(Table 1) Sapropel analysis from ODP Sites 160-964, 160-966, 160-967 and 160-969

Seventeen eastern Mediterranean Pliocene sapropels from ODP Sites 964, 966, 967 and 969, some of which are coeval, have been analysed for their geochemistry. The sapropels are characterized by very high organic carbon contents (up to 30%) which are reported to be the result of both increased productivity and improved preservation. Although the organic matter in the sapropels is mainly of marine origin, the d13Corg values and C/N ratios appear "terrestrial". This is the result of anaerobic organic matter degradation which preferentially removed nitrogen- and 13C-rich organic components. A comparison with Ti/Al profiles, which mimic the precession index, and a calculation of organic carbon accumulation rates indicate that sedimentation rates were at most 30% lower or at most 50% higher during sapropel formation. Thus, sapropel formation lasted from between 2000 and 10,000 years at Site 964 to between 4500 and 12,000 years at Site 967. A synthesis of new data and a comparison with existing models indicates that productivity, which increased due to extra nutrients supplied as a result of winter mixing and as a result of enhanced input by the Nile, was the driving mechanism behind sapropel formation. The resulting sapropel formation was simultaneous at different depths, but lasted longer in the part of the basin closest to the Nile.

(Appendix) Carbon and nitrogen isotope ratios of ODP Site 160-964

Nitrogen isotopes of chlorins, degradation products of chlorophyll, reflect the isotopic composition of nutrient N utilized by marine phytoplankton communities. Here we show that in sediments of the eastern Mediterranean Pleistocene and Holocene, values of d15N for chlorins and total nitrogen vary in concert, with a consistent offset of ~5 per mil reflecting the fractionation imparted during chlorophyll biosynthesis. Samples from the Integrated Ocean Drilling Program Sites 964 and 969 were analyzed at a sampling resolution of ~4-10 cm, clustered around sapropel events 2, 3, 4 and 5 (~100-170 ka). In low organic content sediments, chlorin values of ~0 per mil coincident with total nitrogen values of ~+ 5 per mil indicate that the latter reflects the original biomass and is not a consequence of diagenetic isotope enrichment. In sapropel horizons, the chlorin and total nitrogen values are 5 per mil more negative (~-5 per mil and ~ 0 per mil, respectively), resembling previously-reported, modern-day water-column particulates (~0 per mil). We suggest that nutrient conditions in the Eastern Mediterranean correspond to three scenarios and that the similarity between sapropel and modern-day bulk d15N is coincidental. Organic-poor marl sediments formed under oligotrophic conditions where surface productivity resulted from upwelling of Atlantic-sourced nitrate. Sapropels were characterized by enhanced diazotrophy that was likely fueled by increased riverine P fluxes to surface waters. Present-day conditions are dominated by anthropogenic N sources. These scenarios agree with a model of sapropel formation in which stratification caused by increased fresh-water inputs led to N fixation due to P:N nutrient imbalance. Enhanced production combined with stratification promoted and maintained anoxic deep waters, consequently increasing organic matter preservation. Such a model may be relevant to interpreting other episodes of intense organic matter deposition in past oceans.

The sapropel record of the eastern Mediterranean Sea

Research on sediments recovered during Ocean Drilling Leg 160 has concentrated on two issues: the first concerned the stratigraphy of sapropel formation, the second was oriented to clarify specific processes that explain sapropel origin. Progress has been made in the construction of stratigraphic composites out of sedimentary sequences from individual holes at each of the palaeoceanographic sites. On the composites, initial work has resulted in the establishment of high-resolution and intermediate-resolution stratigraphies for three sites (963, 964, 967); correlation of sedimentary cycles to astronomical (insolation) cycles extends the stratigraphies to Sites 969 and 966. The sapropel occurrences in the marine and land sequences over the entire Eastern Mediterranean are correlated; with the resolution that can be obtained from isotope studies, groups of sapropels occurred simultaneously over the entire basin. In detail, however, the temporal and facies patterns of sapropel sequences differ between individual sites and depositional basins. The differences may be related to effects of water depth, diagenesis, and post-depositional tectonic attenuation of sequences. Studies on the geochemistry and facies of sapropels agree that anoxic conditions favoured preservation of organic matter in sapropels, caused the enrichment of trace metals associated with sapropels, and helped to preserve primary sedimentary structures. Besides, all evidence is consistent with elevated fluxes of organic matter and associated elements during sapropel events.