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Biogenicity
... Biogenicity is a property referring to any chemical and/or morphological signatures preserved in various geological formations that are created by either extant or extinct organisms. Indications supporting the biogenicity are also controlled by biologically important factors such as temperature, light and nutrients (McLoughlin 2015). ...
... Another aspect to keep in mind is that particular biogenicity criteria have been adapted for various specimens of biosignatures found on Earth. Scientists distinguish the above-mentioned criteria for several structures such as microfossils, stromatolites and microcavities (McLoughlin 2015). ...
For a better understanding of how to search for an extraterrestrial life, scientists
study hidden biospheres on Earth. The subseafloor crust is recognized as a vast
microbial habitat and it is hypothesized that extremophilic microorganisms, occurring
there, can be the first living organisms on Earth. Those extremophiles do not
require oxygen due their ability to derive bioavailable energy from fluid-rock
interactions, resembling conditions on Mars. Hence, in this study, geological samples
from such environments are analysed.
Overall, this report examines a concept of biogenicity and evaluates a set of
methods used for the determination of biologic origin. Fossilized microbial remains
were discovered in unconsolidated sediments from the volcaniclastic apron of Gran
Canaria and in aragonite veins in ultramafic rocks from the North Pond at the Mid-
Atlantic Ridge. Mentioned sediments and rocks were collected during Ocean Drilling
Program (ODP) Leg 157 and 209. The fossil record from Gran Canaria is consistent
with Foraminifera. The microbial remains from North Pond are consistent with
Frutexites microstromatolites. Both fossilized communities have characteristic
compositions associated with carbonaceous matter (CM) and different configurations
of trace elements such as Si, Al, Mg, Mn, Ni, Fe, and Co. This study confirms the
biologic origin of the fossilized remains and shows that the applied methods are
suitable for astrobiological application.
The importance of the search for life within the universe underpins many of the objectives of ESA and NASA. Fundamental to this objective is the recognition and understanding of the early fossil record on planet Earth. The history of study of ‘microfossils’ from the 3465 Ma Apex cherts of the Warrawoona Group in Western Australia is reviewed, as are the criteria put forward to test for for biogenicity of early microfossils. We propose that the null hypothesis of an abiotic or prebiotic origin of ancient microscopical artefacts is sustained until mutually supporting, contextural morphological and geochemical evidence for a biological origin is presented and an abiotic origin is falsified.
McKay et al. [(1996) Science 273, 924-930] suggested that carbonate globules in the meteorite ALH84001 contained the fossil remains of Martian microbes. We have characterized a subpopulation of magnetite (Fe(3)O(4)) crystals present in abundance within the Fe-rich rims of these carbonate globules. We find these Martian magnetites to be both chemically and physically identical to terrestrial, biogenically precipitated, intracellular magnetites produced by magnetotactic bacteria strain MV-1. Specifically, both magnetite populations are single-domain and chemically pure, and exhibit a unique crystal habit we describe as truncated hexa-octahedral. There are no known reports of inorganic processes to explain the observation of truncated hexa-octahedral magnetites in a terrestrial sample. In bacteria strain MV-1 their presence is therefore likely a product of Natural Selection. Unless there is an unknown and unexplained inorganic process on Mars that is conspicuously absent on the Earth and forms truncated hexa-octahedral magnetites, we suggest that these magnetite crystals in the Martian meteorite ALH84001 were likely produced by a biogenic process. As such, these crystals are interpreted as Martian magnetofossils and constitute evidence of the oldest life yet found.
This paper deals with the difficulty of decoding the origins of natural structures through the study of their morphological features. We focus on the case of primitive life detection, where it is clear that the principles of comparative anatomy cannot be applied. A range of inorganic processes are described that result in morphologies emulating biological shapes, with particular emphasis on geochemically plausible processes. In particular, the formation of inorganic biomorphs in alkaline silica-rich environments are described in detail.
Micron-sized cavities created by the actions of rock-etching microorganisms known as euendoliths are explored as a biosignature for life on early Earth and perhaps Mars. Rock-dwelling organisms can tolerate extreme environmental stresses and are excellent candidates for the colonization of early Earth and planetary surfaces. Here, we give a brief overview of the fossil record of euendoliths in both sedimentary and volcanic rocks. We then review the current understanding of the controls upon the distribution of euendolithic microborings and use these to propose three lines of approach for testing their biogenicity: first, a geological setting that demonstrates a syngenetic origin for the euendolithic microborings; second, microboring morphologies and distributions that are suggestive of biogenic behavior and distinct from ambient inclusion trails; and third, elemental and isotopic evidence suggestive of biological processing. We use these criteria and the fossil record of terrestrial euendoliths to outline potential environments and techniques to search for endolithic microborings on Mars.
Stromatolites are morphologically circumscribed accretionary growth structures with a primary lamination that is, or may be, biologically influenced (biogenic). They are found in Archean sedimentary carbonate rocks, almost always associated with extensive volcanic sequences. Thirty-two occurrences have been reported from n small regional clusters representing the world’s principal preserved Archean cratons: North America 16, Africa 7, Australia 5, Asia 3, and Europe s; none are presently known from Archean rocks of South America and Antarctica; less than two dozen of the occurrences are viewed as definitely Archean and stromatolitic. The earliest stromatolite records date back to nearly 3.5 Ga, and their worldwide distribution and abundance increase as time progresses.
Morphological types include structures with flat, convex-up, concave-up, and globoidal laminae; stacking patterns producing nodular, columnar (unbranched as well as branched), and oncoidal forms are represented. The observed diameters of the structures show a gradual increase in size as the stratigraphic column is ascended, spread over two orders of magnitude in geon 34 (centimetric to decimetric), but ranging over six orders of magnitude by geon 25 (sub-millimetric to dekametric). Unlike Proterozoic stromatolites, most are developed in limestones rather than dolostones, with sideritic/ankeritic and cherty types also present. Microfossils are only very rarely preserved. Ministromatolites with radial-fibrous microstructure, probably almost exclusively the result of chemical precipitation, developed after 3.0 Ga, as did mesoscopic aragonite/calcite crystal fans, indicating carbonate supersaturation of ambient Meso-and Neoarchean ocean waters.
Archean microfossils are notoriously difficult to recognize. Most that have been reported have subsequently proved to be either younger contaminants or abiogenic pseudofossils. To avoid mistaking contaminants for genuine Archean objects, a set of rigorous collecting and preparation procedures should be followed. To avoid mistaking pseudofossils for authentic microfossils, a hierarchical series of recognition criteria can be used. Six classes of microfossil-like objects from a ~3500 m.y. old chert-barite unit in the Warrawoona Group at North Pole, Western Australia, are evaluated using the above technique. -from Author
The word ‘stromatolite’ should only be applied to organosedimentary structures predominantly accreted by sediment trapping, binding and/or in situ precipitation as a result of the growth and metabolic activities of benthic, principally prokaryotic, micro-organisms. Structures of uncertain origin that resemble stromatolites should be called ‘stromatoloids’. This cautious approach would eliminate the currently common assumption that structures with mesoscopic morphological similarities to microbially accreted sedimentary structures must be biogenic, a misconception that hampers investigations into the antiquity of life.A hierarchical series of meso- and microstructural attributes of stromatolites can be used to assign gradually increasing probabilities of biogenicity to stromatoloids. This method is particularly useful for interpreting ancient noncolumnar stromatoloids with poor microstructural preservation. In a range of Early Archaean pseudocolumnar, nodular and stratiform stromatoloids from North Pole studied using this method, none could be proved to be stromatolites and only a few are probable or possible stromatolites. As these stromatoloids closely resemble previously reported structures from North Pole interpreted as stromatolites, we consider that the evidence for the existence of life c. 3500 my ago at North Pole is less definitive than previously supposed.
The identification of fossils or biogenic sedimentary structures in rocks of Archean age is difficult, because similar lithological features could rise from purely physical or chemical processes alone. Therefore it is important to define criteria that serve the secure definition of a fossil or structure in question as of biological origin. Such criteria have been established for stromatolites and microfossils.This contribution discusses the 6 criteria of biogeneicity of ‘microbially induced sedimentary structures’ (MISS). Those structures are found in sandy deposits of early Archean age to the present, and rise from the interaction of benthic microbiota with physical sediment dynamics. The six criteria for their biogeneicity are: (i) MISS occur in rocks of not more than lower greenschist facies; (ii) in stratigraphic sections, MISS correlate with turning points of regression–trangressions; (iii), MISS correlate with a characteristic depositional facies that enhances the development and the preservation of microbial mats; (iv), the distribution of MISS correlates with the ancient average hydraulic pattern; (v), the geometries and dimensions of fossil MISS correspond to that of the modern ones; (vi), the MISS include at least one of 9 specific microtextures.
Stromatolites and wrinkle structures are often taken to be an important indicator for early life. While both may be shaped by microbial mat growth, this can be open to doubt, so that the contribution of abiotic processes in their construction always needs to be established (Grotzinger & Knoll, 1999). We here report laboratory spray deposition experiments that can generate stromatolites and wrinkle structures in the absence of microbes. These minicolumnar and sometimes branched stromatolites are produced artificially by the aggregation of a synthetic colloid in a turbulent flow regime. They self-organize at the relatively low particle concentrations found in the outer parts of a spray beam. This contrasts with adjacent stratiform deposits that are produced by high rates of colloid deposition and relatively low sediment viscosities found in the centre of a spray beam. These stratiform laminae become subsequently wrinkled during hardening of the colloid. These results support numerical models that together suggest that physicochemical processes are capable of generating laminated sedimentary structures without the direct participation of biology. Geological environments where comparable abiogenic stromatolites and wrinkle structures may be found include: splash-zone silica sinters, desert varnish crusts and early Archean cherts formed from silica gel precursors.
Precambrian fossils, pseudofossils, and problematica in Canada
- Hj Hofmann