Ga Gunflint Research Articles

Erratum: High mass resolution fs-LIMS imaging and manifold learning reveal insight into chemical diversity of the 1.88 Ga Gunflint chert

An erratum on: Lukmanov RA, de Koning C, Schmidt PK, Wacey D, Ligterink NFW, Gruchola S, Grimaudo V, Neubeck A, Riedo A, Tulej M and Wurz P (2022) High Mass Resolution fs-LIMS Imaging and Manifold Learning Reveal Insight Into Chemical Diversity of the 1.88 Ga Gunflint Chert. Front. Space Technol. 3:718943. doi: 10.3389/frspt.2022.718943An omission to the funding section of the original article was made in error. The following sentence has been added: "Open access funding was provided by the University of Bern".The original version of this article has been updated.

Evolutionary diversification of paleoproterozoic prokaryotes: New microfossil records in 1.88 Ga Gunflint Formation

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.

High Mass Resolution fs-LIMS Imaging and Manifold Learning Reveal Insight Into Chemical Diversity of the 1.88 Ga Gunflint Chert

Extraction of useful information from unstructured, large and complex mass spectrometric signals is a challenge in many application fields of mass spectrometry. Therefore, new data analysis approaches are required to help uncover the complexity of such signals. In this contribution, we examined the chemical composition of the 1.88 Ga Gunflint chert using the newly developed high mass resolution laser ionization mass spectrometer (fs-LIMS-GT). We report results on the following: 1) mass-spectrometric multi-element imaging of the Gunflint chert sample; and 2) identification of multiple chemical entities from spatial mass spectrometric data utilizing nonlinear dimensionality reduction and spectral similarity networks. The analysis of 40′000 mass spectra reveals the presence of chemical heterogeneity (seven minor compounds) and two large clusters of spectra registered from the organic material and inorganic host mineral. Our results show the utility of fs-LIMS imaging in combination with manifold learning methods in studying chemically diverse samples.

Acritarch-like Microorganisms from the 1.9 Ga Gunflint Chert, Canada.

Fossil evidence of eukaryotic life older than 1.8 Ga has long been debated because known fossils of that age usually lack cellular micro- and ultra-structures that bear strong affinities to eukaryotes. These include fossils of the ∼1.9 Ga Gunflint Chert microbiota that, despite being exceptionally well preserved, have suffered from cellular degradation, which poses challenges to studying their delicate cellular structures. In this study, we use an extended-focal-depth imaging technique, in combination with scanning electron microscopy, to document multiple types of large (10–35 μm diameter), cyst-like bodies based on distinctive details such as (1) radially arranged internal strands similar to those in some acritarchs and dinoflagellates; (2) regularly spaced long tubular processes, stubby pustules, and/or robust podia on the cell surface; (3) reticulate cell-wall sculpturing such as scale-like tubercles, pits, and ridges; and (4) internal bodies that may represent membrane-bound organelles. These micro- and ultra-structures provide strong morphological evidence for the presence of protists in the late Paleoproterozoic.

Multiwavelength Ablation/Ionization and Mass Spectrometric Analysis of 1.88 Ga Gunflint Chert.

The investigation of chemical composition on planetary bodies without significant sample processing is of importance for nearly every mission aimed at robotic exploration. Moreover, it is a necessary tool to achieve the longstanding goal of finding evidence of life beyond Earth, for example, possibly preserved microbial remains within martian sediments. Our Laser Ablation Ionization Mass Spectrometer (LIMS) is a compact time-of-flight mass spectrometer intended to investigate the elemental, isotope, and molecular composition of a wide range of solid samples, including e.g., low bulk density organic remains in microfossils. Here, we present an overview of the instrument and collected chemical spectrometric data at the micrometer level from a Precambrian chert sample (1.88 Ga Gunflint Formation, Ontario, Canada), which is considered to be a martian analogue. Data were collected from two distinct zones-a silicified host area and a carbon-bearing microfossil assemblage zone. We performed these measurements using an ultrafast pulsed laser system (pulse width of ∼180 fs) with multiple wavelengths (infrared [IR]-775 nm, ultraviolet [UV]-387 nm, UV-258 nm) and using a pulsed high voltage on the mass spectrometer to reveal small organic signals. We investigated (1) the chemical composition of the sample and (2) the different laser wavelengths' performance to provide chemical depth profiles in silicified media. Our key findings are as follows: (1) microfossils from the Gunflint chert reveal a distinct chemical composition compared with the host mineralogy (we report the identification of 24 elements in the microfossils); (2) detection of the pristine composition of microfossils and co-occurring fine chemistry (rare earth elements) requires utilization of the depth profiling measurement protocol; and (3) our results show that, for analysis of heterogeneous material from siliciclastic deposits, siliceous sinters, and cherts, the most suitable wavelength for laser ablation/Ionization is UV-258 nm.

Nanoscale 3D quantitative imaging of 1.88 Ga Gunflint microfossils reveals novel insights into taphonomic and biogenic characters

Precambrian cellular remains frequently have simple morphologies, micrometric dimensions and are poorly preserved, imposing severe analytical and interpretational challenges, especially for irrefutable attestations of biogenicity. The 1.88 Ga Gunflint biota is a Precambrian microfossil assemblage with different types and qualities of preservation across its numerous geological localities and provides important insights into the Proterozoic biosphere and taphonomic processes. Here we use synchrotron-based ptychographic X-ray computed tomography to investigate well-preserved carbonaceous microfossils from the Schreiber Beach locality as well as poorly-preserved, iron-replaced fossil filaments from the Mink Mountain locality, Gunflint Formation. 3D nanoscale imaging with contrast based on electron density allowed us to assess the morphology and carbonaceous composition of different specimens and identify the minerals associated with their preservation based on retrieved mass densities. In the Mink Mountain filaments, the identification of mature kerogen and maghemite rather than the ubiquitously described hematite indicates an influence from biogenic organics on the local maturation of iron oxides through diagenesis. This non-destructive 3D approach to microfossil composition at the nanoscale within their geological context represents a powerful approach to assess the taphonomy and biogenicity of challenging or poorly preserved traces of early microbial life, and may be applied effectively to extraterrestrial samples returned from upcoming space missions.

Hematite replacement and oxidative overprinting recorded in the 1.88 Ga Gunflint iron formation, Ontario, Canada

Abstract The 1.88 Ga Gunflint Formation in Ontario, Canada, has played a key role in the development of current models for the deposition of iron formations. The presence of hematite-rich iron formation intercalated with chert stromatolites containing purported cyanobacterial microfossils sparked the idea that biology was the principal driver of Fe2+ oxidation and iron deposition. However, despite the abundance of hematite in the Gunflint Formation, a primary depositional origin has not been established. Here we present evidence for the replacement of Fe-silicate granules by hematite in drill core intersecting the Gunflint Formation. Iron-oxide replacement proceeded inwards from granule boundaries and along intergranular fractures, producing iron oxide–rich rims around Fe-silicate cores. The abundance of organic matter in shaly iron formation implies that the iron-rich mudstones experienced anoxic diagenesis and that coexisting hematite was not depositional but formed after burial. Widespread distribution of the alteration textures indicates that this was a large-scale process and that much of the hematite is not primary. Lifting the veil of oxidative overprinting reveals an iron-rich sediment that was originally more reduced and dominated by Fe(II)-rich minerals. Our results imply that a major assumption underpinning the original model for biological iron oxidation as the driver of iron formation deposition may be flawed, raising broader questions about the origin of hematite in other iron formations and the role of biology in iron deposition in the early oceans.

Evidence for widespread oil migration in the 1.88 Ga Gunflint Formation, Ontario, Canada

Abstract The abundance of Precambrian organic-rich shales, heated beyond the oil and gas window, requires that enormous volumes of hydrocarbons were generated and transported through the ancient crust. However, the former passage of fluid hydrocarbons rarely leaves a trace, so evidence for this process in the early Precambrian rock record is sparse. Here, we report the widespread presence of solidified oil (pyrobitumen) in the iron formation of the 1.88 Ga Gunflint Formation, Ontario, Canada. Petrographic textures indicate at least two phases of oil migration, an early phase marked by pyrobitumen in granules and intergranular pores, synchronous with synsedimentary silica cementation, and a later phase restricted to crosscutting fractures. The paragenetic relationships between the pyrobitumen and iron oxides indicate that oil migration commenced before hematite and some magnetite growth. Our evidence for early oil migration can be explained by petroleum generation during the 1.86–1.80 Ga Penokean orogeny, expelling hydrocarbons generated in the core of the fold-and-thrust belt outwards and updip through the sediments of the Gunflint Formation.

Snapshot of an early Paleoproterozoic ecosystem: Two diverse microfossil communities from the Turee Creek Group, Western Australia.

Eighteen microfossil morphotypes from two distinct facies of black chert from a deep-water setting of the c. 2.4Ga Turee Creek Group, Western Australia, are reported here. A primarily in situ, deep-water benthic community preserved in nodular black chert occurs as a tangled network of a variety of long filamentous microfossils, unicells of one size distribution and fine filamentous rosettes, together with relatively large spherical aggregates of cells interpreted as in-fallen, likely planktonic, forms. Bedded black cherts, in contrast, preserve microfossils primarily within, but also between, rounded clasts of organic material that are coated by thin, convoluted carbonaceous films interpreted as preserved extracellular polymeric substance (EPS). Microfossils preserved within the clasts include a wide range of unicells, both much smaller and larger than those in the nodular black chert, along with relatively short, often degraded filaments, four types of star-shaped rosettes and umbrella-like rosettes. Large, complexly branching filamentous microfossils are found between the clasts. The grainstone clasts in the bedded black chert are interpreted as transported from shallower water, and the contained microfossils thus likely represent a phototrophic community. Combined, the two black chert facies provide a snapshot of a microbial ecosystem spanning shallow to deeper-water environments, and an insight into the diversity of life present during the rise in atmospheric oxygen. The preserved microfossils include two new, distinct morphologies previously unknown from the geological record, as well as a number of microfossils from the bedded black chert that are morphologically similar to-but 400-500Ma older than-type specimens from the c. 1.88Ga Gunflint Iron Formation. Thus, the Turee Creek Group microfossil assemblage creates a substantial reference point in the sparse fossil record of the earliest Paleoproterozoic and demonstrates that microbial life diversified quite rapidly after the end of the Archean.

Iron minerals within specific microfossil morphospecies of the 1.88\u2009Ga Gunflint Formation

Problematic microfossils dominate the palaeontological record between the Great Oxidation Event 2.4 billion years ago (Ga) and the last Palaeoproterozoic iron formations, deposited 500–600 million years later. These fossils are often associated with iron-rich sedimentary rocks, but their affinities, metabolism, and, hence, their contributions to Earth surface oxidation and Fe deposition remain unknown. Here we show that specific microfossil populations of the 1.88 Ga Gunflint Iron Formation contain Fe-silicate and Fe-carbonate nanocrystal concentrations in cell interiors. Fe minerals are absent in/on all organically preserved cell walls. These features are consistent with in vivo intracellular Fe biomineralization, with subsequent in situ recrystallization, but contrast with known patterns of post-mortem Fe mineralization. The Gunflint populations that display relatively large cells (thick-walled spheres, filament-forming rods) and intra-microfossil Fe minerals are consistent with oxygenic photosynthesizers but not with other Fe-mineralizing microorganisms studied so far. Fe biomineralization may have protected oxygenic photosynthesizers against Fe2+ toxicity during the Palaeoproterozoic.

X-ray microtomography as a tool for investigating the petrological context of Precambrian cellular remains

Abstract A wide spectrum of tomographic techniques now exists for studying palaeontological specimens, but the suitability of these methods for assessing Earth's oldest prokaryotic life has not been comprehensively investigated. We evaluated the ability of X-ray computed tomography – specifically X-ray microtomography – to reveal the morphology and petrological context of Precambrian microfossils, pseudofossils and biosedimentary structures, all of which are important in the origin and early evolution of life of Earth. The materials tested came from the Pilbara Craton of Western Australia (the 3.49 Ga Dresser Formation, the 3.46 Ga Apex chert and the 3.43 Ga Strelley Pool Formation) and the 1.88 Ga Gunflint Formation of Ontario, Canada. These units chart key developments in palaeobiology. The oldest formations contain profoundly controversial microfossil-like objects and microbially-induced sedimentary structures, whereas definitive prokaryotes are found in the youngest formations. We demonstrate that the imaging of individual microfossils and pseudofossils currently lies at the limits of the capabilities of laboratory-based X-ray microtomography and requires beneficial taphonomy. However, microtomography does provide a good overview of their petrological context at flexible spatial scales, although the quality of the data obtained from mesoscopic microbially-induced sedimentary structures and stromatolites depends largely on their style of preservation. Supplementary material: A zipped Drishti volume for all CT scans, a 0.7z split zip file of one Drishti volume and a HDMI supplementary movie showing digital visualizations for all of the scans are available at https://doi.org/10.5281/zenodo.58161

Molecular preservation of 1.88 Ga Gunflint organic microfossils as a function of temperature and mineralogy.

The significant degradation that fossilized biomolecules may experience during burial makes it challenging to assess the biogenicity of organic microstructures in ancient rocks. Here we investigate the molecular signatures of 1.88 Ga Gunflint organic microfossils as a function of their diagenetic history. Synchrotron-based XANES data collected in situ on individual microfossils, at the submicrometre scale, are compared with data collected on modern microorganisms. Despite diagenetic temperatures of ∼150–170 °C deduced from Raman data, the molecular signatures of some Gunflint organic microfossils have been exceptionally well preserved. Remarkably, amide groups derived from protein compounds can still be detected. We also demonstrate that an additional increase of diagenetic temperature of only 50 °C and the nanoscale association with carbonate minerals have significantly altered the molecular signatures of Gunflint organic microfossils from other localities. Altogether, the present study provides key insights for eventually decoding the earliest fossil record.

Chemical and textural overprinting of ancient stromatolites: Timing, processes, and implications for their use as paleoenvironmental proxies

Shale-normalized Rare Earth Element (REESN) patterns and transition metal abundances in banded iron formations (BIF) and shales have long been used as paleoenvironmental proxies to investigate the evolving geochemistry of Precambrian oceans. Apparent similarities between the REESN of modern stromatolites and contemporary seawater values also mean that some ancient stromatolites could record paleo-ocean chemistry. To test this hypothesis, we present comparative mineralogical, textural, and spectral analyses of multifurcate, silicified and iron oxide/carbonate-rich, stromatolites from the ca.1.88Ga Gunflint Formation (Ontario, Canada). Previous studies show that these organo-sedimentary structures represent the initiation of shallow marine sedimentation in the Animikie Basin on the southern tip of the Superior Province. We present evidence that these structures were originally stabilized by aragonite and Mg-bearing carbonates. Fabrics indicative of stromatolite accretion under local hypersaline conditions are also present, with La anomalies suggestive of laterally variable freshwater runoff. The admixture of partially evaporated seawater and silica-oversaturated deep seawater led to pervasive silicification of metastable carbonates and preservation of less soluble early diagenetic carbonate cements. Bulk rock transition metal contents point to lateral variability and dissimilar partitioning and fractionation during mineral formation, yet Ce anomalies amongst samples are similar. Considering the pronounced differences between the REE partitioning between Fe oxyhydroxide precipitates and carbonates, our results point to hematite and siderite as secondary minerals that postdate the stabilization of these structures by carbonates and silica. Stratabound relations of the Fe-rich stromatolites with diabase sills and dikes indicate that the pore water fluids evolved towards a localized state of hematite and siderite oversaturation towards the Mesoproterozoic. If hematite and siderite are both early diagenetic phases, then the Ce anomalies of these stromatolites may be a poor measure of the redox conditions of the shallow areas of the Animikie Basin at the time of accretion.

Fossils and astrobiology: new protocols for cell evolution in deep time

AbstractThe study of life remote in space has strong parallels with the study of life remote in time. Both are dependent on decoding those historic phenomena called ‘fossils’, here taken to include biogenic traces of activity and waste products. There is the shared problem of data restoration from incomplete data sets; the importance of contextual analysis of potentially viable habitats; the centrality of cell theory; the need to reject the null hypothesis of an abiogenic origin for candidate cells via morphospace analysis; the need to demonstrate biology-like behaviour (e.g., association with biofilm-like structures; tendency to form clusters and ‘mats’; and a preference for certain substrates), and of metabolism-like behaviour (e.g., within the candidate cell wall; within surrounding ‘waste products’; evidence for syntrophy and metabolic cycles; and evidence for metabolic tiers). We combine these ideas into a robust protocol for demonstrating ancient or extra-terrestrial life, drawing examples from Earth's early geological record, in particular from the earliest known freshwater communities of the 1.0 Ga Torridonian of Scotland, from the 1.9 Ga Gunflint Chert of Canada, from the 3.4 Ga Strelley Pool sandstone of Australia, and from the 3.46 Ga Apex Chert also of Australia.

Ion microprobe analyses of δ18O in early quartz cements from 1.9 Ga granular iron formations (GIFs): A pilot study

The low δ18O values of Precambrian cherts have been widely used to infer that temperatures were higher and/or seawater δ18O was lower compared to today's oceans. However, the Precambrian cherts presented as evidence for these temperatures are neomorphosed from amorphous precursors that originally precipitated from ocean water, suggesting diagenetic alteration is an important cause of the widespread low-δ18O values. Many pores between sand-size clasts in granular iron formations (GIFs) are filled with coarsely crystalline quartz cements that are texturally primary. In unmetamorphosed GIF samples from the 1.9Ga Gunflint and Sokoman iron formations, synsedimentary clasts of GIF and high minus-cement porosities (the volume of pore space occupied by recognizable cement, inversely proportional to the degree of compaction) indicate such cements precipitated directly from pore water near the depositional interface. Growth bands identified in quartz cement crystals using cathodoluminescence (CL) imaging further confirm their void-filling and unrecrystallized nature. In order to test the effect of neomorphism on the δ18O of silica in these rocks, unrecrystallized primary quartz cements and neomorphic quartz in adjacent clasts were both analyzed by ion microprobe. Values of δ18O range from 23.5 to 26.4‰ VSMOW in cements and 21.3–26.8‰ in clasts in samples from the Gunflint Iron Formation. Samples from the Sokomon Iron Formation record deeper, more evolved pore waters and in some cases contain lower δ18O cements. In individual pores of Gunflint samples, cement values vary only slightly more than analytical uncertainty from edge to center and no consistent increasing or decreasing trends were observed. Overall, the δ18O of the cements are similar to the most elevated δ18O reported from cherts of similar age, but span a substantially smaller range. This suggests most of the lower values reported previously reflect late-diagenetic conditions at greater burial depths. The shallow, possibly restricted seas above the Gunflint and Sokoman GIFs appear to have been warmer and may have been lower in δ18O than present-day seawater, but they may not have been representative of global seawater.

Microscale oxygen isotope variations in 1.9 Ga Gunflint cherts: Assessments of diagenesis effects and implications for oceanic paleotemperature reconstructions

Variations in the oxygen isotope composition (δ 18O) of five cherts from the 1.9 Ga Gunflint iron formation (Canada) were studied at the micrometer scale by ion microprobe to try to better understand the processes that control δ 18O values in cherts and to improve seawater paleotemperature reconstructions. Gunflint cherts show clearly different δ 18O values for different types of silica with for instance a difference of ≈15‰ between detrital quartz and microquartz. Microquartz in the five samples is characterized by large intra sample variations in δ 18O values, (δ 18O of quartz varies from 4.6‰ to 6.6‰ at the 20 μm scale and from ≈12‰ to 14‰ at 2 μm scale). Isotopic profiles in microquartz adjacent to hydrothermal quartz veins demonstrate that microquartz more than ≈200 μm away from the veins has preserved its original δ 18O value. At the micrometer spatial resolution of the ion probe, data reveal that microquartz has preserved a considerable δ 18O heterogeneity that must be regarded as a signature inherited from its diagenetic history. Modelling of the δ 18O variations produced during the diagenetic transformation of sedimentary amorphous silica precursors into microquartz allows us to calculate seawater temperature ( T sw at which the amorphous silica precipitated) and diagenesis temperature ( T diagenesis at which microquartz formed) that reproduce the δ 18O distributions (mean, range and shape) measured at micrometer scale in microquartz. The two critical parameters in this modelling are the δ 18O value and the mass fraction of the diagenetic fluid. Under these assumptions, the most likely ranges for T sw and T diagenesis are from 37 to 52 °C and from 130 to 170 °C, respectively.

A transmission electron microscopy study of silica and kerogen biosignatures in approximately 1.9 Ga gunflint microfossils.

Microfossils preserved in chert from the;1.9 Ga Gunflint Formation (Schreiber Beach, Ontario, Canada) were studied with transmission electron microscopy (TEM) and analytical TEM (ATEM). Our goals were to uncover the style of silicification relative to the distribution of organic matter, and to evaluate the distribution and evolution of organic matter, at submicroscopic spatial scales. Petrographically the microfossils typically display filamentous or coccoidal morphologies, and consist of quartz crystals surrounded by kerogen along grain boundaries. ATEM analysis revealed that quartz associated with kerogen consists of 200-500nm-sized, round crystallites, whereas the chert matrix is comprised of randomly oriented, polygonal microquartz (5-10 microm). Silica spheroids found within some fossils consist of quartz subgrains in an amorphous to poorly crystalline matrix, suggesting that precipitation of opaline silica on organic matter occurred with subsequent but incomplete transformation to quartz. Some coccoidal microfossils surround large euhedral quartz crystals (up to 5 microm in diameter) that appeared to have influenced the distribution of kerogen during crystal growth. These euhedral quartz crystals commonly contain elongated (50-100 nm) iron-rich crystallites. Energy-loss, near-edge structure analysis of kerogen associated with a coccoidal microfossil showed that it is composed of amorphous carbon with no evidence of graphitization. TEM results revealed significant differences in the style of silicification between microbe-shaped microfossils and their surrounding chert matrix, as well as the presence of amorphous kerogen.

Fossil actinomycete in Eocene-Oligocene Dominican amber

Actinomycetes are Gram-positive prokaryotes that tend to form branching and fragmenting filaments, which in some groups form a sizable mycelium. They make up a large and important part of modern terrestrial microfloras but are not known extensively as fossils, although they have a long fossil history. Actinomycete-like fossils appear several times in the Precambrian: in the middle Precambrian Gowganda Formation of Ontario (Jackson, 1967), in the 2.0 Ga Gunflint Chert of Ontario (Lanier, 1987), and possibly in a lichen-like symbiosis in the 2.8 Ga Witwatersrand rocks of South Africa (Hallbauer and Van Warmelo, 1974), among others. Direct fossil evidence of actinomycetes is very rare in the Phanerozoic, and some “fossil” actinomycetes may be later contaminants (Knoll, 1977; Smoot and Taylor, 1983). Hyphae identified as actinomycetes are known from rod-like bodies identified as nematodes inside a decaying scorpion from the lower Carboniferous of Scotland (Stoermer, 1964), and from the interior of fern phloem cells from the Pennsylvanian (Smoot and Taylor, 1983). UnmineralizedActinomyces-like cells are known from calcite in bituminous lake-bed sediments from the early Cretaceous of Nevada (Bradley, 1963), and similar, poorly preserved fossils of a form calledActinomyciteshave been reported from the Jurassic of Scotland (Ellis, 1915). Actinorhizal nodules, formed by actinomycetes symbiotic with plant roots, have been described from the late Pleistocene (Baker and Miller, 1980).

Stable isotope geochemistry of cherts and carbonates from the 2.0 Ga gunflint iron formation: implications for the depositional setting, and the effects of diagenesis and metamorphism

Abstract δ 18 O values for all but three of 97 samples of chert along a ∼220 km transect through the 2.0 Ga Gunflint Iron Formation of the Animikie Basin, North America, range from 21.3 to 24.7%o. There is no correlation between δ 18 O and the type of chert (i.e., deeper-water lutitic chert, shallower water arenitic chert, and peritidal algal chert). The fact that the Gunflint Iron Formation has been exposed to thermal conditions only slightly above burial diagenesis suggests that the δ values of the chert have not been significantly affected by metamorphism. Field and petrographic observations, and the small δ 18 O range of all the chert types suggests that silicification, lithification, and any conversion to quartz chert from a hydrous silica precursor occurred close to the sediment-water interface. The uniform δ 18 O values of all the different chert types suggest that all cherts precipitated over a relatively common temperature interval (∼20°C) from a common parent water having a relatively uniform δ 18 O. The isotopic data suggest silica saturation and precipitation was not the result of the mixing of seawater and meteoric water, or the result of evaporative processes. Siderite in the deep-water, banded facies of the Gunflint displays a δ 18 O range of −6.0 to −2.6%o, whereas siderite in the more shoreward, organic-rich, shale facies is significantly enriched in 13 C ( δ 13 C=−2.5 to +0.5%o). δ 18 C of siderite does not appear to have been altered as a result of neomorphic recrystallization, and authigenic microsparitic siderite along stylolites displays the same δ 13 C range as primary and neomorphosed siderite. However, neomorphic and authigenic siderite are depleted in 18 O by up to 7%o relative to primary, unaltered microspheroidal siderite. The lower δ 13 C of banded facies siderite (relative to the shale facies siderite) does not seem to be the result of thermal decarboxylation or anaerobic oxidation (abiotic or bacterial) of organic matter, but rather is better explained by primary precipitation in an ocean system that was layered with respect to carbon isotopes. The more basinward banded facies siderite precipitated from a 13 C depleted, Fe 2+ -rich, hydrothermal water mass that was moved by advective upwelling from submarine regions of magmatic/volcanic-hydrothermal activity onto the continental shelf. The siderite of the shale facies precipitated from a more near-shore, shallow seawater mass that was comparatively enriched in 13 C ( δ 13 C≈0±2% o ). Ankerite in the Gunflint Iron Formation occurs as a replacement of primary siderite in the banded facies, and as a replacement of greenalite and chert in the arenite facies. Many ankerite samples have δ 13 C values that are similar to those of the shale facies siderite which suggests that shallow seawater was a major component of the diagenetic fluid. Later neomorphism of the replacement ankerite at higher burial temperatures and probably in the presence of 18 O depleted waters resulted in a wide range of δ 18 O values (13.8–22.5%o). The unique mineralogy of the upper most Gunflint Iron Formation (calcite, serpentine and magnetite) compared to the majority of the Formation, the very low δ values, and overlying diabase sills suggest that the non-ferroan nature of the carbonate in the upper Gunflint is the result of contact metamorphism.

Petrological and stable isotope studies of carbonate and sulfide minerals from the Gunflint Formation, Ontario: evidence for the origin of early Proterozoic iron-formation

Abstract The ∼ 1.9 Ga Gunflint Formation, in the Thunder Bay district of Ontario, principally comprises iron-formation, overlain by an iron-poor limestone/dolostone member (the Upper Limestone Member). Rocks are virtually unmetamorphosed, retaining primary textures, except locally around diabase sills. The Gunflint presents an opportunity to study both unmetamorphosed and metamorphosed iron-formation and to study the changes from iron-rich to iron-poor sedimentation. Of the carbonate minerals in iron-rich units, siderite formed at or near the sediment/water interface, with ankerite and calcite forming later during diagenesis and during metamorphism around sills. Heaviest δ13C values of siderite, and of dolomite in the Upper Limestone Member, are consistently near 0ℵ throughout the stratigraphic section, indicating that marine bicarbonate was the source of carbon. A spread of values between 0 and −7ℵ for unmetamorphosed rocks is the result of the incorporation of oxidized organic matter during diagenesis, whereas δ13C values more negative than −7ℵ are the result of metamorphism. The heaviest δ18O values from unmetamorphosed rocks, −5.3ℵ (PDB) for siderite, −6.1ℵ for ankerite, and −6.7ℵ for calcite, are considered to represent the original marine composition. Lighter values (to −17ℵ) represent isotopic exchange reactions with pore fluids at higher temperatures and/or isotopic exchange with 18O-depleted meteoric water. Low S/C ratios and a relatively narrow range of positive δ34S values in pyrite throughout most of the formation are consistent with bacterial sulphate reduction of seawater containing sulphate at a significantly lower concentration than the modern ocean. Locally, higher sulfide contents and a wider range of δ34S values were the product of the introduction of sulfur after deposition. It is proposed that the Gunflint Formation was deposited in a stratified water column with anoxic bottom water. The depositional basin had restricted communication with the open ocean and was affected by distal volcanism. Hydrothermal activity associated with the volcanism provided a large source of dissolved iron, and possibly silica, which helped buffer O2 and sulphate in water to low levels. Low concentrations of sulphate limited the generation of H2S that would otherwise have restricted the solubility of iron. During periods of increased hydrothermal activity, the anoxic/oxic water boundary moved upwards, permitting transport of iron to the shallow shelf, where it was precipitated as siderite, iron hydroxides, iron-silicates, or pyrite depending on physico-chemical conditions. The transition to the overlying limestone marks a decrease in hydrothermal activity with a contraction of the redox boundary.