CONCLUSIONS AND DIRECTIONS FOR FURTHER RESEARCH

The positive ¹³C excursions in sapropels at Sites 969 and 974 allow us to rule out terrigenous nutrients and terrigenous organic matter as basinwide factors in sapropel formation. The available nitrogen and carbon isotope data are consistent with sapropel formation through either enhanced preservation in the sediments or increases in surface productivity through upwelling. Although Sites 969 and 974 display very similar properties, a better knowledge of the temporal relationship between the sapropels at these sites will be required to distinguish between sapropel formation mechanisms involving a single, Mediterranean-wide event and those involving separate eastern and western events.

Our inability to identify a single mechanism for sapropel formation at all three sites does not mean that these sapropels are unrelated. On the contrary, all the mechanisms examined here could have been triggered by freshwater flooding, possibly caused by African monsoonal rains and (for interglacial sapropels) by glacial meltwater or Black Sea overflow. For example, in an estuarine circulation regime, basin-wide anoxia would be consistent with regional upwelling and localized freshwater influx in the eastern Levantine Basin, producing a negative ¹³C shift at Site MD 84641 and a positive ¹³C shift at the other two sites.

These conclusions reveal two important gaps in knowledge. First, stratigraphic correlation between eastern and western Mediterranean sediments is necessary to constrain the spatial extent of sapropel forming events in the two basins. Second, additional work on the isotopic effects of diagenesis, and the use of additional biomarkers to compare organic matter in sapropels, nonsapropel sediments, and modern-day plankton, may help to distinguish between upwelling and enhanced preservation as mechanisms for sapropel formation.