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

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 bona fi de microfossils. In addition, amorphous carbonaceous material occurs as a grain boundary phase between quartz crystals and within fl uid 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 FischerTropsch-type (FTT) synthesis reactions in an ancient hydrothermal vent.

Fossils and astrobiology: new protocols for cell evolution in deep time

“…Unfortunately, such chemical tests are far from being a safe criterion for cellular organization when used alone (e.g., Schopf et al 2010). Pseudofossils can also be built from degraded organic matter (Brasier et al 2002;De Gregorio & Sharp, 2006). This approach therefore needs to be supplemented by other tests, including the search for chemical signals within small mineral grains in the immediate surroundings of candidate cells (Table 1, criterion IV-B), as outlined below.…”

Ancient graphite in the Eoarchean quartz–pyroxene rocks from Akilia in southern West Greenland I: Petrographic and spectroscopic characterization

“…Raman spectroscopy of carbonaceous material from the Archean Apex chert (Schopf et al, 2002(Schopf et al, , 2005Brasier et al, 2002Brasier et al, , 2005, which has experienced relatively low-grade greenschist-facies metamorphism, was insufficient to determine its origin unambiguously (Pasteris and Wopenka, 2003). However, correlated analysis of microtomed sections of this carbonaceous material using TEM and STXM demonstrated that it was structurally similar to kerogen from the 1.878 Ga Gunflint chert, even if an FTT origin could not be completely ruled out (De Gregorio et al, 2006;De Gregorio and Sharp, 2009). In this study, correlated micro-analyses of graphite in the Akilia Qp rock led to the interpretation that at least some of the graphite formed from fluid-deposition, but that the origin of graphite associated with apatite is ambiguous.…”

Multiple Generations of Carbon in the Apex Chert and Implications for Preservation of Microfossils

“…300-425°C) (Schopf, 1993;Blatt et al, 2005;Brasier et al, 2005;Van Kranendonk, 2006). These rocks also include the Apex chert, a carbon-containing unit (Brasier et al, 2002(Brasier et al, , 2005De Gregorio and Sharp, 2006;Marshall et al, 2011) that has been proposed to contain the oldest putative microbial fossils on Earth (Schopf and Packer, 1987;Schopf, 1993). While the majority of studies have focused primarily on these structures (e.g., Schopf and Packer, 1987;Brasier et al, 2002;Schopf et al, 2002;Marshall et al, 2011), others have addressed the mode of formation and alteration of the Apex chert (e.g., Van Kranendonk and Pirajno, 2004;Brasier et al, 2005;Van Kranendonk, 2006).…”

“…While the majority of studies have focused primarily on these structures (e.g., Schopf and Packer, 1987;Brasier et al, 2002;Schopf et al, 2002;Marshall et al, 2011), others have addressed the mode of formation and alteration of the Apex chert (e.g., Van Kranendonk and Pirajno, 2004;Brasier et al, 2005;Van Kranendonk, 2006). This paper presents a Raman spectroscopic examination of the carbonaceous material (CM) present within the Apex chert in a paragenetic context, a combined approach that allows for the determination of the relative age of formation and thermal grade of the CM recently identified within the matrix of the Apex chert (Brasier et al, 2002(Brasier et al, , 2005De Gregorio and Sharp, 2006;Marshall et al, 2011). Furthermore, these data suggest that the formation and alteration history of CM in other Archean rocks may be more complex than previously thought and that an understanding of the degree of alteration of CM found within Archean deposits is critical when prospecting for evidence of the earliest life on Earth.…”