Table 7. Framework for interpreting the observed compositional and isotopic trends at Sites 969 and 974.
Mechanism for
sapropel formation
Predicted signature
in sapropels
Existing evidence for
and against this mechanism
Influx of terrigenous organic material Negative 13Corg excursion (13C of terrestrial Corg -27) +Plant debris, terrestrial biomarkers, & high C/N found in selected Eastern Mediterranean cores and sapropels (Deroo et al., 1978; Sigl et al., 1978).

Unchanged 13Ccarbonate +Negative 13C and 15N excursions in sapropels (Calvert et al., 1992).

Negative 15N excursion (15N of terrestrial PON 0) - 13Csap has marine signature of -20 to -23 (Fontugne and Calvert, 1992).

Increased C/N ratio - No positive 13C gradient away from Nile.
Influx of terrigenous nutrients and DIC, resulting in enhanced surface productivity Negative 13Corg and 13Ccarbonate excursions (lighter and more abundant DIC) +Negative 13Corg,13Ccarbonate, and 15N excursions in sapropels (Calvert et al., 1992).

Negative 15N excursion (more abundant & possibly lighter nitrate) +Oxygen isotope and other evidence for freshwater flooding during sapropel formation.

No elevated C/N ratio +Light 13C of G. ruber carbonate at many eastern and western Mediterranean sites (Vergnaud-Grazzini et al., 1986 and Thunell et al., 1990).
Upwelling-supplied increase in surface nutrients and DIC, resulting in enhanced surface productivity Heavier 13C (production increases more than CO2(aq)) +Negative 13C and 15N excursions in sapropels (Calvert et al., 1992).

Lighter 13Ccarbonate +Freshwater flooding likely triggered estuarine circulation and E. Med. upwelling.

Lighter 15N (more nitrate) +Light 13C of G. ruber carbonate at many eastern and western Mediterranean sites (Vergnaud-Grazzini et al., 1986 and Thunell et al., 1990).

Diatom bloom (upwelled waters are Si-rich) ±Localized high diatom and silicoflagellate abundance in E. Med. sapropels (Sutherland et al., 1984).
Deep-water anoxia and enhanced preservation Heavier 13C (diagenetic effect on C is lessened) +15Nsap, chlorin 15Nmodern plankton, suggesting 15N excursions in sapropels are a diagenetic signal (Sachs, 1996).

Lighter 15N (diagenetic effect on N is lessened) +Freshwater flooding likely triggered estuarine circulation and Eastern Mediterranean anoxia.

Possibly unchanged 13Ccarbonate