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Pelagites and Organic-Rich Sediments |
Division of Sedimentology, NMNH, Smithsonian Institution, Washington, DC, 20560, USA
Sapropel is the term often applied in a general manner to dark, organic-rich layers of late Quaternary age in the eastern Mediterranean. The DSDP Leg 42A studies have more precisely defined sapropel layers as those containing more than 2.0% organic carbon by weight, and sapropelic layers as those that contain between 0.5 and 2.0% organic carbon. The present study further focuses on extensive vertical and lateral petrological variations of these and associated organic-rich layers. Herein the different sediment lithofacies that form both sapropel and sapropelic sequences in radiocarbon-dated cores distributed throughout the basin are defined. Organic carbon and carbonate carbon content and their ratio are useful criteria; some sedimentary structures and textures also help define sediment types.
The recognition of the dominant vertical successions of lithofacies that form typical sapropel and sapropelic sequences is important; these provide further evidence that favour marked episodic water mass stratification and associated stagnation. The lithofacies succession typically includes the following, from base up: greyish mud becoming darker upward and passing into a grey ooze which is separated from the overlying dark grey to black sapropel (or sapropelic) layer by a sharp contact; the latter layer, in turn, is topped by a thin grey ooze and an orange-brown oxidized layer. These sediments, poor in benthonic faunas, are interbedded within lighter, well-oxygenated open marine lithofacies.
Sapropel and sapropelic sequences comprise sediment types which are largely hemipelagic in origin (suspension settling through the water column); variations within such vertical successions may be directly related to palaeoceanographic events. Regional study, however, reveals numerous variations displayed by these two sequences, and thus further classification of the different organic-rich layers is needed. Lithofacies members are sometimes absent, or poorly developed, or repeated as a result of transport processes (mostly gravity-flow mechanisms) and/or specific oceanographic conditions affecting a depositional site. On the basis of the above, we identify episapropel and episapropelic sequences. These are defined as highly irregular sequences which include sediment types that are locally introduced by sediment gravity-flows, commonly mud-carrying turbidity currents. A more accurate assessment of the diverse organic-rich sequences in cores across the basin helps us to better understand the nature of short-term stagnation-oxygenation cycles that affected the eastern Mediterranean, and may also shed light on some black shales of open marine origin preserved in the rock record.
