石田 章純

Abstract The nitrogen isotope compositions of two samples returned from the asteroid Ryugu were determined using a stepwise combustion method, along with Ivuna (CI) and Y‐980115, a CI‐like Antarctic meteorite, as references. The two Ryugu samples A0105‐07 and C0106‐07 showed bulk δ¹⁵N values of +1.7 ± 0.5‰ and +0.2 ± 0.6‰, respectively, significantly lower than Ivuna with +36.4 ± 0.4‰, but close to Y‐980115 with +4.0 ± 0.3‰. The Ryugu samples are further characterized by C/N and ³⁶Ar/N ratios up to 3.4× and 4.9× the value of Ivuna, respectively. Among all Ryugu samples and CI chondrites, a positive correlation was observed between nitrogen concentrations and δ¹⁵N values, with samples with lower nitrogen concentrations exhibiting lower δ¹⁵N. This trend is explained by a two‐component mixing model. One component is present at a constant abundance among all CI‐related samples, with a δ¹⁵N value around 0‰ or lower. The other varies in abundance between different samples, and exhibits a δ¹⁵N value of +56 ± 4‰. The first ¹⁵N‐poor endmember is seemingly tightly incorporated into a carbonaceous host phase, whereas the ¹⁵N‐rich endmember can be mobilized and decoupled from carbon, potentially because it is in the form of ammonia. Asteroid materials with volatile compositions that are similar to those reported here for the Ryugu samples are attractive candidates for the volatile sources among Earth's building blocks.

Simultaneous analysis of carbon and nitrogen isotope ratios by SIMS was applied for the first-time to a natural diamond from the Kelsey Lake kimberlite, State Line Distinct, Colorado (UWD-1). This in situ procedure is faster, reduces sample size for analysis, and measures both isotope ratios from a single similar to 10 mu m diameter pit, a critical advantage for zoned diamonds. The carbon isotope ratio (expressed as delta C-13(VPDB)) of the bulk UWD-1 crystal, determined by the conventional combustion method in the present study, is -5.9 parts per thousand +/- 0.2 parts per thousand (VPDB, 2s). Nitrogen mass fraction ([N]) and isotope ratio (expressed as delta N-15(Air)) were determined by stepwise combustion and gas-source mass-spectrometry, resulting in 553 +/- 64 mu g g(-1) and -6.7 parts per thousand +/- 1.1 parts per thousand (Air, 2s), respectively. Secondary ions of C-12(2)-, (CC-)-C-12-C-13, (CN-)-C-12-N-14, and (CN-)-C-12-N-15 were simultaneously measured by SIMS using three Faraday cups and one electron multiplier. The spot-to-spot reproducibility of delta C-13 and delta N-15 values for the UWD-1 (178 spots on sixteen chips, 10 mu m spots), were 0.3 parts per thousand and 1.6 parts per thousand, respectively (2s). While (CN-)-C-12-N-14/C-12(2)- ratios, which are an indicator for [N], varied up to 12% among these sixteen chips, such variation did not correlate with either delta C-13 or delta N-15 values. We propose that UWD-1 is a suitable reference sample for microscale in situ analysis of delta C-13 and delta N-15 values in diamond samples.

The Hayabusa2 spacecraft returned to Earth from the asteroid 162173 Ryugu on 6 December 2020. One day after the recovery, the gas species retained in the sample container were extracted and measured on-site and stored in gas collection bottles. The container gas consists of helium and neon with an extraterrestrial 3He/4He and 20Ne/22Ne ratios, along with some contaminant terrestrial atmospheric gases. A mixture of solar and Earth's atmospheric gas is the best explanation for the container gas composition. Fragmentation of Ryugu grains within the sample container is discussed on the basis of the estimated amount of indigenous He and the size distribution of the recovered Ryugu grains. This is the first successful return of gas species from a near-Earth asteroid.

The near-Earth carbonaceous asteroid (162173) Ryugu is expected to contain volatile chemical species that could provide information on the origin of Earth's volatiles. Samples of Ryugu were retrieved by the Hayabusa2 spacecraft. We measure noble gas and nitrogen isotopes in Ryugu samples, finding they are dominated by pre-solar and primordial components, incorporated during Solar System formation. Noble gas concentrations are higher than those in Ivuna-type carbonaceous (CI) chondrite meteorites. Several host phases of isotopically distinct nitrogen have heterogeneous abundances between the samples. Our measurements support a close relationship between Ryugu and CI chondrites. Noble gases produced by galactic cosmic rays, indicating ~5 Myr exposure, and from implanted solar wind, record the recent irradiation history of Ryugu after it migrated to its current orbit.

The evolutionary history of early prokaryotes is recorded in Paleoproterozoic sedimentary rocks. The ca. 1.88 Ga Gunflint Formation is considered key to constrain the course of Paleoproterozoic microbial evolution. However, whether the multicellularity of prokaryote and eukaryote was already present by the Gunflint age remains uncertain. Here, we report novel morphotypes of prokaryotes including colonial, ellipsoidal, spherical, with intracellular inclusions (ICIs), spinous-type, and tail-bearing type, in the Gunflint stromatolitic chert. Biogenicity of such morphotypes was indicated based on their unique microstructures with the parallel C, N, and S distributions and lack of evidence of their post-depositional artifact origin. The new finding of colonial-type microbes in the Gunflint Formation indicates global flourishment of the colonial-type in this age. Moreover, unknown spherical cell-like structures with ICIs were identified, along with microfossils bearing strong similarities to cyanobacterial akinetes. ICIs were more enriched in N-bearing organic compounds than cell wall organic matter. Those ICIs were interpreted as biological contracted protoplasts. These new findings suggest that Paleoproterozoic prokaryotes were more diverse and complex than previously considered and had already acquired adaptability to survive drastic environmental changes. Furthermore, the protruding appendages in the novel spine- and tail-bearing type microfossils likely provided them with advantages in nutrient access and motility respectively, resulting in the promotion of the intercellular interactions. This suggests that functional evolution toward eukaryotes had already started in the Gunflint age.

We report here hourly variations of Mg/Ca, Sr/Ca, and Ba/Ca ratios in a Mediterranean mussel shell (Mytilus galloprovincialis) collected at the Otsuchi bay, on the Pacific coast of northeastern Japan. This bivalve was living in the intertidal zone, where such organisms are known to form a daily or bidaily growth line comprised of abundant organic matter. Mg/Ca ratios of the inner surface of the outer shell layer, corresponding to the most recent date, show cyclic changes at 25–90 μm intervals, while no interpretable variations are observed in Sr/Ca and Ba/Ca ratios. High Mg/Ca ratios were probably established by (1) cessation of the external supply of Ca and organic layer forming when the shell is closed at low tide, and (2) the strong binding of Mg to the organic layer, but not of Sr and Ba. Immediately following the great tsunami induced by the 2011 Tohoku earthquake, Mg/Ca enrichment occurred, up to 10 times that of normal low tide, while apparent Ba/Ca enrichment was observed for only a few days following the event, therefore serving a proxy of the past tsunami. Following the tsunami, periodic peaks and troughs in Mg/Ca continued, perhaps due to a biological memory effect as an endogenous clock.

<title>Abstract</title> We report here hourly variations of Mg/Ca, Sr/Ca and Ba/Ca ratios in a Mediterranean mussel shell (<italic>Mytilus galloprovincialis</italic>) collected at the Otsuchi bay, on the Pacific coast of northeastern Japan. This bivalve was living in the intertidal zone, where such organisms are known to form a daily or bidaily growth line comprised of abundant organic matter. Mg/Ca ratios of the inner surface of the outer shell layer, corresponding to the most recent date, show cyclic changes at 25-90 mm intervals, while no valuable variations are observed in Sr/Ca and Ba/Ca ratios. High Mg/Ca ratios were probably established when the bivalve was located at low tide with a reduced supply of Ca from seawater. Immediately following the great tsunami induced by the 2011 Tohoku earthquake, Mg/Ca enrichment occurred, up to 10 times that of normal low tide, while apparent Ba/Ca enrichment was observed for only a few days following the event, implementing a proxy of the past tsunami. Following the tsunami, periodic peaks and troughs in Mg/Ca continued, perhaps due to a biological memory effect as an endogenous clock.

Geological and geochemical studies were performed on ca.12 Ma Mn-rich rocks in the Hokuroku district in Japan to constrain the origin of Mn-rich hydrothermal fluids and precipitation pathways of Mn oxides. Mn-rich rocks occur in hyaloclastites of rhyolite lavas in the studied areas. Colloform textures of Mn oxides were common in those Mn-rich rocks. Carbonaceous layers uniquely alternate with Mn oxides in the colloform textures. The Mn oxides layers in the colloform textures were composed of aggregates of nano-scale todorokite, birnessite, and pyrolusite. The carbonaceous layers also host apatite nano-crystals. These findings imply that the carbonaceous layers most likely represent fossilized microbial mats of Mn-oxidizing bacteria. The close association of nano scale Mn oxides with organic matter, clay, and silica minerals suggests that Mn oxides initially precipitated near venting areas of low-temperature hydrothermal fluids.The early Mn oxides were trapped in the hyaloclastites and experienced diagenetic modifications combined with hydrothermal alteration, seawater penetration, and microbial activities in the hyaloclastites. Such secondary processes further enriched Mn in the hyaloclastites, forming various Mn silicates (e.g., pennantite and shirozulite) and veins made of Mn (II and III) silicate and oxide/silicate (e.g., rhodonite and braunite).Later hydrothermal fluids also brecciated the massive Mn-rich rocks. Rhodochrosite and rhodonite occurred in such brecciated Mn-rich rocks or sheared hyaloclastites preferentially. The carbon isotope compositions of rhodochrosite ranged from -26.7 parts per thousand to -20.8 parts per thousand (VPDB). The mudstone and early colloform textures in the hyaloclastites contained appreciable amounts of organic matter. Such organic matter was degraded by microbial processes under the influence of the later hydrothermal activities, followed by the formation of rhodochrosite. These findings contribute not only to understanding the formation mechanisms of hydrothermal Mn-rich rocks, but also to future exploration of Mn deposits.

Statoliths of Aurelia coerulea were successfully labeled with S-34 and Sr and were visualized for the first time by nanoscale secondary ion mass spectrometry (NanoSIMS) and electron probe microanalysis (EPMA). Medusae were incubated in artificial seawater with an enrichment of S-34 and Sr for 25 hours. Statocysts for analyses were fixed at two time-points: immediately after the exposure (zero-day) and after 3 weeks of incubation in non-enriched seawater (post-incubated). In the analyses by NanoSIMS, deposition of S-34 was observed as an inner ring enclosed within the crystal in post-incubated specimens. In zero-day specimens, S-34/S-32 ratio in the peripheral region of statolith was much higher than those in the core region. In the analyses by EPMA, small statoliths with dense deposition of Sr were located in the basal part of statocysts in zero-day specimens. The larger statoliths tended to be located more distant from the basal part. These findings were consistent with the hypothesis that the basal part of statocysts is the area of origin of statoliths. Statoliths without deposition of Sr tend to be larger in size than those with deposition, suggesting the saturation of growth after crystals attain a certain size.

Sedimentological observations from the Paleoproterozoic Huronian Supergroup are suggested to mark the rise in atmospheric oxygen at that time, which is commonly known as the Great Oxidation Event (GOE) and typically coupled with a transition from mass-independent fractionation (MIF) to mass-dependent fractionation (MDF) of sulfur isotopes. An early in situ study of S three-isotopes across the Huronian Supergroup by Papineau et al. ( 2007 ) identified a weak MIF-MDF transition. However, the interpretation and stratigraphic placement of this transition is ambiguous. In this study, all four S isotopes were analyzed for the first time in two Huronian drill cores by secondary ion mass spectrometer (SIMS), and both Δ33S and Δ36S were calculated. Based on improved precision and detailed petrography, we reinterpret the dominance of pyrrhotite in the studied sections, which was previously proposed as "early authigenic" in origin, as resulting from regional metamorphism. Small but analytically resolvable nonzero values of Δ33S (from -0.07‰ to +0.38‰) and Δ36S (from -4.1‰ to +1.0‰) persist throughout the lower Huronian Supergroup. Neither pronounced MIF-S signals nor a MIF-MDF transition are seen in this study. Four scenarios are proposed for the genesis of small nonzero Δ33S and Δ36S values in the Huronian: homogenization by regional metamorphism, recycling from older pyrite, dilution by magmatic fluids, and the occurrence of MDF. We argue that the precise location of the MIF-MDF transition in the Huronian remains unsolved. This putative transition may have been erased by postdepositional processes in the lower Huronian Supergroup, or may be located in the upper Huronian Supergroup. Our study highlights the importance of integrated scanning electron microscopy and secondary ion mass spectrometry techniques in deep-time studies and suggests that different analytical methods (bulk vs. SIMS) and diagenetic history (primary vs. metamorphic) among different basins may have caused inconsistent interpretations of S isotope profiles of the GOE successions at a global scale. Key Words: Great Oxidation Event (GOE)-Secondary ion mass spectrometer (SIMS)-Paleoproterozoic-Sulfur isotopes-Mass independent fractionation (MIF). Astrobiology 18, 519-538.

Two species of spinose planktonic foraminifera (Globigerinoides ruber and Globigerina bulloides), cultured under controlled seawater temperatures (19, 21, 23, and 25 degrees C) with newly precipitated walls labeled with Ca isotopes, were subjected to quantitative analyses of Mg/Ca, Sr/Ca and Ba/Ca ratios at a spatial resolution of 2 pm. Mg mapping of calcite walls showed banding structures parallel to the wall surfaces for both species. Significant Mg accumulation was evident, particularly on walls precipitated at higher temperature, together with irregularly shaped chambers. Our experiment reproduced little cross-sectional variation in Sr/Ca, suggesting constant ratio levels in the calcification fluid and seawater. The Ba/Ca, however, showed different behavior in the two species: a relatively constant profile in Gs. tuber but a positive correlation with Mg/Ca in Ga bulloides. The different geochemical properties in the two species cultured under the same conditions suggest different elemental regulation processes in the calcification sites. Positive coupling of Mg-Ba (but constant Sr/Ca) is unlikely to be explained only by a Mg-specific removal process, whereas Sr and Ca transport may follow a similar pathway.

The vestiges of life in Eoarchean rocks have the potential to elucidate the origin of life. However, gathering evidence from many terrains is not always possible, and biogenic graphite has thus far been found only in the 3.7-3.8 Ga (gigayears ago) Isua supracrustal belt. Here we present the total organic carbon contents and carbon isotope values of graphite (δ13Corg) and carbonate (δ13Ccarb) in the oldest metasedimentary rocks from northern Labrador. Some pelitic rocks have low δ13Corg values of -28.2, comparable to the lowest value in younger rocks. The consistency between crystallization temperatures of the graphite and metamorphic temperature of the host rocks establishes that the graphite does not originate from later contamination. A clear correlation between the δ13Corg values and metamorphic grade indicates that variations in the δ13Corg values are due to metamorphism, and that the pre-metamorphic value was lower than the minimum value. We concluded that the large fractionation between the δ13Ccarb and δ13Corg values, up to 25‰, indicates the oldest evidence of organisms greater than 3.95 Ga. The discovery of the biogenic graphite enables geochemical study of the biogenic materials themselves, and will provide insight into early life not only on Earth but also on other planets.

This report describes spatial and temporal variations of helium, carbon, and nitrogen isotopes measured in CO2-rich bubbling gases from natural springs associated with Mt. Ontake (central Honshu, Japan) from November 1981 through September 2015. During the entire period, the He-3/He-4 ratio decreased concomitantly with increasing distance of the sampling site from the central cone. In contrast, the CO2/He-3 ratios and the delta C-13 values of CO2 increased with distance. These spatial trends became more pronounced after the September 27, 2014 Ontake eruption, suggesting reactivation of the volcano plumbing system with enhanced emission of magmatic volatiles, although the spatial trend of nitrogen isotopes disappeared. The 3He/4He ratios of the most proximal site to the central cone remained constant until 2000, apparently increasing from June 2003 through October 2014. They became constant soon after the eruption until September 2015. The ten-year He-3 enhancement might have been a precursor of the 2014 Ontake eruption. In contrast, delta C-13 values of CO2 at the same site remained constant through this period. The lack of delta C-13 anomaly might be attributable to 1) negligibly small amounts of magmatic CO2 introduced into the source of hot springs compared to ambient CO2 in the air-saturated aquifer, or 2) close resemblance of the carbon signature of magmatic CO2 to that of ambient CO2. Therefore its addition cannot change the overall delta C-13 value of the sampled hot springs. At distal sampling sites, no measurable change of helium, carbon, or nitrogen isotopes was observed in relation to the 2014 Ontake eruption, suggesting that the effect of this eruption on the Ontake hydrothermal system was geographically localized. (C) 2016 Elsevier B.V. All rights reserved.

Shallow submarine volcanoes have been newly discovered near the Tokara Islands, which are situated at the volcanic front of the northern Ryukyu Arc in southern Japan. Here, we report for the first time the volatile geochemistry of shallow hydrothermal plumes, which were sampled using a CTD-RMS system after analyzing water column images collected by multi-beam echo sounder surveys. These surveys were performed during the research cruise KS-14-10 of the R/V Shinsei Maru in a region stretching from the Wakamiko Crater to the Tokara Islands. The 3He flux and methane flux in the investigated area are estimated to be (0.99-2.6) × 104 atoms/cm2/sec and 6-60 t/yr, respectively. The methane in the region of the Tokara Islands is a mix between abiotic methane similar to that found in the East Pacific Rise and thermogenic one. Methane at the Wakamiko Crater is of abiotic origin but affected by isotopic fractionation through rapid microbial oxidation. The helium isotopes suggest the presence of subduction-type mantle helium at the Wakamiko Crater, while a larger crustal component is found close to the Tokara Islands. This suggests that the Tokara Islands submarine volcanoes are a key feature of the transition zone between the volcanic front and the spreading back-arc basin.

We measured the hydrogen isotopic composition (D/H ratios) and U-Pb chronology of phosphate minerals in a young Martian meteorite: LAR 06319. D/H ratios of melt-inclusions in the meteorite were also investigated to evaluate the presence of water reservoirs on Mars in recent times. The total Pb/U dating of multiple grains of apatite and merrillite yield a concordant date of 167 +/- 57 Ma, interpreted as the crystallization age, suggesting that both apatite and merrillite in this meteorite have preserved their igneous histories. The D/H ratios of an apatite grain show good reproducibility yielding a delta D value of 4250 +/- 120%, whereas those of merrillite show larger D/H variations with the maximum delta D value of 5260 +/- 790%. However, mafic glass in melt-inclusions shows extremely large variation in delta D, ranging from ca. 1070% to 6830 +/- 460%. The different D/H signatures recorded in these phases reflect the contributions of different hydrous components with distinct D/H ratios, possibly incorporated at different times. It is inferred that several isotopically distinct water reservoirs exist in the present Martian surface/sub-surface system.

Some benthic foraminiferal species are reportedly capable of nitrate storage and denitrification, however, little is known about nitrate incorporation and subsequent utilization of nitrate within their cell. In this study, we investigated where and how much (15)N or (34)S were assimilated into foraminiferal cells or possible endobionts after incubation with isotopically labeled nitrate and sulfate in dysoxic or anoxic conditions. After 2 weeks of incubation, foraminiferal specimens were fixed and prepared for Transmission Electron Microscopy (TEM) and correlative nanometer-scale secondary ion mass spectrometry (NanoSIMS) analyses. TEM observations revealed that there were characteristic ultrastructural features typically near the cell periphery in the youngest two or three chambers of the foraminifera exposed to anoxic conditions. These structures, which are electron dense and ~200-500 nm in diameter and co-occurred with possible endobionts, were labeled with (15)N originated from (15)N-labeled nitrate under anoxia and were labeled with both (15)N and (34)S under dysoxia. The labeling with (15)N was more apparent in specimens from the dysoxic incubation, suggesting higher foraminiferal activity or increased availability of the label during exposure to oxygen depletion than to anoxia. Our results suggest that the electron dense bodies in Ammonia sp. play a significant role in nitrate incorporation and/or subsequent nitrogen assimilation during exposure to dysoxic to anoxic conditions.

Insolation is an important component of meteorological data because solar energy is the primary and direct driver of weather and climate. Previous analyses of cultivated giant clam shells revealed diurnal variation in the Sr/Ca ratio, which might reflect the influence of the daily light cycle. We applied proxy method to sample from prehistoric era, a fossil giant clam shell collected at Ishigaki Island in southern Japan. The specimen was alive during the middle Holocene and thus exposed to the warmest climate after the last glacial period. This bivalve species is known to form a growth line each day, as confirmed by the analysis of the Sr enrichment bands using EPMA and facilitated age-model. We analyzed the Sr/Ca, Mg/Ca and Ba/Ca ratios along the growth axis, measuring a 2-μm spot size at 2-μm interval using NanoSIMS. The Sr/Ca ratios in the winter layers are characterized by a striking diurnal cycle consisting of narrow growth lines with high Sr/Ca ratios and broad growth bands with low Sr/Ca ratios. These variations, which are consistent with those of the cultivated clam shell, indicate the potential for the reconstruction of the variation in solar insolation during the middle Holocene at a multi-hourly resolution.

Nephrite in the Fengtien area of the eastern part of the Central Mountain Range, Taiwan, is associated with antigorite-serpentinite within the Yuli belt, a late Cenozoic subduction-accretionary complex related to the eastward subduction of the South China Sea plate forming the Luzon arc. Diopsidite and clinozoisite rock are two other metasomatic components accompanying nephrite between serpentinites and the greenschist-facies country rock (carbonaceous material-)quartz-mica schist. Detrital zircons were separated from one clinozoisite rock sample, formed through metasomatic replacement after mica-quartz schist at temperatures of 320-420 degrees C or slightly lower, which is lower than the metamorphic temperature conditions of the Yuli belt. Most of the detrital zircons have thin zircon rims less than 15-20 mu m wide. These zircon rims, considered as newly formed during metasomatism leading to nephrite/diopsidite/clinozoisite rock formation, were dated by a high lateral resolution secondary ion mass spectrometer (CAMECA NanoSIMS NS50). The resulting U-238/Pb-206-Pb-204/Pb-206 inverse isochron gave an age of 3.3 +/- 1.7Ma. The collision of the Eurasian continental margin with the Luzon arc has been suggested to have begun at ca. 6.5Ma in the Taiwan area. The nephrite formation processes therefore clearly post-dated South China Sea plate subduction. The present date, substantiated by the metamorphic and metasomatic temperature information, demonstrates that the fluid-rock interaction forming Fengtien nephrite would have taken place during a Barrovian-type metamorphic overprint resulting from arc-continent collision, leading to the exhumation of the Yuli belt. This conclusion on nephrite formation with regard to regional tectonics can serve as a working model for future studies on other nephrite deposits with similar occurrences, mostly embedded within Mesozoic or older subduction-accretionary complexes. The Fengtien nephrite deposit is therefore the youngest one of its kind exposed on Earth's surface.

This report describes in situ ion microprobe U-Pb dating of a protoconodont, an early Cambrian phosphate microfossil, using laterally high-resolution secondary ion mass spectrometry (NanoSIMS). On a single fragment of a fossil (approximately 850 mu m x 250 mu m) derived from a sedimentary layer in the Meishucunian Yuhucun Formation, Yunnan Province, southern China, 23 spots provide a U-238/Pb-206 isochron age of 547 +/- 43 Ma (2 sigma, MSWD = 1.9), which is consistent with the depositional age, 536.5 +/- 2.5 Ma estimated using zircon U-Pb dating of interbedded tuffs. However, five spots on a small region (approximately 250 mu m x 100 mu m) in the same protoconodont yield an isochron age of 417 +/- 74 Ma (2 sigma, MSWD = 0.31), apparently younger than the formation age. The younger age might be attributable to a later hydrothermal event, perhaps associated with Caledonian orogenic activity recorded in younger zircon with an age of 420-440 Ma. We also measured Sr-87/Sr-86 ratios of the protoconodont by NanoSIMS. In the older domain, 19 spots give the Sr-87/Sr-86 ratio of 0.71032 +/- 0.00023 (2 sigma), although seven spots on the younger region provide the ratio of 0.70862 +/- 0.00045; this is significantly less radiogenic than the older domain. This is the first report of U-Pb age and Sr isotope heterogeneity within a single fragment of micro-fossil (215). (C) 2014 Elsevier Ltd. All rights reserved.

The aim of this study is to get information of gas geochemical characteristics at hydrothermal vent sites in the adjacent region of Tokara Islands by using helium isotopes and methane concentration and its stable carbon isotope ratio to estimate the 3He flux and understand the mechanism of carbon cycling.

Some graphite contained in the 3.7-billion-year-old metasedimentary rocks of the Isua Supracrustal Belt, Western Greenland(1), is depleted in C-13 and has been interpreted as evidence for early life(2). However, it is unclear whether this graphite is primary, or was precipitated from metamorphic or igneous fluids(3,4). Here we analyse the geochemistry and structure of the C-13-depleted graphite in the Isua schists. Raman spectroscopy and geochemical analyses indicate that the schists are formed from clastic marine sediments that contained C-13-depleted carbon at the time of their deposition. Transmission electron microscope observations show that graphite in the schist occurs as nanoscale polygonal and tube-like grains, in contrast to abiotic graphite in carbonate veins that exhibits a flaky morphology. Furthermore, the graphite grains in the schist contain distorted crystal structures and disordered stacking of sheets of graphene. The observed morphologies are consistent with pyrolysation and pressurization of structurally heterogeneous organic compounds during metamorphism. We thus conclude that the graphite contained in the Isua metasediments represents traces of early life that flourished in the oceans at least 3.7 billion years ago.

Nitrogen isotopic analyses were conducted on two kerogenous samples from the Gunflint Formation (ca. 1.9 Ga) using the stepwise combustion technique to evaluate a potential analytical problem for the carbonaceous samples and to assess if this method is appropriate for the analysis of ancient rocks. Two discriminated (delta N-15 plateaux are identified for each sample with mean values of +5.0 parts per thousand and +7.3 parts per thousand for sample 0708, and +6.1 parts per thousand and +5.2 parts per thousand for sample 0704, respectively. The direction of the delta N-15 shift is opposite in each sample. This characteristic excludes the possibility of analytical artifact as the source of isotope fractionation and metamorphism isotope fractionation. The two delta N-15 plateaux observed for each of the samples are characterized by different activation energies for the co-released carbon. These results suggest that similarly aged sedimentary rocks may contain at least two types of organic matter that record different source information for delta N-15.