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 extensive nitrogen fixation. This evidence is reconciled with previous indications of high productivity by invoking a model of sapropel formation in which increased river discharge facilitates development of a specialized phytoplankton 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 harbor diazotrophic bacterial symbionts, but does not support eutrophication of surface waters by enhanced river runoff or a circulation reversal.
Sachs
JP
Department of Environmental Science, Barnard College, Columbia University, 3009 Broadway, New York, NY 10027, USA. Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
Repeta
DJ