Περίληψη:
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, micropalaeontological and geochemical studies of the dark-coloured layers and the cream/brownish sediments that occur above and below them. Part of the signal may be removed by early diagenetic processes. The extent and direction of these processes are ultimately controlled by the amount and reactivity of organic matter. The interval of dark colour associated with a sapropel is often somewhat thicker 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-sapropel' layer of variable thickness underlies most sapropel layers. A few centimeters above the most recent sapropel S1, is usually found a clear dark-brown layer 2-3 cm thickness is usually found, which has a diffuse, often mottled, 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 darkbrown layer to the visible upper S1 boundary, there 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., and 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 climatic (humidity) conditions. Using barite-Ba as a paleo-productivity indicator, enhanced fluxes, and hence increases in accumulation rates of organic carbon to the seafloor, must have occurred from approximately 9 to 5 ky BP. The perfect correlations between observed C(org) and calculated C(org) from the C(org)/Ba relation in the visible S1 interval, and the total lack of such correlation in the interval between the darkbrown layer and the visible S1 layer are remarkable. It seems, therefore, that S1 deposition lasted from 9 to 5 ky BP but the organic carbon for the upper part has been removed. The double barium peak visible in a number of sediments studied in high-resolution, may be related to the '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., Dimitrov, P., Gorur, N., Sakinc, M., and 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/Insolation Index. However, the relative value of an insolation maximum and initial C(org) content of the corresponding sapropel are not always related in a simple way. This again points to additional (possibly hydrographic) factors determining (the intensity of) sapropel formation. Understanding the mechanisms of sapropel formation and their subsequent preservation is not only necessary if our reconstructions of palaeoenvironmental conditions in the Eastern Mediterranean are to be improved, but may also assist in understanding the present-day situation and in forecasting possible future developments. The observed paleoenvironmental variations are discussed in a geochemical and environmental context, and are illustrated using typical examples from the eastern Mediteranean.
Συγγραφείς:
De Lange, G.J.
Van Santvoort, P.J.M.
Langereis, C.
Thomson, J.
Corselli, C.
Michard, A.
Rossignol-Strick, M.
Paterne, M.
Anastasakis, G.
