1,721,018 research outputs found
Microfacies characterization of stromatolites from Lake Ashenge in the East African Rift System
No full textTraces of early Life from the Barberton Greenstone Belt, South Africa
No full textWe review the evidence for traces of Paleoarchean (3.47–3.2 Ga) life in the Barberton Greenstone Belt (BGB), South Africa. Morphological, organic, geochemical, isotopic, and mineralogical biosignatures are preserved in massive and stratiform volcano-sedimentary cherts, in hydrothermal and chemical sedimentary rocks, and in quartz-rich sandstones. The well-preserved lithologies of the BGB record a broad diversity of features interpreted as the remnants of anaerobic life forms, including chemotrophs, benthic phototrophs, and possible planktonic organisms. Inferred photosynthetic fossils include planar to low-amplitude, finely laminated microbial mats and stromatolites from marine and terrestrial settings. Rarely preserved, putative, filamentous microfossils are very small compared with present-day phototrophs. Putative chemotrophic fossils are more enigmatic and generally restricted to nutrient-rich hydrothermal environments. Relatively large (up to some hundreds of microns) lenticular and spherical carbonaceous microfossils are interpreted as possible planktonic forms. Many proposed microfossils are of simple morphology and poorly preserved, thus it is difficult to completely discount abiogenic mechanisms for their formation
Carbonate–silica interplay: unraveling microbial synergy and fossil preservation in alkaline lacustrine stromatolites
No full textControls on Preservation of Stromatolites from Extreme Alkaline Environments (Lake Ashenge, Tigray, Ethiopia)
No full textStromatolites from extreme environment as tools for astrobiological exploration, study case of Lake Ashenge, East African Rift system
No full textStromatolites are ideal ecosystems of microbe–sediment synergy and, therefore, represent a fundamental resource for the recognition of biosignatures and investigation of astrobiological targets with regard to environment and life interactions. Even though they are one of the oldest records of life on Earth, at present, modern occurrences of stromatolites are restricted to extreme environments, such as hypersaline lakes. This work aims to characterize stromatolite features that can be used as biosignatures, seeking to understand their morphogenetic and preservation processes. Samples were collected in Lake Ashenge, a hyperalkaline high-altitude lake located in the Tigray Region of Ethiopia.
Two distinct microfacies were observed in the Lake Ashenge stromatolites: a) columnar stromatolites; and b) bulbous stromatolites. Both microfacies feature of slightly laminated microfabric composed of intercalated thicker layers of microsparite and thinner layers of calcite micrite within which exceptionally well-preserved microbial fossils including filamentous and coccoidal cyanobacteria-dominated colonies were recognized. Columnar stromatolite layers show mostly parallel or interlaced vertical arrangements of filamentous microbes which suggest different directions of water flow, demonstrating that the spatial arrangements of microbes reflect the dynamics of the environment. Bulbous structures are composed either of colonies of shrub-like filaments along their entire length and height grading into a vertical parallel arrangement at the top. This type of arrangement suggests self-sustainability of the microbial community, improving the resistance of the structures, in order to endure physical processes in the environment such as currents and wave action or predation. All samples show extracellular polymeric substances (EPS) produced by microbes normally associated with filamentous microbes. In this context, EPS may serve to support and stabilise the biofilm and contribute to the formation of columnar and bulbous structures.
Mars 2020 Perseverance rover recorded carbonate occurrences in Martian paleoenvironments from Jezero Crater which have been compared with microbialites and tufas with the potential to preserve biosignatures similar to those found in Lake Ashenge samples. Therefore, although stromatolite morphology itself configures a macroscopic biosignature which may be detected in Mars rover images, the two scales of observation (millimetres- centimetres vs micrometric) identified biosignatures in the stromatolites of Lake Ashenge could also reflect different conditions of the dynamic environment during their development. In this regard, Lake Ashenge illustrates an important modern environment for astrobiological studies and has potential analogy to ancient lacustrine environments on Mars
Palaeolacustrine environments as windows on microbial biosignature preservation on the Precambrian Earth and Noachian–Hesperian Mars
No full textLiassic geodynamic evolution of the transect Midelt-Errachidia (Central High Atlas, Morocco)
No full textThis work presents the data gathered through a stratigraphic, sedimentologic and geodynamic study of the Liassic formations cropping out along the Midelt-Errachidia transect. This area belongs to the eastern part of the Moroccan Central High Atlas.
This work was inspired to the following steps:
- to recognize the lithostratigraphic succession from the analysis of the different selected field sections;
- to define a local biostratigraphy by using ammonites, foraminifera and algae to establish the age of the sedimentary units within each section;
- to characterize through the facies analysis, the sedimentary environments during the Liassic interval of time;
The purpose of this work was to propose correlation diagrams for the corresponding formations and draw, from Early Sinemurian to Early Toarcian, palaeogeographic maps showing the Liassic historical evolution of this Atlasic area.
During the Sinemurian, the regional geodynamic processes involved the growth of microbialitic sponge-bearing micritic mounds. These build-ups that will be here detailed represent a key-step within the development of the studied atlasic basin.
The collected data and their interpretation lead us to propose a geodynamic evolution diagram involving main steps that will be illustrate and their correlation with a second order transgression-regression cycles. The end of the different regressive periods corresponds to a general structuring phase with a related geometrical change in the shape of the basin. In this frame, two main structuring steps have been distinguished:
- the first step, corresponding to the unconformity D2 at the transition between Early and Upper Sinemurian, is the responsible of the breakdown of the Early Liassic platform, and leads to the creation of a central subsiding trough. The Sinemurian carbonates mounds are closely linked to this fracturing event.
- the second step corresponding to the unconformity D11 at the base of the Domerian Spinatum zone, occurs on the edges of the trough, and indicates a centrifugal migration of the fracturing wave mainly towards the South. This step induces the creation of the asymmetrical southward collapsed basin.
The three transgressive periods are indicative of subsiding and maximum opening conditions of the sedimentary environments. They are capped by the intra Sinemurian D1 unconformity, the Middle Carixian D10 unconformity and the D16 unconformity, at the base of the Toarcian Bifrons zone.
The deposits related to the last maximum transgression suggest that the beginning of the sedimentary homogenization process within the whole basin (central trough and lateral platforms) moving to the following Aalenian-Bajocian complete infilling
Biosignature Investigation at Lake Ashenge, Ethiopia: Implications for Martian Paleolacustrine Environments
No full textJezero crater is a paleolacustrine system on Mars, active until the Early Hesperian era (~3.5 Ga). It features a closed-basin structure with distinct mafic and ultramafic spectral signatures and an elevated zone containing unique lake margin magnesium carbonates that could potentially hold evidence of ancient life. Terrestrial field analogue sites are key for investigating the environmental and mineralogical factors affecting potential biosignature preservation on Mars, but such sites are rare. Lake Ashenge is an alkaline closed-basin lake, developed on the basaltic plateau of Tigray, NE Ethiopia. Living and fossil stromatolites composed of magnesium carbonates occur on its shores. Our aim is to characterize Lake Ashenge stromatolites, in terms of their morphogenetic and preservational processes, in order to understand stromatolite development in extreme alkaline environments that are potentially analogous to Martian paleolake systems. Our primary focus was analyzing field samples to image entombed biomass mineralized within carbonate phases. This entombed biomass includes microbial mat remnants, organic structures resembling microbes (microfossils), and amorphous organics encapsulated by minerals, which were observed using optical and scanning electron microscopy (SEM). Carbonaceous materials, including biomolecules, in the stromatolites were characterized using Raman microspectroscopy and solid 13C nuclear magnetic resonance (NMR). Thin sections revealed abundant filamentous structures resembling cyanobacteria within layers of micritic and microsparitic Mg-calcite, suggesting a high potential for fossilization. SEM examination detected amorphous organics resembling extracellular polymeric substances (EPS) and organic tubes, interpreted as filamentous cyanobacterial sheaths, commonly filled with a Mg-silicate phase, contributing to the preservation of sheath molds. We identified Mg-calcite as the main phase forming the stromatolites using X-ray diffraction and Raman spectroscopy. Mg-calcite precipitates subaqueously in biomass-rich environments, significantly increasing the likelihood of entombment of organic materials and enhancing biosignature preservation. Abundant EPS in the studied stromatolites further contributes to a higher probability of biosignature preservation, as more microbes could potentially be concentrated, entombed and preserved. NMR spectra showed aliphatic structures, corresponding to the algaenan biomacromolecule, which is present in the cell walls of cyanobacteria and known to be resistant to degradation. Lake Ashenge offers a potential modern analogue for habitable environments on ancient Mars. The importance of Mg-calcite, other calcium carbonates, and minor Mg-silicate phases in these systems should be considered when selecting rock formations and outcrops for sample collection at the margins of Jezero crater
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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