# Microstructure-specific carbon isotopic signatures of organic matter from ~3.5 Ga cherts of the Pilbara Craton support a biologic origin.

Navot Morag, Kenneth H. Williford, Kouki Kitajima, Pascal Philippot, Martin J. van Kranendonk, Kevin Lepot, Christophe Thomazo, John W. Valley

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► To cite this version:

Navot Morag, Kenneth H. Williford, Kouki Kitajima, Pascal Philippot, Martin J. van Kranendonk, et al.. Microstructure-specific carbon isotopic signatures of organic matter from ~3.5 Ga cherts of the Pilbara Craton support a biologic origin.. Precambrian Research, 2016, 275, pp.429-449. 10.1016/j.precamres.2016.01.014. hal-01297306

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8 Microstructure-specific carbon isotopic signatures of organic

9 matter from ~3.5 Ga cherts of the Pilbara Craton support a

10 biologic origin

11 Navot Morag¹*, Kenneth H. Williford¹,², Kouki Kitajima¹, Pascal Philippot³, Martin J. Van

12 Kranendonk⁴, Kevin Lepot¹,⁵, Christophe Thomazo³,⁶, John W. Valley¹

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14 1. NASA Astrobiology Institute, WiscSIMS, Department of Geoscience, University of Wisconsin, 1215 W. Dayton
15 St., Madison, WI 53706, USA.

16 2. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.

17 3. Institut de Physique du Globe de Paris, CNRS and Université Denis Diderot, 4 place Jussieu, 75005 Paris cedex, 18 France.

19 4. School of Biological, Earth and Environmental Sciences, and Australian Centre for Astrobiology, University of
20 New South Wales Australia, Kensington, NSW 2052 Australia.

21 5. Laboratoire Géosystèmes, Université Lille 1, CNRS UMR8217, 59655 Villeneuve d'Ascq, France.

22 6. UMR CNRS/uB5561 Biogéosciences UFR Sciences Vie Terre Environnement, Université de Bourgogne, 6 Bd.
23 Gabriel, 21000 Dijon, France.

24 * - Corresponding author. E-mail address: morag@wisc.edu
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Abstract

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The ~3.5 Ga Dresser Formation from the North Pole Dome of the Pilbara Craton (Western Australia) contains some of the oldest evidence for life on Earth. Here, we present a detailed study of microstructure-specific carbon isotopic composition of organic matter (OM) preserved in Dresser Formation bedded cherts and hydrothermal chert vein using *in situ* Secondary-Ion Mass Spectrometry (SIMS). The OM in these rocks occurs mainly as clots that, together with minor fine OM layers and laminae, are considered primary textures formed prior to host rock lithification. Other than rare OM-rich stylolites, no evidence was found for later OM migration beyond the micrometer scale. Average δ¹³C(OM) values in specific microstructural types range between -33.6‰ and -25.7‰. No correlation is seen between measured δ¹³C values and H/C ratios in the studied OM microstructures. This lack of correlation and the low metamorphic grade of the rocks studied argue against significant modification of OM isotopic composition by later metamorphic alteration. It is thus concluded that the range of δ¹³C values found in the samples represents primary OM isotopic variability. Within some individual samples variable δ¹³C(OM) values are correlated with specific microstructural types. This observation is not consistent with solely abiotic OM formation via Fisher-Tropsch type reactions. When compared with associated δ¹³C(ankerite) values, average δ¹³C(OM) values indicate C isotopic fractionation [Δ¹³C(Ank-OM)] of 25–33‰, which translates to dissolved CO₂-OM isotopic fractionation [Δ¹³C(CO₂-OM)] of 20–30‰. This range of Δ¹³C(CO₂-OM) is consistent with enzymatic C fixation via the Calvin cycle utilized by photoautotrophs and the reductive acetyl-CoA pathway utilized by chemolithoautotrophs. Photosynthetic OM formation is supported by the relatively shallow water
depth inferred for the Dresser environment and the restricted occurrence of stromatolites to
shallow water deposits in this unit, whereas chemolithosynthesis is supported by the abundance
of OM in sub-seafloor hydrothermal chert veins. The range of δ¹³C(OM) values observed in the
samples may therefore represent the remains of different organisms utilizing different C-fixation
pathways. Other biologic effects, such as the growth rate and density of microbial communities,
and further heterotrophic overprinting of the autotrophic biomass may have also contributed to
the observed range of δ¹³C(OM) values.

## Keywords

Dresser Formation, organic matter, carbon isotope ratio, SIMS, microfossils, Pilbara craton

## 1. Introduction

The North Pole Dome of the Pilbara Craton in Western Australia is well-known as the site of
Earth's oldest putative stromatolites and microfossils occurring in ca. 3.5 Ga bedded chert-barite
deposits of the Dresser Formation of the Warrawoona Group (Awramik et al., 1983; Walter et
al., 1980). Although the biogenicity of these early structures have been debated (Buick, 1984,
1990; Buick et al., 1981), there is ample evidence in support of the existence of an early
biosphere in the Dresser Formation, including the link between alleged biogenic structures and
organic matter (OM) distribution (Glikson et al., 2008; Harris et al., 2009; Ueno et al., 2004; Van
Kranendonk, 2011; Van Kranendonk et al., 2008), the isotopic signature of sulfur in microscopic
sulfides, which suggest sulfur-based metabolism (Philippot et al., 2007; Shen et al., 2001; Ueno
et al., 2008), and the carbon isotopic composition of methane in fluid inclusions, which is similar
to that of microbially produced methane (Ueno et al., 2006). One of the prominent features of the
Dresser Formation, and the Warrawoona Group in general, is the common occurrence of black-
gray OM-bearing cherts. These cherts occur both as layered cherts interbedded within the
stratigraphic succession and as hydrothermal veins – sometimes referred to as ‘silica dikes’,
which cut across the local stratigraphy before truncating abruptly at specific overlying
stratigraphic horizons, interpreted as the fossil seafloor, where they usually interfinger with the
bedded cherts (e.g. Lindsay et al., 2005; Nijman et al., 1999; Van Kranendonk, 2006; Van
Kranendonk et al., 2008). The source of OM contained in these cherts is enigmatic and may have
significant implications regarding the emergence of early Archean biosphere – is it biogenic, and
thus indicative of early life (e.g. Ueno et al., 2004), or abiogenic, and perhaps serving as a source
of prebiotic compounds that led to the appearance of life (e.g. Lindsay et al., 2005)? Previous
studies have shown that OM in the chert veins and bedded cherts is low in δ¹³C in a manner
comparable to that of organic carbon fixed by autotrophic organisms (Hayes et al., 1983; Pinti et
al., 2009a; Ueno et al., 2001; Ueno et al., 2004). However, some studies have argued that similar
low values of δ¹³C may form during the production of organic compounds by abiotic processes
similar to the industrial Fischer-Tropsch synthesis and thus concluded that the isotopic
composition of the OM by itself does not provide conclusive evidence for a biogenic origin
(Brasier et al., 2002; Lindsay et al., 2005; McCollom and Seewald, 2006).

OM in the Dresser Formation bedded cherts and associated hydrothermal chert veins shows
variable bulk (mm- to cm-scale) δ¹³C values, between -38 and -29‰ VPDB, interpreted as
resulting from varying degrees of post-depositional metasomatic alteration (oxidation) (Ueno et
al., 2004) or hydrothermal/ metamorphic alteration (Pinti et al., 2009a). A detailed study of OM
preserved in these rocks using organic petrology and scanning and transmission electron
microscopy has revealed the occurrence of several morphologically-distinct OM populations,
indicating potentially different sources and/or subsequent degradation/maturation processes (Glikson et al., 2008). These findings demonstrate the inherent difficulty in interpretation of bulk OM data and the need to consider the petrographic context of the carbonaceous microstructures.

*In situ* (micron-scale) C-isotope analysis using Secondary-Ion Mass Spectrometry (SIMS) may provide further important data about the origin of OM since it is capable of distinguishing between different generations of OM in the cherts, whose unique isotopic composition may be averaged or completely obscured by conventional bulk analysis. Hitherto, *in situ* C isotope analyses of OM in the Warrawoona Group have targeted only rare carbonaceous filaments, which may represent fossil bacteria, and associated carbonaceous clots in two chert vein samples (Ueno et al., 2001). Furthermore, these analyses were standardized against a sample of crystalline graphite, which differs from the kerogenous OM found in the cherts, creating differences in the instrumental mass bias and thus potential errors in analysis (House et al., 2000; McKeegan et al., 1985; Williford et al., 2013).

This study presents a new, comprehensive data set of kerogen-standardized *in situ* SIMS δ¹³C analyses of specific OM microstructures and associated carbonates for unweathered samples recovered from the PDP2b and 2c drillcores (Van Kranendonk et al., 2008) of the lowermost chert-barite unit of the ~3.5 Ga Dresser Formation, which represent Earth's oldest proposed fossiliferous sedimentary rocks (Ueno et al., 2001; Van Kranendonk, 2011; Van Kranendonk et al., 2008; Walter et al., 1980). Our data provide new insights as to the origin, migration and alteration of OM preserved in these rocks.
## 2. Geologic setting and sampling

The Dresser Formation is a package of interbedded chert-barite units and pillow basalts within the lower part of the Warrawoona Group of the Pilbara Supergroup (Van Kranendonk et al., 2007). Exposed only in the North Pole Dome, the Dresser Formation is preserved as a ring of hills, about 14 km in diameter, centered around the younger North Pole Monzogranite that was emplaced into the core of the dome at ~3.46 Ga (Van Kranendonk et al., 2008) (Fig.1). The potentially fossiliferous, bedded chert-barite unit at the base of the Dresser Formation varies between 4 and 60 m in thickness (Van Kranendonk et al., 2008). It is composed of predominantly bedded chert, thick units of coarsely crystalline barite, conglomerate, sandstone and carbonates (Buick and Dunlop, 1990; Hickman, 1983; Nijman et al., 1999; Van Kranendonk, 2006; Van Kranendonk et al., 2008). The bedded chert-barite unit is underlain by pervasively hydrothermally altered komatiitic basalt transected by numerous veins of black-gray chert, varying up to ~20 meters wide and up to 2 kilometers long, which are thought to represent conduits of hydrothermal circulation (Isozaki et al., 1997; Nijman et al., 1999; Ueno et al., 2004; Van Kranendonk, 2006).

Two Pb–Pb model ages of ~3.49 Ga on galena in barite were previously cited as the age of the formation (Thorpe et al., 1992a), but recent U–Pb zircon dating of a felsic volcaniclastic sandstone from the top of the lowermost chert unit of the formation has yielded a maximum depositional age of 3.481 ± 0.002 Ga, which is interpreted to represent the age of deposition of this unit (Van Kranendonk et al., 2008). <sup>147</sup>Sm–<sup>143</sup>Nd data from hydrothermally-altered metabasalts and basaltic metakomatiites, stratiform barite and black chert veins (from the studied PDP drill cores) yielded an isochron age of 3.49 ± 0.10 Ga, confirming the U-Pb and Pb-Pb ages
(Tessalina et al., 2010). Overlying volcanics and related intrusions in the North Pole Dome have been dated at between 3.46 and 3.44 Ga (Amelin et al., 2000; Thorpe et al., 1992b).

Regional studies of metamorphic assemblages in the North Pole metabasalts show a locally variable pattern of metamorphic overprint. Peak metamorphic temperatures range from ~150°C up to ~350°C and show a general repeated pattern of increasing temperature with stratigraphic depth within the sequences of basalt-chert packages that comprise the North Pole Dome section (Kitajima et al., 2001; Terabayashi et al., 2003). However, in places the metamorphic conditions also vary considerably horizontally, within the sedimentary beds and basalt flows in relation with the occurrence of hydrothermal veins. This style of metamorphism was interpreted to result from repeated episodes of hydrothermal circulation within volcanic packages separated by silicified sediments that acted as aquacludes to hydrothermal fluid circulation (Van Kranendonk, 2006).

The Raman spectra acquired for the OM in different OM-bearing chert units of the North Pole Dome are generally similar to those of kerogen from lower greenschist facies metasedimentary rocks (Marshall et al., 2012; Ueno et al., 2004; Wacey et al., 2011b), although lower-temperature hydrothermal conditions of ~100 – 300°C have been reported from the study of mineral assemblages (kaolinite-quartz: Van Kranendonk, 2006; Van Kranendonk and Pirajno, 2004) and fluid inclusions in cherts and adjacent metabasalts (Foriel et al., 2004; Harris et al., 2009).

The samples studied here come from the drill cores of the Pilbara Drilling Project site #2 (PDP2) collected in 2004 as part of a scientific collaboration between the Institut de Physique du Globe de Paris and the Geological Survey of Western Australia (for details see Van Kranendonk et al., 2008). Six samples were selected from the bedded chert-carbonate member 6 of the PDP2b
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and PDP2c drill cores and one sample was selected from a dark-gray chert vein transecting the
altered komatiitic basalt below the bedded chert-barite unit in drill core PDP2c (Fig. 2).

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**3. Methods**

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**3.1. Microscopy**

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Approximately 100-µm thick, doubly polished thin sections were prepared from each sample.
After preliminary optical inspection under a microscope using transmitted light the sections were
detached from the carrier glass and fragments (1-3 mm wide) of selected areas were broken off,
embedded in circular 25.4 mm diameter epoxy mounts, and ground and repolished to expose OM
features of interest. The OM microstructures to be analyzed in these mounts were selected,
studied and imaged using a Nikon Eclipse E600 microscope under transmitted light and reflected
light. Scanning electron microscope (SEM) images were acquired in backscattered electron
(BSE) mode on thinly gold-coated (~5 nm) samples using a Hitachi S3400 SEM operated at 10-
15 kV and a working distance of 10 mm. BSE images were recorded before and after SIMS
analyses.

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**3.2. Bulk carbon isotope analysis**

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Bulk rock samples were powdered to <60 µm using a zirconium oxide crusher. Powders were
homogenized, sieved and aliquots were used for organic carbon isotope analyses.

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Total organic carbon content (TOC) and organic carbon isotope composition [δ¹³C(OM)]
were measured on carbonate-free residues. Sample powders were rinsed by dichloromethane-
ethanol (9:1) to remove any modern organic contaminant and reacted with HCl (6N) at room
temperature overnight followed by 4 hours at 80°C to remove carbonates. Residues were rinsed
with deionized distilled water to a neutral pH, centrifuged (3500 rpm for 5 minutes), and dried at 60°C overnight. Aliquots of dried decarbonated samples (100 – 200 mg) were then sealed in evacuated quartz tubes together with CuO wires and combusted at 950°C for 6 hours. The produced CO₂ was purified in a vacuum line and yield was measured manometrically, allowing TOC determination with a reproducibility better than ± 0.03 wt.% (1σ). Carbon isotope composition of the purified CO₂ gas was measured on a delta+XP mass spectrometer at the Institut de Physique du Globe de Paris with a precision better than ± 0.4 ‰ (1σ) based on duplicate analyses of samples and standards. Results are reported in the delta notation in ‰, relative to V-PDB (Sup. data Table S1).

### 3.3. SIMS carbon isotope analysis

SIMS analyses of carbon isotope ratios were performed on selected OM microstructures identified in the thin sections studied. Prior to analysis, a ~200 nm gold coat was applied to facilitate charge compensation during SIMS analysis, and all samples were degassed at 10⁻⁹ torr in the SIMS air-lock sample chamber for >12 hours. Analyses were carried out using a CAMECA IMS 1280 at the WiscSIMS laboratory, UW-Madison, following the procedure described in Williford et al. (2013) and Lepot et al. (2013). A ¹³³Cs⁺ primary ion beam with 20 kV total impact voltage and ~2.5 nA intensity was focused into an ellipse of ~15 µm maximum dimension at the sample surface. Secondary ions were counted simultaneously using the following detector setup: ¹²C⁻ [(Faraday cup (FC) at the L'2 position with a 10¹¹ Ω resistor, mass resolution M/ΔM = 2200], ¹³C⁻ [axial electron multiplier (EM), M/ΔM = 4000], and ¹³CH⁻ (EM at the H2 position, M/ΔM = 2200). DTFA (first deflector of transfer optics for centering secondary ions in the field aperture) X and Y parameters were adjusted by rastering the primary
beam over a 25 × 25 µm area near each target area and applying the dynamic transfer plate voltage synchronized with primary beam rastering to obtain stable secondary ion intensity. Each analysis consisted of 70 s of presputtering and a total counting time of 160 s (80 measurement cycles of 2 s each). Values of δ<sup>13</sup>C<sub>Raw</sub> (in ‰) represent the <sup>13</sup>C/<sup>12</sup>C isotopic ratio measured by SIMS reported against the value for VPDB before correction for instrumental bias.

Standardization of measurements is critical for establishing the accuracy of SIMS analyses, a problem arising chiefly from systematic bias that occurs during analysis. This instrumental bias can be measured and corrected by bracketing sample analyses with analyses of a working standard of known isotopic composition and with a chemical composition and matrix similar to that of the sample (e.g. Eiler et al., 1997; Kita et al., 2009). In certain crystalline materials, the crystal structure and orientation can also affect bias (Huberty et al., 2010; Kozdon et al., 2010).

In the case of SIMS carbon isotope analysis, the bias for graphite is different than that for amorphous carbonaceous matter such as kerogen or coal (by as much as 10‰; House et al., 2000). Moreover, an orientation effect of several per mil has been observed in C isotopic measurements of individual graphite crystals in the WiscSIMS laboratory (T. Ushikubo, unpublished data). A strong systematic correlation between SIMS instrumental bias and H/C ratio was also reported within two suites of natural organic carbon samples analyzed in two different laboratories (Sangély et al., 2005; Williford et al., 2015). For this study we used an epoxy-embedded chip of the carbonaceous chert sample PPRG-215 (Walter et al., 1983), the same sample used as a working standard by House et al. (2000), Williford et al. (2013) and Lepot et al. (2013) to measure carbon isotope compositions of individual Precambrian microfossils.

Each set of ~12 sample analyses was bracketed by eight analyses of the PPRG-215 standard (Sup. data Table S2). Instrumental bias (α<sub>SIMS</sub>) was calculated by comparing the average δ<sup>13</sup>C<sub>Raw</sub>
for the eight bracketing analyses of PPRG-215 to the published value of -31.5‰ (VPDB; Hayes et al., 1983) using the following equation (see also Kita et al., 2009):

$$ \alpha_{SIMS} = \frac{1 + (\delta^{13}C_{Raw} / 1000)}{1 + (-31.5 / 1000)} \quad (1) $$

This factor, $\alpha_{SIMS}$, was then used to correct raw values from sample analyses using the following equation:

$$ \delta^{13}C_{VPDB} = [(1 + \delta^{13}C_{Raw} / 1000) / \alpha_{SIMS} - 1] \times 1000 \quad (2) $$

The analytical error for each OM sample analysis was calculated as the quadratic addition of the internal precision, calculated as two standard errors (2SE) on the sample's 80 measurement cycles, and the external precision, calculated as the two standard deviations (2SD) on the eight bracketing standard analyses.

The $^{12}$C count rates for 70 analyses of the PPRG-215 standard ranged between $2.4 \times 10^6$ and $18.5 \times 10^6$ counts per second (cps) (Sup. data Table S2). Analyses with low carbon concentrations ($^{12}$C count rates < $1 \times 10^6$ cps ≈ 10% of the average count rates for bracketing standards) were rejected, as internal precision decreases significantly relative to external precision (reproducibility of bracketing standards) at such low $^{12}$C count rates, largely due to Poisson counting statistics (Williford et al., 2013). No correlation was found between $^{12}$C cps and $\delta^{13}C_{Raw}$ values in the studied OM samples (Sup. data Fig. S1).

In some of the studied samples, coexisting carbonates [ankerites; 15–28 mol. % Fe (percentage of Fe from total cations – Ca, Sr, Mg, Fe, Mn), Fe/(Fe+Mg) =0.33–0.61] were analyzed during the organic carbon analytical session using similar instrumental settings. The cation composition within the dolomite-ankerite solid solutions introduce systematic differences in bias of the measured C isotope ratios, sometimes referred to as the matrix effect (Valley and
Kita, 2009). In order to correct for this matrix effect, we applied a correction scheme based on the correlation of matrix-induced bias and Fe content in carbonates within a set of three carbonate standards. Carbonate analyses were bracketed by eight analyses of the UWC-3 calcite standard. Two secondary standards of dolomite, UW6220 [0.1 mol. % Fe, Fe/(Fe+Mg) =0.002], and ankerite, UWAnk-1 [24.6 mol. % Fe, Fe/(Fe+Mg) =0.52], were analyzed at the end of the session. A calibration curve for Fe content was fitted by Hill equation using the UW6220 and UWAnk-1 standards following the method described by Śliwiński et al. (in press). The matrix dependent bias was calculated for each carbonate analysis pit using this calibration curve normalized to the average αSIMS attained for the eight bracketing analyses of UWC-3, and the crystal's specific cation composition measured adjacent to each SIMS analysis pit by electron microprobe (EMP; Sup. data Table S3).

## 4. Results

### 4.1. OM microstructures

Several different types of OM microstructures were identified and classified within the studied samples. Some of the samples contained only one type of OM microstructure, whereas others included a variety of OM microstructures. The following is a description of individual samples and the OM microstructures found within them.

* Sample 84.4 (PDP2b) is a bedded chert, perhaps representing a silicified carbonate unit, comprising alternating light and dark-colored layers that differ in their OM content (Fig. 3a). Up to ~5 vol. % carbonate is present as isolated euhedral to subhedral crystals of dolomite-ankerite. OM in this sample occurs as: (1) Discrete clots of amorphous shape and relatively small size, typically < 50 µm, comprised of nano to microscale granular OM distributed
mainly along the grain boundaries of micro-quartz crystals, 1-10 µm in diameter. The clots
occur both in the darker, OM-rich beds, together with dispersed layered OM (see below) and
in the lighter-colored beds, which are otherwise depleted in OM (Fig. 3a). Some of the clots
in the OM-depleted beds display irregular, yet more distinct shapes with relatively
concentrated thread-like OM domains. Based on these observations the clots in this sample
were divided into OM-rich beds 'Type-1' clots and OM-poor beds 'Type-2' clots (Fig. 4a
and b, respectively). (2) Dispersed layered OM that comprises the darker OM-rich layers.
Sub-micrometer scale OM in these layers is sparsely distributed mostly along micro-quartz
grain boundaries (Fig. 4c). (3) OM concentrated along serrated fronts that run more or less
parallel to the bedding plains and resemble stylolitic fronts (Fig. 3a). These stylolitic fronts
usually separate light OM-depleted layers from darker OM-rich layers and probably
represent migration and concentration of OM during primary carbonate dissolution. The OM
along these stylolites occurs as lumps of concentrated OM of variable size (Fig. 4d). Along
some of the well-developed fronts OM lumps may reach relatively large size up to ~30 µm
(Sup. data Fig. S2). The most massive lumps were not targeted in this study as they were too
concentrated to be analyzed under the instrument analytical settings (i.e. the use of EM
detectors for measurements of ¹³C and ¹³CH intensity), but they appear to be texturally
similar to smaller (< 5 µm) lumps that were analyzed. Stylolites are often accompanied by
chlorite (Fig. 4d). (4) OM filling rare small micron-scale cracks/fissures (Sup. data Fig. S3;
a10_1b). (5) OM enclosed in the core of euhedral carbonate rhombs (Sup. data Fig. S2). This
form of OM was not analyzed in this study due to the difficulty in distinguishing the C signal
of OM from that of the host carbonate.
287 • **Sample 84.5 (PDP2b)** is a bedded chert/silicified carbonate comprised of two main beds, a dark-colored OM-rich bed overlaying a light-colored bed devoid of OM. The two beds are separated by a dark well-developed stylolite front containing massive chlorite (Fig. 3b). Rare (< 5 vol. %) ankerite [~25 mol. % Fe, Fe/(Fe+Mg) =0.53] is present as isolated euhedral crystals. OM is present as: (1) Widespread discrete clots of amorphous shape and variable size, typically 50-200 µm, comprising a dense network of nano- to micro-granular OM distributed mainly along micro-quartz grain boundaries (Fig. 4e). The clots occur only in the darker OM-rich bed. (2) OM concentrated along the stylolitic front (Fig. 3b; Sup. data Fig. S2). OM in stylolites was not analyzed in this sample as it resided in massive chlorite matrix, whose effect on the instrument bias cannot be corrected, and was too concentrated to be analyzed under the instrument analytical settings.

298 • **Sample 84.6 (PDP2b)** is a bedded chert/silicified carbonate (Fig. 3c) containing < 5 vol. % carbonates, which occur as euhedral crystals of dolomite-ankerite, containing small (< 10 µm) inclusions of siderite. OM in this sample occurs as discrete clots of relatively concentrated OM of amorphous shape and variable size, typically 50-100 µm in length, comprised of nano-to micro-granular OM (Fig. 4g). Some nano-granular OM is distributed along micro-quartz grain boundaries. A stylolitic front within this sample is marked by the presence of chlorite, however, no OM concentration is seen along this front and it does not separate OM-rich and OM-depleted regions as in other samples (Fig. 3c; Sup. data Fig. S3).

306 • **Sample 84.7 (PDP2b)** is a dark-colored, very OM-rich, bedded chert/silicified carbonate (Fig. 3d). The rock contains 5-10 vol. % carbonate, which occurs as large, mostly subhedral to anhedral crystals of ankerite [15-28 mol. % Fe, Fe/(Fe+Mg) =0.33-0.61]. Some of the carbonates are partly altered to Fe-oxides and chlorite. Rare sulfides (pyrite, chalcopyrite) are
also present as isolated euhedral crystals. OM microstructures in this sample include: (1) Clots of relatively concentrated OM of amorphous shape and variable size, up to ~500 µm long, comprised of nano- to micro-granular OM. The clots appear to be faintly laminated, with their elongated axis mostly parallel to the main bedding. (2) Laminated OM, which appears as fine laminae, typically thinner than 50 µm, of amorphous OM surrounding grains of silicified clastic (clay/silt-sized) material (Fig. 3d). The clots and laminae occur at separate beds, although the transition between those beds is not well defined. The laminae microstructures may resemble the network-forming laminae described in the Dresser Formation by Noffke et al. (2013), who interpreted them to be typical of microbial mat growth. The OM in the clots and laminae is rather similar, comprised of nano- to micro-granular texture (Fig. 4h).

* Sample 93.5a (PDP2c) is a partly silicified bedded carbonate comprised of a bed/vein of coarse-grained dolomite-ankerite partly replaced by chlorite with rare quartz and TiO₂, mostly devoid of OM, and a bed of intensely silicified, finer grained carbonates (Fig. 3e) comprising about 50% microcrystalline quartz and ~50% subhedral dolomite-ankerite with small (< 10 µm) inclusions of siderite. OM occurs in the latter as rare, amorphous clots of OM of relatively small size, < 50 µm, comprised of nano- to micro-scale granular OM (Fig. 4i). The clots occur in both carbonate and quartz matrix, however, in this study we have analyzed δ¹³C only in clots that reside in the quartz matrix because of the difficulty in distinguishing the C signal of OM and that of the host carbonate.

* Sample 93.5b (PDP2c) is a bedded chert/silicified carbonate containing large patches of subhedral carbonates (dolomite-ankerite; ~20-30 vol. %) and smaller rare (~5 vol. %) ankerite rhombs [24-27 mol. % Fe, Fe/(Fe+Mg) =0.53-0.56] scattered in the quartz matrix
(Fig. 3f). Scattered chlorite (~5 vol. %) is also found in the chert matrix. OM in this sample is present as: (1) Discrete clots of relatively concentrated OM of amorphous shape and variable size, typically 50-100 µm, comprised of nano- to micro-granular OM (Fig. 4j). (2) Faint layers of dispersed OM comprised of nanometer scale OM distributed along micro-quartz grain boundaries. The OM clots and layered OM occur mostly within separate beds (Fig. 3f). (3) OM concentrated along rare stylolites, comprised of small, few micron-scale chunks of OM mostly embedded in chlorite matrix (Fig. 4j).

* Sample 111.9 (PDP2c) is a chert vein comprised of dark grey OM-rich chert (Fig. 3g). Up to ~5 vol. % carbonate is present as isolated rhombs of ankerite [23-27 mol. % Fe, Fe/(Fe+Mg) =0.52-0.59]. The OM in this sample occurs as widespread discrete clots of amorphous shape and variable size, typically 50-200 µm, comprised of a network of nano- to micro-granular OM. Some nano-granular OM is distributed along micro-quartz grain boundaries (Fig. 4k).

To summarize, OM in the Dresser Formation chert/silicified carbonate samples studied include six major microstructure types: Clots of amorphous shape and variable size, comprised of nano- to micro-granular OM. The OM clots may be divided into (1) Type-1 clots, which are common in all samples and are of amorphous shape with OM occasionally distributed along micro-quartz grain boundaries (e.g. Fig. 4a, e, g, i-k), and (2) Type-2 clots, which are found only in the OM-poor beds of sample 84.4 and are of more distinct shape, sometimes containing concentrated, thread-like OM domains (e.g. Fig. 4b). (3) Layers of sparsely distributed OM comprised of nanometer-scale OM (e.g. Fig. 4c). (4) Fine laminae of nano- to micro-granular OM (e.g. Fig. 4h). (5) OM concentrated along stylolitic fronts, comprised of variable-size chunks of OM often in association with chlorite (e.g. Fig. 4d). (6) OM enclosed in the core of euhedral carbonate rhombs (e.g. Sup. data Fig. S2). The clots (Type-1) are by far the most common OM
356
357

microstructure occurring in all OM-bearing rocks studied and are sometimes the only OM-
microstructure present.

358

**4.2. Carbon isotope data**

359
360
361
362
363
364
365
366
367
368
369

A total of 97 sample analyses (80 analyses of OM plus 17 analyses of carbonates) and 70 standard analyses (46 analyses of OM plus 24 analyses of carbonates) were performed during the analytical session. The complete SIMS dataset for the studied samples is given in Table 1. For each sample we present the relations between δ¹³C values and raw ¹³CH/¹³C ratios (which serve as a proxy for H/C ratio) of individual OM analysis pits in Figure 5, and the distribution of δ¹³C(OM) values in different OM microstructural types as histograms in Figure 6. The probabilities associated with Student's t-test that two sets of OM δ¹³C values, from different samples and/or different microstructural types, come from populations that have the same mean value are summarized in Table 2, together with the absolute difference between the two sets average δ¹³C(OM) values. The following section is a detailed description of δ¹³C(OM) values in specific OM microstructural types in the different samples.

370
371
372
373
374
375
376
377
378

* 84.4 – Individual spots δ¹³C(OM) values in this sample show an overall range of 13.2‰ (-37.6 to -24.4‰) (Fig. 5a). Values of δ¹³C(OM) in the Type-1 and Type-2 clots show bimodal distribution with average values of -27.6 and -33.6‰, respectively, which are statistically distinct at >99% confidence level. The Type-2 clots also have distinct δ¹³C(OM) values (at >95% confidence level) compared to clots in all other samples, excluding sample 84.5, usually being 5 – 6‰ lower. The layered OM show an intermediate range of δ¹³C values, averaging at -30.1‰. OM in the stylolites yielded higher δ¹³C values, averaging at -25.7‰, which are statistically distinct at >95% confidence level from all other OM microstructures found in this sample (Fig. 6a; Table 2). The bulk OM δ¹³C value of -25.60‰ measured on 18
379 this sample is similar within the error (2SE) to that found for the stylolites OM (Fig. 6a). This
380 observation is consistent with the massive nature of OM lumps along the stylolitic fronts,
381 which likely comprise most of the OM in this sample.

382 • 84.5 – The clots in this sample show an approximately normal distribution of δ¹³C(OM)
383 values with an average of -31.6‰ (Fig. 6b), which is statistically distinct (at >95%
384 confidence level) compared to clots in all other samples, excluding the Type-2 clots in
385 sample 84.4, usually being 3 – 4‰ lower. The average value of these clots is also lower than
386 the bulk OM δ¹³C value of -28.95‰, measured on this sample (Fig. 6b). We note that this
387 sample also contains massive OM lumps within a chlorite matrix along stylolitic fronts,
388 which were not targeted in this study (because of the sample-standard matrix mismatch, as
389 discussed above in section 4.1). This stylolitic OM may have a lower δ¹³C value than
390 associated clots (as seen in sample 84.4) and is more likely to dominate the bulk composition
391 given its massive appearance. One *in situ* analysis of carbonate by SIMS from this sample
392 yielded a δ¹³C value of -1.5±0.7‰.

393 • 84.6 – The clots in this sample yielded an average δ¹³C(OM) value of -28.2‰ (Fig. 6c). One
394 analysis of OM in a stylolite yielded δ¹³C(OM) value of -31.0±2.3‰. However, this value
395 should be treated with caution as the OM in this spot was mostly in chlorite matrix (Sup. data
396 Fig. S3; this is also indicated by higher ¹³CH/¹³C ratio), for which the instrument mass bias
397 correction may not be applicable.

398 • 84.7 – Clots and laminated OM in this sample show overlapping distribution of δ¹³C(OM)
399 values averaging at -27.5 and -26.6‰, respectively, which are statistically indistinct (Fig. 6d;
400 Table 2). The average δ¹³C(OM) of both clots and laminae in this sample, -27.1‰, is slightly
401 lower than the bulk OM δ¹³C value of -24.33‰ measured on this sample (Fig. 6d). Five *in*
402 *situ* analyses of carbonates from this sample yielded relatively homogeneous $\delta^{13}C$ values
403 with an average of 0.0‰. Another carbonate analysis yielded a much lower value of -9.6‰.
404 However, post-analysis BSE imaging of this spot (84.7 a2_3a) indicates that it hit OM
405 enclosed within the carbonate crystal (Sup. data Fig. S3) and was consequently rejected.

406 • 93.5a – The clots in this sample yielded an average $\delta^{13}C(OM)$ value of -28.4‰ (Fig. 6e).

407 • 93.5b – Clots and layered OM in this sample show partly overlapping distribution of
408 $\delta^{13}C(OM)$ values averaging at -27.3 and -30.6‰, respectively, which are statistically distinct
409 at >95% confidence level. $\delta^{13}C(OM)$ values in the stylolites show intermediate distribution
410 with an average of -28.6‰, which is statistically indistinct from those of the clots and the
411 layered OM (Fig. 6f; Table 2). The average $\delta^{13}C(OM)$ of all studied microstructures in sample
412 93.5 (a+b), -28.2‰, is slightly lower than the bulk organic OM $\delta^{13}C$ value of -26.95‰
413 measured on this sample (Fig. 6f). However, the average $\delta^{13}C(OM)$ value of the clots, which
414 contain much more OM compared with the layered OM, -27.5‰, is similar within the error
415 (2SE) to the bulk OM value. Three *in situ* analyses of carbonates from this sample yielded
416 relatively homogeneous $\delta^{13}C$ values with an average of -0.4‰. Another carbonate analysis
417 yielded a notably lower value of -7.5‰. However, post-analysis BSE imaging of this spot
418 (93.5b a2_5c) suggests that it included OM enclosed within the carbonate crystal (Sup. data
419 Fig. S3) and was consequently rejected.

420 • 111.9 – The clots in this sample show a relatively large range of raw $^{13}$CH/$^{13}$C ratios,
421 between 0.082 and 0.119, but no correlation is seen with $\delta^{13}C(OM)$ values (Fig. 5g), which
422 are approximately normally distributed with an average value of -28.3‰. This average
423 value is slightly higher than the bulk OM $\delta^{13}C$ value of -31.05‰, measured in this sample
(Fig. 6g). Six *in situ* analyses of carbonates from this sample yielded relatively homogeneous $\delta^{13}C$ values with an average of -1.0‰.

The total distribution of $\delta^{13}C(OM)$ values from this study is presented in Figure 7 alongside previous $\delta^{13}C(OM)$ values reported in the Dresser and the Strelley Pool formations of the Pilbara Craton. The total range of individual *insitu* $\delta^{13}C(OM)$ values found in this work is between -37.6 and -24.4 (quite interestingly, both the highest and lowest values are from the same sample, 84.4), with a prominent concentration of $\delta^{13}C(OM)$ values between -29 and -26‰ (Fig. 7a). This range of values is wider than that previously reported for the Dresser Formation using bulk OM analysis, -38.3 to -29.4‰ (Pinti et al., 2009a; Ueno et al., 2004), and is shifted toward more positive values. However, it is entirely consistent with the range of bulk OM $\delta^{13}C$ values obtained for the samples (Fig 7e). These differences may thus reflect spatial heterogeneities of OM isotopic compositions within the Dresser Formation. Unlike the samples studied by Ueno et al. (2004), the samples studied here did not show any correlation between TOC content and bulk $\delta^{13}C(OM)$ values (Sup. data Table S1). The range of $\delta^{13}C(OM)$ values found here is also significantly higher compared to previously reported, graphite-standardized SIMS data for the Dresser Formation OM, which range from -42.3 to -30.5‰ (Ueno et al., 2001) (Fig. 7c). The range of $\delta^{13}C(OM)$ values obtained here is generally consistent with that obtained using the same methods and calibration standards for the younger Strelley Pool Formation of the Pilbara Craton (-39.9 - -25.8‰; Lepot et al., 2013), although the Dresser $\delta^{13}C(OM)$ values are skewed toward the lower part of this range whereas the Strelley Pool Formation data show a more even distribution (Fig. 7b). Lepot et al. (2013) carbonaceous chert-standardized SIMS data also show an offset toward higher $\delta^{13}C$ values compared with graphite-standardized SIMS data from the Strelley Pool Formation (-46.0 - -32.9‰; Wacey et al., 2011a) (Fig. 7d). This systematic ~7‰
shift of graphite-standardized OM SIMS data toward lower δ¹³C values is consistent with the
difference in SIMS instrumental bias between graphite (-31‰) and carbonaceous chert (-38‰)
reported by House et al. (2000) and a similar trend of increasingly negative bias with higher OM
H/C ratio has also been shown by Sangély et al. (2005) and Williford et al. (2015). It is therefore
concluded the lower δ¹³C values attained from graphite-standardized OM analyses result from
erroneous bias correction due to sample-standard matrix mismatch, as discussed in section 3.2.

453 **5. Discussion**

454 **5.1. Primary nature of OM**

The OM clots, which occur in all studied samples, are distributed more or less sporadically
within the chert/ silicified carbonate matrix. There is no network of OM seen connecting the
clots and they appear as discrete separated textures. Most of the clots are completely enclosed by
micro-crystalline quartz (chert) matrix and are isolated from any mineral-filled veinlets or
fissures. The occurrence of similar clots in silicified Archean sediments was interpreted as
representing ripped-up microbial mats (Lepot et al., 2013; Tice and Lowe, 2006). Alternatively,
bitumen formed by thermal cracking may also polymerize into similar clotted textures. Such
“pyrobitumen nodules/globules” are thought to form by rapid maturation during hydrothermal
silicification (Buick et al., 1998) or around radioactive mineral grains through radiation-induced
polymerization of migrating hydrocarbons (Buick et al., 1998). The OM clots studied here have
irregular shape, which is very different from the spherical, smooth-surfaced bitumen globules
described in the Warrawoona Group by Buick et al. (1998) and interpreted as oil droplets formed
by thermal maturation of older OM during hydrothermal heating. In fact, these bitumen globules
coexist with kerogen clots similar to those described here, which are interpreted as the (in situ)
469 bitumen source (Buick et al., 1998). The studied clots also lack any radioactive mineral core as
470 seen in the bitumen nodules formed by immobilization of migrating hydrocarbons (Buick et al.,
471 1998). We therefore interpret the OM clots studied here as primary OM microstructures, such as
472 disrupted microbial mats, or OM that polymerized and/or flocculated in the water column and
473 later settled into the sediment. In some samples, the clots occur mostly or solely within defined
474 OM-rich beds (e.g. samples 84.4 and 84.5; Fig. 3a and b). This bedding of OM clots indicates
475 their early entrainment during sediment deposition, providing additional support to their primary
476 nature (see also Marshall et al., 2007). Likewise, the fine layering/lamination presented by the
477 layered OM and OM laminae (e.g. in samples 84.7 and 93.5b; Fig. 3d and f) indicates their
478 sedimentary origin. Hence, these microstructures are also considered primary OM textures.

479 Although the OM clots, layers and laminae are considered to be of primary origin, later
480 modification of their OM during diagenesis and hydrothermal/ metamorphic alteration is still
481 possible, and might have also overprinted the original isotopic signature. In the following
482 sections we will first evaluate the potential effect of these later processes on the original OM
483 isotopic composition, and then discuss in detail the potential origin(s) of primary OM.

484 **5.2. Alteration of OM**

485 Hydrothermal alteration played an important role during metamorphic overprinting of the
486 North Pole metabasalts and sedimentary sequence. However, hydrothermal activity is also
487 closely associated with the early stages of deposition of the Dresser Formation bedded chert-
488 barite unit and precipitation of the chert veins. Under these complex circumstances it may be
489 difficult to distinguish early alteration processes taking place during or soon after OM formation,
490 under the same chemical and physical conditions, from later alterations in response to recurring
491 hydrothermal/metamorphic events. Alteration of OM during hydrothermal/metamorphic events
492

could result in modification of its isotopic composition via three main mechanisms: (1)

493

preferential loss of isotopically light carbon compounds during thermal maturation (Des Marais,

494

2001; Ueno et al., 2004); (2) isotopic exchange with ¹³C enriched inorganic C, principally from

495

carbonate minerals (Valley, 2001; van Zuilen et al., 2007); and (3) mixing of isotopically

496

variable OM due to OM migration. The possible effects of these processes on the studied

497

samples are discussed below.

498

### 5.2.1. Effect of thermal maturation

499

Thermal maturation of OM during diagenesis and/or metamorphism can change the δ¹³C

500

value of OM by preferential breaking of the ¹²C–¹²C bonds over the ¹²C–¹³C bonds, which results

501

in loss of low-δ¹³C CH₄ forming a ¹³C-enriched residue. During this process, OM will also lose

502

heteroatoms such as H, N, O and S, leading to the formation of inverse correlation between δ¹³C

503

and H/C-ratio in the residual OM (Des Marais, 2001; Ueno et al., 2004; van Zuilen et al., 2007).

504

This inverse correlation is not seen in any of the samples studied (Fig. 5), indicating that the

505

variability of δ¹³C(OM) values in our samples is not the result of variable degrees of thermal

506

alteration. Nevertheless, the Dresser OM is typically mature kerogen and may still have been

507

subjected to (uniform) thermal maturation, as suggested by the Raman spectra obtained for a

508

kerogen clot in one of the studied samples (Sup. data Fig. S4), which is similar to that of OM

509

from Alpine metasedimentary rock metamorphosed at ca. 320°C (Lahfid et al., 2010). Such

510

thermal maturation may have led to the formation of δ¹³C(OM) values that are slightly higher

511

relative to the original isotopic composition by up to 3‰ (Des Marais, 2001).
512 **5.2.2. Effect of isotopic exchange**

513 Isotopic exchange between OM and carbonates mediated by C-bearing fluids could have
514 caused ¹³C enrichment if hydrothermal/metamorphic alteration was sufficiently intense (e.g.

515 Schidlowski et al., 1983; Ueno et al., 2002; Valley and O'Neil, 1981; Wada and Suzuki, 1983).

516 In the studied bedded chert/ carbonate samples, extensive hydrothermal alteration is indicated by
517 the replacement of original sediment by hydrothermal silica and ankerite. This alteration took
518 place at a relatively early stage, during the circulation of hydrothermal fluids that accompanied
519 sediment accumulation (Van Kranendonk et al., 2008). Fluid inclusion data suggest that the
520 bedded cherts at the top of the Dresser Formation were formed from relatively cool aqueous fluid

521 at ~120°C (Harris et al., 2009). Higher temperatures of about 300 – 350°C were recorded in the
522 deeper levels of the hydrothermal system by studies of fluid inclusion and epithermal textures in
523 silica veins (Harris et al., 2009), and the mineral assemblages in underlying greenstone basalts

524 (Terabayashi et al., 2003). Temperatures of about 300 – 350°C are consistent with the Raman
525 spectra obtained for the Dresser OM (Sup. data Fig. S4; Ueno et al., 2004), which are similar to
526 that of OM from lower greenschist facies metasediments (Wopenka and Pasteris, 1993; Yui et

527 al., 1996). While isotopic exchange between organic carbon and carbonate carbon may take
528 place under lower-greenschist facies conditions (Dunn and Valley, 1992), it would have been

529 very sluggish and is unlikely to significantly affect OM isotopic composition, especially in rocks
530 that have undergone earlier silicification, where carbonates where mostly replaced and OM is
531 encased in a quartz matrix. Furthermore, the lack of correlation between the bulk δ¹³C(OM)

532 value and observed carbonate content in the studied samples supports the absence of substantial
533 isotopic exchange in these samples.
534 **5.2.3. OM migration**

535 Displacement of OM during early hydrothermal alteration of the original carbonate sediment
536 or (re-)crystallization of hydrothermal silica may result in accumulation of OM along grain
537 boundaries, veinlets or voids. This type of small-scale migration of OM has been previously
538 described in the Pilbara Apex chert unit, where it was argued that carbonaceous microfossil-like
539 microstructures were produced by the displacement of (abiologic) OM within a matrix of
540 recrystallizing silica (Brasier et al., 2002; Brasier et al., 2005). Distribution of fine-grained OM
541 along microquartz grain boundaries is seen in most of the samples studied, indicating some
542 displacement of OM, probably during silicification of the original carbonate and/or quartz
543 recrystallization. However, this re-distribution of OM is limited to within only a few microns
544 beyond the original clot/layer boundaries. In contrast, OM-rich stylolites suggest OM migration
545 at the mm- to cm-scale. The occurrence of highly concentrated OM along stylolitic fronts in
546 samples 84.4, 84.5 and 93.5b is thought to result from accretion of residual OM following the
547 dissolution of the host carbonate. The occurrence of massive, up to 100 µm thick, chlorite seams
548 along some of the stylolites (Sup. data Fig. S2) suggests their possible relation to later
549 hydrothermal/metamorphic events. However, chlorite can also form during relatively low
550 temperature (130 – 300°C) hydrothermal alteration (e.g. Cathelineau and Nieva, 1985) and in
551 some samples scattered chlorite occurs within the chert matrix (84.6, 93.5b; Sup. data Fig. S3). It
552 is therefore possible that chlorite formed before stylolites and was concentrated along the
553 stylolite fronts during host carbonate dissolution, much like the associated OM. In any case,
554 formation of the stylolites (and chlorite) must have occurred at a relatively early stage, prior to,
555 or during silicification of the rock, since they were formed in mostly carbonate matrix. Stylolite
556 OM in sample 93.5b shows an average δ¹³C value of -28.6‰, intermediate to that of associated
clots (-27.3‰) and layered OM (-30.6‰; Fig. 6f), which can be easily explained by mixture of these components during migration and concentration of OM. However, in sample 84.4 the stylolitic OM, which probably comprises most of the OM in this sample, yielded an average δ¹³C value of -25.7‰ (similar to the bulk OM value of -25.60‰) that is heavier than the average δ¹³C(OM) values of all other studied OM microstructures (Fig. 6a). Therefore, this stylolitic OM isotopic composition cannot be explained by simple mixing of OM from the associated microstructure types. Localized modification of OM along the stylolitic fronts by isotopic exchange with carbonate-bearing fluid formed by enclosing carbonate minerals dissolution (leading to stylolite formation) is possible but suggested to be unlikely due to the relatively low temperatures (< 175°C) experienced by the rocks during early stages of hydrothermal alteration, as discussed above. It is more likely that sediments dissolved during formation of these stylolites included a distinct source of primary OM that had slightly higher δ¹³C values compared to that of the studied microstructures.

## 5.3. Origin of OM

The discussion above has shown that the heterogeneous δ¹³C values observed in our samples are probably not the result of later alteration processes and thus represent the complex nature of the primary OM. Processes that may produce such primary δ¹³C heterogeneities include variable (biotic and abiotic) organic carbon-fixation paths, biological metabolism and separation of different biomolecule classes with distinct biosynthetic fractionations (Lepot et al., 2013).

Previous evidence for a biogenic origin of the Dresser OM is not conclusive. Dresser OM occurs in bedded silicified carbonates as well as hydrothermal chert veins that formed below the sediment-water interface and it lacks bona fide microfossil structures. Furthermore, the Dresser
579 Formation is dated to ~3.5 Ga, which would make its OM one of the earliest known relicts of
580 life, if indeed biogenic, as claimed by a number of previous authors (e.g. Glikson et al., 2008;
581 Ueno et al., 2004). Supportive of a biogenic origin of the Dresser OM are: the occurrence of
582 putative stromatolites and other microbially-induced sedimentary structures in the Dresser rocks
583 (Noffke et al., 2013; Van Kranendonk et al., 2008), including significant morphological and
584 habitat variations that may indicate diverse communities (Van Kranendonk, 2011); the isotopic
585 signature of sulfur in microscopic sulfide grains (Philippot et al., 2007; Shen et al., 2001; Ueno et
586 al., 2008), which suggest a sulfur-based metabolism for at least some microbes; the relatively
587 low δ¹³C values reported for the Dresser OM (Pinti et al., 2009a; Ueno et al., 2004), which are
588 similar to δ¹³C values in OM from younger sedimentary rocks of certain biological origin, as
589 well as the magnitude of carbon isotope fractionation between OM and carbonate, which is
590 consistent with that found in modern autotrophic organisms (Ueno et al., 2004 and references
591 therein).

592 In the following subsections we will examine the potential origins of OM in the Dresser
593 Formation and the importance of later heterotrophic degradation processes using its texture-
594 specific carbon isotope composition in conjunction with previous data regarding possible OM
595 formation processes.

596 **5.3.1. Abiologic origin**

597 Reduced carbonaceous compounds can form abiotically in the natural environment during
598 metamorphic processes such as thermal decomposition of siderite (McCollom, 2003; van Zuilen
599 et al., 2003), reactions between CO₂ and CH₄, or direct reduction of CO₂ in metamorphic fluids
600 (Luque del Villar et al., 1998; Naraoka et al., 1996). Organic compounds might also form by
601 Fischer-Tropsch type (FTT) reactions during hydrothermal alteration of ultramafic rock (Horita
28
and Berndt, 1999; McCollom and Seewald, 2006). This latter process is of particular interest as it
has been previously invoked as the source for OM in Pilbara cherts (e.g. Brasier et al., 2002;
2005; Lindsay et al., 2005).

The industrial Fischer Tropsch process uses a metal catalyst to convert carbon monoxide and
hydrogen into liquid hydrocarbons. In the geological literature, the term FTT reaction is used
more generally to refer to the hydrogenation of inorganic carbon to form organic compounds,
often without specifying the nature of the carbon source, the catalyst, the medium in which the
reaction occurs or whether the products are volatile or solid (McCollom, 2013). In most cases,
dissolved CO₂ is inferred to be the primary carbon source for abiotic organic synthesis in
geologic systems. The H₂ required for this reaction might be produced by serpentinization when
water circulating through ultramafic rocks forms serpentine and ferrous iron in olivine is altered
to ferric iron in magnetite (e.g. McCollom and Bach, 2009; Sleep et al., 2004). Accordingly, FTT
synthesis was invoked to explain the emissions of CH₄ observed in some serpentinite-related
hydrothermal systems, such as along mid-ocean ridges (e.g. Lost City, Logatchev, and Rainbow
in the Atlantic Ocean; Etiope and Schoell, 2014). As the Dresser cherts were deposited/silicified
by repeated pulses of hydrothermal fluids that circulated through ultramafic komatiitic basalts
(Fig. 2) (Van Kranendonk, 2006), it might be argued that H₂ was present in the hydrothermal
fluids, providing the reductant needed for FTT reactions, as suggested for modern mid-ocean
ridge environments. However, there is no evidence for extensive serpentinization of the
underlying komatiitic basalts, which are completely replaced by a carbonate-white mica–pyrite
assemblage, with leucoxene mantling igneous ilmenite (Van Kranendonk et al., 2008). It thus
seems that the amount of H₂ present in the Dresser hydrothermal fluids was rather limited. In
addition, it is questionable whether appropriate catalysts for FTT reactions existed in the Dresser
hydrothermal system. In experimental studies under hydrothermal conditions, FTT synthesis has been demonstrated only in the presence of native metal (Fe-Ni) alloys (McCollom, 2013 and references therein). Reduced metals are exceedingly rare in the crust of the Earth. While the trace occurrence of native metal alloys (Fe, Ni, Cu, Zn and Sn) was mentioned from the Pilbara Apex chert by Brasier et al. (2002) and by later Pinti et al. (2009b, one occurrence), these metals are not documented and have not been verified by other studies of the Dresser Formation cherts (Kozdon et al., 2010; Ueno et al., 2004; Van Kranendonk et al., 2008; this study) and surrounding metavolcanics (Kitajima et al., 2001; Terabayashi et al., 2003). Likewise, these highly reduced phases were probably unstable under the temperatures, redox conditions and sulfide concentrations represented by the chert vein mineral assemblages (Ueno et al., 2006; Ueno et al., 2004).

Even if FTT reactions could have taken place within the Dresser hydrothermal system, their potential to produce the amount and type of OM observed in the samples of this study is questionable. In modern serpentinization-related hydrothermal systems where abiotic FTT hydrocarbons formation has been suggested, hydrocarbons occur almost exclusively as methane (Charlou et al., 2002; Proskurowski et al., 2008). In the few places where small amounts of heavier hydrocarbons do occur, they usually consist of simple alkanes from ethane to pentane (e.g. Etiope and Schoell, 2014). These light organic fluids are substantially different from the heavy organic solids (kerogen, bitumen) comprising the Dresser OM (Buick et al., 1998; Ueno et al., 2004) (although methane fluid inclusions have also been reported from Dresser hydrothermal chert veins by Ueno et al., 2006). McCollom and Seewald (2006) have suggested that labile organic compounds could have been synthesized deeper in the hydrothermal system (possibly in the presence of unseen metal catalysts) and subsequently transported to their present location at
the upper part of the system, where they have precipitated to form solid organic compounds
following conductive cooling or mixing with seawater. Further thermal maturation during
metamorphism was also suggested to convert the original organic compounds into their present
form (i.e. kerogen). However, we find this scenario inconsistent with the OM distribution,
textures and range of δ¹³C(OM) values reported in this study. Had the OM been formed by
precipitation from hydrothermal fluids we would expect to find it mainly in voids and along
fissures and not distributed as discrete clots or forming layers and laminae within bedded
sediments. Moreover, had the OM been formed entirely by continuous precipitation of simpler
hydrocarbons, we would not expect to see any correlation between δ¹³C(OM) values and specific
OM microstructural types as demonstrated in the studied samples. Thus, the *in situ* carbon
isotope data for OM argue strongly against a completely abiotic process.

### 5.3.2. *Biologic origin*

All of the samples studied contain rhombs of dolomite-ankerite, whose euhedral form, chemical zoning and the occurrence of quartz inclusions indicate growth during sediment silicification (Fig. 4f and l; Sup. data Fig. S3). In three of the samples, including the chert-vein sample, these ankerites show similar average chemical compositions of ~25 mol. % Fe [Fe/(Fe+Mg) = 0.53-0.55, Sup. data Table S3], as well as similar average δ¹³C values of 0 to -1‰ (Table 1; Fig. 8). Assuming isotopic equilibrium between dissolved inorganic carbon (DIC) in the precipitating fluids and the ankerites in the samples, the δ¹³C(Ank) data can be used to estimate the δ¹³C of CO₂(aq) in the hydrothermal fluids for a given temperature. While CO₂(aq) may form only part of the total DIC, we focus on this phase as it is the most available for biogenic fixation. If we can assume that OM was formed in hydrothermal fluid similar to those who have precipitated the ankerite, the δ¹³C(CO₂) value may be used to calculate the C isotope
fractionation between CO₂(aq) and OM [($\Delta^{13}$C(CO₂ – OM)] in the studied samples. We note that
making this estimation is not trivial, even when supported by high-spatial-resolution in situ data.
Ankerite (and quartz) probably replaced original carbonates in most of the bedded samples and
may therefore be late to primary OM formation. Nevertheless, the OM found in the hydrothermal
chert vein sample (111.9) probably formed in the hydrothermal fluid. The OM clots in this
sample are texturally and isotopically similar to OM clots found in all of the other samples
studied suggesting formation in the presence of similar fluids. It was previously suggested that
the shallow seafloor environment, where the Dresser bedded sediments were deposited, was
dominated by fluids vented from the hydrothermal system (e.g. Harris et al., 2009; Van
Kranendonk, 2006; Van Kranendonk and Pirajno, 2004). For the calculation of $\delta^{13}$C(CO₂) value,
we have used the dolomite–CO₂(g) fractionation scheme of Horita (2014) and the CO₂(g) –
CO₂(aq) fractionation factor published by Mook et al. (1974). Assuming temperatures in the
range of 110 – 142°C for the hydrothermal fluids (Harris et al., 2009), CO₂(aq) in equilibrium
with the average ankerite isotopic composition would have $\delta^{13}$C value of between -6 and -4‰.
These values should be treated with caution as the dolomite–CO₂(g) fractionation was calibrated
for dolomite and may differ from that of high-Fe ankerite. However, they provide the best
estimate for $\delta^{13}$C(CO₂) values in the hydrothermal fluids, resulting in $\Delta^{13}$C(CO₂ – OM) of
between 20 and 30‰ for the studied microstructures. This range of isotopic fractionation is
compatible with the range of isotopic fractionation observed in modern autotrophs utilizing the
reductive pentose phosphate (Calvin) cycle (mostly photoautotrophs) or the reductive acetyl-
CoA pathway (mostly chemolithoautotrophs) (House et al., 2003; Pearson, 2010 and references
therein).
Both photosynthesis and chemosynthesis are viable OM-formation processes in the environment represented by the Dresser sediments and veins. The shallow water depth, indicated by the occurrence of ripples, desiccation cracks and the presence of coniform stromatolites (Buick and Dunlop, 1990; Van Kranendonk et al., 2008; Walter et al., 1980), supports the possibility of photoautotrophy, whereas the association with hydrothermal fluids rich in reduced chemical species such as H₂S, Fe⁺² (Harris et al., 2009; Van Kranendonk, 2006; Van Kranendonk and Pirajno, 2004) and possibly H₂ or CH₄ (as discussed above in section 5.3.1) suggest that chemolithoautotrophs could have also thrived in this environment. We note that in the Dresser Formation, OM is particularly abundant within the hydrothermal chert veins, which occur below the sediment surface (Ueno et al., 2004). The possibility that OM formed in the water column was introduced in these veins by simple gravitative accumulation in open fissures, or by precipitation of labile or colloidal (i.e. nanoscale) OM circulating through the hydrothermal system, seems less consistent with the higher concentration of OM in the chert veins compared with the bedded sediments (Ueno et al., 2004; Sup. data Table S1). Therefore, it may be concluded that OM was formed in the hydrothermal veins. In this case, the main OM-formation process was probably chemolithoautotrophic production. This conclusion is consistent with the occurrence of methane with extremely low δ¹³C values ( <- 56‰) in fluid inclusions within the Dresser chert veins that was interpreted as evidence for microbial methanogenesis (Ueno et al., 2006), although the validity of these low δ¹³C (CH₄) values as proof for microbial origin have been debated (Etiope and Sherwood Lollar, 2013; Sherwood Lollar and McCollom, 2006). OM in the chert vein sample (111.9) occurs as clots whose morphological appearance and isotopic composition (average -28.3‰) are similar to those of OM clots occurring in all the bedded chert samples, excluding the Type-2 clots in sample 84.4 and the clots in sample 84.5, which yielded
significantly lower δ¹³C(OM) values (Table 2). This may imply that chemolithoautotrophic production was the main OM formation process also in the bedded cherts. Nevertheless, the occurrence of stromatolites in the Dresser Formation sediments is more consistent with photoautotrophic organisms (Van Kranendonk, 2011; Walter et al., 1980). We also note that the overall range of δ¹³C(OM) values found in the Dresser Formation is similar to that reported for the younger (~3.4 Ga) Strelley Pool Formation of the Pilbara Craton (Lepot et al., 2013) (Fig. 7), which is widely regarded as containing photoautotrophic stromatolites (e.g. Allwood et al., 2007). A simplified model of the earliest-known inhabited environment on Earth may therefore be considered, where H₂S, Fe²⁺ and possibly H₂ and CH₄ are continuously being produced by hydrothermal circulation through the young basaltic crust, providing electron donors for both chemolithoautotrophs and anoxygenic photoautotrophs. A similar scenario has also been proposed for younger (3.2–3.4 Ga) early Archean OM-bearing cherts of the Barberton Greenstone Belt in South Africa (van Zuilen et al., 2007). Accordingly, the heterogeneous δ¹³C(OM) values reported here for some of the Dresser bedded chert/ carbonate samples may be explained by different organisms utilizing different C-fixation paths. Alternatively, differences in δ¹³C(OM) values may represent changes in Δ¹³C(CO₂-OM) arising from changes in the availability of CO₂ for the autotrophic organisms. Rapid growth of microbial communities in conjunction with restricted permeability in the sediments, or pulsed fluid flow in hydrothermal conduits may limit the amount of CO₂ in these environments. This may result in consumption of a larger fraction of the dissolved CO₂ leading to smaller Δ¹³C(CO₂-OM) values (e.g. Deuser et al., 1968; Calder and Parker, 1973). Shifts in the magnitude of Δ¹³C(CO₂-OM) in methanogens were also shown to occur during growth of the organism communities, as a function of cell density (House et al., 2003).
The range of values observed in the samples may also be explained by separation of different classes of biomolecules from the biomass of a single autotrophic organism, as well as from the biomass of a complex ecosystem (Hayes, 2001; Pearson, 2010). Biosynthetic processes within a single cell form micro-reservoirs comprised of different classes of organic molecules, which may have δ¹³C(OM) values distinct from that of the whole cell (Hayes, 2001; Sakata et al., 1997; Teece and Fogel, 2007). These distinct micro-reservoirs can sometimes be distinguished by in situ δ¹³C(OM) analysis of well-preserved microfossils as shown by Williford et al. (2013). These classes of molecules have distinct reactivity and may polymerize separately during early degradation and diagenesis processes (Close et al., 2011; Damsté et al., 1998; Galimov, 2006; Williford et al., 2013). Nevertheless, we have found no correlation between average δ¹³C(OM) values and raw ¹³CH/¹³C ratio in the different microstructures studied here suggesting that the microstructure-specific isotopic compositions do not result from condensation and/or preservation of specific classes of biomolecules.

Glikson et al. (2008) have compared the OM preserved in the Dresser cherts with the remains of modern hyperthermophilic methanogens from an active seafloor hydrothermal environment using organic petrography, transmission electron microscopy (TEM) and SEM coupled with energy-dispersive X-ray spectroscopy (EDS). These authors report considerable morphological and textural similarity between the Dresser OM and the thermally degraded remains of the organism formed after heating the cultures to >100°C, which resulted in disintegration of most cellular structures. It is likely that the OM clots abundant in the samples studied here represent a similarly degraded form of the original OM. Morphologically similar OM clots from the Strelley Pool Formation were also described as a polymerized residue of degraded microbial OM (Lepot et al., 2013). Glikson et al. (2008) have suggested that thermal degradation of primary OM in the
762 Dresser samples resulted in higher δ¹³C(OM) values due to preferential loss of low-δ¹³C CH₄.

763 However, this process is expected to result in correlated lower H/C ratios with higher δ¹³C, which are not observed in any of our samples, as discussed above in section 5.1 and shown in figure 5. Heterotrophic degradation on the other hand is also capable of increasing OM δ¹³C by a few permil (Pearson, 2010), but without necessarily producing a distinct correlation with OM H/C ratio. Therefore, the range of δ¹³C(OM) values in the Dresser samples may also arise, at least partly, due to the effect of variable heterotrophic degradation. A similar process has been invoked to explain the higher than bulk-rock δ¹³C values found *in situ* for some of the OM clots in the Strelley Pool Formation samples studied by Lepot et al. (2013). Yet, unlike in the Strelley Pool Formation, where pristine OM microstructures such as lenses and spheres have been preserved, it seems that in the Dresser Formation samples studied here, all cellular textures were degraded to form amorphous OM clots. Nevertheless, we note that some of the Type-2 clots of sample 84.4, which yielded the lowest δ¹³C(OM) values in this study, comprise thread-like portions of denser OM (e.g. 84.4\_a9\_4a; Fig 4b), which may represent a more pristine, less degraded form of the original autotrophic OM.

777 **6. Conclusions**

778 1. Through petrographic study of the Dresser Formation cherts we have described six distinct OM microstructural types: OM clots, which may be subdivided into Type-1 and -2 clots, layered OM, laminated OM, OM concentrated along stylolites and OM enclosed in carbonate rhombs. Our study identifies most of the OM microstructures in these rocks as indigenous and syngenetic, formed within unconsolidated sediments or hydrothermal veins during their deposition/precipitation. Later migration of OM within the rocks
during hydrothermal/metamorphic alteration events occurred over a very limited,
micrometer scale, or is confined along distinct stylolitic fronts formed by the dissolution
of primary carbonate, prior to silicification.

2. Average δ¹³C(OM) values in the studied microstructural types range between -33.6‰ and -25.7‰. Thermal maturation or isotope exchange reactions with C-bearing fluids probably did not significantly alter the original δ¹³C value of OM as is indicated by the lack of a negative correlation between δ¹³C values and H/C ratios in OM, and the relatively low temperature alteration of the host rock. It is thus concluded that the ~7‰ range of average δ¹³C(OM) values found for the studied microstructures represents primary variability in OM isotopic composition.

3. Some of the samples studied show a relatively wide range of δ¹³C(OM) values, which correlate to microstructure types. This observation is not consistent with the hypothesis that all OM formed by precipitation of abiotically formed simple organic compounds. Likewise, there is no evidence that hydrogen, required as a source of reduction in most abiotic hydrocarbon-forming reactions, was present in the hydrothermal fluids and the native metal alloys required as catalysts for abiotic FFT reactions were not detected in the rocks studied here or in nearby metabasalts.

4. Average Δ¹³C(CO₂ – OM) values obtained in this study for different OM microstructure types range between 20 and 30‰. This range of isotopic fractionation is consistent with enzymatic C fixation via the Calvin cycle utilized by photoautotrophs and the reductive acetyl-CoA pathway utilized by chemolithoautotrophs. Photoautotrophy is suggested by the relatively shallow water depth inferred for the Dresser environment and the occurrence of stromatolites in this unit, whereas chemolithotrophy is supported by the
abundance of OM in the chert veins that are interpreted as sub-surface hydrothermal conduits. Therefore, the range of δ¹³C(OM) values observed in some of the Dresser Formation samples may be explained by different primary OM formation paths. The observed δ¹³C(OM) heterogeneities may also reflect other biogenic processes, such as differences in biomass density and heterotrophic overprinting of the primary (i.e. autotrophic) OM.

## 813 Acknowledgements

We thank Noriko Kita and Jim Kern for assistance with the ion microprobe, John Fournelle and Phil Gopon for assistance with the SEM and EPMA, Maciej Śliwiński for assistance with the carbonate SIMS standards and data processing, and Brian Hess for expert sample preparation.

Chris House provided our standard chip of PPRG-215. Funding for this study was provided by the NASA Astrobiology Institute (NM, KW, KK, KL, JV) and the Labex UnivEarths program of Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02, PP, KL, CT). Work by KW at the Jet Propulsion Laboratory, California Institute of Technology was funded by a grant from the National Aeronautics and Space Administration. MVK acknowledges support from the University of New South Wales. The WiscSIMS Lab is partly supported by NSF-EAR-1053466, -1355590. Drillcore samples were made available through the collaborative PDP drilling project organized by the Institute de Physique de Globe de Paris and the Geological Survey of Western Australia. This is Publication number XXXX of the Australian Research Council Centre of Excellence for Core to Crust Fluid Systems.
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1080 **Figure captions**

1081 **Figure 1:** Geological map of the North Pole Dome in the Pilbara craton, Western Australia
1082 showing the location of the PDP drill site. Modified after Van Kranendonk (2010).

1083 **Figure 2:** Stratigraphic logs of drillcores PDP2b and PDP2c showing the location of the samples studied (red diamonds). Numbers to the left of each log indicate depth in meters down the drill hole. Sample numbers in red are keyed to depth. Modified after Van Kranendonk et al. (2008).

1086 **Figure 3:** Low-magnification transmitted-light images of OM microstructures in the samples studied. Scale bars are 1 mm.

1088 **Figure 4:** Matching transmitted light (TL; left) and Back-Scattered Electron (BSE; right) images of selected SIMS pits. a, b, c and d – Type 1 clot, Type 2 clot, layered OM and stylolite, respectively, in sample 84.4; e and f – clot and carbonate, respectively, in sample 84.5; g – clot in sample 84.6; h – laminated OM in sample 84.7; i – clot in sample 93.5a; j – clot and stylolite in sample 93.5b; k and l – clot and carbonate, respectively, in sample 111.9. The light hue ovals in the BSE images are the SIMS analysis pits. The δ¹³C(OM)~VPDB~ values obtained for the pits are shown with the BSE images. Abbreviations: Au, remnants of gold coating (not part of the original assemblage); Carb, carbonate (dolomite-ankerite), Chl, chlorite; OM, organic matter; Py, pyrite; Qtz, quartz. The complete dataset of TL and BSE images for all studied SIMS pits is given in Sup. data Figure S3.
1098 **Figure 5:** SIMS δ¹³C(OM) (‰ VPDB) values plotted vs. raw ¹³CH/¹³C ratios in the studied samples. The δ¹³C and ¹³CH/¹³C<sub>Raw</sub> error bars indicate analytical uncertainty as defined in the text.

1101 **Figure 6:** Histograms of δ¹³C values (‰ VPDB) for specific OM microstructures in the studied samples. Dashed grey lines with adjacent numbers in Italic indicate the bulk δ¹³C values for insoluble OM in each sample.

1104 **Figure 7:** Summary of OM C-isotope data in the Pilbara Craton from previous work and this study. The data are presented as histograms of δ¹³C(OM) (‰ VPDB) divided into different formations (Dresser and Strelley Pool) and different analytical methods: a and b – SIMS microanalysis using OM-bearing chert as the standard (SIMS-OM, see text for details); c and d – SIMS microanalysis using graphite as the calibration standard (SIMS-G), e and f – bulk OM analysis (Bulk). The black dashed line indicates the average δ¹³C value of all OM analyses in this study and the gray shaded area marks the range included within 2SD.

1111 **Figure 8:** Histograms of δ¹³C (‰ VPDB) values in OM (all microstructures) and ankerites in the studied samples.
<figure><img src="image_3.png" /></figure>

**Figure 1:** Geological map of the North Pole Dome, showing the location of the PDP2 drillsite. Modified after Van Kranendonk (2010).
Figure 2: Stratigraphic logs of drillcores PDP2b and PDP2c showing the location of the samples studied (red diamonds). Numbers to the left of each log indicate depth in meters down the drillhole. Modified after Van Kranendonk et al. (2008).

<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 3:** Lower magnification transmitted light images of OM micro textures in the samples studied. Scale bars are 1 mm.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 4</figcaption></figure>
<figure><img src="image_8.png" /></figure>

Clot

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

Carbonate

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

Clot

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

Laminated OM

<figure><img src="image_15.png" /></figure>
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>
<figure><img src="image_24.png" /></figure>

**Figure 4:** Transmitted light (TL; left) and Back-Scattered Electron (BSE; right) images of selected SIMS pits. a, b, c and d – Type 1 clot, Type 2 clot, layered OM and stylolite, respectively, in sample 84.4; e and f – clot and carbonate, respectively, in sample 84.5; g – clot in sample 84.6; h – laminated OM in sample 84.7; i – clot in sample 93.5a; j – clot and stylolite in sample 93.5b; k and l – clot and carbonate, respectively, in sample 111.9. The light hue ovals in the BSE images are the SIMS analysis pits. The $\delta^{13}C_{VPDB}$ values obtained for the pits are shown with the BSE images. Abbreviations: Au, remnants of gold coating (not part of the original assemblage); Carb, carbonate (dolomite-ankerite), Chl, chlorite; OM, organic matter; Py, pyrite; Qtz, quartz. The complete dataset of TL and BSE images for all studied SIMS pits is given in Sup. data Figure S3.
<figure><img src="image_25.png" /><figcaption>Figure 5</figcaption></figure>

**Figure 5: SIMS δ¹³C (‰ VPDB) values plotted vs. raw ¹³CH/¹³C ratios in the studied samples. The δ¹³C and ¹³CH/¹³C<sub>Raw</sub> error bars indicate analytical uncertainty as defined in the text.**
<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>

**Figure 6:** Histograms of δ¹³C values (‰ VPDB) for specific OM microstructure in the studied samples. Dashed grey lines with adjacent numbers in Italic indicate bulk δ¹³C values for insoluble OM in each sample.
<figure><img src="image_33.png" /><figcaption>**Figure 7:** Summary of OM C-isotope data in the Pilbara Craton from previous works and this study. The data are presented as histograms of δ¹³C(OM) (‰ VPDB) divided into different formations (Dresser and Strelley Pool) and different analytical methods: a and b – SIMS microanalysis using OM-bearing chert as the standard (SIMS-OM, see text for details); c and d – SIMS microanalysis using graphite as the calibration standard (SIMS-G), e and f – bulk OM analysis (Bulk). The black dashed line indicates the average δ¹³C value of all OM analyses in this study and the gray shaded area marks the range included within 2SD.</figcaption></figure>
<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>

<figure><img src="image_36.png" /></figure>

**Figure 8:** Histograms of δ¹³C (‰ VPDB) values in OM (all microtextures) and ankerites in the studied samples.
<table><caption>**Table 1:** Summary of SIMS session analytical data - Samples.</caption><thead><tr><th>Date</th><th>Sample</th><th>Spot</th><th>microtexture</th><th>δ<sup>13</sup>C<sub>VPDB</sub> (‰)</th><th>±2SD* (‰)</th><th>α<sub>SIMS</sub>**</th><th>Bias (‰)</th><th>δ<sup>13</sup>C<sub>RAW</sub> (‰)</th><th>±2SE*** (‰)</th><th><sup>13</sup>CH/<sup>13</sup>C†</th><th><sup>12</sup>C CPS (x10<sup>6</sup>)</th></tr></thead><tbody><tr><td>6/18/2014</td><td>84.4</td><td>a1_1a</td><td>stylolite</td><td>-25.6</td><td>4.6</td><td>0.960</td><td>-40.31</td><td>-64.9</td><td>2.3</td><td>0.087</td><td>1.6</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a1_1b</td><td>layered</td><td>-29.1</td><td>4.5</td><td>0.960</td><td>-40.31</td><td>-68.3</td><td>2.2</td><td>0.077</td><td>2.1</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a1_2a</td><td>layered</td><td>-28.2</td><td>4.4</td><td>0.960</td><td>-40.31</td><td>-67.3</td><td>1.9</td><td>0.095</td><td>2.1</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a1_2b</td><td>layered</td><td>-26.2</td><td>4.3</td><td>0.960</td><td>-40.31</td><td>-65.5</td><td>1.7</td><td>0.077</td><td>2.1</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a1_3a</td><td>Type 1 clot</td><td>-26.7</td><td>4.1</td><td>0.960</td><td>-40.31</td><td>-65.9</td><td>1.1</td><td>0.087</td><td>4.9</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a1_4a</td><td>Type 1 clot</td><td>-27.8</td><td>4.1</td><td>0.960</td><td>-40.31</td><td>-67.0</td><td>1.2</td><td>0.095</td><td>5.2</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a1_5a</td><td>layered</td><td>-28.0</td><td>6.4</td><td>0.960</td><td>-40.31</td><td>-67.1</td><td>4.9</td><td>0.148</td><td>0.2</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a1_5b</td><td>layered</td><td>-33.5</td><td>7.6</td><td>0.960</td><td>-40.31</td><td>-72.5</td><td>6.3</td><td>0.068</td><td>1.1</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a1_6a</td><td>layered</td><td>-32.0</td><td>4.5</td><td>0.960</td><td>-40.31</td><td>-71.0</td><td>2.1</td><td>0.067</td><td>1.5</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a1_6b</td><td>layered</td><td>-31.5</td><td>4.6</td><td>0.960</td><td>-40.31</td><td>-70.5</td><td>2.3</td><td>0.087</td><td>1.1</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a6_1a</td><td>Type 1 clot</td><td>-27.1</td><td>4.2</td><td>0.960</td><td>-40.31</td><td>-66.3</td><td>1.4</td><td>0.084</td><td>5.3</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a6_2a</td><td>Type 2 clot</td><td>-34.3</td><td>4.0</td><td>0.960</td><td>-40.31</td><td>-73.3</td><td>0.9</td><td>0.083</td><td>8.2</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a7_1a</td><td>stylolite</td><td>-24.4</td><td>4.2</td><td>0.959</td><td>-41.10</td><td>-64.5</td><td>0.9</td><td>0.089</td><td>7.4</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a7_1b</td><td>stylolite</td><td>-26.2</td><td>4.1</td><td>0.959</td><td>-41.10</td><td>-66.2</td><td>0.6</td><td>0.073</td><td>35.9</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a7_1c</td><td>stylolite</td><td>-25.2</td><td>4.1</td><td>0.959</td><td>-41.10</td><td>-65.2</td><td>0.6</td><td>0.072</td><td>21.7</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a7_2a</td><td>stylolite</td><td>-27.2</td><td>4.1</td><td>0.959</td><td>-41.10</td><td>-67.2</td><td>0.6</td><td>0.066</td><td>27.1</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a7_2b</td><td>Type 1 clot</td><td>-27.9</td><td>4.5</td><td>0.959</td><td>-41.10</td><td>-67.9</td><td>1.8</td><td>0.075</td><td>3.2</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a8_4a</td><td>Type 1 clot</td><td>-28.7</td><td>4.3</td><td>0.959</td><td>-41.10</td><td>-68.6</td><td>1.2</td><td>0.071</td><td>5.7</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a9_2a</td><td>Type 2 clot</td><td>-29.8</td><td>4.2</td><td>0.959</td><td>-41.10</td><td>-69.7</td><td>0.8</td><td>0.077</td><td>12.2</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a9_3a</td><td>Type 2 clot</td><td>-31.6</td><td>4.2</td><td>0.959</td><td>-41.10</td><td>-71.4</td><td>0.8</td><td>0.079</td><td>13.8</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a9_4a</td><td>Type 2 clot</td><td>-37.6</td><td>4.2</td><td>0.959</td><td>-41.10</td><td>-77.1</td><td>0.8</td><td>0.071</td><td>9.1</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a10_1a</td><td>Type 2 clot</td><td>-34.5</td><td>4.2</td><td>0.959</td><td>-41.10</td><td>-74.2</td><td>0.9</td><td>0.072</td><td>14.6</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a10_1b</td><td>crack</td><td>-33.9</td><td>4.1</td><td>0.959</td><td>-41.10</td><td>-73.6</td><td>0.6</td><td>0.128</td><td>21.7</td></tr><tr><td>6/18/2014</td><td>84.4</td><td>a10_6a</td><td>Type 2 clot</td><td>-33.9</td><td>4.1</td><td>0.959</td><td>-41.10</td><td>-73.6</td><td>0.6</td><td>0.064</td><td>21.8</td></tr><tr><td>6/19/2014</td><td>84.5</td><td>a3_1a</td><td>clot</td><td>-30.2</td><td>3.1</td><td>0.959</td><td>-41.34</td><td>-70.3</td><td>0.7</td><td>0.078</td><td>26.2</td></tr><tr><td>6/19/2014</td><td>84.5</td><td>a3_1b</td><td>clot</td><td>-32.1</td><td>3.1</td><td>0.959</td><td>-41.34</td><td>-72.2</td><td>0.5</td><td>0.087</td><td>27.4</td></tr><tr><td>6/19/2014</td><td>84.5</td><td>a3_2a</td><td>clot</td><td>-32.6</td><td>3.1</td><td>0.959</td><td>-41.34</td><td>-72.5</td><td>0.5</td><td>0.081</td><td>32.8</td></tr><tr><td>6/19/2014</td><td>84.5</td><td>a3_2b</td><td>clot</td><td>-35.7</td><td>3.1</td><td>0.959</td><td>-41.34</td><td>-75.6</td><td>0.5</td><td>0.073</td><td>53.0</td></tr><tr><td>6/19/2014</td><td>84.5</td><td>a3_4a</td><td>clot</td><td>-32.6</td><td>3.2</td><td>0.959</td><td>-41.34</td><td>-72.6</td><td>1.0</td><td>0.077</td><td>7.8</td></tr><tr><td>6/19/2014</td><td>84.5</td><td>a3_4b</td><td>clot</td><td>-31.3</td><td>3.1</td><td>0.959</td><td>-41.34</td><td>-71.3</td><td>0.9</td><td>0.096</td><td>10.1</td></tr><tr><td>6/19/2014</td><td>84.5</td><td>a3_4c</td><td>carbonate</td><td>-1.5</td><td>0.7</td><td>0.953</td><td>-46.90</td><td>-48.3</td><td>0.4</td><td>0.001</td><td>30.2</td></tr><tr><td>6/19/2014</td><td>84.5</td><td>a2_2a</td><td>no OM</td><td>-25.6</td><td>4.2</td><td>0.959</td><td>-41.34</td><td>-65.9</td><td>3.0</td><td>0.079</td><td>0.5</td></tr><tr><td>6/19/2014</td><td>84.5</td><td>a2_3a</td><td>clot</td><td>-27.2</td><td>3.2</td><td>0.959</td><td>-41.34</td><td>-67.4</td><td>1.0</td><td>0.088</td><td>6.2</td></tr><tr><td>6/19/2014</td><td>84.5</td><td>a2_3b</td><td>clot</td><td>-30.9</td><td>3.1</td><td>0.959</td><td>-41.34</td><td>-71.0</td><td>0.7</td><td>0.084</td><td>13.2</td></tr><tr><td>6/18/2014</td><td>84.6</td><td>a1_1a</td><td>clot</td><td>-30.1</td><td>2.6</td><td>0.958</td><td>-41.53</td><td>-70.4</td><td>0.6</td><td>0.100</td><td>15.9</td></tr></tbody></table>
<table><thead><tr><th>Date</th><th>Sample</th><th>Spot</th><th>microtexture</th><th>δ<sup>13</sup>C<sub>VPDB</sub> (‰)</th><th>±2SD* (‰)</th><th>α<sub>SIMS</sub>**</th><th>Bias (‰)</th><th>δ<sup>13</sup>C<sub>RAW</sub> (‰)</th><th>±2SE*** (‰)</th><th><sup>13</sup>CH/<sup>13</sup>C†</th><th><sup>12</sup>C CPS (x10<sup>6</sup>)</th></tr></thead><tbody><tr><td>6/17/2014</td><td>84.6</td><td>a1_2a</td><td>clot</td><td>-30.0</td><td>2.6</td><td>0.958</td><td>-41.53</td><td>-70.3</td><td>0.7</td><td>0.095</td><td>6.8</td></tr><tr><td>6/18/2014</td><td>84.6</td><td>a2_2a</td><td>clot</td><td>-26.5</td><td>1.6</td><td>0.959</td><td>-40.91</td><td>-66.3</td><td>0.8</td><td>0.099</td><td>12.6</td></tr><tr><td>6/18/2014</td><td>84.6</td><td>a2_3a</td><td>clot</td><td>-25.5</td><td>1.5</td><td>0.959</td><td>-40.91</td><td>-65.3</td><td>0.5</td><td>0.103</td><td>27.9</td></tr><tr><td>6/18/2014</td><td>84.6</td><td>a2_4a</td><td>clot</td><td>-27.6</td><td>1.5</td><td>0.959</td><td>-40.91</td><td>-67.4</td><td>0.5</td><td>0.091</td><td>32.2</td></tr><tr><td>6/18/2014</td><td>84.6</td><td>a2_4b</td><td>stylolite</td><td>-31.0</td><td>2.3</td><td>0.959</td><td>-40.91</td><td>-70.6</td><td>1.8</td><td>0.136</td><td>2.4</td></tr><tr><td>6/18/2014</td><td>84.6</td><td>a2_4c</td><td>clot</td><td>-26.8</td><td>1.8</td><td>0.959</td><td>-40.91</td><td>-66.6</td><td>1.1</td><td>0.090</td><td>10.0</td></tr><tr><td>6/18/2014</td><td>84.7</td><td>a1_2a</td><td>clot</td><td>-26.6</td><td>2.6</td><td>0.958</td><td>-41.53</td><td>-67.0</td><td>0.5</td><td>0.093</td><td>49.0</td></tr><tr><td>6/18/2014</td><td>84.7</td><td>a1_3a</td><td>clot</td><td>-26.6</td><td>2.6</td><td>0.958</td><td>-41.53</td><td>-67.0</td><td>0.4</td><td>0.098</td><td>27.5</td></tr><tr><td>6/18/2014</td><td>84.7</td><td>a1_4a</td><td>clot</td><td>-26.6</td><td>2.9</td><td>0.958</td><td>-41.53</td><td>-67.0</td><td>1.4</td><td>0.112</td><td>6.8</td></tr><tr><td>6/18/2014</td><td>84.7</td><td>a2_1a</td><td>clot</td><td>-29.5</td><td>2.6</td><td>0.958</td><td>-41.53</td><td>-69.8</td><td>0.6</td><td>0.095</td><td>13.5</td></tr><tr><td>6/18/2014</td><td>84.7</td><td>a2_2a</td><td>clot</td><td>-28.1</td><td>2.6</td><td>0.958</td><td>-41.53</td><td>-68.4</td><td>0.5</td><td>0.101</td><td>18.3</td></tr><tr><td>6/18/2014</td><td>84.7</td><td>a2_4a</td><td>no OM</td><td>-31.6</td><td>4.0</td><td>0.958</td><td>-41.53</td><td>-71.8</td><td>3.1</td><td>0.111</td><td>0.6</td></tr><tr><td>6/18/2014</td><td>84.7</td><td>a3_1a</td><td>lamina</td><td>-26.1</td><td>2.6</td><td>0.958</td><td>-41.53</td><td>-66.6</td><td>0.6</td><td>0.100</td><td>29.8</td></tr><tr><td>6/18/2014</td><td>84.7</td><td>a3_2a</td><td>lamina</td><td>-26.0</td><td>2.6</td><td>0.958</td><td>-41.53</td><td>-66.5</td><td>0.6</td><td>0.103</td><td>11.9</td></tr><tr><td>6/18/2014</td><td>84.7</td><td>a3_3a</td><td>lamina</td><td>-25.7</td><td>2.6</td><td>0.958</td><td>-41.53</td><td>-66.1</td><td>0.5</td><td>0.082</td><td>40.8</td></tr><tr><td>6/18/2014</td><td>84.7</td><td>a4_1a</td><td>lamina</td><td>-28.5</td><td>2.6</td><td>0.958</td><td>-41.53</td><td>-68.8</td><td>0.5</td><td>0.103</td><td>27.7</td></tr><tr><td>6/19/2014</td><td>84.7</td><td>a1_2b</td><td>carbonate</td><td>-0.4</td><td>3.0</td><td>0.954</td><td>-46.42</td><td>-46.8</td><td>0.5</td><td>0.004</td><td>33.1</td></tr><tr><td>6/19/2014</td><td>84.7</td><td>a1_2c</td><td>carbonate</td><td>2.1</td><td>3.0</td><td>0.954</td><td>-46.42</td><td>-44.4</td><td>0.4</td><td>0.002</td><td>30.2</td></tr><tr><td>6/19/2014</td><td>84.7</td><td>a1_4b</td><td>carbonate</td><td>0.3</td><td>3.0</td><td>0.954</td><td>-46.43</td><td>-46.2</td><td>0.3</td><td>0.004</td><td>26.0</td></tr><tr><td>6/19/2014</td><td>84.7</td><td>a2_3a</td><td>carbonate<sup>1</sup></td><td>-9.6</td><td>3.0</td><td>0.954</td><td>-46.43</td><td>-55.6</td><td>0.5</td><td>0.023</td><td>48.7</td></tr><tr><td>6/19/2014</td><td>84.7</td><td>a2_3b</td><td>carbonate</td><td>0.0</td><td>3.0</td><td>0.954</td><td>-46.39</td><td>-46.4</td><td>0.4</td><td>0.003</td><td>30.4</td></tr><tr><td>6/19/2014</td><td>84.7</td><td>a2_3c</td><td>carbonate</td><td>-2.2</td><td>3.0</td><td>0.954</td><td>-46.42</td><td>-48.5</td><td>0.4</td><td>0.003</td><td>32.4</td></tr><tr><td>6/19/2014</td><td>93.5a</td><td>a3_1a</td><td>clot</td><td>-28.3</td><td>3.0</td><td>0.960</td><td>-40.41</td><td>-67.6</td><td>0.5</td><td>0.107</td><td>24.2</td></tr><tr><td>6/19/2014</td><td>93.5a</td><td>a3_6a</td><td>clot</td><td>-25.8</td><td>2.9</td><td>0.960</td><td>-40.41</td><td>-65.1</td><td>0.5</td><td>0.102</td><td>39.4</td></tr><tr><td>6/19/2014</td><td>93.5a</td><td>a3_5a</td><td>clot</td><td>-30.0</td><td>3.0</td><td>0.960</td><td>-40.41</td><td>-69.2</td><td>0.8</td><td>0.092</td><td>11.5</td></tr><tr><td>6/19/2014</td><td>93.5a</td><td>a3_2a</td><td>clot</td><td>-29.3</td><td>3.0</td><td>0.960</td><td>-40.41</td><td>-68.6</td><td>0.6</td><td>0.089</td><td>30.5</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a1_1a</td><td>clot</td><td>-27.0</td><td>2.9</td><td>0.960</td><td>-40.41</td><td>-66.3</td><td>0.5</td><td>0.110</td><td>20.8</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a1_1b</td><td>clot</td><td>-27.5</td><td>2.9</td><td>0.960</td><td>-40.41</td><td>-66.8</td><td>0.5</td><td>0.106</td><td>38.7</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a1_1c</td><td>layered</td><td>-31.8</td><td>3.3</td><td>0.960</td><td>-40.41</td><td>-70.9</td><td>1.7</td><td>0.083</td><td>3.1</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a1_2a</td><td>clot</td><td>-27.3</td><td>3.1</td><td>0.960</td><td>-40.41</td><td>-66.6</td><td>1.0</td><td>0.100</td><td>9.7</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a1_3a</td><td>clot</td><td>-27.1</td><td>3.1</td><td>0.960</td><td>-40.41</td><td>-66.4</td><td>1.0</td><td>0.098</td><td>8.0</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a1_6a</td><td>layered</td><td>-28.5</td><td>3.6</td><td>0.960</td><td>-40.41</td><td>-67.8</td><td>2.1</td><td>0.091</td><td>1.3</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a1_6b</td><td>layered</td><td>-23.6</td><td>5.4</td><td>0.960</td><td>-40.41</td><td>-63.1</td><td>4.6</td><td>0.093</td><td>0.3</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a1_6c</td><td>layered</td><td>-31.5</td><td>3.1</td><td>0.960</td><td>-40.41</td><td>-70.6</td><td>1.1</td><td>0.091</td><td>4.5</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a2_2a</td><td>clot</td><td>-26.9</td><td>2.2</td><td>0.959</td><td>-41.08</td><td>-66.9</td><td>0.8</td><td>0.110</td><td>13.3</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a2_4a</td><td>stylolite</td><td>-28.9</td><td>3.6</td><td>0.959</td><td>-41.08</td><td>-68.7</td><td>2.9</td><td>0.093</td><td>1.6</td></tr></tbody></table>
<table><thead><tr><th>Date</th><th>Sample</th><th>Spot</th><th>microtexture</th><th>δ<sup>13</sup>C<sub>VPDB</sub> (‰)</th><th>±2SD* (‰)</th><th>α<sub>SIMS</sub>**</th><th>Bias (‰)</th><th>δ<sup>13</sup>C<sub>RAW</sub> (‰)</th><th>±2SE*** (‰)</th><th><sup>13</sup>CH/<sup>13</sup>C†</th><th><sup>12</sup>C CPS (x10<sup>6</sup>)</th></tr></thead><tbody><tr><td>6/19/2014</td><td>93.5b</td><td>a2_4b</td><td>stylolite</td><td>-27.9</td><td>2.4</td><td>0.959</td><td>-41.08</td><td>-67.8</td><td>1.3</td><td>0.094</td><td>4.5</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a2_5a</td><td>clot</td><td>-29.2</td><td>2.1</td><td>0.959</td><td>-41.08</td><td>-69.1</td><td>0.6</td><td>0.096</td><td>17.6</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a3_2a</td><td>carbonate</td><td>-0.1</td><td>3.0</td><td>0.954</td><td>-46.40</td><td>-46.5</td><td>0.3</td><td>0.001</td><td>33.3</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a3_2b</td><td>clot</td><td>-28.2</td><td>2.1</td><td>0.959</td><td>-41.08</td><td>-68.1</td><td>0.5</td><td>0.099</td><td>28.2</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a3_3a</td><td>clot</td><td>-25.9</td><td>2.1</td><td>0.959</td><td>-41.08</td><td>-65.9</td><td>0.6</td><td>0.091</td><td>26.0</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a2_2b</td><td>carbonate</td><td>-0.8</td><td>3.0</td><td>0.954</td><td>-46.40</td><td>-47.2</td><td>0.4</td><td>0.001</td><td>30.7</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a2_2c</td><td>carbonate</td><td>-0.4</td><td>3.0</td><td>0.954</td><td>-46.39</td><td>-46.7</td><td>0.3</td><td>0.001</td><td>30.6</td></tr><tr><td>6/19/2014</td><td>93.5b</td><td>a2_5c</td><td>carbonate<sup>1</sup></td><td>-7.5</td><td>3.0</td><td>0.954</td><td>-46.40</td><td>-53.6</td><td>0.5</td><td>0.015</td><td>35.8</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a1_1a</td><td>clot</td><td>-26.7</td><td>3.9</td><td>0.958</td><td>-42.06</td><td>-67.6</td><td>0.5</td><td>0.089</td><td>30.5</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a1_1b</td><td>clot</td><td>-28.7</td><td>3.9</td><td>0.958</td><td>-42.06</td><td>-69.5</td><td>0.3</td><td>0.082</td><td>39.9</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a1_2a</td><td>clot</td><td>-27.9</td><td>3.9</td><td>0.958</td><td>-42.06</td><td>-68.8</td><td>0.6</td><td>0.102</td><td>13.3</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a1_2b</td><td>clot</td><td>-30.0</td><td>4.1</td><td>0.958</td><td>-42.06</td><td>-70.8</td><td>1.3</td><td>0.083</td><td>3.6</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a1_3a</td><td>clot</td><td>-24.5</td><td>3.9</td><td>0.958</td><td>-42.06</td><td>-65.5</td><td>0.5</td><td>0.113</td><td>29.0</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a1_4a</td><td>clot</td><td>-27.2</td><td>3.9</td><td>0.958</td><td>-42.06</td><td>-68.1</td><td>0.5</td><td>0.106</td><td>51.7</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a1_4b</td><td>clot</td><td>-27.6</td><td>3.9</td><td>0.958</td><td>-42.06</td><td>-68.5</td><td>0.6</td><td>0.087</td><td>19.5</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a1_5a</td><td>clot</td><td>-27.4</td><td>3.9</td><td>0.958</td><td>-42.06</td><td>-68.3</td><td>0.5</td><td>0.103</td><td>30.7</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a2_1a</td><td>clot</td><td>-30.8</td><td>3.9</td><td>0.958</td><td>-42.06</td><td>-71.6</td><td>0.6</td><td>0.107</td><td>15.3</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a2_2a</td><td>clot</td><td>-28.2</td><td>3.9</td><td>0.958</td><td>-42.06</td><td>-69.1</td><td>0.5</td><td>0.108</td><td>33.3</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a2_3a</td><td>clot</td><td>-31.3</td><td>4.0</td><td>0.958</td><td>-42.06</td><td>-72.0</td><td>1.0</td><td>0.082</td><td>7.1</td></tr><tr><td>6/18/2014</td><td>111.9</td><td>a2_4a</td><td>clot</td><td>-28.9</td><td>3.9</td><td>0.958</td><td>-42.06</td><td>-69.7</td><td>0.5</td><td>0.119</td><td>29.4</td></tr><tr><td>6/19/2014</td><td>111.9</td><td>a1_1c</td><td>carbonate</td><td>-3.2</td><td>3.0</td><td>0.954</td><td>-46.40</td><td>-49.5</td><td>0.6</td><td>0.005</td><td>33.6</td></tr><tr><td>6/19/2014</td><td>111.9</td><td>a1_1d</td><td>carbonate</td><td>0.6</td><td>3.0</td><td>0.954</td><td>-46.39</td><td>-45.8</td><td>0.3</td><td>0.002</td><td>31.9</td></tr><tr><td>6/19/2014</td><td>111.9</td><td>a1_3b</td><td>carbonate</td><td>-0.5</td><td>3.0</td><td>0.954</td><td>-46.41</td><td>-46.9</td><td>0.3</td><td>0.002</td><td>31.4</td></tr><tr><td>6/19/2014</td><td>111.9</td><td>a1_3c</td><td>carbonate</td><td>-0.8</td><td>3.0</td><td>0.954</td><td>-46.42</td><td>-47.2</td><td>0.3</td><td>0.001</td><td>31.5</td></tr><tr><td>6/19/2014</td><td>111.9</td><td>a2_2b</td><td>carbonate</td><td>-0.5</td><td>3.0</td><td>0.954</td><td>-46.38</td><td>-46.9</td><td>0.3</td><td>0.001</td><td>30.7</td></tr><tr><td>6/19/2014</td><td>111.9</td><td>a2_2c</td><td>carbonate</td><td>-1.2</td><td>3.0</td><td>0.954</td><td>-46.41</td><td>-47.6</td><td>0.3</td><td>0.001</td><td>29.5</td></tr></tbody></table>

* Analytical error is calculated as the quadratic addition of the external precision (calculated as 2SD of δ¹³C values from eight bracketing analysis of the PPRG-215-2B std., Sup. data Table S1) and the internal precision for each pit (2SE - see below).

** Instrumental bias (αSIMS) is calculated by comparing the average of d¹³CRAW values for eight bracketing PPRG-215-2B standard analyses (Sup. data Table S1) to the measured bulk value of -31.5‰ (Hayes et al., 1983).

*** Internal precision (2SE) is calculated as two standard errors of δ¹³C values from 80 measurement cycles for each sample analysis pit.

† Mass 14 (¹³CH) was measured simultaneously with ¹²C and ¹³C and used to calculate ¹³CH/¹³C.

1 Post-analysis BSE imaging of this spot suggests that it hit OM enclosed in the carbonate crystal (See Sup. data Fig. S3).
<table><caption>Table 2: Summary of Student's t-tests.</caption><thead><tr><th></th><th>84.4 T1 C</th><th>84.4 T2 C</th><th>84.4 Lyr</th><th>84.4 Stl</th><th>84.5 C</th><th>84.6 C</th><th>84.7 C</th><th>84.7 Lam</th><th>93.5a C</th><th>93.5b C</th><th>93.5b Lyr</th><th>93.5b Stl</th><th>111.9 C</th></tr></thead><tbody><tr><td>84.4 T1 C</td><td></td><td>6.0</td><td>2.4</td><td>1.9</td><td>3.9</td><td>0.1</td><td>0.2</td><td>1.1</td><td>0.7</td><td>0.4</td><td>3.0</td><td>0.9</td><td>0.6</td></tr><tr><td>84.4 T2 C</td><td>0.00</td><td></td><td>3.5</td><td>7.9</td><td>2.0</td><td>5.9</td><td>6.2</td><td>7.0</td><td>5.3</td><td>6.3</td><td>3.0</td><td>5.1</td><td>5.3</td></tr><tr><td>84.4 Lyr</td><td>0.08</td><td>0.05</td><td></td><td>4.4</td><td>1.5</td><td>2.3</td><td>2.6</td><td>3.5</td><td>1.7</td><td>2.8</td><td>0.5</td><td>1.5</td><td>1.8</td></tr><tr><td>84.4 Stl</td><td>0.01</td><td>0.00</td><td>0.01</td><td></td><td>5.9</td><td>2.0</td><td>1.7</td><td>0.9</td><td>2.6</td><td>1.6</td><td>4.9</td><td>2.8</td><td>2.6</td></tr><tr><td>84.5 C</td><td>0.00</td><td>0.16</td><td>0.30</td><td>0.00</td><td></td><td>3.8</td><td>4.1</td><td>5.0</td><td>3.2</td><td>4.3</td><td>1.0</td><td>3.0</td><td>3.3</td></tr><tr><td>84.6 C</td><td>0.92</td><td>0.00</td><td>0.12</td><td>0.04</td><td>0.01</td><td></td><td>0.3</td><td>1.2</td><td>0.6</td><td>0.5</td><td>2.9</td><td>0.8</td><td>0.5</td></tr><tr><td>84.7 C</td><td>0.81</td><td>0.00</td><td>0.08</td><td>0.05</td><td>0.01</td><td>0.79</td><td></td><td>0.9</td><td>0.9</td><td>0.2</td><td>3.1</td><td>1.1</td><td>0.8</td></tr><tr><td>84.7 Lam</td><td>0.17</td><td>0.00</td><td>0.05</td><td>0.30</td><td>0.00</td><td>0.33</td><td>0.35</td><td></td><td>1.8</td><td>0.7</td><td>4.0</td><td>2.0</td><td>1.7</td></tr><tr><td>93.5a C</td><td>0.45</td><td>0.01</td><td>0.31</td><td>0.03</td><td>0.04</td><td>0.62</td><td>0.42</td><td>0.16</td><td></td><td>1.1</td><td>2.2</td><td>0.2</td><td>0.1</td></tr><tr><td>93.5b C</td><td>0.50</td><td>0.00</td><td>0.05</td><td>0.02</td><td>0.00</td><td>0.55</td><td>0.77</td><td>0.30</td><td>0.19</td><td></td><td>3.3</td><td>1.3</td><td>1.0</td></tr><tr><td>93.5b Lyr</td><td>0.02</td><td>0.12</td><td>0.78</td><td>0.00</td><td>0.54</td><td>0.07</td><td>0.03</td><td>0.02</td><td>0.17</td><td>0.00</td><td></td><td>2.0</td><td>2.3</td></tr><tr><td>93.5b Stl</td><td>0.13</td><td>0.02</td><td>0.39</td><td>0.01</td><td>0.07</td><td>0.50</td><td>0.23</td><td>0.06</td><td>0.87</td><td>0.06</td><td>0.14</td><td></td><td>0.3</td></tr><tr><td>111.9 C</td><td>0.48</td><td>0.00</td><td>0.12</td><td>0.01</td><td>0.00</td><td>0.58</td><td>0.40</td><td>0.12</td><td>0.93</td><td>0.16</td><td>0.07</td><td>0.79</td><td></td></tr></tbody></table>

Below the diagonal black line: The numbers in the boxes show the probability associated with Student's t-test that the two sets of OM δ¹³C values, from different samples and/or different microstructural types, come from populations that have the same mean value. Probabilities lower than 0.05 and 0.01 (highlighted in orange and red, respectively) indicate that the two sets are statistically distinct at >95% and >99% confidence level, respectively. Above the diagonal black line: The absolute difference between average δ¹³C values for the two sets. Higher values are marked by darker background. Abbreviations: C – OM clots; T1 – type 1; T2 – type 2; Lyr – layered OM; Stl – OM in stylolites; Lam – OM laminae.
## 1 Highlights

2 *In situ* carbon isotopic analysis of ~3.5 Ga organic matter and coexisting carbonate

3 δ¹³C values in specific organic microstructures range between -33.6‰ and -25.7‰

4 The range of δ¹³C(OM) values represents primary variability of organic matter

5 Correlation between δ¹³C values and microstructures argues against abiotic origin

6 The range of δ¹³C(OM) and Δ¹³C(Carb-OM) values supports variable C-fixation paths

7

8