Raman spectroscopy on microbial carbonates: enhancing understanding of thrombolite formation processes and identification of preserved microbiological and microtextural features in carbonates

EMILIANO RIVAROLA; FERNANDO JAVIER GOMEZ; BENJAMIN TUTOLO; ESTELA CECILIA MLEWSKI

Leysin

Conferencia; 2023 M-FED Microbialite: Formation - Evolution - Diagenesis; 2023

Microbial carbonates, known as microbialites, represent unique geological formationsholding valuable information about past environments. Understanding their formation andidentifying preserved microbiological features (microtextural and compositional) withincarbonates is crucial for reconstructing Earth’s history and advancing our knowledge ofmicrobial life evolution. Here, we show how Raman spectroscopy can contribute to ourunderstanding of modern and ancient thrombolites. Raman spectroscopy is a powerfulnon-destructive analytical technique that provides molecular information about the composition and structure of materials. Its ability to identify organic compounds and mineralogyat the microscale makes it particularly suitable for the study of microbialites. Microbescan leave distinct structural and chemical signatures in carbonate minerals (organic remnants and mineralized microbial cells) that can thus be detected and mapped by Ramanspectroscopy.We acquire Raman spectra and integrate this information with other, complementary techniques (polarized light petrography, SEM, isotopic analysis) to study non-laminated clottedmicrotextures that characterize thrombolites from Strobel Lake (Santa Cruz, Argentina).Raman microscopy provides particularly useful insights into the origin of organic components filling micropores and mesopores, which thereby helps us to unravel the intricateprocesses involved in thrombolite formation.Modern thrombolites from Strobel Lake are colonized by both biofilms and pustular tosubspherical (mm to cm size) cyanobacterial communities (Nostocales-like). The thrombolite framework is represented by clots that usually show a mottled microtexture givenby irregularly shaped darker-colored material. In thin-section, mesopores between clots(framework porosity) can be partially or fully filled by detrital and carbonate sediments,bioclasts, vegetal remains and black colored, subcircular and polylobate dark-colored balls(without a clear internal structure under polarized light microscopy). Gray mottled micriteand microsparite also cements detrital particles and fills the mesopores. Raman analysisallowed identification of monohydrocalcite composing the clots and organic material (pigments) related to cyanobacteria (in the black-colored subspherical structures). The darkirregular spots observed within the mottled clots were also identified as organic materialthat, when degraded, leave micropores that could be potentially considered as biosignatures. The close association of organic material and clot microtextures suggests somebiological influence in the thrombolite framework construction.