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Enhanced rainfall in the Mediterranean region during the last Sapropel Event

M. Paterne,J. Duplessy,L. Labeyrie,M. Arnold,M. Fontugne,C. Pierre

Published 1998 in Oceanographic Literature Review

ABSTRACT

Mer Méditerranée Paléoclimatologie Paléocéanographie Evénement sapropèle Isotope Nejib KALLEL a, b, Martine PATERNE a, Jean-Claude DUPLESSY a, Colette VERGNAUD-GRAZZINI tc, Claude PUJOL d, Laurent LABEYRIE a, Maurice ARNOLD a, Michel FONTUGNE a and Catherine PIERRE c. tC. VERGNAUD-GRAZZINI died on May 1996. a Centre des Faibles Radioactivités, Laboratoire mixte CNRS-CEA, 91198 Gif-sur-Yvette cedex, France. b Ecole Nationale d'Ingénieurs, B.P. "W" , 3038 Sfax, Tunisie. c Laboratoire d'Océanographie Dynamique et de Climatologie, Université Pierre et Marie Curie, 4, place Jussieu, 75252 Paris cedex 05, France. d Département de Géologie et Océanographie, CNRS, URA 197, Université de Bordeaux 1, 33405 Talence, France. Received 05/01/96, in revised form 21/02/97, accepted 04/03/97. 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 best 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 sirnilar to present in the eastern basin, whereas they were colder than today by about l.5 °C in the Alboran Sea and 2.5 °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 isotopie equilibrium with ambient water, we reconstructed Mediterranean surface water 8180 and salinity during the last sapropel event (S 1). ln 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 bad 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 silllevel. The Mediterranean outftow 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 ,, surface sai inities 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

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