Seasonal salinity changes during Mediterranean sapropel deposition 9000 years B.P.: Evidence from isotopic analyses of individual planktonic foraminifera

摘要

Using stable isotopic analyses of individual tests of planktonic foraminifera with different seasonal and depth habitats, we have investigated temporal variations in surface water salinities in the eastern Mediterranean during the past 13,000 years. Low oxygen isotopic values document the presence of anomalously low salinity conditions in the eastern Mediterranean at about 13,000 years B.P., 4000 years before the deposition of the youngest sapropel. By 11,000 years B.P., during the deposition of presapropel sediments, low‐salinity surface waters were more pervasive but were restricted to summer months and only affected the surface of the water column. At the onset of sapropel deposition about 9000 years B.P., surface salinities in the eastern Mediterranean had decreased by almost 4‰ and had extended to depths of at least 75 m. The low‐salinity surface lens was not, however, a permanent feature of the eastern Mediterranean during sapropel deposition. Low‐salinity surface conditions occurred in the fall and winter seasons, but normal sea surface salinities returned during spring months on a seasonal and/or interannual basis. Thus, a low‐salinity surface lens could have existed continuously for months or decades at a time but would have been interrupted intermittently by a return to normal conditions. 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 salinities returning occasionally during spring months, a low‐salinity surface lens may have greatly inhibited the production of Levantine intermediate water, prevented the formation of Adriatic deep water, and consequently, reduced ventilation of the eastern