Biosignatures link microorganisms to iron mineralization in a paleoaquifer

Abstract

Concretions, preferentially cemented masses within sediments and sedimentary rocks, are records of sediment diagenesis and tracers of pore water chemistry. For over a century, rinded spheroidal structures that exhibit an Fe(III) oxide-rich exterior and Fe-poor core have been described as oxidation products of Fe(II) carbonate concretions. However, mechanisms governing Fe(III) oxide precipitation within these structures remain an enigma. Here we present chemical and morphological evidence of microbial biosignatures in association with Fe(III) oxides in the Fe(III) oxide-rich rind of spheroidal concretions collected from the Jurassic Navajo Sandstone (southwest United States), implicating a microbial role in Fe biomineralization. The amount of total organic carbon in the exterior Fe(III) oxides exceeded measured values in the friable interior. The mean δ 13 C value of organic carbon from the Fe(III) oxidecemented exterior, δ 13 C of −20.55‰, is consistent with a biogenic signature from autotrophic bacteria. Scanning electron micrographs reveal microstructures consistent with bacterial size and morphology, including a twisted-stalk morphotype that resembled an Fe(II)-oxidizing microorganism, Gallionella sp. Nanoscale associations of Fe, O, C, and N with bacterial morphotypes demonstrate microorganisms associated with Fe(III) oxides. Together these results indicate that autotrophic microorganisms were present during Fe(III) oxide precipitation and present microbial catalysis as a mechanism of Fe(III) oxide concretion formation. Microbial biosignatures in rinded Fe(III) oxide-rich concretions within an exhumed, Quaternary aquifer has broad implications for detection of life within the geological record on Earth as well as other Fe-rich rocky planets such as Mars, where both Fe(II) carbonate and Fe(III) oxide-rich concretions have been identifi ed.

Life and Liesegang: Outcrop-Scale Microbially Induced Diagenetic Structures and Geochemical Self-Organization Phenomena Produced by Oxidation of Reduced Iron

“…They noted that a number of morphological characteristics associated with these microbes, taken collectively, were strong indicators of biogenicity and were, therefore, biosignatures. Weber et al (2012) were able to link biosignatures to some unusual iron-oxide mineralization in the Navajo sandstone and proposed that a variety of unusual concretions were the product of microbial activity. Among the more successful attempts to find large-scale biosignatures has been the description of microbially induced sedimentary structures (MISS) (e.g., Noffke, 2009).…”

“…(2) Mineral cements are archives in which records of fluid-mineral interaction are stored, a fact that has prompted considerable interest in the origin of carbonate and silicate mineral cements (e.g., Harwood et al, 2013;Tomkinson et al, 2013). IOCs have not, with recent exceptions (Chan et al, 2012, and references therein;Weber et al, 2012;Vasconcelos et al, 2013;Reiners et al, 2014), been subjected to the detailed study that is commonplace for carbonate or silicate cements. We are not aware, for example, of any attempts to test the Liesegang hypothesis of iron-oxide cementation in a specific formation.…”

“…We have observed iron encrusted bacilli-like structures in IOCs previously . Given the distinctive morphology and evidence for preservation of Gallionella and similar species (e.g., Weber et al, 2012;Krepski et al, 2013), however, we focused on the twisted stalk morphology as a biosignature indicator for past metabolically active iron-oxidizing microbiota.…”

Biological and Geochemical Development of Placer Gold Deposits at Rich Hill, Arizona, USA

“…Manganese and iron are elements commonly associated with redox chemosynthetic bacteria in the shallow subsurface, particularly within deposits of metals [42][43][44][45] and aquifers [46]. Indeed, some are reactive Mn-oxide biominerals (e.g., birnessite), capable of mobilization of gold by oxidizing Au(0) and Au(I) to Au(III) complexes, thereby driving the transformation of gold nuggets [47,48].…”

A peculiar reworking ofOphiomorphashafts in the Miocene Nangang Formation, Taiwan

“…Studies on extant Thalassinidean ghost shrimp have revealed slightly reducing conditions in the wall material related to the organic material in the mucus (Bird, Boon, & Nichols, 2000). Microbial growth accelerates the oxidation of Fe 2+ to Fe 3+ by acting as a catalyst (Weber et al, 2012), as the microbes require Fe 2+ for obtaining their energy at depths where the availability of organic carbon is minimal. This implies that the overall environment in the walls was oxidising, with the available oxidising pore waters giving rise to iron bearing hydroxides and oxyhydroxide minerals such as goethite.…”