The transition toward the Messinian evaporites identified by biomarker records in the organic-rich shales of the Tripoli Formation (Sicily, Italy)

Pre-evaporitic Messinian deposits, consisting of a cyclic alternation of diatomites with lime mudstones, and laminated marls, crop out all over the Mediterranean area, representing important archives of deteriorating environmental conditions. The peculiar section of Torrente Vaccarizzo (Sicily) shows several intervals of organic-rich shales, exceptionally rich in lipid biomarkers, interbedded mostly with diatomites, laminites, and lime mudstones. The research is dealing with geochemical analyses on organic rich-shales to characterize the depositional environments and the organic matter source, as well as to investigate the biological archives predating the extreme saline conditions of the Messinian Salinity Crisis. Biomarker composition along with petrographic results reflect a change from oxic/dysoxic to dysoxic/anoxic conditions towards younger strata. Although a minor terrestrial input is displayed by saccharides, angiosperm biomarkers such as amyrins, dicarboxylic acids, and lignin degradation, most of the n-alkanes, isoprenoids, n-alkanoic acids, steroids, hopanes, and tocopherols are produced by marine hypersaline organisms. Kerogen investigation from Rock Eval pyrolysis revealed a moderate-good oil potential and it pointed to a thermally immature organic matter, which is also confirmed by multiple geochemical parameters (e.g. hopane, sterane). Bituminite at this stage of thermal maturity is not prone to hydrocarbon generation. In general, its hydrocarbon generation potential will depend on the degree of bacterial alteration. This work highlights the relevance in using biomarkers to characterize the response of biological communities to deteriorating conditions.