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 |
|