Silica (SiO₂) is a common mineral that can preserve biological materials through a process known as silicification, and its composition and context can reflect past microbial activity.
Silica-rich deposits form in hydrothermal and aqueous environments, where biological materials such as microbial mats or biofilms can become rapidly encased in amorphous or crystalline silica. These silica precipitates may preserve fine biological textures and geochemical signals over geologic timescales. While abiotic silica is widespread, biologically influenced silica often exhibits distinctive structures, distributions, or associations.
Argumentation that supports the use of this feature as a biosignature based on biological prevalence.
PRO arguments and evidence highlight the presence of the feature in and/or due to life. CON arguments and evidence highlight how biological prevalence cannot justify the use of this feature as a biosignature.
Biological Signal Strength - Background
Argumentation that supports the use of this feature as a biosignature based on signal strength.
PRO arguments and evidence highlight the strength of signals such abundance, rate, structure, patterns, and intensity to be indicative of life. CON arguments and evidence highlight how the strength of specific signals are not justifications to support a biological origin.
Abiotic Prevalence - Background
Argumentation that refutes the use of this feature as a biosignature due to abiotic prevalence.
PRO arguments and evidence highlight the presence of the feature due to abiotic processes. CON arguments and evidence highlight how abiotic prevalence cannot refute the use of this feature as a biosignature.
Abiotic Signal Strength - Background
Argumentation that refutes the use of this feature as a biosignature based on signal strength of abiotic processes.
PRO arguments and evidence highlight the strength of signals such abundance, rate, structure, patterns, and intensity to be caused by abiotic processes. CON arguments and evidence highlight how the strength of specific signals from abiotic processes do not refute a potential biological origin.
Diatoms are a type of phytoplankton that have cell walls composed of silica, indicating they have a biogenic origin. They are crucial to silicon distribution in oceans. The walls have two silica ‘valves’, overlapping bands of silica, and tubing that form up the cellular shape. The system is highly dependent on large amounts of silica for survival and DNA replication.
Evidence is Sourced from:
Title
Evolution of the diatoms: major steps in their evolution and a review of the supporting molecular and morphological evidence
Authors
Linda K Medlin
Abstract
Over the years, many reviews of different aspects of diatom biology, ecology and evolution have appeared. Since 1993 many molecular trees have been produced to infer diatom phylogeny. In 2004, Medlin & Kaczmarska revised the systematics of the diatoms based on more than 20 years of consistent recovery of two major clades of diatoms that did not correspond to a traditional concept of centrics and pennates and established three classes of diatoms: Clade 1 = Coscinodiscophyceae (radial centrics) and Clade 2 = Mediophyceae (polar centrics + radial Thalassiosirales) and Bacillariophyceae (pennates). However, under certain analytical conditions, an alternative view of diatom evolution, a grades of clades, has been recovered that suggests a gradual evolution from centric to pennate symmetry. These two schemes of diatom evolution are evaluated in terms of whether or not the criteria advocated by Medlin & Kaczmarska that should be met to recover monophyletic classes have been used. The monophyly of the three diatom classes can only be achieved if (1) a secondary structure of the small subunit (SSU) rRNA gene was used to construct the alignment and not an alignment based on primary structure and (2) multiple outgroups were used. These requirements have not been met in each study of diatom evolution; hence, the grade of clades, which is useful in reconstructing the sequence of evolution, is not useful for accepting the new classification of the diatoms. Evidence for how these two factors affect the recovery of the three monophyletic classes is reviewed here. The three classes have been defined by clear morphological differences primarily based on gametangia and auxospore ontogeny and envelope structure, the presence or absence of a structure (tube process or sternum) associated with the annulus and the location of the cribrum in those genera with loculate areolae. New evidence supporting the three clades is reviewed. Other features of the cell are examined to determine whether they can also be used to support the monophyly of the three classes.
In the Waiotapu, New Zealand in the Champagne Pools, silica was often formed on EPS that was secreted by microorganisms.
When assessing the siliceous sinters formed within the Champagne Pools in New Zealand, it was found that there was significant growth of this silica surrounding exopolymeric substances (EPS). Samples from the pools were collected on site, grown over a year’s time, and analyzed using several different types of microscopy. After testing it was found that often, though not always, EPS production and silicification occurred in areas with colonies of filamentous microorganisms, indicating that the silicification may be a bioproduct of those life forms.
Evidence is Sourced from:
Title
Silicifying biofilm exopolymers on a hot-spring microstromatolite: templating nanometer-thick laminae
Authors
Kim M Handley, Sue J Turner, Kathleen A Campbell, Bruce W Mountain
Abstract
Exopolymeric substances (EPS) are an integral component of microbial biofilms; however, few studies have addressed their silicification and preservation in hot-spring deposits. Through comparative analyses with the use of a range of microscopy techniques, we identified abundant EPS significant to the textural development of spicular, microstromatolitic, siliceous sinter at Champagne Pool, Waiotapu, New Zealand. Examination of biofilms coating sinter surfaces by confocal laser scanning microscopy (CLSM), environmental scanning electron microscopy (ESEM), cryo-scanning electron microscopy (cryo-SEM), and transmission electron microscopy (TEM) revealed contraction of the gelatinous EPS matrix into films (approximately 10 nm thick) or fibrillar structures, which is common in conventional SEM analyses and analogous to products of naturally occurring desiccation. Silicification of fibrillar EPS contributed to the formation of filamentous sinter. Matrix surfaces or dehydrated films templated sinter laminae (nanometers to microns thick) that, in places, preserved fenestral voids beneath. Laminae of similar thickness are, in general, common to spicular geyserites. This is the first report to demonstrate EPS templation of siliceous stromatolite laminae. Considering the ubiquity of biofilms on surfaces in hot-spring environments, EPS silicification studies are likely to be important to a better understanding of the origins of laminae in other modern and ancient stromatolitic sinters, and EPS potentially may serve as biosignatures in extraterrestrial rocks.
The biotic environment of El Tatio, Chile hosts opaline silica deposits that have many similarities to the ‘Home Plate’ deposits on Mars.
El Tatio is a site that hosts acid-sulfate-chloride hot springs which lead to the formation of opaline silica sinters in nodular clumps within discharge channels for the springs. Biofilms often can form on these deposits as they support the growth of life. These deposits have a surface fabric that appears smooth, but on the undersides have microporosity similar to the Home Plate deposits. Spectroscopy has shown that a thin layer of halite exists on Home Plate deposits, which is similarly seen on inters at El Tatio as the springs evaporate and condensates form. Both sites are also suspected to be volcanic hydrothermal environments. Despite this evidence, proof of biotic origin has not been established for these features on Mars, likely due to the current harsh environment.
Evidence is Sourced from:
Title
Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile
Authors
Steven W Ruff, Jack D Farmer
Abstract
The Mars rover Spirit encountered outcrops and regolith composed of opaline silica (amorphous SiO2·nH2O) in an ancient volcanic hydrothermal setting in Gusev crater. An origin via either fumarole-related acid-sulfate leaching or precipitation from hot spring fluids was suggested previously. However, the potential significance of the characteristic nodular and mm-scale digitate opaline silica structures was not recognized. Here we report remarkably similar features within active hot spring/geyser discharge channels at El Tatio in northern Chile, where halite-encrusted silica yields infrared spectra that are the best match yet to spectra from Spirit. Furthermore, we show that the nodular and digitate silica structures at El Tatio that most closely resemble those on Mars include complex sedimentary structures produced by a combination of biotic and abiotic processes. Although fully abiotic processes are not ruled out for the Martian silica structures, they satisfy an a priori definition of potential biosignatures.
eThe distribution of intracellular silica within bacteria is verified between different forms of silica in similar bacteria, indicating that some bacteria may have lost the genes enabling this silica in evolution or different strands vary in origin.
The distribution of intracellular silica within bacteria is verified between different forms of silica in similar bacteria, indicating that some bacteria may have lost the genes enabling this silica in evolution or different strands vary in origin.
A magnetotactic bacterium (MTB), WYHC-5, was studied to observe how silica globules may have come to form in the bacteria. It was hypothesized that a gene in the bacteria potentially allowed for this intracellular silicification, indicating that intracellular biosilicification may have been prevalent in the early formation of bacteria on Earth and there may be an evolutionary origin including this gene. It was found that several other strands of MTB could form similar silica structures. However, though a silica deposition gene was found, genes that enable silica transport were not found in WYHC-5 despite being found in other bacteria. This odd distribution of genes and abilities between similar bacteria could indicate that the genes enabling silicification could potentially be evolutionarily eliminated from strands of bacteria over time. Therefore, finding bacterial life on other planets through a search of silica could lead to some being missed as the gene to create silica globules may have evolutionarily left over time.
Evidence is Sourced from:
Title
Intracellular silicification by early-branching magnetotactic bacteria
Authors
Jinhua Li, Peiyu Liu, Nicolas Menguy, Xingliang Zhang, Jian Wang, Karim Benzerara, Lianjun Feng, Lei Sun, Yue Zheng, Fanqi Meng, Lin Gu, Eric Leroy, Jialong Hao, Xuelei Chu, Yongxin Pan
Abstract
Biosilicification—the formation of biological structures composed of silica—has a wide distribution among eukaryotes; it plays a major role in global biogeochemical cycles, and has driven the decline of dissolved silicon in the oceans through geological time. While it has long been thought that eukaryotes are the only organisms appreciably affecting the biogeochemical cycling of Si, the recent discoveries of silica transporter genes and marked silicon accumulation in bacteria suggest that prokaryotes may play an underappreciated role in the Si cycle, particularly in ancient times. Here, we report a previously unidentified magnetotactic bacterium that forms intracellular, amorphous silica globules. This bacterium, phylogenetically affiliated with the phylum Nitrospirota, belongs to a deep-branching group of magnetotactic bacteria that also forms intracellular magnetite magnetosomes and sulfur inclusions. This contribution reveals intracellularly controlled silicification within prokaryotes and suggests a previously unrecognized influence on the biogeochemical Si cycle that was operational during early Earth history.
eMany species of bacteria, algae, and micro-organisms are silicified in laboratory conditions in a manner that is consistent with authentic microfossils found in Precambrian cherts.
Many species of bacteria, algae, and micro-organisms are silicified in laboratory conditions in a manner that is consistent with authentic microfossils found in Precambrian cherts.
Francis et. all chose 30 different species of bacteria, algae, and micro-organisms that are suspected to create fossils similar to those in the eukaryotic database from Precambrian cherts and attempted to silicify them in order to compare the laboratory-derived fossil to naturally occurring sample. Using an equal amount of culture medium and ethyl silicate, each sample was given time to silicify for between 2 weeks to 6 months. At the end of this process, it was found that, though not all the bacteria silicified, some of the silicified bacteria were reminiscent of microfossils from Precambrian cherts.
Evidence is Sourced from:
Title
On the experimental silicification of microorganisms II. On the time of appearance of eukaryotic organisms in the fossil record
Authors
S Francis, L Margulis, E S Barghoorn
Abstract
On the basis of ultrastructural, biochemical and genetic studies, bacteria and blue green algae (Kingdom Monera, all prokaryotes) differ unambiguously from the eukaryotic organisms (Fungi, plants sensu stricto) and protists or protoctists, (Copeland, 1956). The gap between eukaryotes and prokaryotes is recognized as the most profound evolutionary discontinuity in the living world. This gap is reflected in the fossil record. Fossil remains of Archaean and Proterozoic Aeons primarily consist of prokaryotes and the Phanerozoic is overwhelmingly characterized by fossils of the megascopic eukaryotic groups, both metazoa and metaphyta. Based on the morphological interpretation of microscopic objects structurally preserved in Precambrian cherts, the time of appearance of remains of eukaryotic organisms in the fossil record has been claimed to be as early as 2.7 · 109 years ago, (Kaźmierczak, 1976). Others suggest chronologies varying between 1.7 to 1.3 · 109 (Schopf et al., 1973) or a time approaching 1.3 · 109 years (Cloud, 1974).
There is general agreement that many of the Ediacaran faunas, which have been dated at about 680 m.y. are fossils of megascopic soft-bodied invertebrate animals. Since all invertebrates are eukaryotic, the ca. 680 m.y. date for deposition of these animal assemblages may represent the earliest appearance of eukaryotic organisms. But the question remains as to whether there is definitive evidence for eukaryotic cells before this “benchmark” of the late Precambrian.
An excellent discussion of this particular problem as especially relating to acritarchs extending from rocks of Upper Riphean through Vendian and into the basal Cambrian is presented in recent studies by Vidal (1974, 1976) in Late Precambrian microfossils from the Visingsö rocks of southern Sweden.
Previous work on the laboratory silicification of wood and algal mat communities (Leo and Barghoorn, 1976) suggested that further analysis of “artificial fossils” might be of aid in the interpretation of fossil morphology toward the ultimate solution of this problem. Thus the procedure developed by one of us (ESB) for laboratory wood silicification was adapted to various smaller objects.
By successive immersions of wet cellular aggregates, colonies of various organisms and abiotic organic microspheres in tetraethyl orthosilicate, silicified cells and structures are produced which bear an interesting resemblance to ancient chert-embedded microfossils. Our observation of these microorganisms and proteinoid microspheres silicified in the laboratory as well as of degrading microorganisms, both eukaryotic and prokaryotic, have led us to conclude that many, if not all, of the criteria for assessing fossil eukaryotic microorganisms are subject to serious criticism in interpretation. We studied a large variety of prokaryotic algae, some eukaryotic algae, fungi, protozoa, and abiotic organic microspheres stable at essentially neutral pH. In some cases, degradation and/or silicification systematically altered both size and appearances of microorganisms. By the use of monoalgal cultures of blue-green algae, features resembling nuclei, chloroplasts, tetrads, pyrenoids, and large cell size may be simulated. In many cases individual members of these cultures show so much variation that they may be mistaken as belonging to more than one species. The size ranges for silicified prokaryotic and eukaryotic algae overlap. Several prokaryotes routinely yielded spherical or filamentous structures that resembled large cells. Because of genuine large sizes (e.g., Prochloron), shrinkage, systematic alteration or congregation of unicells to form other structures we find sizes to be of very limited use in determining whether an organism of simple morphology was prokaryotic or eukaryotic. Although some “prebiotic proteinoid microspheres” (of Fox and Harada, 1960) are impossible to silicify with our laboratory methods, those stable at neutral pH (Hsu and Fox, 1976) formed spherical objects that morphologically resemble silicified algae or fungal spores. Many had internal structure. We conclude that even careful morphometric studies of fossil microorganisms are subject to many sources of misinterpretation. Even though it is a logical deduction that eukaryotic microorganisms evolved before Ediacaran time there is no compelling evidence for fossil eukaryotes prior to the late Precambrian metazoans.
eSilicification in the Proterozoic commonly occurred in locations that had good silica availability, sediment permeability, and significant amounts of organic matter.
Silicification in the Proterozoic commonly occurred in locations that had good silica availability, sediment permeability, and significant amounts of organic matter.
Any environment with large amounts of silica available for use, such as areas with volcanism or large deposits or dissolved SiO2, has a potential to undergo silicification, however the process favors areas with large amounts of organic matter. This was first noticed during analysis of the Draken Conglomerate Formation, where organics-rich areas were silicified and poor areas had instead been calcified, despite the full area being exposed to the materials that could have triggered either process. Many models have been made to try and characterize how presence of organics affects silica growth patterns, but no one solution appears to be favored at the time of writing. But this still indicates that there is a direct connection between silicification and presence of organisms.
Evidence is Sourced from:
Title
Exceptional preservation of photosynthetic organisms in silicified carbonates and silicified peats
Authors
A H Knoll
Abstract
Carbonaceous cherts in Proterozoic carbonate sequences provide an exceptionally clear record of early microbial life, but one that is significantly biased with respect to the range of environments inhabited by contemporary organisms. Many of the best preserved Proterozoic microfossil assemblages come from microbial mats and organicrich muds that accumulated in protected coastal areas where a combination of high productivity, limited water circulation, and, often, hypersalinity limited post mortem degradation. The close distributional relationship between early diagenetic silica and organic matter can be explained in terms of a model developed by Leo and Barghoorn for the silicification of wood. Three factors appear to control the distribution of early diagenetic chert in Proterozoic sequences: sediment permeability, availability of silica in ground water solution, and locally high concentrations of organic matter in near-surface sediments. Of these, organic content appears to impose the major environmental bias. In terms of their excellent preservation, geochemistry of formation, and limited environmental coverage, Phanerozoic silicified peats bear comparison with their Proterozoic counterparts. Swamp dwellers may be the plants most likely to be preserved exceptionally well, but they may also be the plants least likely to give rise to new populations that will become ecologically widespread and evolutionarily important in subsequent periods. Allochthonous elements in permineralized peats may be unusually important to palaeobotany because they combine the exceptional preservation conferred by peat permineralization with ecological representation of floodplain and upland evolutionary cradles rather than swampland museums.
The process of silicification can destroy features of organisms that indicate biogenic origin.
A series of glass slides were placed at the edge of an acidic hot spring, and after 90 hours these slides were analyzed for growth. On these slides, silicified, partially silicified, and unsilicified microbes had all grown, with unsilicified microbes often wrapping around silicified microbes. Though the unsilicified and silicified versions of the same microbes should be easily compared to each other, the process of silicification altered the microbes so much that microbes of each type could not be matched unless attached to each other. The author also notes that many of the silicified microbes would be unlikely to be even identified as of biological origin.
Evidence is Sourced from:
Title
Microbial silicification in Iodine Pool, Waimangu geothermal area, North Island, New Zealand: implications for recognition and identification of ancient silicified microbes
Authors
Brian Jones, Kurt O Konhauser, Robin W Renaut, Raymond S Wheeler
Abstract
Silicified microbes provide evidence for some of the earliest life forms on Earth. They are extremely important to understanding the early development of life and the conditions that allowed its development. Such discussions commonly rely on comparisons with extant taxa and therefore depend upon the preservation style of the microbes and, in particular, the preservation of the taxonomically important features. Silicified microbes are deceptive: they commonly appear to be well preserved even though their taxonomically critical features have been destroyed by silicification. An understanding of the early taphonomic processes that influence microbial silicification can be obtained by studying extant microbes that are being silicified in modern hot spring pools. Iodine Pool, located in the Waimangu geothermal area on the North Island of New Zealand, is ideal for this purpose. The spring water has a temperature of 69–100 8C, a pH of 8.3–9.0, and 440–457 ppm SiO2. Glass slides left in the shallow marginal waters of this pool for 90 h became covered with thin layers of opaline silica, discrete opal-A spheres, unsilicified microbes, pseudofilaments, partly silicified microbes and silicified microbes. The rapidly silicified microbes appear well preserved with their general morphology, diameter, length, and presence or absence of septa being readily apparent. Most of the silicified microbes, however, lack the key features that would allow accurate comparisons with extant taxa. Only two of the silicified microbes can be tentatively allied with the unsilicified forms, despite being found side by side on the same glass slide. These problems in identifying modern, rapidly silicified microbes suggest that identifications of ancient silicified microbes can be problematic and must be treated with caution.
Silicification occurs more frequently in areas with organic matter, but the exact relation between these is unknown and can be hard to detect in areas with low amounts of organic matter.
[Patchiness]
Silicification in the Proterozoic commonly occurred in locations that preserved significant amounts of organic matter.
Any environment with large amounts of silica available for use, such as areas with volcanism or large deposits or dissolved SiO2, has a potential to undergo silicification, however the process favors areas with large amounts of organic matter. This was first noticed during analysis of the Draken Conglomerate Formation, where organic-rich areas were silicified and organic-poor areas had instead been calcified, despite the full area being exposed to materials that could have triggered either process. Many models have been made to try and characterize how presence of organics affects silica growth patterns, but no one solution appears to have been favored at the time of writing. But this still indicates that there is a direct connection between silicification and preservation of organisms. Therefore, if no organic matter is currently present, biogenic silica may not be able to be identified.
Evidence is Sourced from:
Title
Exceptional preservation of photosynthetic organisms in silicified carbonates and silicified peats
Authors
A H Knoll
Abstract
Carbonaceous cherts in Proterozoic carbonate sequences provide an exceptionally clear record of early microbial life, but one that is significantly biased with respect to the range of environments inhabited by contemporary organisms. Many of the best preserved Proterozoic microfossil assemblages come from microbial mats and organicrich muds that accumulated in protected coastal areas where a combination of high productivity, limited water circulation, and, often, hypersalinity limited post mortem degradation. The close distributional relationship between early diagenetic silica and organic matter can be explained in terms of a model developed by Leo and Barghoorn for the silicification of wood. Three factors appear to control the distribution of early diagenetic chert in Proterozoic sequences: sediment permeability, availability of silica in ground water solution, and locally high concentrations of organic matter in near-surface sediments. Of these, organic content appears to impose the major environmental bias. In terms of their excellent preservation, geochemistry of formation, and limited environmental coverage, Phanerozoic silicified peats bear comparison with their Proterozoic counterparts. Swamp dwellers may be the plants most likely to be preserved exceptionally well, but they may also be the plants least likely to give rise to new populations that will become ecologically widespread and evolutionarily important in subsequent periods. Allochthonous elements in permineralized peats may be unusually important to palaeobotany because they combine the exceptional preservation conferred by peat permineralization with ecological representation of floodplain and upland evolutionary cradles rather than swampland museums.
eCarbonaceous material in cherts may exist in different phases (from different time periods), possibly resulting from pulses of hydrothermal silica of different temperatures.
Carbonaceous material in cherts may exist in different phases (from different time periods), possibly resulting from pulses of hydrothermal silica of different temperatures.
This paper discusses the use of Raman spectroscopy to identify carbonaceous material in the Apex Chert. The result of the analyses indicates that there are two different phases of CM in the rock samples analyzed. The different generations add ambiguity to the potential biogenicity of the CM. After viewing the different phases, the authors note that one phase of CM is more thermally altered than the other, hinting at possible scenarios where one phase was deposited and preserved after a thermal event. Regardless of whether the CM was transported to the chert from somewhere else, or if the hydrothermal vent pulsed silica out at different times, these scenarios do not support biogenicity.
Evidence is Sourced from:
Title
Multiple Generations of Carbon in the Apex Chert and Implications for Preservation of Microfossils.
Authors
Alison Olcott Marshall, Julienne R. Emry, Craig P. Marshall.
Abstract
While the Apex chert is one of the most well-studied Archean deposits on Earth, its formation history is still not fully understood. Here, we present Raman spectroscopic data collected on the carbonaceous material (CM) present within the matrix of the Apex chert. These data, collected within a paragenetic framework, reveal two different phases of CM deposited within separate phases of quartz matrix. These multiple generations of CM illustrate the difficulty of searching for signs of life in these rocks and, by extension, in other Archean sequences.
Silica samples in the Apex Chert are found at depths where photosynthetic organisms can’t exist.
The argument of hydrothermal silica is used again to emphasize the abiotic nature of Apex Chert samples. When judging different forms of silica, certain stratiforms are noted to be derived from seawater silica, while silica filaments in black chert veins are shown to be from hydrothermal fluids mixing with seawater at depth. The depth is the key part of this argument, as photosynthetic organisms would not be found on sub-seafloor hydrothermal vents. It is too deep a place to get an accurate amount of sunlight to undergo photosynthesis. Instead, Fischer-Tropsch synthesis is proposed for how the CM was created. This process is abiotic in nature, supporting the argument that silica, and therefore the CM was abiotic in nature.
Evidence is Sourced from:
Title
Self-Assembled Silica-Carbonate Structures and Detection of Ancient Microfossils.
Authors
J.M Garcia-Ruiz, S. T. Hyde, A. M Carnerup et al.
Abstract
We have synthesized inorganic micron-sized filaments, whose microstucture consists of silica-coated nanometer-sized carbonate crystals, arranged with strong orientational order. They exhibit noncrystallographic, curved, helical morphologies, reminiscent of biological forms. The filaments are similar to supposed cyanobacterial microfossils from the Precambrian Warrawoona chert formation in Western Australia, reputed to be the oldest terrestrial microfossils. Simple organic hydrocarbons, whose sources may also be abiotic and indeed inorganic, readily condense onto these filaments and subsequently polymerize under gentle heating to yield kerogenous products. Our results demonstrate that abiotic and morphologically complex microstructures that are identical to currently accepted biogenic materials can be synthesized inorganically.
eChert samples taken from Mount Ada Basalt in Western Australia show evidence of hydrothermally altered organics, however they differ greatly from actual microfossils discovered in the same region.
Chert samples taken from Mount Ada Basalt in Western Australia show evidence of hydrothermally altered organics, however they differ greatly from actual microfossils discovered in the same region.
Using a combination of Raman spectroscopy, Scanning/transmission electron microscopy, and other geochemical analyses, carbonaceous material in the Mount Ada Basalt was evaluated. Silicon isotopic composition, when analyzed alongside Rare Earth Element composition, points to the precipitation of silica from hydrothermal vents (with a very small contribution from seawater). This data, alongside the presence of certain elements like Nickel and Iron, express favorability of microbial life. Raman data indicates similar peak metamorphic temperatures to organic microfossils found in the Strelley Pool formation. The similarity between Mount Ada and Strelley Pool samples ends there, mainly due to the latter samples having different molecular, morphological and textural attributes. The reasoning behind these differences has been debated, although the presence of aforementioned elements like Iron and Nickel (in the form of Fe-Cr-Ni alloys) in the reducing conditions of a hydrothermal system could have resulted in the abiotic formation of hydrocarbons. Reactions such as polymerization could also form longer hydrocarbon chains, subsequently producing the complex organic samples found in the Mount Ada Basalt. This results in a false positive evaluation because despite the data corroborating a biotic origin, the samples from Mount Ada differ greatly from actual microfossils from Strelley Pool.
Evidence is Sourced from:
Title
Organo-mineral associations in chert of the 3.5 Ga Mount Ada Basalt raise questions about the origin of organic matter in Paleoarchean hydrothermally influenced sediments.
Authors
Alleon, J., Flannery, D.T., Ferralis, N. et al.
Abstract
Hydrothermal and metamorphic processes could have abiotically produced organo-mineral associations displaying morphological and isotopic characteristics similar to those of fossilized microorganisms in ancient rocks, thereby leaving false-positive evidence for early life in the geological record. Recent studies revealed that geologically-induced alteration processes do not always completely obliterate all molecular information about the original organic precursors of ancient microfossils. Here, we report the molecular, geochemical, and mineralogical composition of organo-mineral associations in a chert sample from the ca. 3.47 billion-year-old (Ga) Mount Ada Basalt, in the Pilbara Craton, Western Australia. Our observations indicate that the molecular characteristics of carbonaceous matter are consistent with hydrothermally altered biological organics, although significantly distinct from that of organic microfossils discovered in a chert sample from the ca. 3.43 Ga Strelley Pool Formation in the same area. Alternatively, the presence of native metal alloys in the chert, previously believed to be unstable in such hydrothermally influenced environments, indicates strongly reducing conditions that were favorable for the abiotic formation of organic matter. Drawing definitive conclusions about the origin of most Paleoarchean organo-mineral associations therefore requires further characterization of a range of natural samples together with experimental simulations to constrain the molecular composition and geological fate of hydrothermally-generated condensed organics.
Introduction Silica (SiO₂) is a geologically abundant mineral that, under the right conditions, can entomb and preserve biological materials, making it a key medium in the fossilization of early life and a target in the search for ancient biosignatures. Silica occurs naturally in both crystalline (e.g., quartz) and amorphous forms (e.g., opal-A) and is particularly prevalent in hydrothermal, volcanic, and aqueous environments. In these settings, silica can precipitate from supersaturated fluids, sometimes influenced or accelerated by biological activity.
Mechanisms and Formation Silica becomes biologically relevant in environments where supersaturation allows for precipitation—often driven by cooling, evaporation, pH changes, or the presence of organic matter. Microbial biofilms and extracellular polymeric substances (EPS) can nucleate and bind dissolved silica, promoting the formation of amorphous silica layers. In hot spring [read more]Introduction Silica (SiO₂) is a geologically abundant mineral that, under the right conditions, can entomb and preserve biological materials, making it a key medium in the fossilization of early life and a target in the search for ancient biosignatures. Silica occurs naturally in both crystalline (e.g., quartz) and amorphous forms (e.g., opal-A) and is particularly prevalent in hydrothermal, volcanic, and aqueous environments. In these settings, silica can precipitate from supersaturated fluids, sometimes influenced or accelerated by biological activity.
Mechanisms and Formation Silica becomes biologically relevant in environments where supersaturation allows for precipitation—often driven by cooling, evaporation, pH changes, or the presence of organic matter. Microbial biofilms and extracellular polymeric substances (EPS) can nucleate and bind dissolved silica, promoting the formation of amorphous silica layers. In hot spring settings, microbial mats are often rapidly coated with opaline silica, leading to the preservation of cellular textures and sedimentary structures. This process, known as silicification, can preserve microorganisms, filaments, or stromatolitic forms in fine detail.
Biogenic Signals Silicified deposits may contain biosignatures in the form of preserved textures (e.g., microbial filaments, mat laminae), isotopic patterns (e.g., carbon or sulfur isotopes), or organic inclusions. On Earth, some of the oldest traces of life—dating back over 3.4 billion years—are found in silica-rich deposits, such as those in the Apex chert or Strelley Pool Formation. Biosilicification can also be active, as in diatoms, radiolarians, and sponges, which incorporate silica biologically to form shells or spicules. Even passive silicification of microbial communities may produce diagnostic morphologies, spatial associations, or geochemical contexts suggestive of life.
Abiotic Influences and Ambiguity Silica is not inherently biogenic. It precipitates under a wide range of abiotic conditions—particularly in volcanic terrains, hydrothermal outflows, and evaporitic systems. Abiotic silica can form sinters, nodules, or other sedimentary features that resemble biological structures in gross morphology. Additionally, post-depositional alteration may overprint original textures or isotopic signatures, complicating interpretation. As such, biosignature claims from silicified samples require multi-line evidence: textural, chemical, and contextual.
Why This Matters Silica is one of the best natural media for preserving microscopic life, especially in planetary environments where liquid water and volcanic activity coincide. Mars, in particular, hosts silica-rich deposits near former hydrothermal systems, making it a prime target for astrobiology. Detection of finely laminated or morphologically complex silica deposits—especially in conjunction with organic compounds, isotopic shifts, or mineralogical layering—may strongly indicate former biological presence. Understanding how silica interacts with life, and how those interactions persist through geologic time, is central to evaluating this mineral as a biosignature scaffold.
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