Paleoenvironmental changes at the Messinian–Pliocene boundary in the eastern Mediterranean (southern Cyprus basins): significance of the Messinian Lago-Mare

Abstract

The study of the late Messinian–Early Pliocene sediments in south Cyprus (Pissouri and Polemi basins) and the comparison with the offshore coeval deposits drilled in the ODP Leg 160 boreholes, provide new data illustrating the environmental changes which occurred in the eastern Mediterranean at the end of the salinity crisis. The Messinian–Pliocene transition is marked by a very rapid environmental change indicated by sharp variations in lithology, sedimentology, microfaunal assemblages and stable isotope composition. The latest Messinian interval is dominated by the ‘Lago-Mare’ oligo-mesohaline environments. The most striking feature in the Pissouri Basin is the intercalation between the uppermost gypsum bed and the base of the Zanclean deposits of four main horizons of paleosols interbedded with carbonates and conglomerates. Paleosols indicate periods of subaerial exposure long enough to permit pedogenesis to develop. Conglomerates indicate intense erosion of the Troodos Massif and its sedimentary cover as a result of the water level drop, tectonic activity and more humid climatic conditions. Thus, several subaerial exposure phases occurred in the marginal areas like the Pissouri Basin whereas shallow oligohaline conditions persisted in the deeper parts of the basin, as indicated at ODP Site 968. Both in the Pissouri Basin and on the flank of the Erathostenes Seamount the late Messinian paleosols are directly overlain by the lowermost Pliocene sediments deposited in well-oxygenated deep marine conditions. This drastic change of water depth shows that the water level of the late Messinian saline to brackish lakes dropped far below that of the world ocean, at least more than several hundred meters. Both these base level fluctuations and the oligohaline conditions caused intensive erosion–karstification of the upper gypsum beds as also reported from many other Mediterranean regions. Moreover, the study of the Polemi sections shows that oligohaline conditions had already started during the deposition of the upper gypsum sub-unit. Typical brackish and fresh water assemblages are present in the last two gypsum layers and intergypsum beds. This indicates that freshwater dilution, i.e. the Lago-Mare conditions, resulted from a progressive change of the hydrological budget which started during deposition of the upper evaporites and climaxed after the deposition of the uppermost gypsum layer (classical Lago-Mare facies). During this time, periods of increased evaporation over precipitation led to deposition of the uppermost gypsum layers. This evolution is interpreted as being due both to the severance from the open ocean and to the increasing contribution of the runoff–precipitation versus marine inputs in the hydrological budget of the Mediterranean.