Molecular palaeontology of eastern Mediterranean sapropels: Evidence for photic zone euxinia

Abstract

Organic geochemical data are presented for two Pliocene sapropels recovered during Ocean Drilling Program (ODP) Leg 160 in the Eastern Mediterranean Basin. One sapropel from Site 969 is exceptionally rich in organic carbon (TOC up to 30%) and the other one, from Site 967, has a more typical organic carbon content (TOC up to 8%). The majority of the solvent extractable organic matter of these sapropels can be ascribed to autochthonous sources. Higher plant-derived organic compounds are present in only minor amounts. Pyrolysis gas chromatography–mass spectrometry (GC-MS) analyses of the bulk organic matter also indicates that the contribution of terrestrial organic matter to these sapropels is insignificant. The presence of biomarkers derived from green sulfur bacteria, which are indicative of anoxygenic photosynthesis, show that during sapropel formation anoxia was not merely confined to the lower parts of the water column but, in fact, extended into the photic zone, resulting in an almost completely euxinic water column, a setting similar to the present-day Black Sea. Furthermore, it is proposed that these paleoenvironmental conditions have been a major controlling factor in the accumulation of organic matter in these sapropels.