The genesis of actively growing siliceous stromatolites: Evidence from Lake Specchio di Venere, Pantelleria Island, Italy

This study documents the attributes of siliceous stromatolites growing in the Lake Specchio di Venere, on thevolcanic island of Pantelleria, Italy, in a setting characterized by very shallow cold waters and pools and byscattered hydrothermal activity, which exhales mainly CO2, at emission point temperatures of 34 to 58 °C.The saturation indexes indicate that the lake waters are saturated with respect to tridymite, cristobalite,chalcedony and quartz, and slightly undersaturated with respect to amorphous silica. Common roughlylaminated and poorly lithified stromatolites show scanning electron microscope (SEM) evidence for silicifiedmicrobial mat structures, including biofilms, filamentous and coccoid cells, and extracellular polymericsubstances (EPS). The screening of bacterial 16S rRNA libraries indicates that autotrophic and heterotrophicbacterial communities colonize surface and core levels of the stromatolites. Locally the stromatolites show agranular non-porous fabric, where filaments and silica sheets are not apparent. Inhomogeneity in thestromatolite fabric corresponds with varying DNA content and different structures of the colonizing bacterialcommunities, non-porous stromatolite levels being microbially colonized to a lesser extent. Based on FourierTransform (FT)–Raman and FT-infrared investigations, the laminated stromatolite contains early diagenetictridymite in addition to amorphous silica (opal-A), whereas the non-porous stromatolite shows anessentially amorphous character. The laminated stromatolite is thought to form at relatively low rates ofsilica precipitation and with a possible microbial mediation in terms of microbial cells and their EPSaccelerating the onset of amorphous silica nucleation. It is suggested that high porosity probably favoured aconsistent flux of silica-rich fluids that triggered the opal-A to tridymite transformation, thus strengtheningthe preservation of biosignals. Non-porous stromatolite growth could reflect temporal or localised changes inenvironmental conditions that caused variations in the degree of silica saturation in the lake waters inducingabiotic silica accumulation. Accelerated opal-A deposition could have obscured primary filamentous fabricsand limited the flux of pore fluid needed for sustaining the process of silica maturation.