The Messinian "Vena del Gesso" evaporites revisited: characterization of isotopic composition and organic matter

The "Vena del Gesso" (Gessoso−Solfifera Fm., Messinian) is a 227 m−thick ridge along the western Romagna Apennines (Italy) consisting of up to 16 selenite cycles separated by shales and minor carbonate. The TOC values of these deposits range between 0.087−0.016% (gypsum) and 3% (shales). Organic matter is dominated by black debris associated with continental debris. Algae and dynocysts are rare (<1%). The amount of amorphous organic matter is low but it may reach up to ~40%. The 87Sr/86Sr of gypsum and carbonate vary from 0.708890 to 0.709024, yielding non−oceanic values with several exceptions that plot within error of coeval oceanic values only in the upper part of the section (from the 6° bed).The sulphur isotope composition of gypsum range between δ34S=+21.8 and +23.7‰ and may represent precipitation of δ34S−enriched gypsum due to the fractionation effect or recycling of coeval gypsum with contributions of brine−sediment redox variations.The isotope values of carbonates show a large variability (−6.4<+6.05‰; −14.68<+2.5‰) suggesting a complex origin by mixing of marine and non−marine waters with a significant contribution of reduced organic matter.These data point to an evaporite basin dominated by continental waters which received significant phases of marine recharge in the upper part together with a marked facies change. Because seawater recharges and a similar facies change are present in other Messinian sections, it follows that we have new possible geochemical and facies markers to correlate the Lower Evaporites across the Mediterranean.