Mediterranean Foraminiferal Changes as Related to Paleoceanography and Paleoclimatology

Abstract

The modern distribution of planktonic and benthic foraminifera in a semienclosed sea such as the present-day Mediterranean is closely related to water mass movement, nutrients, and oxygen supply. Temperature and salinity playa secondary role. Diversification of the assemblages differs from west to east depending on basin conditions and sill depths. The stratigraphic record indicates that both long- and short-term fluctuations affected benthic and planktonic life of the Mediterranean from the early Miocene to the Recent. From early to middle Miocene, the composition of assemblages is similar from west to east, indicating accessibility to the Indo-Pacific and Atlantic oceans, with an increase in temperature and a rise in sea-level. In the late middle Miocene, sea-level lowering induced differentiations in the planktonic and benthic assemblages from west to east. From the late Miocene (Tortonian) to the Messinian, a short transgression was followed by a general regression. A change in assemblages indicates a more oxygenated water column in the west and depletion of nutrients and oxygen in the east. In Messinian time, rapid sedimentological changes in small tectonically controlled basins correlate with fluctuations in the foraminiferal assemblages. From west to east, some assemblages of shallow-water benthic species are recorded, below the evaporite deposits, in relation to a lowering of sea-level. Tectonic activity and vertical movements are recorded in the Eastern Mediterranean, from the late Messinian to the earliest Pliocene. In the Western Mediterranean, the MiocenePliocene boundary is marked by some marine regressive facies along the coast; evaporites and salt are deposited in deep basins. From this time onward, none of the tropical planktonic or benthic species is recorded in the Mediterranean. Early Pliocene transgression is progressive but rapid. Diversification of benthic foraminiferal assemblages depends on the configuration of the basins. From the early Pliocene to the middle Pliocene, there is an increase in the temperature and a rise in sea-level. With some exceptions, exchanges between the Atlantic Ocean and the Mediterranean from west to east are relatively direct. In the late middle Pliocene and the late Pliocene, a decrease in the temperature and a fall in the sea-level cause differences between west and east, with a maximum in the late Pliocene. Rapid fluctuations of temperature in the Pleistocene induce changes in the pelagic and benthic foraminiferal assemblages. The role of interrelated physico-chemical factors in the sediment-water interface and in the water column remains poorly defined. These factors are responsible for the distribution of microfaunal communities, with regard to the general evolution of the Mediterranean and the paleogeography of the basins.