A morphogram for silica-witherite biomorphs and its application to microfossil identification in the early earth rock record
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2018Referencia bibliográfica
Rouillard J, García-Ruiz J-M, Gong J, van Zuilen MA. A morphogram for silica-witherite biomorphs and its application to microfossil identification in the early earth rock record. Geobiology. 2018;16:279–296. [http://hdl.handle.net/10481/52802]
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This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement nº 646894) and under the ERC Seventh Framework Programme FP7/2007-2013 (grant agreement n° 340863). JMG-R also acknowledges the Ministerio de Economía y Competitividad of Spain through the project CGL2016-78971-P. We acknowledge the analytical platform PARI and Stefan Borenstazjn for SEM imaging. Prof. Y. Tsukii and The Protist Information Server (http://protist.i.hosei.ac.jp/) are acknowledged for the use of pictures of cyanobacteria. This is IPGP contribution n° 3912. We are grateful to two anonymous reviewers for their helpful comments.Resumen
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.