New Insights Into Upper Messinian Microbial Carbonates: A Dendrolite-Thrombolite Build-Up From the Salento Peninsula, Central Mediterranean.

Upper Messinian carbonates recently recorded in the Salento Peninsula (southern Italy, central Mediterranean) contain microbial facies, including textures never previously described in the Late Miocene of the Mediterranean. This study focuses on the geometry and internal fabrics of a 3 × 28 m build-up of coalescent dendrolite and thrombolite, to examine its formation and the possible microbes involved, and to reconstruct its growth dynamics and related palaeoenvironmental conditions. Salento dendrolites have centimetric dendritic growth forms with a microlaminated, originally aragonitic, microstructure. The thrombolites, in contrast, are characterized by larger mesoclots with arborescent, anastomose growth patterns and a distinctive microfabric of small, originally calcitic, spheroids with a sparry nucleus surrounded by acicular crystals. Bio-geochemical analyses (UV epifluorescence, micro-Raman spectroscopy and SEM-EDS) reveal the presence of organic matter intimately associated with both dendrolite and thrombolite textures, supporting a biotic origin. The sedimentary context and microfabrics suggest that cyanobacteria may have played a major role in the formation of these structures, together with heterotrophic microbes, mainly sulfate-reducing bacteria, in the dendrolite. Build-up geometries, stratigraphic setting, and analysis of the associated sediment suggest that the dendrolite-thrombolite framework developed in a small, shallow-water lagoon, under moderate to high energy, variable salinity, and possibly high sedimentation rate. Salento dendrolite-thrombolite build-up appears to be the only known example of large microbial bioconstruction made by microlaminated dendrolites.