Earth's earliest and deepest purported fossils may be iron-mineralized chemical gardens.

Sean Mcmahon

doi:10.1098/rspb.2019.2410

Abstract

Recognizing fossil microorganisms is essential to the study of life's origin and evolution and to the ongoing search for life on Mars. Purported fossil microbes in ancient rocks include common assemblages of iron-mineral filaments and tubes. Recently, such assemblages have been interpreted to represent Earth's oldest body fossils, Earth's oldest fossil fungi, and Earth's best analogues for fossils that might form in the basaltic Martian subsurface. Many of these putative fossils exhibit hollow circular cross-sections, lifelike (non-crystallographic, constant-thickness, and bifurcate) branching, anastomosis, nestedness within ‘sheaths’, and other features interpreted as strong evidence for a biological origin, since no abiotic process consistent with the composition of the filaments has been shown to produce these specific lifelike features either in nature or in the laboratory. Here, I show experimentally that abiotic chemical gardening can mimic such purported fossils in both morphology and composition. In particular, chemical gardens meet morphological criteria previously proposed to establish biogenicity, while also producing the precursors to the iron minerals most commonly constitutive of filaments in the rock record. Chemical gardening is likely to occur in nature. Such microstructures should therefore not be assumed to represent fossil microbes without independent corroborating evidence.

Highlights

In line with previous studies of tubular chemical gardens using iron salts [30,32,33], the Raman, EDX, and X-ray diffraction (XRD) analyses in the present study suggest that biomorphs produced by reacting ferrous sulfate with either sodium carbonate or sodium silicate solutions were composed largely of iron oxyhydroxides
This work has shown that the self-organizing behaviour of pH-driven inorganic chemical reactions can produce ironmineralized filaments and tubes closely analogous in both morphology and composition to numerous microstructures found in diverse rocks of all ages, which have hitherto been interpreted as fossil microorganisms
It would be rash to conclude that these geologic assemblages exclusively represent mineralized chemical gardens, but the present results show that this abiotic mode of origin must be considered as a plausible ‘null hypothesis’ to be rejected or not on a case-by-case basis depending on the evidence

Summary

Introduction

Filaments and tubes composed predominantly of nano- and microcrystalline iron (oxyhydr)oxides and iron (alumino)silicates occur as dense assemblages in diverse rocks of all ages, including submarine hydrothermal chert ( jasper) beds and veins [1,2,3,4,5]; fractures, veins, vesicles, and amygdales in numerous marine and terrestrial basalts [6,7,8,9,10]; mineralized cavities in limestones [8,11,12]; and the porous oxidation zones of metal ore bodies [7,8]. I report the facile experimental production of microscopic filamentous iron-mineral biomorphs that replicate key morphological features previously thought to imply a biotic origin for compositionally similar filaments in the rock record These structures result from the well-known phenomenon of chemical gardening ( known as ‘silica gardening’), whereby the dissolution of a ‘seed’ metal salt into an alkaline carbonate or silicate solution produces a pocket of acidic fluid enclosed by a gelatinous membrane of hydrous metal carbonate or silicate together with metal (oxyhydr)oxides [32,33,34,35]. These processes have been studied for nearly a century and are increasingly well understood, yet their potential to form large populations of microscopic iron-mineralized filaments closely resembling purported fossil assemblages has rarely been recognized [28,36]

Results

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Conclusion