SEASONAL SALINITY CHANGES DURING MEDITERRANEAN SAPROPEL DEPOSITION 9000 YEARS BP - EVIDENCE FROM ISOTOPIC ANALYSES OF INDIVIDUAL PLANKTONIC-FORAMINIFERA

Using stable isotopic analyses of individual tests of planktonic foram inifera with different seasonal and depth habitats, we have investigat ed temporal variations in surface water salinities in the eastern Medi terranean during the past 13,000 years. Low oxygen isotopic values doc ument the presence of anomalously low salinity conditions in the easte rn Mediterranean at about 13,000 years B.P., 4000 years before the dep osition of the youngest sapropel. By 11,000 years B.P., during the dep osition of presapropel sediments, low-salinity surface waters were mor e pervasive but were restricted to summer months and only affected the surface of the water column. At the onset of sapropel deposition abou t 9000 years B.P., surface salinities in the eastern Mediterranean had decreased by almost 4 parts per thousand and had extended to depths o f at least 75 m. The low-salinity surface lens was not, however, a per manent feature of the eastern Mediterranean during sapropel deposition . Low-salinity surface conditions occurred in the fall and winter seas ons, but normal sea surface salinities returned during spring months o n a seasonal and/or interannual basis. Thus, a low-salinity surface le ns could have existed continuously for months or decades at a time but would have been interrupted intermittently by a return to normal cond itions. Any triggering mechanism proposed for Mediterranean sapropels must account for the seasonal fluctuations and ephemeral nature of low -salinity conditions in the eastern Mediterranean. Despite normal sali nities returning occasionally during spring months, a low-salinity sur face lens may have greatly inhibited the production of Levantine inter mediate water, prevented the formation of Adriatic deep water, and con sequently, reduced ventilation of the eastern Mediterranean.