstrated in Precambrian rocks. To clarify the originality of this paper, we have\nrewritten these sentences in the revised paper (Lines 305-309).\nReviewer #2 (Remarks to the Author):\nReview report on the manuscript entitled \"Abiotic synthesis of graphitic carbon biosignatures in the\nEoarchean Saglek-Hebron metasedimentary rocks”\nThe study presents new data on metamorphic graphite in samples from rocks of the Saglek-Hebron\nGneiss Complex. The study presents a thorough analysis of graphite using various relevant\ntechniques to deduce the isotopic fractionation and composition and structure of the graphite and\nthe geochemical setting in which it is found. This is then used to deduce a likely metamorphic\ntransformation history, that in several cases involve abiotic processes. The authors argue that while\nthe process that have transformed the carbon material was abiotic, the final state of the carbon\nmaterial is only consistent with the original source material being biogenic in origin. The purported\nclaim is then that biological material was produced sometime during the formation of the rock, 3.9\nbillion years ago, and then this material was transformed by abiotic metamorphic process into the\ncomposition and state observed today.\nThe claim of original biogenicity is primarily based on carbon isotope fractionation, assuming that\nancient biological processes generated lighter carbon deposits, similar to modern biological\nprocesses. While we agree with this assumption and we think the paper represents a thorough,\nscientific sound treatment of the data, the assumption is not universally accepted. An objection often\nraised when it comes to conclusions based primarily on carbon isotope fractionation in old rocks, is\nthe possibility of later contamination through fluid migration or introduced otherwise, particular for\nthis set of rocks. Nevertheless we think that the data, the analysis and the complex intricate\nmetamorphic picture presented in the manuscript mitigates this concern.\nThe text is well referenced and comments on previous studies by Tashiro, T. et al. (2017) who\nreported similar isotope fractionations in carbon material from the same set of rocks, though the\nissue of later introduction of carbon was more ambiguous in those cases. Nevertheless, the\nmanuscript presents valuable data and analysis deserving of publication.\nAnswer: We thank R2 for recognizing the valuable contribution of our study. We have\nacknowledged and added the underlying assumption that ancient biological processes generated\nisotopically light carbon deposits similar to modern biological processes in the revised paper (Lines\n198-200)\nSpecific points:\n1)The poorly crystalline graphite (PCG) is identified and suggested to be formed from the\nprecipitation from C-H-O fluids. It would be interesting to see the distribution of PCG on Figure 1.\nDoes it only occur in the fluid-associated graphite (Gra3)?\nAnswer: We thank R2 for valuable suggestion, which highlighted a possibly confusing aspect.\nRaman images and TEM observations reveal that some graphitic carbons consist of both crystalline\ngraphite and PCG. While most Gra is PCG, PCG also occurs in Gra . Hence, the graphite types\nIII II\nare defined primarily by their petrographic modes of occurrence, rather than their crystallinity, and\nPCG is not readily distinguishable from crystalline graphite in all occurrences of graphitic carbons\n(sometimes they are mixed together). So, in this contribution, we document the petrographic types\nof graphite, calcite + graphite, and some PCG-bearing fluid inclusions (red circles) in Figure 1g. We\nhave clarified this in the revised paper.\n2) It is implied that the fluid + graphitic carbon represents a system where syngenetic organic matter\nis mobilized in C-H-O fluids and redeposited as abiotic PCG and graphite elsewhere (line 232-239),\nand that some of the graphite represents the primitive organic matter. Is this directly reflected in the\nGra1, Gra2, Gra3 association? And does this imply that Gra1 is the graphitized ‘primitive OM’?\nThe absence of heteroatoms from the Gra1 as suggested by Supplementary Figure 6 could be due\nto loss following metamorphism.\nAnswer: We agree with R2 that the Gra could lose heteroatoms during metamorphism. The Gra is\nI I\nmainly crystalline graphite representing the graphitized ‘primitive OM’, while Gra is mainly PCG\nIII\nrepresenting abiotic deposition from C-H-O fluids derived from the retrograde cracking and\noxidation of syngenetic organic matter.\n3) The oxidation of graphite is loosely suggested in the text to explain the patterns of graphite-calcite\nassociations observed in Figure 1. However the mechanism for oxidation of graphite is not explained\n– is the oxidation a recent happening? Or is the association between graphite and calcite\nbetter explained by synchronous precipitation of graphite and calcite during retrograde\nmetamorphism (line 213-215)?\nAnswer: We recognize this confusion. Observations of the petrographic mode of occurrence with\nmore Gra and Gra near the central zone of quartz-rich band and more graphite-free calcite grains\nI II\nnear the Fe-silicate + magnetite portions of the Saglek-Hebron BIF (Fig. 1g) suggest that graphite\nwas oxidized by trivalent iron in Fe-minerals during retrograde metamorphism. So R2’s second\nsuggestion is correct in our view, and it is unlikely that this is a recent (i.e. modern) phenomenon\nbecause graphite is mostly unreactive in air. We further note that the graphite associated with calcite\nhas variable D/G peak intensity ratios ranging from 0.05 to 0.80, which further suggest that the\ngraphite + calcite associations formed from both the oxidation (for crystalline graphite) and\nsynchronous precipitation of graphite and calcite from the C-H-O fluids (for PCG) during retrograde\nmetamorphism. To make this clearer, we have added the mechanism for oxidation of graphite in the\nrevised paper (Lines 127-128).\n4) In line 180 it is stated that ‘lack of magnetite + graphite association clearly excludes’ the\nformation of graphite from decomposition of Fe-carbonates. However, there is magnetite in all the\nsamples studied.\nAnswer: We agree that magnetite is ubiquitously present in all the Saglek-Hebron BIF and marble\nsamples. However, our point is that graphite is not documented to be petrographically associated\nwith magnetite in the Saglek-Hebron BIF (Fig. 1g). While the decomposition of Fe-carbonates\nwould produce graphite associated with magnetite, as seen in the Saglek marble specimen (Fig. 4),\nthis interpretation is inconsistent with the absence of magnetite + graphite association in the Saglek-\nHebron BIF. We clarified this on lines 212-213.\nFig2: I suggest changing the color of the fluid inclusion bar in subfigure O to something else (grey\nperhaps, to match Fig3). At first glance at the Fig2, the reader is lead to think that the blue colors in\nthe Raman maps indicates Fluid inclusions.\nAnswer: We thank R2 for this constructive suggestion. We have changed the color of the fluid\ninclusion bar to grey.\nSupplemental information:\nFigS1: Maybe add labels on b, c and d or arrows pointing into a\nAnswer: We have added labels based on your suggestion.\nFigS3: What is what here? It is not clear to me why and you can distinguish between fluid inclusions\nand graphite inclusions based on this figure. And the Gra I, GraII etc.? Maybe indicate G and D\npeak as well (or lack off).\nAnswer: We understand the possible confusion here. Optical microscopy and Raman spectroscopy\nwere combined to distinguish fluid inclusions from graphite in this study. To make this clearer, we\nhave indicated Gra, Gra , Gra , G and D bands in the revised figure.\nI II III\nFigS5: Scales are impossible to see and the use of cyan in text makes it unreadable.\nAnswer: We have changed the colour of scales to black, and replaced cyan with red in text to make\nit more readable.\nFigS6: Line 107: Why do you want to make this statement here?\nAnswer: We agree that this interpretive statement should be in the discussion. We have deleted it\nfrom this caption.\nFigS8: What is the scale on a) and b). Again Cyan text on white background is not working well.\nWas the Raman spectra recorded on this particular sample? I guess they where extracted from h) and i)\nAnswer: We have corrected them to be more readable, and clarified the Raman spectra were indeed\nextracted from h) and i).\nResponse to Reviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part\nof the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts.\nAnswer: Thank you for your contribution to this peer-review process."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I thank the authors for improving their manuscript. The authors have addressed most of my\ncomments, and I recommend the paper for publication, provided they address a remaining concern\nabout the Raman data.\nI have no doubt that CH4 is associated with CO2 in some of the fluid inclusions. However, I remain\nskeptical about the attribution of the \"peak\" at ~2915 cm-1 to CH4 when associated with graphitic\ncarbon: the \"peak\" attributed to CH4 in Fig. 2c (and Supp. Fig. 3l,n,p) is confused with noise. The\nsignal-to-noise ratio is too low to support the claim that CH4 is found associated with graphitic\ncarbon. This should be removed from the figures mentioned above and from the text.", "reviewer_2": "We are good with general changes made to the manuscript. However, one change appear to be a bit\noff:\nThe sentence starting line 213:\nhowever, the lack of the petrographic type of graphite associated with magnetite clearly excludes\nthis reaction in the Saglek-Hebron BIF.\nGot us confused. We suggest it gets reformulated. Maybe something like:\nhowever, the lack of the petrographic association of the type of graphite with magnetite clearly\nexcludes this reaction in the Saglek-Hebron BIF.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part\nof the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "All of us would like to thank the editors and reviewers for your considerations about this manuscript\nand constructive comments. We have revised it very carefully based on all of the comments. Detailed\nresponses are provided in a point-by-point fashion as follows.\nResponse to Reviewer #1 (Remarks to the Author):\nI thank the authors for improving their manuscript. The authors have addressed most of my\ncomments, and I recommend the paper for publication, provided they address a remaining concern\nabout the Raman data.\nI have no doubt that CH is associated with CO in some of the fluid inclusions. However, I remain\n4 2\nskeptical about the attribution of the \"peak\" at ~2915 cm-1 to CH when associated with graphitic\n4\ncarbon: the \"peak\" attributed to CH in Fig. 2c (and Supp. Fig. 3l,n,p) is confused with noise. The\n4\nsignal-to-noise ratio is too low to support the claim that CH is found associated with graphitic\n4\ncarbon. This should be removed from the figures mentioned above and from the text.\nAnswer: Thank you for recommending to publish our paper. Based on your suggestion, we have\nrevised this Raman identification of CH associated with graphitic carbon. We would argue however\n4\nthat the narrow peak at ~2915 cm-1 should be derived from CH in fluid inclusions rather than noise,\n4\nbecause 1) graphitic carbons do not have the narrow peak at ~2915 cm-1, however they have this\nnarrow peak only when associated with fluid inclusion having strong CO peaks, and 2) such a CH\n2 4\npeak associated with graphitic carbons has also been identified in early Archean gneiss from Akilia\n(Lepland et al., 2011) and in eclogite from Western Tianshan, China (Zhang et al., 2023).\n Lepland, A. et al. Fluid-deposited graphite and its geobiological implications in early Archean\ngneiss from Akilia, Greenland. Geobiology 9, 2–9 (2011).\n Zhang, L. et al. Massive abiotic methane production in eclogite during cold subduction. Natl.\nSci. Rev. 10, nwac207 (2023).\nResponse to Reviewer #2 (Remarks to the Author):\nWe are good with general changes made to the manuscript. However, one change appear to be a bit\noff:\nThe sentence starting line 213:\nhowever, the lack of the petrographic type of graphite associated with magnetite clearly excludes\nthis reaction in the Saglek-Hebron BIF.\nGot us confused. We suggest it gets reformulated. Maybe something like:\nhowever, the lack of the petrographic association of the type of graphite with magnetite clearly\nexcludes this reaction in the Saglek-Hebron BIF.\nAnswer: Based on your suggestion, we have reformulated this sentence in the revised paper.\nResponse to Reviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part\nof the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts.\nAnswer: Thank you for your contribution to this peer-review process."}]

Decision letter