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

Abstract

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 high in lipid biomarkers, interbedded mostly with diatomites, laminites, and lime mudstones. This research deals with geochemical analyses of 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. The biomarker compositions with the petrographic results reflect a change from oxic/dysoxic to dysoxic/anoxic conditions towards younger strata. Although a minor terrestrial input is displayed by saccharides and angiosperm biomarkers, such as amyrins, dicarboxylic acids, and lignin degradation products, most of the n-alkanes, isoprenoids, n-alkanoic acids, steroids, hopanes, and tocopherols are produced by marine hypersaline tolerant organisms. Sulfurization of organic compounds (mainly isoprenoids and n-alkanols) complements the pyrite mineralogy also indicating anoxia. The assessment of kerogen by Rock Eval pyrolysis revealed a moderate-good oil potential and indicated 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, but depends on the degree of bacterial alteration. This work highlights the relevance of using biomarkers to characterize the response of biological communities to deteriorating environmental conditions.