Palaeoenvironments of the Mediterranean Basin at the Messinian hypersaline/hyposaline transition: evidence from natural radioactivity and microfacies of post-evaporitic successions of the Adriatic sub-basin

At the end of the Messinian salinity crisis, changes in the palaeoceanography and palaeoclimate induced a transition from hypersaline to hyposaline conditions in the Mediterranean water body. Detailed investigation of natural radioactivity and microfacies analyses of two early post-evaporitic Messinian sections show that the transition occurred in poorly oxygenated and well-stratified water masses. These transitional deposits, which mainly consist of marls and CaCO3-rich horizons, are generally well laminated and totally barren in benthic and planktonic fauna. The presence of barite, small (5–8 lm) authigenic framboidal pyrite, and high values of U indicate persistent anoxic conditions during the early postevaporitic Messinian stage in the Adriatic sub-basin of the Mediterranean Sea. The presence of post-evaporitic welllaminated barren deposits from both Mediterranean borderland sections and ODP sites suggests that these anoxic conditions extended to other sub-basins of the Mediterranean region.