Orbitally-controlled oscillations in planktic communities and cyclic changes in western Mediterranean hydrography during the Messinian

Abstract

We use quantitative analyses of the planktic foraminiferal assemblages and stable isotope analyses of the Sorbas section in the western Mediterranean to reconstruct the cyclical changes in surface and deep water hydrographic conditions during the Late Miocene prior to the Messinian salinity crisis. Oscillations in winter and summer temperatures linked to cyclical fluctuations in the hydrographic conditions and nutrient availability were the main mechanisms driving the cyclical changes in the planktic foraminiferal assemblages during the Messinian. The winter intensification and southward expansion of the northern cool and dry winds during precession maxima lowered sea surface temperatures (below≃14–15°C), favoring water convection and the upward mixing of nutrients with the consequent increase in the proportion of cold, eutrophic water foraminifera. The low summer temperatures (below 24°C) inhibited the growth of warm, oligotrophic water foraminifera, as occurs today in the Mediterranean. By contrast, at times of precession minima the low influence of the high latitude air masses caused winter temperatures to remain relatively high which, combined with the lower surface salinities, led to the formation of a permanent pycnocline. Winter conditions prevented the growth of cold, eutrophic water foraminifera, while high summer temperatures and the formation of a permanent pycnocline stimulated the growth of warm, oligotrophic water foraminifera. The prevalence of stable water stratification during winter due to the large density gradient between the surface and intermediate waters prevented deep water formation and slowed down the rates of oxygen supply to the bottom, resulting in the formation of the sapropels. A sharp decrease in δ 13C of benthic and planktic foraminifera that occurred between 6.8 and 6.7 Myr is related to an increase in the residence time of Mediterranean waters. Because similar changes have been observed in other regions of the Mediterranean, we conclude that this change records a significant reduction in the Atlantic–Mediterranean water exchange at that time. Mediterranean climatic amplification increased throughout the Messinian as the Atlantic–Mediterranean water exchange was progressively more limited and oscillations in the ratio of planktic/benthic foraminifera and of warm-oligotrophic/cold-eutrophic species are good records of this increasing amplification, which culminated at 5.95 Myr with the deposition of gypsum–pelitic layers that mark the onset of the Messinian salinity crisis.