ROCK SURFACE DIAGENETIC BIOSIGNATURES INDUCED BY FUNGAL-CARBONATE SUBSTRATES INTERACTION AS LIFE INDICATORS

Rock surfaces exposed to fungal attack can produce significant diagenetic biosignatures that can be evaluated by various analytical instruments. In a 30-day experimental study, carbonate substrates (dolomites and limestones) were exposed in vitro to fungal attack. The analyses of fungal material, thin sections and chips under optical microscopy, scanning electron microscope (SEM), x-ray diffraction (XRD), energy dispersive x-ray (EDX), Raman spectroscopy and stable isotopes (C and O) revealed an extensive microbial diagenesis of the substrates. The results pointed to strong characteristic diagenetic biosignatures: Intense substrate de-micritization and micritization by oxalates, grain bridging and cementation, crystal replacement, void filling, formation of inter- and intragranular porosity, permeability enhancement and significant mineral neoformations: α- and β-glushinskite, neo-dolomite, weddellite, whewellite, a variation of calcium oxalate hydrate, calcite and possibly struvite. The selective precipitation of biominerals on grain boundaries formed typical alveolar - honeycomb structures. Advanced stages of diagenesis revealed complete digestion and replacement of original lithologies by the newly formed biomineralized substrates. These new substrates, produced a micro-scale “biological stratification” when they were deposited directly on the original attacked surfaces. Stable isotopes analyses indicated an isotopic fractionation between the mineral neoformations within the fungal mass and the attacked and unattacked surfaces. In sedimentary profiles, these isotopic differences could perhaps define a biological isotopic signature. The results of this study suggest that fungal interaction with carbonate substrates can produce a pattern of measurable biosignatures that are specific to microbial activity. Similar biosignatures on earth or on other planets could indicate the presence of primitive life forms.