Enhanced rainfall in the Mediterranean region during the last sapropel event

Sea-surface temperature (SST) estimates for the last 10,000 years have been derived from foraminiferal fauna variations in ten cores from the Mediterranean Sea. For the eastern cores, modern analogs of fossil assemblages are found in the eastern Mediterranean Sea. In the Alboran and Tyrrhenian Seas, the bet analogs originate from modern fauna located either in the western Mediterranean Sea or in the North Atlantic Ocean. During the last sapropel event, centred at about 8,000 years B.P., SSTs were similar to present in the eastern basin, whereas they were colder than today by about 1.5 degrees C in the Alboran Sea and 2.5 degrees C in the Tyrrhenian Sea. Oxygen isotope measurements agree with ecological studies to show that Globigerina bulloides and Globigerinoides ruber alba grow their shell respectively in April-May and October-November. Assuming that these species deposited their shell in isotopic equilibrium with ambient water, we reconstructed Mediterranean surface water delta(18)O and salinity during the last sapropel event (S-1). In contrast with the modern pattern in which evaporation dominates and salinity increases from west to east, the surface salinity during S-1 became almost homogeneous over the whole basin. This pattern suggests that the freshwater budget (precipitation plus runoff minus evaporation, P + R - E) was nearly equilibrated and that the Mediterranean Sea had ceased to be a concentration basin. In the western Mediterranean Sea, the observed cooling can account for the formation of intermediate and deep waters with densities only slightly higher than that of the Atlantic subsurface water at the Gibraltar sill level. The Mediterranean outflow was then drastically reduced and the residence time of the Mediterranean deep water increased. In the eastern basin, no significant temperature change occurred during the Holocene. During the sapropel event S-1, surface salinities were not significantly different from those of the western basin and deep water colder and denser than those of the western basin could not form. At the level of the Siculo-Tunisian Strait, the cold western intermediate water penetrated the deep eastern Mediterranean Sea. As a consequence, a permanent pycnocline was established between surficial and intermediate waters, preventing winter overturning and the supply of dissolved oxygen in the deeper part of the basin. This hydrological structure was responsible for the establishment of anoxia and the preservation of organic matter at the bottom in the eastern Mediterranean Sea.