Palaeo-environmental variations in eastern Mediterranean sediments: a multidisciplinary approach in a prehistoric setting

Not only the occurrence of distinct organic-rich intervals (sapropels). but also the relative contents of key major and minor elements and isotopes in the sediments of the eastern Mediterranean, appear to be cyclic and to be astronomically associated, interpretations of the environmental conditions leading to sapropel formation are based on results from sedimentological, m icropalacontological and geochemical studies of the dark-coloured layers an d the cream/brownish sediments that occur above and below them. Part of the signal may be removed by early diagenetic processes. The extent and direct ion of these processes are ultimately controled by the amount and reactivit y of organic matter. The interval of dark colour associated with a sapropel is often somewhat th icker than that defined by the >2% C-org definition and usually has a sharp colour change at upper and lower boundaries, A grey so-called 'proto-sapro pel' layer of variable thickness underlies most sapropel layers. A few cent imeters above the most recent sapropel S1, is usually found a clear dark-br own layer 2-3 cm thickness is usually found, which has a diffuse, often mot tled, upper boundary and a relatively abrupt colour transition at its lower boundary. The colour is characteristic of Mn oxyhydroxide enrichments. In the interval from the dark-brown layer to the visible upper S1 boundary, th ere is usually an increasingly red-brownish colour. The distinct upper manganese Marker-Bed has been related to the Santorini ( Minoan) eruption in 3356+/-18 BP (Bruins, H. J., & Van Der Plicht, J. (1996 ). The Exodus enigma. Nature, London. 382, 213-214), but is more likely to be associated with a Basin-wide re-ventilation event induced by changing cl imatic (humidity) conditions. Using barite-Ba as a paleo-productivity indicator, enhanced fluxes, and hen ce increases in accumulation rates of organic carbon to the seafloor, must have occurred from approximately 9 to 5 ky BP. The perfect correlations bet ween observed C-org and calculated C-org from the C-org/Ba relation in the visible S1 interval, anti the total lack of such correlation in the interva l between the darkbrown layer and the visible S1 layer are remarkable. It s eems, therefore, that S1 deposition lasted from 9 to 5 ky BP but the organi c carbon for the upper part has been removed. The double barium peak visibl e in 3 number of sediments studied in high-resolution, may be related to th e `Flooding' of the Black Sea that occurred around 7150 BP (Ryan, W. B. F., Pittman, W. C., Major, C. O., Shimkus, K., Moskalenko, V., Jones, G. A., D imitrov, P., Gorur, N., Sakinc, M., & Yuce, H. (1997. An abrupt drowning of the Black Sea shelf. Marine Geology, 138, 119-126). Using various 'proxies', the (initial) occurrence of eastern Mediterranean sapropels appears cyclic and to be strongly related to the Monsoonal/Insola tion Index. However, the relative value of an insolation maximum and initia l C-org content of the corresponding sapropel are not always related in a s imple way. This again points to additional (possibly hydrographic) factors determining (the intensity of) sapropel formation. Understanding the mechanisms of sapropel formation and their subsequent pre servation is not only necessary if our reconstructions of palaeoenvironment al conditions in the Eastern Mediterranean are to be improved. but may also assist in understanding the present-day situation and in forecasting possi ble future developments. The observed paleoenvironmental variations are discussed in a geochemical a nd environmental context, and are illustrated using typical examples from t he eastern Mediteranean. O 1999 Elsevier Science Ltd. All rights reserved.