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Oligotrophy and nitrogen fixation during eastern mediterranean sapropel events
1Department 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.
Science (New York, N.Y.)
|January 5, 2000
Summary
Nitrogen isotope data from fossil chlorophyll indicate nutrient-depleted surface waters and high nitrogen fixation during late Pleistocene sapropel formation in the Mediterranean Sea. This suggests a specialized phytoplankton bloom, not eutrophication, fueled sapropel deposition.
Area of Science:
- Paleoceanography
- Marine Geochemistry
- Biogeochemistry
Background:
- Late Pleistocene sapropels in the eastern Mediterranean Sea suggest periods of high productivity.
- Previous studies indicated potential nutrient enrichment or circulation changes.
- The exact mechanisms driving sapropel formation remain debated.
Purpose of the Study:
- To investigate the source of organic matter and nutrient conditions during late Pleistocene sapropel formation.
- To reconcile geochemical evidence with existing models of Mediterranean sapropel deposition.
- To test hypotheses regarding eutrophication and circulation patterns.
Main Methods:
- Nitrogen isotopic measurements of fossil chlorophyll.
- Analysis of organic-rich sediment cores (sapropels) from the eastern Mediterranean.
- Geochemical interpretation of isotopic data in the context of paleoceanographic models.
Main Results:
- Nitrogen isotope data reveal stratified, nutrient-depleted surface waters.
- Evidence points to significant nitrogen fixation by microorganisms.
- Sapropel formation is linked to a specialized phytoplankton bloom and mass sinking events.
- Results do not support widespread eutrophication or circulation reversal.
Conclusions:
- Sapropel formation was driven by a unique phytoplankton community adapted to stratified, nutrient-poor conditions.
- Nitrogen fixation played a crucial role in supporting this ecosystem.
- The findings challenge previous interpretations of enhanced riverine input or major circulation shifts as primary drivers.
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