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

Periodic Reporting for period 2 - STROMATA (Micro-pyrites associated with organic material in ancient stromatolites: a new proxy attesting for their biogenicity)

Reporting period: 2018-08-01 to 2020-01-31

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
During the period February to end of July 2018, two master students have worked on the project. One (Maryline Imbault) did a detailed stratigraphic and geochemical study of the Moodies sediment (one of the sample target of the task 4). She has performed major and trace element geochemistry on the sandstone, as well as SEM (secondary electron microscope) imaging and Raman characterization of the mineralogy. Marilyne Imbault has also conducted a characterization of the organic matter and has reconstructed the temperature of the thermal maturity (between 350 and 400 °C). This study has allowed us to determine the samples suitable for the SIMS and NanoSIMS study.
The other master student Marie Noelle Decraene has developed the Fe isotope analyses by SIMS using the new capability of the Hyperion Source recently installed on the Cameca 1280 in CRPG-CNRS, Nancy. She has performed several tests and defined a new analytical protocol. She also has analyzed Archean samples from the Tumbiana Formation (2.7 Ga, Australia) and modern microbialite from Cuba (Cayo Coco Lagoon). Her work was therefore part of the task 3 and 4 of the project. Marie Noelle will continue her work as a PhD student at UNIL. She will start in October 2018.

Jennyfer Miot (IMPMC) has conducted experiments according to the task 2. These are long time running experiments and we will analyze them in 2020.
During this period, I have conducted more SEM imaging of nanopyrites from both modern and Archean samples (Cuba, Alchichica and Buck Reef). I also have written two papers one about the Fe respiration in the Archean (to be submitted to PNAS) and one on the sulfate reduction in modern microbial mats (to be submitted to Geochemical Perspective Letters).
So far, we have validated the analytical protocol for S isotopes measurements on the NanoSIMS. The next months will be then used for measuring Archean and modern samples.