Characterization of Sulfur-Rich Microbial Organic Matter in Jurassic Carbonates Using Laser-Assisted Mass Spectrometry.

Laser desorption-ionization mass spectrometry (MS) shows great potential for in situ molecular analysis of planetary surfaces and microanalysis of space-returned samples or (micro)fossils. Coupled with pyrolysis gas chromatography-mass spectrometry (Py-GC-MS) in ESA's ExoMars project, this technique could help assess further the origin of sulfur-bearing organic matter (OM) recently detected on Mars. To unravel this potential, we analyzed sulfurized microbial OM from ca. 150 million year-old carbonates with laser desorption-ionization mass spectrometry (single- and two-step: LDI-MS and L2MS), in comparison with time-of-flight secondary-ion mass spectrometry (ToF-SIMS), gas chromatography-mass spectrometry (GC-MS), and Py-GC-MS. We show that LDI-MS and L2MS readily detect sulfur-bearing moieties such as (alkyl)thiophenes and (alkyl)benzothiophenes. The mineral matrix, however, made the identification of sulfur-bearing molecules challenging in our L2MS experiment. The dominance of small aromatic hydrocarbons (≤14 carbons) in the LDI-MS and L2MS of the extracted soluble and insoluble OM and of the bulk rock is consistent with the low thermal maturity of the sediment and contrasts with the predominance of larger polycyclic aromatic structures commonly observed in meteorites with these techniques. We detected inorganic ions, in particular VO+, in demineralized OM that likely originate from geoporphyrins, which derive from chlorophylls during sediment diagenesis. Finally, insoluble OM yielded distinct compositions compared with extracted soluble OM, with a greater abundance of ions of mass-to-charge ratio (m/z) over 175 and additional N-moieties. This highlights the potential of laser-assisted MS to decipher the composition of macromolecular OM, in particular to investigate the preservation of biomacromolecules in microfossils. Studies comparing diverse biogenic and abiogenic OM are needed to further assess the use of this technique to search for biosignatures.

Distinguishing cellular from abiotic spheroidal microstructures in the ca. 3.4 Ga Strelley Pool Formation.

The morphogenesis of most carbonaceous microstructures that resemble microfossils in Archean (4-2.5 Ga old) rocks remains debated. The associated carbonaceous matter may even-in some cases-derive from abiotic organic molecules. Mineral growths associated with organic matter migration may mimic microbial cells, some anatomical features, and known microfossils-in particular those with simple spheroid shapes. Here, spheroid microstructures from a chert of the ca. 3.4 Ga Strelley Pool Formation (SPF) of the Pilbara Craton (Western Australia) were imaged and analyzed with a combination of high-resolution in situ techniques. This provides new insights into carbonaceous matter distributions and their relationships with the crystallographic textures of associated quartz. Thus, we describe five new types of spheroids and discuss their morphogenesis. In at least three types of microstructures, wall coalescence argues for migration of carbonaceous matter onto abiotic siliceous spherulites or diffusion in poorly crystalline silica. The nanoparticulate walls of these coalescent structures often cut across multiple quartz crystals, consistent with migration in/on silica prior to quartz recrystallization. Sub-continuous walls lying at quartz boundaries occur in some coalescent vesicles. This weakens the "continuous carbonaceous wall" criterion proposed to support cellular inferences. In contrast, some clustered spheroids display wrinkled sub-continuous double walls, and a large sphere shows a thick sub-continuous wall with pustules and depressions. These features appear consistent with post-mortem cell alteration, although abiotic morphogenesis remains difficult to rule out. We compared these siliceous and carbonaceous microstructures to coalescent pyritic spheroids from the same sample, which likely formed as "colloidal" structures in hydrothermal context. The pyrites display a smaller size and only limited carbonaceous coatings, arguing that they could not have acted as precursors to siliceous spheroids. This study revealed new textural features arguing for abiotic morphogenesis of some Archean spheroids. The absence of these features in distinct types of spheroids leaves open the microfossil hypothesis in the same rock. Distinction of such characteristics could help addressing further the origin of other candidate microfossils. This study calls for similar investigations of metamorphosed microfossiliferous rocks and of the products of in vitro growth of cell-mimicking structures in presence of organics and silica.

New preparation techniques for molecular and in-situ analysis of ancient organic micro- and nanostructures.

Organic microfossils preserved in three dimensions in transparent mineral matrices such as cherts/quartzites, phosphates, or carbonates are best studied in petrographic thin sections. Moreover, microscale mass spectrometry techniques commonly require flat, polished surfaces to minimize analytical bias. However, contamination by epoxy resin in traditional petrographic sections is problematic for the geochemical study of the kerogen in these microfossils and more generally for the in situ analysis of fossil organic matter. Here, we show that epoxy contamination has a molecular signature that is difficult to distinguish from kerogen with time-of-flight secondary ion mass spectrometry (ToF-SIMS). This contamination appears pervasive in organic microstructures embedded in micro- to nano-crystalline carbonate. To solve this problem, a new semi-thin section preparation protocol without resin medium was developed for micro- to nanoscale in situ investigation of insoluble organic matter. We show that these sections are suited for microscopic observation of Proterozoic microfossils in cherts. ToF-SIMS reveals that these sections are free of pollution after final removal of a <10 nm layer of contamination using low-dose ion sputtering. ToF-SIMS maps of fragments from aliphatic and aromatic molecules and organic sulfur are correlated with the spatial distribution of organic microlaminae in a Jurassic stromatolite. Hydrocarbon-derived ions also appeared correlated with kerogenous microstructures in Archean cherts. These developments in analytical procedures should help future investigations of organic matter and in particular, microfossils, by allowing the spatial correlation of microscopy, spectroscopy, precise isotopic microanalyses, and novel molecular microanalyses such as ToF-SIMS.

Iron minerals within specific microfossil morphospecies of the 1.88 Ga Gunflint Formation.

Problematic microfossils dominate the palaeontological record between the Great Oxidation Event 2.4 billion years ago (Ga) and the last Palaeoproterozoic iron formations, deposited 500-600 million years later. These fossils are often associated with iron-rich sedimentary rocks, but their affinities, metabolism, and, hence, their contributions to Earth surface oxidation and Fe deposition remain unknown. Here we show that specific microfossil populations of the 1.88 Ga Gunflint Iron Formation contain Fe-silicate and Fe-carbonate nanocrystal concentrations in cell interiors. Fe minerals are absent in/on all organically preserved cell walls. These features are consistent with in vivo intracellular Fe biomineralization, with subsequent in situ recrystallization, but contrast with known patterns of post-mortem Fe mineralization. The Gunflint populations that display relatively large cells (thick-walled spheres, filament-forming rods) and intra-microfossil Fe minerals are consistent with oxygenic photosynthesizers but not with other Fe-mineralizing microorganisms studied so far. Fe biomineralization may have protected oxygenic photosynthesizers against Fe2+ toxicity during the Palaeoproterozoic.

Metal-induced malformations in early Palaeozoic plankton are harbingers of mass extinction.

Glacial episodes have been linked to Ordovician-Silurian extinction events, but cooling itself may not be solely responsible for these extinctions. Teratological (malformed) assemblages of fossil plankton that correlate precisely with the extinction events can help identify alternate drivers of extinction. Here we show that metal poisoning may have caused these aberrant morphologies during a late Silurian (Pridoli) event. Malformations coincide with a dramatic increase of metals (Fe, Mo, Pb, Mn and As) in the fossils and their host rocks. Metallic toxins are known to cause a teratological response in modern organisms, which is now routinely used as a proxy to assess oceanic metal contamination. Similarly, our study identifies metal-induced teratology as a deep-time, palaeobiological monitor of palaeo-ocean chemistry. The redox-sensitive character of enriched metals supports emerging 'oceanic anoxic event' models. Our data suggest that spreading anoxia and redox cycling of harmful metals was a contributing kill mechanism during these devastating Ordovician-Silurian palaeobiological events.

Biogenicity of morphologically diverse carbonaceous microstructures from the ca. 3400 Ma Strelley pool formation, in the Pilbara Craton, Western Australia.

Morphologically diverse structures that may constitute organic microfossils are reported from three remote and widely separated localities assigned to the ca. 3400 Ma Strelley Pool Formation in the Pilbara Craton, Western Australia. These localities include the Panorama, Warralong, and Goldsworthy greenstone belts. From the Panorama greenstone belt, large (> 40 µm) lenticular to spindle-like structures, spheroidal structures, and mat-forming thread-like structures are found. Similar assemblages of carbonaceous structures have been identified from the Warralong and Goldsworthy greenstone belts, though these assemblages lack the thread-like structures but contain film-like structures. All structures are syngenetic with their host sedimentary black chert, which is associated with stromatolites and evaporites. The host chert is considered to have been deposited in a shallow water environment. Rigorous assessment of biogenicity (considering composition, size range, abundance, taphonomic features, and spatial distributions) suggests that cluster-forming small (<15 µm) spheroids, lenticular to spindle-like structures, and film-like structures with small spheroids are probable microfossils. Thread-like structures are more likely fossilized fibrils of biofilm, rather than microfossils. The biogenicity of solitary large (>15 µm) spheroids and simple film-like structures is less certain. Although further investigations are required to confirm the biogenicity of carbonaceous structures from the Strelley Pool Formation, this study presents evidence for the existence of morphologically complex and large microfossils at 3400 Ma in the Pilbara Craton, which can be correlated to the contemporaneous, possible microfossils reported from South Africa. Although there is still much to be learned, they should provide us with new insights into the early evolution of life and shallow water ecosystems.

Early Archaean microorganisms preferred elemental sulfur, not sulfate.

Microscopic sulfides with low 34S/32S ratios in marine sulfate deposits from the 3490-million-year old Dresser Formation, Australia, have been interpreted as evidence for the presence of early sulfate-reducing organisms on Earth. We show that these microscopic sulfides have a mass-independently fractionated sulfur isotopic anomaly (Delta33S) that differs from that of their host sulfate (barite). These microscopic sulfides could not have been produced by sulfate-reducing microbes, nor by abiologic processes that involve reduction of sulfate. Instead, we interpret the combined negative delta34S and positive Delta33S signature of these microscopic sulfides as evidence for the early existence of organisms that disproportionate elemental sulfur.