Molecular fossils and other organic markers as palaeoenvironmental indicators of the Messinian Calcare di Base Formation: normal versus stressed marine deposition (Rossano Basin, northern Calabria, Italy)

A multidisciplinary study has been carried out on the Late Miocene (Messinian) Calcare di Base Formation in northern Calabria, Italy, with the aim of understanding the depositional conditions of these enigmatic carbonate sediments that have been interpreted as evaporitic or diagenetic limestone. The research has been developed through sedimentological (microfacies), organic petrography and geochemical analyses (palynofacies, Rock-Eval pyrolysis and gas chromatography/mass spectrometry). The carbonates studied preserve their original mineralogy (aragonite) and microstructure. The prevailing microfacies consists of clotted peloidal micrite with antigravitative fabric. Larger and darker cylindrical–subcylindrical micritic grains, attributable to faecal pellets, are more or less randomly dispersed in the peloidal micrite. The faecal pellets have been ascribed to copepods because zooclasts of these arthropods were observed in the palynofacies. Bright epifluorescence characterizes the faecal pellets and peloidal micrite, indicating their high content of organic matter. Detrital and stromatolitic microfacies occur rarely (around a few percent). The detrital microfacies is characterized by silt-sized grains organized in thin, sometimes bioturbated, graded laminae. The stromatolitic microfacies shows planar to gently curved/wrinkled laminae organized in dark and light couplets. The associated sedimentary organic matter shows low maturity, and preserves the original signatures of the source organisms. Rock-Eval pyrolysis revealed a transitional composition between Type II and Type III kerogen, suggesting a mixture of marine and terrigenous organic matter input. The molecular biogeochemical data indicate three main biological signatures: algae, bacteria and higher plants. Organic markers together with sedimentary evidence indicate a marine depositional scenario influenced by freshwater input rather than evaporitic concentration. We conclude that the most probable bacterial process involved in carbonate precipitation of the Calcare di Base, considering the freshwater input and consequent probable enrichment in organic matter and Ca2+ ions, is ammonification of amino-acids in aerobic conditions.