The transition from normal marine to evaporitic conditions recorded in a cold seep environment: The Messinian succession of Northern Italy

Messinian pre-evaporitic seep carbonates outcropping in the Tuscan-Romagna area share a common upward stratigraphic trend: massive seep carbonates rich in large lucinids, modiolids and planktonic foraminifera pass upward to horizontally laminated carbonates free of macrofauna and foraminifera. A transitional macrofauna-free facies marks the passage in some sections, sometimes with faint lamination. Thin section petrography, SEM-EDS analyses, and X-ray diffraction revealed a high textural and mineralogical variability between the studied sections and facies. In general, the upward facies transition is associated with a switch from massive packstone/grainstone with complex carbonate mineralogy to laminated mudstone microfacies with monomineralic, mostly dolomitic, carbonate composition. The δ13C values of all facies indicate domination of methane-derived carbon, thus demonstrating a methane-charged sedimentary setting with precipitation of authigenic carbonates induced by anaerobic oxidation of methane. The transitional and laminated facies contain several structures typical of microbial sediments, such as peloids, filamentous features, clotted micrite, fibrous cement, and frequent fenestrae; web-like structures are remnants of cyanobacterial mats and resemble Entophysalidaceae thriving at the bottom of the Messinian sea. The distinctive depositional and biotic switch (abrupt disappearance of macrofauna and planktonic foraminifera, predominance of dolomite over calcite, absence of aragonite, presence of horizontal lamination) reflects important environmental changes taking place in the basin. The rise of δ18O from the massive, through transitional, to the laminated facies suggests either a drop of bottom-water temperature or an increase of salinity. Nevertheless, other paleoenvironmental changes must have co-occurred in order to account for the dramatic facies shift observed. We hypothesize that this change could reflect the development of water-column stratification with respect to both salinity and oxygen concentration shortly before the Messinian Salinity Crisis