Ancient Cherts Research Articles

Silicon and oxygen isotope fractionation in a silicified carbonate rock

Silicon isotope fractionation during silicification is poorly understood and impedes our ability to decipher paleoenvironmental conditions from Si isotopes in ancient cherts. To investigate isotope fractionation during silica-for‑carbonate replacement we analyzed the microscale Si and O isotope composition in different silica phases in a silicified zebra dolostone as well as their bulk δ18O and Δ’17O compositions. The subsequent replacement of carbonate layers is mimicked by decreasing δ18O and δ30Si. The textural relationship and magnitude of Si and O isotope fractionation is best explained by near-quantitative silica precipitation in an open system with finite Si. A Rayleigh model for silicification suggests positive Ɛ30/28Si during silicification, conforming with predictions for isotope distribution at chemical equilibrium from ab-initio models. Application of the modelled Ɛ30Si-T relationship yields silicification temperatures of approx. 50 °C. To reconcile the δ18Ochert composition with these temperatures, the δ18O of the fluid must have been between −2.5 and − 4 ‰, compositions for which the quartz phases fall close to the oxygen equilibrium fractionation line in three-isotope space. Diagenetic silica replacement appears to occur in O and Si isotopic equilibrium allowing reconstructions of temperatures of silicification from Si isotopes and derive the δ18O composition of the fluid – a highly desired value needed for accurate reconstructions of the temperature- and δ18O histories of the oceans.

Labyrinth patterns in Magadi (Kenya) cherts: Evidence for early formation from siliceous gels

Abstract Sedimentary cherts, with well-preserved microfossils, are known from the Archean to the present, yet their origins remain poorly understood. Lake Magadi, Kenya, has been used as a modern analog system for understanding the origins of nonbiogenic chert. We present evidence for synsedimentary formation of Magadi cherts directly from siliceous gels. Petrographic thin-section analysis and field-emission scanning electron microscopy of cherts from cores drilled in Lake Magadi during the Hominin Sites and Paleolakes Drilling Project in 2014 led to the discovery of two-dimensional branching “labyrinth patterns” in chert, which are a type of fractal “squeeze” pattern formed at air-liquid interfaces. Labyrinth patterns preserved in chert from Lake Magadi cores indicate invasion of air along planes in dewatering gels. These patterns support the precipitation of silica gels in the saline-alkaline Lake Magadi system and syndepositional drying of gels in contact with air as part of chert formation. Recognizing cherts as syndepositional has been critical for our use of them for U-Th dating. Identification of labyrinth patterns in ancient cherts can provide a better understanding of paleoenvironmental and geochemical conditions in the past.

Effect of diagenetic phase transformation on the silicon isotope composition of opaline sinter deposits of Geysir, Iceland

Detectible δ30Si variations in present-day chemical silica deposits have stimulated the application of silicon isotopes to infer environmental conditions from ancient equivalents. Interpretations of δ30Si signatures remain problematic in view of potential post-depositional changes, of which magnitudes and underlying mechanisms are largely unknown. A critical issue in the interpretation of isotope data from cherts concerns the extent to which early-diagenetic processes modify original δ30Si signatures. Here, we report δ30Si variations in opal-A, opal-A/CT and opal-CT from fossil sinter deposits in an active discharge apron in the Geysir geothermal field, Iceland. Opal-A samples show an average δ30Si of −0.7±0.2‰, while opal-CT samples are isotopically lighter with an average δ30Si of −2.0±0.4‰, implying a sizable shift of approximately 1.3‰ between the different phases. This shift can be explained by repetitive dissolution/re-precipitation processes, diffusive transport or temperature differences during phase transitions. On average, the fossil opal-A tends to be less negative in δ30Si than amorphous silica that recently precipitated from the hydrothermal water. The difference can be attributed to primary variability in isotopic fractionation that accompanies precipitation out of spring water at the surface, or to a post-depositional release of surface 28Si at the onset of diagenetic formation. Our results corroborate the perception that original silicon isotope signatures of silica, acquired during chemical precipitation from a saturated solution, may not be preserved in the geological record, and that post-depositional changes must be taken into account when interpreting data from ancient chert deposits.

The silicon isotope record of early silica diagenesis

The heavy isotopes of silicon are strongly enriched in some of the youngest, early diagenetically formed porcellanite layers from the Southwest Indian Ridge (Pleistocene) and the Maud Rise (Pliocene). These porcellanite layers are composed of opal-CT and were formed by the conversion of amorphous silica (opal-A) from siliceous sediment via dissolution–reprecipitation. Their bulk δSi30 values range between 1.7 and 2.3‰. Detritus-poor siliceous sediment surrounding these layers is significantly lower at −0.3 to 1.5‰. Sequential chemical extractions of bulk siliceous sediment show (i) preferential dissolution of diatoms featuring higher δSi30 than radiolaria and Al–Si components. The detailed investigation of porcellanite layers by micro-scale Si isotope and Al/Si analyses using UV femtosecond laser ablation ICP mass spectrometry show that (ii) precipitation of authigenic aluminum silicates enriched in light Si isotopes drives pore waters to even higher δSi30. We suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert. We infer that past environmental conditions can be reconstructed with high fidelity from the stable Si isotope composition of chert when initial seawater Si concentrations were high (such as in the Precambrian). Exchange of Si between layers during phase transformation (from opal-A to opal-CT and from opal-CT to quartz) is impeded when variable amounts of detrital minerals are present, because they control rates of silica phase transformation and hence the timing of dissolution–reprecipitation during burial.

Ultrastructural Heterogeneity of Carbonaceous Material in Ancient Cherts: Investigating Biosignature Origin and Preservation.

Opaline silica deposits on Mars may be good target sites where organic biosignatures could be preserved. Potential analogues on Earth are provided by ancient cherts containing carbonaceous material (CM) permineralized by silica. In this study, we investigated the ultrastructure and chemical characteristics of CM in the Rhynie chert (c. 410 Ma, UK), Bitter Springs Formation (c. 820 Ma, Australia), and Wumishan Formation (c. 1485 Ma, China). Raman spectroscopy indicates that the CM has experienced advanced diagenesis or low-grade metamorphism at peak metamorphic temperatures of 150-350°C. Raman mapping and micro-Fourier transform infrared (micro-FTIR) spectroscopy were used to document subcellular-scale variation in the CM of fossilized plants, fungi, prokaryotes, and carbonaceous stromatolites. In the Rhynie chert, ultrastructural variation in the CM was found within individual fossils, while in coccoidal and filamentous microfossils of the Bitter Springs and formless CM of the Wumishan stromatolites ultrastructural variation was found between, not within, different microfossils. This heterogeneity cannot be explained by secondary geological processes but supports diverse carbonaceous precursors that experienced differential graphitization. Micro-FTIR analysis found that CM with lower structural order contains more straight carbon chains (has a lower R3/2 branching index) and that the structural order of eukaryotic CM is more heterogeneous than prokaryotic CM. This study demonstrates how Raman spectroscopy combined with micro-FTIR can be used to investigate the origin and preservation of silica-permineralized organics. This approach has good capability for furthering our understanding of CM preserved in Precambrian cherts, and potential biosignatures in siliceous deposits on Mars.

Texture-specific Si isotope variations in Barberton Greenstone Belt cherts record low temperature fractionations in early Archean seawater

Sedimentary cherts are unusually abundant in early Archean (pre-3.0Ga) sequences, suggesting a silica cycle that was profoundly different than the modern system. Previously applied for the purpose of paleothermometry, Si isotopes in ancient cherts can offer broader insight into mass fluxes and mechanisms associated with silica concentration, precipitation, diagenesis, and metamorphism. Early Archean cherts contain a rich suite of sedimentological and petrographic textures that document a history of silica deposition, cementation, silicification, and recrystallization. To add a new layer of insight into the chemistry of early cherts, we have used wavelength-dispersive spectroscopy and then secondary ion mass spectrometry (SIMS) to produce elemental and Si and O isotope ratio data from banded black-and-white cherts from the Onverwacht Group of the Barberton Greenstone Belt, South Africa. This geochemical data is then interpreted in the framework of depositional and diagenetic timing of silica precipitation provided by geological observations. SIMS allows the comparison of Si and O isotope ratios of distinct silica phases, including black carbonaceous chert beds and bands (many including well-defined sedimentary grains), white relatively pure chert bands including primary silica granules, early cavity-filling cements, and later quartz-filled veins. Including all chert types and textures analyzed, the δ30Si dataset spans a range from −4.78‰ to +3.74‰, with overall mean 0.20‰, median 0.51‰, and standard deviation 1.30‰ (n=1087). Most samples have broadly similar δ30Si distributions, but systematic texture-specific δ30Si differences are observed between white chert bands (mean +0.60‰, n=750), which contain textures that represent primary and earliest diagenetic silica phases, and later cavity-filling cements (mean −1.41‰, n=198). We observed variations at a ∼100μm scale indicating a lack of Si isotope homogenization at this scale during diagenesis and metamorphism, although fractionations during diagenetic phase transformations may have affected certain textures. We interpret these systematic variations to reflect fractionation during silica precipitation as well as isotopically distinct fluids from which later phases originated. SIMS δ18O values fall in a range from 16.39‰ to 23.39‰ (n=381), similar to previously published data from bulk gas source mass spectrometry of Onverwacht cherts. We observed only limited examples of texture-related variation in δ18O and did not observe correlation of δ18O with δ30Si trends. This is consistent with hypotheses that Si isotope ratios are more resistant to alteration under conditions of rock-buffered diagenesis (Marin-Carbonne et al., 2011). Our results indicate that low temperature processes fractionated silicon isotopes in early Archean marine basins, a behavior that probably precludes the application of chert δ30Si as a robust paleothermometer. The values we observe for facies that sedimentological and petrographic observations indicate formed as primary and earliest diagenetic silica precipitates from seawater are more 30Si-rich than that expected for bulk silicate Earth. This is consistent with the hypothesis that the silicon isotope budget is balanced by the coeval deposition of 30Si-enriched cherts and 30Si-depleted iron formation lithologies. Precipitation of authigenic clay minerals in both terrestrial and marine settings may have also comprised a large 30Si-depleted sink, with the corollary of an important non-carbonate alkalinity sink consuming cations released by silicate weathering.

Nuclear Magnetic Biosignatures in the Carbonaceous Matter of Ancient Cherts: Comparison with Carbonaceous Meteorites

The search for organic biosignatures is motivated by the hope of understanding the conditions of emergence of life on Earth and the perspective of finding traces of extinct life in martian sediments. Paramagnetic radicals, which exist naturally in amorphous carbonaceous matter fossilized in Precambrian cherts, were used as local structural probes and studied by electron paramagnetic resonance (EPR) spectroscopy. The nuclear magnetic resonance transitions of elements inside and around these radicals were detected by monitoring the nuclear modulations of electron spin echo in pulsed EPR. We found that the carbonaceous matter of fossilized microorganisms with age up to 3.5 billion years gives specific nuclear magnetic signatures of hydrogen (¹H), carbon (¹³C), and phosphorus (³¹P) nuclei. We observed that these potential biosignatures of extinct life are found neither in the carbonaceous matter of carbonaceous meteorites (4.56 billion years), the most ancient objects of the Solar System, nor in any carbonaceous matter resulting from carbonization of organic and bioorganic precursors. These results indicate that these nuclear signatures are sensitive to thermal episodes and can be used for Archean cherts with metamorphism not higher than the greenschist facies.

Planning for Mars Returned Sample Science: Final Report of the MSR End-to-End International Science Analysis Group (E2E-iSAG)

Returning samples from Mars to Earth for scientific analysis has been, and continues to be, among the highest priority objectives of planetary science. Partly for this reason, the 2011 Planetary Science Decadal Survey placed high priority on a proposed 2018 rover mission that would conduct careful in situ science and use that scientific information to select and cache samples that could be returned to Earth by a potential future mission. In order to ensure that the potential contributions of the 2018 rover to the proposed MSR Campaign are properly planned, this study was undertaken to consider the science of the MSR Campaign concept from end to end. This white paper is the principal output of the Mars Sample Return (MSR) End-to-End International Science Analysis Group (E2E-iSAG): a group chartered by MEPAG (Mars Exploration Program Analysis Group).

Oxygen and hydrogen isotope evidence for a temperate climate 3.42 billion years ago

Stable oxygen isotope ratios (delta(18)O) of Precambrian cherts have been used to establish much of our understanding of the early climate history of Earth and suggest that ocean temperatures during the Archaean era ( approximately 3.5 billion years ago) were between 55 degrees C and 85 degrees C (ref. 2). But, because of uncertainty in the delta(18)O of the primitive ocean, there is considerable debate regarding this conclusion. Examination of modern and ancient cherts indicates that another approach, using a combined analysis of delta(18)O and hydrogen isotopes (deltaD) rather than delta(18)O alone, can provide a firmer constraint on formational temperatures without independent knowledge of the isotopic composition of ambient waters. Here we show that delta(18)O and deltaD sampled from 3.42-billion-year-old Buck Reef Chert rocks in South Africa are consistent with formation from waters at varied low temperatures. The most (18)O-enriched Buck Reef Chert rocks record the lowest diagenetic temperatures and were formed in equilibrium with waters below approximately 40 degrees C. Geochemical and sedimentary evidence suggests that the Buck Reef Chert was formed in shallow to deep marine conditions, so our results indicate that the Palaeoarchaean ocean was isotopically depleted relative to the modern ocean and far cooler (<or=40 degrees C) than previously thought.

Rapid in situ silicification of microbes at Loburu hot springs, Lake Bogoria, Kenya Rift Valley

Microbial mats, located along the margins of hot‐spring pools and outflow channels at Lake Bogoria, Kenya, are commonly silicified forming friable laminated crusts. Columnar microstromatolites composed of silica and calcite are also forming at several springs in sites of oscillating water level or spray. Silicification of the microbes involves impregnation of organic tissue by very fine amorphous silica particles and encrustation by small (&lt; 2 μm) silica spheroids. Rapid silicification of the microbes, which may begin while some are still alive, can preserve sheaths and in some examples, the filaments, capsules and cells. Although this provides evidence of their general morphology, the biological features that are required for taxonomic identifications are commonly poorly preserved.The silica precipitation results mainly from evaporative concentration and rapid cooling of spring waters that have been drawn upward through the mats and microstromatolites by capillary processes. Almost all the silica at the Loburu springs nucleates on microbial substrates. This affinity of silica for functional groups on microbial surfaces contributes to the rapid silicification of the microbes and their preservation in modern and ancient cherts.

Oxygen isotope study of mid-ocean ridge hydrothermal fluids: Implication for the oxygen-18 budget of the oceans

New data are reported for the 18O composition of hot hydrothermal fluids from various sites with different tectonic settings. The measured δ 18O, in the range 0.53–2.3‰ confirm earlier data suggesting that mid-ocean ridge hydrothermal vents are a major source of 18O to the ocean. In light of this new data, we attempt to re-evaluate the marine 18O budget from direct quantification of the main sources and sinks of oceanic oxygen-18. Within the uncertainty of the flux estimates, 18O inputs and outputs are found to balance each other, showing that oceanic 18O is presently at a steady-state. Using these present fluxes, we examine the past evolution of the 18O composition of the ocean and speculate that the initial 18O oceanic content may have evolved rapidly towards its present value. This suggests, as already pointed out in previous studies, that ocean water may have had a rather constant 18O composition over most of the Earth's history and that δ 18O measurements in ancient cherts and carbonates may primarily record the evolution of the ocean temperature.

Investigating regional lithic procurement using multi-spectral imagery and geophysical exploration

The primary goal of this research was to develop an innovative survey method that could be applied to the prediction, confirmation, and understanding of archaeological sites utilizing aerial and ground-based remote sensing methods in a combined fashion. By drawing on the resources of the University of Colorado Department of Anthropology, the Center for the Study of Earth from Space (CSES), and the Institute of Arctic and Alpine Research (INSTAAR), this combined remote sensing method has been used successfully in locating and evaluating ancient chert quarries in the Horse Prairie region of southwestern Montana. The quarries in this region have served as a source of quality raw material for over 10,000 years, and are found along veins of hydrothermal alteration and in deposits of till. By combining ground-truth data based on electromagnetic induction surveying and conventional aerial photography and principal components image analysis of Landsat imagery, various spectral class signatures were created and then reapplied to the raw spectral data in a supervised maximum-likelihood multispectral classification. Subsequent field checking and spectroscopy data illustrate that this classification successfully predicted the location of previously undocumented quarry areas and also suggests that the quarries are highly restricted geographically within the region. The data obtained from this research will also contribute understanding to such problems as the nature of lithic resource procurement and associated economic relationships between the Great Plains and Great Basin areas of the western USA. In addition, this research method will be applicable to a variety of alpine, Arctic, and semi-arid regions where ground surfaces are exposed and spectral signatures can be established. © 1996 John Wiley & Sons, Ltd.

Mathematical approach to calculate indices for depositional environments of pelagic-hemipelagic-terrigenous sediments on the basis of metallic element concentrations

Modern marine sediments can be classified into terrigenous, hemipelagic, and pelagic types according to the distances from the land for the sites recovered and in terms of lithological characteristics of sediments. Ancient cherts are the best sedimentary rocks to reconstruct ancient depositional environments and hemipelagic and pelagic cherts can be identified in terms of lithologic and stratigraphic characteristics. However, geochemical characteristics of modern sediments and ancient cherts can discriminate them into these three types. Particularly, the degree of correlation between Fe and Mg in clay minerals is the most effective criterion for the classification, because the concentration of pelagic clay minerals in which the Fe/Mg ratio is constant may become indices for the classification into the three types. The correlation coefficient and goodness-of-fit to a certain regression line were used as indices of the classification and calculated for several sets of modern sediments and ancient cherts totalling to more than 400 samples. The correlation coefficients and the student-t values for the coefficients become better classification indices than the goodness-of-fit andF-values of the analysis of variance. Goodness-of-fit andF-values can be only used as subsidiary indices for the main classification indices of the correlation coefficients. The classification on the basis of the degree of correlation between Fe and Mg is effective and this approach can be used for other major chemical elements such as Al, K, and Na, if some sequential values of chemical analyses are available.

Magnetic and Optical Methods for Detecting the Heat Treatment of Chert

The magnetic properties of ancient chert implements can reveal whether they were heated during their manufacture. Our experiments reveal that magnetic susceptibility, saturation isothermal remanent magnetization and magnetic hysteresis parameters are changed markedly by short periods of heating. These tests should be able to detect if an ancient chert artefact was heated during its manufacture. These methods are non-destructive as long as the objects can be fitted into the sample holders used in the magnetic measuring equipment. These tests should be applicable to other cherts and possibly to other rock types because of the ubiquity of the magnetic minerals used in identification. In the cherts that we studied, organic inclusions were also present. These changed greatly during heating, enabling us to distinguish between heated and unheated material in polished sections and, in some cases, in hand specimens. The optical tests are simple to perform but may be destructive, and organic inclusions are rare.

Petrography and Diagenesis of Cherts From Lake Magadi, Kenya

Pleistocene cherts from alkaline Lake Magadi, Kenya, are believed to have formed inorganically from sodium silicate precursors (magadiite--NaSi 7 O 13 (OH) 3 .3H 2 O and/or kenyaite--NaSi 11 O (sub 20.5) (OH) 4 .3H 2 O). They have been cited as modern analogs for cherts found in both ancient lacustrine deposits and Precambrian iron-formations, but few criteria exist to distinguish Magadi-type chert from other cherts in the geological record. We studied approximately twenty chert samples and one magadiite sample from Lake Magadi to develop such criteria and to shed light on the mechanism of the magadiite-quartz conversion. Characteristics of the type Magadi cherts are 1) a groundmass mosaic of fine quartz crystals that vary in orientation from random to rectilinear; 2) finely disseminated inclusions of silicate clays, zeolites, and/or carbonates (predominantly calcite); 3) large crystal molds (probably after trona) concentrated near sample margins; and 4) inward-directed shrinkage cracks and/or more irregular internal voids filled with chalcedony, silicate clays, zeolites, and/or carbonates (predominantly calcite). Shrinkage cracks, which define surface reticulation patterns, are frequently cited as evidence of a Magadi-type origin for ancient lacustrine cherts, but they are not unique to alkaline lake cherts. Based on a reconnaissance of ancient cherts of various origins, no single characteristic appears to be unique to Magadi-type chert, with the possible exception of the rectilinear or grid-work orientation of the quartz crystals. This texture appears to be inherited from the precursor magadiite, which displays a similar extinction pattern due to the presence of 10-20 mu m spherical aggregates of plate-like crystals. This pseudomorphing implies direct volume-for-volume replacement of magadiite by quartz. Void-filling chalcedony and the high densities of the cherts, however, indicate that silica was added during and after the conversion from magadiite to chert. Petrographic observations suggest the magadiite-quartz conversion takes place in multiple stages, with early conversion in brines and later leaching by more dilute meteoric water. A reconnaissance of 35 chert samples from 16 different formations, ranging in age from Pleistocene to Archean, yielded only two samples with convincing grid-work fabrics, both of which had already been interpreted as Magadi-type chert on the basis of other characteristics. This suggests that the distinctive textures which characterize the type Magadi cherts may be useful for recognizing other inorganic alkaline lake cherts in the stratigraphic record.

The Umsoli chert, turbidite testament for a major phreatoplinian event at the onverwacht/fig tree transition (Swaziland supergroup, Archaean, South Africa)

The volcanogenic origin of accretionary grains in the oolitic Umsoli chert is proved by the unequivocal identification of constituent vesicular and pumiceous ash particles, by the discovery of authigenic alkali feldspars and by the K-meta-bentonite parentage of associated illites/celadonites. Ash particle morphology and ash grain-size distribution suggest a phreato-magmatic process of eruption. Sedimentary structures indicate that the felsic ash precursor of this ancient chert was deposited by turbidity currents. The sedimentologic model invokes the transition of parts of pyroclastic falls and base surges into turbidites. Accretionary lapilli forming a major constituent of this silicified tuff-oolite may be indicators of subaerial eruption but they can probably not be used as evidence for subaerial deposition.

Trapped methyl radicals in chert

We have demonstrated recently that electron spin resonance (ESR) spectroscopy can detect a narrow signal from electron gain (E′) centres in flint (Upper Cretaceous lepispheric chert)1. This signal, caused by the effect of natural radiation, is lost on heating and may be useful for dating2. ESR signals from various organic radicals are now reported in a wide variety of cherts. The thermal dependence of the signals may provide valuable information about the duration and temperature of ancient chert heatings.

The development and diversification of Precambrian life

During the past decade, important strides have been made toward deciphering the paleobiology of the Precambrian Eon, the earliest seven-eighths of Earth history. This progress has accrued chiefly from micropaleontological and organic geochemical studies of fine-grained, ancient cherts. Although understanding of the early biota — of its composition, diversity, paleoecology and evolution — still remains far from adequate, three particularly significant generalizations have emerged: (i) Living systems were extant earlier than about 3000 m.y. ago; (ii) between about 3000 and 1000 m.y. ago, the Earth’s biota was dominated by prokaryotic blue-green algae; and (iii) the development of the nucleated, eukaryotic cell type somewhat earlier than 1000 m.y. ago led to a stage of rapid diversification that culminated with the appearance of megascopic life near the close of the Precambrian. Consideration of these generalizations, and of the evidence bearing on them provides a ‘state-of-the-art’ assessment of the current status of Precambrian paleobiology.

Imaging and Life Detection

The use of imaging for the detection of extraterrestrial microorganisms was evaluated by studying the morphology of inorganic particles produced by the reaction of iron with a silicate solution. The resemblance of these particles to certain microorganisms suggests that they could be identified inadvertently as microfossils. Nevertheless, the presence of these kinds of particles in ancient cherts in association with organic matter could be significant since they have been suggested as precursors to living organisms.

Thermal Environments as Models of Precambrian Environments: ABSTRACT

Thermal ecosystems are biologically simple. These systems contain a few species of unicellular organisms and may resemble ecosystems that existed on the ancient earth during the early stages of biological evolution. The analysis of fatty acids from some bacteria-algal mats in streams draining hot springs in Yellowstone National Park show distributions resembling those of fatty acids isolated from ancient cherts. A layered silica deposit, stromatolite, taken from the perimeter of an alkaline hot spring was used as a modern analog of bedded chert deposits. The distributions of fatty acids from the surface, crust, and interior regions of the indicate that the acids are syngenetic with the silica deposition, and that acid distributions have not changed signific ntly during the time of formation of the stromatolite (102-103 years). Comparisons were made of fatty acids and hydrocarbons from natural ecosystems and laboratory cultures of thermophiles. Marked differences were observed between the distributions of the fatty acids and alkanes in the specific samples and of the fatty acids from different ecosystems. End_of_Article - Last_Page 786------------