STABLE ISOTOPE STUDY OF THE CYCLIC DIATOMITE-CLAYSTONES FROM THE TRIPOLI FORMATION, SICILY - PRELUDE TO THE MESSINIAN SALINITY CRISIS

• Published in: Palaeogeography, Palaeoclimatology, Palaeoecology
• Main Authors: Mckenzie, J, Jenkyns, H, Bennet, G
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2016
• Subjects: Antarctic; The Antarctic; Antarc*; Ice Sheet
• Online Access: https://doi.org/10.1016/0031-0182(79)90077-4; https://ora.ox.ac.uk/objects/uuid:b8770ef9-0557-4fbc-8dc8-c23c199130cb

The Tripoli Formation, underlying the Messenian evaporites of Sicily, comprises cyclic alternations of diatomites and claystones that commonly contain dolomite. An oxygen- and carbon-isotope study on the carbonate component of the diatomaceous interbeds records normal marine conditions, whereas the 18O values (+4.0 to +8.5‰) of the dolomite in the claystones signify dolomitization by highly evaporated waters. Extremely negative 13C values (-3.5 to -22.0‰) of this dolomite, certain interbedded limestones, and the overlying Calcare di base denote an organic source of carbon dioxide, the origin of which could be explained by the metabolic activity of sulfate-reducing bacteria in anoxic sediments. This, in turn, implies the former presence of evaporites (?gypsum) before the onset of the Messenian salinity crisis. Eustatic sea-level changes dependent upon the waxing and waning of the Antarctic ice sheet are proposed as the cause of the periodic restrictions in the Tripoli Sea. During high sea-level stands, diatomaceous sedimentation took place under normal marine conditions; during low sea-level stands, restriction and subsequent evaporation of water entering the Tripoli Sea favored the formation of dolomitizing solutions which diagenetically altered the carbonates in the clay-rich sediments. The restriction of the Tripoli Sea during low sea-level stands required the presence of a shallow Atlantic-Mediterranean sill. Progressive tectonic constriction of the connection between the two water bodies, or global eustatic sea-level lowering, is indicated by the extensive dolomitization of the uppermost transitional unit of the Tripoli Formation and the calcitized "evaporite" beds of the Calcare di base. The final phase was total restriction which led to the desiccation of the deep Mediterranean basins and the onset of the Messinian salinity crisis. © 1979.