Corentin Le Guillou

Molecular preservation of 1.88 Ga Gunflint organic microfossils as a function of temperature and mineralogy

The significant degradation that fossilized biomolecules may experience during burial makes it challenging to assess the biogenicity of organic microstructures in ancient rocks. Here we investigate the molecular signatures of 1.88 Ga Gunflint organic microfossils as a function of their diagenetic history. Synchrotron-based XANES data collected in situ on individual microfossils, at the submicrometre scale, are compared with data collected on modern microorganisms. Despite diagenetic temperatures of ∼150–170 °C deduced from Raman data, the molecular signatures of some Gunflint organic microfossils have been exceptionally well preserved. Remarkably, amide groups derived from protein compounds can still be detected. We also demonstrate that an additional increase of diagenetic temperature of only 50 °C and the nanoscale association with carbonate minerals have significantly altered the molecular signatures of Gunflint organic microfossils from other localities. Altogether, the present study provides key insights for eventually decoding the earliest fossil record.

Structure, composition, and location of organic matter in the enstatite chondrite Sahara 97096 (EH3)

The insoluble organic matter (IOM) of an unequilibrated enstatite chondrite Sahara(SAH) 97096 has been investigated using a battery of analytical techniques. As the enstatitechondrites are thought to have formed in a reduced environment at higher temperatures thancarbonaceous chondrites, they constitute an interesting comparative material to test theheterogeneities of the IOM in the solar system and to constrain the processes that could affectIOM during solar system evolution. The SAH 97096 IOM is found in situ: as submicrometergrains in the network of fine-grained matrix occurring mostly around chondrules and asinclusions in metallic nodules, where the carbonaceous matter appears to be moregraphitized. IOM in these two settings has very similar d15N and d13C; this supports the ideathat graphitized inclusions in metal could be formed by metal catalytic graphitization ofmatrix IOM. A detailed comparison between the IOM extracted from a fresh part and aterrestrially weathered part of SAH 97096 shows the similarity between both IOM samples inspite of the high degree of mineral alteration in the latter. The isolated IOM exhibits aheterogeneous polyaromatic macromolecular structure, sometimes highly graphitized, withoutany detectable free radicals and deuterium-heterogeneity and having mean H- and N-isotopiccompositions in the range of values observed for carbonaceous chondrites. It contains somesubmicrometer-sized areas highly enriched in 15N (d15N up to 1600&). These observationsreinforce the idea that the IOM found in carbonaceous chondrites is a common componentwidespread in the solar system. Most of the features of SAH 97096 IOM could be explainedby the thermal modification of this main component.

Mineralogical evolution of Fe–Si-rich layers at the olivine-water interface during carbonation reactions

Abstract Recent studies investigating carbonation of iron-bearing silicates have shown that the rates of these reactions, although formally not depending on oxygen fugacity, are strongly different at different redox states of the system (Saldi et al. 2013; Sissmann et al. 2013). Here we provide a micro- and nanostructural characterization of the olivine/water interface during the carbonation of forsteritic olivine at 150 °C and pCO₂ = 100 bar. When the reaction starts under oxic conditions, the observed temporal sequence of interfacial layers consists of: a hematite/SiO2(am) assemblage, Fe-rich phyllosilicates with mixed Fe valence and a non-passivating Fe-free amorphous SiO2 layer, which allows the formation of ferroan magnesite. In contrast, starting at micro-oxic conditions, carbonation rates are much faster, with no real evidence of interfacial layers. Separate deposits of goethite/lepidocrocite in the early stages of the reaction and then formation of magnetite are observed at these conditions, while precipitation of siderite/magnesite proceeds unhindered. The evolution of the redox conditions during the reaction progress controls the sequence of the observed reaction products and the passivating properties of Fe-Si-rich interfacial layers. These findings have important implications for modeling the carbonation of ultramafic rocks under different oxygen fugacity conditions as well as for understanding the technological implications of adding accessory gases to CO2 in carbon capture and storage mineralization processes involving ultrabasic rocks.

Organic molecular heterogeneities can withstand diagenesis

Reconstructing the original biogeochemistry of organic fossils requires quantifying the extent of the chemical transformations that they underwent during burial-induced maturation processes. Here, we performed laboratory experiments on chemically different organic materials in order to simulate the thermal maturation processes that occur during diagenesis. Starting organic materials were microorganisms and organic aerosols. Scanning transmission X-ray microscopy (STXM) was used to collect X-ray absorption near edge spectroscopy (XANES) data of the organic residues. Results indicate that even after having been submitted to 250 °C and 250 bars for 100 days, the molecular signatures of microorganisms and aerosols remain different in terms of nitrogen-to-carbon atomic ratio and carbon and nitrogen speciation. These observations suggest that burial-induced thermal degradation processes may not completely obliterate the chemical and molecular signatures of organic molecules. In other words, the present study suggests that organic molecular heterogeneities can withstand diagenesis and be recognized in the fossil record.

XANES-Based Quantification of Carbon Functional Group Concentrations

X-ray absorption spectroscopy in the soft X-ray range is used in many research fields to identify the nature of functional groups in organic compounds and carbon materials. However, the concentrations of these functional groups have so far remained difficult to quantify. Using X-ray absorption near edge structure (XANES) spectra of reference materials (polymers and compounds of known molecular composition), we established a correlation between measured optical densities and functional groups concentration. This methodology relies on an alternative method for normalization to the total amount of carbon and for deconvolution of the spectra. It allows precisely quantifying the N/C atomic ratio (σ1 = 0.02 atom %) as well as the concentration of [aromatic + olefinic] groups (σ1 = 3.7 atom %), [ketone + phenol + nitrile] groups (σ1 = 2.2 atom %), [aliphatic] groups (σ1 = 11.2 atom %) and [carboxylic] groups (σ1 = 7.4 atom %). We validated this quantification by comparing with nuclear magnetic resonance data obtained on pyrolized lignin samples. We also provide an easy-to-use python program automating XANES-based quantification of carbon functional group concentrations.

Investigation of Organic Matter at The Micron Scale in Carbonaceous Chondrites: a Spyglass to Study The Early Solar System.

Carbonaceous chondrites are primitive meteorites formed 4.56 Gy ago. During the first stages of the solar system, they accreted material from the interstellar space that was processed in the disk and mixed with other solar system components. All these materials were likely millimeter to micrometer grains. Their investigation at fine scale thus provides important clues to understand solar system formation. The matrix of CI, CM and CR carbonaceous chondrites contains abundant hydrated minerals, such as clays, and up to 5 wt.% of organic matter (OM). These meteorites have likely brought volatiles to the early Earth during the late veneer, hence delivering to the Earth potential building blocks of Life. A better understanding of OM contained in chondrites may help to refine scenarios of solar system formation and emergence of Life on Earth. OM and water were likely accreted together as ice grains within asteroids and other planetary embryos, and their abundance is therefore correlated. After accretion, internal heating due to decay of short-lived radionuclides melted the ice and induced fluid circulation that promoted mineralogical and chemical reactions on the asteroidal parent bodies of these chondrites. This aqueous (or hydrothermal) alteration has modified some of the chemical and isotopic signatures that the OM could have acquired during it synthesis, hence blurring the record of protosolar conditions.