17 mars 2023 - 14h45-15h30

Juliette Debrie – Chercheuse postdoctorale à l’IMPMC
Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC) UMR CNRS 7590, Sorbonne Université, Muséum National d’Histoire Naturelle, 4 Place Jussieu 75005 Paris, France

Titre : Mineralogical and fluorescence studies in modern stromatolites : a search for environmental proxies and traces of life

Résumé : Stromatolites are particularly emblematic geobiological materials since they are the oldest evidence of life-mineral interactions, dated at several billion years. They have received strong attention because of the information they can provide on microbial paleobiodiversity and paleoenvironments. Modern marine and lacustrine stromatolites are well-studied and often used as references for the interpretation of ancient stromatolites. However, little is known about modern stromatolites from coastal pond environments, where variations of physico- chemical conditions such as salinity are very significant. This talk will present research carried out within the PhD project STROMAS framework. This project aimed at deciphering how the environment of formation (including abiotic and biotic factors) is recorded in modern stromatolites collected in Mari Ermi, a coastal pond in Western Sardinia. This pond experiences strong chemical variations over time, due to evaporitic/ water supply episodes.

Here we combined a variety of analytical tools allowing, from the bulk to the nanometer-scale, the characterization of the mineralogical composition of these stromatolites and the assessment of the spatial distribution of the mineral phases. Specifically, we used quantified x-ray elemental maps provided by energy dispersive x-ray spectrometry analyses coupled with scanning electron microscopy (SEM-EDXS). The use of SEM-EDXS together with an innovative data treatment allowed (i) the mineral phase recognition and (ii) the mapping of their spatial distribution down to a hundred-nanometer spatial resolution. The results showed that, the seasonal chemical variations of the Mari Ermi ponds can be recorded in the mineralogical composition of the stromatolites at the submicrometer scale.

We also used confocal laser scanning microscopy (CLSM), cathodoluminescence and synchrotron-based X-ray fluorescence (SR-XRF). The hyperspectral data evidenced pervasive fluorescence throughout the stromatolites. Within this fluorescence, different kinds of spectra were detected and attributed to different types of organic and inorganic compounds. It revealed the potential preservation of degraded photosynthetic pigments related to past microbial activity and opened new discussions on the preservation of traces of life.

Le séminaire aura lieu sous format hybride. Si vous souhaitez y assister, vous pouvez demander le lien de connexion à l’organisateur : Laurent Tranchant (laurent.tranchant@synchrotron-soleil.fr).