Hydrothermal circulation and oil migration at the root of the heterogeneous micro-structure of carbonaceous material in the 2.0 Ga Zaonega Formation, Onega Basin, Russia

Organic-rich rocks of the 2.0 Ga Zaonega Formation, Karelia, Russia, have been studied extensively to gain understanding of the global carbon cycle and reconstruction of paleo-environments, directly after the Great Oxidation Event (GOE). This formation has a complex history of alteration, involving pervasive hydrothermal circulation, hydrocarbon generation/migration, and mineral authigenesis. Several previous studies have focused on the description of these secondary effects, and the identification of primary geochemical signals in the carbonaceous phases. Migration and infiltration of organic-rich fluids appear to have had only limited effect on the primary carbon isotope record (δ13Corg). However, the structural variability of carbonaceous material (CM) appears to have been strongly affected, with a range of reported structures including carbon onion-shaped nanostructures and mineral-templated graphite films. Here we present a systematic Raman spectroscopy-based study of the structural variability of CM in a drill core representing the middle and upper strata of the Zaonega Formation. The Raman spectra of CM show a systematic difference in structural order between the bulk carbonaceous matrix (Matrix-CM) and the CM occurring near mineral contacts (Contact-CM), indicating that mineral templating was an important process affecting structural order in the formation. The templating effect was observed on the surface of a wide range of minerals. The difference in structural order between Matrix-CM and Contact-CM can be traced throughout the ca. 400 m stratigraphy. The structural order varied with the degree of alteration and hydrothermal circulation, from highly ordered structures directly above a large gabbro intrusion at the bottom of the stratigraphy to less ordered structures higher up in the sequence. This trend directly correlates with the δ18O trend of secondary calcite, and can be attributed to the decreasing influence and temperature regime of hydrothermal circulation upward in the stratigraphy. The results presented here suggest that organic-rich hydrothermal fluids can locally strongly enhance graphitization of carbonaceous materials, and cause sample-scale heterogeneities in the structural order of organic materials. This has implications for the interpretation of carbonaceous materials in other ancient rocks experiencing circulation of organic-rich hydrothermal fluids.publishedVersio

Depth-dependent δ13 C trends in platform and slope settings of the Campbellrand-Malmani carbonate platform and possible implications for Early Earth oxygenation

Highlights • Carbon cycle of Neoarchean carbonate platform and potential oxygen oasis. • Carbon isotopes reveal a shift to aerobic biosphere and increasing oxidation state. • Rare earth element patterns reveal decrease in open ocean water influx. • Rimmed margin architecture was crucial for evolution of aerobic ecosystems. Abstract The evolution of oxygenic photosynthesis is widely seen as the major biological factor for the profound shift from reducing to slightly oxidizing conditions in Earth’s atmosphere during the Archean-Proterozoic transition period. The delay from the first biogenic production of oxygen and the permanent oxidation of Earth’s atmosphere during the early Paleoproteorozoic Great Oxidation Event (GOE) indicates that significant environmental modifications were necessary for an effective accumulation of metabolically produced oxygen. Here we report a distinct temporal shift to heavier carbon isotope signatures in lagoonal and intertidal carbonates (δ13Ccarb from -1.6 to +0.2 ‰, relative to VPDB) and organic matter (δ13Corg from about -40 to -25 ‰, relative to VPDB) from the 2.58–2.50 Gy old shallow–marine Campbellrand-Malmani carbonate platform (South Africa). This indicates an increase in the burial rate of organic matter caused by enhanced primary production as well as a change from an anaerobic to an aerobic ecosystem. Trace element data indicate limited influx of reducing species from deep open ocean water into the platform and an increased supply of nutrients from the continent, both supporting primary production and an increasing oxidation state of the platform interior. These restricted conditions allowed that the dissolved inorganic carbon (DIC) pool in the platform interior developed differently than the open ocean. This is supported by coeval carbonates from the marginal slope setting, which had a higher interaction with open ocean water and do not record a comparable shift in δ13Ccarb throughout the sequence. We propose that the emergence of stable shallow-water carbonate platforms in the Neoarchean provided ideal conditions for the evolution of early aerobic ecosystems, which finally led to the full oxidation of Earth’s atmosphere during the GOE

In Situ Fe and S isotope analyses in pyrite from the 3.2 Ga Mendon Formation (Barberton Greenstone Belt, South Africa): Evidence for early microbial iron reduction

International audienceOn the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/ Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7-2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28-to 3.26-Gyr-old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ 56 Fe values of −1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass-independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ 33 S values > +3‰, while the other groups display more variable and closer to 0‰ Δ 33 S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ 33 S values in the pyrite core and similar trace element compositions

Spatially heterogeneous argon-isotope systematics and apparent ⁴⁰Ar/³⁹Ar ages in perlitised obsidian

In situ laser ablation Ar-isotope analyses of variably hydrated and devitrified obsidian from the ~ 27 Ma Cochetopa Dome, San Juan, USA, reveal complex interplay between degassing of initial Ar and absorption of atmospheric Ar. These processes have locally modified the Ar-isotope composition of the obsidian and led to spurious, spatially-heterogeneous Ar-isotope and 40Ar/39Ar age data. Small perlite beads exhibit older apparent Ar-ages at the rims than the cores. This is interpreted as an apparent excess of 40Ar at the rims, produced either by a) diffusion of excess 40Ar into the bead during flushing of the lava with excess 40Ar-bearing volcanic gas, or by b) isotopic fractionation during degassing of initial Ar, causing preferential loss of 36Ar over 40Ar at the bead rims. The second interpretation is favoured by a relative enrichment of 36Ar in the core of a perlite bead along a microlite-free (poorly degassed) flow band, and by a lack of age variation in a larger, fresh, well-degassed perlite bead. These isotopic gradients were later overprinted during glass hydration by absorption of Ar with near-atmospheric composition, resulting in elevated 36Ar and reduced radiogenic 40Ar* yields at the rims of perlite beads. These complex interactions essentially represent the mixing of three distinct Ar reservoirs: initial trapped Ar that may or may not be fractionated, an isotopically atmospheric Ar component introduced during hydration, and radiogenic 40Ar*. Such reservoir mixing is the underlying reason for poor correlations on isotope correlation diagrams and the difficulties in validating the composition of the non-radiogenic Ar component. We thus suggest that high 36Ar yields are a combination of the incomplete degassing of initial (possibly magmatic) Ar and the gain of Ar during interaction between the obsidian and meteoric/atmospheric fluids. Our analyses emphasise the challenging nature of 40Ar/39Ar dating obsidian samples, but also point to possible solutions by careful sample characterisation and selection of highly degassed samples

COSPAR Sample Safety Assessment Framework (SSAF)

The Committee on Space Research (COSPAR) Sample Safety Assessment Framework (SSAF) has been developed by a COSPAR appointed Working Group. The objective of the sample safety assessment would be to evaluate whether samples returned from Mars could be harmful for Earth's systems (e.g., environment, biosphere, geochemical cycles). During the Working Group's deliberations, it became clear that a comprehensive assessment to predict the effects of introducing life in new environments or ecologies is difficult and practically impossible, even for terrestrial life and certainly more so for unknown extraterrestrial life. To manage expectations, the scope of the SSAF was adjusted to evaluate only whether the presence of martian life can be excluded in samples returned from Mars. If the presence of martian life cannot be excluded, a Hold & Critical Review must be established to evaluate the risk management measures and decide on the next steps. The SSAF starts from a positive hypothesis (there is martian life in the samples), which is complementary to the null-hypothesis (there is no martian life in the samples) typically used for science. Testing the positive hypothesis includes four elements: (1) Bayesian statistics, (2) subsampling strategy, (3) test sequence, and (4) decision criteria. The test sequence capability covers self-replicating and non-self-replicating biology and biologically active molecules. Most of the investigations associated with the SSAF would need to be carried out within biological containment. The SSAF is described in sufficient detail to support planning activities for a Sample Receiving Facility (SRF) and for preparing science announcements, while at the same time acknowledging that further work is required before a detailed Sample Safety Assessment Protocol (SSAP) can be developed. The three major open issues to be addressed to optimize and implement the SSAF are (1) setting a value for the level of assurance to effectively exclude the presence of martian life in the samples, (2) carrying out an analogue test program, and (3) acquiring relevant contamination knowledge from all Mars Sample Return (MSR) flight and ground elements. Although the SSAF was developed specifically for assessing samples from Mars in the context of the currently planned NASA-ESA MSR Campaign, this framework and the basic safety approach are applicable to any other Mars sample return mission concept, with minor adjustments in the execution part related to the specific nature of the samples to be returned. The SSAF is also considered a sound basis for other COSPAR Planetary Protection Category V, restricted Earth return missions beyond Mars. It is anticipated that the SSAF will be subject to future review by the various MSR stakeholders

Serum potassium and adverse outcomes across the range of kidney function: a CKD Prognosis Consortium meta-analysis.

Aims: Both hypo- and hyperkalaemia can have immediate deleterious physiological effects, and less is known about long-term risks. The objective was to determine the risks of all-cause mortality, cardiovascular mortality, and end-stage renal disease associated with potassium levels across the range of kidney function and evaluate for consistency across cohorts in a global consortium. Methods and results: We performed an individual-level data meta-analysis of 27 international cohorts [10 general population, 7 high cardiovascular risk, and 10 chronic kidney disease (CKD)] in the CKD Prognosis Consortium. We used Cox regression followed by random-effects meta-analysis to assess the relationship between baseline potassium and adverse outcomes, adjusted for demographic and clinical characteristics, overall and across strata of estimated glomerular filtration rate (eGFR) and albuminuria. We included 1 217 986 participants followed up for a mean of 6.9 years. The average age was 55 ± 16 years, average eGFR was 83 ± 23 mL/min/1.73 m2, and 17% had moderate- to-severe increased albuminuria levels. The mean baseline potassium was 4.2 ± 0.4 mmol/L. The risk of serum potassium of >5.5 mmol/L was related to lower eGFR and higher albuminuria. The risk relationship between potassium levels and adverse outcomes was U-shaped, with the lowest risk at serum potassium of 4-4.5 mmol/L. Compared with a reference of 4.2 mmol/L, the adjusted hazard ratio for all-cause mortality was 1.22 [95% confidence interval (CI) 1.15-1.29] at 5.5 mmol/L and 1.49 (95% CI 1.26-1.76) at 3.0 mmol/L. Risks were similar by eGFR, albuminuria, renin-angiotensin-aldosterone system inhibitor use, and across cohorts. Conclusions: Outpatient potassium levels both above and below the normal range are consistently associated with adverse outcomes, with similar risk relationships across eGFR and albuminuria

Stable Isotope Ratios as a Biomarker on Mars

International audienceAs both Earth and Mars have had similar environmental conditions at least for some extended time early in their history (Jakosky and Phillips in Nature 412:237-244, 2001), the intriguing question arises whether life originated and evolved on Mars as it did on Earth (McKay and Stoker in Rev. Geophys. 27:189-214, 1989). Conceivably, early autotrophic life on Mars, like early life on Earth, used irreversible enzymatically enhanced metabolic processes that would have fractionated stable isotopes of the elements C, N, S, and Fe. Several important assumptions are made when such isotope fractionations are used as a biomarker. The purpose of this article is two-fold: (1) to discuss these assumptions for the case of carbon and to summarize new insights in abiologic reactions, and (2) to discuss the use of other stable isotope systems as a potential biomarker. It is concluded that isotopic biomarker studies on Mars will encounter several important obstacles. In the case of carbon isotopes, the most important obstacle is the absence of a contemporary abiologic carbon reservoir (such as carbonate deposits on Earth) to act as isotopic standard. The presence of a contemporary abiologic sulfate reservoir (evaporite deposits) suggests that sulfur isotopes can be used as a potential biomarker for sulfate-reducing bacteria. The best approach for tracing ancient life on Mars will be to combine several biomarker approaches; to search for complexity, and to combine small-scale isotopic variations with chemical, mineralogical, and morphological observations. An example of such a study can be a layer-specific correlation between δ 13C and δ 34S within an ancient Martian evaporite, which morphologically resembles the typical setting of a shallow marine microbial mat

Experimental and theoretical modelling of kinetic and equilibrium Ba isotope fractionation during calcite and aragonite precipitation

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Variations in atmospheric sulphur chemistry on early Earth linked to volcanic activity

International audienceVolcanism on early Earth should have had an impact on atmospheric chemistry, but that impact can be challenging to reconstruct. The isotopic composition of sulphur contained in rocks deposited more than 2.45billion years (Gyr) ago shows both mass-dependent (as denoted by δ34S) and mass-independent (Δ33S) isotopic fractionation. This sulphur is predominantly in the form of suphides. These sulphides show a positive correlation between δ34S and Δ33S between 4.0 and 2.45Gyr ago. Sulphates deposited episodically between about 3.5 and 3.2Gyr ago indicate a more homogeneous sulphur reservoir and show no correlation to the sulphide trend. Here we report sulphur isotope values of sulphide from volcanic ash layers in the 3.2-Gyr-old Mapepe Formation of South Africa. We find a δ34S-Δ33S relationship that deviates from previous sulphide isotope records and instead overlaps with the range of values reported for sulphates. Coexisting sulphates and sulphides of a similar age found in Australia and India show a similar array of δ34S-Δ33S values. We suggest that the occurrence of this δ34S-Δ33S array reflects widespread, ultraviolet-light-triggered photodissociation of sulphur dioxide that was released into the atmosphere by short-lived but intense bursts of subaerial volcanic activity

Structural characterization by Raman hyperspectral mapping of organic carbon in the 3.46 billion-year-old Apex chert, Western Australia

International audienceThe 3.46 billion years old Apex Chert, Pilbara Craton, Western Australia, is well-known for hosting the oldest, highly disputed microfossils on Earth. This rock has a complex history of thermal alteration that includes circulation of hydrothermal fluids, lower greenschist-facies regional metamorphism, and post-metamorphic weathering by meteoric fluids. Carbonaceous material occurs in the sedimentary stratiform part of the chert as well as the underlying intruding hydrothermal black chert veins. In order to identify a least-altered remnant of early life it is necessary to develop a method that enables rapid evaluation of CM structural order on a small spatial scale. Here we present the detailed characterization of CM in the Apex chert by Raman hyperspectral mapping. It is shown that this approach gives better estimates of average Raman band ratios than individual point analyses, and it is demonstrated that significant differences in structure exist between CM in the stratiform part of the Apex chert and CM in an underlying black chert vein. The large Raman map-based datasets also reveal that significant mixing took place between these two end members CM’s, indicating that the Apex chert has been thoroughly altered by hydrothermal fluid circulation. At the brecciated intersection between the stratiform chert and the intrusive hydrothermal chert vein very poorly ordered CM was found that is not in line with lower greenschist-facies regional metamorphism. It is speculated here that this CM represents an organic fraction that was introduced or thoroughly altered by late stage meteoric fluids. Alternatively, the ubiquitous presence of hematite in this sample caused a perturbation in the Raman spectra of the CM, leading to an artifact in the calculated Raman-based band ratios. Overall it can be concluded that the best preserved CM occurs in the stratiform parts of the Apex chert, while earlier discussions on organic microfossils in this rock often focused on parts of the chert that either represented the hydrothermal veins or the brecciated intersection between the veins and the stratiform part