Periodic Reporting for period 4 - STROMATA (Micro-pyrites associated with organic material in ancient stromatolites: a new proxy attesting for their biogenicity)
Reporting period: 2021-08-01 to 2023-01-31
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 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.
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, few analyses have been made but several articles have been written and published. All the post doc and phD students have published their results as first author papers.
- The new analytical protocol for measuring Fe isotope in micropyrites has been calibrated on two different SIMS instruments and published in Rapid Communications in Mass Spectrometry.
- The micropyrites from the Tumbiana stromatolite (2.7 Ga) have been measured for Fe isotope compositions and have recorded a complex redox cycle at local scale, i.e sediment scale. This result has been published at Geochimica and Cosmochemica Acta and confirm that micro pyrite record local process of formation rather than global conditions. This study also confirm the presence of a fully living microbial ecosystem at the the time of the stromatolite formation.
- We have investigated the nature of the organic matter present in the Buck Reef chert (3.5 Ga, South Africa). This chert is interpreted as a fossil microbial mat with putative cyanobacteria. Our study highlights the extreme heterogeneity in the structure of the organic matter preserved in this chert, compatible with a biological origin but also with some abiotical processes. This study has been published in Communications in Earth and Environment and a press release has been done on the blog of the publisher.
- We also have investigated the Moodies sediment (3.2 Ga, South Africa), which are interpreted as record the first terrestrial environment and consist of fossil microbial mats. Unfortunately our study cannot confirm the biological origin of the lamina but rather show the complex post deposit history of these rocks with several events of fluid circulations. This study has been published in Precambrian Research.
-We also have finally published our study on the Mendon pyrite (3.2 Ga South Africa). We have shown that pyrite can preserve pristine isototopic signal even after experienced metasomatic event and that Fe isotope composition can be quite resistant to late fluid circulation and recrystallisatio. We also have interpreted the iron isotope signatures of the Mendon pyrites as reflecting the presence of dissimilatory iron reduction. Our data extend the geological record of dissimilatory iron reduction (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). This study has been published in Geobiology.
In the main time, we have continue to investigate modern microbial mats, stromatolite from the Malmani stromatolite and the middle Marker cherts.
- The new analytical protocol for measuring Fe isotope in micropyrites has been calibrated on two different SIMS instruments and published in Rapid Communications in Mass Spectrometry.
- The micropyrites from the Tumbiana stromatolite (2.7 Ga) have been measured for Fe isotope compositions and have recorded a complex redox cycle at local scale, i.e sediment scale. This result has been published at Geochimica and Cosmochemica Acta and confirm that micro pyrite record local process of formation rather than global conditions. This study also confirm the presence of a fully living microbial ecosystem at the the time of the stromatolite formation.
- We have investigated the nature of the organic matter present in the Buck Reef chert (3.5 Ga, South Africa). This chert is interpreted as a fossil microbial mat with putative cyanobacteria. Our study highlights the extreme heterogeneity in the structure of the organic matter preserved in this chert, compatible with a biological origin but also with some abiotical processes. This study has been published in Communications in Earth and Environment and a press release has been done on the blog of the publisher.
- We also have investigated the Moodies sediment (3.2 Ga, South Africa), which are interpreted as record the first terrestrial environment and consist of fossil microbial mats. Unfortunately our study cannot confirm the biological origin of the lamina but rather show the complex post deposit history of these rocks with several events of fluid circulations. This study has been published in Precambrian Research.
-We also have finally published our study on the Mendon pyrite (3.2 Ga South Africa). We have shown that pyrite can preserve pristine isototopic signal even after experienced metasomatic event and that Fe isotope composition can be quite resistant to late fluid circulation and recrystallisatio. We also have interpreted the iron isotope signatures of the Mendon pyrites as reflecting the presence of dissimilatory iron reduction. Our data extend the geological record of dissimilatory iron reduction (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). This study has been published in Geobiology.
In the main time, we have continue to investigate modern microbial mats, stromatolite from the Malmani stromatolite and the middle Marker cherts.
We have now shown that micro pyrite in microbialite and stromatolite record local process rather than global conditions and that ancient microbial metabolic activities can be preserved even if the sediments have experienced complex post deposit history. We are now exploring how the different pathways of pyrite formation can be preserved in the geological record. We will have new resulst on modern microbialite that will be important to take account for constraining ancient biogeochemical cycles.