M6fossils

We aim at constraining the co-evolution of life and the environments on early Earth, targeting five milestones through life evolution (between 3.4 Ga – 400 Ma, Billion-Million years) linked with important changes in redox conditions and oxygenation. Identifying the fossils of these times has been limited by (1) morphological simplicity, (2) non-diagnostic organic carbon isotope ratio, (3) difficulty to correlate individual fossils with molecular biomarkers analyzed on bulk rocks, (4) difficulty to correlate fossils with geochemical metabolic/environmental proxies from bulk rocks. To overcome these limitations, we will use a combination of micro- to nanoscale characterizations of fossils. We will develop novel microscale molecular methods: Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and microscale Laser-desorption Laser-ionization Mass Spectrometry (µL2MS). Thanks to these innovative techniques we will be able, for the first time, to retrieve molecular information (biomarker and fossil biopolymer composition) on single fossil cells, and to distinguish adjacent cells as well as cell anatomy. These spatially-resolved analyses will identify possible in-lab and weathering contaminations. Complementary nanoscale analytical (spectro)microscopy will be used to analyze anatomy as well as mineral structures informing on post-mortem morphological modifications and biominerals. Metabolic signatures will be investigated using microscale and bulk-rock isotope analyses of organic matter and biominerals. The project builds on the gathered participants’ expertise in the fields of organic and isotope geochemistry, paleontology, nano-mineralogy, mass spectrometry and spectroscopy, and analytical developments. (A) Our selection of samples will allow us to address the effects of diagenetic and metamorphic transformations, as i) all fossils are preserved in quartz, ii) their age gradient is correlated with an increase in organic matter maturity and mineral matrix recrystallization, iii) 412 Ma to 1.6 Ga samples contain comparable microfossils (e.g. cyanobacteria, algae). (B) The 412-410 Ma samples will allow us to build a database of microscale molecular fingerprints on a large diversity of micro-organisms (cyanobacteria, algae, fungi) and specialized cells of macrofossils (plants, animals) correlated with nanoscale anatomical imaging. This will inform on cell structures and compositions in some of the earliest land plants (412-400 Ma) thus constraining the evolution of biopolymers including lignin, which triggered a rise in pO2 (O2 partial pressure). (C) We will compare morphologically identified microfossils and ambiguous morphospecies in 800-700 Ma old rocks with coupled molecular, textural and isotopic criteria. We hope to identify the relative importance of cyanobacteria and micro-algae in the primary photosynthetic production in order to constrain the role of the evolution of algae in the rise of pO2 of the Neoproterozoic Oxygenation Event, which resulted in the evolution of multicellular life. (D) ~1.6 Ga microfossil assemblages, coincident with the earliest eukaryotic fossils, will be characterized to constrain primary production during a period of reduced pO2 using microfossils of increased thermal maturity. (E) Microfossil assemblages of the Great Oxidation Event and its aftermaths between 2.45 and 1.8 Ga, will be studied to constrain metabolisms in environments characterized by important redox and pO2 fluctuations, with a focus on Fe-biomineralization associated with ferruginous conditions characterizing the ocean of this time period. (F) Organic microstructures (2.7 Ga) and enigmatic assemblages of small and large microfossils (3.0-3.4 Ga) will be studied to document primary production, methane and sulfur metabolisms associated with early anoxic and ferruginous environments together with possible early production of O2.