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Micro-pyrites associated with organic material in ancient stromatolites: a new proxy attesting for their biogenicity

Periodic Reporting for period 3 - STROMATA (Micro-pyrites associated with organic material in ancient stromatolites: a new proxy attesting for their biogenicity)

Reporting period: 2020-02-01 to 2021-07-31

The question of the origin of Life is one of the noblest questions that we can ask in science. Understanding the conditions of the origin and the evolution of Life has always been an important prerequisite to answering to where we are from. Identifying Archean fossils from the Earth’s early biosphere is ambitious and has excited researchers for decades. Recovering traces of life in the geological record is challenging due to the scarcity of the oldest rocks (10 % of the Earth’s surface) and the complex post deposit history (diagenesis, metasomatism, metamorphism) of these rocks. Moreover, the first forms of Life were small (microorganisms) with micrometer size and did not form macroscopic fossils, but rather ambiguous, like stromatolite and microbially induced sedimentary structures (MISS). The biogenicity and syngenicity identification of these structures mostly rely on morphological and stable geochemical signature identification by comparison with modern analogs [e.g. 1]. However the biogenicity of stromatolites and MISS dating to the Archean is still controversial [2] and these structures rarely contain definitive microbial body fossils [3]. Furthermore, linking Archean microbial structures to specific metabolisms like oxygenic photosynthesis, microbial sulfate reduction or taxonomic groups remain difficult [2]. Therefore, the question remains open and requires renewed approach to tackle it from a new angle. Stromatolite and MISS can contain small sulfides (with size around 1 micrometer) closely associated to the organic matter laminae in the case of the fossil ones or with the microbial mat in case of the modern ones [4, 5]. Sulfides and especially pyrite (FeS2) can have recorded the influence of microbial metabolism like sulfate-reduction or iron respiration and therefore can be used as a proxy for microbial activities [6-9] even if the precise role for biotic and abiotic processes in the pyrite formation pathway still need to be precised [10]. The present project will question the origin of these sub-micrometer sulfides in order to define new and strong criteria for the biogenicity identification of the stromatolites and MISS. STROMATA proposes new in situ analytical developments allowing the isotopic analysis of small geological objects, micro-pyrite grains in stromatolites or (MISS), that we propose to be more robust markers of early metabolism.

The aim of the STROMATA project is to propose robust criteria for metabolic and biological activities and thus for record of the first traces of Life. This can be subdivided in several sub-questions:
- Were Archean stromatolites and MISS really produced by important and various biological activities?
- Are Fe and S isotope composition of micro-pyrites associated with organic material record metabolic signatures?
- Were isotopic and mineralogical biosignatures of micro-pyrite modified by environmental, post-depositional or fossilization processes?
- Were microbial sulfate reduction and iron respiration biosignatures influenced by the oxygenation of the atmosphere?

To address these questions, a comparative study of Precambrian and modern stromatolites and experimentally produced abiotic pyrites will be performed by detailed mineralogical characterization on the small scale, in situ investigation of various stable isotope proxies (Fe, S) and chemical identification of the organic material. STROMATA will focus on samples, for which the geological, depositional environment and post-deposit history are well constrained (Fig 4). STROMATA is a unique integrated study dedicated to the identification of biological signatures preserved in fossil and modern stromatolites at a micro and nanometer scale. STROMATA will achieve unprecedented advance in the search of biogenicity criteria by this original approach combining isotopic analyses, chemistry characterization and mineralogical description at a very small scale, using state of the art techniques (NanoSIMS, SIMS, Raman, FIB-TEM and XANES). STROMATA will also provide new constraints on the potential evolution of important metabolic pathways (microbial sulfate reduction and iron respiration) as well as on their link with anoxygenic or oxygenic photosynthesis.
During this period, we have developed two analytical protocol to measure precisely Fe and S isotope compositions in micro pyrites. S isotope compositions is now measured by isotope imaging by using the NanoSIMS, a technical paper is currently under preparation. Fe isotope analyses with a spatial resolution of 3 micrometer has been developed in the frame of the PhD project of Marie-Noelle Decraene. She was able to get the same precision than previous analyses that use a 20 microns spot size. An article is currently under revision at Rapid Communication in Mass Spectrometry.
Experiments of pyrites produced in controlled conditions by microbial sulfate reducers and by abiotic reduction have been also conducted at IMPMC and preliminary analyses have been conducted. Two modern microbialites from Cayo Coco (Cuba) and a volcanic lake from Atexcac (Mexico) have been extensively studied for mineralogy and Fe and S isotope compositions. A manuscript is currently under preparation.
For Archean samples, various samples are currently studied by the team for mineralogy, organic carbon chemistry and structure and Fe and S isotope compositions. Studies on the Mendon chert (3.2 Ga, South Africa) and on the Moodies Group(3.2 Ga, South Africa) have been finalised while studies on the Buck Reef (3.4 Ga, South Africa), Middle Marker (3.47 Ga, South Africa) cherts and Strelley Pool (3.5 Ga, Australia) and Malmani (2.5 Ga, South Africa) are still on going.
The results have been presented during various seminar in Swiss Universities (University of Bern, Geneva and ETH Zurich), during the Swiss Geoscience Meeting (2019), the microbialite workshop (2019) and the Goldschmidt Conference (2019, Barcelona).
Besides on the achievement on the development of new analytical protocol, we were able to show that even sedimentary pyrites that have undergone metamorphic process can still record pristine biosignatures of metabolic activities. We have thus identified the presence of Dissimilatory Iron Reduction (DIR) in sedimentary pyrite from the 3.2 Ga Mendon formation in South Africa, published in Geobiology in 2020. These results extend the geological record of DIR back more than 560 million years (Myr) and confirm that microorganisms closely related to the Last Common Ancestor had the ability to reduce Fe(III).
We are now able to measured precisely Fe and S isotope composition in micro pyrites with an high spatial resolution, which open the road for various applications, especially in enviromment and geobiology.
The study on micro pyrites formed in modern microbial mats will bring very important constrain on pyrite formation pathways in microbial consortium both on Fe and S biogeochemical cycles but also on microbial induced mineralization process.
Experiments have already also highlight the complex pyrite formation pathways and will thus bring important data on the range of Fe and S isotope distribution expected during pyrite precipitation.