Oxidative alteration of ferrous smectites and implications for the redox evolution of early Mars.

Surface conditions on early Mars were likely anoxic, similar to early Earth, but the timing of the evolution to oxic conditions characteristic of contemporary Mars is unresolved. Ferrous trioctahedral smectites are the thermodynamically predicted products of anoxic basalt weathering, but orbital analyses of Noachian-aged terrains find primarily Fe3+-bearing clay minerals. Rover-based detection of Fe2+-bearing trioctahedral smectites at Gale Crater suggest that ferrous smectites are the unoxidized progenitors of orbitally-detected ferric smectites. To assess this pathway, we conducted ambient-temperature oxidative alteration experiments on four synthetic ferrous smectites having molar Fe/(Mg+Fe) from 1.00 to 0.33. Smectite suspension in air-saturated solutions produced incomplete oxidation (24-38% Fe3+/ΣFe). Additional smectite oxidation occurred upon re-exposure to air-saturated solutions after anoxic hydrothermal recrystallization, which accelerated cation and charge redistribution in the octahedral sheet. Oxidation was accompanied by contraction of the octahedral sheet (d(060) decreased from 1.53-1.56 Å to 1.52 Å), consistent with a shift towards dioctahedral structure. Ferrous smectite oxidation by aqueous hydrogen peroxide solutions resulted in nearly complete Fe2+ oxidation but also led to partial Fe3+ ejection from the structure, producing nanoparticulate hematite. Reflectance spectra of oxidized smectites were characterized by (Fe3+,Mg)2-OH bands at 2.28-2.30 µm, consistent with oxidative formation of dioctahedral nontronite. Accordingly, ferrous smectites are plausible precursors to observed ferric smectites on Mars, and their presence in late-Noachian sedimentary units suggests that anoxic conditions may have persisted on Mars beyond the Noachian.

Raman imagery: a new approach to assess the geochemical maturity and biogenicity of permineralized precambrian fossils.

Laser-Raman imagery is a non-intrusive, non-destructive analytical technique, recently introduced to Precambrian paleobiology, that can be used to demonstrate a one-to-one spatial correlation between the optically discernible morphology and kerogenous composition of permineralized fossil microorganisms. Made possible by the submicron-scale resolution of the technique and its high sensitivity to the Raman signal of carbonaceous matter, such analyses can be used to determine the chemical-structural characteristics of organic-walled microfossils and associated sapropelic carbonaceous matter in acid-resistant residues and petrographic thin sections. Here we use this technique to analyze kerogenous microscopic fossils and associated carbonaceous sapropel permineralized in 22 unmetamorphosed or little-metamorphosed fine-grained chert units ranging from approximately 400 to approximately 2,100 Ma old. The lineshapes of the Raman spectra acquired vary systematically with five indices of organic geochemical maturation: (1) the mineral-based metamorphic grade of the fossil-bearing units; (2) the fidelity of preservation of the fossils studied; (3) the color of the organic matter analyzed; and both the (4) H/C and (5) N/C ratios measured in particulate kerogens isolated from bulk samples of the fossil-bearing cherts. Deconvolution of relevant spectra shows that those of relatively well-preserved permineralized kerogens analyzed in situ exhibit a distinctive set of Raman bands that are identifiable also in hydrated organic-walled microfossils and particulate carbonaceous matter freed from the cherts by acid maceration. These distinctive Raman bands, however, become indeterminate upon dehydration of such specimens. To compare quantitatively the variations observed among the spectra measured, we introduce the Raman Index of Preservation, an approximate measure of the geochemical maturity of the kerogens studied that is consistent both with the five indices of organic geochemical alteration and with spectra acquired from fossils experimentally heated under controlled laboratory conditions. The results reported provide new insight into the chemical-structural characteristics of ancient carbonaceous matter, the physicochemical changes that accompany organic geochemical maturation, and a new criterion to be added to the suite of evidence by which to evaluate the origin of minute fossil-like objects of possible but uncertain biogenicity.

Laser--Raman imagery of Earth's earliest fossils.

Unlike the familiar Phanerozoic history of life, evolution during the earlier and much longer Precambrian segment of geological time centred on prokaryotic microbes. Because such microorganisms are minute, are preserved incompletely in geological materials, and have simple morphologies that can be mimicked by nonbiological mineral microstructures, discriminating between true microbial fossils and microscopic pseudofossil 'lookalikes' can be difficult. Thus, valid identification of fossil microbes, which is essential to understanding the prokaryote-dominated, Precambrian 85% of life's history, can require more than traditional palaeontology that is focused on morphology. By combining optically discernible morphology with analyses of chemical composition, laser--Raman spectroscopic imagery of individual microscopic fossils provides a means by which to address this need. Here we apply this technique to exceptionally ancient fossil microbe-like objects, including the oldest such specimens reported from the geological record, and show that the results obtained substantiate the biological origin of the earliest cellular fossils known.