A morphogram for silica‐witherite biomorphs and its application to microfossil identification in the early earth rock record

Abstract

Archean hydrothermal environments formed a likely site for the origin and early evolution of life. These are also the settings, however, were complex abiologic structures can form. Low‐temperature serpentinization of ultramafic crust can generate alkaline, silica‐saturated fluids in which carbonate–silica crystalline aggregates with life‐like morphologies can self‐assemble. These “biomorphs” could have adsorbed hydrocarbons from Fischer–Tropsch type synthesis processes, leading to metamorphosed structures that resemble carbonaceous microfossils. Although this abiogenic process has been extensively cited in the literature and has generated important controversy, so far only one specific biomorph type with a filamentous shape has been discussed for the interpretation of Archean microfossils. It is therefore critical to precisely determine the full distribution in morphology and size of these biomorphs, and to study the range of plausible geochemical conditions under which these microstructures can form. Here, a set of witherite‐silica biomorph synthesis experiments in silica‐saturated solutions is presented, for a range of pH values (from 9 to 11.5) and barium ion concentrations (from 0.6 to 40 mmol/L BaCl2). Under these varying conditions, a wide range of life‐like structures is found, from fractal dendrites to complex shapes with continuous curvature. The size, spatial concentration, and morphology of the biomorphs are strongly controlled by environmental parameters, among which pH is the most important. This potentially limits the diversity of environments in which the growth of biomorphs could have occurred on Early Earth. Given the variety of the observed biomorph morphologies, our results show that the morphology of an individual microstructure is a poor criterion for biogenicity. However, biomorphs may be distinguished from actual populations of cellular microfossils by their wide, unimodal size distribution. Biomorphs grown by diffusion in silica gel can be differentiated by their continuous gradient in size, spatial density, and morphology along the direction of diffusion.

On the biogenicity of Fe‐oxyhydroxide filaments in silicified low‐temperature hydrothermal deposits: Implications for the identification of Fe‐oxidizing bacteria in the rock record

“…However, the range of stalk widths within samples from a single locality appears to be relatively narrow. In contrast, abiotic filamentous artifacts may display a large range of widths and a skewed distribution (Hofmann et al, ; Rouillard, García‐Ruiz, Gong, & Zuilen, ). The differences in width distribution in Figure could thus indicate that the filaments in the chimney are of a different origin than the confirmed biogenic stalks in the laminated mounds, although width distributions alone are insufficient to exclude microbial Fe oxidation and stalk mineralization.…”

“…The twisted shape of extracellular iron stalks is a prime example of a diagnostic trait that is linked to physiology and may promote the recognition of Fe‐oxidizing micro‐organisms in the rock record (Chan et al, ). It is, however, well‐known that abiogenic processes are capable of producing filamentous features that may easily be mistaken for remnants of bacteria or biominerals (e.g., Rouillard et al, ). Our comparison of textures from the JMVF mounds and chimney demonstrates that even biosignatures considered quite robust can give rise to substantial ambiguity, as a result of both post‐depositional diagenetic alteration and resemblance to abiogenic features.…”

“…The twisted shape of extracellular iron stalks is a prime example of a diagnostic trait that is linked to physiology and may promote the recognition of Fe-oxidizing micro-organisms in the rock record . It is, however, well-known that abiogenic processes are capable of producing filamentous features that may easily be mistaken for remnants of bacteria or biominerals (e.g., Rouillard et al, 2018 Organic carbon is a key component of life and the detection of C in microfossils has successfully been applied to support the identification of microbes in ancient environments harboring photosynthetic and heterotrophic organisms (e.g., Wacey et al, 2013;Wacey et al, 2012). Considering the lack of preserved C in the studied stalks from the mildly altered JMVF mounds, organic C does not appear to be a useful biosignature in the search for microaerophilic Fe-oxidizing bacteria in hydrothermally affected deposits, despite its presence in stalks from fresh environmental samples (Chan et al, 2009;Suzuki et al, 2011) and cultures exposed to elevated temperatures (Picard et al, 2015).…”

Identifying microbial life in rocks: Insights from population morphometry

“…Two different types of abiogenic objects were chosen: interstitial spaces between clasts in sedimentary rocks and silica–carbonate biomorphs. The importance of silica–carbonate biomorphs for micropaleontology has been put forward in previous studies (García‐Ruiz, Carnerup, Christy, Welham, & Hyde, ; García‐Ruiz et al, ; Rouillard et al, ). They display a wide range of life‐like morphologies; during the Archean, they may have been formed—and preserved—in the same hydrothermal environments where life emerged and first evolved.…”

“…Early life forms lacked sufficient morphologic complexity to be easily distinguished from abiogenic structures, such as pore‐fillings, interstitial spaces between crystals, air bubbles, and fluid inclusions. Furthermore, certain abiotic self‐organized structures, including silica–carbonate biomorphs (Carnerup, ; García‐Ruiz et al, ; Rouillard, García‐Ruiz, Gong, & Zuilen, ), chemical gardens (McMahon, ), carbon–sulfur biomorphs (Cosmidis & Templeton, ), or manganese oxide biomorphs (Muscente, Czaja, Tuggle, Winkler, & Xiao, ) also have the potential to create a variety of shapes, including spheroids, framboids, helicoids, or filaments that resemble individual cells or clusters of cells. The shapes adopted by microbial cells and colonies are therefore not unique to living systems.…”