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.
Highlights
The search for biosignatures in the Early Archean geological record is challenging, as most sedimentary formations from this time interval have been exposed to hydrothermal fluids, and have undergone at least prehnite–pumpellyite facies to lower greenschist facies metamorphism.Under these high temperature and high pressure conditions, rocks have recrystallized and organic matter has undergone carbonization
Examples are exfoliated micas (Wacey, Saunders, Kong, Brasier, & Brasier, 2015), dispersed hematite crystals (Marshall, Emry, & Marshall, 2011; Pinti, Mineau, & Clement, 2009), and migrated hydrocarbons that filled the pore space between botryoidal quartz grains (Brasier et al, 2005)
Most intriguing are self-organized life-like silica–carbonate precipitates (Garcia-Ruiz et al, 2003). These microstructures form spontaneously in the laboratory when alkaline-earth metals are mixed with silica-rich alkaline solutions in the presence of CO2 (García-Ruiz, 1998). They have been synthesized by diffusion of alkaline-earth metals through silica gels
Summary
The search for biosignatures in the Early Archean geological record is challenging, as most sedimentary formations from this time interval have been exposed to hydrothermal fluids, and have undergone at least prehnite–pumpellyite facies to lower greenschist facies metamorphism. In Earth history, the concentration of silica in the oceans was much higher than today (Konhauser, Jones, Reysenbach, & Renaut, 2003; Maliva, Knoll, & Simonson, 2005; Siever, 1992), which is attested by ubiquitous sedimentary chert deposits in Archean greenstone belts (Nijman, De Bruijne, & Valkering, 1998; Van den Boorn, Van Bergen, Nijman, & Vroon, 2007; Van Kranendonk & Pirajno, 2004) These cherts formed either by direct precipitation of colloidal silica from silica-rich seawater, or by fluid-induced silica infiltration/replacement (silicification) of preexisting sediments (Stefurak, Lowe, Zentner, & Fischer, 2014; Van den Boorn et al, 2007). The results of both solution-based and gel-based experiments are subsequently used to define criteria for distinction between biomorphs and microfossils, and to make an assessment of the likelihood that assemblages of biomorphs are associated with, or mistaken for, true microfossils in ancient chert deposits
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
