Biogeochemical signals of unfossilizable organisms in terrestrial problematic carbonates: Relevance to carbonatogenesis and astrobiology

The origin of "Kess-Kess" mounds of the Hamar Laghdad Ridge, SE Morocco, is still under debate. The biogenicity of these structures seems to be related to submarine hydrocarbon seapage or, less probably, to hydrothermal vents in which bacteria and/or archaea play a prominent role in the carbonate biomineralization. To investigate the traces of prokaryotic metabolic activity, which possibly caused the laminated microbialite precipitation, the research was focused on micro-nanomorphology of the very fine dark and white wrinkled laminations and biogeochemical analyses of their organic remains. The geochemical characterization of extracted organic matter was performed through the functional group analyses by FT-IR Spectroscopy. FT-IR parameters indicate a marine origin and low thermal evolution for the organic compounds. The organic matter, inside the laminated micrite, is characterized by the presence of stretching C=C vibrations attributable to alkene and/or unsatured carboxylic acids. These organic compounds are likely to be synthesized by bacteria and/or archaea communities via methanotrophic metabolism. This approach permit to detect biosignatures of microorganisms belonging to communities which flourished in methanotrophic extreme environments, like those that could have characterized the early Earth and Mars.