A novel organic-rich meteoritic clast from the outer solar system

The Zag meteorite which is a thermally-metamorphosed H ordinary chondrite contains a primitive xenolitic clast that was accreted to the parent asteroid after metamorphism. The cm-sized clast contains abundant large organic grains or aggregates up to 20μm in phyllosilicate-rich matrix. Here we report organic and isotope analyses of a large (~10μm) OM aggregate in the Zag clast. The X-ray micro-spectroscopic technique revealed that the OM aggregate has sp2 dominated hydrocarbon networks with a lower abundance of heteroatoms than in IOM from primitive (CI,CM,CR) carbonaceous chondrites, and thus it is distinguished from most of the OM in carbonaceous meteorites. The OM aggregate has high D/H and 15N/14N ratios (δD=2,370±74‰ and δ15N=696±100‰), suggesting that it originated in a very cold environment such as the interstellar medium or outer region of the solar nebula, while the OM is embedded in carbonate-bearing matrix resulting from aqueous activities. Thus, the high D/H ratio must have been preserved during the extensive late-stage aqueous processing. It indicates that both the OM precursors and the water had high D/H ratios. Combined with 16O-poor nature of the clast, the OM aggregate and the clast are unique among known chondrite groups. We further propose that the clast possibly originated from D/P type asteroids or trans-Neptunian Objects

Organic biomorphs may be better preserved than microorganisms in early Earth sediments

The Precambrian rock record contains numerous examples of microscopic organic filaments and spheres, commonly interpreted as fossil microorganisms. Microfossils are among the oldest traces of life on Earth, making their correct identification crucial to our understanding of early evolution. Yet, spherical and filamentous microscopic objects composed of organic carbon and sulfur can form in the abiogenic reaction of sulfide with organic compounds. Termed organic biomorphs, these objects form under geochemical conditions relevant to the sulfidic environments of early Earth. Furthermore, they adopt a diversity of morphologies that closely mimic a number of microfossil examples from the Precambrian record. Here, we tested the potential for organic biomorphs to be preserved in cherts; i.e., siliceous rocks hosting abundant microbial fossils. We performed experimental silicification of the biomorphs along with the sulfur bacterium Thiothrix. We show that the original morphologies of the biomorphs are well preserved through encrustation by nano-colloidal silica, while the shapes of Thiothrix cells degrade. Sulfur diffuses from the interior of both biomorphs and Thiothrix during silicification, leaving behind empty organic envelopes. Although the organic composition of the biomorphs differs from that of Thiothrix cells, both types of objects present similar nitrogen/carbon ratios after silicification. During silicification, sulfur accumulates along the organic envelopes of the biomorphs, which may promote sulfurization and preservation through diagenesis. Organic biomorphs possessing morphological and chemical characteristics of microfossils may thus be an important component in Precambrian cherts, challenging our understanding of the early life record

Molecular preservation of organic microfossils in Paleoarchean cherts

International audienceFossilization processesandthe increase of temperature and pressure conditions associated withburialinevitably alter the original biochemical signatures of organic molecules.At a certain stage, biogenic and abiotic organic structures may become undistinguishable [1].Cherts (i.e.silica-rich rocks)are well known forthe morphologicalpreservationof fossilized microorganisms.Recently,spatially resolvedinvestigations usingsynchrotron-based XANESmicrospectroscopyrevealedthatmolecular information about the organic precursor of3.4 Gamicrofossils, was preservedintheStrelley Pool chert(Pilbara, Western Australia),despite a metamorphichistory so far believed to be incompatible with such preservation(lower greenschist facies-peak temperature ≅300 °C;[2]).Laboratory experiments showed that silica-organic interactions are likely to play a key rolein the molecular preservation of microorganismsfossilized in cherts[3].Altogether,these resultsdemonstratethat ancient organic microfossils may exhibit a high level of chemical preservation in appropriate settings independent of a long and complex geological history.Here, weusespatially resolvedmicrospectroscopy techniques, includingSTXM-based XANES spectroscopy,to investigate the chemical nature andmolecularpreservation ofindividualmicrofossils from the3.4 GaBuck Reef chert(Barberton, SouthAfrica). The latter experiencedslightly higherpeaktemperatureconditions(≅360 °C)during their geological historycompared to the Strelley Pool chert.These molecular data providekeyconstraintsto understandingthe impactof increasing metamorphic temperatureon thepreservation of theorganic moleculescomposing some of the oldest microbial fossils on the Earth

Chemical nature of the 3.4 Ga Strelley Pool microfossils

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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

Towards a representation of complex ecosystems in the ORCHIDEE Land Surface Model

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Towards a representation of complex ecosystems in the ORCHIDEE Land Surface Model

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