Some of the earliest forms of life on Earth produced organic carbon that accumulated in the sedimentary record.

Specimens similar to microfossils in Apex chert have been found in Paleoarchaean hydrothermal units. The optical micrographs and drawings of the microfossils in Apex chert show that the mode of preservation, size, shape, form of occurrence and paleoenvironmental setting are very similar to deposits of similar age. Raman spectra of the putative microfossils in the Apex chert showed “D” and “G” bands, interpreted as kerogenous material that is similar to other permineralized Precambrian fossils rather than graphite.

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Title

Biogenicity of Earth’s earliest fossils: A resolution of the controversy

Authors

Schopf, J. William and Kudryavtsev, Anatoliy B.

Abstract

The abundant and diverse assemblage of filamentous microbial fossils and associated organic matter permineralized in the ~ 3465 Ma Apex chert of northwestern Australia — widely regarded as among the oldest records of life — have been investigated intensively. First reported in 1987 and formally described in 1992 and 1993, the biogenicity of the Apex fossils was questioned in 2002 and in three subsequent reports. However, as is shown here by use of analytical techniques unavailable twenty years ago, the Apex filaments are now established to be bona fide fossil microbes composed of three-dimensionally cylindrical organic(kerogenous-) walled cells. Backed by a large body of supporting evidence of similar age — other microfossils, stromatolites, and carbon isotopic data — it seems clear that microbial life was present and flourishing on the early Earth ~ 3500 Ma ago.

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Using spectroscopy and microscopy tools, organic carbon of biological origin is found to be prevalent in different rock units containing microfossils

High resolution transmission electron microscopy (TEM) and EELS spectra show that the distribution of carbonaceous material along the quartz grain in the Apex chert is similar to kerogen distribution in the Gunflint microfossils. These experiments also showed that the disseminated carbon is structurally amorphous which would be consistent with a biogenic origin.

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Title

The structure and distribution of carbon in 3.5 Ga Apex chert:Implications for the biogenicity of Earth‘s oldest putative microfossils

Authors

Gregorio, Bradley T. De and Sharp, Thomas G.

Abstract

The oldest putative microfossils on Earth occur in the 3.5 Ga Apex chert of the Warrawoona Group, Western Australia. We have analyzed disseminated interstitial carbon found within Apex chert using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) to address the controversy regarding its state of structural disorder. We found that the carbonaceous material is structurally amorphous, with no evidence of graphitization, and contains aromatic domains, most likely as polyaromatic ring structures, similar to preserved kerogen in bonafide microfossils. In addition, amorphous carbonaceous material occurs as a grain boundary phase between quartz crystals and within fluid inclusions in quartz crystals, indicating that hydrocarbons moved through the chert during crystallization and hydrothermal alteration. The results suggest that the carbonaceous material is similar in structure to microfossil kerogen, implying the microbe-like features within Apex chert are also microfossils. However, this kerogen-like material may also be produced abiotically via Fischer- Tropsch-type (FTT) synthesis reactions in an ancient hydrothermal vent.

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The composition of kerogen in Apex fossils is very similar to other 3200-3500 Ma chert units where microfossils were found.

Raman spectral bands showed that while the Apex chert are more geochemically mature than Bitter Springs and Gunflint formations, they are better preserved and less geochemically altered. The Raman spectra of Apex fossils show carbonaceous peaks at a different wavenumbers than graphite. This implies that these fossils do not contain any graphite which was been used as argument against its biogenicity.

Evidence is Sourced from:

Title

Biogenicity of Earth’s earliest fossils: A resolution of the controversy

Authors

Schopf, J. William and Kudryavtsev, Anatoliy B.

Abstract

The abundant and diverse assemblage of filamentous microbial fossils and associated organic matter permineralized in the ~ 3465 Ma Apex chert of northwestern Australia — widely regarded as among the oldest records of life — have been investigated intensively. First reported in 1987 and formally described in 1992 and 1993, the biogenicity of the Apex fossils was questioned in 2002 and in three subsequent reports. However, as is shown here by use of analytical techniques unavailable twenty years ago, the Apex filaments are now established to be bona fide fossil microbes composed of three-dimensionally cylindrical organic(kerogenous-) walled cells. Backed by a large body of supporting evidence of similar age — other microfossils, stromatolites, and carbon isotopic data — it seems clear that microbial life was present and flourishing on the early Earth ~ 3500 Ma ago.

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The distribution of kerogen in Apex chert is similar to the 1.9Ga Gunflint microfossils.

Using high resolution transmission electron microscopy (TEM) and EELS spectra the distribution of carbonaceous material in the Apex chert is found to be similar to kerogen distribution in the Gunflint microfossils. EELS data also showed that the kerogen in Apex is similarly amorphous to the Gunflint micrfossils.

Evidence is Sourced from:

Title

The structure and distribution of carbon in 3.5 Ga Apex chert:Implications for the biogenicity of Earth‘s oldest putative microfossils

Authors

Gregorio, Bradley T. De and Sharp, Thomas G.

Abstract

The oldest putative microfossils on Earth occur in the 3.5 Ga Apex chert of the Warrawoona Group, Western Australia. We have analyzed disseminated interstitial carbon found within Apex chert using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) to address the controversy regarding its state of structural disorder. We found that the carbonaceous material is structurally amorphous, with no evidence of graphitization, and contains aromatic domains, most likely as polyaromatic ring structures, similar to preserved kerogen in bonafide microfossils. In addition, amorphous carbonaceous material occurs as a grain boundary phase between quartz crystals and within fluid inclusions in quartz crystals, indicating that hydrocarbons moved through the chert during crystallization and hydrothermal alteration. The results suggest that the carbonaceous material is similar in structure to microfossil kerogen, implying the microbe-like features within Apex chert are also microfossils. However, this kerogen-like material may also be produced abiotically via Fischer- Tropsch-type (FTT) synthesis reactions in an ancient hydrothermal vent.

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Fischer Tropsch type-processes can abiotically synthesize organic carbon under alkaline, mildly hydrothermal conditions.

Immersion of abiotically-synthesized witherite crystallites in formaldehydephenol mixtures, followed by heating at 125 °C, produced carbonaceous residues, as confirmed through Laser Raman spectroscopy. The obtained spectra resemble the kerogen spectroscopic signals of the microfossils in the old chert from the Warrawoona Group. Such a physiochemical environment required for the organic carbon formation (alkaline medium, silicate and carbonate sources, barium ions, and organic precursors) is unusual at present; however, it was geochemically plausible during Archaean times when these microfossils were formed. Furthermore, the Warrawoona Group is rich in chert, barite, and carbonate minerals supporting this abiotic mechanism for forming carbonaceous microstructures.

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Title

Self-assembled silica-carbonate structures and detection of ancient microfossils

Authors

García-Ruiz, Juan Manuel, Hyde, Stephen T, Carnerup, AM, Christy, AG, Van Kranendonk, MJ, Welham, NJ

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.

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Abiotic hydrothermal alteration and weathering of micas can explain the distributions of kerogen-like carbon observed in the Apex filaments.

The exfoliation of muscovite (the most common form of a group of aluminum silicate minerals known as micas) within the Apex hydrothermal dykes are presented as a potential mechanism that could have created mineral-like objects that resemble the microfossils in the old chert. Further elucidation of a partial decrease of potassium levels and the presence of elements known to be abundant in hydrothermal systems within the Apex filaments supports the idea that hydrothermal dikes could have aided in the separation of mica layers. In this way, organic material from the organic-rich Pilbara black chert dikes may have (i.) helped facilitate the exfoliation of muscovite and (ii.) trapped within the separate muscovite layers. Additionally, the authors attribute the range of coloration within the Apex filaments to differences in iron sulfide or carbon concentration within the separated muscovite sheets.

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Title

Comparative multi-scale analysis of filamentous microfossils from the c. 850 Ma Bitter Springs Group and filaments from the c. 3460 Ma Apex chert

Authors

Wacey, David, Eiloart, Kate, Saunders, Martin

Abstract

Filamentous microfossils belonging to Cephalophytarion from the 850 Ma Bitter Springs Group have previously been used as key analogues in support of a biological interpretation for filamentous objects from the 3460 Ma Apex chert. Here we provide a new perspective on this interpretation by combining Raman data with correlative electron microscopy data from both Cephalophytarion and Apex specimens. We show that, when analysed at high spatial resolution, the Apex filaments bear no morphological resemblance to the younger Bitter Springs microfossils. Cephalophytarion filaments are shown to be cylindrical, comprising chains of box-like cells of approximately constant dimensions with lateral kerogenous walls and transverse kerogenous septa. They exhibit taphonomic shrinkage and folding, possess fine cylindrical sheaths and are permineralized by sub-micrometric quartz grains. They fulfil all established biogenicity criteria for trichomic microfossils. In contrast, Apex filaments do not possess lateral cell walls, are not cylindrical in nature, and vary considerably in diameter along their length. Their kerogenous carbon does not have a cell-like distribution and their chemistry is consistent with an origin as exfoliated phyllosilicate grains. This work demonstrates the importance of high-resolution data when interpreting the microstructure, and origins, of putative Precambrian microfossils.

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Abiotic processes can explain the presence of separate generations of carbonaceous material within the Apex chert. 

Raman spectroscopic analyses of carbonaceous material within the matrix of the Apex chert revealed the presence of two different phases of carbonaceous material with varying thermal alteration histories. This heterogeneity could be explained by various mechanisms, including but not limited to allochthonous carbonaceous materials or the emplacement of carbonaceous material during episodic hydrothermal pulses varying in temperature. Furthermore, Raman microspectroscopy on carbonaceous particles from a collection site 50 km west of the Apex chert, known as the Pilbara Craton, showed intensities within the range of those seen in the analyses of the carbonaceous material within the Apex chert. Such proposed mechanisms and results support the idea that the carbonaceous material could have been transported from nearby regions, showing great potential for the abiotic synthesis of carbonaceous material.

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Title

Multiple generations of carbon in the Apex chert and implications for preservation of microfossils

Authors

Marshall, Alison Olcott, Emry, Julienne R, Marshall, Craig P

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.

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Carbonaceous material in Apex chert has been deposited through multiple generations

The different phases of the different carbonaceous material found in Apex chert implies that the material was deposited at different times. Thermal alteration and metamorphic alteration are suggested as causes for differing phases, but it cannot be attributed to solely by biogenic processes.

Evidence is Sourced from:

Title

Multiple Generations of Carbon in the Apex Chert and Implications forPreservation of Microfossils

Authors

Marshall, Alison Olcott and Emry, Julienne R. and Marshall, Craig P.

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.

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