Oligotrophy and nitrogen fixation during eastern Mediterranean sapropel events

Nitrogen isotopic measurements in fossil chlorophyll from Late Pleistocene organic-rich sediments (sapropels) in the eastern Mediterranean Sea provide geochemical evidence for stratified, nutrient-depleted surface water and e xtensive nitrogen fixation. This evidence is reconciled with previous indic ations of high productivity by invoking a model of sapropel formation in wh ich increased river discharge facilitates development of a specialized phyt oplankton population whose annual mass sinking provides the organic flux to generate sapropels. This interpretation is consistent with the widespread occurrence of mat-forming diatoms that thrive in stratified water and can h arbor diazotrophic bacterial symbionts, but does not support eutrophication of surface waters by enhanced river runoff ora circulation reversal.