Ionian Sea sapropel distribution and late Quaternary palaeoceanography in the eastern Mediterranean

Abstract

SAPROPELS, dark organic-rich mud layers observed in deep-sea cores of the eastern Mediterranean, indicate that oceanographic conditions in this almost completely land-locked sea were at certain times during the Pliocene and Quaternary significantly different from those observed at present. Regional correlation of Quaternary sections in piston cores collected during the past two decades has shown that these distinct grey-to-black layers, ranging from a few millimetres to as much as 50 cm or more thick, are deposits that for the most part cover laterally extensive areas1. It has been suggested that sapropel formation is directly related to phases of stagnation2,3 which occurred synchronously over much of the eastern Mediterranean4. It was proposed that this lithofacies type was associated with well-developed water mass stratification that induced euxinic conditions on the sea floor5,6 and accumulated primarily during periods of climatic warming. Recent studies indicate that some sapropels also formed during cooling trends within interglacial episodes7. Interpretation of Mediterranean palaeoceanographic conditions during late Quaternary stagnation phases requires a precise determination of both the spatial distribution and the age of sapropels. Correlations are generally made by lithologic, microfossil and oxygen isotope methods. The core-to-core variations (due to hiatuses or repetition of section) require a framework of absolute age determination in order to identify specific sapropels reliably over broad regions6. The study reported here is an extension of lithofacies and chronostratigraphic (based on 14C dating) investigations made by the Smithsonian Institution, of numerous piston cores recovered between the Strait of Sicily in the central Mediterranean and the Nile Cone in the eastern Levantine Basin. The regional distribution of late Quaternary sapropels is outlined and their significance discussed. Two recent hypotheses are also considered: (1) that sapropels are absent in the western Ionian Sea; and (2) that stagnant phases were not basin-wide phenomena8. These latter points contradict prevailing hypotheses4,5. Information available in the Ionian Sea must be compared with data in adjacent basins to interpret properly late Pleistocene and Holocene physical oceanographic events in the Mediterranean.